precursor chemistry, thin film deposition, mechanistic studies
TRANSCRIPT
Rational Development of Precursors for MOCVD of TiO2:
Precursor Chemistry, Thin Film Deposition, Mechanistic Studies
Raghunandan Krishna Bhakta, M.Sc.
2005
A dissertation submitted for the degree of Dr. rer. nat. (Doctor rerum naturalium)
in the Faculty of Chemistry at Ruhr- University Bochum,
Germany
Raghunandan Krishna Bhakta, M. Sc.
2005
Rational Development of Precursors for MOCVD of TiO2:
Precursor Chemistry, Thin Film Deposition, Mechanistic Studies
Dissertation Zur Erlangung des Grades eines
Doktors der Naturwissenschaften
der Fakultät für Chemie
an der Ruhr-Universität Bochum,
Deutschland
vorgelegt von
Raghunandan Krishna Bhakta, M. Sc.
2005
This dissertation is based on the experimental work carried out during the period from
April 2001 to September 2004 at the chair of Inorganic Chemistry II, Ruhr University
Bochum, Germany. Herewith I declare that the following work has been carried out
independently by me and all the sources of help and services used during this work have
been reported in the acknowledgement section.
Research Supervisor: Prof. Dr. Roland A. Fischer
Co-supervisor: Jun. Prof. Dr. Anjana Devi
2nd examiner: Prof. Dr. M. Epple
3rd examiner: Prof. Dr. W. S. Sheldrick
Day of examination 02.02.2005
Acknowledgements
I am indebted to many people who contributed in several ways to this work, and
supported me with their cooperation and timely help.
In particular, I wish to express my sincere gratitude to my research supervisor Prof. Dr.
Roland A. Fischer, for providing me an opportunity to work in his research group, for
introducing to an interesting research theme for the present work and for closely
following the work, for his support throughout this work with fruitful suggestions and for
reviewing this thesis.
I equally express my sincere gratitude to my co-supervisor, Jun. Prof. Dr. Anjana Devi,
who directly supervised this work. I thank her for unique personal support in every aspect
of the experimental work, from the precursor synthesis to the CVD, for all the stimulating
ideas, all the deep discussions, for providing support in preparation of various
manuscripts for publications, presentations, posters and for the time she spent in
reviewing this thesis.
This work forms a part of the project funded by Deutsche Forschungs Gemeinschaft
(DFG). I gratefully acknowledge DFG, who funded the project under CVD- SPP-1119.
I thank all my colleagues Andreas Kempter, Andrian Milanov, Jun.-Prof. Dr. Anjana
Devi, Arne Baunemann, Beatrice Buchin, Dr. Christian Gemel, Daniel Rische, Eliza
Gemel, Eva Maile, Eun Jeong Kim, Felicitas Schröder, Dr. Frank Hipler, Dr. Harish
Parala, Heike Kampschulte, Jayaprakash Khanderi, Manuela Winter, Marie-Katrin
Schoeter, Dr. Maxim Tafipolski, Mirza Cokoja, Dr. Rochus Schmid, Sabine Bendix,
Sabine Masukowitz, Stephan Hermes, Stephan Spöllmann, Thomas Kadenbach, Tobias
Steinke, Todor Hikov, Urmila Patil, Ursula Fischer, Ursula Herrmann, Dr. Younsoo Kim,
Dr. Wenhua Zhang for friendly and encouraging work atmosphere, help and cooperation.
I also thank and cherish the support and help received from my former colleagues Dr.
Andreas Wohlfart, Dr. Dana Weiß, Dr. Jurij Weiß, Prof. Dr. Jens Müller, Dr. Julia
Hambrock, Dr. Lianhai Lu, Dr. Nicola Oberbeckmann, Dr. Oliver Segnitz, Dr. Oliver
Stark, Dr. Pia Wennek, Dr. Ralf Becker, and Dr. Ulrike Weckenmann.
I gratefully acknowledge the following colleagues for their help and assistance as noted:
Stephan Hermes, Jutta Schäfer (GC, MS); Karin Bartholomäus (elemental analysis); Dr.
Christian Gemel (hydrolysis studies, scientific discussions); Manuela Winter, Dr. Iris M.
Müller, Dr. Klaus Merz (single crystal X-ray diffraction); Urmila Patil, Dr. Harish Parala
(TGA/DTA); Dr.Andreas Wohlfart, Dr. Harish Parala, Eva Maile, (XRD analysis); Dr.
Hans-Werner Becker (RBS analyses); Dr. Rolf Neuser (SEM); Dr. Frank Hipler (XPS
analysis); Prof. Dr. Jens Müller, Dr. Frank Hipler, Dr. Holger F. Bettinger, Eliza Gemel,
Sabine Bendix (helping in matrix isolation experiments).
My sincere thanks to Dr. Peter Ehrhart, Dr. Reji Thomas, Dr. Stephan Regnery, Prof. Dr.
Rainer Waser (Forschungszentrum Jülich) for their help in thin film depositions using an
industrial tool reactor, for various analyses of thin films at their facility and for a fruitful
collaboration.
I am extremely thankful to Sabine Masukowitz, and Heike Kampschulte for helping me
with administrative and official procedures. I gratefully acknowledge the help extended
by library personnel, mechanical workshop personnel, glass blowers, electrical workshop
personnel and chemical store personnel at the faculty of chemistry, which was
instrumental during my research efforts.
I would like to thank my former employer Rallis India Ltd. for hosting my career for one
year at their facilities. My sincere thanks are due to Dr. G. Ananda Rao, CEDT, Indian
Institute of Science, Bangalore, for his encouragement and support for my research
activities.
I am thankful to my friends, Aravinda, Eva, Ganesh, Janardana, JP, Keshav, Koushik,
Sasi, Shakila, Stephan, Suresh, Uday, Urmila, Vijay, Yathish, and my well-wishers
whose emotional support, encouragement and inspiration were instrumental during all
these years.
I dedicate this work to my parents M. Krishna Bhakta and Radha Bhakta, my wife Sarita,
my sisters, Sucheta, Suman, and Sahana and their families.
List of abbreviations and Acronyms AFM Atomic Force Microscopy
CVD Chemical Vapor Deposition
CFD Computational Fluid Dynamics
DFT Density Functional Theory
DRAM Dynamic Random Access Memory
DTA Differential Thermal Analysis
EDX Energy Dispersive X-ray
IR Infra Red
LPCVD Low Pressure Chemical Vapor Deposition
MBE Molecular Beam Epitaxy
Meaoac Methylacetoacetate
MIS Metal-Insulator-Semiconductor
MO Metal Organic
MOCVD Metalorganic Chemical Vapor Deposition
MS Mass Spectrometry
NMR Nuclear Magnetic Resonance
OEt Ethoxy group
OPri Isopropoxy group
PACVD Plasma Assisted Chemical Vapor Deposition
PECVD Plasma Enhanced Chemical Vapor Deposition
RT Room Temperature
SEM Scanning Electron Microscopy
Tbaoac Tertiary Butyl Acetoacetate
TGA Thermogravimetric analysis
TTIP Titanium Tetraisopropoxide
UV Ultraviolet
XPS X-ray Photoelectron Spectroscopy
XRD X-ray Diffraction
XRF X-ray fluorescence
Contents
Chapter 1
1.0 Introduction 1
1.1 Titanium dioxide 2
1.2 An overview of various thin film deposition technologies 5
1.3 An overview of CVD 7
1.4 CVD kinetics 11
1.5 Growth rate 14
1.6 Precursor for CVD 16
1.7 The liquid injection CVD 17
1.8 Gas-phase chemical species measurements 19
1.8.1 Mass spectroscopy 20
1.8.2 Gas chromatography 21
1.8.3 IR absorption spectroscopy 22
1.8.4 Matrix isolation FTIR spectroscopy 23
1.9 MOCVD of TiO2 thin films 25
1.10 Scope of the present work 26
1.11 References 30
Chapter 2
2.1 Introduction 34
2.2 Experimental section 36
2.3 Results and Discussion 39
2.3.1 Objectives of ligand design 39
2.3.2 Precursor synthesis and properties 43
2.3.3 NMR studies 48
2.3.4 Single crystal X-ray diffraction analysis 49
2.4 References 56
Chapter 3
3.1 Introduction 58
3.2 Experimental section 60
3.2.1 Experiments using home built CVD reactor 60
3.2.2 Experiments using industrial tool CVD reactor 63
3.3 Results and Discussion 65
3.3.1 Deposition of TiO2 thin films using home built CVD reactor 65
3.3.2 Deposition parameters 66
3.3.3 Crystal structure of the films 66
3.3.4 Effect of substrate temperature on growth rate 68
3.3.5 Film composition 70
3.3.6 Film microstructure 72
3.4 Deposition of TiO2 thin films using liquid injection industrial tool 75
3.4.1 Susceptor temperature dependent growth rate, surface roughness
structural and electrical properties 75
3.4.2 Surface roughness 76
3.4.3 Crystal structure 78
3.4.4 Effect of post deposition annealing on the structure and morphology 80
3.4.5 Electrical properties 82
3.5 Deposition of SrTiO3 thin films 85
3.6 References 90
Chapter 4
4.1 Introduction 93
4.2 Experimental section 99
4.2.1 Description of matrix isolation apparatus 101
4.2.2 Photolysis experiments using ultraviolet source 103
4.2.3 Preparation of gaseous mixture of argon and iso-propanol 103
4.3 Results and Discussion 104
4.3.1 Fundamental aspects of the matrix isolation technique 104
4.3.2 Spectroscopic methods 106
4.3.3 Vibrational spectroscopy in the infrared region 106
4.3.4 Sample preparation and concerns 107
4.3.5 In situ generation 108
4.3.6 Chemistry with matrices 109
4.3.7 Rigidity and mobility of matrix material 109
4.3.8 Ultraviolet photolysis of the decomposition products in the matrix 109
4.3.9 The FTIR spectra of [Ti(OPri)4] and iso-propanol 110
4.3.10 Thermolysis of the TTIP 114
4.3.11 Matrix isolation studies on mixed alkoxide complexes of titanium 121
4.3.12 The FTIR spectrum of the ligand tert. butylacetoacetate 122
4.3.13 Thermolysis of tert. butylacetoacetate 123
4.3.14 Thermolysis of [Ti(OPri)2(tbaoac)2] 127
4.3.15 The FTIR spectrum of the ligand 2,2,6,6,-teramethyl-3,5
-heptane dione ligand (Hthd) 136
4.3.16 Thermolysis of the ligand Hthd 137
4.3.17 Thermolysis of [Ti(OPri)2(thd)2] 139
4.4 References 144
Chapter 5
5.1 Introduction 146
5.1.1 Volatility 147
5.1.2 Long term stability 148
5.1.3 Hydrolytic stability 150
5.1.4 Thermal analyses 150
5.2 Experimental section 152
5.3 Results and Discussion 153
5.3.1 Sublimation studies 160
5.3.2 Shelf life 163
5.3.3 Hydrolysis studies 165
5.4 References 166
- 1 -
Chapter 1
Introduction
Sustainable technological development is strongly dependent on new materials with
particular mechanical, chemical, electrical, magnetic, or optical properties. In order to
address this challenge, interdisciplinary research bridging science and technology to
develop new materials, to impart new functional properties, and to provide new
processing methods for the formation of useful objects is under intense focus. It is hard to
achieve the functionality of materials in a macroscopic form employing basic chemical
compounds off the shelf. In such a situation the role of a chemist as a manipulator of the
compounds at a molecular level to design and develop new materials with desired
properties is gaining critical importance.
Of all the functional materials, those under intense investigation can be broadly classified
in to following different types: namely, semiconductors, metals, alloys, ceramics, and
biomaterials.[1] Ceramics and semiconductors are often prepared as surface layers of
different composition from that of the bulk and some times also to impart a specific
functionality to the surface or to act as a protective layer for the bulk material. In
particular, simple inorganic metal oxides, silicates, nitride compounds have been used in
a variety of applications that take advantage of their optical properties, chemical
resistance, high thermal stability, and resistance to environmental degradation. In many
of these applications a ceramic is also coated onto other material, such as plastics,
semiconductors, or metals thus forming a heterogeneous interface. With the general trend
to miniaturize devices, the interest in the interface layers between surface and bulk
material having vast technological significance is gaining momentum.[2]
Within the class of inorganic materials, oxides display perhaps the most diverse range of
functionality. The most commonly observed electrical property observed in oxides is
insulating, and the number of dielectric and ferroelectric oxides that have been realized as
thin films is quite large.[3] The oxide ceramics are the most used materials for technical
applications, particularly in electronic and structural areas. The purity of these materials
is extremely important, especially for electronic and high temperature applications.
- 2 -
Among the oxide class MO2, Titanium dioxide (TiO2) is one of the most widely studied
oxide over the years and the following section gives a brief summary of its properties and
applications.
1.1 Titanium dioxide (TiO2)
Titanium dioxide is perhaps one of the most studied oxides both in bulk and thin film
forms. Pure titanium dioxide is extracted from ilmenite or leuxocene ores. Rutile beach
sand is another source for pure rutile form of TiO2. Titanium dioxide mainly exists in
three different allotropic forms, each with distinguishing electronic, optical, and
structural properties. The crystalline phases of TiO2 are rutile (tetragonal, a = 4.5845 ? , c
= 2.9533 Å), anatase (tetragonal, a = 3.7842 Å, c = 9.5146 Å), and brookite
(orthorhombic, a = 9.184 Å, b = 5.447 Å, c = 5.145 Å).[4]
Other forms exist as well, for example, cotunnite TiO2 has been synthesized under high
pressure and is reported to be the hardest known oxide material.[5] However, mostly rutile
Table 1.1: Important properties of three different phases of TiO2
Phase Refractive
index
Density
(g.cm-3) Crystal structure
Dielectric
constant
Band gap
(eV)
Brookite 2.58 4.23
Orthorhombic
a = 9.184 Å
b = 5.447 Å
c = 5.145 Å
14 2.2
Anatase 2.49 3.79
Tetragonal
a = 3.7842 Å,
c = 9.5146 Å
31 3.2
Rutile 2.903 4.26
Tetragonal
a = 4.5845 ? ,
c = 2.9533 Å
86 3
- 3 -
and anatase phases play important roles in many applications of TiO2. Table 1.1
summarizes some important properties of three different phases of TiO2.
Each titanium ion in anatase is coordinated by six oxygens, and each oxygen ion by three
titaniums. The TiO6 octahedra share edges with four adjacent octahedra – refer to the
central octahedron in the Fig. 1.1 which shows this arrangement. The rutile structure
contains a tetragonal close packing. The structure consists of TiO6 octahedra, with the O
atoms shared by neighboring Ti atoms. Each Ti atom is surrounded by six O atoms and
each O atom is surrounded by three Ti atoms. Brookite phase contains cubic close
packing of the oxygen atoms. Titanium atoms occupy the vacancies of the oxygen-
octahedra in such a way that one octahedron shares three edges with neighboring TiO6
octahedra. Fig. 1.1 shows the atomic arrangement of three TiO2 structures.
Brookite Anatase Rutile
Fig. 1.1: The crystalline phases of TiO2: Brookite (Orthorhombic), Anatase (tetragonal)
and Rutile (tetragonal).
- 4 -
Rutile represents the stable phase at high temperatures and is the easiest to realize as
phase pure crystals or as thin films. As the more stable phase, rutile is easily realized as
an epitaxial film using most film growth techniques.[6-8] Anatase is only metastable in
bulk, but despite this fact, it has been realized as an epitaxial film using different
techniques. Epitaxial anatase films can be grown between 500 and 700 °C over a wide
range of processing pressures.[9-11] These phases possess different electro-optical
properties. For instance, although uniform anatase films can be grown with a quite high
permittivity value (70),[12] the anatase is still characterized by a lower dielectric
constant,[13] higher leakage current,[14] and lower breakdown field strength,[13] than rutile.
At the same time, pure anatase is optically less absorbent than rutile.[15] The crystal
structures of three different phases are shown in Fig. 1.1.
Though the largest commercial application of TiO2 remains as an additive to pigments for
imparting white color to paints, it finds numerous other applications as well. Even in
mildly reducing atmospheres, TiO2 tends to lose oxygen and become sub stoichiometric.
This form of TiO2 acts as a semiconductor and its electrical resistivity can be correlated
to the oxygen content of the atmosphere to which it is exposed.
Hence TiO2 is often used to sense the amount of oxygen (or reducing species) present in
an atmosphere. Over the past four decades remarkable progress has been made in to the
research related to superconductors, photovoltaic and silicon based semiconductors.
Table 1.2: Proposed materials for the high-k applications and their dielectric constants.[16]
Material Bandgap
(eV)
Relative dielectric
constant
SiO2 9 3.9
Al2O3 8.8 9.5-12
ZrO2 5.7-5.8 12-16
HfO2 4.5-6 16-30
La2O3 ~ 6 20.8
Ta2O5 4.4 25
TiO2 3.05 80
- 5 -
Titanium dioxide has been under focus because of its higher dielectric constant which has
been extensively utilized in semiconductor and related technologies. The dielectric
constants of different materials under consideration are given in table 1.2. The
opportunities afforded by titanium dioxide based materials in many applications have
driven significant efforts exploring their formation as thin films. Titanium dioxide thin
films are widely used in construction applications because of their chemical durability
and mechanical resistance.[17]
It is well known that thin films of TiO2 can modify the optical and electrical properties of
glass. TiO2 has been successfully used for optical wave guides and antireflection (AR)
coatings in silicon solar cells, due to its high refractive index and high transparency in the
visible and near IR range.[18] In addition, TiO2 forms one of the main components of high-
? and ferroelectric materials such as SrTiO3, (Ba,Sr)TiO3 and, Pb(Zr,Ti)O3, which are
considered for the fabrication of ultra high density DRAMs, non-volatile computer
memories, sensors, IR detectors etc.[19, 20] Since the research efforts to develop a method
for depositing TiO2 thin films spans over five decades and detailed descriptions are
unwarranted for the present study, only a selected number of notable techniques that have
been used for TiO2 thin film depositions are noted in following section.
1.2 An overview of various thin film deposition technologies
The technology of the film deposition has advanced dramatically during the past 30
years. This advancement was driven primarily by the need for new products and devices
in the electronic and optical industries. The rapid progress in solid-state electronic
devices would not have been possible without the development of new type of film
deposition processes, improved film characteristics and superior film qualities. Thin films
are applied to surfaces using vapor deposition to many types of work pieces (substrates)
materials. There are two major vapor deposition process categories, they are: PVD
(Physical Vapor Deposition) and CVD (Chemical Vapor Deposition). Each of these two
processes does essentially same thing that bring vapor deposition. Basic difference
between PVD and CVD is how the material is transported in vapor form to the substrate
where it is deposited.
- 6 -
Deposition techniques
Physical methods
Sputtering
Co-evaporation
Ion implantation
Molecular beam epitaxy
Pulsed laser deposition
Chemical methods
Solution deposition
Sol-gel
Spin coating
Spray pyrolysis
Chemical vapor deposition
Fig. 1.2: Broad classification of different processes used for the deposition of TiO2
thin films.
In PVD, the particles to be deposited are carried by a physical means to the substrate,
whereas in CVD, the particles are carried through a chemical reaction. PVD involves the
atomic vapor deposition of materials onto a substrate, through physical transport. The
PVD technologies available today are electron beam evaporation, arc vapor deposition,
sputtering, molecular beam epitaxy and pulsed laser deposition.
The thickness of a surface layer can vary from less than a µm in case of ion implantation
or chemical vapor deposition to few mm in case of electrochemical methods.
Correspondingly, process temperatures range from ambient to several hundred degrees
Celsius depending on the process and material to be deposited. The choice of a process is
therefore limited to a range of substrate materials depending on their temperature
sensitivity and the thickness requirement. In case of thin film requirements, the vapor
deposition methods are considered as a better choice over other deposition processes. A
plethora of deposition processes have been utilized for the growth of titanium dioxide
thin films and still new techniques are being introduced. The different deposition
techniques are broadly classified in to two types (Fig. 1.2).
- 7 -
Physical vapor deposition or PVD consists of atom/ion beam techniques. A wide range of
techniques is available, wherein the main goal is to attract metal ions to the surface of a
substrate under the influence of an electrical bias and in the presence of a reactive gas
under low pressures.[21] These are sometimes assisted by sub processes involving
generation and action of a plasma. Broadly the generation of vapors is done in four
different ways. They are evaporation, sputtering, ion-plating and reactive ion-plating.
PVD is a line–of–sight process.[22] In case of highly uniform deposition requirements
over large areas having non planar surfaces, vias, trenches, PVD suffers from this draw
back.[20] As this work is based on the research in to the field of CVD, the details
regarding PVD are kept at a minimum.
Chemical vapor deposition or CVD is a common term used for a set of processes that
involve depositing a solid material from a gaseous phase. There are distinct differences
between CVD and PVD although some similarities do exist. In a PVD process the
precursors are solid, with the material to be deposited being vaporized from a solid target
and deposited onto the substrate. But in case of a CVD process, the precursor for the
material can be a solid, liquid or a gas and/or a combination of these.[20]
Metal organic chemical vapor deposition (MOCVD) which uses vapor phases of different
metal organic compounds as precursors and is the most versatile and promising
deposition technique. It offers a possibility for large area deposition with good
composition control, high degree of uniformity and excellent conformal step coverage
over non planar structures.[23] During the present study, MOCVD was used as a
deposition process for TiO2 thin films and hence will be discussed in detail in the
following sections.
1.3 An overview of Chemical Vapor Deposition
Methods of film formation by purely chemical processes in the gas or vapor phases
include chemical vapor deposition (CVD) and thermal oxidation. CVD is a materials
synthesis process whereby constituents of the vapor phase react chemically near or on a
substrate surface to form a solid product. The deposition technology has become one of
- 8 -
the most important means of creating thin films and coatings of a very large variety of
materials essential to advanced technology particularly solid state electronics where some
of the most sophisticated purity and composition requirements must be met. The main
feature of CVD is its versatility for synthesizing both simple and complex compounds
with relative ease at generally low temperatures.
Both chemical composition and physical structure can be tailored by control of the
reaction chemistry and deposition conditions. Fundamental principles of CVD encompass
an interdisciplinary range of gas phase reaction chemistry, thermodynamics, kinetics,
transport mechanisms, film growth phenomena and reactor engineering.
Chemical reaction types basic to CVD include pyrolysis (thermal decomposition),
oxidation, reduction, hydrolysis, nitride and carbide formation, synthesis reactions,
disproportionation and chemical transport. A sequence of several reaction types may be
involved in more complex situations to create a particular end product. Deposition
variables such as temperature, pressure, input concentrations, gas flow rates and reactor
geometry and operating principle determine the deposition rate and the properties of the
film deposit.
The use of modern chemical vapor deposition technique can be dated back to the year
1880 where the technique was used to strengthen the filaments in incandescent lamps
using carbon or metal depositions on them.[24] The CVD related activities remained more
or less passive for the first half of the last century, but for the last 45 years the growth has
been tremendous. The term chemical vapor deposition was first used in 1960.[25] The
increasing requirements of semiconductor industry have been the driving force in the
development of CVD techniques and very importantly, the intensified efforts to
understand the fundamentals of the CVD processes.
Most CVD processes are chosen to be heterogeneous. That is, they take place at the
substrate surface rather than in the gas phase. Undesirable homogenous reactions in the
gas phase nucleate particles that may form powdery deposits and lead to particle
contamination instead of clean and uniform coatings. The reaction feasibility (other than
reaction rate) of a CVD process under specified conditions can be predicted by
thermodynamic calculations provided reliable thermodynamic data (especially the free
energy of formation) are available. Kinetics control and the rate of reactions depend on
- 9 -
temperature and factors such as substrate orientation. Considerations related to heat, mass
and momentum transport phenomenon are especially important in designing CVD
reactors of maximum efficiency. Since the important physical properties of a given
material are critically influenced by the structure (such as crystallinity) control of the
factors governing the nucleation and structure of growing film is necessary.
CVD has become an important process technology in several industrial fields. As
mentioned earlier, applications in solid-state microelectronics are of prime importance.
Thin films of insulators, dielectrics (oxides, silicates, nitrides), elemental and compound
semiconductors (silicon, gallium arsenide etc.) and conductors (tungsten, molybdenum,
aluminum, refractory metal silicides) are extensively utilized in the formation of solid
state devices. Hard and wear resistance coatings of materials such as boron, diamond-like
carbon, borides, carbides and nitrides, are used for metal protection in metallurgical
applications. Numerous other types of materials, including vitreous graphite and
refractory metals, have been deposited mainly in bulk form or as thick coatings. Many of
these CVD reactions have long been used for coating of substrates at reduced pressure,
often at high temperatures.
Reactors
The reactor system (comparing the reaction chamber and all associated equipment) for
carrying out CVD process must provide several basic functions common to all types of
systems. It must allow transport of the reactant and diluent gasses to the reaction site,
provide activation energy to the reactants (heat, radiation, plasma), maintain a specific
system pressure and temperature allow the chemical processes for film deposition and
proceed optimally, and remove the by-product gasses and vapors. These functions must
be implemented with adequate control, maximal effectiveness and complete safety.
The most sophisticated CVD reactors are those used for the deposition of electronic
materials. Low temperature (below 600 °C) production reactors for normal or
atmospheric CVD (APCVD) include rotary vertical flow reactors and continuous in line
conveyorized reactors with various gas distribution features. They are used primarily for
depositing oxides and binary and ternary silicate glass coatings for solid-state devices.
- 10 -
Reactors for mid-temperature (600 °C – 900 °C) and high temperature (900 °C – 1300
°C) operation are either hot-wall or cold wall types constructed of fused quartz.
Hot wall reactors, usually tubular in shape are used for exothermic processes and have
the advantage of close temperature control. They have been used for synthesizing
complex layer structures of compound semiconductors for microelectronic devices. A
disadvantage is that deposition occurs everywhere on the part as well as on the walls of
the reactors, which requires periodic cleaning or the use of disposable lines. Cold wall
reactors, usually bell-jar shaped, are used for endothermic processes, such as the
deposition of silicon from the halides or the hydrides. Heating is accomplished by RF
induction or by high intensity radiation lamps. Substrate susceptors of silicon carbide
coated graphite slabs are used for RF heated systems.
Reactors operating at low pressures (typically 0.1 – 10 torr) for low pressure CVD
(LPCVD) is the low, mid or high temperature range are resistance heated hot-wall
reactors of tubular, bell-jar, or close spaced design. In the horizontal tubular design, the
substrate slices (silicon device wafers) stand up in a carrier sled and gas flow is
horizontal. The reduced operating pressure increases the mean free path of the reactant
molecules, which allows a closely spaced wafer stacking. The very high packing density
achieved (typically 100-200 wafers per tube) allows a greatly increased throughput,
hence substantially lower product cost. In the vertical bell-jar design, the gas is
distributed over the stand-up wafers, hence there is much gas depletion and generation of
few particles, but the wafer load is smaller (50 to 100 wafers per chamber). Finally, the
close spaced design developed most recently processes each wafer in its own separate,
close space chamber with the gas flowing across the wafer surface to achieve maximum
uniformity. In LPCVD, no carrier gasses are required, particle contamination is reduced
and film uniformity and conformality are better than in conventional APCVD reactor
systems. It is for these reasons that low-pressure CVD is widely used in the high cost-
competitive semiconductor industry for depositing films of insulators, amorphous and
polycrystalline silicon, refractory metals and silicides.
Current developments in CVD focus on low temperature forms of CVD, such as
MOCVD, plasma-CVD, and photo-CVD and atomic layer deposition ALD.[26] These are
assisted processes and are employed widely in the fields of semiconductor industry and
- 11 -
microelectronics, as well as in hard coatings corrosion and wear resistance applications as
lower deposition temperatures now permit the use of a broader spectrum of substrates. In
addition, the need for the better understanding of the process in order to control the
process in a better way has led to increased activities to monitor the deposition
parameters through in situ observations.[23]
CVD - Chemical vapor deposition is a materials-synthesis process in which one or more
vapor-phase chemical components are transported into a reaction chamber. They are
activated thermally or by several other methods, such as plasma or laser stimulation in
the vicinity of the substrate to chemically react at the substrate surface. The reaction
taking place at the vicinity of substrate surface lead to a solid film. The unreacted vapors
and volatile byproducts of decomposition reaction are transported away from the surface.
The CVD process belongs to those vapor transfer processes that are atomistic in nature.
The main features of CVD are its versatility for synthesizing both simple and complex
compounds with relative ease. Both chemical composition and physical structure can be
tailored by control of the reaction chemistry and deposition conditions.[23]
1.4 CVD Kinetics
In CVD, film growth takes place following complex set of chemical reactions taking
place sequentially and continuously. In order to get optimal quality films one needs to
have a better understanding of the theory of CVD. A theoretical analysis is in most cases,
an essential step, which if properly carried out should predict any one for the following
1) Chemistry of the reaction (intermediate steps, by products)
2) Reaction mechanism
3) Composition of the deposit (i.e. stoichiometry)
4) Structure of deposit (i.e. the geometric arrangement of its atoms).
This analysis may then provide guidance for an experimental program and considerably
reduce its scope and save a great deal of time and effort. Such an analysis requires a clear
understanding of the CVD process and a series of several fundamental considerations in
the disciplines of thermodynamics, kinetics, and chemistry is in order. Critical to CVD
- 12 -
Impinging reactant atoms
Nucleation Surface reactions
Surface migration
Condensation
Desorption of volatile reaction products Gas phase reactions
theory are chemical kinetics, fluid mechanics, chemical engineering principles as well as
an understanding of growth mechanisms.
Any vapor deposition technique is based on the principles of mass transfer from one
source to another. Macroscopically following three fundamental steps play critical role in
the growth of a film.
• Transfer of the precursor to the gas phase
• Transport of gas phase to the substrate
• Deposition onto substrate and film growth.
These three steps are either separated in space and time or superimpose with each other,
depending on process requirements.[23]
The individual process steps are displayed in Fig. 1.3. There are efforts to simulate the
situation in CVD reactors from predictions of thermodynamical equilibrium calculations.
But the deposition reaction is almost a heterogeneous reaction. The sequence of events in
the usual heterogeneous process can be described as follows.
• mass transport of the gas phase in to the deposition zone;
Fig. 1.3: The representation of the individual process steps involved in chemical vapor
deposition
- 13 -
• gas phase reactions leading to the formation of film precursors and byproducts;
• mass transport of film precursors on the growth surface;
• surface diffusion of film precursors to growth sites;
• incorporation of film constituents into growing film;
• desorption of byproducts of the surface reactions; and
• mass transport of byproducts in the bulk gas flow region away from the
deposition zone towards the reactor exhaust.
The individual steps take place simultaneously and can not be independently followed.
The slowest of these process steps determines the deposition rate. The close control of
these steps through the vital CVD parameters - substrate temperature, reactor pressure
gas flow rates and gas-phase composition (precursor concentration) determines the film
growth rate and enables the deposition of a wide variety of coatings. The strict quality
requirements (e.g., thickness and compositional uniformity, crystallinity, electrical
properties) imposed on CVD deposits imply the need for stringent control of the CVD
processes. In particular following parameters are controlled.
1. Temperature in the reactor (one or more zones)
2. The quantities and compositions of all gasses or vapors entering the reactor.
3. The time sequencing of variables mentioned in (1) and (2)
4. The pressure in the case of low pressure CVD
Most chemical reactions in CVD are thermodynamically endothermic and/or have a
kinetic energy of activation associated with them. Generally this is an advantage since the
reactions can be controlled by regulating the energy input. However, it does mean that
energy has to be supplied to the reacting system, and traditionally CVD processes have
been initiated and controlled by the input of thermal energy to the substrate. Based on the
energy input, three different methods of energy input in CVD processes are practiced.
a) Thermal CVD
Thermal CVD requires high temperature, generally from 800-2000 °C, which can be
generated by resistance heating, high frequency induction, radiant heating etc. The choice
of heating methods depends largely on factors such as the type of deposition process and
the shape, size, and composition of the substrate material as well as economics.
b) Plasma CVD
- 14 -
In plasma CVD, also known as plasma enhanced CVD (PECVD) or plasma assisted CVD
(PACVD), the reaction is activated by plasma and the deposition temperature is
substantially lower. Plasmas are extremely complex chemical soups and deposition
characteristics can depend markedly on system variables such as gas pressure, flow rate,
RF power and frequency, and reactor geometry and substrate temperature.
c) Laser and photo CVD
Two methods based on photo activation have recently been developed: A laser produces
a coherent monochromatic high energy beam of photons, which can be used effectively to
activate a CVD reaction. Laser CVD occurs as a result of thermal energy from the laser
coming in contact with and heating an absorbing substrate. The wavelength of the laser is
such that little or no energy is absorbed by the gas molecules.
In photo CVD, the chemical reaction is activated by the action of photons, specifically
UV radiation; which have sufficient energy to break the chemical bonds in the reactant
molecules. No heat is required and the deposition may occur essentially at room
temperature. More over, there is no constraint on the type of substrate which can be
opaque, absorbent or transparent. A limitation of photo CVD is the slow rate of
deposition which has so far restricted its applications.
1.5 Growth rate and modeling principles for CVD
The growth rate is primarily determined by the substrate temperature, reactor pressure
and gas-phase composition. When CVD growth rates are plotted against reciprocal of
deposition temperature, the following regimes can be distinguished.[23]
• The growth rate is controlled by surface reaction kinetics at the substrate at lower
temperatures.
• At intermediate temperatures, mass-transfer-kinetics through the stagnant
boundary layer limits the growth rate
• Reduced growth rate at high temperatures due to parasitic reactions, such as
increased decomposition rate on hot reactor walls or through pronounced gas-
phase reactions
The growth rate is primarily determined by the substrate temperature, reactor
pressure, and gas-phase composition. At low temperatures, the growth rate is limited
- 15 -
by chemical kinetics and increases exponentially with temperature according to the
Arrhenius expression,
G = A exp (EA/RT)
Where, G = growth rate
EA = the apparent activation energy of the rate determining step,
R = universal gas constant
T = temperature in K.
Since the rate is limited by chemical kinetics, uniform film thickness can be achieved by
minimizing temperature variations. This is the regime desired by hot wall low pressure
CVD reactors. The growth rate is nearly independent of temperature in the intermediate
temperature regime where mass transport to the surface controls the rate. This
temperature independence is particularly advantageous in cold wall reactors, where it is
often difficult to obtain completely uniform substrate heating. The growth rate may
decrease at high temperatures because of an increased desorption rate and depletion of
reactants on reactor walls. The emergence of an alternative reaction pathway may also
lead to a decline in the temperature-dependent growth rate.
Because of the complexity of transport phenomena and chemical reactions underlying
CVD, models of the process are required to identify rate controlling steps and to link
1/T (K)
Log growth rate (µm/min)
Fig. 1.4: Typical growth rate dependence on substrate temperature
- 16 -
growth uniformity performance to process conditions precursor chemistry. In addition,
accurate models play a significant role in the design of reactors capable of producing high
deposition rates and composition uniformity over large areas. Thermodynamic analysis
has been the traditional approach to CVD process modeling because of relative ease of
determining the system state by equilibrium calculations relative to experiments or
detailed kinetic models. Equilibrium composition at constant temperature and pressure is
generally computed in two ways; by direct minimization of the Gibbs free energy of the
system, subject to elemental abundance and mole number non-negativity constraints; or
by transforming the species mole number variables into a new set of reaction variables
and then minimizing the Gibbs free energy in terms of these new variables.[23]
In the solution of CVD reactor models it is often possible to separate the flow and energy
solution from the mass transfer analysis since CVD reactants often are used in low
concentration in some inert carrier gases. The continuum approach is based upon
Knudsen diffusion concepts and has the advantage of being simple. The Monte-Carlo
approach is versatile, allowing the inclusion of surface transport and gas-phase reactions
besides the surface reactions with variable sticking coefficients.[23]
Because of the low pressures used in LPCVD systems, the fluid flow can be in either the
usual continuum regime or in the transition regime, depending on the relative magnitude
of the mean free path of the reactant molecules and the characteristic dimension of the
reactors, as reflected by the Knudsen number, Kn = ? / d, where ? is the mean free path
of gas molecules and d is the characteristic reactor dimension. For Kn = 0.01 the flow is
dominated by gas molecule collisions and the continuum models apply to such systems.
When Kn > 10, the molecules primarily collide with solid surfaces and the flow is
described as `free-molecular`. Deposition in this regime can not be modeled by the
classical continuum equations but must be described through view factor, computations
similar to those used for the radiation heat transfer or by Monte-Carlo simulations. The
intermediate range of Knudsen numbers, 0.01 < Kn < 10, corresponds to the so-called
transition flow regime where both gas-phase and surface interactions are important. In
this case, solution of the Boltzmann equation or use of specialized Monte-Carlo
simulation techniques must be applied in order to model LPCVD systems accurately.[23]
- 17 -
1.6 Precursors for CVD
During initial stages of CVD process development, the materials deposited employing the
process were simple and the proper reactants referred to as precursors were just taken
“off the shelf”. With increasing demand for new materials as thin films, having two or
more elemental components, efforts have been made to search new precursors depending
on requirements.
Precursors for CVD can be broadly classified into three types:
1. Inorganic precursors: which do not contain any carbon
2. Metal-organic precursors: which possess organic ligands but without metal to
carbon bond.
3. Organometallic precursors: which contain organic ligands wherein metal to
carbon bond exists.[27]
CVD processes using metal organic or organometallic precursors are generally referred to
as metal-organic chemical vapor deposition (MOCVD). The highest potential for future
materials and process development is associated with the use of the metal organic (MO)
compounds. Through molecular engineering, the selected ligands influence the precursor
properties such as thermodynamic stability, kinetic lability, solubility and volatility of the
compound. The ligands have a major influence on the quality of the deposited product, as
ideally, they do control the chemical decomposition reaction taking place.[28]
Inorganic precursors are kinetically or even thermodynamically stable compounds that
need high activation energies to decompose. Decomposition takes place near
thermodynamic equilibrium and only thermodynamically stable phases can be formed.
Inorganic precursors are often prone to aggregation and as a result exhibit lower vapor
pressure. Although the vapor pressure can be increased by increasing the temperature,
care must be taken to avoid premature reaction in the gas phase before reaching the hot
substrate surface. This can lead to irreproducible growth rates and the transport of
unknown species. Particle formation resulting in clogged delivery lines and formation of
powdery products interfere with the growth of clean smooth films.[27]
- 18 -
Metalorganic compounds suitable as precursors for most main-group and transition-metal
elements are now available from a number of commercial sources. In case of specific
requirements with stringent restrictions, the multitude of potential ligands available for
MO compounds offers the possibility to design the precursor and to engineer the
molecular decomposition pathway.
The choice of a precursor is governed by certain general characteristics which can be summarized as follows:
• Good volatility to achieve high transport rates and to accommodate high growth
rates by minimizing intermolecular forces in the condensed state and suppression
of molecular aggregation.
• High purity of precursor to reduce contamination
• Good thermal stability during evaporation and transport in the gas phase to avoid
premature decomposition
• Clean decomposition on pyrolysis to give desired material with minimum
contamination.
• Long term stability for storage
• Non-toxic, non-pyrophoric, non corrosive
• Inexpensive and simple to synthesize.
Precursors should be synthesized through easy routes to achieve high yield using
inexpensive, readily available chemicals. Although significant progress has been made on
synthesis of new compounds, there have been no source compounds satisfying all the
above criteria.[29] Thus, a great deal of work remains to be carried out before all the
challenges are addressed.
1.7 The liquid injection CVD
The lack of vapor pressure and in some cases high thermal stability of the precursors has
driven the development of the liquid injection technique in which reactant gas
composition is set by volumetric metering of liquids followed by flash evaporation.
Accurate and precise mass transport from the source in to the reaction zone is a salient
- 19 -
feature of a CVD process. It is difficult to get an ideal precursor stable at room
temperature with high vapor pressure. There are numerous cases where the precursors are
solids. It has been long established that solid source delivery using conventional bubblers
is mass transfer limited.[30]
Consequently the effective transport rate drops as the precursor is consumed. This is a
non linear phenomenon because the smallest particles have the highest surface area to
volume ratio and they vaporize rapidly. Also, changing the carrier gas flow affects the
transport rate because to a first approximation the boundary layer thickness is inversely
proportional to carrier gas velocity. Finally, several of the precursors are non-ideal and
sublime as clusters of molecules and the average cluster size decreases with decreasing
pressure. Hence the volatility of the precursor varies with pressure. These effects make
precise delivery of low vapor pressure materials using conventional bubblers extremely
difficult. Continuous feed methods such as that developed by Hiskes et al. attempt to
compensate for these limitations.[31]
The liquid delivery technique relies on the flash vaporization of liquid solutions and over
comes the limitations of bubbling. Neat liquids as well as liquid solutions comprised of
solids dissolved in organic or inorganic media can be used with this technique. The liquid
Fig. 1.5: The industrial tool liquid injection reactor with five six inch wafers
having gas foil rotation.
- 20 -
precursor solutions are maintained at room temperature and the composition of the inlet
to the CVD chamber is controlled by one of several methods. In the preferred
embodiment reactant gas composition is controlled through real time volumetric mixing
of the individual precursor solutions. The liquid mixture is then flash vaporized to
generate a homogenous gas at the inlet to the CVD tool. This method ensures process
reproducibility as variations in delivery rate give rise to variations in the over-all
deposition rate but do not impact film composition.[31]
Variations of this approach utilize mixtures of precursors or a single liquid solution of the
desired composition which are metered and then vaporized as a single liquid to deposit an
oxide film by MOCVD. This technique requires the solvents and solutes to be un-reactive
at the vaporization temperature which is a necessary but not sufficient criterion to prevent
homogenous nucleation inside the reaction chamber. An alternative approach to liquid
delivery uses a metering device and individual vaporizer for each liquid stream and
mixing occurs inside the shower head or process chamber. This method suffers from the
limitation that variation in the delivery rate of any of the individual precursor solutions
impact composition control. The industrial tool liquid injection reactor with five six inch
wafers having gas foil rotation used in this study is shown in Fig. 1.5.
1.8 Gas-phase chemical species measurements
Thermal decomposition of the precursor leads to the formation of thin films in a typical
CVD process. Adequate molecular stability of the precursor is required to prevent
premature reaction or decomposition of the precursor during vapor phase transport. The
volatility of a metal compound is a complex function of intermolecular forces (van der
Waals interactions, hydrogen bonds etc.). Precursor designing is a complex issue for
which reliable analytical feed back from different stages of development is necessary.
One such important stage where analysis is essential is, thermolysis of precursor. It is
interesting to know how a metal complex decomposes by the application of thermal
energy. Preferably there should be simple steps leading to formation of required
stoichiometry. The ideal case should be the decomposition of the precursor at the
required temperature process zone without any prior decomposition and leading to highly
- 21 -
oriented growth of the film. But most of the precursors do not meet the ideal conditions.
There are deviations from ideal cases when inter and intra molecular reactions and
rearrangements taking place during thermolysis of the precursor. A detailed feed back is
very helpful in improving design and inclusion/exclusion of moieties suiting the
requirement. Typical study of precursor decomposition has to be done on gas phase of the
molecules. Gas phase studies are generally difficult and require a highly sensitive method
with low response times. This is because, the interaction between different species in the
gas phase leads to a constant change in the composition and species under investigation
during the period of measurement.
The often neglected by-products of the CVD process are volatile gases. Analysis of the
gas phase can also lead to a better understanding of the CVD reaction mechanisms and
the information can be used to refine the process. The spatial distribution of gas-phase
chemical species in a CVD reactor is controlled by chemical reactions, which are coupled
to heat and mass transfer through the strong temperature dependence of chemical reaction
rates.[23] In this section we discuss various techniques that have been used to study gas-
phase chemical species in CVD systems.
Information on the identity and concentrations of chemical species in the gas-phase can
yield significant insights into the mechanism of a process. Monitoring the depletion of the
reactant can give information on overall process efficiency, as well as the gas-phase
decomposition of the reactant. Understanding gas-phase reactions is crucial to identifying
the chemical species incident on the deposition surface, which in turn is important in
understanding how process parameters affect film properties. Measurements of gas phase
species also provide data for extensive testing of CVD models. The ideal probe for
chemical species analysis would be non intrusive, capable of selective detection of a
desired species in the presence of other species, sensitive, quantitative, and capable of
high spatial resolution in the presence of rapidly changing chemical and temperature
fields. While no single technique meets all these requirements, a combination of the
methods discussed below comes very close to this ideal situation.
- 22 -
1.8.1 Mass spectrometry
Mass spectrometry (spectroscopy) is a well-developed analytical technique in which the
gases to be analysed are joined by electron bombardment. The resulting ions are
separated either by time-of-flight, quadrupole or magnetic methods and are counted as a
function of their charge/mass ratio. When a molecule is ionized, it often `cracks´,
producing a number of fragments of different masses with relative intensities that are
characteristic of the molecule and ionization energy.
The major advantages of mass spectrometry are its generality and ability to yield
quantitative measurements. Any chemical system can be studied because all molecules
have mass spectra. Highly reactive intermediates, as well as stable molecules can be
studied. Mass spectrometry can yield accurate numbers for the relative abundance of
different species and, with appropriate calibrations, can give absolute partial pressures
(number densities) as well.[23]
The major disadvantages of mass spectrometry for studying CVD processes are the need
to use sampling probes, which can perturb the system, and the need for independent data
on molecular cracking patterns. Sampling probes are required because mass
spectrometric analysis must be carried out at pressures of 10-5 Torr or less, much lower
than CVD processing pressures. Probes must be carefully designed to ensure
representative sampling of the gas and minimal perturbation of the CVD process. This is
particularly important when studying reactive intermediate species, as they are likely to
undergo further chemical reactions in the sampling probe. In CVD systems, probes are
particularly susceptible to clogging by deposited material. For CVD reactions that are
primarily heterogeneous, the presence of the probe can induce extraneous chemical
reactions in the sampling region and produce misleading results; in addition mass
spectrometry gives little information on the structure of a molecule.
1.8.2 Gas chromatography
Gas chromatography (GC) (vapor-phase chromatography) is a well established technique
for separating and identifying mixtures of chemicals. To analyze a chemical mixture by
GC, part of the sample is injected onto a column that has a carrier gas flowing through it.
- 23 -
Chemical species are separated by differential adsorption on the column (generally a
high-molecular-weight liquid on a solid support or a solid adsorbent) which is chosen for
a particular chemical system. With the proper choice of column conditions, the different
species emerge from the column at different times and are detected, generally by means
of a thermal conductivity or flame ionization detector. A particular chemical is then
identified by its elusion time from the column.
The advantages of using GC technique include its generality as most stable molecules can
be analysed. GC also gives quantitative measurements of species concentrations in a
straight forward manner. With use of the proper calibrations and standards, the identity
and amount of each species in a mixture can be determined.[23] The major disadvantages
of using GC for CVD studies are that sampling is required and that it is not a real-time
analysis technique. Reactive intermediate can not be observed directly because they
undergo reactions during the analysis procedure. The dynamic range of a GC analysis can
be limited and it can be difficult to separate and detect the minor components in a
mixture. In addition, developing an analysis scheme i.e. determining the optimum column
materials and analysis conditions often must be done by trial and error.
A number of optical techniques have been applied to the analysis of deposition systems.
One of the major advantages of using photons to probe CVD processes is that they are
much less intrusive than a physical sampling probe. In addition, optical methods can
provide unambiguous species identification quantitative measurements. The
spectroscopic probes discussed in this section can be divided into those involving
vibrational excitation of the molecules and those that involve electronic excitation of the
molecules. Infrared (IR) absorption spectroscopy, laser Raman spectroscopy and coherent
anti-Stokes Raman spectroscopy (CARS) fall in to the first category while optical
emission spectroscopy, UV/visible absorption spectroscopy and laser-induced
fluorescence (LIF) spectroscopy fall into the second category.[23] During the course of
this work IR spectroscopy was extensively used for the gas phase analysis and hence a
brief description is given below.
- 24 -
1.8.3 IR absorption spectroscopy
In IR absorption spectroscopy, a sample is exposed to a beam of IR radiation, and the
transmission is monitored as a function of the wavelength. This can be done either with
traditional dispersive techniques, Fourier transform methods, or tunable lasers. Molecules
are identified by the characteristic frequencies of IR radiation that they absorb, which
correspond to the excitation of vibrations in the molecule. This technique is widely used
in analytical chemistry, particularly for the identification of organic molecules; and
commercial instruments are readily available. Generality is one of the advantages of IR
absorption technique. With the exception of a few highly symmetric molecules, most
molecules have electric-dipole allowed transitions in the IR region. For studies of CVD
mechanisms, standard IR spectroscopy can be used to analyze gas samples extracted from
the reactor.
IR absorption spectroscopy can also be used for in situ analysis to monitor reactive
intermediate species. In this case, the IR beam passes through the CVD reactor and the
absorption spectrum of the hot, reacting gas is obtained. The disadvantage of this
technique is poor spatial resolution. The measurement is averaged over total path length
and the size of the IR beam can be quite large if conventional incoherent sources are
used. Although this advantage can be mitigated via careful reactor design, this spatial
averaging makes quantitative measurements difficult because of the steep temperature
and concentration gradients present in most CVD reactors.[23]
1.8.4 Matrix isolation FTIR spectroscopy
CVD is a process where the gas phase is highly dynamic and plays a crucial role and has
a bearing impact on the deposited material. As discussed above, during a CVD process
several chemical reactions occur, which rely on the heat and mass transfer through the
strong temperature dependence of chemical reaction rates. To study a system undergoing
constant physico-chemical changes, is an extremely challenging task and need a
combination of analytical methods. CVD processes often use metalorganic precursors
- 25 -
designed for specific purposes. To get an understanding of the process and optimizing the
conditions for depositions mechanistic studies are sought. But decomposition of
metalorganic complexes pose a challenge as the resulting reactive intermediates
sometimes can not be detected simply because the analytical methods lack such
capability. In order to detect reactive intermediates during a chemical reaction, either the
detection method should be fast (as well as sensitive) enough (of the order of few nano
seconds or faster) or there should be a method to preserve the reactive intermediates for
sufficiently long time so that they can be analyzed and characterized.
Fig. 1.6: Schematic diagram of the matrix isolation unit.
In matrix isolation techniques, individual molecules resulting from a chemical reaction
are trapped and isolated from one another in a solid, inert matrix at low temperature while
Vacuum pump
Expander Radiant heat shield
Sample holder
Temperature controller
Compressor
Cooling water supply and drain
Vacuum shroud
Gas lines Expander electrical power cable
Vacuum valve
Instrumentation skirt
Pressure gauge P
- 26 -
their spectrum is measured. According to IUPAC Compendium of Chemical
Terminology, Matrix isolation is “A term which refers to the isolation of a reactive or
unstable species by dilution in an inert matrix (argon, nitrogen, etc.), usually condensed
on a window or in an optical cell at low temperature, to preserve its structure for
identification by spectroscopic or other means.”[32, 33]
Most of the chemical processes result in atoms or molecules which undergo further
reactions. In such a case isolating the primary reaction product suppresses successive
reactions. In matrix isolation technique, reactive intermediates and compounds which are
unstable under normal conditions are frozen in a rigid matrix at about 4 to 40 K mainly
consisting of non-reactive media such as noble gases (Argon, Neon, etc.) or nitrogen.[34]
Conventional techniques fail to detect the reactive intermediates because of their
insensitivity towards low concentrations of the species. Matrix isolation method provides
a possibility to accumulate the reactive intermediates over a period of time so that
spectroscopically detectable concentrations can be achieved.[35] Infrared and visible-
ultraviolet (electronic) spectroscopies are the most widely used tools for detecting and
studying reaction intermediates. Infrared absorptions of molecules trapped in neon or
argon matrices are sharp, with typical band widths (full width at half maximum, FWHM)
of the order of 1 cm-1. Earlier experiments have also demonstrated that at 20 K and below
solid nitrogen and argon are sufficiently rigid so that molecular diffusion and subsequent
chemical reactions are effectively inhibited. Additional advantage is that, nitrogen and
the noble gases are transparent through the entire far infrared spectral region to the
vacuum ultraviolet region. Hence when coupled with FTIR/UV-Visible spectroscopy,
matrix isolation measurements provide a potentially valuable analytical tool.[34] A
schematic diagram of basic matrix isolation unit is shown in Fig. 1.6.
This suits very well with the requirement for studying the decomposition of metal organic
precursors. With the increased life time of reactive intermediates, using different optical
spectroscopic tools, analyses can be done which helps in improving the basic
understanding of decomposition process. In addition, the designer of the precursors for
CVD gets much needed feed back from the decomposition studies and which helps to
include/exclude design changes in the precursor molecule.
- 27 -
1.9 MOCVD of TiO2 thin films
The CVD of TiO2 thin films spans over five decades and a variety of precursors used for
thin film depositions. Mainly the simple halides like [TiCl4], [TiI4][49] used in early
studies have been replaced with alkoxides like [Ti(OiPr)4], [Ti(OEt)4],[13] etc. Simple
compounds like titanium nitrate [Ti(NO3)4], have also been investigated for the
deposition of TiO2 thin films using CVD technique.
The air and moisture sensitivity of halides and alkoxides of titanium has made their
handling difficult during a CVD process. In order to overcome the sensitivity problem,
several chelating ligands were tried in combination with alkoxides of titanium. Table 1.3
lists notable precursors used for the deposition of TiO2 thin films. Though there are
several precursors which can be used for the deposition of TiO2 thin films, improvements
are possible in terms of tuning of thermal stability, chemical reactivity and orderly
decomposition of available precursors by the inclusion/exclusion of moieties in the ligand
sphere. During the course of this work, the effect of fine tuning the ligand sphere on the
Table 1.3: The list of notable precursors reported for the deposition of TiO2 thin films using CVD
Precursor Tb (°C) Minimum Td
(°C)
References
[TiCl4] Not reported 200 36-38
[Ti(OiPr)4] 80-120 250 39-41
[Ti(NO3)4] 120 300 39, 42
[Ti(µ-ONep)(ONep)3]2 22-40 230 43
[{HB(pz)3}Ti(OPri)3] 115 450 44
[TiOPri)2(dmae)2] Liquid injection 400 45
[TiOPri)2(thd)2] Liquid injection 350 46, 47
[Ti(2meip)2] Liquid injection 400 48
Tb = bubbler temperature, Td = Deposition temperature
- 28 -
chemical and physical properties of the precursors were investigated. Following section
provides the details about the scope of the present work.
1.10 Scope of the present work
The growth of TiO2 and related oxides has been successfully demonstrated using the
above mentioned titanium sources with a varying degree of success. From the point of
view of precursor chemistry, the interactions at the molecular level which influence the
precursor purity, volatility and decomposition kinetics needs to be thoroughly
investigated. Due to the complexity involved with respect to the influence of process
parameters, it is worthwhile to retain the established concepts of precursor design rather
than to explore totally new ideas. There is still scope for improvement with respect to the
stability and thermal decomposition of titanium precursors. The use of metal alkoxides as
a synthetic platform for molecular design of suitable precursors for TiO2 thin films has
been demonstrated. The main objective of this work was to engineer the ligand
framework of well established key structures by introducing small and distinct changes in
a systematic manner. Chelating ligands such as ß-ketoesters, malonates and ß-ketoamides
were tested in combination with alkoxides of titanium to yield mixed alkoxide
complexes.
The resulting compounds were characterized for their chemical and physical properties
by NMR, IR, melting point, mass spectrometry, elemental analysis and single crystal X-
ray diffraction. Single crystal X-ray diffraction was employed as a tool to probe and
engineer the structure and function of the novel precursors. The objective in this study
was to systematically study the properties of the building blocks such as size, shape, and
directionality of functional groups with a view to determine how these parameters control
and influence the packing in the crystal lattice. The main focus was to design and
synthesize coordinatively saturated metal complexes with appropriate ligands and to
correlate the design changes to changes in the thermal properties of the precursors. It is
important to control the nuclearity of metal complexes by the use of efficient designing.
This is because nuclearity of the precursor has a significant influence on the thermal
properties of the precursor. The determination of the thermal properties of the compounds
- 29 -
which is a key figure of merit for CVD precursors was carried out in detail using
thermogravimetry (TG) and differential thermal analysis (DTA).
The rationally developed compounds of titanium were then screened for CVD
applications using a homebuilt cold wall CVD reactor. It was demonstrated that by
introducing small and distinct changes in the ligand system, which act as targeted
cleavage points, it was possible to grow TiO2 thin films at low deposition temperatures.
Therefore the scaling up precursor synthesis was taken up (~ 25 g). As these compounds
showed high degree of solubility in common organic solvents, they were dissolved in n-
butyl acetate and used as precursors for liquid injection MOCVD of TiO2 and SrTiO3
employing an industrial tool reactor (MOCVD facilities of Forschungszentrum Jülich).
The benchmark precursor [Ti(OPri)2(thd)2] which is commercially available was used to
grow TiO2 and SrTiO3 to compare the results to those obtained from the engineered
precursors.
1.10.1 Understanding the precursor fragmentation
The main concern of the research community involved in the field of CVD is to prepare
films with the most satisfying properties for the desired application. However, a very
good control of the CVD process is generally difficult as many complex phenomena such
as mass transport, gas phase and surface reactions etc. are involved during deposition.
Among these phenomena, gas phase reactions are particularly important in determining
the material characteristics and properties. A very good understanding of the reaction
pathways and kinetics is thus required for optimization of the process. A powerful
method to analyze relevant molecular decomposition mechanisms, which has not yet
been widely recognized in the CVD community, is to analyze the reaction intermediates
formed using the matrix-isolation–FTIR spectroscopy techniques. Using this technique,
the reaction intermediates formed by the thermolysis of CVD precursors in the gas phase
can be preserved by shock-like quenching of the gas phase onto a cooled matrix window.
(e.g. CsI maintained at 10 K) and are subsequently analyzed by FTIR spectroscopy. This
elegant technique gives insights into the CVD process on a molecular level and thus
provides a rational basis for further precursor development. For this purpose, a matrix
- 30 -
apparatus consisting of a cooling system (Helium closed cycle), vacuum parts and a FTIR
spectrometer was fabricated and used for investigating the molecular mechanisms
involved during precursor decomposition. The most widely used alkoxide of titanium,
[Ti(OPri)4] and its ß-ketoester derivative, [Ti(OPri)2(tbaoac)2] and the commercially
available precursor [Ti(OPri)2(thd)2] were studied for thermal decomposition using matrix
isolation FTIR spectroscopy. By employing the present set up of matrix isolation unit
with a themolysis oven, there is always a possibility of coupling between the
homogeneous gas phase reactions and heterogeneous gas phase reactions occurring on
the surface of the oven. However, the present set up has advantages over other methods
and can be effectively be used in combination with other gas phase or surface studies for
a deeper understanding of the reaction mechanisms. There are relatively very few studies
on the gas phase decomposition studies for organometallic precursors when compared to
a large number of solution based studies. The results obtained from matrix isolation
studies were successful to identify clearly the organic intermediates involved in the
process. Use of FTIR in combination with matrix isolation techniques could provide
insights in to mechanism not considered earlier in such studies.
The work presented here focuses on,
a) the very important rational basis for further precursor development which includes
mechanistic studies on the fragmentation and transformation of the precursors to the final
material and
b) development of the CVD process for technologically useful oxide materials such as
TiO2 and SrTiO3 and the study of these materials for their device related characteristics.
The nature of the work in this field, i.e. bridging precursor chemistry and thin film
processes with mechanistic studies is somewhat unique in the field of CVD and paves
way to give scientific knowledge on the relationship between the molecular structure of
the precursors and the nature of the films obtained.
Although MOCVD growth TiO2 and related oxide materials has been successfully
accomplished with varying degrees of success using above mentioned titanium
complexes, current generation precursors exhibit significant deficiencies. The use of
metal alkoxides as synthetic platform, molecular design of the useful precursors for TiO2
can be achieved. The main goal of the present work was to introduce small changes in the
- 31 -
ligand sphere of the well known ß-diketonate systems and use them for the MOCVD of
TiO2.
First part of the work was to explore new compounds by molecular engineering, a
systematic approach by varying the ligands by introducing small changes. As a result ß-
ketoesters, malonate and ß-ketoamide ligands were tested in combination with alkoxides
and amides of titanium. These precursors were characterized for their chemical and
physical properties by NMR, IR, Mass spectrometry, elemental analysis, and single
crystal X-ray diffraction. After clear confirmation for the formation of the compounds, a
detailed analysis of the thermal properties of the precursors for their suitability for CVD
was carried out using thermo gravimetric and differential thermal analysis.
The second part of the work was to deposit TiO2 thin films using a home built low
pressure horizontal CVD reactor. The available choice of precursors with small variations
in the ligand sphere offered a large possibility for depositing TiO2 thin films. Selected
precursors were tried taking thermal properties as merit and TiO2 thin films were grown
with various temperature and pressure series. Low temperature depositions were possible
with designed precursors without the use of additional oxygen. However, higher carbon
content in the film was noticed.
High solubility of these designed precursors makes them candidates for liquid injection
CVD. One of the precursor [Ti(OPri)2(tbaoac)2], was tried for deposition of TiO2 and
complex oxide SrTiO3 using solution based liquid injection CVD. These experiments
were carried out using MOCVD facilities at Forschungszentrum Jülich. Using the
commercially available precursor, [Ti(OPri)2(thd)2] comparative studies were performed
for depositing TiO2 and SrTiO3 thin films. The second part of the thesis provides details
of the CVD experiments carried out using home built reactor as well as the industrial tool
reactor.
For the macroscopic understanding of the CVD process it s essential that individual steps
involved must be studied carefully. Gas phase of a precursor is one such quite complex
systems for investigation and vitally influential of all processes in a CVD process. Any
design change involved during precursor synthesis has to provide detailed logic for
design inclusion in the molecule. Mechanistic details are often sought in such cases for
fundamental understanding of the functionality of the precursor. Most widely used
- 32 -
alkoxide of titanium, [Ti(OPri)4] and its ß-ketoester derivative, [Ti(OPri)2(tbaoac)2] and
commercially available precursor [Ti(OPri)2(thd)2] were studied for thermal
decomposition using matrix isolation FTIR spectroscopy. The third part of the work
extensively reports the details of the decomposition studies carried out on the above
mentioned TiO2 precursors using MI-IR. There are relatively very few studies on the gas
phase decomposition studies for organometallic precursors when compared to a large
number of solution based studies. The results obtained from matrix isolation studies were
successful to identify clearly the organic intermediates involved in the process. Use of
FTIR in combination with matrix isolation techniques could provide insights in to
mechanism not considered earlier in such studies.
Design changes in the ligand sphere influence the resulting physical properties of the
precursors. Fourth part of the work describes the thermal properties of the newly
developed precursors relevant for the CVD purposes. Simultaneous TG-DTA were
carried out on all of the newly developed complexes to evaluate their suitability as
precursors. A comparison to the thermal properties of the parent alkoxides and the
recently developed titanium precursors were done. Selected precursors were screened
with isothermal studies at different temperatures and sublimation rates were determined.
The inclusion of ester moiety in the ligand sphere induced low decomposition
temperature. In order to evaluate the hydrolytic stability, a comparative study was carried
out with structurally similar ß-diketonate complexes using NMR as an analytical tool.
- 33 -
1.11 References
[1] S. I. Stupp, P. V. Braun, Science 1997, 277, 1242.
[2] S. B. Sinnotta, E. C. Dickey, Materials Science and Engineering 2003, R 43, 1.
[3] D. P. Norton, Materials Science and Engineering 2004, R 43, 139.
[4] U. Diebold, Surface science reports 2003, 48, 53.
[5] L. S. Dubrovinsky, N. A. Dubrovinskaia, V. Swamy, J. Muscat, N. M. Harrison,
R. Ahuja, B. Holm, B. Johansonn, Nature 2001, 410, 654.
[6] S. A. Chambers, Y. Gao, S. Thevuthasan, Y. Liang, R. J. Shivaparan, J. Smith, J.
Vac. Sci. Technol. A, Part2 1996, 14(3), 1387.
[7] P. A. Morris Hotsenpiller, A. Roshko, J. B. Lowekamp, G. S. Rohrer, J. Cryst.
Growth 1997, 174 (1-4), 424.
[8] S. Yamamoto, T. Sumita, T. Yamaki, A. Miyashita, H. Naramoto, J. Cryst.
Growth 1997, 237-239, 569.
[9] B.-S. Jeong, J. D. Budai, D. P. Norton, Thin Solid Films 2002, 422 (1–2), 166.
[10] M. Murakami, Y. Matsumoto, K. Nakajima, T. Makino, Y. Segawa, T. Chikyow,
P. Ahmet, M. Kawasaki, H. Koinuma, Appl. Phys. Lett. 2001, 78 (18), 2664.
[11] S. A. Chambers, C. M. Wang, S. Thevuthasan, I. Droubay, D. E. McCready, A. S.
Lea, V. Shutthanandan, C. F. Windisch Jr., Thin Solid Films 2002, 418, 197.
[12] M. L. Hitchman, J. Zhao, J. Phys. IV 1999, 9, Pr8-357.
[13] H.-K. Ha, M.Yoshimoto, H. Koinuma, B.-K. Moon, H. Ishiwara, Appl. Phys. Lett.
1996, 68, 2965.
[14] J. V. Grahn, M. Linder, E. Fredriksson, J. Vac. Sci. Technol. A 1998, 16, 2495.
[15] J.-S. Chen, S. Chao, J.-S. Kao, G.-R. Lai, W.-H. Wang, Appl. Opt. 1997, 36,
4403.
[16] The National Technology Roadmap for Semiconductors, 3rd edition,
Semiconductor Industry Association, San Jose, CA, 1997.
[17] G. H. Johnson, J. Am. Ceram. Soc. 1953, 36, 97.
[18] K. S. Yeung, Y. W. Lam, Thin solid films 1989, 109, 169.
- 34 -
[19] A. J. Moulson, J. M. Herbert, Electroceramics : Materials, Properties and
Applications, Chapman and Hall, London, 1990.
[20] W. C. Hendricks, S. B. Desu, C. H. Peng, Chem. Mater. 1994, 6, 1955.
[21] R. F. Bunshah, PVD and CVD coatings, ASM international, Ohio, 1992.
[22] J. E. Mahan, Physical vapor deposition, Wiley Interscience, 2000.
[23] M. L. Hitchman, K. F. Jensen, Chemical vapor deposition-Principles and
applications, Academic Press, 1992.
[24] W. E. Sawyer, A. Mann, US. Patent 229,335, 1880.
[25] J. M. Blocher. Jr., J. Electrochem. Soc. 1960, 107, 117C.
[26] P. D. Agnello, IBM J. Res. & Dev. 2002, 46, 317.
[27] T. Kodas, M. J. Hampden-Smith, The chemistry of metal CVD, VCH Publishers,
Weiheim, Germany, 1994.
[28] R. A. Fischer, Chemie i. u. Zeit, 1995, 29, 141.
[29] A. C. Jones, J. Mater. Chem. 2002, 12, 2576.
[30] P. C. van Buskirk, J. Zhang, P. S. Kirlin, Ferroelectric thin film memories,
Science Forum, Tokyo, 1995.
[31] R. Hiskes, S. A. DiCarollis, J. L. Young, S. S. Laderman, R. D. Jacowitz, R. C.
Taber, Appl. Phys. Lett. 1991, 59, 607.
[32] IUPAC, Compendium of Chemical Terminology 1990, 62, 2200.
[33] IUPAC, Compendium of Chemical Terminology 1994, 66, 1138.
[34] M. E. Jacox, Chem. Soc. Rev. 2002, 31, 108.
[35] E. Whittle, D. A. Dows, G. C. Pimentel, J. Chem. Phys. 1954, 22.
[36] G. Hass, Vacuum 1952, 2, 331.
[37] R. N. Ghoshtagore, J. Electrochem. Soc. 1970, 117, 1310.
[38] R. N. Ghoshtagore, J. Electrochem. Soc. 1970, 117, 529.
[39] C. J. Taylor, D. C. Gilmer, D. G. Colombo, G. D. Wilk, S. A. Campbell, J.
Roberts, W. L. Gladfelter, J. Am. Chem. Soc. 1999, 121, 5220.
[40] T. W. Kim, M. Jung, H. J. Kim, T. H. Park, Y. S. Yoon, W. N. Kang, S. S. Yom,
H. K. Na, Appl. Phys. Lett. 1994, 64, 1407.
[41] A. C. Jones, T. J. Leedham, J. Brooks, H. O. Davies, J. Phys. IV 1999, 9, Pr8-
553.
- 35 -
[42] D. C. Gilmer, C. J. Taylor, D. G. Colombo, J. Roberts, G. Haugstad, S. A.
Campbell, H.-S. Kim, G. D. Wilk, M. A. Gribelyuk, W. L. Gladfelter, Chem. Vap.
Deposition 1998, 4, 9.
[43] J. J. Gallegos, T. L. Ward, T. J. Boyle, M. A. Rodriguez, L. P. Francisco, Chem.
Vap. Deposition 2000, 6, 21.
[44] E.-C. Plappert, K.-H. Dahmen, R. Hauert, K.-H. Ernst, Chem. Vap. Deposition
1999, 5, 79.
[45] A. C. Jones, T. J. Leedham, P. J. Wright, M. J. Crosbie, K. A. Fleeting, D. J.
Otway, P. O’Brien, M. E. Pemble, J. Mater. Chem. 1998, 8, 1773.
[46] M. Balog, M. Schieber, J. Cryst. Growth 1972, 17, 298.
[47] D. C. Bradley, R. C. Mehrotra, D. P. Gaur, Metal Alkoxides, Academic Press,
New York, 1978.
[48] Y.-S. Min, Y.-J. Cho, D. Kim, J.-H. Lee, B.-M. Kim, S.-K. Lim, I.-M. Lee, W.-I.
Lee, Chem. Vap. Deposit. 2001, 7, 146.
[49] M. Schuisky, A. Hårsta, J. Phys. IV 1999, 9, Pr8-381.
- 36 -
Chapter 2
Rational development of titanium precursors used for MOCVD of titanium containing oxide thin films
Abstract
The concept of introducing small variations in the ligand sphere of well known structures
for the synthesis of improved metal organic chemical vapor deposition (MOCVD)
precursors for titanium have been studied in detail. The reaction of titanium alkoxides
with ß-ketoester, ß-ketoamide and malonate ligands resulted in complexes having
potential to serve as precursors for MOCVD of TiO2 thin films. Titanium
bis(isopropoxide) bis(methylacetoacetate) (1), titanium bis(ethoxide)
bis(methylacetoacetate) (2), titanium bis(isopropoxide) bis(tert-Butylacetoacetate) (3),
titanium bis(ethoxide) bis(tert-Butylacetoacetate) (4) and titanium bis(isopropoxide)
bis(N,N-diethylacetoacetamide) (5) and bis-[(di-ethylmalonato) tetra(isopropoxy)-µ-
ethoxy-titanium(IV)] (6) Titanium bis(isopropoxide) bis(diethylaminoethoxide) (7),
Titanium bis(ethoxide) bis(diethylaminoethoxide) (8) have been synthesized and
characterized by elemental analysis, NMR, and mass spectrometry. While the
aminoalkoxide complexes were viscous liquids, ß-ketoester/malonate and ß-ketoamide
complexes were crystalline solids. The molecular structure of the compounds as
determined by single crystal X-ray diffraction revealed that complexes with ß-ketoester
and ß-ketoamide ligands exist as monomers while complex with malonate ligand
undergoes trans-esterification reaction which exhibits a centro-symmetric dimeric
structure. This study assumes importance as there are seldom any reports about solid state
structures of ß-ketoesters with titanium alkoxides in monomeric forms. It is shown that
by introducing small changes in the established key structures of the existing ligands, it is
possible to tailor the physical properties of the resulting titanium complexes.
- 37 -
2.1 Introduction
MOCVD offers many attractions for titanium dioxide film growth, including conformal
deposition on a variety of complex substrates, low equipment costs, easy scale-up, lower
growth temperatures, and higher growth rates. However, the success of an MOCVD
process depends critically on the availability of volatile, thermally stable precursors
which exhibit constant vapor pressure and the capacity to selectively form the desired
phase at the substrate surface. Generally low-melting precursors are preferred because on
the one hand solid compounds can be handled with ease at room temperature; on the
other, the liquid form at reservoir operating temperatures affords constant surface area for
stable vapor delivery to the reactor (as discussed in section 1.7). The versatility of
precursor chemistry derives largely from variety of molecular precursors available as
sources.
The chemical vapor deposition of titanium dioxide has been practiced for over five
decades.[1, 2] [TiCl4] has been used as precursor for TiO2 thin films extensively. Using O2
and H2O as reactants thin films of TiO2 have been obtained. However, [TiCl4] is toxic
and requires special equipment and safety installation for handling.[3] The pioneering
work by Bradley and co-workers[4] explored the relationship between the molecular
structure of metal alkoxides and their physical properties such as degree of association,
i.e. nuclearity and volatility. The volatility of metal alkoxides is strongly influenced by
their tendency to form oligomers and clusters. This is related to positive charge on the
metal center, which has a tendency to form bonds with negatively charged oxygen of the
neighboring M(OR)n molecules. It was concluded that, in order to inhibit oligomerization
in metal alkoxides containing large, highly positively-charged metal atoms, bulky
sterically demanding ligands such as iso-propoxide, tert-Butoxide must be employed. So
volatile titanium alkoxides complexes were synthesized and used as precursors for
deposition of TiO2 thin films and one of the most widely used titanium precursor is
titanium tetraisoproxide (TTIP). However, the alkoxide precursors contain unsaturated
four-coordinate metal centers and the alkoxy ligands undergo a facile catalytic hydrolytic
decomposition reaction in the presence of trace water.[5] These complexes are therefore
extremely air and moisture-sensitive, which limits their shelf-life and makes them
- 38 -
difficult to handle and use in MOCVD, especially in the case of solution-based liquid
injection MOCVD applications.
In order to reduce the moisture sensitivity of the Ti-alkoxide precursors, chelating ß-
diketonate groups have been inserted to increase the saturation of the Ti(IV) coordination
sphere. [Ti(OPri)2(acac)2][6] and [Ti(OPri)2(thd)2][7] have been used for the deposition of
TiO2 thin films. [Ti(OPri)2(thd)2] is a monomeric six coordinated complex in solution and
stable in solution than its parent alkoxide [Ti(OPri)]4.[8] Presence of ß-diketonate group
with bulky side chain such as Hthd results in higher thermal stability of the precursor and
[Ti(OPri)2(thd)2] is perhaps most widely used mixed alkoxide-ß-diketonate complex of
titanium.
Another approach to increasing the coordinative saturation of the metal center is to
introduce bidentate donor functionalized ligands such as 2-dimethylaminoethanolate or
diolates such as 2-methylpentane-2, 4-diolate. There are few examples where complexes
other than ß-diketonates or alkoxides have been used for MOCVD of TiO2 and related
oxides; namely monomeric ß-ketiminate complex [Ti(2meip)2] 2meip = 4-(2-
methylethoxy)imino-2-pentonate[9]and [Ti(NO3)]4[10-11] are the notable ones.
The objective of this work was to vary the terminal groups of the well established ß-
diketonate complexes of titanium alkoxides and study the effect on the volatility and
decomposition pathways. In addition, mixed alkoxides of titanium with amino alcohols
were also studied by inclusion of small variations in the ligand periphery. With relative
ease in preparation of metal alkoxides after Bradley[4], the efforts aimed at the precursor
development were turned towards mixed alkoxide-chelating ligand complexes, mainly ß-
diketonates. In a deliberate effort to include targeted “cleavage points” to bring about
orderly thermolytic decomposition, we tried ß-ketoesters, ß-ketoamides and malonates, as
chelating ligands. In addition, the use of aminoalcohols in combination with titanium
alkoxides was expected to act as donor functionalized chelating ligand. The ligands used
for complexation were, methylacetoacetate (meaoac), tert-Butylacetoacetate (tbaoac), N,
N-diethylacetoacetamide, (deacam) di-ethylmalonate (deml) and diethylaminoethanol
(deae). Inclusion of ester/malonate/amide moiety in the side chain of the ß-keto structure
is expected to weaken the metal-oxygen bond of the chelating ring thus leading to facile
thermal fragmentation of the complexes. It was expected that these complexes would
- 39 -
combine the chemical stability derived from ß-keto structure and facile fragmentation
pattern induced by ester moiety embedded therein and volatility derived from alkoxy
ligands.
2.2 Experimental Section
For precursor synthesis all manipulations were performed utilizing oven-dried reaction
vessels and Schlenk techniques under inert atmosphere of purified argon or in a glove
box. Solvents were dried under N2 by standard methods and stored over 4 Å molecular
sieves. Proton- 1H and carbon- 13C NMR spectra were recorded for all synthesized
compounds. The spectra were referenced to residual protic impurities of the internal
solvent and corrected to tetramethylsilane. The integration of peaks and peak intensity
analyses were done using Mestrec® software version 2.30. Bruker Advance DPX 200 and
Bruker Advance DPX 250 spectrometers were used.
Elemental analysis (Elemental, CHNSO Vario EL, Hanau) and mass spectra were
provided by the Spectroscopy and Chromatography Analytical section of the Faculty of
Chemistry at the Ruhr-University of Bochum. Electron Ionization (EI) mass spectra were
recorded using ionization energies between 24 eV to 70 eV using CHS-Mass
spectrometer “Varian MAT” (Bremen). Output spectra was given as specific masses
(m/z) based on abundant isotopes, 1 1H, 12
6C 14 7N 16 8O, 48
22Ti. Infrared data were
collected on a 1650 Perkin-Elmer spectrometer. The alkoxides of titanium namely,
Titanium(IV) isopropoxide (Aldrich) and Titanium (IV) ethoxide (Fluka) were purchased
and used as such without further purification, but stored in a glove box excluding air and
moisture during storage. Ligands used in the study were purchased from Aldrich and
distilled prior to use.
Synthesis of [Ti(OPri)2(meaoac)2] (1). About 2.32 ml (0.02 mol) of methyl acetoacetate
ester was added to 2.97 ml (0.01 mol) of Ti(OPri)4, the resulting yellowish clear solution
was stirred for 2 hours and kept in a refrigerator at -20 °C. Colourless needles were
crystallised after 24 hours. The crystals were washed with cold pentane and dried in
vacuum. Yield 87%. C16H28O8Ti calculated C, 48.50, H, 7.12; Found C, 47.87, H, 7.35.
- 40 -
1H NMR ( 200 MHz, C6D6, 25 °C) δ 1.4 (12H, d, OCH(CH3)2) ; δ 1.9 (6H, s, CO(CH3));
δ 3.6 (6H, s, OCH3); δ 5.1 (2H, s, COCHCO ); 5.3 (2H, hept, CH(CH3)2); 13C{1H}
NMR 200 MHz C6D6 25 °C. 186(CH3CO), 173(COO), 89(COCHCO), 79(CH -OPri),
51(OCH3), 29(CH3 lig), and 26(CH3 -OPri). Mass spectrometry. (EI+) m/z 337, 25% [1 -
isopropyl group]; 281, 100% [Ti(meaoac)2]; 211, 35% [Ti2(meaoac)].
Synthesis of [Ti(OEt)2(meaoac)2] (2). About 2.32 ml (0.02 mol) of methyl acetoacetate
ester was added to 2.09 ml (0.01 mol) of Ti(OEt)4, and the resulting clear solution was
kept in a refrigerator at -20 °C. Slightly yellow colored crystalline solid was formed
which was filtered and dried in vacuum. Yield 80%. Elemental analysis (%):C14H24O8Ti,
calculated C, 45.65, H, 6.57.; Found C, 45.69, H, 6.49. 1H NMR (200 MHz, C6D6, 25
°C) δ 1.4 (6H, t, OCH2CH3) ; δ 1.8 ( H, s, CO(CH3)) ; δ 3.5 (6H, s, OCH3) ; δ 4.7 (4H, q,
OCH2CH3) ; δ 5.1 (2H, s, COCHCO); 13C{1H}-NMR (200 MHz C6D6 25 °C) δ[ppm] =
185 (CH3CO), 173 (COO), 88(COCHCO), 73 (CH2 –ethoxy), 51 (OCH3 -lig), 29(CH3 -
lig), 18(CH3-Ethoxy). Mass spectrometry: (EI+) m/z 323, 20% [2 – Ethoxy group]; 278,
25% [Ti(meaoac)2]; 211, 15% [Ti2(meaoac)]; 162, 100% [Ti(meaoac)].
Synthesis of [Ti(OPri)2(tbaoac)2] (3). About 3.3 ml (0.02 mol) of the tert. Butyl
acetoacetate ester was added to 2.97 ml (0.01 mol) of Ti(OPri)4. The resulting yellowish
clear solution was stirred for 2 hours and stored at -20 °C. Pale yellow needles were
crystallized after 24 hours, and were washed with cold pentane and dried in vacuum.
Yield, 84%. Elemental analysis (%):C22H40O8Ti calculated C, 55,0, H, 8.39. Found C,
53.99, H, 8.39. 1H-NMR (200 MHz, C6D6, 25 °C) δ[ppm] = 1.5 (12H, d, OCH(CH3)2) ;
1.7 (18H, s, C(CH3)3); 1.9 (6H, s, CO(CH3)); 5.2 (2H, hept, CH(CH3)2); 5.3 (2H, s,
OCCHCO); 13C{1H}-NMR (200 MHz C6D6 25 °C) δ[ppm] = 187 (CH3CO), 172 (COO),
91 (CHCO), 81 (C tBut), 79 (CH -OPri), 29 (CH3), 26 (CH3 -OPri) and 25 ( CH3 -tBut).
Mass spectrometry: (EI+) m/z 422, 15% [3- tBut]; 362, 15% [Ti(tbaoac)2]; 267, 100%
[Ti(OPri)(tbaoac)]; 225, 20%[Ti(OPri)3]; 165, 20% [Ti(OPri)2]; 57, 20% C(CH3)3.
Synthesis of [Ti(OEt)2(tbaoac)2] (4). About 3.3 ml (0.02 mol) of tert. Butyl acetoacetate
ester was added to 2.09 ml (0.01 mol) of Ti(OEt)4, the resulting yellowish clear solution
- 41 -
was stirred for 2 hours and kept in a refrigerator at -20 °C. Off white solid was
crystallised after 24 hours as pale yellow needles. Washed with cold pentane and dried in
vacuum. Yield, 83%. C20H36O8Ti, Calculated C, 53,10, H, 8.02. Found C, 53.14, H, 8.22. 1H NMR ( 200 MHz, C6D6, 25 °C) δ 1.5 ( 6H, t, OCH2CH3 ) ; δ 1.7 ( 18H, s, C(CH3)3 );
δ 1.9 ( 6H, s, CO(CH3) ); δ 4.8 ( 4H, q, OCH2CH3 ) ; δ 5.2 ( 2H, s, COCHCO ); 13C{1H}
NMR (200 MHz C6D6 25°C). 186 (CH3CO), 173(COO), 90(CHCO), 81 (C tBut), 73
(CH2 Ethoxide), 29(CH3), 28(CH3 tBut), and 19(CH3 Ethoxide). Mass spectrometry. (EI+)
m/z 08, 3% [4- CO2]; 362, 3% [Ti(tbaoac)2]; 239, 10% [Ti3(OPri)2]; 57 100% tBut.
Synthesis of [Ti(OPri)2(deacam)2] (5). A diluted solution of 3.14 ml (0.02 mol) of
diethyl acetamide in 20 ml of hexane was added to a diluted solution of Ti(OPri)4 [2.97
ml (0.01 mol) in hexane (20 ml)]. The mixture was refluxed for 12 hrs at 68 °C and then
the resulting mixture was stored in the refrigerator at -20 °C for 24 hours. Brown colored
solid crystallized which was washed repeatedly with cold hexane which resulted in pale
yellow crystalline product. Dissolved in hot hexane and was allowed to recrystallize
slowly at -20 °C to yield off white crystals. Yield, 43%. C22H42O6N2Ti, Calculated C,
55,19, H, 8.78, N, 5.85. Found C, 55.14, H, 8.82, N, 5.78. 1H-NMR (250 MHz, C6D6, 25
°C) d 1.90 (6H, s, CH3 deacam), 1.45 (6H, d, CH3 OPri, 1J ~ 5.97 Hz), 0.7 (6H, t,
NCH2CH3a,), 0.95 (6H, t, NCH2CH3b,), 2.64 (4H, q, NCH2a CH3), 3.07 (4H, q, NCH2b
CH3), 4.66 (2H, s, CH OPri), 4.75 (2H, s, CH deacam) 13C{1H} NMR (200 MHz C6D6 25
°C). d 41 (NCH2aCH3), 42 (NCH2bCH3), 27 (CH3 OPri), 26 (CH3 deacam), 13.02
(NCH2CH3a), 13.07 (NCH2CH3b), 86 (CH deacam), 70 (CH OPri), 185 (COCH3
deacam), 168 (CO deacam). Mass spectrometry. (EI+) m/z 419, 23% [5- OPri]; 362,
15% [Ti(deacam)2]; 239, 10% [Ti3(OPri)2]; 85 100% [deacam-NEt2]
Synthesis of [Ti2(µ-OEt)2(OPri)4(deml)2] (6). About 3.1 ml (0.02 mol) of diethyl
malonate was added to a solution of 2.97 ml (0.01 mol) Ti(OPri)4 in 10 ml hexane. The
colourless solution turned slightly yellow and warm. After two hours of stirring, the
mixture was cooled down to -25 °C for 24 hours. Slightly yellow, cubic crystals were
formed and dried in vacuum after the removal of the solvent. Yield, 88 %. 1H-NMR
(250MHz, Toluene, 25 °C) Unidentified peaks (refer results and discussion part): d
- 42 -
[ppm] = 5.6 – 4.0 (multiplet), 3.866 (q), 3.046 (s), 3.036 (s), 3.026 (s), 1.7-1.0 (multiplet),
0.971 (q), 0.859 (multiplet); 13C-NMR (250 MHz, C6D6, RT, 20000 scans): can not be
interpreted.
Synthesis of [Ti(OPri)2(deae)2] (7). A diluted solution of 2.66 ml (0.02 mol) of diethyl
aminoethanol in 20 ml of hexane was added to a diluted solution of titanium
tetraisopropoxide 2.97 ml (0.01 mol) in hexane (20 ml). The resulting solution was
refluxed at 70 °C for two hours. Solvent and volatile products were removed under low
pressure resulting in a clear, yellow oily liquid as product. Yield, 94 %. 1H NMR ( 200
MHz, C6D6, 25 °C) δ δ 1.0 (12H, t, NCH2CH3 ) δ 1.5 ( 12H, d, OCH(CH3)2), δ 2.8 (12H,
q + t, NCH2CH3+ OCH2CH2NEt2), δ 4.6( 4H, t, O-CH2CH2N), δ 5.0 (2H, sep,
OCH(CH3)2). 13C{1H}-NMR (200 MHz C6D6 25 °C) δ[ppm] 11.5 (N-CH2-CH3)2, 26.67
(OCH(CH3)2), 47.51 (NCH2CH3), 57.23 (OCH2CH2NEt2), 68.04 (OCH2CH2NEt2),
77.13 (OCH(CH3)2). Mass spectrometry. (EI+) m/z Expected mass 398.8. 340 (2%) [7-
2Et], 279 (4%) [7 – lig. deae], 86 (100%) remains speculative.
Synthesis of [Ti(OEt)2(deae)2] (8). A diluted solution of 2.09 ml (0.01 mol) of titanium
tetraethoxide in 20 ml of hexane was added to a diluted solution of 2.66 ml (0.02 mol) in
hexane (20 ml). Resulting solution was refluxed at 70 °C for two hours. The volatile
products were removed under low pressure resulting in a clear, yellow oily liquid as
product. Yield, 96 %. TiC16H38O4N2 Calculated C, 51,89, H, 10.34, N, 7.56. Found C,
51.56, H, 10.39, N, 7.96. 1H NMR ( 200 MHz, C6D6, 25 °C) δ 0.9 ( 12H, t, NCH2CH3 ),
δ 1.3 ( 6H, t, OCH2CH3 ), δ 2.5 ( 8H, q, NCH2CH3 ), δ 2.8 ( 4H, t, OCH2CH2NEt2 ) δ
4.7 (8H, q+q, OCH2CH3+ OCH2CH2NEt2). 13C{1H}-NMR (200 MHz C6D6 25 °C)
δ[ppm] 12.16 (N-CH2-CH3)2, 20.23 (OCH2CH3), 48.38 (N-CH2-CH3)2, 57.9
(OCH2CH2NEt2), 70.06 (OCH2CH2NEt2), 72.37 (OCH2CH3). Mass spectrometry. (EI+)
m/z remains speculative with 100% peak at 86 and highest m/z peak ( 2%) at 326.
Expected mass 370.25.
- 43 -
2.3 Results and Discussion
2.3.1 Objectives of ligand design
The design and realization of new molecular MOCVD precursors offers a considerable
synthetic challenge. It is a challenging task in terms of complex chemistry but it provides
an inspiration for the construction of advanced materials, as well as it is a fascinating
research topic in its own right. Precursor design and synthesis involves new molecules or
known molecules with inclusion of design changes characterized by intricate structures,
optimized to their function for material applications.The most important precursor design
requirements that ideally must be satisfied are volatility, thermal stability, clean
decomposition, which are discussed in detail in section 1.6. It is highly unlikely that a
single precursor meets all the requirements for an ideal precursor. So designing a
precursor by including rational variations in precursor molecule according to process
requirements helps to a considerable extent in optimizing the process.
Volatility of a chemical precursor is based on weak attractive interaction between
molecules. In an ideal case monomeric, neutrally charged, complex is preferred. This is
because, higher molecular masses tend to reduce volatility; similarly polar groups and
polarizable groups also affect volatility.[12] The ideal ligand system for early transition
metals in a high oxidation state should have optimal steric bulk for stabilizing the metal
center. The selection of ligands with electron donor properties and optimal bulk not only
avoids oligomerization but also helps in stabilizing the electrophilic metal center. It is
well known that the volatility of a precursor is a complex function of intermolecular
forces (van der Waals interactions, p-stacking or hydrogen bonds) which not only depend
on the molecular weight and geometry but also (for solids) on the lattice structure.
Control of the polymerization in the solid state and in the vapor by optimizing the steric
bulk of the ligands, manipulation of ligands, represent the main directions for tailoring
volatility.
Indeed, the thermal behavior of these complexes is extensively influenced by ligand
dissociation reactions taking place during the transport into the vapor phase. The extent
of dissociation during or before evaporation depends on the stability of the complexes.
- 44 -
The complexes based on monodentate donors such as alkoxides dissociate completely.
The partial substitution of alkoxide ligands by ß-diketonates, the latter acting essentially
as chelating ligands, could be a means to reduce polymerization and thus to enhance the
stability. Decreased susceptibility towards air and moisture could be an added advantage.
2.3.2 Precursor synthesis and properties
The most common precursors for metal oxide are compounds which already contain an
M-O linkage, namely alkoxides [M(OR)n]m, carboxylates [M(O2CR)n]m or ß-diketonates
[M(OCRCHCR´O)n]m where n = oxidation state and m = molecular complexity.[4] These
three ligand types are considered versatile because of the backbone flexibility and the
variations possible in substituents. During the course of this study alkoxides and ß-
Fig 2.1: Molecular structure of [Ti(OPri)2(meaoac)2]. Important bond lengths [? ] and bond angles [deg] are listed here.
Bond lengths (Å) Bond Angles (deg.)
Ti-O(11) 2.126(4) O(16)-Ti-O(26) 100.83(17) Ti-O(25) 1.970(4) O(15)-Ti-O(25) 161.11(15) Ti-O(16) 1.775(4) O(21)-Ti-O(11) 78.99(14) Ti-O(21) 2.114(4) O(26)-Ti-O(21) 90.79(16) Ti-O(26) 1.782(4) C(24)-O(25)-Ti 133.5(4) O(11)-C(12) 1.224(6) O(16)-Ti-O(15) 98.33(18) C(12)-C(13) 1.412(8) O(15)-Ti-O(11) 82.72(15)
- 45 -
ketoesters were employed for synthesizing the titanium complexes, hence details of
carboxylates are not discussed.
Metal alkoxides contain at least one M-O-C structural unit. Titanium with a Pauling
electronegativity of 1.5 does not have strong polar bonds between metal and oxygen. As a
result, alkoxides of titanium have limited nuclearities and simple titanium tetraalkoxides
are all volatile, including the titanium tetramethoxide, which is a solid at room
temperature that sublimes at 190 °C.[13] The demands for large ligands for minimizing the
nuclearity and low molecular weight are conflicting to each other and not easily achieved.
Titanium tetraisopropoxide is the most volatile of the titanium alkoxides a property
attributed to its low molecular weight and the steric effect of the ligand which restricts
nuclearity (molecular complexity) of the complex.[14] It is accepted that titanium
Fig. 2.2: Molecular structure of [Ti(OPri)2(tbaoac)2]. Important bond lengths [? ] and bond angles [deg] are as follows
Bond lengths (Å) Bond Angles (deg.)
Ti-O(16) 1.784(2) O(15)-Ti-O(25) 161.85(10) Ti-O(21) 2.118(2) O(26)-Ti-O(11) 167.78(10) Ti-O(26) 1.787(3) O(16)-Ti-O(26) 101.41(11) Ti-O(15) 1.976(3) O(26)-Ti-O(25) 100.07(12) C(14)-O(15) 1.286(4) O(25)-Ti-O(11) 83.29(10) O(11)-C(12) 1.249(4) O(11)-Ti-O(21) 78.68(9) C(12)-O(121) 1.337(4) C(12)-O(121)-C(121) 122.1(3) O(121)-C(121) 1.486(4) C(14)-C(13)-C(12) 122.3(4) C(22)-C(23) 1.418(5) C(14)-O(15)-Ti 134.4(2) C(23)-C(24) 1.358(5)
- 46 -
tetraisopropoxide has a nuclearity of approximately 1.2 by earlier studies.[15] Titanium
tetraethoxide is a tetramer in solid state and a trimer in solution.[16] These two alkoxides
are used in the present study for inducing optimum volatility to the resulting precursor
complexes.
Fig. 2.3: Molecular structure of [Ti(OEt)2(tbaoac)2]. The dotted line shows the bond for
the disordered methyl group. Important bond lengths [? ] and bond angles [deg]
are listed here.
Bond lengths (Å) Bond Angles (deg.)
Ti-O(5) 2.130(4) O(3)-Ti-O(6) 160.98(17)
Ti-O(52) 1.788(4) O(52)-Ti-O(2) 88.77(17)
Ti-O(51) 1.797(4) C(2)-O(2)-Ti 128.7(4)
Ti-O(6) 1.966(4) O(51)-Ti-O(2) 170.85(19)
Ti-O(3) 1.966(4) O(3)-Ti-O(2) 82.68(17)
O(4)-C(7) 1.342(7) C(4)-O(3)-Ti 131.9(4)
O(4)-C(6) 1.481(7) O(2)-Ti-O(5) 81.49(15)
C(7)-C(8) 1.406(8) O(6)-Ti-O(2) 84.26(17)
C(51)-C(52) 1.351(18)
C(51)-C(52') 1.29(2)
- 47 -
The ß-ketoesters always exist in keto and enol forms in equilibrium with each other [17] as
shown in the scheme 2.1.
O O
R1
R2
OR3
O O
R1
R2
OR3
H O O
R1
R2
O
H R3
Scheme 2.1.
The methane proton in the keto form and the hydroxyl proton in the enol form of ß-
ketoesters are acidic and their removal generates 1,3-ketoesterate anions which are source
of an extremely broad class of coordination compounds. 1,3-ketoesterate anions are
powerful chelating species and form complexes with virtually every transition and main
group element. There has been a large scope for this chemistry and has been reviewed
several times.[18-21]
The reaction between a ß-ketoester and tetraalkoxy-titanium dates back to more than half
a century.[22,23] The first two studies on such type of reaction could only predict the
formation of a desired compound but failed to either isolate the pure compound or to
provide convincing analytical data. But these studies could shed some light on the
possibility and access to such compounds. Subsequently, during the same decade the first
convincing structural study based on IR spectroscopy and cryoscopy could predict the
structure to be monomeric and octahedral coordination around titanium metal center.[24]
It remained unclear whether the complex has cis-substituted or trans-substituted
structure. However, Bradley and Holloway conclusively ruled out the possibility of the
trans configuration in favor of cis configuration. Based on proton NMR studies they
found that all of the derivatives of [Ti(ß-diketonate)2(OR)2] type complexes existed only
in the cis (optically active ) form over a wide range of temperatures. Activation energies
for intramolecular exchange of ligands in these fluxional molecules showed that steric
hindrance of the alkoxy group increased the energy of activation but did not promote the
trans- form [25] In the case of reactions of ß-diketonates or ß-ketoesters with titanium
alkoxides, bis-ß-diketonate or ketoester derivatives were the final products even when the
excess of these ligands were used. The non-replaceability of the third or fourth alkoxy
- 48 -
groups with ß-diketonates or ketoesters may most probably due to the preferred
coordination number of 6 for titanium in the bis –derivatives and has been reported
earlier [24]
Titanium prefers a coordination number six to form stable complexes. Complexes having
coordination number four are sensitive towards hydrolysis by moisture. This is evident in
most of the tetra-alkoxy compounds of titanium. Alkoxy ligands being monodentate,
bond through oxygen atom to the metal center. ß-ketoester and ß-ketoamides act as
bidentate ligands. Reactions of tetra alkoxy titanium with two equivalents of ß-ketoesters/
ß-ketoamides resulted in stable six coordinated complexes. The representative scheme of
such a reaction is given in schemes 2.2 and 2.3 respectively.
Fig. 2.4: Molecular structure of [Ti(OPri)2(deacam)2].
Important bond lengths [? ] and bond angles [deg] are listed here.
Bond lengths (Å) Bond Angles (deg.)
Ti-O(115) 1.8086(16) O(111)-Ti-O(110) 100.84(7) Ti-O(111) 1.8178(16) O(15)-Ti-O(11) 82.58(6) Ti-O(16) 2.0394(15) O(115)-Ti-O(16) 169.97(7) Ti-O(11) 2.0737(16) O(115)-Ti-O(111) 99.32(7) C(17)-N(17) 1.345(3) C(123)-N(12)-C(121) 117.29(19) N(17)-C(171) 1.464(3) O(11)-C(12) 1.271(3) C(14)-O(15) 1.299(3)
- 49 -
O O
N
Ti(OPri)4 + 2 O
O
N
Ti
O
O NO O
+ 2 HOPri
Complex 5 Scheme 2.3
Comparison of the pKa values of ß-ketoester with a series of substituted ß-diketonates
shows that the Lewis acidity at the metal center increases, moving from ß-diketonates to
ß-ketoesters and malonates.[26] In such a situation, it is likely that ß-ketoester can undergo
a facile trans-esterification reaction in the presence of catalytically active Lewis acidic
centers, such as titanium metal. Interestingly during our studies the use of malonic ester
as ligand led to trans-esterification reaction and ligand moieties were exchanged between
two types of ligands surrounding the metal. On the contrary when ß-ketoesters were used
as ligands we did not encounter such type of reactions. Interestingly, the reaction with
malonic esters follows a different path altogether. It was hoped that use of malonates,
being bulky ligands, stabilizes the metal center by restricting the nuclearity to monomer.
So the ratio of metal to ligand was altered to favor monomeric products, but interestingly
these efforts resulted in the same dimeric complex 6. The reaction of malonates with
titanium alkoxides is shown in the reaction scheme 2.4.
O O
R2 OR3
Ti(OR1)4 + 2 + 2 R1OH
R1=iPr R2, R3 = CH3 Complex 1
O
O
R2
OR3
O
O
R2
OR3Ti
R1O OR1
R1=Et R2, R3 = CH3 Complex 2
R1=iPr R2, R3 = tBut Complex 3
R1= Et R2, R3 = tBut Complex 4
Scheme 2.2
- 50 -
O O
O OTi
O
O
O
O O
O
O
O
O
O
O
OO
O
2Ti(OPri)4 +2 + 2 EtOH
Complex 6 Scheme 2.4
Fig. 2.5: Molecular structure of [Ti2(µ-OEt)2(OPri)4(deml)2].
Important bond lengths [? ] and bond angles [deg] are listed here.
Bond lengths (Å) Bond Angles (deg.)
Ti(1)-O(6) 1.791(3) O(6)-Ti(1)-O(5) 97.09(13) Ti(1)-O(7) 1.802(3) O(6)-Ti(1)-O(7) 96.44(14) Ti(1)-O(5) 2.027(3) O(6)-Ti(1)-O(1) 86.54(12) Ti(1)-O(1) 2.109(3) O(5)-Ti(1)-O(1) 81.38(12) Ti(1)-O(8) 2.080(3) O(7)-Ti(1)-O(1) 172.53(12) Ti(1)-Ti(1) 3.2113(16) O(8)-Ti(1)-O(5) 157.89(12) O(41)-C(41) 1.450(6) O(8)-Ti(1)-O(1) 84.55(11) C(2)-C(3) 1.391(6) O(7)-Ti(1)-O(8) 101.69(12) O(1)-C(2) 1.250(5) C(4)-C(3)-C(2) 120.3(4) C(4)-O(5) 1.262(5)
- 51 -
Due to the presence of catalytically active Lewis acidic metal center Ti, the trans-
esterification reaction was observed. Logic for this can be derived from the fact that
malonate ligands have higher pKa values compared to ß-ketoesters and thus render the
metal center more Lewis acidic. The octahedral coordination of titanium results from
bridging of alkoxide ligands of low steric hindrance, i.e. in this case, ethoxide moieties
drawn from the periphery of the malonate ligand used in the reaction. The reaction is
facile and slightly exothermic at room temperature.
The compounds with ß-ketoesters/malonates/amides were off white to pale yellow in
appearance and were crystalline solids. The purified products were sublimed at moderate
temperatures and the melting points were below 90 °C. In addition the complexes were
highly soluble in organic solvents. All these factors make these compounds attractive for
MOCVD applications. Donor functionalized ligands of the type amionoalkoxides, where
the amino group acts as a donor is used widely for sol gel processes.[12] Donor
functionalized ligand are assumed to be less easily separated from the metal center by
hydrolysis than monodentate ligands because of chelate effect. Scheme 2.5
O
N O
N
RO OR
HO
NTi(OR)4 + 2
-2HOR
Complex 7 R = OPri
Complex 8 R= OEt Scheme 2.5
The use of such type of ligands to synthesize titanium complexes has been demonstrated
earlier and employed for TiO2 deposition.[27] The synthesis of aminoalkoxide classes of
compounds was simple and was reproducible in high yields. The complexes of
aminoalkoxides were viscous liquids which form glassy solid at -77 °C. The higher
temperatures required (>150 °C) for vaporizing these complexes deprived their use as
MOCVD precursors but perhaps suitable for liquid injection CVD. Nevertheless, these
- 52 -
complexes have been used to synthesize TiO2 nano particles using a non hydrolytic
approach.[28]
2.3.3 NMR studies
The room temperature 1H and 13C NMR spectra of compounds 1-8 except 6 in benzene-d6
are structurally significant without any dynamic solution behavior. Sharp singlets were
observed for the protons of methyl, tert. Butyl and methyne of the tbaoac ligand. A septet
(2H) as well as a doublet (12H) refers to the isopropoxide ligands. Similarly a quartet
(4H) and a triplet (6H) were clearly visible for ethoxide complexes. The 13C NMR was in
good agreement with the proposed monomeric structures as well. As a typical example
the 1H NMR spectrum for the compound [Ti(OEt)2(tbaoac)2] is depicted in Fig. 2.6.
Complex 6 showed very complex NMR behaviour. 1H and 13C-NMR spectra, taken from
the crystals in benzene-d6 or toluene-d8 are complex and could be hardly interpreted. The
reason is that 6 undergoes highly fluxional exchange reactions of the alkoxy groups in
solution and probably forms species with different nuclearities. 1H spectra of 6 in
toluene-d8 at temperatures between -40 °C and 65 °C failed to provide the solution for the
ppm (t1)1.02.03.04.05.06.07.08.0
-CH-
-CH2
-
-CH3
tBu
-CH3
Fig. 2.6: 1H NMR spectrum of the compound [Ti(OEt)2(tbaoac)2] in benzene-d6
- 53 -
complexity. Only four peaks could be identified which were readily assigned to
isopropoxy group and ethoxy group in an agreeable resolution.
2.3.4 Single crystal X-ray diffraction analysis
Data collections for compounds 1, 3, 4, 5 and 6 were performed on a Bruker AXS CCD
1000 diffractometer, equipped with a cryogenic nitrogen cold stream to prevent loss of
solvent and using graphite monochromated Mo-Ka radiation (0.71073 ? ). The crystals
were mounted on glass capillaries. The overall molecular geometries with atomic labeling
are illustrated in Figs. 2.1, 2.2, 2.3, 2.4 and 2.5 respectively. Relevant crystallographic
details are summarized in Table 2.1. The structures were solved by direct method using
SHELXL-97[29] software package and refined by full matrix least-squares methods based
on F2 with all observed reflections. Empirical absorption corrections were applied to the
data by SADABS (version 2.03).[30] All non hydrogen atoms were refined anisotropically
and all the hydrogen atoms were placed in calculated positions.
Single-crystal X-ray diffraction experiments reveal that the titanium complexes (1, 3, 4
and 5) formed with the ß ketoesters/ ß-ketoamides as ligands, are monomeric, a property
which is preferred for better volatility. Crystals of complex 2 failed to give enough
reflections for determining the structure. The structures of the four compounds 1, 3, 4 and
5 carry similar features. The mononuclear crystal structures with ß-diketo systems are
rarely reported in the literature. Though there are several studies wherein based on NMR
studies the structure was predicted to be monomeric. Because of similarity in structural
features, the general observations will be discussed.
- 54 -
Compounds are 1, 3, 4 and 5 are mononuclear with octahedral geometry surrounding the
metal center. While compounds 1, 3 and 5 crystallize in the monoclinic space group
P2(1/n), the compound 4 crystallizes in orthorhombic, P2(1)2(1)2(1) space group and
form a distorted six coordinated environment at the titanium center (Fig. 2.3). The two ß-
ketoester ligands as well as the two alkoxy ligands are arranged cis to each other, with
both ester moieties (amide moiety in case of ß-ketoamides) arranged trans to the alkoxy
ligands. In all complexes it was observed that one of the oxygen to metal bond is shorter
by about 0.2 Å. This is due to trans effect induced by alkoxy ligands on these bonds. So
the bond angles O-Ti-O always deviate from the ideal octahedral angles of 90°. The
deviation varies between 80° to 100° in different complexes. In all cases, it was observed
that ß-ketoesters/ß-ketoamides tend a small bite angle (~ 82°) and these result in
expansion of external angle O-Ti-O subtended by oxygens of alkoxy groups (~100°).
Fig. 2.7: Distorted octahedral geometry surrounding metal center. Bite angles
subtended by ß-ketoesters are smaller than 90°; in typical case (~83°) and
one of the metal-oxygen bonds from ß-ketoesters are smaller by 0.2 Å.
The cis arrangement of ligands around metal center was observed with
alkyl groups of side chains pointing away from each other.
- 55 -
As a typical example, octahedral geometry surrounding the titanium center is depicted in
Fig. 2.7.
In the solid state, compound 6 (Fig. 2.5) exists as a dimer and crystallizes in space group
P2(1)/c. Ti is six coordinated in which each titanium atom is surrounded by two oxygen
atoms of chelating ß-ketoester group in an ?2 fashion, oxygen of two terminal
isopropoxide groups and by two µ-oxo atoms of bridging ethoxide groups. The chelating
ligands on each Ti atom are trans to each other. The geometry around each titanium
center is distorted octahedral (Fig. 2.8). A distance of 3.2113(16) Å exists between two
metal centers which are non bonding.
Bridging position is occupied by ethoxy moieties for the simple reason that they are
sterically least demanding. One more reason is that the bridging position, aside from
preferring the less-hindered ligand, also demands the more Lewis basic oxygen. In this
regard, oxygen of ethoxy moiety is readily available for saturation and thus preferred to
oxygen from malonates. Selected bond lengths and angles are reported in the figure
captions.
Fig.2.8: The distorted octahedral geometry around titanium centers. Centro
symmetric structure has malonate groups pointed in opposite directions.
- 56 -
Molecular packing in the unit cell is an important factor which influences the thermal
property. Minimum molecular interactions in the unit cell are contributive to the volatility
of the complex. The ß-ketoesterate complexes reported above show similar packing
arrangements. As a representatitive example crystal packing for complex 3 is shown in
Fig. 2.9. It shows the projections of the three-dimensional packing arrangement of 3
along the b axis. It appears that the bending of the basal skeleton is related to the bulky
tert. Butyl group attached to the ß-keto functional group. And the bulky tert. Butyl groups
are found to point in the same direction and are away from the basal plane and always
away from each other. As a result, each individual molecule is packed in an alternated
layer structure, composed of a parallel arrangement of individual units. Moreover, the
observed curvature is a natural consequence of the face-to-back stacking of the
molecules, wherein, tert. Butyl groups point into sterically unencumbered regions.
Fig. 2.9: Crystal packing for 3, four units are packed in a cell. Crystals are arranged
with face to back type stacking.
- 57 -
Table 2.1: Crystal data and refinement details of compounds 1 and 3.
Compound 1 3
Empirical formula C16 H28 O8 Ti C22 H40 O8 Ti
Formula weight 396.28 480.44
Crystal size [mm] 0.28 x 0.15 x 0.10 0.30 x 0.18 x 0.15
T,K 213(2) 213(2)
Crystal system Monoclinic Monoclinic
Space group P2(1)/n P2(1)/n
a, Å 7.753(5) 10.646(6)
b, Å 19.451(13) 16.019(6)
c, Å 13.751(6) 15.992(6)
alpha 90 90
beta 93.13(5) 90.44
gamma 90 90
V, Å3 2071(2) 2727(2)
Z 4 4
Calc. Density [g/cm3] 1.271 1.170
m (Mo-Ka) mm-1 0.448 0.352
F (100) 840 1032
Theta range [°] 2.83 to 25.19 1.80 to 25.04
Index range -9<=h<=9
-22<=k<=23
-11<=l<=16
-7<=h<=12
-18<=k<=19
-19<=l<=18
No. of reflections 11608 12176
Unique reflections 3675 4783
Observed reflections
[I>2 sigma (I)]
1763 2168
No. of parameters 239 292
Largest diff. peak/hole
[e Å-3 ]
0.411 and -0.448 0.286 and -0.329
R1, wR2 [I >2 (I)] 0.0708 / 0.1634 0.0632 / 0.1076
Goodness of fit 0.965 0.865
- 58 -
Table 2.2: Crystal data and refinement details of compounds 4, 5 and 6.
Compound 4 5 6
Empirical formula C20 H36 O8 Ti C22 H42 N2 O6 Ti C34 H68 O14 Ti2
Formula weight 452.39 478.47 796.68
Crystal size [mm] 0.25 x 0.15 x 0.10 0.30 x 0.25 x 0.22 0.40 x 0.34 x 0.09
T,K 203(2) 100(2) 208(2)
Crystal system Orthorhombic Monoclinic Monoclinic
Space group P2(1)2(1)2(1) P2(1)/n P2(1)/c
a, Å 8.095(5) 16.7363(16) 11.971(4)
b, Å 15.964(7) 17.6227(13) 16.674(4)
c, Å 19.022(10) 18.3725(15) 11.029(5)
alpha 90 90 90
beta 90 102.085(8) 103.50(3)
gamma 90 90 90
V, Å3 2458(2) 5298.6(8) 2140.5(14)
Z 4 8 2
Calc. Density [g/cm3] 1.222 1.200 1.236
m (Mo-Ka) mm-1 0.386 0.358 0.431
F (100) 968 2064 856
Theta range [°] 2.73 to 25.07 2.75 to 25.00 2.13 to 27.50
Index range -9<=h<=9
-12<=k<=18
-13<=l<=22
-19<=h<=19
-20<=k<=20
-21<=l<=21
-15<=h<=15
-21<=k<=19
-13<=l<=14
No. of reflections 6112 77867 5662
Unique reflections 4285 9310 4607
Observed reflections
[I>2 sigma (I)]
2387 7652 2489
No. of parameters 283 579 248
Largest diff. peak/hole
[e Å-3 ]
0.321 and -0.307 1.565 and -0.441 0.295 and -0.363
R1, wR2 [I >2 (I)] 0.0653 / 0.1204 0.0578 / 0.1546 0.0749 / 0.1795
Goodness of fit 0.963 1.102 1.028
- 59 -
2.4 Summary
The rational design, synthesis and characterization of several MOCVD precursors for
TiO2 have been carried out. Keeping in view of increasing the sublimation rates and
lowering the decomposition temperature, several ligands were used for synthesizing the
titanium complexes. Variations in the ligand sphere on the side chain of the well known
ß-keto systems have been carried out. ß-ketoesters, ß-ketoamides, malonates and
aminoalkoxides have been used as ligands for synthesizing the mixed alkoxide
complexes of titanium.
Titanium bis(isopropoxide) bis(methylacetoacetate) (1), titanium bis(ethoxide)
bis(methylacetoacetate) (2), titanium bis(isopropoxide) bis(tert-Butylacetoacetate) (3),
titanium bis(ethoxide) bis(tert-Butylacetoacetate) (4) and titanium bis(isopropoxide)
bis(N, N-diethylacetoacetamide) (5) and bis-[(di-ethylmalonato) tetra(isopropoxy)-µ-
ethoxy-titanium(IV)] (6) Titanium bis(isopropoxide) bis(diethylaminoethoxide) (7),
titanium bis(ethoxide) bis(diethylaminoethoxide) (8) were synthesized and characterized
by elemental analysis , NMR and mass spectrometry.
The use of ß-ketoesters, and ß-ketoamide ligands led to six coordinated complexes,
namely titanium bis(isopropoxide) bis(methylacetoacetate) (1), titanium
bis(isopropoxide) bis(tert-Butylacetoacetate) (3), titanium bis(ethoxide) bis(tert-
Butylacetoacetate) (4) and titanium bis(isopropoxide) bis(N, N-diethylacetoacetamide)
(5), as determined by single crystal X-ray diffraction. The six coordinated complexes
showed distorted octahedral geometry around titanium center. In such an arrangement,
the alkoxy groups positioned cis to each other and so are the ß-keto ester/ß-ketoamide
groups.
With the use of malonate ligands, the trans-esterification reaction was observed. This led
to the exchange of alkoxy groups from the malonate ligand periphery to that of parent
alkoxide. The resulting complex bis-[(di-ethylmalonato) tetra(isopropoxy)-µ-ethoxy-
titanium(IV)] (6) showed a dynamic behavior in NMR studies and could not be
understood even by variable temperature NMR studies. The single crystal X-ray
diffraction studies showed the formation of dimeric centrosymmetric complex. The
bridging positions were occupied by ethoxy moieties which are sterically least
- 60 -
demanding. The complexes 1-6 were all solids and showed high solubility (~ 0.8 g/ml) in
common organic solvents. These complexes could be sublimed at moderate temperatures
and the melting points were below 90 °C.
Donor functionalized ligands of the type aminoalcohols, where amino group acts as a
donor, resulted in complexes titanium bis(isopropoxide) bis(diethylaminoethoxide) (7),
titanium bis(ethoxide) bis(diethylaminoethoxide) (8). The synthesis of these class of
compounds was simple and was reproducible in high yields. These complexes were
viscous liquids and attempts to measure X-ray diffraction at low temperatures (-77 °C)
resulted in glassy solids. It is demonstrated that by introducing small variations in the
ligand sphere, it is possible to change the physical and chemical properties of the
resulting complexes.
- 61 -
2.5 References
[1] G. Hass, Vacuum 1952, 2, 331.
[2] R. N. Ghoshtagore, A. J. Norieka, J. Electrochem. Soc. 1970, 117, 1310.
[3] R. N. Ghoshtagore, J. Electrochem. Soc. 1970, 117, 529.
[4] D. C. Bradley, R. C. Mehrotra, D. P. Gaur, Metal Alkoxides, Academic Press,
New York, 1978.
[5] D. C. Bradley, Chem. Rev. 1989, 89, 1317.
[6] M. Balog, M. Schieber, J. Cryst. Growth 1972, 17, 298.
[7] H. Yamazaki, T. Tsuyama, I. Kobayashi, Y. Sugimori, Jpn. J. Appl. Phys. 1992,
31, 2995.
[8] P. Comba, H. Jakob, B. Nuber, B. K. Keppler, Inorg. Chem. 1994, 33, 3396.
[9] Y.-S. Min, Y.-J. Cho, D. Kim, J.-H. Lee, B.-M. Kim, S.-K. Lim, I.-M. Lee, W.-I.
Lee, Chem. Vap. Deposit. 2001, 7, 146.
[10] R. C. Smith, T. Ma, N. Hoilien, L. Y. Tsung, M. J. Bevan, L. Colombo, J.
Roberts, S. A. Campbell, W. L. Gladfelter, Adv. Mater. Opt. Electron. 2000, 10,
105.
[11] D. G. Colombo, D. C. Gilmer, V. G. J. Young, S. A. Campbell, W. L. Gladfelter,
Chem. Vap. Deposit. 1998, 4, 220.
[12] W. A. Herrmann, N. W. Huber, O. Runte, Angew. Chem. Int. Ed. Engl. 1995, 34,
2187.
[13] I. D. Varma, R. C. Mehrotra, J. Chem. Soc. (London) 1960, 2966.
[14] S. M. Damo, K. -C. Lam, A. Rheingold, M. A. Walters, Inorg. Chem. 2000, 39,
1635.
[15] D. C. Bradley, R. C. Mehrotra, W. Wardlaw, J. Chem. Soc. 1952, 5020.
[16] W. R. Russo, W. H. Nelson, J. Am. Chem. Soc. 1970, 92, 1521.
[17] R. L. Toung, C. Wentrup, Tetrahedron 1992, 48, 7641.
[18] R. C. Mehrotra, R. Bohra, D. P. Gaur, Metal ß-diketonates and allied derivatives,
Academic publishers New York, 1978.
[19] K. C. Joshi, V. N. Pathak, Coord. Chem. Rev. 1977, 22, 37.
- 62 -
[20] D. P. Graddon, Coord. Chem. Rev. 1969, 81, 79.
[21] B. Bock, K. Flatau, H. Junge, M. Kuhr, H. Musso, Angw. Chem.Int. Ed. Engl.
1971, 10, 225.
[22] F. Schmidt, Angew. Chem. 1952, 64, 536.
[23] R. E. Reeves, L. W. Mazzeno, Jr., J. Am. Chem. Soc. 1954, 76, 2533.
[24] A. Yamamoto, S. Kambara, J. Am. Chem. Soc. 1957, 79, 4344.
[25] D. C. Bradley, C. E. Holloway, J. Chem. Soc., Chem. Comm. 1965, 13, 284.
[26] J. W. Bunting, J. P. Kanter, J. Am. Chem. Soc. 1993, 115, 11705.
[27] A. C. Jones, T. J. Leedham, P. J. Wright, M. J. Crosbie, K. A. Fleeting, D. J.
Otway, P. O´Brien, M. E. Pemble, J. Mater. Chem. 1998, 8, 1773.
[28] H. Parala, A. Devi, R. Bhakta, . J.Mater. Chem., 2002, 12, 1625.
[29] G. M. Sheldrick,, Program for crystal structure analysis ed., University of
Göttingen, Germany, 1997.
[30] G. M. Sheldrick, SADABS, Program for area detector adsorption correction,
Institute for Inorganic Chemistry, University of Göttingen, Germany, 1997.
- 63 -
Chapter 3
Chemical vapor deposition of TiO2 an SrTiO3 thin films using
rationally developed titanium precursor
Abstract
The MOCVD of TiO2 thin films using newly developed precursors namely, Titanium
bis(isopropoxide) bis(tert-Butylacetoacetate) [Ti(OPri)2(tbaoac)2] (A), titanium
bis(ethoxide) bis(tert-Butylacetoacetate) [Ti(OEt)2(tbaoac)2] (B), and Titanium
bis(isopropoxide) bis(methylacetoacetate) [Ti(OPri)2(meaoac)2] (C), is discussed in
detail. Initial screening of the precursors was carried out using a homebuilt horizontal
cold wall CVD reactor. The films were analyzed by XRD, SEM, XPS and RBS. The
growth rates were determined based on weight gain method considering the bulk density
of anatase phase of TiO2. Comparison of growth rates at different deposition
temperatures were done under similar deposition conditions. One of the precursors,
[Ti(OPri)2(tbaoac)2] (A), which showed high solubility in common organic solvents was
tested in a liquid injection industrial MOCVD tool. The film depositions were compared
to the films obtained using a standard commercially available Ti precursor,
[Ti(OPri)2(thd)2]. Films were analyzed for crystallinity, surface roughness, surface
morphology, and electrical characteristics. It was found that new precursor efficiently
incorporates Ti into growing films at lower temperatures compared to standard precursor.
The surface roughness was lower in comparison with standard precursor. Equivalent
oxide thickness (EOT) and dielectric constant of the films were measured and it was
found that EOT of ~ 2 nm and dielectric constant of ~35 were obtained. After successful
testing for TiO2 depositions, the precursor [Ti(OPri)2(tbaoac)2] A, was tested for complex
oxide SrTiO3 depositions using standard [Sr(thd)2] precursor. Polycrystalline, off
stoichiometric films were obtained at a deposition temperature of 500 °C. Low deposition
temperatures (<600 °C) resulted in Sr rich films. At deposition temperatures above 600
°C stoichiometric Sr:Ti ratio could be observed resulting in crystalline films with (200)
and (110) orientation, exhibiting required electrical properties.
- 64 -
3.1 Introduction
In an effort to explore new and improved titanium precursors by molecular engineering, a
systematic approach by varying the ligands by introducing small changes was pursued in
our laboratory. Use of ß-ketoester instead of ß-diketonate ligands in combination with
alkoxides of titanium resulted in mononuclear complexes (refer chapter 2). Titanium
bis(isopropoxide) bis(tert-Butylacetoacetate) [Ti(OPri)2(tbaoac)2] (A), titanium
bis(ethoxide) bis(tert-Butylacetoacetate) [Ti(OEt)2(tbaoac)2] (B), Titanium
bis(isopropoxide) bis(methylacetoacetate) [Ti(OPri)2(meaoac)2] (C), are three of the
newly developed precursors. The thermal properties of these compounds were promising
enough for MOCVD applications. All the three are low melting compounds; possess a
sufficient temperature window between volatility and decomposition. The onsets of
volatilization and decomposition temperatures were lower when compared to the ß-
diketonate derivatives. The compounds can be sublimed at low temperatures (< 100 °C)
under mild conditions (3.2 x 10-2 torr) which shows promise for MOCVD applications.
These compounds showed extremely high solubility in common organic solvents making
them suitable for liquid injection as CVD as well.
In the case of MOCVD processes, optimization of deposition conditions can be difficult
and time consuming given the large number of process variables involved.[1-4] In such a
situation, it is worthwhile to explore the possibility of depositions using lab scale reactors
and when the results are promising enough, the precursor can be scaled up and tested in
industrial scale reactors. Following this line the above mentioned precursors were tested
for TiO2 depositions using the homebuilt CVD reactor and results are reported in
following sections. Among the newly developed precursor complexes, titanium
bis(isopropoxide) bis(tert-Butylacetoacetate), [Ti(OPri)2(tbaoac)2] (A), showed the most
promising deposition behavior using the home built CVD reactor. Therefore the precursor
was scaled up to large batches (~ 25 g.) and was tested for TiO2 thin film deposition
using an industrial tool reactor. Complex oxides of titanium are important in many
ternary perovskites such as BaTiO3, SrTiO3 and the quaternary materials [Pb(Zr,Ti)O3]
(PZT),[(Pb,La)(Zr,Ti)O3] (PLZT) which are predicted to be useful in next generation
- 65 -
non-volatile computer memories.[5-9] So after successful TiO2 depositions, the precursor
was tested for SrTiO3 depositions using standard Sr precursor [Sr(thd)2]. In an effort to
evaluate the performance of the new precursor A, the depositions were compared with
deposition results obtained using a commercially available bench mark titanium
precursor, namely titanium bis(isopropoxide) bis(tetramethylheptanedionate)
[Ti(OPri)2(thd)2]. The thin films were tested for their morphology and electrical
properties, keeping in view, their use for device applications.[10-17]
3.2 Experimental Section
3.2.1 Experiments using a home built CVD reactor
Thin films of TiO2 were generally grown on Si(100) substrates (1 cm x 1 cm). Prior to
film deposition the substrates were degreased in trichloroethylene and rinsed with
deionized water followed by rinsing in hot acetone and iso-propanol. They were dried
using pressurized air. The substrates were weighed before and after the depositions in
order to determine the weight gained due to film deposition and subsequently to calculate
the thickness of the film. A home built horizontal, cold wall, low pressure reactor was
used to deposit titanium dioxide (TiO2) thin films. A schematic diagram of the reactor is
shown in Fig. 3.1. The CVD reactor consisted of a quartz tube, at the center of which the
substrates are placed on a SiC coated graphite susceptor. Substrate heating was
accomplished by an inductive heating arrangement attached to a radiation pyrometer. The
quartz tube was attached to a glass vaporizer (bubbler) by means of O-ring joints, and the
vaporizer was placed in an air bath that can be heated up to 150 °C. The precursor was
filled into the vaporizer in a glove box and generally about 200 mg. of freshly filled
precursor was used for each deposition. The path between the precursor evaporator and
deposition chamber is maintained as short as possible to avoid any decomposition in the
reactor lines or on the walls. This path is maintained at precursor evaporator temperature
by using an air bath. The reactor chamber is surrounded by water jacket operating
approximately at 90 °C to avoid the condensation of the precursor. Thin film depositions
were generally carried out at reduced pressure, and therefore a turbo molecular pump was
used for this purpose. The reactor pressure was controlled using a motor driven throttle
- 66 -
valve and the deposition pressure was varied between 0.1 -100 mbar. Typically, a base
pressure of 2.8 x 10-5 mbar could be achieved. The gas flow in the reactor was monitored
and controlled by mass flow controllers (MFC). Prior to film deposition, the substrates
were cleaned by heating the substrate to 800 °C for about 30 minutes under N2 using a
bypass line. Then temperature was lowered to the required value. The onset of deposition
was marked by opening the vaporizer valves and bubbling nitrogen as the carrier gas. A
typical deposition lasted for 90 minutes. At the end of 90 minutes, the valves (inlet and
outlet) were closed and substrates were slowly cooled to room temperature under flowing
nitrogen (using bypass line).
The substrates were removed carefully and stored for analyses. The substrates were
weighed again to estimate the weight again due to film deposition. A microbalance
delivering up to five decimal values of a gram was used for weight measurements. Bulk
density ? = 3.94 g/cm-3 was assumed to determine average thickness of the TiO2 thin
films using the equation
AWt ρ/∆= (equation 3.1)
MFC MFC
Turbomolecular pump
Cooling trap
Valve for pressure control
Air bath
SiC coated graphite susceptor
Inductive heating
Substrate
Water cooling
Reactive gas Carrier gas
Capacitance manometer
Pressure gauge
Precursorreservoir
MFC MFC
Turbomolecular pump
Cooling trap
Valve for pressure control
Air bath
SiC coated graphite susceptor
Inductive heating
Substrate
Water cooling
Reactive gas Carrier gas
Capacitance manometer
Pressure gauge
Precursorreservoir
Fig. 3.1: Schematic diagram of the homebuilt CVD reactor used for TiO2 thin film
deposition
- 67 -
where ? W is the weight of the deposited film in grams and A is the surface area of the
substrate.
The reactor was cleaned with acetone and iso-propanol and loaded with a new set of
substrates and the vaporizer was charged with fresh precursor and the reactor was
evacuated.
Table 3.1: Deposition parameters for precursors A, B and C using home built CVD
reactor.
X-ray diffraction analyses were carried out on all of the deposited films, employing a D8-
Advance Bruker axs diffractometer. CuKa radiation (? = 1.5418 Å) with Nickel filter was
used as the source. High angle XRD measurements were carried out with ?-2? geometry
in the range 20-60° using a position sensitive detector. The film composition was
analysed by energy dispersive analysis of X-rays (EDAX) and Rutherford back-scattering
(RBS) and X-ray photoelectron spectroscopy (XPS). The RBS spectra were measured
with the 2 MeV single charged He-beam of the 4MV Dynamitron-Tandem laboratory in
Bochum. A beam intensity of about 20 nA incident to the sample perpendicular to the
surface was used. The back-scattered particles were measured at an angle of 170° by a Si-
detector with a resolution of 16 keV. The film thickness was also determined using the
RBS data. The surface morphology and composition of the films was studied using a
scanning electron microscope attached with EDX system. This facility was provided by
faculty of Geology, Ruhr University, Bochum, employing LEO Gemini SEM 1530. X-
ray photoelectron spectroscopy analyses were carried out with a modified Fisons X-ray
Precursor
Vaporizer
temperature
(°C)
Deposition
temperature
(°C)
Deposition
time
(min)
Reactor
pressure
(mbar)
A [Ti(OPri)2(tbaoac)2] 85-90 350-800 90 10 & 50
B [Ti(OEt)2(tbaoac)2] 85-90 350-800 90 10 & 50
C [Ti(OPri)2(meaoac)2] 85-90 350-800 90 10 & 50
Carrier gas N2 = 100 sccm, substrate = Si(100)
- 68 -
photoelectron spectrometer equipped with an Al Ka X-ray source and a CLAM3 electron
energy analyzer. Substrates were loaded into the chamber and evacuated overnight for 12
hours at 10-9 mbar. Survey X-ray photoelectron spectra and high resolution XPS spectra
were collected for desired elements.
3.2.2 Experiments using an industrial tool CVD reactor
System description
Most of the experimental reactors currently used for the development of mass production
tools use the conventional single wafer showerhead designs.[18-19] In contrast to the
current trend towards single substrate reactors, which can be integrated with cluster tools
along with other manufacturing process steps, present AIXTRON systems use the
Planetary Reactor®. These planetary systems are capable of offering extremely high
throughput due to possible batch mode processing resulting in low cost of ownership.
This reactor type was already well established, because it offers good homogeneity, high
efficiency of the precursors. For the deposition of complex-oxide films like SrTiO3, the
reactor is combined with a liquid delivery system from ATMI (LDS-300B).
The vaporizer is placed in the immediate vicinity of the reactor. The line to the reactor
was kept as short as possible and held at the evaporation temperature in order to avoid
condensation of the precursors which results into particle generation and subsequent yield
loss. As shown in Fig. 1.5 the wafers are placed on a coated graphite susceptor that
rotates typically at 8 rpm and carries five smaller plates (satellites) which rotate at 40 rpm
by gas foil rotation. The gas inlet is placed central above the reactor providing a pure
horizontal gas flow direction which makes this reactor a radial flow system. The oxidizer
gases enter the reactor just below the nozzle separate from the precursors in order to
avoid premature reactions. The homogenous heating of the graphite susceptor is achieved
by infrared lamps positioned below the rotating disk.
The desorbed products as well as the non reacted molecules are transferred through a ring
line, into a cold trap. The reactor operates under low pressure at ~2 mbar in order to
increase the gas diffusivity and prevent pre-reactions. The pumping system consists of a
- 69 -
A, [Ti(OPri)2 (tbaoac)2] 0.05 mol in butyl acetate
Precursor solutions
[Sr(thd)2] 0.05 mol in butyl acetate
Injection pulse length and period
0.8 ms and 0.32 s
Argon carrier flow 1000 sccm
Vaporization temperature 170-240°C
Process pressure 1-1.5 mbar
Susceptor temperature 350-800°C
O2 flow 200 sccm
root or booster pump and a rotary pump for pumping larger gas loads like the carrier gas.
The length of the injection pulses was kept at 0.8 ms, which corresponds to 5 µL/ pulse
and the delay between pulses was 160 ms. The deposition characteristics of the new
precursor were compared to those of a commercial [Ti(OPri)2(thd)2] precursor.
[Ti(OPri)2(tbaoac)2] was dissolved in n-butyl acetate (0.05 molar solution) and oxygen
(200 sccm) was used for the deposition of TiO2 films. Deposition pressure was in the
range 1 to 1.5 mbar. Films were deposited on Pt/ZrOx/SiOx/Si and on SiOx/p-Si
substrates. The process conditions are summarized in table 3.2.
Table 3.2: Standard MOCVD deposition conditions used for deposition of TiO2 and
SrTiO3 thin film depositions.
X-ray fluorescence (XRF, RIGAKU ZSX-100e) was used for the determination of molar
amount of the individual element in the deposited films. The Ti incorporation in the films
was determined by X-ray fluorescence (XRF) and film thickness was deduced from the
measured areal mass density of Ti atoms by assuming the density of TiO2 anatase phase,
3.8 g/cm3. Films were deposited on Pt/ZrOx/SiOx/Si substrates. Crystal structure of the
films was studied with the aid of X-ray diffraction (Philips Analytical) employing grazing
angle and Bragg-Brentano geometry with Cu-Kα radiation. Surface morphology of the
- 70 -
films was studied with AFM (SIS pico station). Electrical properties of the metal-
insulator-semiconductor (MIS) structures, Capacitance-Voltage (C-V) characteristics
were obtained using HP4284 LCR meter by sweeping the voltage from inversion to
accumulation and back.
3.3 Results and Discussion
3.3.1 Deposition of TiO2 thin films using homebuilt CVD reactor
The rationally developed precursors were screened for MOCVD applications using a
home built cold wall CVD reactor. Inclusion of ester moiety in the side chain of the ß-
keto structure is expected to act as a cleavage point and facilitate lower deposition
temperatures (refer chapter 2). During the course of this work, it was found that the
complexes with small variations in the ligand sphere showed significant differences in the
hydrolytic stability and thermal behavior. These observations drove us to test a series of
precursors with small variations for CVD of titanium dioxide thin films using the home
built reactor.
Fig. 3.2 shows the TG curves for three different precursors A, B and C used in this study
and is compared with standard precursors. It can be seen from the TG curves, that the
onset of volatilization for the rationally developed precursors (A,B,C) occur at lower
temperatures compared to the bench mark precursor [Ti(OPri)2(thd)2]. The vaporization
temperature is lowered by about 100 °C and a monotonic weight loss is observed. The
residue left behind in the case of precursor A is negligible (<2%) indicating a clear
volatilization of the precursor. (More details on the thermal properties of the rationally
developed precursors are given in chapter 5).
- 71 -
100 200 300 400
0
20
40
60
80
100
CBA
Wei
ght [
%]
Temperature [°C]
[Ti(OPri)2(tbaoac)
2]
[Ti(OEt)2(tbaoac)
2]
[Ti(OPri)2(meaoac)
2]
[Ti(OPri)2(thd)
2]
[Ti(OPri)4]
Fig. 3.2: Thermogravimetric analysis of three different precursors A, [Ti(OPri)2(tbaoac)2],
B, [Ti(OEt)2(tbaoac)2], and C, [Ti(OPri)2(meaoac)2] compared with parent
alkoxide [Ti(OPri)4] and bench mark titanium precursor [Ti(OPri)2(thd)2].
3.3.2 Deposition parameters
Thin films of TiO2 were deposited in the temperature range 350-800 °C and at two
different reactor pressures (10 and 50 mbar). The deposition conditions employed for
TiO2 film deposition are listed in table 3.1. No additional reactive gas (oxygen) or water
vapor used during film deposition. The vaporizing temperatures for all the three
precursors were maintained at similar temperature range, so we expect the uniformity in
the transport rate to the reactor zone. The films grown at 350 °C were dark in color and
dull in appearance whereas films grown at temperatures above 400 °C had a shiny
appearance with a bluish green color. The highest growth rate was observed in case of
precursor A [Ti(OPri)2(tbaoac)2], at 500 °C where the growth rate was of the order of 0.6
µm/hr.
- 72 -
3.3.3 Crystal structure of the films
The crystalline properties of the film were examined by X-ray diffraction (XRD). The
XRD analyses of TiO2 films grown at different temperatures are shown in Figs. 3.3 to
3.5. Films grown at 350 °C did not show any reflexes indicating that the films were
amorphous in nature. The temperature onset for crystallization was 400 °C as seen the
appearance of (111) reflection corresponding to the anatase phase of TiO2. With increase
in substrate temperatures, the films were more crystalline and the anatase phase was
predominant till 600 °C. The appearance of rutile phase in addition to anatase phase was
evident at temperatures above 700 °C. It should be noted that oriented and textured films
of TiO2 with the rutile phase (110) were observed at 800 °C. A complete phase transition
Fig. 3.3: X-ray diffraction patterns of TiO2 films with [Ti(OPri)2(tbaoac)2] precursor on
Si (100) substrates deposited under various susceptor temperatures.
20 30 40 50 60
Inte
nsity
(ar
b. u
nits
)
2 theta [deg]
400 °C
rutil
e (2
11)
rutil
e (2
20)
anat
ase
(200
)
Si(2
11)
anat
ase
(200
)
rutil
e (1
10)
anat
ase
(101
)
Si(2
11)
800 °C
500 °C
700 °C
600 °C
- 73 -
to a more stable phase occurs at this temperature. The reflexes remain broad along the
series probably due to the small crystallite size.
Precursors B and C were also screened for the TiO2 film depositions. The XRD plots of
TiO2 films grown, using precursors B and C are shown in Figs. 3.4 and 3.5 respectively.
As in the case of precursor A, the TiO2 films were amorphous below 400 °C. Above 400
°C, the films were polycrystalline with anatase phase dominant till about 600 °C in case
of precursor B (Fig. 3.4). The rutile phase begins to appear and at 700 °C and 800 °C
rutile phase is predominant with some traces of anatase with lowered intensity.
However, in contrast to precursor A and B, the films grown using precursor C consisted
of brookite phase (201) and (111) at 400 °C. At higher temperature the reflexes of anatase
and rutile were observed consistent to those obtained from precursors A and B.
20 30 40 50 60
400 °C
500 °C
600 °C
Inte
nsity
(ar
b. U
nits
)
2 theta [deg]
rutil
e (2
11)
anat
ase
(101
)
rutil
e (1
01)
rutil
e (1
10)
anat
ase
(004
) rutil
e (1
11)
700 °C
800 °C
Fig. 3.4: X-ray diffraction patterns of TiO2 films with [Ti(OEt)2(tbaoac)2] precursor on Si
(100) substrates deposited under various susceptor temperatures.
- 74 -
3.3.4 Effect of substrate temperature on growth rate
Film growth by CVD is strongly dependent on substrate temperature, not only because
surface reactions are temperature-activated, but also because reaction pathways could be
altered by temperature. Therefore, the growth rate of TiO2 films studied as a function of
substrate temperatures for A [Ti(OPri)2(tbaoac)2], B [Ti(OEt)2(tbaoac)2] and C
[Ti(OPri)2(meaoac)2]. Fig. 3.6 shows the Arrhenius plot of growth rate with different
precursors.
Three precursors behave differently along the series. It was observed that precursor A
[Ti(OPri)2(tbaoac)2] has maximum growth rate at 500 °C (~ 0.6 µm/hour) and thereafter
the growth rate decreases to ~ 0.3 µm/hour at 800 °C. Precursors B [Ti(OEt)2(tbaoac)2]
and C [Ti(OPri)2(meaoac)2] have maximum growth rates at 600 °C, 0.45 µm/hour and
20 30 40 50 60
broo
kite
(201
)
anat
ase(
101)
rutil
e(22
0)
anat
ase(
112)
Si(1
11)
Si(2
11)
rutil
e(11
0)
anat
ase(
200)
broo
kite
(111
)800 °C
400 °C
600 °C
2 theta [deg]
Inte
nsity
[arb
. Uni
ts]
700 °C
500 °C
Fig. 3.5: X-ray diffraction patterns of TiO2 films with [Ti(OPri)2(meaoac)2] precursor on
Si (100) substrates deposited under various susceptor temperatures.
- 75 -
0.25 µm/hour respectively. While the growth rate of precursor B [Ti(OEt)2(tbaoac)2]
decreases rapidly to 0.11 µm/hour at 800 °C, that of precursor C [Ti(OPri)2(meaoac)2] is
not so rapid and has a growth rate of 0.25 µm/hour. It has to be noted that growth rates
were calculated based on density of anatase phase of the films and assuming that the
depositions were uniform over the surface of the substrate. Since the films contain rutile
phase and some carbon impurities, the growth rates determined by other methods may
deviate from these results.
0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
0.1
0.2
0.3
0.4
0.5
0.6
Gro
wth
rat
e [µ
m/h
our]
1000 /T [K-1]
[Ti(OPri)2(tbaoac)
2]
[Ti(Et)2(tbaoac)
2]
[Ti(OPri)2(meaoac)
2]
Fig. 3.6: Arrhenius plot of growth rates with precursors, A [Ti(OPri)2(tbaoac)2],
B [Ti(OEt)2(tbaoac)2] and C [Ti(OPri)2(meaoac)2].
- 76 -
1200 1000 800 600 400 200 00
10000
20000
30000
40000 Ti LMM
O KLL
O 2S
Ti 3STi 3P
C 1S
Ti 2P
O 1S
Ti 2S
Inte
nsity
[cou
nt/S
ec]
Binding Energy [eV]
XPS Survey spectrum
470 465 460 455 450
1500
2000
2500
3000
Ti 2p(3/2)
Ti 2p(1/2)
Inte
nsity
[Cou
nts/
Sec
]
Binding Energy [eV]
XPS Ti 2p
Fig. 3.7: A XPS survey and high resolution spectra of TiO2 thin film grown on Si(100) at
700 °C using precursor A [Ti(OPri)2(tbaoac)2] (b) Titanium Ti2p region (c) O1s
region
536 534 532 530 528 526 524
2000
2500
3000
3500
4000
Inte
nsity
[Cou
nts/
Sec
]
Binding Energy [eV]
XPS O 1S
(a)
(b) (c)
- 77 -
3.3.5 Film composition
Routine film composition analyses could not be done for all films and hence only results
from representative samples are reported here. Fig. 3.7 (a) shows a XPS survey spectrum
of a sputtered-cleaned TiO2 film grown using precursor A [Ti(OPri)2(tbaoac)2], at 700 °C
where the signals can be attributed to titanium, oxygen and some traces of carbon.
Detailed spectra for the titanium Ti 2p region and for the oxygen O1s region are shown in
Fig. 3.7 (b) and 3.7 (c) respectively. A gauss peak-fit gives binding energies of 530.4 eV
for O1s and 458.9 eV for Ti 2p, in agreement with the values reported for TiO2 in the
literature.[ 21]
0 200 400 600 800 1000-500
0
500
1000
1500
2000
2500
3000
3500
4000
O - edge at interface and Si - edge (substrate)below threshold
Ti - edge (surface)
O - edge
Ti - edge (interface)
Pulser
Experimental curve Simulated curve
Yie
ld
Channel Number
Fig. 3.8: RBS spectrum of TiO2 thin film grown using precursor A
[Ti(OPri)2(tbaoac)2], the red line represents the values simulated for
TiO2 and black line represents the experimentally obtained spectra.
- 78 -
In the RBS spectrum of the same film, the signals from Ti as well as O in the layer could
be clearly identified as depicted in Fig. 3.8. The comparison with the simulation gave no
hint to a sizeable additional contamination and supports the stoichiometric atomic ratio of
Ti to O as ½. The thickness of the film was found to be of the order 3.6 µm.
In addition to XPS and RBS, EDX analyses were carried out on selected films. Only
peaks belonging to titanium and oxygen could be detected in high intensity. No sizeable
carbon contamination could be detected. Fig. 3.9 shows the EDX spectrum obtained for
the TiO2 film deposited using the precursor A [Ti(OPri)2(tbaoac)2].
3.3.6 Microstructure of the films
The film morphology is determined by the relative rates of precursor vapor transport,
decomposition reaction, surface diffusion, and lattice incorporation.[22] The growth of a
thin film is a non equilibrium phenomenon where kinetics and thermodynamics play
essential role in determining the microstructure of the film. The morphology of the films
was analyzed by scanning electron microscopy, SEM. As a typical case the films grown
between temperatures 400 and 800 °C using precursor A [Ti(OPri)2(tbaoac)2] at a reactor
Fig. 3.9: The EDX spectrum of the TiO2 films deposited using the precursor A [Ti(OPri)2(tbaoac)2]
- 79 -
pressure of 10 mbar is discussed here. At low temperatures, the microstructure is nearly
equiaxed and the texture shows very little preferred orientation. However, as the
temperature increases, the energy of the surface diffusion increases, so are the growth
rate, resulting in more rapid lattice incorporation on low energy crystal planes. Anatase
films were observed in this temperature regime (XRD) and granular structure was
observed with dense packing. At temperatures above 700 °C high degree of orientation
was observed with rutile crystalline forms. The film morphology shows densely packed
bigger crystallites grown at the cost of smaller crystallites as depicted in Fig. 3.10.
Fig. 3.10: SEM micrographas of films obtained using precursor A
[Ti(OPri)2 (tbaoac)2], at different susceptor temperatures and at a reactor
pressure of 10 mbar.
These initial experiments with new set of precursors were performed in order to test the
ability of precursors to deposit TiO2 thin films. The preliminary experiments conducted
using the homebuilt reactor had varying degree of success and paved the way for their
detailed study using commercial scale reactor. The precursor A [Ti(OPri)2(tbaoac)2],
showed promise for titanium dioxide films over a wide range of temperatures. As shown
in TGA, (Fig. 3.2) this precursor sublimes cleanly with negligible residue under normal
500 °C 600 °C
700 °C 800 °C
500 °C
- 80 -
pressure and has high solubility (0.8 g/ml) in most of the common organic solvents hence
it was decided to scale up the synthesis of this precursor and use for liquid injection CVD
using commercial scale reactor.
3.4 Deposition of TiO2 thin films using a liquid injection industrial tool
3.4.1 Susceptor temperature dependent Growth rate, Surface roughness, Structural
and Electrical properties
The efficiency of a precursor is defined as the ratio of the amount (moles) of the
respective elements (titanium in present case) in the films deposited on all the five
wafers, (as determined by XRF analyses), to the amount (moles) of the element in the
consumed precursor. The XRF results indicate almost the same maximum efficiency in
the case of both precursors, but the temperature range for this was different in both cases
300 350 400 450 500 550 600 650 700 750 8000
10
20
30
40
50
2
4
6
8
10
12
Effi
cien
cy [%
]
Susceptor Temperature [oC]
[Ti(OPri)2(tbaoac)
2] on Si
[Ti(OPri)2(tbaoac)
2] on Pt
[Ti(OPri)2(thd)
2] on Si
[Ti(OPri)2(thd)
2] on Pt
Gro
wth
rat
e (n
m/m
in)
Fig. 3.11: Precursor efficiency of commercially available [Ti(OPri)2(thd)2]
precursor and newly developed [Ti(OPri)2(tbaoac)2] precursor on
Silicon and Pt/Zr/Si substrates under identical conditions. (as the
period is fixed the growth rate scales with the efficiency)
- 81 -
( as shown in Fig. 3.11). [Ti(OPri)2(tbaoac)2] showed maximum value between 450 °C to
550 °C, where as [Ti(OPri)2(thd)2] showed this behaviour above 600 °C. This lower
growth temperature of TiO2 films with [Ti(OPri)2(tbaoac)2] compared to [Ti(OPri)2(thd)2]
precursor, making it useful for the technologically important low temperature
depositions. The [Ti(OPri)2(tbaoac)2] precursor has a lower efficiency at high deposition
temperatures probably due to the decomposition at elevated temperatures (>700 °C).
Efficiency of the precursors on Si and Pt/Zr/Si substrates prepared under identical
conditions were at variance (Pt showed higher efficiency) at lower deposition
temperatures than at higher temperature. This may be due to the temperature difference
on the Pt and Si substrates. We expect Pt substrates are at a higher temperature than the
Si substrates, and hence in the kinetically controlled low temperature region (exponential
growth with temperature) higher deposition was expected on the Pt coated silicon
substrates. In other words, this difference was due to the variation of reactant
concentration on the substrate surface due to difference in temperature on the Si and
Pt/Zr/Si substrates. Convergence of the efficiencies on both substrates at high
temperatures supports this argument, as at high reactor temperatures, both substrates may
be at the same temperature.
3.4.2 Surface roughness
Surface roughness of the films deposited at various reactor temperatures were studied
with AFM for both type of precursors. The average roughness showed strong dependence
on the susceptor temperature and the substrate. Films deposited on Si substrates had
lower surface roughness compared to those deposited on platinized substrates.
In the case of films prepared with [Ti(OPri)2(thd)2] precursor, this dependency was more
drastic compared to the films deposited with [Ti(OPri)2(tbaoac)2] precursor. For films
from [Ti(OPri)2(thd)2] precursor, RMS roughness variation from 0.35 nm to 7 nm for Si,
and 1.14 nm to 8 nm for Pt/Zr/Si substrates were observed. Fig. 3.12 shows the AFM
micrographs of the films deposited using two precursors at 600 °C on Si substrates. In
this case, for films deposited at low temperature (<550 °C) and high temperature (700-
- 82 -
750 °C), the RMS roughness were found to be lower than those deposited in the medium
temperature range (550-650 °C).
Fig. 3.12: AFM micrographs of TiO2 films grown using [Ti(OPri)2(tbaoac)2]
and [Ti(OPri)2(thd)2] precursors on Si substrates deposited at 600 °C.
The probable reason for this behaviour may be attributed to the crystalline grain growth
as a function of temperature which can also be correlated with the XRD results as
crystalline phase starts at a temperature of 550 °C. As the susceptor temperature
increased grain growth increased and reached a maximum value in the medium range and
then again reduced due to the fine-grained structure at elevated temperatures. This
argument also supported with the XRD results. In the low temperature region lower
roughness is due to the amorphous nature of the film and high temperature this behaviour
may be due the effect of growth rate/efficiency on the grain growth. High growth rate
(10.22 nm/min on Pt at 600 °C) may result in the formation of localized clusters of grains
or the coalition of these clusters to form bigger grains in the case of medium
temperatures. We observed a slightly low growth rate (~ 9.67 nm/min on Pt at 750 °C)
for films at high temperature. This low growth rate along with the high mobility of
adatoms at high temperature might have influenced the fine-grained growth and hence the
lower roughness was observed in the crystalline films at high temperatures. On the other
hand, roughness of the films deposited with [Ti(OPri)2(tbaoac)2] precursor didn’t show
large dependence on the susceptor temperature. The roughness slightly increased with the
[Ti(OPri)2(tbaoac)2] Si- 600 °C
[Ti(OPri)2(thd)2] Si - 600 °C
- 83 -
temperature and which can be explained due to the growth of crystalline phase and the
subsequent grain growth. The difference in the surface morphology of the TiO2 films
with two different precursors may be due to the difference in the thermal characteristics
and reaction steps leading to the formation dense films on the substrate surface.
3.4.3 Crystal structure
X-ray diffraction studies were performed to understand the onset of crystallization
temperatures on different substrates and with both precursors and the results are depicted
in Fig. 3.13 (a) and 3.13 (b).The reflections observed at 2θ = 28.3° and 29.95° for films
on Pt/Zr/Si are due to the Zr adhesion layer (ZrOx) as it is present in the amorphous case
also and absent in the films on Si substrates. Films deposited at 450 and 500 °C were
amorphous for both type of substrates and crystallization in the tetragonal ‘anatase’ phase
starts at a deposition temperature of 550 °C. Another low temperature ‘metastable’
orthorhombic phase, brookite was not observed in the crystallinity evolution. So in the
thin film forms, amorphous and anatase phases were stabilized in the present deposition
process. As the deposition temperature increased intensity of the most prominent peak
(2θ = 25.28°) corresponding to the (101) phase increased and other reflection (200),
(105) and (211) were also started to appear at 2θ = 48.05°, 53.89° and 55.06°,
respectively; suggesting polycrystalline nature of the films. The films on Si showed an
additional peak around 37.87° corresponding to the anatase (004) reflection. In the case
platinized substrate a weak reflection corresponding to the rutile form of the TiO2 (110)
was observed at 2θ = 27.3° and this was not present for the films on Si substrates. In
both form of substrates intensity of reflections increased with susceptor temperature.
Similar XRD patterns of TiO2 films on Pt substrates can be seen for the films deposited
with the [Ti(OPri)2(tbaoac)2] precursor, but at high deposition temperatures, contrary to
the [Ti(OPri)2(thd)2] case, anatase (101) peak intensity reduced. This may be due to the
low efficiency of the [Ti(OPri)2(tbaoac)2] precursor at high deposition temperatures (refer
to Fig.3.11), which results in low thickness as the films were deposited under identical
condition with the same number of pulses.
- 84 -
Fig. 3.13: (a) X-ray diffraction patterns of TiO2 films with [Ti(OPri)2(thd)2] precursor on
Pt/Zr/Si and (b) with [Ti(OPri)2(tbaoac)2] precursor on Pt/Zr/Si deposited under various
susceptor temperatures.
20 30 40 50 60
(b)A
(211
)
A(2
00)R(1
10)
A(1
01)
Pt (
200)
Pt (111)ZrOx
350oC
400oC
450oC
500oC
550oC
650oC
750oC
TiO2/Pt/Zr/Si(100)
700oC
600oC
Inte
nsity
(ar
b.un
its)
2θ (Deg.)
20 30 40 50 60
0
50
100
150
200
250(a)
R(1
10)
A(2
11)
A(1
05)
A(2
00)
A(1
01)
Pt (200)Pt (111)ZrO
x 450oC
500oC
550oC
650oC
750oC
TiO2/Pt/Zr/Si(100)
700oC
600oC
Inte
nsity
(arb
.uni
ts)
2θ (Deg.)
- 85 -
Also, in the case of films with [Ti(OPri)2(tbaoac)2] precursor crystallization started in the
anatase phase at 500 °C, which is 50 °C lower than [Ti(OPri)2(thd)2] precursor. Here, in
the case of films on Si substrates at higher temperature (>700 °C) films contains rutile
phase also, as the small peak at 2θ = 27.4° is corresponds to the rutile (110) reflection
and hence the films have two phases. On Pt substrates this trend is weakly seen at 750 °C
depositions. Compared to the films on Si from [Ti(OPri)2(thd)2] precursor,
[Ti(OPri)2(tbaoac)2] showed a different pattern. First, the appearance of rutile phase at
and above 700 °C of deposition. Anatase (004) reflection was absent in the case of
[Ti(OPri)2(tbaoac)2] precursor. Also (105) & (211) reflections were also not so well
resolved as in the case of [Ti(OPri)2(thd)2]; suggesting a lower grain size in the case of
films with [Ti(OPri)2(tbaoac)2] precursor. This lower grain size may be the reason for the
smaller surface roughness in the case of films with [Ti(OPri)2(tbaoac)2] precursor as
compared [Ti(OPri)2(thd)2] precursor.
Fig. 3.14: X-ray diffraction patterns of TiO2 films with [Ti(OPri)2(tbaoac)2] precursor on
Pt/Zr/Si substrates deposited at 550°C and then annealed (room temperature
annealing for 20 min in O2) at various temperatures.
20 30 40 50 60
A(004) A(112)
R(2
20)R
(211
)
R(2
10)
R(1
11)
R(2
00)
Pt
r
r
n
r
n
R(1
01)
A(211)A(200)
R(1
10)
A(101)
ZrOx
as.depo.
800oC
550oC
650oC
750oC
TiO2/Pt/Zr/Si(100)
700oC
600oCInte
nsity
(arb
.uni
ts)
2θ (Deg.)
- 86 -
3.4.4 Effect of post deposition annealing on the structure and morphology
Rutile, technologically preferred form has higher refractive index and dielectric constant [33] than anatase could not be obtained in the single phase (in situ) due to the process
limitation of MOCVD system and low efficiency of the precursors at high temperatures.
This can be achieved by thermally induced transformation of anatase phase at
temperature higher than 800 °C as can be seen from the X-ray diffraction depicted in the
Fig. 3.14. X-ray diffraction shows as deposited film (550 °C) and the films annealed at
550 °C have anatase as crystalline phase. As the annealing temperature is increased to
600°C reflexes for rutile phase starts to grow at the cost of anatase phase. This trend was
observed till 750 °C and at 800 °C films were converted to rutile phase structure. Surface
morphology reveals grain growth with annealing temperature as expected due to the
growth of rutile phase. Larger grains were obtained for the films annealed at 800 °C
which corresponds to the tetragonal rutile single phase.
Fig. 3.15: SEM cross sectional view of the films grown at 550 °C and then annealed at
different temperatures.
SEM cross sectional view of the films annealed at different temperatures also showed
similar behaviour as can be seen from the images shown in Fig. 3.15. Closely packed
550 °C as deposited
annealed at 650°C annealed at 750°C
annealed at 550°C
- 87 -
grains with columnar growth were observed in the cross-sectional view of the SEM
micrographs. Furnace annealing even at 850 °C resulted in the mixed phase where as
room temperature annealing at 800 °C resulted in only rutile phase as observed in the
XRD analyses.
3.4.5 Electrical properties
Electrical measurements in terms of C-V and I-V were done on the films in the MIS
configuration. C-V characteristics of the TiO2 films deposited at various susceptor
temperatures with [Ti(OPri)2(tbaoac)2] precursors were done first to understand the effect
of EOT (equivalent oxide thickness)and effective dielectric constant with the deposition
temperature. Fig. 3.16 shows the high frequency C-V measurements performed on 49.1
nm2 circular capacitor patterns. The capacitance was measured at 100 kHz as a function
of gate voltage and capacitor was swept from inversion to accumulation and back to
inversion to check the amount of hysteresis.As is evident from Fig.3.16, there was no
significant hysteresis as the crystallization of initial amorphous TiO2 films took place.
This suggests that introduction of grain boundaries or crystal/amorphous boundaries do
not significantly increase the density of carrier trapping defects sites that contribute to C-
V hysteresis at these frequencies. The films deposited at 500 and 600 °C showed almost
same characteristics, except an increase in the accumulation capacitance. This is due to
increase in the crystallinity in the films with deposition temperature temperatures as
observed with the XRD. In contrast, the films deposited at 650 °C showed a reduction in
the accumulation capacitance preferably due to the amorphous interface oxide growth at
higher temperatures. Also the shape of this curve and the one deposited at 450 °C was
found to be different compared to others, stretched along the X-axis, suggesting a large
interface traps, in it.
- 88 -
-2 -1 0 1 2
0.00E+000
1.00E-010
2.00E-010
3.00E-010
4.00E-010
5.00E-010
6.00E-010
7.00E-010
Cap
acita
nce
(F)
Bias Voltage (V)
450oC 500oC 600oC 650oC
-2 -1 0 1 2
0.00E+000
2.50E-010
5.00E-010
Cap
acita
nce
[F]
Bias Voltage [V]
450oC 500oC 600oC 650oC
Fig.3.16: C-V characteristics of TiO2/ Si as a function of deposition temperatures (a) as-
deposited (b) post deposition top Pt annealing at 550 °C for 15 min, only for the
films which did not show the saturation.
(a)
(b)
- 89 -
450 500 550 600 6500
2
4
6
8
10
Target requirement
Oxide thickness tox
[nm]
24.7 27.6 18.7 21.7
EOT ε
r
Susceptor temperature [°C]
EO
T [n
m]
0
10
20
30
40
50
Die
lect
ric c
onst
ant [
ε r]
Fig.3.17: Equivalent oxide thickness (EOT) and dielectric constant of films deposited at
various temperatures. Dotted line represents the requirement for EOT.
For a gate dielectric of thickness Td and relative dielectric constant k, EOT (equivalent
oxide thickness) is defined by EOT = Td/k/3.9 where 3.9 is the relative dielectric constant
of thermal SiO2. The ideal gate capacitance per unit area of the gate dielectric of
thickness Td is the same as that of a gate dielectric made up of thermal SiO2 with a
thickness of EOT. The equivalent oxide thickness and dielectric constant of the TiO2 thin
films deposited at various temperatures were measured. Particularly, the electrical studies
of ultra-thin films on Si substrates showed promising properties; EOT of ~ 2 nm and
dielectric constant ~ 35 were obtained. The variation of EOT and dielectric constant as a
function of deposition temperature is depicted in the Fig. 3.17.
Room temperature leakage current behaviour was measured at various stages of micro
structural evolutions and the representative results are shown in Fig. 3.18 for TiO2 films
deposited onto Si(100) substrates at different susceptor temperatures.
- 90 -
-2 -1 0 1 2
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1Le
akag
e cu
rren
t (A
/cm
2 )
Applied Voltage (V)
Deposition temperatures 450oC 500oC 600oC 650oC
Fig.3.18: Leakage current characteristics of TiO2 deposited on Si structures
3.6 Deposition of SrTiO3 thin films
The compatibility of the Ti precursor with the standard thd-precursor for the group-II
metals was investigated by depositing SrTiO3 thin films. In addition, it was of special
interest to investigate whether the advantageous low temperature deposition behaviour of
the new Ti precursor could be preserved during SrTiO3 depositions. Deposition
conditions are listed in table 3.2.
Fig. 3.19 summarizes the efficiency of the metal incorporation, (the ratio of total amount
of Ti or Sr in the film to the amount of Ti or Sr in the injected precursor), into the films at
different temperatures. The precursors were injected alternatively with a period of 0.45 s
and an offset between Sr and Ti of 0.225 s in order to reduce gas phase reactions.
- 91 -
Fig. 3.19: Variation of efficiency of the precursor and stoichiometry of the film with
susceptor temperature on Pt substrates (Period = 0.45 s and delay between Sr
and Ti injections was 0.225 s)
It can be seen from the figure that efficiency of both precursors was improved as the
susceptor temperature increased and from the trend high temperature deposition yields
stoichiometric films of SrTiO3 with the same injection rate for Sr and Ti. But at low
temperature especially the efficiency of the Ti precursor was drastically decreased
compared to the deposition of pure TiO2.
The behavior of the Sr precursor in combination with [Ti(OPri)2(thd)2] is given as a
reference line and there is only a small decrease. Hence, the drastic reduction in the Ti
incorporation in the films resulted in Sr rich films. There seems to be some interaction
and some hindering of the Ti incorporation at low temperatures in the presence of the Sr
precursor. In this temperature region reactions occur near the substrate or on the substrate
450 500 550 600 650 700 7500
5
10
15
20
25
30
35
40
45
Ti-Eff Sr-Eff Sr-Eff with [Ti(OPri)
2(thd)
2]
Sr/Ti ratio
Susceptor Temperature (°C)
Eff
icie
ncy
(%)
1
3
5
7
9
11
13
15
SrTiO3/Pt/ZrO
x/SiO
x/Si
Sto
ichi
omet
ry (S
r/Ti
)
- 92 -
in such a way that kinetically controlled adsorption of the precursor molecule and the
subsequent desorption of the organic ligand.
0 2 4 6 8 10 120
10
20
30
40
50 Ti-Eff Sr-Eff Sr/Ti ratio
Delay between Sr and Ti pulses (s)
Eff
icie
ncy
(%)
1.0
3.0
5.0
7.0
9.0(a)
Sto
ichi
omet
ry (S
r/Ti
)
0 2 4 6 8 10 120
10
20
30
40
50
Ti-Eff Sr-Eff Sr/Ti ratio
Delay between Sr and Ti pulses (s)
Eff
icie
ncy
(%)
1
3
5
7
9
11
13
15(b)
Sto
ichi
omet
ry (S
r/Ti
)
Fig. 3.20: Efficiency and Sr/Ti ratio as function delay time for the films deposited at
(a) 450 °C, (b) 500 °C on Pt substrates. (Ti:Sr pulse ratio = 1:1).
- 93 -
0.0 0.2 0.4 0.6 0.8 1.00
5
10
15
20
25
30
35
40
45
50(a)
B C
Delay between Sr and Ti pulses (s)
Eff
icie
ncy
(%)
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
Sr/Ti
Sto
ichi
omet
ry (S
r/Ti
)
1.0 1.1 1.2 1.3 1.4 1.50.01
0.1
1
10
(b)
EA~ 0.9 eV
1/de
lay
time
(s-1)
1000/T(K-1)
Fig. 3.21: Efficiency and Sr/Ti ratio as function delay time for the films deposited at
(a) 650 °C on Pt substrates. (Ti:Sr pulse ratio = 1:1) and (b) Arrhenius plot
of desorption rate.
- 94 -
Hence the presence of each precursor affect the deposition of the other, with one more
sensitive compared to the other. Since the layer growth at 400 °C to 500 °C is of high
interest for technological applications like the manufacture of high-k embedded
capacitors and as it is shown possible from the efficiency of the individual precursors,
depositions were tried to obtain stoichiometric films at these temperatures. The delay
between the Sr and Ti injection were increased, while maintaining the same injection
rate, towards the direction of individual deposition.
The results for depositions at 450 °C, 500 °C and 650 °C are depicted in the Figs. 3.20
(a), 3.20 (b) and 3.21 (a). As can be seen from the figure, 650 °C deposition resulted in
stable efficiencies and stoichiometric STO deposition with a very small delay (0.2 s)
between the Sr and Ti injection into the vaporizer. At 500 °C a plateau in the Sr/Ti curve
was observed after approximately 5 s delay, however, an additional change of the 1:1
ratio of the Sr and Ti pulses would be necessary for obtaining exactly stoichiometric
films. At 450 °C this saturation could be observed only after a delay above 10 s. As gas
phase reactions can be excluded due to the pulse separation, this behavior suggests that
adsorption and desorption reactions on the substrate surface affect the incorporation of
the cations in the growing films. The corresponding cleaning rate of the surface, 1/delay
time, is plotted in Fig. 3.21 (b) in an Arrhenius diagram.
A straight line with activation energy of about 0.9 eV could be observed. This effective
activation energy is less compared with the activation energy for the TiO2 deposition of
~1eV discussed above, and therefore this desorption process becomes the rate limiting
reaction, resulting in increase in Sr content at low temperatures. Although there was some
success in the deposition of stoichiometric films by additional correction of the Sr/Ti
injection rates, the low temperature deposition (T < 450 °C) demands long deposition
times, which are undesirable for industrial throughput. But a more compatible Sr
precursor with newly developed Ti-precursor may result in stoichiometric deposition of
SrTiO3 at low temperatures.
X-ray diffraction of the film deposited at three different temperatures, 450 °C, 500 °C
and 650 °C is shown in Fig. 3.22. Grazing angle XRD patterns (Fig.3.22 (a)) reveal an
amorphous nature for the film deposited at 450 °C. At 500 °C we observe a
polycrystalline phase although this film is strongly off-stoichiometric, Sr/Ti ratio of about
- 95 -
1.7. Peaks at 2θ = 22.78°, 32.42°, 46.48° and 57.79° are corresponding to the (100),
(110), (200) and (211) reflections of the cubic SrTiO3 phase. Additional peaks, apart from
the substrate peak, in the XRD pattern of this film could be attributed to SrOx phases.
Stoichiometric films deposited at 650 °C didn’t show any reflection corresponding to a
crystalline phase, due to the texturing and hence Bragg-Brentano XRD was taken for this
film and is shown in Fig. 3.22 (b). θ-2θ scan shows crystalline peaks with some texturing
along the (200)- and (110)-direction. In this case the (200)-reflection can be distinguished
from Pt and attributed to SrTiO3 considering the ratio of the 100 and 200 reflections.
Fig. 3.22: X-ray diffraction of SrTiO3 on Pt/ZrOx/SiOx/Si substrates: (a) grazing angle
XRD at three different temperatures (b) θ-2θ scan on the film deposited at
650 °C.
20 30 40 50 60
(a)SrO
xt
tt (211
)
(110
)
(100
)
(200
)
Pt (111)ZrOx 450oC
500oC
650oC
SrTiO3/Pt/Zr/Si(100)
Inte
nsity
(arb
.uni
ts)
2θ (deg.)
20 30 40 50 600
200
(b) θ-2θ scan
Pt (111)ZrOx
(200
)
(110
)
(100
)
Inte
nsity
(arb
.uni
ts)
2θ (deg.)
- 96 -
3.6 Summary
The newly developed titanium complexes namely, titanium bis(isopropoxide) bis(tert-
Butylacetoacetate) [Ti(OPri)2(tbaoac)2] (A) titanium bis(ethoxide) bis(tert-
Butylacetoacetate) [Ti(OEt)2(tbaoac)2] (B), Titanium bis(isopropoxide)
bis(methylacetoacetate) [Ti(OPri)2(meaoac)2] (C), were tested for TiO2 depositions using
home built horizontal cold wall reactor. No additional reactive gases like oxygen or water
vapor were used for depositions. Thin film depositions were carried out in the
temperature range 350 °C to 800 °C and reactor pressures of 10 mbar and 50 mbar were
used. Nitrogen was used as carrier gas (100 sccm) and depositions were carried out on
Si(100) substrates. The deposited TiO2 films were analyzed by XRD, SEM, EDX, XPS
and RBS for crystal structure, microstructure, composition and thickness. Weight gain of
the substrate was used to determine the thickness of the films assuming the density of
bulk anatase phase.
Films grown at 350 °C did not show any XRD reflexes because of amorphous nature of
the films. The temperature onset of crystallization was 400 °C with all the three
precursors. At 400 °C of deposition temperature, anatase phase was observed in case of
precursors [Ti(OPri)2(tbaoac)2] (A) and [Ti(OEt)2(tbaoac)2] (B), while depositions with
precursor [Ti(OPri)2(meaoac)2] (C) showed only brookite phase at this temperature. With
increasing temperature the transition from anatase phase to rutile phase was observed.
The rutile phase was predominant from 700 °C up to 800 °C but some traces of anatase
phase with lower (XRD) intensity was observed.
Growth rates as determined by weight gain of the substrates was highest (~ 0.6 µm/hour)
in case of precursor [Ti(OPri)2(tbaoac)2] (A) at a temperature of 500 °C. Precursors
[Ti(OEt)2(tbaoac)2] (B) and [Ti(OPri)2(meaoac)2] (C) showed highest growth rates of 0.5
µm/hour and 0.3 µm/hour at 600 °C respectively. Since the films contain rutile phase and
some carbon impurities, the results may deviate from the results obtained by other
methods. Film composition of the films grown at 700 °C using [Ti(OPri)2(tbaoac)2] (A)
as determined by XPS showed the presence of titanium, oxygen and some traces of
carbon. In RBS spectrum of the same film, showed the signals from Ti as well as O.
Simulated curves for TiO2 were compared with experimentally observed curves and it
- 97 -
was found that there was no hint for sizeable carbon contamination and the stoichiometric
atomic ratio of Ti to O was found to be ½ . Microstructure of the films as observed in
SEM shows microcrystalline morphology at low temperatures. At high temperatures
granular structure was observed with dense packing.
With the initial testing of the precursors it was concluded that the precursors can be used
for the deposition of TiO2 thin films. The precursor [Ti(OPri)2(tbaoac)2] (A) was highly
soluble in most of the organic solvents and showed promise for deposition over wide
range of temperatures. Hence it was decided to test this precursor using liquid injection
industrial scale reactor. The TiO2 depositions using industrial scale reactor were carried
out at MOCVD facilities at Forschungszentrum Jülich.
The comparison between the performances of the commercially available titanium
precursor [Ti(OPri)2(thd)2] and newly developed precursor [Ti(OPri)2(tbaoac)2] (A) were
made. Precursors were dissolved in butyl acetate and solutions of 0.05 M concentration
were prepared. Depositions were carried out from 350 °C to 800 °C using oxygen as
reactive gas. Films were deposited on Pt/ZrOx/SiOx/Si and on SiOx/p-Si substrates.
Deposition pressure of 1-1.5 mbar was employed. In addition to TiO2 thin films, complex
oxide SrTiO3 depositions were investigated using standard Sr precursor [Sr(thd)2].
Composition of the films determined by XRF results indicated that [Ti(OPri)2(tbaoac)2]
(A) showed maximum efficiency of titanium incorporation in to growing films between
450 °C to 550 °C, where as the [Ti(OPri)2(thd)2] precursor showed the this behavior
above 600 °C. Surface roughness as determined by AFM showed that at in the
temperature range of 550 °C-650 °C the films had higher roughness compared to films
deposited at higher or lower temperatures. The average roughness showed strong
dependency on the susceptor temperature and the substrate. Onset of crystallization
temperature using precursor [Ti(OPri)2(thd)2] was found to be 550 °C. But the newly
developed precursor [Ti(OPri)2(tbaoac)2] showed the onset to be at 500 °C .
Crystallization of anatase films starts at 500 °C with prominent reflex (2 ? = 25.28°). At
higher temperature (>700 °C) reflexes due to rutile (110) (2 ? = 27.4°) were also
detected.
Electrical measurements in terms of C-V and I-V were done on the films in the MIS
configuration. No significant hysteresis was observed as the crystallization of initial
- 98 -
amorphous phase took place. Films deposited at 650 °C showed a reduction in the
accumulation capacitance. Electrical studies of ultra-thin films on Si substrates showed
promising properties; EOT of ~ 2 nm and dielectric constant of ~ 35 were obtained.
The compatibility of newly developed precursor [Ti(OPri)2(tbaoac)2] (A) and
[Ti(OPri)2(thd)2] with [Sr(thd)2] were investigated for the deposition of SrTiO3 thin films.
The [Sr(thd)2] and titanium precursors were injected alternatively with an offset period
of 0.225 s. Efficiency of both precursors to incorporate titanium in to growing films
increased with increasing temperature. At high deposition temperature (>600 °C)
stoichiometry between Sr:Ti in the 1:1 ratio could be achieved. At lower temperatures
stoichiometric ratio could not be achieved and films had titanium deficiency. In order to
reduce the interaction between the two precursors, the delay between injections were
increased up to 5 s. It was observed that stoichiometric films are possible at 500 °C with
approximately a 5 s delay between injections.
- 99 -
3.7 References
[1] R. N. Ghoshtagore, A. J. Norieka, J. Electrochem. Soc. 1970, 117, 1310.
[2] A. C. Jones, Materials Science in Semiconductor Processing 1999, 2, 165.
[3] T. Kodas, M. J. Hampden-Smith, The chemistry of metal CVD, VCH Publishers,
Weiheim, Germany, 1994.
[4] M. L. Hitchman, K. F. Jensen, Chemical vapor deposition-Principles and
applications, Academic Press, 1992.
[5] A. R. Teren, J. A. Belot, N. L. Edleman, T. J. Marks, B. W. Wessels, Chem. vap.
deposition. 2000, 6, 175.
[6] C. S. Hwang, Mater. Sci. Eng. 1998, B, 56, 178.
[7] C. S. Hwang, S. O. Park, H. J. Cho, C. S. Kang, H. K. Kang, S. I. Lee, M. Y. Lee,
Appl. Phys. Lett. 1995, 67, 2819.
[8] M. Shimizu, M. Fujimoto, T. Katayama, T. Shiosaki, K. Nakaya, M. Fukagawa,
E. Tanikawa, Mater. Res. Soc. Symp. Proc. 1993, 310, 255.
[9] K. Tominaga, A. Shirayanagi, T. Takagi, M. Okada, Jpn. J. Appl. Phys. 1993, 32,
4082.
[10] T. Carlson, G. L. Giffin, J. Phys. Chem. 1986, 90, 5896.
[11] B. E. Yoldas, T. W. O´Keeffe, Appl. Opt. 1979, 18, 3133.
[12] M. A. Butler, D. S. Ginley, J. Mater. Sci. 1980, 15, 19.
[13] T. Matsunaga, R. Tomoda, T. Nakajima, T. Komine, Appl. Environ. Microbiol.
1988, 54, 330.
[14] S. I. Borenstain, U. Arad, I. Lyubina, A. Segal, Y. Warschawer, Thin Solid Films
1999, 75, 2659.
[15] P. S. Peercy, Nature 2000, 406, 1023.
[16] D. Wang, Y. Masuda, W. S. Seo, K. Koumoto, Key Eng. Mater. 2002, 214, 163.
[17] K. Abe, S. Komatsu, Jpn. J. Appl. Phys. 1992, 31 Part 1., 2985.
[18] C. S. Kang, H.-J. Cho, C. S. Hwang, B. T. Lee, K.-H. Lee, H. Horii, W. D. Kim,
S. I. Lee, M. Y. Lee, Jpn. J. Appl. Phys. 1997, 36, 6946.
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[19] T. Horikawa, M. Tarutani, T. Kawahara, M. Yamamuka, N. Hirano, T. Sato, S.
Matsuno, T. Shibano, F. Uchikawa, K. Ono, T. Oomori, MRS Symp. Proc. 1999,
3, 541.
[20] A. Sherman, Chemical Vapor Deposition For Microelectronics, Noyes
Publications, Park Ridge, 1987.
[21] D. Briggs, M. P. Seah, Practical Surface Analysis, Vol. 1, 2 ed., John Wiley &
Sons, Chichester, 1996.
[22] J. A. Venables, G. L. Price, Epitaxial growth, Academic Press, NY, 1975.
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Chapter 4
Investigations into thermal decomposition of the precursors
using matrix isolation – FTIR techniques
Abstract
The thermal decomposition of precursors used for CVD plays an important role in the
process. The thermal decomposition of the precursors is studied using several techniques.
Mostly decomposition studies are tried either in situ using mass spectroscopy or by the
use of combinational studies like GC-MS, temperature desorption studies etc. Techniques
employed so far to study mechanism of decomposition of Titanium tetraisopropoxide
[Ti(OPri)4], (TTIP) were either in the bulk phase or on a surface. In a MOCVD process,
gas phase of a precursor plays an important role which determines final film quality and
stoichiometry. Present work describes the investigation of the gas phase decomposition of
CVD precursors using matrix isolation (MI) technique coupled with FTIR spectroscopy.
TTIP the most volatile alkoxide of titanium has been studied for thermal decomposition
using MI–IR technique. As a supplementary experiment, the matrix isolation of iso-
propanol was carried out. During the course of this work new class of mixed alkoxide
titanium complexes having ß-ketoester were synthesized. It was speculated that, an ester
moiety on the side chain of the ß-diketonate system will act as a “cleavage point”
facilitating easy decomposition of the precursor complex. In order to ascertain these facts
the mixed ß-diketonate/ß-ketoester complexes like [Ti(OPri)2(thd)2], [Ti(OPri)2(tbaoac)2],
were studied for gas phase decomposition behavior, so that the difference between
decomposition behavior of the two complexes could be understood. In addition, the
thermal decomposition studies of ligands Htbaoaoc (tert.-Butyl acetoacetate) and Hthd
(2,2,6,6- tetramethyl-3,5- heptanedione) were also studied to gain insights into their
decomposition mechanism.
The mixed alkoxide-ß-diketonate/ketoester complexes decompose through the formation
of acylketenes as intermediates. Matrix isolation studies coupled with FT-IR
- 102 -
spectroscopy techniques were very useful to analyze these short-lived intermediates.
Efforts were made to suggest corresponding gas phase decomposition mechanisms based
on these studies. With the help of matrix isolation investigations it was able to reveal new
decomposition pathways not previously considered by other researchers. In addition this
chapter presents shortly the details of the matrix isolation technique and the issue
concerning the use of thermolysis oven used during the study.
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4.1 Introduction
Precursor designing is a complex issue for which reliable analytical feedback from
different stages of development is necessary.[1] One such important stage where analysis
is essential is thermolysis of the precursor. It is interesting to know how a metal complex
decomposes by the application of thermal energy. Preferably there should be simple steps
leading to formation of required stoichiometry. The ideal case should be the
decomposition of the precursor at the required temperature without any prior
decomposition and leading to the formation of high quality films. There are deviations
from the ideal case where intra and inter molecular reactions and rearrangements taking
place during thermolysis of the precursors. A detailed analytical feedback is very helpful
in improving design and inclusion/exclusion of ligand moieties suiting the requirement.
Therefore mechanistic studies on precursor decomposition are crucial to understand a
CVD process. Typically study of precursor decomposition mechanism has to be done on
gas phase of the molecules. Gas-phase studies are generally difficult due to
multidimensional problems encountered during these studies.[2] For e.g. the molecules in
the `gas phase´ require sensitive method with low response times. Reason being,
interactions between different species with gas phase leads to constant change in the
composition and species under investigation during the period of measurement.[3]
Most of the mechanistic studies of the decomposition of organometallic precursors have
been carried out in solutions in organic solvents.[4] In addition, the efforts are often
directed towards understanding the precursor behavior using surface
adsorption/desorption during a CVD process.[5-6] In such cases the role of the gas phase
of a precursor is often neglected. There are several empirical methods, which can be used
to study the decomposition mechanisms. But cautious approach has to be used, as CVD
process itself is often associated with extremely complex physical and chemical growth
kinetics. Also the amount/concentration of gas-phase species required for detailed
analysis has to be high or method of detection should be highly sensitive. The most
commonly used methods are in situ molecular beam mass spectrometry and in situ
Fourier transform infrared spectroscopy.[7-8] Though these analytical methods are
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successful to certain extent, they still lack qualitative measurement of transient species
involved in gas phase decomposition of a MOCVD precursor.
The matrix isolation (MI) technique is particularly well suited for the study of such
transient species, which react rapidly or decompose under normal conditions. In matrix
isolation technique, the molecules or reactive intermediates are shock-like frozen in an
inert gas matrix at low temperatures (5-20 K).[9] Since these frozen matrices are
accessible in principle, for spectrometric analysis for indefinite length of period, different
analytical tools can be used according to the convenience and the need to analyze the
matrices. Specifically, for the gas phase generated by a MOCVD precursor, which is one
of the highly dynamic systems, matrix isolation techniques are very helpful for retaining
the transient species. Matrix isolation technique in combination with vibrational
spectroscopy in the infrared region (IR) is reasonably well suited combination, wherein
the qualitative analysis of the reactive intermediates can be performed.
Basically the MI apparatus consists of a cooling system (having cryostat with closed
Helium cycle) to cool the matrix deposition window (CsI and associated accessories)
maintained at 10 K and a high vacuum system capable of attaining a vacuum of the order
10 -7 mbar. Inert gas systems form a matrix, act as a trap for the molecules and ions
resulting from the thermolysis of the precursors. Analytical part mainly consists of FTIR,
ESR, and laser induced fluorescence etc. In MI experiments, the precursor is vaporized
by using different methods. The vapors are passed by the action of carrier gas (inert)
through an oven of aluminum oxide where they get thermolyzed over a wide range of
temperatures. Resulting fragments are trapped in a highly dilute matrix of inert gases at
low temperatures (15-20 K). The whole system is maintained at reduced pressure within a
range of 10-5 to 10-7 mbar. So the matrix isolation set up strongly resembles a hot wall
CVD reactor. The themolysis oven of the matrix isolation set up basically acting as a hot
zone of the reactor and the exhaust gases are trapped inside the matrix. By using
absorption spectroscopy, a broad spectral range from the far infrared to the UV can be
explored in a single series of experiments. The results are compared with well established
results from the small molecules and fragments and trapped ions and identified in most
cases.
- 105 -
By changing the experimental conditions like temperature of deposition window
(annealing), photolysis in addition to thermolysis, helps to determine isomers of the same
species. Intensity ratios of different species represent the concentration of species in the
matrix. Valuable information about possible reaction path can be deduced from detailed
analysis of the intensity ratios. In addition to the MI experiments, quantum mechanical
calculations can help in determining the possible equilibrium structures corresponding to
a particular atomic composition and to simulate their IR spectra. Relevant advantages of
MI over other analytical methods: highly reactive intermediates can be detected as they
are frozen in a dilute matrix of inert gases; Identification of fragments with short life
time; Use of optimal sensitive methods for detection; Use of low pressure conditions
more close to a real time CVD process; Use of broad range of thermolysis temperatures
to get an insight in to the process, possibility to get an idea of photolysis in addition to
thermolysis.
4.1.1 Thermolysis oven of the matrix isolation apparatus and its limitations
The oven used for thermolyses experiments in matrix isolation apparatus consists of an
Al2O3 tube having twin channels of 1 mm diameter. One of the channels is used for the
flow of the gaseous mixture and the other is used for inserting thermocouple to measure
the temperature of the oven without being in contact with the gaseous mixture. The rear
end of the Al2O3 tube is attached to the vacuum line and front end is directed towards the
matrix cooled window. The last 15 mm of the Al2O3 tube is heated using resistive
heating. A heat shield is provided to minimize the convection of heat of the oven to the
matrix window. The whole set up is enclosed in a stainless steel case to be attached to
matrix vacuum lines. This assembly can be effectively used to thermolyze the precursors
and carry out matrix isolation of the resulting thermolysis products. But at the same time,
the Al2O3 tube used in the oven acts as a heated substrate and depositions of materials are
observed inside the oven surface. In order to ascertain the fact that there is indeed
coupling of gasphase and surface reactions taking place certain parameters like residence
time of the gaseous molecules inside the oven, mean free path of molecules and Knudsen
number need to be specified. Following sections describe these issues in detail.
- 106 -
4.1.2 Residence time of the gas phase molecules in the channel of the matrix oven
The oven used in the matrix apparatus is a narrow Al2O3 tube with an orifice having a
diameter of 1 mm. The inner surface of the Al2O3 tube coming in contact with gas phase
of the precursors is potentially active surface upon which reactions can take place. In
order to minimize the surface reactions inside the oven, it is important for the gas phase
molecules to have minimum possible residence time. The mean residence time (t) of the
molecules inside the matrix oven can be defined by,
(equation 4.1)
Where Vr is the volume of the channel of the matrix oven and v is the volumetric flow
rate of gases through it.
In all the experiments the argon flow maintained through the container into the oven at
the rate of 1.25 sccm and operating pressure is of the order of 10-6 mbar. A flow of 1.25
sccm at atmospheric pressure corresponds to 1.25 x 109 cm3/minute of volumetric gas
flow into the oven, which is maintained at 10-6 mbar (calculated at constant temperature).
The diameter of the oven channel is 1 mm (dimension provided by the supplier) and total
length of the oven is 35 mm of which the last 15 mm are heated to required temperature.
The total volume of the channel of the oven therefore is 0.0275 cm3. The total length of
the oven is considered to be the critical dimension for all calculative purposes. The
residence time of the molecules in the channel of the oven therefore can be calculated
from the above mentioned equation 4.1 as,
0.0275/1.25 x 109 = 3.66 x 10-13 sec.
The residence time also depends on various parameters like, temperature, pressure,
sticking coefficient of the precursor molecules, surface diffusion etc, which are not
considered in the calculations above.
ντ rV
=
- 107 -
4.1.3 Coupling of gas-phase and surface/wall reactions
Because of the low pressures used in matrix isolation systems, the gas flow can be in
either the usual continuum regime or in the transition regime, depending on the relative
magnitude of the mean free path of the reactant molecules and the characteristic
dimension of the system, as reflected by the Knudsen number. The Knudsen number is
proportional to {length of mean free path (?) / characteristic dimension (L)} and is used
in momentum and mass transfer in general and very low pressure gas flow calculations in
particular. It is normally defined in the following form:
L
Knλ
= (equation 4.2)
For Kn = 0.01 the flow is dominated by gas molecule collisions and the continuum
models apply to such systems. When Kn > 10, the molecules primarily collide with solid
surfaces and the flow is described as `free-molecular`. Gas flows in this regime can not
be modeled by the classical continuum equations but must be described through view
factor, computations similar to those used for the radiation heat transfer or by Monte-
Carlo simulations. The intermediate range of Knudsen numbers, 0.01 < Kn < 10,
corresponds to the so-called transition flow regime where both gas-phase and surface
interactions are important. In this case, solution of the Boltzmann equation or use of
specialized Monte-Carlo simulation techniques must be applied in order to model low
pressure systems accurately.
The diameter of the argon is 4.17 Å and the mean free path ? of the argon atoms inside
the oven are given by the equation,
PNdRT
A22π
λ = (equation 4.3)
where d is critical dimension of the oven, P is the operating pressure of the matrix
isolation unit and T is the temperature. At 27 °C and at 10-6 mbar of operating pressure
the mean free path of the argon is found to be 53.63 m (using equation 4.3). For argon
atoms experiencing a mean free path of 53.63 m at 10-6 mbar, the Knudsen number, Kn is
found to be ~ 1532.28 (using equation 4.2). This high Kn is in accordance with studies
- 108 -
which deal with gas transport characteristics through microchannels operating at low
pressures. It was reported that these microchannel gas flows are characterized by high
Knudsen numbers. High Knudsen number gas flows are characterized by ‘rarified’ or
‘microscale’ behavior; wherein significant gas-phase as well as surface/wall reactions are
possible. Because of significant non-continuum effect, traditional CFD (computational
fluid dynamics) techniques are often inaccurate for analyzing high Kn gas flows. The
direct simulation Monte Carlo (DSMC) method offers an alternative to traditional CFD
which retains its validity in slip and transition flow regimes.[10]
This clearly indicates that during the process of matrix isolation, the gas phase of the
precursors not only undergoes collisions in the gas phase but also with the inner surface
of the oven. Reactions that take place at the surface of the oven are known as
heterogeneous reactions. Reactions that take place in the gas phase are known as
homogeneous reactions. In short, though the homogeneous reactions are much more
desirable than heterogeneous reactions occurring at the surface, during matrix isolation
the coupling between these two types of reactions is unavoidable using present set up and
a complete gas phase analyses are thus hindered. The lower residence time of the
molecules on the surface of the oven may de-couple these heterogeneous reactions to
some extent but the surface reactions do occur in considerable number. Nevertheless, the
technique of matrix isolation can be used as a complementary to other gas phase studies,
surface decomposition techniques and a useful picture of the reaction mechanism can be
derived.
In the course of this study the mechanistic studies on selected precursors of titanium were
carried out using MI-IR techniques. The most commonly used precursors for deposition
of TiO2 thin films using MOCVD are titanium alkoxides. Titanium alkoxide complexes
have been prepared from a variety of alcohols.[20] The simple titanium tetra alkoxides are
all volatile, but among them titanium tetraisopropoxide [Ti(OPri)4] is the most volatile of
the titanium alkoxides, a property that it owes to its low molecular weight and the steric
effects of the ligand which restricts the nuclearity of the complex.[20] Titanium
tetraisopropoxide [Ti(OPri)4] is a liquid at or above room temperature and its nuclearity
has long been accepted as being approximately 1.4.[20] Titanium tetraisopropoxide is the
most widely used precursor for TiO2 thin film depositions using MOCVD. Therefore the
- 109 -
gas phase decomposition of the [Ti(OPri)4] is of interest to understand the CVD processes
involving this precursor. Though there are several studies[4,7,11] on the decomposition of
the [Ti(OPri)4] complex on surface and in the bulk phase, the gas phase studies have not
been reported so far. So it was appropriate to study the gas phase decomposition of
[Ti(OPri)4] using matrix isolation technique. MI technique can be a powerful method to
analyze relevant molecular decomposition mechanism and this methodology has not yet
been widely recognized within CVD community.
Titanium tetraisopropoxide [Ti(OPri)4], has Ti(IV) center which is coordinatively
unsaturated. Therefore, [Ti(OPri)4] is susceptible for attack of air or moisture resulting in
facile hydrolysis. This sensitivity towards air and moisture is reduced by reacting
[Ti(OPri)4] with chelating ligands such as ß-diketonates wherein two of the alkoxy
ligands are replaced by bidentate chelating ligands resulting in full coordinatively
saturated titanium complexes. The most widely used chelating ligand is thd (2,2,6,6,
tetramethyl-3,5-heptane-dione) and resulting complex, [Ti(OPri)2(thd)2] is reported to be
six coordinated and stable complex based on NMR studies.[12] It is one of the most widely
used standard precursors for the deposition of titanium containing oxide thin films using
MOCVD.[13] During our efforts to synthesize new precursors for MOCVD of titanium
dioxide thin films, ß-ketoester, tert. Butylacetoacetate (tbaoac) was used as chelating
ligand in combination with titanium tetraisopropoxide [Ti(OPri)4], resulting in
monomeric six coordinated stable complex [Ti(OPri)2(tbaoac)2]. Thin films of TiO2 were
deposited using this precursor and it was found that the new precursor was able to
efficiently incorporate titanium into growing films at temperatures as low as 350 °C.
(refer chapter 2 and 3 for details)
So it was thought that gas phase decomposition of these precursors would be helpful in
understanding the difference in behavior. Though there are reports about decomposition
mechanism involving the decomposition of the standard complex [Ti(OPri)2(thd)2], there
is a lack of clear understanding of the decomposition mechanism involving the reactive
intermediates in the process.[15] The main aim of the MI-IR in present case was to provide
qualitative information about the gas phase during the thermal decomposition of the
titanium MOCVD precursors under isolated conditions.
- 110 -
During this study the complexes were investigated for decomposition in the gas phase
using MI-IR techniques. As a preliminary set of the experiments, the individual ligands
used in the study were studied for their thermal decomposition behavior using MI-IR
techniques. A series of thermolysis experiments were conducted for these ligands and the
starting complex [Ti(OPri)4] starting from ambient till the decomposition temperature.
Most of the IR bands resulting from the thermal decomposition of the ligands were
assigned to different species based on literature database available over last forty years.
The decomposition of [Ti(OPri)4] was found to occur through the formation of iso-
propanol and propene as intermediates but the variation was observed depending on
temperature wherein acetone and water were also observed as decomposition products.
The ligands having ß-keto structure were found to decompose through the formation of
Acylketene intermediates. While the complex containing tert. Butylacetoacetate
decomposed through the formation of acetylketene intermediate, the complex containing
ligand thd, decomposed through the formation of pivaloyl ketene. The matrices
containing these intermediates resulted in complex spectra because of various conformers
possible for these species. Bands due to these conformers could be assigned with the help
of photolysis studies. In addition, DFT calculations were carried out and experimental
spectra were compared with simulated spectra of the species which helped to a great
extent to identify the species. In later stage the metal complexes having these ligands
were subjected to a series of thermolysis experiments and assignments of the IR bands
were done based on preliminary studies on each ligand set. A tentative mechanism was
proposed for the decomposition of the titanium complexes in the gas phase based on
these observations. This study throws some light on the reaction mechanism of the gas
phase decomposition of above mentioned precursors through the formation of acylketene
intermediates which were not considered by other reports so far.
- 111 -
4.2 Experimental section
To investigate the molecular mechanism involved during precursor decomposition a
matrix isolation apparatus was designed and fabricated. (Fig. 4.1) The following section
describes the details of the apparatus.
4.2.1 Description of matrix isolation apparatus
The matrix isolation apparatus consists of a vacuum line (Pfeiffer TMH 261; Pfeiffer
DUO 5) and an ARS 8200 cryogenic closed cycle system (ARS cryogenics Inc.). The
starting compound can be kept at constant temperature in a small stainless steel vaporizer
connected to high vacuum line through Swagelok® fittings.
Fig. 4.1: The matrix isolation unit used during the present study.
- 112 -
Fig. 4.2: (a) Schematic diagram of the refrigeration unit (b) Arrangement of spectroscopic
and photolysis windows attached to a vacuum shroud. (Top view)
Helium pressure connections
Rotatable seals
Heater lead through Thermocouple lead through
Valve power supply
Radiation shield (80 K)
Very cold station (8 K)
Cooled window
Spectroscopic window
Warm flange (a)
Cold station (77 K)
Thermolysis oven
Spectrometer source Spectrometer detector
Photolysis window
Cooled window
(b)
- 113 -
A small (~25 mm) window (optically polished cesium iodide suitable for infrared work)
was suspended at the tip of the cryostat within the vacuum shroud (Figs. 4.2 (a) and (b))
and can be cooled to temperatures as low as 9 K. Vacuum windows (CsI 40 mm
diameter, 3 mm thick) on the chamber permit spectroscopic measurements of samples
prepared within. Additional ports permit the admission of the inert gaseous matrix
material (usually argon for IR work) and vacuum ultraviolet light for sample photolysis.
The thermolysis oven consists of an Al2O3 tube with two parallel, inner channels (outer
diameter of 4 mm; inner diameter of 1 mm each) where one of the inner channels is
equipped with a thermocouple (Thermocox: NiCrSi/NiSi) and the other channel is for the
argon/compound mixture. With this set up, reliable thermolysis temperatures could be
measured during the experiment without a contact between the gaseous mixture and the
thermocouple. The oven is constructed in such a way that the heat radiation from the
Al2O3 tube is not conducted to the matrix by having a metal shield surrounding the Al2O3
tube wherein only one small orifice is provided for the passage of gaseous mixture from
the oven and directed to the cold window.
Argon (Linde 6.0) was used as the carrier gas and passed over the compound using a
mass flow controller (flow =1.25 sccm) and the gaseous mixture was passed through an
Al2O3 tube (inner diameter of 1mm; heated by tungsten wire coiled around the last 15
mm). The hot end of the pyrolysis oven was 25 mm away from the cooled CsI window to
assure that a maximum amount of volatile fragments emerging from the oven were
trapped in the matrix. The cold-trap, designed both to prevent back-diffusion of vapors
from the pumps to the very cold sample mount and to protect the pumps from reactive
vapors released when the matrix evaporates. The cold trap was, `flow-through` type with
no abrupt changes of direction or constrictions was attached between vacuum pump and
the cold head. The steps in a typical experiment were; first, the sample window was
cooled to 8-9 K and positioned to face the beam axis of the spectrometer where a
background spectrum of the bare cooled window was recorded.
Afterwards, the window was rotated to face the sample deposition ports. Precursor vapor
was generated by sublimation of precursor sample which was placed in a stainless steel
vaporizer and attached to the system through standard stainless steel Swagelok® vacuum
fittings. The inert gas flows from a cylinder, through a length of stainless steel tubing,
- 114 -
(gas flow controlled by mass flow controller) through a mass flow controller and into the
precursor container. The two vapor streams coalesce and are carried through the oven and
then freeze at the surface of the cold window. Use of a great excess of inert gas ensures
that in the resulting solid solution, the sample molecules are effectively isolated in the
inert matrix. Once a suitable amount of sample has been deposited (generally 45 minutes
to one hour at a flow rate of 1.25 sccm), the cold window was returned to the first
position and its spectrum was recorded and difference spectrum against background
spectrum was recorded. With the operating pressure of the order of the 10-6 mbar, and the
flow rates of 1.25 sccm, the residence times of the gas-phase molecules are calculated to
be around 3.66 x 10-13 sec. The short residence time and the high dilution in the argon gas
flow are expected to limit the surface reactions inside the oven surface, though these
reactions can not be avoided to a certain extent. Oven temperatures ranging from room
temperature (RT) to 1000 °C were used. The IR spectra of the matrices, cooled down to
10 K, were recorded on a Bruker EQUINOX 55 with a KBr beam splitter in the range of
400 to 4000 cm-1 with a resolution of 1.0 cm-1.
4.2.2 Photolysis experiments using an ultraviolet source
The experimental arrangement for photolysis is rather simple. An aperture in the various
heat shields surrounding the matrix is provided, together with a window (quartz, optically
polished 40 mm diameter, 3 mm thick) sealed to the outer vacuum shroud. The ultraviolet
lamp is mounted so that radiation is directed on to the matrix through the window and the
aperture. In an ideal case, one may monitor the disappearance of the precursor (indicated
by the disappearance of corresponding IR bands) or the growth of the product without
interrupting the photolysis. But in the present system employed in our laboratory, the
photolysis window is at right angles to the spectroscopic windows in which case the
matrix and cooling system must be rotated for photolysis and then back again before the
progress of photolysis can be monitored spectroscopically.
4.2.3 Preparation of gaseous mixture of argon and iso-propanol
A 2L-glass balloon having two flanges, one attached to the pressure gauge and the other
to the inlet valve for gases, was dried for 12 hours at 150 °C and evacuated under high
- 115 -
vacuum to remove traces of volatiles that could interfere with the measurements. With
the help of a pressure controller, a gaseous mixture of highly volatile sample (iso-
propanol, propionic anhydride, diisopropyl ether etc.) and argon in the ratio 1:1200 was
filled into the glass balloon which was then attached to the matrix isolation apparatus.
The drop in pressure was taken as a guide to control the flow of the gaseous mixture in to
the matrix isolation instrument.
4.3 Results and Discussion
4.3.1 Fundamental aspects of the matrix isolation technique
It is important to know the structure and other properties of individual molecules,
although matter is rarely found in the form of isolated molecules. Intermolecular
interactions dominate the physical nature of the matter in the solid and liquid phases, and
are experimentally observable even in gases, where they are smallest. In general, the
`molecular` properties of a substance can only be deduced from gas-phase studies.[15] The
intermolecular interactions are strongest between chemically reactive species such as
most atoms, free radicals and “high temperature monomers”, all of which can be studied
in the gas phase only at low concentrations and high effective temperatures. Even under
such extreme conditions some species are so reactive that they exist for only a few micro-
or milliseconds after they are formed, so that the study of their molecular properties is a
difficult matter.[15]
The technique of matrix isolation is one result of attempts to overcome some of the
difficulties associated with the study of very reactive molecules. Matrix isolation is a
technique developed over the last five decades which enables reactive, short-lived
molecules to be trapped in a solid matrix (of inert gases) and studied spectroscopically.
In essence, the method involves the trapping of the molecule in a rigid cage of a
chemically inert substance (the matrix) at a low temperature. The rigidity of the cage
prevents diffusion of reactive molecules, which would lead to reaction with other such
species. The inertness of the matrix material prevents loss of reactive molecules by
reaction with their environment. The low temperature, besides contributing to the rigidity
- 116 -
of the cage, serves to reduce that rate of possible internal rearrangements that require any
activation energy. Under such conditions molecules that normally have very short
lifetimes can be preserved indefinitely and studied at leisure.[9]
In practice, few materials other than the rare gases and molecular nitrogen are chemically
inert enough to serve as matrices for most reactive species. The formation of a rigid
matrix implies the use of temperatures not exceeding above one-third of the melting point
of the solid, i.e. temperatures of 9 K for neon, 29 K for argon, 40 K for krypton, 55 K for
xenon or 26 K for nitrogen. As the lowest temperature attainable using liquid nitrogen as
coolant is 63 K, the triple point of nitrogen, the most inert materials available can only be
used as matrices if colder refrigerants are employed. Only liquid hydrogen and liquid
helium are suitable; they are usable over the ranges 12-13 K and 2-5 K under `boil off`
pressures that can be controlled to adjust the temperature of the liquid. The necessity for
the use of such low temperatures has controlled the development of the technique of
matrix isolation.[15]
In the early 1950s Broida in Washington and Pimental in Berkeley began to use matrix
isolation technique in the study of atoms and reactive molecules, but the method spread
only slowly until the wider availability of liquid helium in the early 1960s and the advent
of microrefrigerators in the late 1960s made it possible for matrix isolation experiments
to be performed widely.[15]
The matrix isolation technique necessarily involves a combination of several distinct
technologies, each of which interacts with the others. The most basic factor, the low
temperature needed to give a rigid matrix, implies cryogenic technology, and in turn
requires the use of high vacuum techniques without which low temperatures can not be
maintained conveniently. The nature of the matrix, the low temperature and the need to
isolate the sample in a vacuum all imply that only spectroscopic methods can be used to
study matrix-isolated species, and the experimental technique is to a large extent
dominated by the need to expose the sample to the spectrometer at the same time as
cooling it in a high vacuum.
A matrix-isolated species is indeed isolated, in the sense that one can only operate on it
under conditions where the matrix is not disrupted. This essentially limits means of
characterization and study to non-destructive spectroscopic methods. These may be
- 117 -
further limited by the matrix itself, which must not interfere with the spectrum of the
species under investigation.
4.3.2 Spectroscopic methods
The essence of most spectroscopic techniques is the absorption or emission of
electromagnetic radiation in resonance with a transition between two discrete states of a
molecule or atom. The two states may, for example, be different in the electronic, the
vibrational or the rotational parts of the molecular wavefunction; for atoms only the
electronic wavefunction is relevant.
The main spectroscopic methods used to study matrix-isolated species are electronic
absorption and emission spectroscopy in the visible and ultraviolet regions, vibrational
absorptions spectroscopy in the infrared region and electron spin resonance (e.s.r)
spectroscopy. Electronic and vibrational absorption studies are usually carried out on
samples deposited on cooled windows transparent to the corresponding radiation. During
the course of this study vibrational spectroscopy in the infrared region was studied and
hence will be detailed in following section.
4.3.3 Vibrational spectroscopy in the infrared region
There are only two classes of substances (isolated atoms and homonuclear diatomic
molecules) which have no vibrational spectrum in `the infrared` region. Commercial
spectrometers are available covering the range containing all known stretching vibrations
(~ 4000 to 100 cm-1); most deformation vibrations fall in the same range, though they are
lower in energy than the stretching vibrations involving the same atoms. In practice the
region in which KBr is transparent and can be used as a window material for the matrix
and within the spectrometer (? > 400 cm-1) has been far more extensively studied than the
`far infrared` below 400 cm-1, but even the latter region is strictly available for study and
is becoming more thoroughly investigated.[9]
The ideal matrix material is rather more limited for infrared region than for ultraviolet
studies, only nitrogen, oxygen and the rare gases being completely transparent over the
normal range. Window materials pose little problem in the normal infrared region.
Potassium bromide, which is cheap and readily available, is used down to 400 cm-1, while
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cesium iodide or bromide may be used to lower frequencies (down to 250 or 200 cm-1).
The cesium salts are much more expensive but mechanically rather stronger than
potassium bromide. Interestingly cesium iodide transmits radiation down to 120 cm-1
when cooled to matrix temperatures. At wavenumbers below about 400 cm-1 polythene
can be used, though it suffers from several disadvantages. It has an absorption band at 72
cm-1, conducts heat very poorly, and is mechanically weak. Silicon and Germanium
windows may be used below 200 cm-1, and quartz is also useful, transmitting far-infrared
radiation below about 200 cm-1. Most of these `far-infrared` windows suffer the great
disadvantage that they do not transmit in the normal infrared region, so that the
comparison of the two region must be made using two separate samples. For this reason
potassium bromide or cesium iodide windows are preferred wherever possible. Then it is
assured that at least one infrared band from any matrix-isolated species other than an
atom or a homonuclear diatomic molecule.[15]
4.3.4 Sample preparation and concerns
The sample itself must be carefully prepared for high temperature studies. In particular it
must be free from more volatile impurities, since these would vaporize preferentially and
might mask the spectrum of the sample or prevent the formation of the desired species.
The sample container and all hot parts of the oven must be similarly clean; even so, traces
of water, carbon dioxide and so on arise from fingerprints and similar minor
contamination of surfaces during sample preparation and loading the container. These,
and carbon monoxide released during the heating of metal parts of the oven, can
effectively be removed only by prolonged degassing under high vacuum at a temperature
just below that required for sample evaporation before cooling down the apparatus and
beginning the deposition of the matrix.
Wherever possible the sample is arranged so that the evaporated molecules have a
straight-line path to the cold window. If this is not done, a considerable amount of
material will condense at the obstruction, unless this is heated to a temperature above that
of the precursor container.
Any method used for heating a precursor must overcome a number of problems. These
include:
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• The sample container must not itself vaporize or react with the sample or any
other material in contact with it;
• The amount of radiation reaching the cold window from the hot parts of the oven
must be minimized;
• The transfer of heat from the oven itself to its surroundings by any method
(radiation or conduction) must be minimized, not only to prevent damage to
vacuum seals and the like but also to maintain the efficiency of the oven and
maximize the sample temperature.
An even simpler experiment than that using an oven to generate reactive species is
practicable when the desired species can be made by the vaporizing a volatile precursor
that can be handled at room temperature. In this case, a mixture of matrix gas and the
precursor is prepared and passed through a heated tube (oven) before condensation of the
matrix. The presence of the large excess of the matrix gas is usually helpful, as it reduces
the mean free path of the unstable species and thus reduces the probability of wall
reactions and aggregation before deposition.[15]
4.3.5 In situ generation
In many ways it is easier to prepare a matrix containing the more-or-less stable precursor,
and decompose the precursor in the matrix. The problems of heat transfer from an oven to
the matrix and of loss of reactive species by wall collisions and aggregation before and
during deposition are thereby avoided, and it is much easier to ensure thorough mixing of
matrix gas with a stable precursor than with a rapidly flowing gas stream containing
reactive species.
This last consideration is critical, as the importance of interactions with nearest neighbors
is bound to be increased by any uneven distribution of matrix-isolated species. It is
common-place of elementary physics that a gas expands to fill the volume available to it,
and it is usually assumed that this process is an instantaneous one and is independent of
the presence of another gas. It is found, on the contrary, that the rate of the mixing
process depends on the gases involved, and diffusion through heavy gases like xenon is
particularly slow. Adequate mixing requires either a long time or a very turbulent gas
- 120 -
flow if the diffusion is slow, and it is more satisfactory to allow the matrix gas and stable
precursor to mix in a large bulb than simply to inject one gas into a stream of the other.
Uniformity is easier to achieve in a spherical bulb than in a cylindrical vessel.
During our experiments we had to handle at least three of such volatile precursors. When
cooling down of the precursor container was ineffective for maintaining proper dilution
of the matrix, we had to resort to glass balloon preparative methods, which resulted in
nice spectra with proper dilutions.
4.3.6 Chemistry with matrices
Once the precursor and thermolysis products are isolated in the matrix it is quite difficult
to decompose them by any means other than photolysis, unless the matrix also contains
some reactive intermediate that can react chemically with the precursor after diffusion.
Other methods involving bombarding the matrix with high energy particles are best
thought of as special types of photolysis. These two issues are discussed briefly in
following sections.
4.3.7 Rigidity and mobility of matrix material
In a matrix isolation experiment it is important that no changes should take place while
spectra are being recorded, and this implies that the matrix must remain rigid and prevent
any diffusion of reactive species. A useful rule –of –thumb is that a matrix may be taken
as rigid below 30 % of its melting point. Below this temperature essentially no
rearrangement of matrix atoms or diffusion of isolated species is expected. In the range
from 30 % to 50 % of the melting point, the process of annealing of the matrix may
occur. This is basically the rearrangement of the matrix structure at the atomic level
towards the most stable crystal structure. Thus grain growth begins in this temperature
range, while large trapped species will cause a local rearrangement to give the most
stable possible cage. Small trapped species such as atoms and diatomics may be able to
take part in the lattice rearrangement to such an extent that segregation at grain
boundaries or reaction with neighboring trapped species occurs.
Above 50 % of the melting point, the matrix must be regarded as non-rigid. Diffusion of
trapped species now begins, and the ultimate effect is that all impurities will be
- 121 -
segregated at grain boundaries and reactive species will be lost, as diffusion will occur
until reaction results in formation of a large and stable species. This ultimate stage can
rarely be achieved in practice because the vapor pressure of the matrix solid becomes too
high and the matrix evaporates.[15]
4.3.8 Ultraviolet photolysis of the precursor in the matrix
The photolysis of a precursor in a matrix can only be efficiently carried out using
radiation that is strongly absorbed by the molecule, and that has sufficient energy to
break chemical bonds. In a few cases it may be possible to photolyze a colored precursor
using visible radiation, but in generally ultraviolet radiation is required. A wide variety of
ultraviolet lamps is available, and most useful ones are discussed here.
All ultraviolet lamps depend on an electrical discharge through a vapor to excite the
ultraviolet radiation. The characteristics of the radiation emitted are controlled by the
nature of the vapor and its pressure. Low pressure lamps tend to produce predominantly
atomic emission lines, because the vapor is largely behaving as isolated atoms under the
high temperature / low pressure conditions. Thus, if the lamp is filled with hydrogen at
low pressure, the emission is largely the atomic hydrogen line at 121.6 nm, while if it is
filled with mercury vapor, the emission lines at 184.9 nm and 253.7 nm are excited. High
pressure lamps on the other hand give emission spectra consisting of broader bands,
which may be continuous or may be made up of many lines. In either case the effect is to
give a broad spectrum of emission. The high pressure hydrogen lamp gives a many-line
spectrum below 160 nm and above 500 nm, as well as a true continuum between 160 nm
and 500 nm. The high pressure mercury lamp gives broad bands over the range 250 nm to
470 nm.
The windows through which the radiation passes between the discharge and the matrix
(including the lamp envelope) are of great importance in governing the transmission of
various wavelengths. Glass transmits only a little way in to the ultraviolet region and is
generally useless for photolysis experiments. Sodium chloride and potassium bromide
both transmit radiation down to 200 nm wavelength and make cheap windows for
irradiation, but they fog easily in damp conditions and are too weak mechanically to be
used for lamp envelopes or windows for matrix vacuum shroud. Quartz is preferred for
- 122 -
this purpose as its high strength and chemical and mechanical stability suits it for use in
discharge tubes as well as for windows.[9]
4.3.9 Limitations of matrix isolation techniques for studying gas phase
decomposition of a CVD precursor
Matrix isolation studies can be performed on relatively simple molecules. This is mainly
because interpretation of the data resulting from complex molecules is difficult. Even
with the best efforts to solve the spectra, understanding of the decomposition mechanism
has to be speculated based on intermediates observed during the decomposition reactions.
Added to these problems, most of the CVD processes use reactive gases or more than one
molecular precursor for depositing required material. These factors result in highly
complex reactions taking place either in gas phase or on the substrates. Matrix isolation
technique can not be effectively used for the purpose of studying with additional gases or
with precursors containing two different ligand systems. The best way of using this
technique is in combination with other in-situ methods like in situ MS or GC-MS which
provide complementary data in order to get better understanding.
4.3.10 The FTIR spectra of [Ti(OPri)4] and iso-propanol
Vibrational spectroscopy is an excellent method for structural analysis and for the
determination of molecular interactions. [Ti(OPri)4] (TTIP) is one of the most widely
studied transition metal oxide precursors. The vibrational spectrum is under detailed
investigation with new assignments which are not in agreement with each other.[16-18]
Titanium alkoxides are known to form dimeric, trimeric and higher molecularity species,
which is a response to the electron deficient nature of the transition metal in these
tetracoordinated compounds.[19] This is achieved through the formation of alkoxide
bridges, where the degree of association or “molecular complexity” is dependent on the
steric constraints of the ligand. Employing ebullioscopic measurements it was found that,
[Ti(OPri)4] has a molecular association of 1.4.[20]
The IR spectra of the matrix isolated TTIP is shown in the figure 4.3. And corresponding
assignments for observed bands are given in table 4.1.
- 123 -
Given the presence of four isopropoxy ligands per TTIP molecule, the likelihood of
extensive coupling of the C-O and C-C stretching modes of each isopropoxy ligand, and
the possibility of molecular association, results in a complex spectrum. To reduce the
complexity of the spectra it was necessary to compare the vibrational spectra of TTIP
with that of corresponding alcohol, i.e. iso-propanol. Any differences between these two
spectra can then be correlated with structural differences between the molecules, to help
in understanding of the vibrational spectrum of the TTIP.
Surprisingly there are no reports about the spectra of matrix isolated iso-propanol in the
range 400 to 400 cm-1. So as a supplementary experiment, highly diluted iso-propanol in
argon (1:1000) was matrix isolated using glass balloon preparative method (see
experimental section). The assignment of observed bands is given in table 4.2. The most
intense band in case of matrix isolated iso-propanol appears at 1253.1 cm-1 which could
be assigned to ?s(CCC) mode. The ?(O-H) mode is detected at 3639.4 cm-1. Despite good
dilution, we observed two bands 3482.6 and 3507.1 cm-1, which we assume are due to
molecular associations of iso-propanol.
4000 3500 3000 1500 1000 500
0.0
0.4
0.8
1.2
1.6
2.0
A
Wavenumber [ cm-1]
Fig. 4.3: FT-IR spectrum of matrix-isolated [Ti(OPri)4] (TTIP).
- 124 -
Increased drying time of the glass balloon, and dilution to 1:1200 failed to avoid these
associations of iso-propanol observed in the matrix and still we could observed the bands
at 3482.6 and 3507.1 cm-1. Nevertheless, the IR Spectrum of matrix isolated iso-propanol
was effectively used for the comparison of the bands in the complex TTIP. The IR
spectrum of matrix isolated TTIP, showed presence of small amounts of iso-propanol.
These bands showed typical half band widths at 1253.1, 1077.2, 948.9 and 3639.4 cm-1
which were readily assigned to that of iso-propanol.
Table 4.1: IR frequencies for (TTIP) in the bulk phase (literature reported) and matrix-isolated in solid argon.
bulk phase
frequencies
[cm-1][21]
assignment observed
frequencies
[cm-1]
relative
intensity†
430 (CCC) sym. def. 412 vw
509 (Ti-O) str. as. 509 vw
557 (Ti-O) str. as. 557 w
583 (Ti-O) str. as. 580 sh
611 (Ti-O) str. as. 616 s,b
849 (CCC), (Ti-O) str. 853 s
940 (C-O), (Ti-O) str. 947 m
988 (C-O), (Ti-O) str. 1008 vs, b
1115 (CH3), (Ti-O) str. 1132 vvs
1161 (CH3) str. 1165 mw
1331 (C-H) str. 1334 m,b
1362 (CH3) str. s. 1365 m
1376 (CH3) str. s. 1378 m
1450 (CH3) bend. as. 1451 vw
1463 (CH3) bend. as. 1464 vw
2866 (CH3), (C-H) str. 2871 w
2912 (CH3) str. 2916 w,sh
2930 (CH3) str. as. 2937 m
2968 (CH3) str. as. 2980 m
s = strong, vs = very strong, m = medium, b = broad, sh = shoulder, w = weak, vw = very weak, vvw = very very weak
- 125 -
Other bands listed in the table 4.1 showed broad band widths and most intense one
among these bands could be detected at 1115 cm-1. Tentative assignments of these bands
were made based on bulk phase studies of this molecule reported in the literature.[21]
There are significant shifts in the wavenumbers assigned in the bulk phase studies to the
one observed in the gas phase using matrix isolation technique. The shifts observed were
in the range 800 to 1200 cm-1, particularly ?(C-O), ?as(Ti-O) and ?r(CH3),?as(Ti-O)
modes showed shifts of around 15 to 20 cm-1.
4000 3500 3000 2500 2000 1500 1000 500
0.00
0.25
0.50
0.75
1.00
1.25
1.50
A
Wave number [cm-1] Fig. 4.4: Matrix isolated iso-propanol. Sample prepared by glass balloon
preparative method, having a 1:1000 dilution in argon.
- 126 -
Table 4.2: IR frequencies of matrix-isolated iso-propanol in solid argon and
corresponding assignments.
observed
frequencies
[cm-1]
assignments relative
intensities†
414.1 ds(CCC) bend. m
813.7 ?s(CCC) str.s. m
948.9 (CH3) vvs
1077.2 (C-O), str. m
1128.3 (C-O), str. vvw
1134.7 (CCC) str.as. w
1165.3 (CH3) m
1253.1 (CCC) str.s. vvs
1380.4 (CH3) bend.s. w
1461.4 (CH3) bend. as. vw
1469.5 (CH3) bend. as. w
2921.3 (CH3), (C-H) str. m
2937.4 (CH3) str.s m
2968.2 (CH3) str. as. m
2988.0 (CH3) str. as. s
3482.6 association m
3507.1 association m,b
3639.4 (O-H) str. s
s = strong, vs = very strong, m = medium, b = broad, sh = shoulder, w = weak,
vw = very weak, vvw = very very weak
- 127 -
4.3.11 Thermolysis experiments on TTIP
The high vacuum thermolysis of TTIP, with matrix isolation techniques is described.
Since the experiments were conducted under dynamic conditions where in the TTIP is
highly diluted in the argon gas, it was expected that the secondary reactions are unlikely
to occur. The container for TTIP had to be cooled to 0 °C in order to suppress the high
vapor pressure and maintain a proper dilution in the matrix. Initially the intact TTIP
molecule was matrix isolated and a spectrum was recorded in order to ascertain the
existence of intact molecule and then compare it with the spectra of thermolysis products.
Initial thermolysis experiments were carried out in steps of 100 °C to note the dynamic
region where most of the decomposition reactions were taking place. Under matrix
conditions, in the temperature range of 300 to 400 °C there were distinct developments in
terms of appearance/disappearance of different species. Hence, this region was studied
closely with increase in temperature steps of 20 °C till 400 °C.
While first new IR bands started to appear at oven temperatures as low as 100 °C, the
bands belonging to starting material TTIP completely disappear at 390 °C indicating the
completion of thermolysis. From 100 to 300 °C of oven temperatures it was observed that
bands assigned to iso-propanol were growing in intensity along with bands of the starting
complex TTIP. The formation of iso-propanol was indicated by the appearance of strong
bands at 3639.4, 948.9 and 1253.1 cm-1 which were assigned to ?(O-H), ?r(CH3) and
?s(CCC) modes, respectively. The strong bands observed which are in good agreement
with spectra of matrix-isolated iso-propanol. These bands along with 14 other bands are
listed in table 4.2. It must be noted that even though the iso-propanol was being observed,
the broad bands belonging to intact complex molecule TTIP still existed.
Up to 310 °C of oven temperature, bands belonging to propene could be observed only in
low intensities. At 330 °C, all the bands belonging to propene appear in very good
intensity. And thereafter, propene bands persist and are observed till 400 °C with good
intensity. The major bands for propene were observed at 1453.3 and 908.8 cm-1 which
were assigned to das(CH3) and ? (CH2) modes respectively. A list of frequencies observed
for propene and assignments have been given in table 4.3 which are compared to the
matrix-isolation work of Barnes et el. Furthermore we could reproduce all the bands,
except the band at 1212 cm-1, assigned to propene in this temperature range.
- 128 -
One point to be noted here is that, though the concentration of iso-propanol was lowered
at temperatures higher than 390 °C, (which was noted by comparison of band intensities),
but that of propene are slightly increased. In addition there was an increase in the
intensities of bands due to water at 1623.8 and 1608.0 cm-1. The bands belonging to
acetone start appearing at a temperature of 320 ° C and these bands were in very less
intensity till 390 °C. The most intense bands appeared at 1721.5 and 1216.4 cm-1 which
are in good agreement with bands for matrix isolated acetone.[23]
Table 4.3. FT-IR frequencies of matrix-isolated propene in solid argon and corresponding assignments.
literature reported frequencies
[cm-1][22]
assignment observed frequencies?
[cm-1]
relative intensities
3091 (CH2) str. as. 3091.8 s 3036 (CH) str.s. 3036.7 s 2983 (CH2) str.s. 2983.8 s 2941 (CH3) str.as. 2940.8 s 2923 (CH3) str.as. 2922.7 s 2859 (CH3) str.s. 2858.8 m 1650 (C=C) str. 1650.4 s 1453 (CH3)
bend.as. 1453.3 vs
1439 (CH3) bend.as.
1438.8 s
1415 (CH) bend. 1415.3 m 1373 (CH3) str.s. 1373.2 w 1212 (CH2) wag. Not observed 1043 ( CH3) tor. 1043.4 s 998 (CH) bend. 998.1 s 932 (CC) str. 932.5 m 908 (CH2) wag. 908.8 vs 578 ( CH2) tor. 578.4 s
s = strong, vs = very strong, m = medium, b = broad, sh = shoulder, w = weak,
vw = very weak, vvw = very very weak
- 129 -
A couple of bands could not be assigned to any species. The intense one is at 1820.1
accompanied by weak bands at 916.4, 1893.0 and 1768.30 cm-1. These bands showed no
group behavior and even with thorough observation of these bands we failed to assign
them to any of the species.
Though earlier flash vacuum pyrolysis studies accompanied by NMR and GC-MS
analysis showed the presence of diisopropyl ether in 2% concentration, in our studies we
failed to observe this species.[4] The bands of TiO2, TiOx or that of TiOxHy were not
observed in the matrices. We speculate that any oxides of titanium formed during the
decomposition might have deposited on the inner surface of the Al2O3 oven tube which is
Table 4.4: FT-IR frequencies of matrix-isolated acetone and corresponding assignments.
literature reported
frequencies [cm-1][23]
assignment observed frequencies [cm-1]
relative intensities
529.0 (C-O) bend. 528.9 w 882.3 (CH3) wag. 882.3 w 1091.6 (CH3) wag. 1091.6 w 1216.4 (C-C) str.as. 1216.5 s 1354.0 (CH3) bend.s. 1353.9 s 1361.8 (CH3) bend.s. 1361.6 s 1406.8 (CH3) bend.as. 1406.8 w 1429.4 (CH3) bend.as. 1429.4 w 1721.5 (C-O) str. 1721.8 vs
s = strong, vs = very strong, m = medium, b = broad, sh = shoulder, w = weak, vw = very weak, vvw = very very weak
Ti(OC3H7)4 TiO2 + 4C3H6 + 2H2O (> 330 °C)
Ti(OC3H7)4 TiO2 + 2C3H6 + 2HOC3H7 (<330 °C)
Scheme 4.1
Scheme 4.2
- 130 -
hottest part of the oven and effectively failed to travel the length of the oven tube to be
trapped by the matrix. The reaction mechanism for the formation of TiO2 films using
TTIP with the help of MI-IR technique can be classified in to two different temperature
regimes.
These mechanisms are based on observation of different species in high and low
temperature region. In low temperature region the bands due to TTIP started to decrease,
and those of iso-propanol, propene and water started to increase with the temperature.
Also during our experiments iso-propanol was the first product to be detected at
temperatures as low as 100 °C. With increasing steric crowding around a-carbon the
formation of corresponding alcohol is more likely to occur as depicted in reaction scheme
4.1.
Ti
PriO O
CH3C
H
C
H
HH
O
Ti
C
C
H H
H CH3
Ti
PriO OH Ti
O
H
Scheme 4.4
C OH
H3C
H3C
Ti O
C
CH3
HCH
H H C OH
H3C
H3C H
Ti
O
C
C
HH
HH3C
(C3H7O) (OC3H7)
Scheme 4.3
H3C
CH
H3C
OH CH3C CH3
O
+ H2
- 131 -
This reaction (path 1 in scheme 4.6) possibly was dominating at low temperature region
up to 300 °C where predominantly iso-propanol could be observed in matrices. And
propene bands are weak in this temperature range. At temperatures higher than 320 °C,
both reactions (1 and 2 in scheme 4.6) might likely to have occurred as, we could observe
the bands due to propene with growing intensity in addition to bands due to iso-propanol.
It is also possible that TTIP decomposes straight to propene and iso-propanol in the
temperature range 100 to 320 °C according to reaction scheme shown in scheme 4.8.
Formation of acetone can be reasoned out in two different ways. One possibility is
through the dehydration of iso-propanol at 320 °C (scheme 4.3) leading to the formation
of acetone and hydrogen. Second possibility is the decomposition of TTIP directly into
acetone, propene and hydrogen. (scheme 4.9) During temperature range 320-390 °C
bands due to propene are the most intense ones and bands of iso-propanol were observed
to be decreased in favour of increasing intensity for water bands. The strong presence of
propene bands above 320 °C supports the fact that straight decomposition of TTIP in
toTiO2, propene and water is predominant. (scheme 4.2). One more possibility would be
that the water released in the reaction shown in reaction schemes 4.5 and 4.2 could then
react with TTIP forming iso-propanol, as shown in reaction scheme 4.7
Ti O
C
CH3
HC
HH
H
OH
HO
H
Ti O
C
C
HH
HH3C
Scheme 4.5
Ti O CH
OC 3H7 CH 3
(C3H7O)2
CH 2 H
Ti O(C3H7O)2 C3H7OH H2CHC CH 3+ +
1
2
Scheme 4. 6
- 132 -
Ti(OC3H7)4 + 2H2O TiO2 + 4 (CH3)2CHOH
Scheme 4.7
Ti(OC3H7)4 TiO2 + 2 (CH3)2CHOH + 2 C3H6
Scheme 4.8
Ti(OC3H7)4 TiO2 + CH3COCH3 + 2H2 + C3H6
Scheme 4.9
1800 1600 1400 1200 1000 800
Wavenumber [cm-1]
100 °C
A 200°C
Oven temperature
400°C
Fig. 4.5: An overview spectra of thermolysis of [Ti(OC3H7)4] (TTIP). Bands
due to TTIP disappear and product bands could be seen at higher
temperatures.
- 133 -
An important point to be considered here is the role of gas phase collisions proposed by
Griffin.[24-26] The gas phase collisions could have an important role in the reaction
mechanism. It was proposed that due to collision of TTIP molecule with another gas
phase TTIP molecule or carrier gas (argon) molecule, it gets activated. This activated
molecule can undergo any of the above mentioned reactions. During our studies we could
detect only stable species and no hints were found for existence of radicals in the
reaction. Also dilution of the precursor molecule in argon increases the mean free path
which generally hinders collision to a large extent.
1800 1600 1400 1200 1000 800
0.0
0.1
0.2
0.3
A
IP
PI
HH
P
I
P
p
IPP
HH
I
I
I I
I
A
A
AA
XX
Wave number [cm-1]
Figure 4.6: Spectrum of thermolysed [Ti(OC3H7)4] at 400 °C of oven temperature. Most
informative range 1900-800 cm-1 is represented. A: acetone, H: water, P:
propene, I: isopropanol, X: Unidentified bands.
- 134 -
4.3.12 Matrix isolation studies on mixed alkoxide complexes of titanium
The experiments to study molecular fragmentations of [Ti(OPri)2(tbaoac)2] were carried
out in the above mentioned matrix-isolation apparatus; and the matrix-isolated species
were analysed by FIIR spectroscopy. Fig. 1.6 shows the schematic of the instrument
employed in this study. The matrix-isolation apparatus can be envisioned as a small CVD
model reactor, where the Al2O3 tube of the thermolysis oven acts as a substrate. In such a
set up the possibility of coupling between the gas phase and surface reactions, which are
certainly important, cannot be ruled out. Titanium tetra isopropoxide (TTIP) and Htbaoac
are the starting compounds used for the synthesis of the precursor [Ti(OPri)2(tbaoac)2]
(refer chapter 2 for details). In order simplify the process of identifying the fragments
generated by the decomposition of the precursor, independent studies on the
fragmentation of TTIP and Htbaoac were carried out. The results of molecular
mechanism involved in the fragmentation of TTIP, which is as such one of the widely
used precursor for deposition of TiO2, are reported in detail in a previous section 4.3.1. In
this section, the fragmentation of the ligand Htbaoac and [Ti(OPri)2(tbaoac)2] precursor
are discussed.
4.3.13 The FTIR spectra of ligand tert. Butylacetoacetate
In general, ß-dicarbonyl compounds, which include diketones, ketoaldehydes, ketoacids
and their esters and amides, may exist in five tautomeric forms: the diketo form (A), two
cis-enolic forms (B and C) and two trans-enolic forms (D and E in Scheme 4.10).[27] The
interconversion of structures B and C does not require the dissociation of the
intramolecular hydrogen bond and it reduces to the migration of a proton between two
oxygen atoms. Therefore it is very fast. All other tautomeric transformations in this
system as a rule are slow processes. For ß-diketones, the non-bonded van der Waals
interactions between X and Y becomes important. The tautomeric equilibria for the ß-
diketones then favor the enol tautomers.[27] In case of these compounds, it must be also
considered whether there is an enol form present or not. There are experimental and
theoretical evidence for existence of enol form. A carbonyl group is conjugated to the
ester and the hydrogen is bonded in such a way that resonance leads to a negative charge
on the carbonyl oxygen and to a positive charge on the atom which carries the bonding
- 135 -
proton. Because of hydrogen bonding, resonance is increased, resulting in weakening of
the C=O and C=C bonds. The characteristic C=O stretching bands appear at 1750 and
1728 cm-1 for keto tautomer. The band at 2988 cm-1 originated by the CH stretching
vibration of the enol form indicates the presence of this structure. The observed
frequencies in main spectral region, 1800 to 800 cm-1 together with band assignments are
given in table 4.5.
C
O
X CH
C
O
Y
Z
O
CC
O
YX
Z
HO
CC
O
YX
Z
H
O
X
CC
Y
OH
Z
HOC
X
C O
Y
Z
A B C
D E
Scheme 4.10
4000 3500 3000 2500 2000 1500 1000 500
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
A
Wavenumber [cm-1]
Fig. 4.7: FT-IR spectrum of matrix isolated tert. Butylacetoacetate (Htbaoac).
- 136 -
4.3.14 Thermolysis of Htbaoac
Gas-phase pyrolysis of ß-ketoesters using MI-IR spectroscopy had been studied in detail
by Wentrup et al.[28-30] It was shown that ß-ketoesters fragment to give α-oxoketenes
(acylketenes) (scheme 4.11). The thermolysis reaction proceeds through the enol
tautomer b, and presumably through the intermediate c (scheme 4.11). Based on these
known results it is expected that Htbaoac fragments to give a mixture of s-Z and s-E
isomers of acetylketene and tert-butanol (R1 = But, R2 = H, R3 = Me in Scheme 4.11).
High vacuum thermolyses of Htbaoac were carried out in the temperature range of
ambient to 750 °C. While the first new IR bands were seen at temperatures as low as 150
°C, the bands belonging to starting material completely disappear at 650 °C indicating the
completion of the thermolysis. Figure 4.8 shows a representative IR spectrum of matrix-
isolated gas-phase species from a thermolysis of Htbaoac at 650 °C. The most intense IR
bands appear in the range of 2100 to 2200 cm-1 indicating the presence of ketene
intermediates. From the enlargement of the ketene CO stretching region in Fig. 4.8 it can
Fig. 4.8: Thermolysis spectrum of Htbaoac at an oven temperature of 650 °C.
A: acetone, H: water, 2mp: 2-methylpropene: P2O: 1-propene-2-ol,
gBu:gauche 1-butene, cBu: cis 1-butene, sEa: s-E-acetylketene,
sZa: s-Z-acetylketene, ack: acetylketene, tba: tert-butanol.
2000 1600 1200 800 400
0.0
0.1
0.2
ack
ack
tba
tba
P2O
tba
P2O
P2O
tba
A
A
ack
P2O
wavenumbers [cm-1]
4000 3600 3200 2800 2400
0.0
0.1
0.2
sEa
gBu
2mp
2mp
cBu
sEasE
a
2mp
2mp
H
sZa
sZa
+ tb
asZa
sZa
+ sE
a
CO
2
2mp
tba
+ P
2O
gBu
H HH
A
Wavenumber [cm-1]
- 137 -
be seen that the most intense bands are found at 2132.7 and 2142.8 cm-1. These values
match well with those published for acetylketene.[30] In accordance with the literature, the
two main IR frequencies are always accompanied by two minor IR bands at 2137.1 and
2147.6 cm-1. Based on experimental and theoretical data, Wentrup et al. could assign the
two IR bands at 2143 and 2148 cm-1 to s-Z-acetylketene and those at 2133 and 2137 cm-1
to s-E-acetylketene.[30]
Each acetylketene isomer shows a distinct pattern of IR bands. Besides the ketene CO
stretches discussed so far, the acetyl group discloses its identity by intense carbonyl
bands at 1681 cm-1 (s-Z-acetylketene) and 1698 cm-1 (s-E-acetylketene), again in perfect
agreement with the published values.[30] Furthermore, we could reproduce all published
IR frequencies of the two acetylketenes in our thermolysis experiments (Fig. 4.8 and
Table 4.6). There are frequencies between 750 cm-1and 400 cm-1 which show similar
behavior with acetylketene bands. On photolysis, the bands belonging to acetylketene
disappear. This characteristic photolysis behavior of acetylketene was helpful in
assigning these bands which were not assigned in earlier studies. The frequencies of
2160 2150 2140 2130 2120
0.2
0.4
0.6
0.821
47
2142 21
32
2137
2149
A
Wavenumber [cm-1] Fig. 4.9: Ketene region of the spectrum. Two strong bands of acetyl ketene
showing two conformers along with CO band at 2149 cm-1.
- 138 -
newly assigned bands are listed in the table 4.6. In addition to the ketene IR bands, Fig.
4.9 shows a shoulder at 2149 cm-1. Depending on the thermolysis conditions, a second
weak IR band at 2139 cm-1 appears in this region. These two IR bands are indicative for
CO trapped in argon matrices.[32] CO2 is also one of the reaction products; IR bands at
2345 and 2339 cm-1 unequivocally show its presence (Fig. 4.8). Based on the known
fragmentation of ß-ketoesters (Scheme 4.11), tert-butanol should be formed by the
fragmentation of Htbaoac. With the aid of published matrix IR data[31] we could assign
several IR bands to tert-butanol. The most intense band appears at 1213 cm-1 followed by
a band at 1139 cm-1; a complete list of assigned IR bands is given in table 4.7. Our data
match very well with those reported in the literature.[31]
Two different C4 alkenes could be identified among the thermolysis products, 2-
methylpropene and 1-butene. In accordance with the literature, the most intense IR band
of 2-methylpropene appears at 887 cm-1.[22] Other less intense IR bands could be assigned
to this alkene and they match well with reported values (see experimental section for
details).[22] 1-Butene is known to exist as cis and gauche conformers.[22] Bands due to the
gauche conformer (strong bands at 796, 631, 998, cm-1) were intense compared to the
bands of the cis conformer (554, 1444 cm-1). At oven temperature of 300 °C, bands
belonging to acetone and 1-propen-2-ol were detected. Acetone has an intense band at
O O
R3
R2OR1
O O
R2R3 OR1
O O
OR3
R2
R1HH
O
R2
CO
R3
O
R2
C
O
R3
a b c
+ + R1OH
s-Z-acylketene s-E-acylketene Scheme 4.11: Thermal decomposition of a ß-ketoester leading to two conformers of
ketenes.
- 139 -
1721 cm-1 while 1-propen-2-ol show strong absorptions at 1181 and 1002 cm-1 which
match very well with reported values.[33] While acetone bands are weak at all
thermolysis temperatures, those of 1-propen-2-ol are higher in intensity over the entire
temperature range. Water is formed during the course of the thermolysis of Htbaoac. The
intensities of IR bands indicative of matrix-isolated H2O increase with increasing
thermolysis temperatures. The intensities of those IR bands are far more intense than
those of H2O of the ‘background’. Scheme 4.12 summarises the identified species of the
vacuum thermolysis of Htbaoac.
Table 4.5: FT-IR frequencies of matrix isolated ligand Htbaoac and assignments.
observed
frequencies [cm-1]
relative
intensity†
assignment
1750 vs C=O str.
1728 vs C=O str.
1652 m
1635 m OCC enol str.
1453 s CH3 scissors
1414 m (H3)C-O str.
1394 s
1369 s CH3 in phase def.
1326 s CH2 wagging
1266 s C-O(O) str.
1206 w CH3 twisting
1180 s CH2 twisting
1148 s CCC str.
1028 m (H3)C-O str.
985 m C-C(O) str.
940 m C-CH3 str.
852 m CCO def.
801 m CCO def.
s = strong, vs = very strong, m = medium, b = broad, sh = shoulder, w = weak,
vw = very weak, vvw = very very weak
- 140 -
Table 4.6: Literature reported and experimental IR frequencies of s-Z- acetylketene and
s-E- acetylketene between 4000 and 400 cm-1
s-Z-acetylketene (sZa) s-E-acetylketene (sEa)
reported frequencies
[cm-1]
observed
frequencies
[cm-1]
intensities† reported frequencies
[cm-1]
observed
frequencies
[cm-1]
intensities†
3095 3095.4 m 3084 3084.8 m
2143 2142.8 vvs 2133 2132.7 vvs
1681 1681.3 vs 1698 1698.4 vs
1434-1421 1434-1421 br 1434-1421 1434-1421 br
1379 1379.1 w 1365 1364.7 s
1345 1345.0 s 1343 1344.4 s
1169 1169.6 s 1221 1221.0 s
1015 1015.0 w 1081 1081.0 w
956 955.9 m 1021 1021.3 w
889 889.0 w 997 997.0 m
663.3 (m), 661 (m), 636.5 (m), 641.2 (s), 600.9 (s), 607.5 (w), 528.8 (m), 512.3 (s), 497 (m), 488 (w), cm-1,
Frequencies from 750 to 400 cm-1are assigned based on photolysis studies and quantum mechanical
calculations (see text for details). s = strong, vs = very strong, m = medium, b = broad, sh = shoulder, w =
weak, vw = very weak, vvw = very very weak
O O
O
O
C
O
HO
H
CO
H3CH3C
H3C
H3CO
H3C
H2C
OH
H2O CO CO2
H2C CH
H3C CH2H2C CH
H3C
CH2
H3C
C
H3C
CH2
H3CC
H3C CH3
OH
Scheme 4.12: High vacuum thermolysis products of Htbaoac at 650 °C
- 141 -
Table 4.7: List of reported IR frequencies of tert. butanol in argon matrices compared with observed frequencies from this work.
reported
frequencies [cm-1]
observed frequencies
[cm-1]
intensity† assignment
3626.5 3626.4 s 3622.3 3622.3 m
(OH) str.
2988.0 2988.0 b 2973.1 2972.8 s 2942.7 2942.7 m
(CH3) str.s.
2907.6 2907.6 s (CH3) str.s. 2888.5 2888 vvw 2875.5 2875 vvw
(CH3) str.s.
1490 Not Observed vw 1476.6 1476 sh (CH3) def.as. 1469.3 1469 sh (CH3) def.as. 1464.4 1464 w (CH3) def.as. 1450 1450 m (CH3) def.as.
1391.6 1391 sh 1372.5 1371.8 s
1367.1 1367 sh
(CH3) def.s.
1328.2 1328 b (OH) def. 1241.5 1241 sh 1213.5 1213.5 vs
(CCC) str.as.
1183 Not observed w (assoc) str.s. 1145.1 Not observed sh 1139.5 1139.5 vs
(CO) str.s.
1027.1 1027 vw 1019 1019 s
(assoc) str.s.
1013.4 1013.4 m 921.2 Not observed s
(CH3) def.as.
914.6 914 b (CH3) def.s. 747.8 747 vw 745.9 745 w
(CCC) str.s.
461 Not observed vw 456 456 w
(CCO) def.as.
418 418 vw (CCC) def.s.
s = strong, vs = very strong, m = medium, b = broad, sh = shoulder, w = weak,
vw = very weak, vvw = very very weak
- 142 -
Table 4.8:List of reported IR frequencies of 2-methylpropene in argon
matrices compared with observed frequencies from this work.
literature reported
frequency [cm-1]
observed frequency
[cm-1] assignment
3085 3019
3085 3019 CH str.
2942 2983 2996
2942 2983 2995
CH3 asym str.
2884 2893
2884 2892 CH3 asym def.
2871 2860
2871 2860 CH3 sym str
1655 1655 C=C str 1442 1461
1442 1461 CH3 asym def.
1377 1383
1377 Not observed CH3 sym def.
1416 887
1278
1415.8 887
1278.4 CH2 wag.
1053 1058 1141
Not observed 1058
Not observed CH3 rock
970 802
970.3 801.8 CC str.
O(RO)2Ti
O
R
CR1R2
CH2
H
(RO)2Ti=O + ROH + CH2=CR1R2
Scheme 4.13
- 143 -
4.3.15 Thermolysis of [Ti(OPri)2(tbaoac)2]
Thermolysis experiments with the complex [Ti(OPri)2(tbaoac)2] were performed between
100 to 600 °C. Thermolysis products could be seen already at oven temperatures as low
as 200 °C. Bands assigned to the starting material disappeared completely at an oven
temperature of 500 °C, indicating the completion of thermolysis.
Fig. 4.10 shows a typical IR spectrum of matrix-isolated species from the thermolysis of
[Ti(OPri)2(tbaoac)2] at 500 °C. Like in the case of the ligand Htbaoac, intense IR bands at
2143, 2133, 1681, and 1698 cm-1 clearly show that a mixture of s-Z- and s-E-acetylketene
has formed. However, the similarity between the fragmentation of [Ti(OPri)2(tbaoac)2]
and Htbaoac goes beyond the ketene formation. All species that have been identified in
the case of the free ligand were found in matrices of the thermolysis experiments of the
Table 4.9: List of reported IR frequencies of But-1-ene in argon matrices compared
with observed frequencies from this work.
literature reported
frequency [cm-1]
Gauche Cis
observed frequency
[cm-1]
Gauche Cis
assignment
2886 2889 2886 br 2889 br Str sym CH2
1439 1444 1439 sh 1444 br CH2 bend
1414 1421 1415 1421 CH2 bend
1316 1323 1316.5 1323 br CH2 wag
1262 1258 1262.58 1256.78 CH2 twist
1175 1180 1175 sh 1180 sh CH2 rock
1076 1128 Not observed Not observed CC str
998 976 998 976 CH=CH wag
796 835 796 835 CH2 rock
631 554 631 554 =CH twist
- 144 -
Ti precursor too; i.e. acetone, 1-propen-2-ol, cis and gauche 1-butene, 2-methylbutene,
tert-butanol, CO, CO2, and H2O (see assignments in Fig. 4.10).
A new IR absorption at 3639 cm-1, right in the area for O-H stretches, hinted to a
formation of an alcohol and we speculated that iso-propanol was formed. As reported in
section 4.3, in order to obtain reliable IR data of iso-propanol , we measured IR spectra of
iso-propanol in solid argon in a separate set of experiments (refer experimental section).
Based on these data, we could identify iso-propanol among the thermolysis products by
the set of its four most intense IR bands at 3639, 2988, 1253, and 949 cm-1. In addition to
iso-propanol, we found a second new product, namely propene. The most intense IR band
of this alkene appears at 908 cm-1. A list of detected IR absorptions for propene is given
2000 1600 1200 800 400
0.0
0.1
0.2
0.3
A
sZa
wavenumbers [cm-1]
4000 3600 3200 2800 2400
0.0
0.1
0.2
0.3
CO
2
2mp
+ tb
a
P2O
+ IP
+ P
tba2m
p
ack
tba
+ sE
a +
P
IPIP
2mp
P
ack
sEa
IP
sZa
A
sZa
sEa
gBu
sZa
+ sE
a
H
H H
H
H IP
Fig. 4.10: Thermolysis spectrum of [Ti(OPri)2(tbaoac)2] at an oven temperature
of 500 °C. A: acetone, H: water, P: propene, 2mp: 2-
methylpropene: IP: isopropanol, P2O:1-propene-2-ol, gBu: gauche
1-butene, cBu, cis 1-butene, sEa: s-E-acetylketene, sZa: s-Z-
acetylketene, ack: acetylketene, tba: tert-butanol.
- 145 -
in the experimental section, which is in good agreement with published values.[22] IR
bands at 757, 1598, 1694, 1820, and 2088 cm-1 are relatively weak but show typical half-
band widths of matrix-isolated small molecules. Up to date, we could not assign these IR
bands to any species.
Table 4.10: List of reported IR frequencies of 1-propen-2-ol in argon matrices compared with observed frequencies from this work.
literature reported
frequencies
[cm-1]
observed
frequencies
[cm-1]
assignment
3622 3622 OH str
2992 2992 CH2 asy. Str
2978 2978 CH2 sym str
2950 2948 CH2 asym str
2921 2922 CH3 asym str
2835 2837 CH3 sym str
1673 1673 C=C str
1466 1465 CH3 def
1439 1439 CH3 def
1379 1379 CC str.
1331 1331 CO str.
1181 1181 OCC asym str.
1050 Not observed CH3 rock
1002 1002 CH3 rock/CO str
963 963 CH2 def
848 848 CH2 def
821 Not observed OCC sym str.
697 Not observed C=CH2 torsion
494 494 CCOC framework
478 478 OH wag
395 395 C-OH torsion
- 146 -
The singularly important task is to rationalize the formation of the detected species.
Matrix-isolation FTIR spectroscopy allows the identification of intermediates, but not the
reaction mechanism, which must therefore remain speculative. The fragmentation of the
ligand Htbaoac seems to follow the known fragmentation pattern of β-ketoesters,
resulting in acetylketene and tert-butanol (Scheme 4.11).[28-30] A second fragmentation
path is illustrated in scheme 4.14. It is feasible that the ester Htbaoac directly eliminates
2-methylbutene to give acetyl acidic acid. We could not identify the latter species, which
presumably undergoes a fast decarboxylation reaction to the detected CO2, acetone, and
1-propene-2-ol. Additionally, the intermediate acetyl acidic acid might eliminate water to
give acetylketene.
Wentrup et al. found that the ease of fragmentation of β-ketoesters correlates with the
availability of the enol form b (scheme 4.11).[29] The Ti precursor [Ti(OPri)2(tbaoac)2]
already contains the β-ketoester in enol form. In analogy to the fragmentation depicted in
schemes 4.11 and 4.14, a possible fragmentation of the Ti precursor is illustrated in
scheme 4.15.
There are several possible explanations for the formation of iso-propanol and tert-butanol
in the thermolysis of [Ti(OPri)2(tbaoac)2]. Hydrolysis of a [Ti]-O-R complex with gas-
phase water or surface OH groups would result in an elimination of ROH. Additionally,
O
O O
H
O
O OH
- CO2
O OH
- H2O
O
CO
Scheme 4.14: Proposed fragmentation of Htbaoac, resulting in acetylketene and
2-methylpropene.
- 147 -
the proposed titanium complex d (Scheme 4.15, route B), which contains a carboxyl
group, could intramolecularly undergo hydrolysis to eliminate one equivalent of ROH.
The flash vacuum pyrolysis of titanium complexes of the type [Ti(OR)4] has been
investigated using glass and quartz tubes at 550 and 700 °C.[4] Volatile reaction products
were collected in a liquid-N2 trap and subsequently analysed by NMR, GC, and GC-MS
techniques.
The isopropyl complex [Ti(OPri)4] was found to decompose at 550 °C into propene, iso-
propanol, small amounts of acetone and diisopropyl ether. At 700 °C the fragmentation
seems to be cleaner resulting only in two products propene and iso-propanol. For the tert-
butyl compound [Ti(OBut)4] at both pyrolysis temperatures 2-methylbutene and tert-
butanol were the only volatile products. Obviously, all titanium complexes of our
investigation that exhibit at least two alkoxy groups could fragment according to scheme
4.13. Of course, this includes the proposed intermediate titanium compounds of scheme
4.15. There is neither hints nor spectroscopic evidences for the trapping of TiO2 in the
matrices. We speculate, that Al2O3 oven used in the experiment acts as substrate, where
O
O O
[Ti][Ti] O
O
CO
[Ti] = Ti(OPri)2(tbaoac)
path A
O
O O
[Ti]path B
H
O
O O
[Ti]H
d
- CO2
[Ti] O
Scheme 4.15: Proposed fragmentation pathways of Ti(OPri)2(tbaoac)2]
- 148 -
TiO2 could be deposited. We did not find any indications that free radicals like R or RO
play a role in the process. The predominant formation of alkenes and alcohols was
interpreted in form of equation (4.13).
4.3.16 The FTIR spectra of ligand 2,2,6,6,-tetramethyl-3,5-heptane dione (Hthd)
As mentioned in section 4.3.12, ß-diketones may exist in five tautomeric forms. Nyquist
has reported that carbon oxygen absorption band of the chelate form of ß-diketones
occurs in the region 1606-1620 cm-1 accompanied by much weaker absorption between
1712 and 1720 cm-1 assigned to the carbon oxygen absorption of the ß-diketonate
structure.[34] The FT-IR spectrum of the matrix isolated Hthd ligand is shown in figure
4.11.
The spectrum is void of absorption bands in the region associated with keto form. Instead
a strong absorption (multiplet) is found centered at 1621 cm-1. It is seen clearly that the
ligand Hthd exists mainly in enolic form as evident by the strong bands at 1621 and 1601
cm-1. A less intense band at 1727 cm-1 represents the ß-diketone form of the ligand. The
band at 2988 cm-1 originated by the CH stretching vibration of the enol form indicates the
presence of this structure.
O OO O
H
Scheme 4.16
- 149 -
4.3.17 Thermolysis of ligand Hthd
The ligand Hthd has been previously studied by other groups for thermal decomposition
using matrix isolation techniques. But the focus has been to investigate the chemistry of
pivaloyl ketenes generated during such a reaction. [35] In addition, Wentrup et al. reported
the flash vacuum pyrolysis of the a-tert-butyl-ß-ketoesters resulting in the formation of
pivaloyl ketenes.[29] But a clear mechanism involving the a-oxoketenes is not reported so
far. Based on known results so far we understand that the ß-diketonates with a-tert-butyl
groups exist predominantly in enol forms and decompose only sluggishly to afford tert-
butyl a-oxoketenes. High vacuum thermolyses of Hthd were carried out in the
temperature range of ambient to 1000 °C. It was found that Hthd is a stable molecule
compared to ß-ketoester ligand Htbaoac. There were no thermolysis products till the oven
temperature of 400 °C and spectra indicate the presence of intact molecule up to 400 °C.
New IR bands could be seen at 500 °C along with the bands of starting material Hthd.
4000 3500 3000 2500 2000 1500 1000 500
0.0
0.5
1.0
1.5
2.0
2.5
3.0
A
Wavenumber [cm-1] Fig. 4.11: FTIR spectra of matrix-isolated ligand 2,2,6,6,-tetramethyl-3,5-heptane
dione (Hthd)
- 150 -
Even at 1000 °C the bands belonging to starting material appear in mid level intensity
and assignments of the bands are hindered to some extent. Due to limitations posed by
the overheating of the oven element and necessity to maintain the matrix at low
temperatures, further experiments with higher temperatures could not be performed.
Figure 4.12 shows a representative IR spectrum of matrix-isolated gas-phase species from
a thermolysis of Hthd at 1000 °C. The most intense IR bands appear in the range of 2100
to 2200 cm-1 indicating the presence of ketene intermediates. From the enlargement of the
ketene CO stretching region in Fig. 4.12 it can be seen that the most intense bands are
found at 2142.7 and a shoulder with lower intensity at 2134.6 cm-1. In accordance with
the literature, these two main IR frequencies are always accompanied by two minor IR
bands at 1667 and 1681 cm-1. Based on experimental and theoretical data,[29] R. L.
Toung, et al. could assign the two IR bands at 2142.7 and 2134.6 cm-1 to s-Z-
pivaloylketene and s-E-pivaloylketene respectively. It was observed that s-Z-
pivaloylketene is more favored conformer and hence bands belonging to s-Z-
pivaloylketene were observed in higher intensities compared to those of s-E-
pivaloylketene.[29]
O O
O
H
C
O
O
H
CO
CH2
C
H3C CH3
H3C
HC CH2
(4.17)s-E-pivaloylketene s-Z-pivaloylketene
Scheme 4.17: Thermolysis products of Hthd ligand at 1000 °C.
- 151 -
There are no published data over whole mid IR range for pivaloylketene. The bands of
pivaloylketene in the ketene region reported by the thermolysis of the standard ß-
ketoester ligand and those generated by the thermolysis of Hthd ligand found to match
very well. DFT calculations were carried out using Gaussian 98 [37] on s-E-pivaloyketne
and s-Z-pivaloylketne employing B3LYP/6-31 G(d) level calculations and simulated
spectra are compared with observed spectrum. The comparison of observed and
simulated spectra is given in table 4.11. Besides pivaloylketene, 2-methylpropene was
observed in high intensity at a temperature of 500 °C. The temperature range from 500
°C to 900 °C is dominated by the bands of these two species. At 1000 °C, bands due to
propene could also be detected.
2200 2000 1800 1600 1400 1200 1000 800 600 400
0.0
0.1
0.2
0.3
0.4
0.5
0.64000 3800 3600 3400 3200 3000 2800 2600 2400 2200
0.0
0.1
0.2
0.3
0.4
0.5
* ** *
Wavenumbers [cm-1]
p pk
X X X
2mp
p
CO
2
p
H
H
2mp
2mp
ppk
2mp
p
H
H
pk
2mp
+pk
+ p
2mp
2mp
+ p
pk
pk +
CO
A
Fig. 4.12: Thermolysis spectrum of Hthd at an oven temperature of 1000 °C.†
(* indicates the bands due to starting material)
H: water, P: propene, 2mp: 2-methylpropene: pk-pivaloylketene, X-
unidentified bands
- 152 -
4.3.18 Thermolysis of [Ti(OPri)2(thd)2]
A series of high-vacuum thermolyses of [Ti(OPri)2(thd)2] was carried out. Between
ambient temperature and oven temperatures of 400 °C the IR spectra of the isolated
complex remains relatively unchanged. Around 500 °C of oven temperature, new IR
bands appeared, indicating the beginning of the thermal decomposition. Thermolyses
experiments up to 1000 °C were carried out. Even at this temperature the bands due to
starting material still exist but their intensities are lowered. As mentioned above, due to
limitations posed by operating at higher temperatures, further high temperature
experiments were not carried out. But data collected up to 1000 °C convincingly provides
the evidence of pivaloyl ketene as intermediate formed during thermal decomposition.
Right from 500 °C up to 1000 °C the bands due to pivaloylketene could be detected in
varying intensities.
2150 2145 2140 2135 2130
0.0
0.1
0.2
0.3
0.4
2134
.6
2149
2138
.7
A
Wavenumbers [cm-1]21
42.7
Fig. 4.13: Ketene region of the spectrum, the most intense band at 2142.7 cm-1 is
assigned to s-Z-pivaloylketene. Bands due to CO2 can be seen at 2138.7
and 1249 cm-1. The weak band at 2134.6 is assigned to s-E-
pivaloylketene.
- 153 -
Table 4.11: B3LYP/6-31 G(d) level calculations of frequencies pivaloylketene conformers
and comparison with observed frequencies
IR frequencies s-Z-pivaloylketene
(cm-1)
IR intensities (km/mol)
IR frequencies s-E-
pivaloylketene (cm-1)
IR intensities (km/mol)
Observed frequencies for pivaloylketene
(cm-1)
Relative intensity
556.8 16.1 576.8 29.2 432.6 w
588.1 27.6 768.6 16.3 612 w
756.1 10.2 1041.0 30.6 629.1 w
896.3 11.9 1094.8 101.7 637.8 w
1002.9 103.8 1157.3 91.2 658.4 w
1074.5 64.2 1279.0 15.3 661.3 s
1391.7 255.9 1377.7 75.4 663.2 s
1434.6 17.1 1507.2 11.0 901 w
1507.1 10.7 1517.6 23.3 916 w
1517.6 33.4 1718.7 417.4 932.5 w
1710.9 218.2 2204.4 816.7 997.8 s
2223.1 1075.3 3028.7 23.2 1058.5 m
3024.4 24 3030.6 13.4 1171.9 w
3026.9 18.9 3039.2 21.8 1207 w
3037.9 24.2 576.8 29.2 1230.2 w
3089.8 33.5 768.6 16.3 1355.1 w
3097.7 49.6 1041.0 30.6 1362.7 w
3104.2 33.2 1094.8 101.7 1373.8 m
1380.3 m
1439.3 w
1667 w
1681 w
2134.6 m
2142.7 vvs
3036.5 m
3064.7 m
- 154 -
2000 1800 1600 1400 1200 1000 800 600 400
0.00
0.04
0.08
0.12
4000 3600 3200 2800 2400 2000
0.00
0.04
0.08
0.12
Wavenumbers [cm-1]
pk
IP +
p2m
p
pk
IP
p
2mp
?
X
X
X
X
2mp
A
IP +
pk
IP
cBuIP
AIPA
CO2
p
gBu
2mp
p
IP
p A
IP
IPIP
pk &CO
pk
pk
HH
H
HH
**
A
Fig. 4.14: Thermolysis spectrum of [Ti(OPri)2(thd)2] at an oven temperature of
1000 °C. (* indicate the bands due to starting material) A: acetone, H: water,
P: propene, 2mp: 2-methylpropene: IP: isopropanol, P2O: 1-propene-2-ol,
gBu: gauche 1-butene, cBu, cis 1-butene, pk-pivaloylketene, X-unidentified
bands.
The most intense bands observed were those of iso-propanol; the most intense band of
iso-propanol was detected at 1253.4 cm-1 and other bands due to iso-propanol listed in
table 4.2. Two different C4 alkenes could be identified among the thermolysis products,
2-methylpropene and 1-butene. 2-methylpropene has intense band at 887 cm-1. Also other
bands assigned to 2-methylpropene show high intensity throughout the thermolyses
series. Bands due to 2-methylpropene are listed in table 4.8. The ketene region of the
spectrum shows the bands due to pivaloylketene appearing at 2142.7 and 2134.6 cm-1.
These bands are accompanied by bands at 2138.7 and 2149 cm-1 which are due to CO
trapped in matrices. The intense bands of the spectrum are at 2345 and 2339 cm-1 which
are assigned for CO2 trapped in matrix. Propene was observed at 100 °C and it has
- 155 -
intense band at 908.7 cm-1 and other bands due to propene are listed in table 4.3. The
intense band assigned to acetone is detected at 1721 cm-1. Other bands due to acetone are
listed in table 4.4. Small amounts of cis-butene and trans-butene were also detected
though all the bands due to these species were not detected; the most intense ones were
clearly visible (refer table 4.9).
The photolysis experiments were carried out along the temperature series to identify the
bands belonging to pivaloylketene. But under different photolysis conditions the new
bands started to appear in the ketene region which increased the complexity of the spectra
and bands due to conformers of pivaloylketene could not be separated as done in case of
acetylketenes. In addition the bands due to other products started to disappear as well
with increasing number of new bands throughout the spectrum. In order to rationalize the
formation of detected species during the thermolysis of the [Ti(OPri)2(thd)2], the
understanding of the structural aspects of the titanium complexes is necessary. Mixed
alkoxide complexes of titanium with ß-diketonate chelating ligands are widely studied. It
has been established that the alkoxy ligands induce a strong trans-effect on the chelating
ligands thus ß-diketonates form a shorter bite angles to the titanium center. One of the
two titanium-oxygen bonds is shorter than the other in the chelate ring. This elongation of
the bond occurs trans to alkoxy ligand attached to the metal center. We speculate that,
this bond is susceptible for cleavage during thermal decomposition. As a result, the
chelate ring opens up and eliminates pivaloylketene and isobutene in the process. The
cleavage of second titanium–oxygen bond leaves behind highly reactive Ti(III) center.
This [R1]2[R2] Ti(III) complex has highly reactive Ti(III) center and we reason out that it
gets easily hydrolyzed by gas phase water or surface OH groups leading to the formation
of iso-propanol.
As experienced in the earlier case there are neither hints nor spectroscopic evidences for
the trapping of TiO2 in the matrices. We speculate, that Al2O3 oven used in the
experiment acts as substrate, where TiO2 could be deposited. Also we could not find any
indications that free radicals like R or RO play a role in the process. The predominant
formation of alkenes and alcohols was interpreted in form of equation (4.13). During our
studies at 100 °C we could observe the propene and formation of propene by thermal
- 156 -
decomposition of 2-methylpropene at high temperatures has already been reported
through a radical mechanism proposed in scheme 4.18.[36]
The pivaloylketene intermediate offers a new insight in to the precursor chemistry of
Hthd ligand. The observations during the thermoylses indicate the formation of
pivaloylketene as intermediate over a temperature range of 500 to 1000 °C. The
annealing of matrices to 28 K lead to increase in intensity of CO and CO2 bands at the
cost of bands due to pivaloylketene. This strongly indicates the nature of intermediate
pivaloylketene which is short-lived species in the gas phase. Also this study sheds some
light on the role of Hthd ligand in titanium precursor chemistry. The most plausible
pathway for the decomposition of [Ti(OPri)2(thd)2] is shown in scheme 4.17. The Hthd
ligand not only provides high stability by chelating to the metal center but also on
thermal decomposition, it helps in reducing the metal center and rendering it more
reactive.
O O
O O
H H
-H
O
CO
H
CH3
CH3
C HH
H
-H
CH2
H3C CH3
Scheme 4.16: Decomposition mechanism of Hthd ligand through the formation
of 2-methylpropene and pivaloylketene intermediates.
- 157 -
[R1]2[R2]Ti O C
C
CH
C
O
C
[R1]2[R2]Ti O C
C
CH
CO
C
[R1]2[R2]Ti(III)O C
C
CH
CO
O
C
C CC
O
H
R1 = OPri , R2 = thd[R1]2[R2]Ti
O
O
++
-H
Scheme 4.17: Decomposition mechanism of the complex [Ti(OPri)2(thd)2]
through the formation of pivaloylketene intermediates.
- 158 -
Band notations:
Following notations were used in the above figures:
† A: acetone, H: water, P: propene, 2mp: 2-methylpropene: IP: isopropanol, P2O: 1-
propene-2-ol, gBu: gauche 1-butene, cBu, cis 1-butene, sEa: s-E-acetylketene, sZa: s-Z-
acetylketene, ack: acetylketene, tba: tert-butanol.
Band intensities: s (strong), vs (very strong), br (broad), w (weak), vw (very weak), m
(medium).
H3C
H3C
CH2 H3C + H2C C CH3
H2C C CH3
H3C
H3C
CH2 +
H2CHC CH3
+
H2C C CH2
CH3
Scheme 4.18: High temperature decomposition of 2-methylpropene to form
propene. No radical intermediates were detected as reported in the
literature.[36]
- 159 -
4.4 Summary
The use of matrix isolation coupled with FTIR as a tool for studying the fragmentation of
the MOCVD precursor is demonstrated. To investigate the molecular mechanism
involved in the decomposition of precursor a matrix isolation apparatus was designed and
fabricated. The smaller dimensions of the thermolysis oven used in the present study and
the need for using low pressures in matrix isolation set up increases the Knudsen number
to be at 1532.28 which clearly indicate the heterogeneous surface reactions taking place
inside the oven surface. There is unavoidable coupling between homogeneous gas phase
reactions and heterogeneous surface reaction in such a system.
The thermolysis studies on selected titanium precursors were carried out using MI-IR
techniques. Titanium tetraisopropoxide (TTIP), [Ti(OPri)2(thd)2] are the standard
precursors used for the deposition of TiO2 thin films and newly developed precursor
[Ti(OPri)2(tbaoac)2] were the three systems investigated for thermal decomposition using
MI-IR techniques. The main aim of employing the MI-IR technique to above mentioned
systems is to provide qualitative information about the gas phase during thermal
decomposition of titanium precursors under isolated conditions.
TTIP has been studied for decomposition using several methods and it was found that
iso-propanol is one of the thermolysis products. In the absence of any data on matrix
isolated iso-propanol in the mid IR range, we had used glass balloon method of dilution
to get FTIR spectrum of matrix isolated iso-propanol. Based on this spectrum the iso-
propanol formed during the thermal decomposition of TTIP and other alkoxide
complexes, we could identify IR peaks belonging to iso-propanol.
Thermolyses experiments of TTIP were carried out between ambient temperature and
600 °C. A small amount of iso-propanol was found at temperatures as low as 100 °C. The
most dynamic changes were observed during temperature of 300-400 °C and hence this
range was studied in steps of 20 °C. The themolysis was complete at 390 °C indicated by
the disappearance of bands due to starting material. Three different temperature regimes
where different products could be seen were identified. Up to 300 °C bands due to iso-
propanol were predominant and the bands due to propene were in low intensity. Above
- 160 -
320 °C of oven temperature bands due to propene intensified with increasing temperature
till 400 °C. In addition to iso-propanol and propene, bands due to acetone appear from a
temperature of 320 °C and persist along with propene bands till 390 °C. Water was
observed as one of the reaction products indicated by the increase in the intensity of
bands compared to background water of the matrix. There was no hint for possible
trapping of TiO2 in the matrices. It was speculated that TiO2 was deposited on the inner
wall of the matrix oven and so it could not have been trapped in the matrices. Based on
these observations a probable mechanism for the decomposition under different
temperature regimes was proposed.
Matrix isolation studies on the thermolyses of newly developed precursor
[Ti(OPri)2(tbaoac)2] were carried out from ambient to 750 °C. In order to identify the
bands due to different products, a series of thermolyses experiments with ligand Htbaoac
was carried out. It was found that the ligand Htbaoac undergoes thermal decomposition
forming acetylketene intermediates (having the most intense bands in the region 2120-
2150 cm-1) and tert. Butanol. It was found that the acetylketene has two conformers s-E-
acetylketene and s-Z-acetylketene having almost equal energies. So, bands due to these
conformers were detected in almost equal intensities. Other thermolyses products like cis
and trans butane, 2-methyl propene and acetone, 1-propen-2-ol were also observed which
were not reported in earlier studies. Having known the decomposition products of parent
alkoxide TTIP and the ligand Htbaoac it was relatively easy for analysis spectra
generated by the thermolyses of complex [Ti(OPri)2(tbaoac)2]. Based on these
observations the plausible mechanism for the decomposition of the complex
[Ti(OPri)2(tbaoac)2] was proposed. Though there were few bands which could not be
assigned to any species, we failed to assign them to single isolated species. In addition,
we have not observed any bands due to free radicals. Thermoylses of the well known
titanium precursor complex [Ti(OPri)2(thd)2] were carried out. It was thought that a
comparative study on the decomposition of well known precursor and the newly
developed precursor would help us to improve the understanding of the designing issues
in terms of inclusion of ester moieties to well known ß-keto systems.
As a supplementary experiment, the ligand Hthd was thermolysed in a series of
experiments from ambient to 1000 °C. The ligand Hthd started to decompose only at 500
- 161 -
°C. The thermolysis is not complete till 1000 °C where we could detect bands due to
starting material along with bands due to the products. The most intense bands due to
products appear in the range 2120-2150 cm-1 and it was found that pivaloyl ketene has
been formed as an intermediate. The most intense bands at 2142.7 cm-1 and 2134.6 cm-1
were assigned to two different conformers of pivaloylketenes namely s-Z-pivaloylketene
and s-E-pivaloylketene. In addition to ketenes, the thermolysis experiments resulted in
the formation of 2-methylpropene and propene were detected.
The complex [Ti(OPri)2(thd)2] was studied for thermal decomposition from ambient to
1000 °C. Up to an oven temperature of 400 °C, the spectrum of the [Ti(OPri)2(thd)2]
remains relatively unchanged. Bands due to products could only be seen at temperatures
above 500 °C. The most intense bands are assigned to pivaloylketene, iso-propanol, 2-
methyl propene, propene, and acetone. Several bands due to starting material were
detected even at temperature of 1000 °C. Based on the above observations the tentative
mechanism involving the cleavage of metal to oxygen bond of the thd ligand and
subsequent formation of pivaloylketene intermediates were proposed.
- 162 -
4.5 References
[1] M. L. Hitchman, K. F. Jensen, Chemical vapor deposition-Principles and
applications, Academic Press, 1992
[2] B.-O. Cho, J. Wang, J. P. Changa, J. Appl. Phys. 2002, 92, 4238.
[3] N. Dietz, H. Born, M. Strassburg, V. Woods, Mat. Res. Soc. Symp. Proc. 2004,
798, Y10.45.1.
[4] M. Nandi, D. Rhubright, A. Sen, Inorg. Chem. 1990, 29, 3065.
[5] S.-I. Cho, C.-H. Chung, S. H. Moon, J. Electrochem. Soc. 2001, 148, C599.
[6] S. Cho, -I., C.-H. Chung, S. H. Moon, Thin solid films 2002, 409, 98.
[7] C. P. Fictorie, J. F. Evans, W. L. Gladfelter, J. Vac. Sci. Technol. A 1994, 12,
1108.
[8] J. P. A. M. Driessen, J. Schoonman, K. F. Jensen, J. Electrochem. Soc. 2001,
148, G178.
[9] I. R. Dunkin, Matrix-isolation techniques, Oxford University Press, Oxford New
York Tokyo, 1998.
[10] Fang Yan, PhD. Thesis, Drexel University, 2003.
[11] S.-I. Cho, C.-H. Chung, S. H. Moon, Thin solid films 2002, 409, 98.
[12] P. Comba, H. Jakob, B. Nuber, B. K. Keppler, Inorg. Chem. 1994, 33, 3396.
[13] A. C. Jones, J. Mater. Chem. 2002, 12, 2576.
[14] A. E. Turgambaeva, V. V. Krisyuk, S. V. Sysoev, I. K. Igumenov, Chem. Vap.
Deposition 2001, 7, 121.
[15] Stephen Cradock , A. J. Hinchcliffe, Matrix isolation : a technique for the study of
reactive inorganic species Cambridge Univ. Press, 1975.
[16] C. G. Barraclough, D. C. Bradley, J. Lewis, I. M. Thomas, J. Chem. Soc. 1961,
2601.
[17] O. Poncelet, J. C. Robert, J. Guilment, Mater. Res. Soc. Symp. Proc. 1992, 271,
249.
[18] V. A. Zeitler, C. A. Brown, J. Phys. Chem. 1957, 61, 1174.
[19] J. Livage, M. Henry, C. Sanchez, Prog. Solid State Chem. 1989, 18, 259.
- 163 -
[20] D. C. Bradley, R. C. Mehrotra, P. D. Gaur, Metal alkoxides 1978, Academic
press: New York, NY, 1978. p 63.
[21] P. D. Moran, G. A. Bowmaker, R. P. Cooney, Inorg. Chem. 1998, 37, 2741.
[22] A. J. Barnes, J. D. R. Howels, Faraday Transactions 1973, 69, 532.
[23] A. Engdahl, Chem. Phys. 1993, 178, 305.
[24] K. L. Siefering, G. L. Griffin, J. Electrochem. Soc. 1990, 137, 814.
[25] K. L. Siefering, G. L. Griffin, J. Electrochem. Soc. 1990, 137, 1206.
[26] Q. Zhang, G. L. Griffin, Thin Solid Films 1995, 263, 65.
[27] M. M. Schiavoni, H. E. Di Loreto, A. Hermann, H.-G. Mack, S. E. Ulic, C. O.
Della Vedova, J. Raman Spectrosc. 2001, 32, 319.
[28] B. Freiermuth, C. Wentrup, J. Org. Chem. 1991, 56, 2286.
[29] R. L. Toung, C. Wentrup, Tetrahedron 1992, 48, 7641.
[30] C. O. Kappe, W. M.W., C. Wentrup, J.Org.Chem. 1995, 60, 1686.
[31] J. K.-À. Tommola, Spectrochimica Acta 1978, 34A, 1077.
[32] H. Dubost, L. A. Marguin, Chem. Phys. Lett. 1972, 17, 269.
[33] X. K. Zhang, J. M. Parnis, E. G. Lewars, R. E. March, Can. J. Chem. 1997, 75,
276.
[34] R. A. Nyquist, The Interpretation of Vapor Phase Infrared Spectra, Group
Frequency Data, Vol. 1, Sadtler Research Laboratories, Philadelphia,
Pennsylvania, 1984.
[35] V. A. Nikolaev, Y. Frenk`h, I. K. Korobitsyna, J. Org. Chem. USSR 1978, 14,
1338, 1147, 1433.
[36] M. Szwarc, The Journal of chemical physics 1949, 17, 292.
[37] Gaussian 98, Revision A.11.1, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E.
Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery,
Jr., R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K.
N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B.
Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y.
Ayala, Q. Cui, K. Morokuma, P. Salvador, J. J. Dannenberg, D. K. Malick, A. D.
Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, A. G.
Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.
- 164 -
Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A.
Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W.
Wong, J. L. Andres, C. Gonzalez, M. Head-Gordon, E. S. Replogle, and J. A.
Pople, Gaussian, Inc., Pittsburgh PA, 2001.
- 165 -
Chapter 5
Thermal properties and hydrolysis behavior of newly
developed titanium precursors Abstract Rationally developed precursors have been screened for their thermal properties in detail.
The newly developed precursors were analyzed by simultaneous TG-DTA analyses. The
simultaneous TG-DTA was performed under ambient of nitrogen and under normal
pressure. The onset of volatilization of the precursors were found to be between 80-150
°C which are lower compared to commercially available titanium precursor
[Ti(OPri)2(thd)2]. Clean volatilization was observed mostly in single step with small
amount of residue being left behind.
The sublimation rates of three precursors [Ti(OPri)2(tbaoac)2] (A), [Ti(OEt)2(tbaoac)2]
(B), [Ti(OPri)2(meaoac)2] (C), which were used for TiO2 depositions using home built
CVD reactor were carried out using isothermal studies. The precursor
[Ti(OPri)2(tbaoac)2] (A) was studied for shelf life using TG analyses and it was found
that residue left behind after TG increased with ageing.
Hydrolytic stability of the selected ß-ketoester complexes was investigated using NMR as
an analytical tool. A comparison to standard ß-diketonate complexes with similar ligand
system was done. It was found that ß-keto ester complexes are susceptible for hydrolysis
at a faster rate compared their ß-diketonate analogues.
- 166 -
5.1 Introduction
Precursors play a crucial role for the development of materials by a CVD process. From
the point of view of precursor chemistry, the interaction at the molecular level which
influences the precursor purity, volatility and long term stability as well as reactivity and
decomposition kinetics needs to be investigated. The problem associated with the CVD
of multicomponent oxides is the combination of individual precursors as they have large
difference in their thermal and chemical reactivity. It is necessary to design evenly
matched individual precursor with regard to their thermal properties. The basic thermal
properties essential for a molecular precursor are discussed briefly in following sections.
5.1.1 Volatility
The volatility of a compound is a complex function of intermolecular forces (van der
Waals interactions, pi-stacking or hydrogen bonds) which are affected by molecular
weight, geometry and, for solids, lattice structure.[1] The monomeric precursor molecules
are preferred because they tend to be more volatile compared to oligomeric molecules.[2]
In order to control the oligomerization and to increase volatility, tuning of the steric bulk
around metal center is widely practiced. In addition the manipulation of Lewis acid–base
reactions and thus formation of adducts and the use of fluorinated ligands help to certain
extent to tailoring volatility.[3] Highly fluorinated ligands are known to increase volatility.
It is reported that the substitution of H, CH3, or CR3 groups with fluorine ligands causes
intrermolecular repulsion due to the resulting negative “charge envelope”- and thus
contribute to increase in volatility. [4-5] It is well known fact that the polar groups or the
polarizable groups and high molecular masses tend to reduce volatility. To achieve the
volatility, the strength of the polar interactions has to be minimized. This concept is well
utilized in using donor functionalized ligands. Preferably the donor groups like ether, or
amino groups are widely considered as donor moieties for inducing volatility in resulting
complexes.
Volatility of a substance is best characterized by its vapor pressure. If vapor pressures of
precursor compounds are known, it would be easy to compare their volatilities and find
- 167 -
the right evaporation temperature for each precursor so that the precursor flux would be
maintained in the reactor at the right level.[6] However, vapor pressures of many possible
precursor compounds are unknown due to the difficulty of measuring low vapor pressures
of solid substances. Vapor pressure of a liquid is measured easily by observing the
pressure and the temperature at which the liquid boils. Measurement of vapor pressure of
a solid is much more complicated and is usually done with special equipment.[7]
In CVD, the volatility of precursors plays an important role in determining the properties
of a deposited film.[8] Gaseous precursors are preferable to liquids and solids as they can
be readily metered and transported to the reactor.[9] In practice, however, precursors are
often solids or liquids. Liquid and solid precursors should have high vapor pressures and
should not decompose at the vaporizing temperature. Solid precursors are not desired in
CVD because the sublimation rate of a solid may not be constant due to particle sintering
effects and therefore a homogeneous precursor flux may be difficult to maintain.
In conventional CVD of group III-V compound semiconductors, the precursors are
vaporized from a stainless steel container (bubbler) held at moderate temperatures (~ 60-
80 °C) and precursor vapors are carried by the carrier gas flow to the deposition zone.
But in case of oxide materials precursors often possess very low vapor pressure (< 1 torr
at room temperature). This naturally demands higher evaporation temperatures to
maintain the precursor flux to the reaction zone. This means the transport lines leading to
the reactor need to be maintained at higher temperature to prevent the condensation of the
precursor. So according to the need, reactor design has to be modified which needs
additional effort to optimize each precursor for a CVD process.[3]
In addition, incase of the multicomponent system, the individual precursors used should
have matching thermal properties i.e. the temperature of volatilization of individual
precursors and their decomposition temperatures should be as near as possible. The
difference in volatilization and decomposition temperatures of individual precursors
results in difficulty in handling them. Premature reactions taking place on the reactor
walls or lines pose a difficult problem to handle. Thus the CVD of multicomponent
oxides becomes even more difficult with these added process variables.
- 168 -
5.1.2 Long term stability
In a CVD process, generally the precursor container is held at an evaporation temperature
as long as the deposition is taking place. This can be for few minutes to several hours.
The ideal precursor should withstand this treatment and still should retain its physical and
chemical characteristics. Added to this, the precursors used in CVD are not freshly
prepared for every run. They are stored under the conditions specified by the
manufacturer before putting them to use. Precursors should be stable to certain period of
time and should perform efficiently during this period.
However, it is difficult to find all these properties in a single precursor. The precursor
may undergo chemical and physical change after prolonged heating. Thereafter it may
not retain same efficiency as before. Any change in the physical and chemical property of
the precursor affects the film quality and reproducibility. Storage of precursor is an
important issue which needs to taken care. There are several possibilities wherein the
precursor degradation can occur. One important possibility is the reaction with air,
moisture and light. Precursors having highly unsaturated metal centers react easily with
air and moisture leading to hydrolysis of the precursor. There are several methods to
prevent and stabilize the metal center such as using appropriate chelating agents,
polyether adducts etc. But one can not expect them to be stable for infinite period in air or
moisture. There is a certain time period within which they withstand the attack by oxygen
and moisture but thereafter they hydrolyze. This leads to inconsistency in depositions.
Also air and moisture and light sensitive precursors need to be studied for stability by the
supplier prior to putting them to use.
As the sublimation rate of precursor is strongly dependent on temperature, variation of
vaporizer temperature, under conditions of constant carrier gas flow rate provides a
means to examine film growth by CVD as a function of precursor partial vapor pressure
under conditions of kinetically limited growth. The poor stability of the precursor often
leads to inconsistent run-to-run reproducibility.[10]
In a CVD process where precursors are solid materials, the sublimation rate of the
precursor in the flowing carrier gas ambient actually determines the rate of precursor
delivery into the reactor. The sublimation rate of a solid precursor is easier to determine
- 169 -
and more useful for developing a CVD process than a knowledge of the equilibrium
vapor pressure at a given temperature.[10]
Most of the compounds used as precursors for oxide materials are solids and their
stability is usually estimated by repeated sublimations. The traditional delivery
techniques used for solids and/or liquids with low vapor pressures are direct sublimation
or inert or reactive gas-bubbler methods. These require well-controlled, high-temperature
delivery lines in order to avoid condensation of the precursors. The complexity of the
delivery systems increases with the number of lines required for a multicomponent
material. The mass transport of a solid is a function of the surface area of the powder and
therefore the effective transport rate drops as the solid is consumed.
Moreover, some precursors for oxide materials are unstable at high temperature over long
periods of time. The quest for more stable and more volatile oxide precursors needed for
dielectric, multicomponent ferroelectrics and optical thin films is an example of the
challenges in precursor chemistry in order to ensure reproducibility of the deposits, scale-
up and thus industrial applications. During the course of this work, the focus has been to
synthesize new and improved precursors for the CVD of titanium dioxide thin films.
Therefore the objective of this work has been to study the effect of the variation of the
terminal groups of the mixed alkoxy-ß-ketoester complexes of titanium on their volatility,
thermal stability and decomposition.
5.1.3 Hydrolytic stability
Compounds synthesized during this work are not indefinitely air/moisture stable. The
reaction of a solid, air-sensitive compound with moisture or oxygen upon exposure to air
depends, besides on chemical factors, on several other factors such as the particle size,
relative humidity, temperature, and turnover of the air, just to cite a few of them. These
reactions are normally carried out in a less defined manner, and many of the important
parameters are not exactly known. Therefore, experiments were performed to estimate
quantitatively whether an inclusion of ester moiety in the ligand sphere has significant
influence on the hydrolytic stability of the precursors.
When a new precursor is tested for CVD applications, it would be important to know the
right evaporation temperature so that the flow of vapors and consumption could be tuned
- 170 -
to an appropriate level already from the very beginning of the experiments. The above
mentioned issues associated with thermal properties of CVD precursors have to be
analyzed in order to have a qualitative data on the volatility, stability, reactivity of a
precursor before employing them as CVD precursors.
In general, TG (thermogravimetric) measurements have proven to be highly valuable
when evaluating the properties of possible precursor compounds.[11] Dynamic
measurements are used as a routine to evaluate the thermal stability and volatility of the
compounds. Isothermal measurements give information about evaporation rates at certain
temperatures. With thermal analyses performed under reduced pressure, the low pressure
CVD process conditions may be simulated.
5.1.4 Thermal analyses
A group of techniques in which a physical property of a substance and/or its reaction
products is measured as a function of temperature whilst the substance is subjected to a
controlled temperature program.[12-14] There are over a dozen thermal analysis methods
which differ in properties measured and the temperature programs. For the purpose of
evaluating the thermal properties relevant to CVD we have used thermogravimetry and
differential thermal analysis (DTA).
In a thermogravimetric analysis, the mass of a sample in a controlled atmosphere is
recorded continuously as a function of temperature or time as the temperature of the
sample is increased (usually linearly with time). A plot of mass or mass percent as a
function of time is called a thermogram.
Differential thermal analysis is a technique in which the difference in temperature
between a substance and a reference material is measured as a function of temperature
while the substance and reference material are subjected to a controlled temperature
program. Usually, the temperature program involves heating the sample and reference
material in such a way that the temperature of the sample Ts is increased linearly with
time. The difference in temperature ? T between the sample temperature and the reference
temperature Tr (? T = Tr - Ts) is then monitored and plotted against sample temperature to
give a differential thermogram.[15]
- 171 -
A single thermal method does not always give sufficient information to allow conclusive
analysis. For example, a downward peak in a DTA experiment means an endothermic
change is occurring at a particular temperature range. One can not infer whether this is a
chemical reaction or a physical change such as melting, or whether any gases are
evolved. A TG experiment on the same sample may show a mass loss over this
temperature range, thereby ruling out melting, but still not identifying any volatiles. So
the combination of several analytical methods gives better profile of the changes taking
place. If any two of the techniques are performed on a single sample at the same time,
then they are known as simultaneous techniques.
Successful synthesis of new class of precursors, (refer chapter 2) has led us to test the
physical and chemical properties of this precursors relevant to CVD using simultaneous
TG-DTA analyses. In addition to thermal properties, the hydrolytic stability of the
selected precursors was carried out using NMR as an analytical tool.
5.2 Experimental section
The titanium complexes, titanium bis(isopropoxide) bis(methylacetoacetate)
[Ti(OPri)2(meaoac)2] (1), titanium bis(ethoxide) bis(methylacetoacetate)
[Ti(OEt)2(meaoac)2] (2), titanium bis(isopropoxide) bis(tert-Butylacetoacetate)
[Ti(OPri)2(tbaoac)2] (3), titanium bis(ethoxide) bis(tert-Butylacetoacetate)
[Ti(OEt)2(tbaoac)2] (4) and titanium bis(isopropoxide) bis(N,N-diethylacetoacetamide)
[Ti(OPri)2(deacam)2] (5) and bis-[(di-ethylmalonato) tetra(isopropoxy)-µ-ethoxy-
titanium(IV)] [Ti2(µ-OEt)2(OPri)4(deml)2] (6) were synthesized by according to the
procedure reported in the literature.[16] (The synthesis and chemical characterization are
described in chapter 2).
- 172 -
The complexes were recrystallized in hexane to obtain high purity single crystals of
these complexes. The melting points of the precursor complexes were determined in glass
capillaries and are given in table 5.1. The precursors were purified by repeated
sublimation and the temperature of sublimation at a reduced pressure of 3.0 x 10-2 mbar is
listed in table 5.1 for all the newly synthesized titanium compounds.
The thermal characteristics of the compounds relevant to their suitability as precursors for
CVD were studied by simultaneous thermogravimetric analysis and differential thermal
analysis. Thermal measurements were made using a Seiko 6300S11 system. About 10-15
mg of the finely powdered sample were weighed inside a glove box in to aluminum
crucibles. Analyses were made under pre-purified nitrogen flowing at a rate of 300
ml/min. All samples were heated at a typical heating rate of 5 °C/min.
Thermogravimetry was also used to determine the sublimation rates of three different
precursors used for CVD applications. This was done by recording the mass of the
sample as a function of time after equilibrating the sample temperature at a chosen value.
Hydrolysis studies were carried out using Bruker Advance DPX 250 NMR spectrometer.
1 mmol of the compounds were weighed into the NMR tube and molar amounts of water
were added using micro syringe. NMR was taken immediately after the addition of water
and growing hydrolysis product, iso-propanol was monitored over a period of six hours.
Table 5.1: Melting points and sublimation temperatures (at 3.0 x 10-2 mbar) for
various titanium complexes.
Precursor complex Melting point (°C)
Sublimation temperature
(°C) [Ti(OPri)2(meaoac)2] (1) 59 85
[Ti(OEt)2(meaoac)2] (2) 57 80
[Ti(OPri)2(tbaoac)2] (3) 58 85
[Ti(OEt)2(tbaoac)2] (4) 55 80
[Ti(OPri)2(deacam)2] (5) 88 115
[Ti2(µ-OEt)2(OPri)4(deml)2] (6) 89 110
- 173 -
5.3 Results and Discussion
There are several metalorganic precursors used for the deposition of titanium containing
oxide thin films. The most widely used precursors are the halides such as TiCl4, the
alkoxides such as [Ti(OPri)4] and the mixed alkoxide-ß-diketonate complex
[Ti(OPri)2(thd)2]. Of all these precursors [Ti(OPri)2(thd)2] is the most widely used
precursor for thin film deposition of titanium dioxide. This is because the compound
possesses the volatility associated with alkoxides and the stability associated with the ß-
diketonates.
In addition, the thermal properties are more closely compatible with other precursors
such as [Sr(thd)2] and [Ba(thd)2] which are used widely for thin film depositions of
BaSrTiO3.
The bench mark precursor, [Ti(OPri)2(thd)2] has been investigated for thermal behavior in
earlier studies.[17] The DTA curve shows three endothermic peaks. First one at 56 °C,
second one at 176 °C and the third one at 224 °C (Fig. 5.1). The only exothermic peak
was centered at 391 °C. The endothermic peak at 176 °C was assigned to the melting and
the one at 224 °C was assigned for the vaporization of the precursor. Also the exothermic
75 150 225 300 375 450
0
20
40
60
80
100
5.3 %
Temperature [°C]
Wei
ght [
%]
-2
0
2
4
6
DT
A [µV
]
Fig. 5.1: Simultaneous TG-DTA curves for bench mark titanium precursor
[Ti(OPri)2(thd)2].
- 174 -
peak at 391 °C was assigned to decomposition of the precursor. The bench mark
precursor shows notable thermal stability compared to parent alkoxide [Ti(OPri)4].
Increased thermal stability of [Ti(OPri)2(thd)2] precursor was successfully utilized for the
deposition of multicomponent oxide thin films using [Sr(thd)2] and [Ba(thd)2] precursors
which showed compatibility in thermal stabilities. Though [Sr(thd)2] and [Ba(thd)2] are
thermally stable, prolonged heating of these precursors during vaporization led to
oligomerization thus reducing the volatility of the precursor.[18]
The simultaneous TG-DTA curves of complexes 1-6 are shown in Fig. 5.2 to 5.7
respectively. The temperature onset of volatilization for the compound,
[Ti(OPri)2(meaoac)2] (1) (Fig. 5.2) begins at 150 °C and there are two steps observed in
the TG curve. The step observed at 150 °C could be attributed to the volatilization and at
220 °C to the onset of decomposition which is evident by the appearance of an
exothermic peak at the same temperature. Beyond 225 °C there is no change in weight
loss observed and the amount of residue left behind is quite small (6.3 %). A sharp
100 200 300 400
0
20
40
60
80
100
6.28 %
Temperature [°C]
Wei
ght [
%]
-6
-4
-2
0
2
4
6
8
10
12
DT
A [µV
]
Fig. 5.2: Simultaneous TG-DTA curves of [Ti(OPri)2(meaoac)2]
- 175 -
endotherm in the DTA curve at 45 °C corresponds to the melting point of the compound
which was verified by the melting point determination using the capillary mode.
Although the baseline of the DTA curve for the compound [Ti(OEt)2(meaoac)2] (2) (Fig.
5.3) is poor, there is an endotherm at 57 °C which may be attributed to the melting point
of the compound 2, which was confirmed by melting point measurements by capillary
mode. The precursor seems to decompose in the temperature range 200 to 265 °C. A rest
mass of about 16% is left behind at temperatures above 250 °C.
The compound [Ti(OPri)2(tbaoac)2] (3) was the most widely studied precursor complex
during this work. As depicted in Fig. 5. 4 (a), the compound volatilizes monotonically
without any steps during volatilization. As can be seen from the sharp endotherm in
DTA, the compound melts at 58 °C and volatilizes thereafter. The onset of volatilization
begins at 150 °C and is complete at around 230 °C with almost no compound left behind
indicating that the compound vaporizes completely without decomposition. The residue
left behind is less than 2 % above 250 °C. In order to test the behavior of the complex in
air, one set of TG-DTA analysis was carried out on the precursor complex
100 200 300 4000
20
40
60
80
100
16.0 %
Temperature [°C]
Wei
ght [
%]
-10
0
10
20
30
40
50
60
DT
A [µV
]
Fig. 5.3: Simultaneous TG-DTA curves of [Ti(OEt)2(meaoac)2]
- 176 -
[Ti(OPri)2(tbaoac)2] (3), as shown in Fig. 5.4 (b).Though the TG curve shows no
significant deviation from the curve taken with the nitrogen flow, the differences were
observed in DTA curve.
100 200 300 400 500
0
20
40
60
80
100
Temperature [°C]
Wei
ght [
%]
-4
-2
0
2
4
6
8
10
(a)
1.6 %
DT
A [µV
]
100 200 300 400 500
0
20
40
60
80
100
Temperature [°C]
Wei
ght [
%]
-4
-2
0
2
4(b)
6 %
DT
A [µV
]
Fig. 5.4. Simultaneous TG-DTA curves for [Ti(OPri)2(tbaoac)2] (a) under flowing argon
(b) in the absence of argon gas flow
- 177 -
In order to evaluate the stability of the precursor when exposed to air, TG-DTA analyses
were carried out under ambient atmosphere. It is seen that the compound shows no
significant change in the TG curve when compared to Fig. 5.4 (a). There were only some
differences observed in the DTA curve and the residue left behind was slightly higher (~
6 %) compared to that carried out under inert atmosphere (1.6 %) This shows that the
precursor is having a better stability compared to the parent alkoxide. One important
point to be noted in such studies is that the sensitivity of the compound towards air or
moisture could not be quantified based only on these experiments. Other analytical
techniques are required wherein controlled exposure to air and moisture would help to
certain extent for quantitative analyses.
The simultaneous TG-DTA curves for compound [Ti(OEt)2(tbaoac)2] (4) are shown in
Fig. 5.5. Apparently mass loss is monotonic without any steps. The onset of volatilization
begins in considerable amount at 170 °C and nearly complete at 225 °C. There are two
endothermic peaks in corresponding DTA curve. First one beginning at 56 °C is sharp
and indicative of melting of the precursor. The second endotherm starts at around 170 °C
and ends at around 225 °C. This peak corresponds to bulk sublimation of the precursor.
The exothermic peak starting at 229 °C could be attributed to the decomposition of the
precursor. Above 280 °C, no mass loss was recorded. The mass loss is not total with
100 200 300 400 500
0
20
40
60
80
100
3.7 %
Temperature [°C]
Wei
ght [
%]
-6
-4
-2
0
2
4
DT
A [µV
]
Fig. 5.5: Simultaneous TG-DTA curves for [Ti(OEt)2(tbaoac)2]
- 178 -
residue stabilizing beyond 280 °C at about 3.7 % of the original mass. It should be noted
that in most of the newly developed precursors there is significant weight loss and there
is not enough material to show any characteristic DTA peaks because of the small
amount of residue left behind at high temperatures.
The compound [Ti(OPri)2(deacam2] (5) melts at 64 °C as indicated by the endothermic
peak in the DTA curve. It volatilizes thereafter and in a single step till 400 °C. The
maximum volatilization takes place between 170 and 280 °C where most of the mass loss
was recorded in the TG curve. The decomposition of the compound is marked by a broad
exothermic peak in the DTA curve which begins at around 300 °C. The formation of
TiO2 corresponds to 16.5 % of the initial mass. A residue of around 16.5 % of the initial
mass is left behind which could be TiO2 formed by the decomposition of the precursor
assuming it to be cleanly decomposed.
The TG-DTA data for the complex [Ti2(µ-OEt)2(OPri)4(deml)2] is shown in the Fig. 5.7
[Ti2(µ-OEt)2(OPri)4(deml)2] has a dimeric structure with bridging ethoxy moieties. Also
the metal center is surrounded by three different environments influenced by three
different types of ligands, namely ethoxide, iso-propoxide and diisopropyl malonate. This
100 200 300 400 5000
20
40
60
80
100
16.5 %
Tempearature [°C]
Wei
ght [
%]
-2
0
2
4
6
8
10
12
DT
A [µV
]
Fig. 5.5: Simultaneous TG-DTA curves for [Ti(OPri)2(deacam2].
- 179 -
may lead to a dynamic thermal behavior. The endothermic peaks could be probably
attributed to the loss of the ligands as the metal center is surrounded by three types of
ligands. The complex melts at 89 °C marked by an endothermic peak in the DTA curve.
The mass loss occurs even before melting. Most of the mass loss occurs within
temperature range of 150-275 °C. Two endotherms were observed centered around 200
and 275 °C. TG curve shows inflexion points between 225 °C and 275 °C. At
temperatures beyond 275 °C no considerable weight loss was observed with a rest mass
of 15 % is left behind.
During the course of this work, the design changes were included in to the well known ß-
keto systems in the form of ester moieties. In order to evaluate the performance of these
precursors, a comparison of their thermal properties was made. For comparison the two
of the reported precursors were synthesized and thermal properties were analyzed.
Fig. 5.8 shows the comparison of the parent alkoxide [Ti(OPri)4], the bench mark
precursor [Ti(OPri)2(thd)2], and recently reported titanium precursor [Ti(2meip)2] with
newly developed precursor [Ti(OPri)2(tbaoac)2]. The chelating ligands such as Hthd and
Hmeip stabilize the metal center to a great extent. The effect of chelation to these ligands
50 100 150 200 250 300 350 4000
20
40
60
80
100
14.8 %
Temperature [°C]
Wei
ght [
%]
-4
-2
0
2
4
6
8
10
12
14
DT
A [µV
]
Fig. 5.7: Simultaneous TG-DTA curves for [Ti2(µ-OEt)2(OPri)4(deml)2] (6)
- 180 -
is that the volatilization is also shifted to higher temperatures. The enhanced chemical
stability has to be compromised for decreased volatility.
It can be seen from the Fig. 5.8 the onset of volatilization of the newly developed
precursor is shifted to higher temperature compared to the parent alkoxide [Ti(OPri)4] and
the temperature onset of volatilization is lower compared to bench mark precursor. Thus
by the inclusion of small design changes in the well established ligand structures it is
possible to tune the thermal properties to a certain extent. And the newly developed
precursors show superior thermal properties like clean volatilization, lower
decomposition temperature, sufficient window between volatilization and decomposition
and negligible residue.
100 200 300 400 500
0
20
40
60
80
100
Wei
ght [
%]
Temperature [°C]
[Ti(OPri)2(thd)
2]
[Ti(Meip)2]
[Ti(OPri)2(tbaoac)
2]
[Ti(OPri)4]
Fig. 5.8: Comparison of TG curves of [Ti(OPri)2(tbaoac)2] (3) with the TG curves of
standard and recently developed titanium precursors
- 181 -
5.3.1 Sublimation studies
Three different precursors [Ti(OPri)2(meaoac)2] (1), [Ti(OPri)2(tbaoac)2] (3) and
[Ti(OEt)2(tbaoac)2] (4) were used for deposition of TiO2 thin films by CVD. All these
precursors are solids, and in a CVD process where precursor is a solid material, the
sublimation rate of the precursor in the flowing carrier gas ambient actually determines
the rate of the precursor delivery into the reactor. Therefore detailed sublimation studies
were necessary for the above mentioned precursors to determine the weight loss of each
complex as a function of time at different sublimation temperatures.
For practical use of as a CVD precursor, the sublimation (vaporization) rate at the chosen
temperature must be steady over periods of time involved in typical CVD growth runs,
i.e., 2-3 hours. As depicted in Fig. 5.9, the mass loss is constant over long periods of time
at three different temperatures. This ensures constant mass transport during a CVD
growth process. The sublimation rate increases as a function of temperature.
0 50 100 150 200 25050
60
70
80
90
100
150 °C
125 °C
Wei
ght [
%]
Time [min]
100 °C
Fig. 5.9: Mass loss as a function of time a three different temperatures for
[Ti(OPri)2(meaoac)2]
- 182 -
Table 5.2: Sublimation rates of [Ti(OPri)2(meaoac)2] as a function of time at different
temperatures (at atmospheric pressure) as obtained from thermogravimetric
analysis carried out at atmospheric pressure.
Table 5.2 gives the various sublimation rates in mg/min for [Ti(OPri)2(meaoac)2] (1), at
three different temperatures. With increase of sublimation temperature from 150 °C to
175 °C the sublimation rate increased almost four times over from 3.69 x 10-3 mg/min to
18.61 x 10-3 mg/min.
Fig. 5.10: Mass loss as a function of time a three different temperatures for
[Ti(OPri)2(tbaoac)2]
0 50 100 150 200
50
60
70
80
90
100
110
175 °C
125 °C
Wei
ght [
%]
Time [Min]
150 °C
Temperature
(°C)
Sublimation rate
(mg/min)
100 1.35 x 10-3
125 3.70 x 10-3
150 18.60 x 10-3
- 183 -
Table 5.3: Sublimation rates of [Ti(OPri)2(tbaoac)2] as a function of time at different
temperatures as obtained from thermogravimetric analysis carried out at
atmospheric pressure.
The mass loss as a function of time for the precursor [Ti(OPri)2(tbaoac)2] (3)
[Ti(OEt)2(tbaoac)2] (4) and is shown in Fig. 5.10 and 5.11 and corresponding
sublimation rates are given in tables 5.3 and 5.4 respectively. It can be seen from these
figures that mass loss is constant over long periods of time and over the temperature
range use. From the Fig. 5.10 the sublimation rate at 175 °C is not constant but is slightly
deviated. The precursor sublimes at a much higher rate at 175 °C.
0 50 100 150 200 25075
80
85
90
95
100
120 °C
100 °C
85 °C
Wei
ght [
%]
Time [min] Fig. 5.11: Mass loss as a function of time a three different temperatures for
[Ti(OEt)2(tbaoac)2]
Temperature
(°C)
Sublimation rate
mg/min
125 3.70 x 10-3
150 10.75 x 10-3
175 15.40 x 10-3
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Table 5.4: Sublimation rates of [Ti(OEt)2(tbaoac)2] as a function of time at different
temperatures (at atmospheric pressure) as obtained from thermogravimetric
analysis carried out at atmospheric pressure.
5.3.2 Shelf life
The long term stability (shelf life) of the precursor [Ti(OPri)2(tbaoac)2] was studied using
thermogravimetry. One batch of precursor was stored for the purpose in the glove box to
prevent it from degradation. This study was essential because this precursor was used
extensively for deposition of TiO2 and SrTiO3 thin films. The storage either as a solid or
in solution using some organic solvent is essential in order to employ this precursor for
various depositions. In order to address the stability over long term some quantification is
needed. This prompted us to study the precursor over a period of two years. Fig. 5.12
shows the TG curves for the precursor over a period of 2 years. From the TG curves one
can observe that volatilization temperature in case of aged samples is shifted to higher
temperatures by about 20 °C. The decomposition temperature range of all the three
samples remains between 220 at 260 °C. In addition, the residues left behind by the
precursor increased with ageing of the precursor.
Temperature
(°C)
Sublimation rate
mg/min
85 2.20 x 10-3
100 5.90 x 10-3
120 9.25 x 10-3
- 185 -
Fig. 5.12: Stability of [Ti(OPri)2(tbaoac)2] in storage; determined by the TG analysis.
It can be rationalized by the fact that the precursor crystallites agglomerate over the
period of time, correspondingly the surface area and hence volatility is affected. In order
to ascertain the chemical stability of the precursor, NMR studies were done on the aged
precursor. The aged samples showed no change when compared to the freshly prepared
sample. Also the NMR studies on the precursor left over in the evaporator after CVD
showed no change when compared to freshly prepared sample. This is advantageous for a
CVD process as the precursor is stable for repeated heating cycles under inert
atmosphere.
5.3.3 Hydrolysis studies
Newly developed precursor systems have to be tested for stability in air and moisture.
This is because the large scale depositions are usually carried out on the shop floor by
non-chemists and sometimes the precursors are handled in situations wherein air and
moisture can not be excluded. But the data on the stability of the precursor in air and
50 100 150 200 250 300 350 400 450
0
20
40
60
80
100
8.3 %
Residues
3.9 %1.6 %
Wei
ght [
%]
Temperature [°C]
After 2 years After 1 year Freshly prepared
- 186 -
moisture certainly provides a guideline for handling the precursor in an appropriate
manner.
Fig. 5.13: Hydrolysis studies on the precursors [Ti(OPri)2(tbaoac)2] and
[Ti(OPri)2(meaoac)2] in comparison with well known ß-diketonate
complexes.
During the course of this work, new titanium precursors were developed by the inclusion
of design changes in the ligand sphere of well known ß-diketonate systems in the form of
ester moieties. These changes in the ligand sphere have resulted in the formation of
monomeric structures and found to possess superior thermal properties as discussed
earlier in this chapter. Two of the newly developed precursors namely
[Ti(OPri)2(meaoac)2] (1) and [Ti(OPri)2(tbaoac)2] (3) were studied for hydrolysis
behavior and compared with the similar ß-diketonate complexes. It was thought that the
hydrolysis studies would give us a hint about dependency of hydrolytic stability on the
bulk of the side chain as well on the ester moiety embedded in the side chain. Fig. 5.13
shows the results, when the deliberate hydrolysis was carried out in millimolar amounts
using NMR as an analytical tool.
0 50 100 150 200 250 300 350 400
0
10
20
30
40
50
60
70
80
90
100
Hyd
roly
sis
prod
uct [
%]
Time [Min]
[Ti(OPri)2(acac)
2]
[Ti(OPri)2(meaoac)
2]
[Ti(OPri)2(thd)
2]
[Ti(OPri)2(tbaoac)
2]
- 187 -
Two ß-diketonate complexes [Ti(OPri)2(acac)2] and [Ti(OPri)2(thd)2] were selected
because of their wide acceptance as titanium precursors and proximity to the ligand
system used in this study. It can be seen from the Fig. 5.13 that the hydrolysis is fastest in
case of [Ti(OPri)2(meaoac)2] and slowest in case of [Ti(OPri)2(thd)2]. The hydrolysis rates
of [Ti(OPri)2(tbaoac)2] and [Ti(OPri)2(acac)2] are comparable. It can be explained in a
simple way that inclusion of ester moiety in the side chain of ß-keto system, stabilizes the
metal center in monomeric form, but is susceptible for faster hydrolysis than its alkyl
counterparts. This is evident by comparison of hydrolysis curves of [Ti(OPri)2(meaoac)2]
and [Ti(OPri)2(acac)2]. In case of ß-ketoester complex the hydrolysis is complete in two
hours while that of ß-diketonate complex was hydrolyzed only 80 % in five hours.
Interestingly, the bulk on the side chain of the ligand seems to play significant role in
stabilizing the complex against hydrolysis. This is evident from the fact that
[Ti(OPri)2(thd)2] complex is more stable to hydrolysis even after 5 hours indicating only
about 20 % of hydrolysis products where as the [Ti(OPri)2(acac)2] is hydrolyzed almost
90 % during the same period.
- 188 -
5.4 Summary
During the course of present study several precursors were developed and tested for CVD
applications. The most important criteria to use them as precursors for CVD application
are thermal properties. Simultaneous TG-DTA studies were carried out on all of the
newly developed precursors. During the course of this study the precursors titanium
bis(isopropoxide) bis(tert-Butylacetoacetate) [Ti(OPri)2(tbaoac)2] (A), titanium
bis(ethoxide) bis(tert-Butylacetoacetate) [Ti(OEt)2(tbaoac)2] (B), Titanium
bis(isopropoxide) bis(methylacetoacetate) [Ti(OPri)2(meaoac)2] (C), were used for the
deposition of TiO2 thin films using home built horizontal cold wall reactor. The
simultaneous TG-DTA was performed under ambient of nitrogen and under normal
pressure. Most of the newly developed precursors showed onset of volatilization between
80-150 °C.
The sublimation rates of these precursors were determined at different temperatures
based on isothermal studies. It was found that the precursor [Ti(OPri)2(tbaoac)2] (A) has
highest sublimation rate of 15.4 x 10-3 mg/min at 175 °C. Sublimation rate of
[Ti(OEt)2(tbaoac)2] (B) at 120 °C was found to be 9.25 x 10-3 mg/min. The sublimation
rate of [Ti(OPri)2(meaoac)2] (C) was found to be of the order of 18.60 x 10-3 mg/min at
150 °C. The compound [Ti(OPri)2(tbaoac)2] (A) was extensively studied for shelf life
after storing for three years under argon ambient. It was found that ageing of the
precursor increased the residue after TG-DTA analysis by about 7% of initial mass. But
the volatilization trend was observed to be preserved with slight deviation from the
freshly prepared samples.
Hydrolysis studies carried on the newly developed precursors using NMR as analytical
tool. The hydrolytic stabilities were compared with standard [Ti(OR)2(ß-diketonate)2]
types of precursors. It was found that inclusion of ester moieties led to decreased
hydrolytic stability of the precursors. Use of homoleptic ß-diketonates as ligands
provided better hydrolytic stability in addition to higher thermal stability. Also the bulk
on the side chain of the ester moiety found to have an effect on the hydrolytic stability of
the precursors. The more bulky ß-ketoesterate complex [Ti(OPri)2(tbaoac)2] has better
hydrolytic stability compared to less bulkier [Ti(OPri)2(meaoac)2].
- 189 -
5.5 References
[1] L. G. Hubert-Pfalzgraf, H. Guillon, Appl. Organometal. Chem. 1998, 12, 221.
[2] M. Becht, T. Gerfin, K. H. Dahmen, Chem. Mater. 1993, 5, 137.
[3] A. C. Jones, J. Mater. Chem. 2002, 12, 2576.
[4] A. P. Purdy, A. D. Berry, R. T. Holm, M. Fatemi, D. K. Gaskill, Inorg. Chem.
1989, 28, 2799.
[5] R. Gardiner, D. W. Brown, P. S. Kirlin, A. Rheingold, Chem. Mater. 1991, 3, 45.
[6] A. Niskanen, T. Hatanpää, M. Ritala, M. Leskelä, J. Therm. Anal. Cal. 2001, 64,
955.
[7] G. V. Sidorenko, D. N. Suglobov, Soviet Radiochemistry 1983, 24, 646.
[8] A. N. Gleizes, Chem. Vap. Deposition. 2000, 6, 155.
[9] R. A. Gardiner, P. C. van Buskirk, P. S. Kirlin, Mater. Res. Soc. Symp. Proc.
1994, 335, 221.
[10] A. Devi, Ph. D. Thesis, Indian Institute of Science 1997.
[11] T. Ozawa, Thermochim. Acta 1991, 174, 185.
[12] J. O. Hill, For Better Thermal Analysis and Calorimetry III, ICTA, 1991.
[13] R. C. Mackenzie, Thermochim. Acta 1979, 28, 1.
[14] W. W. Wendlandt, Thermal Analysis, 3rd ed., Wiley, New York, 1985.
[15] D. A. Skoog, F. J. Holler, T. A. Nieman, Principles of Instrumental Analysis,
Saunders College Publishing, 1997.
[16] R. J. Errington, J. Ridland, W. Clegg, R. A. Coxall, J. M. Sherwood, Polyhedron
1998, 17, 659.
[17] J.-H. Lee, S.-W. Rhee, J. Electrochem. Soc. 2001, 148 (6), C409.8
[18] S. R. Drake, M. B. Hursthouse, K. M. Abdul Malik, A. S. Miller, J. Chem. Soc.,
Chem. Comm. 1993, 478.
- 190 -
Appendix
General characterization techniques
NMR analysis
Proton- 1H and carbon- 13C NMR spectra were recorded for all synthesized compounds.
The spectra were referenced to residual protic impurities of the internal solvent and
corrected to tetramethylsilane. The integration of peaks and peak intensity analyses were
done using Mestrec® software version 2.30. Bruker Advance DPX 200 and Bruker
Advance DPX 250 spectrometers were used.
CHN analysis
The elemental analyses were performed in the spectrometry and chromatography section
at the Ruhr University Bochum using Elemental, CHNSO Vario EL, Hanau.
Mass spectrometry
Electron Ionization (EI) mass spectra were recorded using ionization energies between 24
eV to 70 eV using CHS-Mass spectrometer “Varian MAT” (Bremen). Output spectra was
given as specific masses (m/z) based on abundant isotopes, 1 1H, 12
6C 14 7N 16 8O, 48
22Ti.
Thermal analysis
Thermal measurements were made using a Seiko 6300S11 system. About 10-15 mg of
the finely powdered sample were weighed inside a glove box in to aluminum crucibles.
Analyses were performed under pre-purified nitrogen flowing at a rate of 300 ml/min. All
samples were heated at a typical heating rate of 5 °C/min.
X-ray diffraction analysis
X-ray diffraction anaylsis were carried out employing a D8-Advance Bruker axs
dffractometer using CuKa radiation (? = 1.5418 Å). XRD instrument consists of CuKa as
X-ray source with nickel filter, and a Goebel mirror with a parallel plate collimator and
- 191 -
OED detector. High angle XRD measurements were carried out with ?- 2 ? geometry in
the range 20 – 80° using position sensitive detector.
Matrix isolation
The matrix isolation apparatus consists of a vacuum line (Pfeiffer TMH 261; Pfeiffer
DUO 5) and an ARS 8200 cryogenic closed cycle system (ARS cryogenics Inc.). The
starting compound is kept at constant temperature in a small stainless steel vaporizer
connected to high vacuum line through Swagelok® fittings. A small (~25 mm) window
(optically polished cesium iodide suitable for infrared work) is suspended at the tip of the
cryostat within the vacuum shroud and can be cooled to temperatures as low as 9 K.
Vacuum windows (CsI 40 mm diameter, 3 mm thick) on the chamber permit
spectroscopic measurements of samples prepared within. Additional ports permit the
admission of the inert gaseous matrix material (usually argon for IR work) and vacuum
ultraviolet light for sample photolysis. Typically, argon (purity, 6.0) is used as the carrier
gas and passed over the compound using a mass flow controller (flow =1.25 sccm) and
the gaseous mixture is passed through an Al2O3 tube (inner diameter of 1mm; heated by
tungsten wire coiled around the last 15 mm). The hot end of the pyrolysis oven is
stationed 25 mm away from the cooled CsI window to ensure that a maximum amount of
volatile fragments emerging from the oven can be trapped in the matrix. The IR spectra
of the matrices, cooled down to 10 K, is recorded on a Bruker EQUINOX 55 with a KBr
beam splitter in the range of 400 to 4000 cm-1 with a resolution of 1.0 cm-1.
Scanning electron microscopy
Scanning electron microscopy (SEM) analyses were carried out using LEO-1530 Gemini
SEM instrument equipped with an energy dispersive X-ray analysis unit, Oxford ISIS
EDX system. Prior to analyses, metallic Au was evaporated on the surface of the
specimen to form a conducting layer to avoid electrostatic charging. This facility was
provided by faculty of Geology, Ruhr University, Bochum. Surface morphology of the
films was studied with AFM (SIS pico station situated at Forschungszentrum Jülich).
- 192 -
X-ray photoelectron spectroscopy
XPS analyses were carried out with a modified Fisons X-ray photoelectron spectrometer
equipped with an Al Ka X-ray source and a CLAM3 electron energy analyzer. The pass
energy was set to 50 eV. The typical operating pressure was less than 10-8 mbar. All
binding energies were referenced to the substrate signal. Survey X-ray photoelectron
spectra and high resolution spectra were recorded for desired elements.
Electrical measurements
LCR (Inductance/Capacitance/Resistance) meter
The HP 4284 LCR meter is a general purpose LCR meter used for evaluating LCR
components, materials, and semiconductor devices over a wide range of frequencies
(20Hz to 1MHz) and test signal levels (5 mV to 2Vrms, 50µA to 20mArms). The LCR
meter is mostly controlled automatically by a computer with an HP-IB interface (standard
interface for HP equipment for automatic test system). The probe station consists of a
microscope, a wafer chuck and a manipulator. The wafer chuck is connected to the
ground and equipped with vacuum for wafer holding. The manipulator is a device that
enables the user to move the tip in any designated direction. Three knobs move tip: up
and down (1), left and right (2), back and forth (3).
Electrical properties of the metal-insulator-semiconductor (MIS) structures, Capacitance-
Voltage (C-V) characteristics are obtained using HP4284 LCR meter by sweeping the
voltage from inversion to accumulation and back. These analyses were carried out using
facilities situated at Forschungszentrum Jülich.
X-ray fluorescence analyses
X-ray fluorescence (XRF, RIGAKU ZSX-100e) was used for the determination of molar
amount of the individual element in the deposited films. For a particular energy
(wavelength) of fluorescent light emitted by an element, the number of photons per unit
time (generally referred to as peak intensity or count rate) is related to the amount of that
analyte in the sample. The counting rates for all detectable elements within a sample are
usually calculated by counting, for a set time, the number of photons that are detected for
the various analytes "characteristic" X-ray energy lines. By determining the energy of the
- 193 -
X-ray peaks in spectrum of a sample, and by calculating the count rate of the various
elemental peaks, it is possible to qualitatively establish the elemental composition of the
sample and to quantitatively measure the concentration of these elements. These analyses
were carried out using the facilities situated at Forschungszentrum Jülich.
- 194 -
List of publications
1. Synthesis of nano-scale TiO2 particles by a nonhydrolytic approach,
H. Parala, A. Devi, R. Bhakta, R. A. Fischer, J. Mater. Chem., 2002, 1625-1627.
2. Mononuclear Mixed-ß-Ketoester-alkoxide Compound of Titanium as a
Promising Precursor for Low Temperature MOCVD of TiO2 Thin Films
R. Bhakta, F. Hipler, A. Devi, S. Regnery, P. Ehrhart and R. Waser, Chem. Vap.
Deposition, 2003, 9, 295.
3. MOCVD of TiO2 thin films using a new class of metalorganic precursors
R. Bhakta, U. Patil and A. Devi, Electrochem. Soc. Proc. 2003, 08, 1477.
4. MOCVD of TiO2 thin films and studies on the nature of molecular
mechanisms involved in the decomposition of [Ti(OPri)2(tbaoac)2]
R. Bhakta, R. Thomas, F. Hipler, H. F. Bettinger, J. Müller, P. Ehrhart and A.
Devi, J. Mater. Chem. 2004, 14, 3231.
5. High k dielectric materials by metalorganic chemical vapor deposition;
growth and characterization
R. Thomas, S. Regnery, P. Eharhart, R. Waser, U. Patil, R. Bhakta, and A. Devi,
Ferroelectrics, 2005, in press.
6. Engineered precursors for MOCVD of titanium containing oxide thin films;
precursor chemistry and thin film growth
R. Bhakta, R. Thomas, A. Baunemann, M. Winter, P. Ehrhart, R. Waser and
A. Devi, submitted to Chemisty of Materials, 2004.
7. Gas phase decomposition mechanism involved in the thermal decomposition
of [Ti(OPri)2(thd)2] using matrix isolation technique
R. Bhakta, H.F. Bettinger, J. Müller, and A. Devi, under preparation.
- 195 -
Presentations at conferences
MOCVD of TiO2 thin films using a new class of metalorganic precursors
R. Bhakta, U. Patil, S. Regnery, F. Hipler, R. A. Fischer, P. Ehrhart, R. Waser, and
A. Devi. Electrochemical Society, 203rd meeting and EUROCVD XIV Paris, France
April 27- May 2, 2003 (Poster)
MOCVD of TiO2 thin films and studies on the nature of molecular mechanisms
involved in the decomposition of [Ti(OPri)2(tbaoac)2]
From Molecules to Materials: Materials Discussion 7, Queens Mary College, London,
13-15 September, 2004. (Oral presentation)
- 196 -
Personal details
Name Raghunandan Krishna Bhakta
Date of birth 20. 06. 1974
Place of birth Chitradurga, India
Nationality Indian
Marital status Married
Academic details
1980-1987 Primary school, MHPS, Nilekani, Sirsi, India
1987-1990 High school, Ave Maria, Sirsi, India
1990-1992 Pre university, MMAS College Sirsi, India
1992-1995 B.Sc., MMAS College, Sirsi, India
1995-1997 M.Sc., Mangalore University, Mangalore, India
Professional experience
1997-1998 Rallis India Ltd. Bangalore, India, as research fellow
1998-2001 Centre for Electronics Design and Technology, Indian Institute of
Science, Bangalore, India, as project assistant
April 2001 Registered for Ph.D at Ruhr University Bochum, Germany