Preparation of undoped and some doped ZnO

thin films by SILAR and their characterization

Dissertation submitted to The University of Burdwan in partial

fulfillment of the requirements for the degree of Doctor of

Philosophy in Science (Physics)

SHAMPA MONDAL

Department of Physics

The University of Burdwan

Burdwan

West Bengal, INDIA

2013

Dedicated to my parents & my son

THE UNIVERSITY OF BURDWAN GOLAPBGA, BURDWAN: 713104

WEST BENGAL, INDIA

Dr. Partha Mitra Phone: 0342 2657800 (O)

Associate Professor Fax : +91 342 2657800

Materials Science Laboratory e-mail: mitrapartha1@rediffmail.com

Department of Physics

Date:

Certificate from the Supervisor

This is to certify that the research work incorporated in the dissertation entitled

“Preparation of undoped and some doped ZnO thin films by SILAR and their

characterization” has been carried out at The University of Burdwan, Burdwan by

Shampa Mondal, under my supervision. Mrs. Mondal has followed the rules and

regulations as laid down by The University of Burdwan for the fulfillment of

requirements for the degree of Doctor of Philosophy in Science. Any other worker

anywhere has not published the results included in this dissertation.

(Dr. Partha Mitra)

ACKNOWLEDGEMENT

It is a distinct pleasure to express my deepest sense of gratitude to my research

supervisor, Dr. P. Mitra, Department of Physics, The University of Burdwan for kindly

suggesting me this challenging problem. His efficient guidance and constructive criticism

encourage me all along and helping through the course of the work. It was a privilege to

carry out the present research work under him.

Similar gratitude also goes to the Head of the Dept. Prof. S. Das for his kind co-

operation. The friendly and stimulating discussion with Dr. S. K. Pradhan, Dr. M. Pal,

Dr. S. Mukherjee and Dr. P. K. Chakraborty contributed greatly to the progress of my

work. I greatly acknowledge the help and guidance of all other faculty members of the

Department of Physics right from the beginning of my research work.

My thanks also goes to Mr. S. Patra, Mr. S. Bandyopadhyay, Mr. S. Lala, Mr. A.

Nandy, Smt. A. Sen, Mr. S. Sain, Mr. U. K. Bhaskar, Smt B. Ghosh and Mr. A. Banerjee

for their help during the experiment. My regards also goes to Dr. A. Dutta and Mr. K. P.

Kanta for their encouragement and suggestions during the progress of the work. With a

sense of gratitude, I am thankful to all the office and library staff of the Department of

Physics and the technical staff for all the help and cooperation.

My graceful thanks goes to my husband Mr. B. Roy Choudhury, my brothers Mr.

S. Mondal, Mr. A. Samanta and Mr. A. Roy Choudhury and my parent in-laws Mr. M.

K. Roy Choudhury and Smt. C. Roy Choudhury for their continuous inspiration

throughout the work.

I have no sufficient word to express my sincere gratitude to my parents Late R. N.

Mondal and Late K. Mondal for their blessings. I am grateful to my family members,

close relatives, all my colleagues of A.K.P.C Mahavidyalaya, well wishers and friends,

for their constant encouragement.

January 2013 (Shampa Mondal)

PREFACE

Preparation and characterization of ZnO and doped ZnO polycrystalline thin film

via different techniques have attracted considerable attention due to their wide

application prospects in various electronic and optoelectronic devices. Consideration of

simplicity, economy and input energy suggest that thin films of these materials be

deposited by a low temperature and simpler chemical route. The present work was taken

up to prepare ZnO and doped ZnO thin films following a relatively new and less

investigated wet chemical technique called Successive ion layer adsorption and reaction

(SILAR). The structural and morphological property of ZnO thin films synthesized by

SILAR has been studied and influence exerted by some metal doping on structural

properties, optical band gap and electrical resistance has been investigated.

The thesis contains nine chapters. Chapter 1 contains an introduction to ZnO and

its properties and importance of thin films of ZnO and doped ZnO. Different physical and

chemical techniques to prepare thin films have been discussed. The chapter ends with a

discussion on general characteristics of SILAR technique. Chapter 2 presents a brief

review of literature on preparation of ZnO and doped ZnO by different techniques and

their properties. Chapter 3 discusses the instrumental techniques used for characterization

of prepared materials in the present work. Theoretical consideration for evaluation of

preferred orientation and particle size has been discussed here. Characterization of ZnO

thin films prepared by SILAR has been discussed in chapter 4. Chapter 5 presents the

preparation of Cd doped ZnO thin films, their structural characterization and evaluation

of band gap energy. The preparation of Mn doped ZnO thin films and the influence of

Mn incorporation on structural properties and optical band gap of ZnO is discussed in

chapter 6. Chapter 7 deals with the preparation and characterization of Al doped ZnO

(AZO) thin films. Structural, morphological and electrical characterization of AZO thin

films has been discussed here. The influence of Ni incorporation in ZnO has been

discussed in chapter 8. Chapter 9 presents the concluding remarks and scope of future

work.

i

CONTENTS

Page No.

CHAPTER 1: Introduction

1.1 Zinc oxide and its properties 1

1.2 Thin films of ZnO and doped ZnO 6

1.3 Thin films and their deposition techniques 10

1.4 The technique of SILAR 16

References 20

CHAPTER 2: Literature review and Aim of the work

2.1 Review of Literature 24

2.2 Aims and objectives of the present work 39

References 40

CHAPTER 3: Instrumental techniques and Theoretical considerations

3.1 Instrumental techniques 48

3.1.1 X-ray Diffraction (XRD) Analysis 48

3.1.2 Electron Microscopes: SEM and TEM 51

3.1.3 Ultraviolet – Visible (UV-VIS) spectroscopy 52

3.1.4 Energy dispersive X-ray spectroscopy (EDS or EDX) 53

3.2 Theoretical considerations

3.2.1 Preferred orientation 53

3.2.2 Particle size estimation 54

References 56

CHAPTER 4: Preparation of ZnO thin films by SILAR and their

characterization

4.1 Introduction 57

ii

4.2 Preparation of ZnO thin films 58

4.2.1 Preparation of bath solutions 60

4.2.2 Optimization of pH and Concentration of the zincate baths 62

4.2.3 Deposition of ZnO films 63

4.2.4 Film thickness and its measurement 66

4.3 Structural characterization by XRD: Evaluation of particle size 69

4.4 Electron microscope studies 76

4.5 EDX and FTIR studies 80

4.6 Discussion of results on ZnO thin films 83

References 84

CHAPTER 5: Preparation of Cd doped ZnO thin films by SILAR and

their characterization

5.1 Preparation of Cd doped ZnO (Cd:ZnO) films 86

5.2 Structural characterization: Evaluation of particle size 87

5.3 SEM and EDX studies 89

5.4 Optical band gap evaluation of Cd:ZnO films 92

5.5 Discussion of results on Cd:ZnO thin films 94

References 95

CHAPTER 6: Preparation of Mn doped ZnO thin films by SILAR and

their characterization

6.1 Preparation of films and thickness measurements 96

6.2 Structural characterization: Evaluation of particle size and

strain 100

6.3 SEM and EDX studies 104

6.4 Evaluation of band gap from optical absorption 108

6.5 Discussion of results on Mn:ZnO thin films 111

References 112

iii

CHAPTER 7: Preparation of Al doped ZnO (AZO) thin films by SILAR

and their characterization

7.1 Preparation of AZO films 113

7.2 Structural characterization by XRD: Evaluation of TC(002) 114

7.3 Band gap evaluation from optical absorption 120

7.4 Electrical resistance measurements 121

7.5 Electrical resistance measurements in presence of LPG 125

7.6 Discussion of results on AZO thin films 130

References 132

CHAPTER 8: Preparation of Ni doped ZnO thin films by SILAR and

their characterization

8.1 Preparation of Ni doped ZnO (NZO) films 134

8.2 Structural characterization by XRD: Evaluation of particle size 135

8.3 SEM & EDX studies 138

8.4 Band gap evaluation from Optical absorption 140

8.5 Electrical characterization 141

8.6 Discussion of results on Ni:ZnO thin films 143

References 144

CHAPTER 9: Summary, Conclusions and Scope of future work

9.1 Summary and Conclusions 145

9.2 Scope of future Work 150

List of Publications 151

1

CHAPTER 1

Introduction

1.1 Zinc oxide and its properties

The earliest commercially produced semiconducting materials belong to II-VI

compounds. These are cadmium sulphide (CdS), cadmium selenide (CdSe), zinc oxide

(ZnO), tin dioxide (SnO2), zinc selenide (ZnSe), zinc telluride (ZnTe) etc. The ionicity of

such compounds is very high compared to elemental semiconductors such as silicon ( )Si

or germanium (Ge) and also III-V compounds. Polycrystallinity with crystallite

dimensions of the order of minority carrier diffusion length and good optical quality are

some of the important features which make the II-VI materials potential candidates for

their applications in optical and electronic devices. ZnO is one of the most versatile II-VI

materials and have long been subjects of investigation. The material ZnO is known since

the Bronze Age [1] and is an important topic of research in the 21st century. This is also

one of the most important materials that we come across in our day-to-day lives. Zinc

white is used as a pigment in paints and in coatings for paper. Some of the favorable

aspects of ZnO include its radiation hardness, abundance in nature and nontoxicity,

biocompatibility, excellent piezoelectric and semiconducting properties among many

others. Such multi-functional properties of ZnO make it suitable for applications in

electronic and optoelectronic devices.

2

The distinctive features of the material includes non-stoichiometric defect

structure, wide band gap (WBG) with high optical transparency in the visible region,

large variability of conductivity and high surface sensitive catalytic activity under

different atmospheres and high voltage-current nonlinearity etc. [2-4]. Besides being a

wide band gap semiconductor with a bandgap of around 3.2-3.37eV at room temperature

300K [5-6] and exciton binding energy of 60 meV (almost three times greater than that of

GaN, another most widely used WBG compound), it has several other aspects. ZnO

shows anisotropy in crystal structure and strong absorption in the ultraviolet range.

Accordingly it has got several potential applications in windows for photovoltaic solar

cell and heterojunction solar cells, surface acoustic wave (SAW) devices, IR reflective

coatings, piezo-electric and guided optical wave devices, blue and UV light emitting

diodes, phosphors, solid state gas sensors and transducers [7-14]. In transparent

conducting oxide form, the material have got applications in solid state display devices,

resistors, selective absorber components in solar collectors and in a number of electronic

and opto-electronic devices [15]. Being a large direct band gap material, it can transmit

most of the terrestrial sunlight (85%-95%) over the complete solar spectrum.

At ambient pressure and temperature, ZnO crystallizes in the hexagonal wurtzite

structure [7] having a 6-mm symmetry as shown in figure 1.1. It has a hexagonal lattice

belonging to the space group P633mc and is characterized by two interconnecting

sublattices of 2Zn

+ and 2O

− such that each Zn ion is surrounded by oxygen tetrahedra and

vice-versa. The bulk unit cell contains two Zn cations and two O anions. The crystal can

be viewed as a sequence of O-Zn double layers, which are stacked along c-axis. In fact,

the layers occupied by zinc atoms alternate in the lattice with layers occupied by oxygen

atoms. The effective ionic charges are about 1 to 1.2, which results in a polar c-axis

(Figure 1.1). The mean lattice constants are 3.25a = Å and 5.206c = Å [7]; the values

depend slightly on the stoichiometry of the oxide composition. The Zn-O distance is

1.992 Å parallel to the c-axis and nearly similar (~1.973 Å) in the other three directions

of the tetrahedral arrangement of nearest neighbours. Extensive literature review of the

3

different properties of ZnO (primarily single crystal ZnO) has been repoted by Hirschald

and his co-workers [16].

Figure 1.1: Crystal structure of ZnO (Hexagonal structure with 6-mm symmetry) [7]

The room temperature band gap value of ZnO corresponds to a strong absorption

in the ultraviolet range (λ≤387 nm). The nature of the absorption shows that the band gap

is of direct type. In 1969, Rossler [17] first reported the bulk band structure of ZnO by

Korringa-kohn-Rostoker calculations. In 1973, Bloom and Ortenburger [18] reported an

empirical pseudopotential calculation. In 1977, Chelikowsky [19] published the first self

consistently determined bulk band structure using a non-local pseudo-potential approach.

Using the Empirical Tight Binding Model (EBTM), Ivanov and Pollman [20] evaluated

the surface electronic structure.

Stoichiometric zinc oxide has the band structure typical of an insulator material.

When excess of zinc atoms are present, which often happens, ZnO becomes an n-type

semiconductor. Accordingly, irrespective of the preparation technique used, ZnO has a

c

a

4

characteristic n-type conductivity which results from stoichiometric deviation. The n-type

character can be deduced from the sign of the Hall coefficient and the thermoelectric

power. One of the problems in the quantitative determination of non-stoichiometric factor

δ in 1ZnO δ− is to achieve uniformity of defect concentrations in the sample. The non-

stoichiometric composition arises due to the presence of excess zinc in the form of zinc

interstitial (the excess zinc atom goes to interstitial space) and/or oxygen vacancy (the

excess zinc atom go into a lattice site resulting in the formation of an oxygen vacancy) as

point defects [21-22]. Both zinc interstitial ( )iZn and oxygen vacancy ( )o

V occupied by

electron pairs can serve as donor levels giving rise to conduction electrons resulting in n-

type conductivity in ZnO following the ionization schemes:

(1.1)i i

Zn Zn e+ −→ + →

(1.2)o o

V V e+ −→ + →

In equations (1.1) and (1.2), i

Zn+ and

oV

+ are singly ionised zinc interstitial and

oxygen vacancy respectively and e− is the electron released in the above ionization

processes. Both i

Zn+ and

oV

+can act as donor states through double ionization processes

at enhanced temperatures.

The electrical behaviour of ZnO is sufficiently modified by adsorption and

desorption of oxygen species from ambient air on the surface and this is particularly

important for thin film form of the material. The adsorption process may be either

physical (physisorption) or chemical (chemisorption) in nature. Physically adsorbed

oxygen forms a surface acceptor site for a conduction electron. The process does not

involve any transfer of electron between the adsorbate and the oxide material. The

chemisorption process involves capture or trapping of free electrons (conduction

electrons) by the adsorbed oxygen species. This leads to a surface double layer that is

5

actually formed between the charge transferred to the adsorbed gas and the opposing

charge, remaining in the semiconductor. The process is therefore an electronic transfer

process and the adsorbtion process thus directly controls the carrier density.

It has been observed that different adsorbed species become activated at different

temperature ranges. For instance, the following conversion scheme for oxygen species as

the function of increasing temperature, has been suggested [23]

2O (room temperature) → 2O

− (upto 200

oC) → 2O

− (upto 400oC) → 22O

− (above 400oC)

The trapping of conduction electrons to form negatively charged oxygen species

may be represented as 2 2O e O−

+ → , 2 2O e O− −+ → etc. The chemisorbed oxygen on the

surface and at the grain boundaries thus acts as an acceptor-like trap state causing a large

reduction in the electrical conductivity of the oxide material. A surface barrier is

produced by the chemisorbed oxygen species through electron exchange with the oxide

material and the grain boundary barrier height is modulated. The overall resistance is thus

governed by the non-stoichiometric defect states (acting as donor states for conduction

electrons) and chemisorbed oxygen species (acting as acceptor states for conduction

electrons). From a practical viewpoint, the most important consequence of chemisorption

is the ability of the oxide semiconductors to catalyse gas phase chemical reactions on

their surfaces. The resistance of the material is sensitive to the coverage of adsorbed

oxygen and any factor that changes this coverage will change the resistance. For a

semiconducting metal oxide, this can result in a measurable change in the electrical

conductivity, a phenomenon that is the basis of gas sensing sensor. The sensors in

resistive mode make use of this change in resistivity occuring on interaction with the gas

molecules. On exposure to a reducing gas, the trapped electrons are returned to the

conduction band due to the interaction of the reducing gas molecules and the

chemisorbed oxygen [24]. So the resistance of the oxide material changes. This effect has

been interpreted as the mechanism of gas sensing. The reducing gas molecule itself gets

oxidized in this process.

6

1.2 Thin films of ZnO and doped ZnO

Due to its versatility, ZnO has drawn considerable attention and has been

prepared and investigated in various physical forms such as single crystals,

sintered/ceramic pellets, thick films, thin films and nanostructures etc. [6, 9-10, 14, 25-

28]. Among different physical forms, the thin films of ZnO find a multitude of

immensely important applications in electronic and optoelectronic devices such as

photothermal conversion systems, transparent conductors, gas sensors for toxic and

combustible gases and heat mirrors among many others [11-13, 24]. It is also being

considered as a potential candidate in the new frontiers of research like spintronics [27].

Thin films form the basic for many electronic components and are of particular interest

for fabrication of large area arrays. Thus most of the device applications require ZnO in

polycrystalline thin film form. Simultaneous occurrence of high optical transparency

(≥80%) in the visible region and high conductivity may be conveniently obtained by

controlling the non-stoichiometry and/or dopants. Thus the thin films have been widely

studied during the last few decades because of their technological applications,

particularly in the field of semiconductor electronics.

A thin film can be visualised as a near surface region of a material whose

properties are different from those of the bulk. In general any solid or liquid system

possesing at most two-dimensional order of periodicity may be called a thin film. Thus a

thin film is a microscopically thin layer of material that is deposited onto a substrate [29].

The substrate may be glass, mica, metal or ceramic etc. Though a thin film, in general,

has a thickness of 1.0 µm or less, in practice, the thickness of a thin film may range from

a few hundred angstroms to several microns (0.01 - 10 µm). Films typically used in thin-

film applications range from a few angstroms to 100 µm thick (the width of a human

hair). Films of the order of few nanometer thicknesses are possible to fabricate in which

case they are called ‘Ultra thin films’. Thin materials may also be formed from a liquid or

a paste, in which case it is called a ‘thick film’.

7

The physical properties of polycrystalline thin films are different from those of

bulk single crystals in the sense that they are modified by their thickness as well as the

crystallite size. The physical properties include structural properties, electrical properties,

optical properties, mechanical properties etc. The change in structural properties includes

change in lattice parameters, partuicle size or grain size, stress, strain, etc. The change in

optical properties occurs in terms of band gap and other optical constants. The change in

electrical properties refers to change in carrier density, mobility etc. Also the way the

film is prepared affects its microstructure and properties. These perturbations affect the

electrical properties much more than optical properties since the band structure is

unaltered inside the bulk. In addition to thickness and crystallite size, the lattice impurity

and other structural defects also affect the electrical properties (e.g. conductivity) of the

films [30].

For polycrystalline thin films, the electrical properties are modified by the grain

boundaries as well. For polycrystalline oxide materials, such as zinc oxide, the electrical

properties are further modified by the adsorption of oxygen at the grain boundaries and

also on the surface. These grain boundaries generally contain fairly high density of

interface states which trap carriers from the bulk of the grain and scatter free carriers by

virtue of their inherent disorders and the presence of trapped charges. The interface states

result in a space charge limited region at the grain boundaries [31] which results in

potential barriers to charge transport.

Pure zinc oxide thin films have certain limitations in their application. They are

not stable against corrosive environments and in humid ambient and lack stability in

terms of thermal edging in air [32-33]. Adsorption of oxygen in the films modifies its

electrical conductivity and also modifies the surface morphology. To stabilize the ZnO

system against such changes and also to widen the potential areas where ZnO thin films

can be applied, dopant ions have to be incorporated into them to obtain certain desired

properties like wider or narrower band gap, higher optical absorbance, lower or higher

melting point, ferromagnetism, etc. Therefore polycrystalline ZnO films have been doped

8

with metals of group I. group II, group III and group V. Accordingly doped ZnO thin

films with improved stability and suitable structural, electrical and optical properties are

in constant demand for their potential application prospects.

Polycrystalline films of ZnO have been doped to enhance their properties with

Lithium ( )Li [34], tin ( )Sn [35], cadmium (Cd) [36], manganese (Mn) [37], silver (Ag)

[38], copper (Cu) and iron (Fe) [39], gallium (Ga) [40], indium (In) [41], aluminium (Al)

[15, 42], nickel (Ni) [43], phosphorous (P) [44-45], nitrogen [46] etc. Doping with IB

acceptors (Cu, Ag, Au) reduces the emission in the UV region and intensifies in the

visible region [47]. Doping with silver, phosphorous, nitrogen etc. has been primarily

done with the objective to get stable p-type conductivity [38, 44,46]. Doping with Gr. III

metal ions (such as Al , In ) is particularly done to get high transparency, stability and

high conductivity. On the other hand doping with Fe, Cu, Co etc. has been primarily

carried out to study their magnetic properties. Copper doping also shows interesting

thermoluminescence properties [48]. Another group III metal nickel (Ni) has been doped

for ferromagnetic study.

Mn doped ZnO (Mn:ZnO) is an extremely important material for its coexisting

magnetic, semi-conducting and optical properties [37]. Mn:ZnO is regarded as promising

material for spintronic applications as it shows room temperature ferromagnetism [49]. It

has also been utilized as a material for the manufacture of solar cells, transparent

electrodes, gas sensors, varistors, piezoelectric transducers, etc, due to its behaviour as a

dilute magnetic semiconductor (DMS) [50]. However the microstructural effects of Mn

doping in ZnO thin film are not well established [37].

Cd doping has been found to impart stability and also Cd doped ZnO films are

useful humidity sensors [51]. Cadmium oxide possesses cubic structure and a narrow

direct band gap of 2.3 eV, whereas ZnO possesses is a wide band gap of 3.2 eV [52-53].

Hence, it is possible to modify the physical properties of ZnO upon mixing with CdO.

Due to variation of the bandgap with doping percentage of Cd, they can be used as an

9

excellent candidate for the preparation of quantum wells, superlattices and other

configurations that involved bandgap engineering [51]. Cd doped ZnO nanowires show a

positive temperature coefficient of resistance effect, which is quite abnormal to pure ZnO

nanowires [54]. Although cadmium doped ZnO is one of the promising candidates in the

field of optoelectronics and also for the fabrication of ZnO based devices, the knowledge

of the physical properties of Cd doped ZnO has been very limited until recent times [55].

Doping with Al is primarily done to achieve high transparency, stability, high

conductivity and also, because it enhances the gas-sensing properties of the ZnO thin

films, which have immensely important industrial and domestic applications for detecting

hazardous gases, such as LPG [42, 56]. The enhancement of bandgap energy due to Al

incorporation offers the possibility to tailor its optical property. The bandgap

enhancement is particularly significant for nanocrystalline thin films. For transparent

conducting oxide (TCO) thin films like ZnO, it is always desired to improve the electrical

conduction without affecting its excellent optical properties. As such, it is very important

to optimize the process parameters of film growth and doping levels to have a desired

enhanced device performance. Al is chosen as dopant material because of its easy and

abundant availability. AZO films have got potential applications in solar cells, solid-state

display devices, optical coatings, heaters, defrosters, chemical sensors etc. [57-58].

Accordingly, synthesis of polycrystalline AZO thin films has been widely carried out

using different techniques.

Ni doped ZnO is considered as an important II-VI diluted magnetic

semiconductor (DMS) material due to its unique magneto-electrical and magneto-

transport properties [59]. Room temperature photoluminescence has been observed for

Ni-doped ZnO films. However the mechanism of conductivity change due to Ni

incoporation is still inconclusive [60]. The microstructural and optical properties of ZnO

are very much sensitive to the method of preparation, the type and amount of dopants.

Thus synthesis and characterization of doped ZnO thin films via different techniques

have attracted considerable attention.

10

1.3 Thin films and their deposition techniques

Thin films can be synthesized by many different processes. A thin film deposition

technique involves three steps: (i) Creation of atomic/molecular/ionic species, (ii)

transport of these species through a medium, and (iii) condensation of the species on a

substrate. The growth of a thin film can take place by different modes. One such is layer-

by-layer mode. In this case thin film is formed layer by layer on the substrate. This is

followed by formation of three-dimensional nuclei. Another possible mode is direct

three-dimensional growths of discrete nuclei. Depending on whether the species has

been created by a physical or a chemical process, thin film deposition techniques can be

broadly divided into two categories: physical and chemical [29]. In physical methods the

film material is moved from a target source with some form of energy to the substrate.

Chemical film fabrication methods involve chemical reactions and the precursors are

mostly components undergoing reaction at the substrate surface or in the vicinity of the

substrate.

In physical deposition technique, film is formed by atoms directly transported

from source to the substrate through gas phase [29]. The physical routes include different

forms of sputtering and evaporation. Commercial physical deposition systems require a

low-pressure environment and are classified as Physical vapor deposition or PVD. The

different physical vapor deposition systems includes evaporation, thermal evaporation,

electron beam evaporation, sputtering, reactive PVD etc.The material to be deposited is

placed in such a way that particles of material escape from the surface. They are then

allowed to arrive on a substrate to form a solid layer. The whole system is kept in a

vacuum deposition chamber. Since particles tend to follow a straight path, films

deposited by physical means are commonly directional, rather than conformal.

In evaporation technique, the material to be deposited is evaporated and the

evaporant vapor is allowed to impinge on the surface of the substrate. The evaporant

condenses on the substrate and is absorbed on it. This is done in a high vacuum. The

11

temperature of a material for evaporation may be raised by direct or indirect heating. In

thermal evaporation technique, an electric resistance heater is used for this purpose.

Electron beam evaporation fires a high-energy electron is used to boil a small spot of

material. Molecular beam epitaxy is a particular sophisticated form of thermal

evaporation. In this technique, slow streams of an element are directed at the substrate, so

that material deposits one atomic layer at a time [29]. The beam of material can be

generated by either physical means or by a chemical reaction (chemical beam epitaxy).

Sputtering is a popular method and one of the most flexible deposition techniques

for adhering thin films onto a substrate. In this technique, energetic ions in plasma

(usually a noble gas, such as Argon) are used to knock out or eject a few atoms at a time

from a "target". The ejection process, known as sputtering, takes place as a result of

momentum transfer between the impinging ions and the atoms of the target surface. The

sputtered atoms are condensed on a substrate to form a film. Since the process is not one

of evaporation, it is particularly suitable for compound or mixtures compared to

evaporation techniques where different components would tend to evaporate at different

rates. Different versions of sputtering are used by researchers. These are direct current

(dc) sputtering, where a dc current is used, radio frequency (rf) sputtering, where a rf

current is used and dc magnetron sputtering where a magnetic field is also applied. The

magnetic field is applied to confine the path of the ions.

In dc sputtering, a dc voltage is applied between the cathode (target) and anode

(on which the substrate is placed). The sputtered atoms reach the substrate with

randomized direction and energies due to collisions with gas atoms. If the cathode is an

insulator material, dc sputtering is not possible owing to building up of positive surface

charges. A high frequency rf field is applied in this case. Arrangements in which the

applied field is perpendicular to each of the electric and magnetic field is termed as

magnetron sputtering [29]. In reactive sputtering, a small amount of some non-noble gas

such as oxygen or nitrogen is mixed with the plasma-forming gas. After the material is

sputtered from the target, it reacts with this gas, so that the deposited film is a different

12

material, i.e. an oxide or nitride of the target material. Pulsed Laser deposition systems

work by an ablation process. Pulses of focused laser light vaporize the surface of the

target material and convert it to plasma.

Some methods fall outside these two categories i.e. physical and chemical

techniques. These are Chemical vapor deposition (CVD), Oxidation, and Plating etc. In

fact, reactive evaporation and sputtering is also referred to as hybrid techniques where

PVD and CVD are combined. In CVD, the film is formed through chemical reaction on

the surface of substrate followed by surface absorption. The technique generally uses a

gas-phase precursor, often a halide or hydride of the element to be deposited. The

reactive gas is introduced into the chamber and is allowed to decompose by heat or

plasma. The decomposition requires 800-1300oC [29]. The different CVD techniques are

Low-Pressure CVD (LPCVD), Plasma-Enhanced CVD (PECVD), Atmosphere-Pressure

CVD (APCVD) and Metal-Organic CVD (MOCVD)

In the case of MOCVD, an organometallic gas is used. PECVD uses an ionized

vapor (or plasma) as a precursor. The ionized plasma is used to transfer energy to the

reacting gases resulting in decomposition. Commercial PECVD relies on electromagnetic

means (electric current, microwave excitation), rather than a chemical reaction, to

produce plasma [29].0 Plating relies on liquid precursors, often a solution of water with a

salt of the metal to be deposited. Some plating processes are driven entirely by reagents

in the solution (usually for noble metals), but by far the most commercially important

process is electroplating.

Conventional physical rotes (vacuum techniques such as sputtering and

evaporation) renders better control over stoichiometry produces uniform and compact

films and generally produces good quality transparent films. They are generally safe (no

toxic gas emissions) and performs high deposition rate at room temperature. However

they require expensive capital instruments. Accordingly they are difficult to expand to

large scale. Chemical techniques of thin film deposition involving aqueous solution on

13

the other hand are cost-effective compared with vapor-phase techniques and simple. Thus

they offer the desirable cheapness and possibility of scaling up to industrial level.

Accordingly, chemical techniques have come out to be a good alternative for material

preparation during the past few decades. In chemical deposition techniques, a liquid

precursor undergoes a chemical change at a solid surface, leaving a solid layer. Thin

films from chemical deposition techniques tend to be conformal, rather than directional.

The different category of chemical deposition techniques include: Spray pyrolysis, Sol

gel, Chemical Bath deposition (CBD), Electroless deposition, Electrodeposition,

Anodization, Electrophoresis and SILAR etc.

Spray pyrolysis is a method of depositing films having thicknesses in the region

between thin film and thick film. Film deposition is carried out by spraying a solution

containing soluble salts of the constituent atoms of the desired compounds onto a

substrate. The substrate is maintained at elevated temperatures (typically 300-700°C).

The sprayed droplet reaches the hot substrate and undergoes pyrolytic (endothermic)

decomposition and forms a single crystallite or a cluster of crystallites of the product. The

thermal energy required for decomposition is provided by the hot substrate. The other

volatile by-products and the excess solvent escape in the vapor phase. Post deposition

sintering helps to the recombination of the constituent species and clustering. Finally a

coherent film is obtained. Several parameters affecting the deposition mechanism and

film properties are solution concentration, solution flow rate (spray rate), substrate

temperature, nature of the substrate, sprayer tip to substrate distance etc. Spray deposited

films generally have a rough microstructure. Oxides, sulphides and selenides are prepared

by this technique.

Electroless is a process typically used to obtain thick (micrometers) metal

structures on metallic or nonmetallic substrates. The process offers simplicity and

cheapness. In this method, the substrate on which the film to be deposited is chemically

activated and introduced in a solution containing a reducible form of the ion of the

desired metal. The ions are reduced at the substrate surface and the insoluble metal atoms

14

are incorporated into the surface. The process is often referred to as autocatalytic coating

technique. The occurrence of chemical changes owing to the passage of electric current

through an electrolyte is termed electrolysis. The deposition of any substance on an

electrode as a consequence of electrolysis is called electrodeposition. The phenomenon

of electrolysis is governed by Faraday’s laws [29].

In anodization technique, the metal to be anodized is made an anode and

immersed in an oxygen-containing electrolyte. The electrolyte may be aqueous,

nonaqueous or fused salt. The pH of the electrolyte plays an important role in obtaining a

coherent film. Growth may take place at constant voltage or at constant current. In

electrophoresis technique, electrically charged particles suspended in a liquid medium

are deposited on an electrode. The as-deposited films are loosely adherent coatings of

powder. Further post deposition treatment leads to adherent, compact and mechanically

strong surface coating. The electrophoresis technique is used to the deposition of both

conductors and nonconductors, including metals, alloys, salts, oxides, polymers etc. Sol-

gel method gained much interest because of its simplicity, low processing temperature,

stoichiometry control and its ability to produce uniform, chemically homogenous films

over large areas that can provide integration with other circuit elements. The substrate is

dipped in sol and gel to get the film [29].

In chemical bath deposition (CBD) method, deposition of thin films occurs due

to substrate maintained in contact with dilute chemical bath containing cationic and

anionic solutions [29, 61]. The film formation on substrate takes place when ionic

product (IP) exceeds solubility product (SP). However this results into precipitate

formation in the bulk of the solution which cannot be eliminated.

According to the solubility product principle, in a saturated solution of a weakly

soluble compound, the product of the molar concentrations of its ions (each concentration

term being raised to a power equal to the number of ions of that kind) is called the ionic

product. This is a constant at a given temperature and this constant is called the solubility

15

product. When the solution is saturated i.e. at equilibrium, IP SP= . But when the ionic

product exceeds the solubility product (in a supersaturated solution) i.e. SPIP⟩ ,

precipitation occurs [29, 62]. Under this situation ions combine on the substrate to form

nuclei. The kinetics of growth of a thin film in this process is determined by the ion-by-

ion deposition on the immersed surfaces. Initially, the film growth rate is negligible

because an incubation period is required for the formation of critical nuclei from a

homogeneous system onto a clean surface. Once nucleation occurs, the rate rises rapidly

until the rate of deposition equals rate of dissolution i.e. IP SP= . Consequently, the film

attains a terminal thickness. It seems that precipitate formation and wastage of material is

a common problem in CBD since ions combine to form nuclei in the solution also.

It is seen that different techniques ranging from simple to sophisticate ones has

been used to dope ZnO. Among the various chemical techniques, Spray pyrolysis is a

high temperature process and choice of suitable precursor solution is often not

convenient. Sol-gel is considered to be superior to most other conventional chemical thin

film deposition techniques for fabricating stoichiometric polycrystalline and uniformly

doped semiconducting thin films. However it requires costly precursors or sophisticated

reaction setups and the choice of the solvent is also often not convenient. Electroless

deposition is characterized with poor coverage whereas precipitate formation and wastage

of material is a common problem in chemical bath deposition (CBD). A relatively less

utilized and less investigate chemical technique is SILAR (Successive ionic layer

adsorption and reaction or Successive Ion layer adsorption and recation) [62-63]. In this

technique, thin films are obtained by immersing the substrate into separately placed

cationic and anionic precursors. Thus precipitate formation and wastage of material is

avoided in this technique. Accordingly, SILAR is often termed as modified version of

chemical bath deposition (modified CBD). Film formation takes place when SPIP⟩ .

Temperature, solvent, and particle size affect the solubility product [64]. The rate at

which nuclei forms on the substrate surface depends on the degree of supersaturation (S)

which is the ratio of IP and SP.

16

1.4 The technique of SILAR

The most used solution technique and also one of the oldest methods for thin film

growth is chemical bath deposition (CBD), sometimes called chemical deposition (CD),

or chemical solution deposition (CSD). CBD has been widely used for the deposition of

metal sulphides for various applications [65]. In CBD all the precursor ions (cations and

anions) are present at the same time in the reaction vessel. Typically CBD has a so-called

terminal thickness indicating a point where the growth of thin film is stopped due to

depletion of precursors in the solution. The film formation on substrate takes place when

ionic product exceeds solubility product. This results into precipitate formation in the

bulk of solution, which can not be eliminated. This causes the unnecessary formation of

precipitation and loss of material.

In SILAR, on the other hand, thin films are obtained by immersing the substrate

into separately placed cationic and anionic precursors for reaction at chosen temperatures.

Between every immersion the substrate is rinsed in distilled water or ion exchanged water

to avoid homogeneous precipitation in the solution. Sequential reaction on the substrate

surface under optimized conditions of concentration and pH of the reacting solutions

results in the formation of the film. Thus, precipitation formation i.e wastage of material

is avoided in SILAR method.

The SILAR method is a relatively less used and less investigated method. The

method was initially reported by Call et. al. [66] as chemical deposition method. It was

then used for copper oxide film deposition by Ristov et. al. [67]. The name SILAR was

ascribed to this method by Nicolau [68] since it involves ion-by-ion deposition and

discussed in subsequent papers of Nicolau and co-workers [69-70] and Ristov et. al. [71],

which deals with ZnS, CdZnS, CdS and ZnS thin films. Later on the technique has been

extended by many workers primarily to deposit sulphide thin films (ZnS, CdS, PbS, CuS,

MnS etc. and their doped versions) [62, 72-76] and not much effort has been made to

deposit oxide thin films and their doped version by this technique.

17

The basic building blocks in SILAR are ions instead of atoms. The substrate can

be introduced into various reactants for a specific length of time depending on the nature

and kinetics of the reaction. The immersion-reaction cycle can be repeated for any

number of times, limited only by the inherent problems associated with the deposition

technique and the substrate-thin film interface. The technique is called SILAR since it

involves adsorption of a layer of complex ion on the substrate followed by reaction of the

adsorbed ion layer. The different parameters that can affect the film growth process are

the nature of the bath solution, concentration of the bath solution and its pH value, nature

of the substrate and temperature of deposition. By proper optimisation of the deposition

parameters, good quality film can be achieved. Apart from being a relatively less studied

and less used process, it is an extremely simple to carry out. The thickness can be easily

controlled easily by varying the number of deposition cycles and thus both thin and thick

films can be prepared by this method.

In spite of its simplicity SILAR has number of advantages [62]: (i) unlike vapour

deposition method, SILAR does not require vacuum at any stage; (ii) The deposition can

be carried out at or close to room temperature; (iii) unlike high power methods such as

radio frequency magnetron sputtering, it does not cause local over heating that can be

detrimental for materials to be deposited and (iv) there are virtually no restrictions on

substrate material, dimensions or its surface profile. Thus, any insoluble surface to which

the solution has free access will be a suitable substrate for the deposition making the

technique convenient for large area deposition.

Adsorption of a substance on the surface of another substance is the basis of

SILAR [62]. Adsorption may be expected when two heterogeneous phases are brought

into contact with each other. In SILAR method, one is only concerned with adsorption in

liquid-solid sytem. The adsorption is a surface phenomenon between ions and surface of

the substrate and is possible due to attraction force between ions in the solution and

surface of the substrate. These forces may be cohesive or van-der-waals or chemical

attractive forces. Atoms or molecules of substrate surface are not surrounded by atoms or

18

molecules of their kind on all sides. So, they posses unbalanced or residual force and thus

can hold ions on them. The factors like concentration nd temperature of solution,

pressure, type of the substrate, area of the substrate etc. affect the adsorption process. The

reaction between pre-adsorbed cations and newly adsorbed anions forms the thin films of

desired material [62]. Figure 1.2 schematically presents SILAR growth.

Figure 1.2: Schematic of SILAR method [62]

In the first step of SILAR process, the cations present in the precursor solution are

adsorbed on the surface of the substrate (step a: adsorption of cations). In the next step,

excess adsorbed ions are rinsed away from the diffusion layer (step b: water rinsing of

loosely bound cations in deionized or distilled water). In the third step, the anions from

anionic precursor solution are introduced to the system (step c: adsorption of anions and

reaction of pre-absorbed cation with newly absorbed anion). In the last step of a SILAR

deposition cycle, the excess and unreacted species and the reaction byproduct from the

diffusion layer are removed (step d). This gives a material composed of two layers: the

inner (positively charged cations) and outer (negatively charged anions) layers. These

two layers form the Helmholtz electric double layer [62]. The substrate can be introduced

19

into various beakers containing the ionic precursors for a specific interval, withdrawn and

reintroduced into another beaker for reaction. By repeating these cycles a thin layer of a

material can be grown.

The maximum increase in the film thickness for one complete reaction cycle

(dipping cycle) is theoretically one monolayer. Dividing the measured overall film

thickness, by number of reaction cycles a numerical value of growth rate can be

determined [62]. A homogenous precipitation in the solution can result if the measured

growth rate exceeds the lattice constant of the material. In practice, however, the

thickness increase is typically less than or greater than a monolayer. The factors affecting

the growth phenomena are the quality of the precursor solutions, their pH and

concentrations, counter ions, individual rinsing and dipping times. In addition,

complexing agent and pretreatment of the substrate have been shown to affect the SILAR

growth.

It appears from the above discussion that SILAR is based on sequential reaction

of cations and anions at the substrate surface. Rinsing follows each reaction, which

enables heterogeneous reaction between the solid phase and the solvated ions in the

solution. If the anionic bath is water, SILAR reduce to a two-step process.

20

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24

CHAPTER 2

Literature review and Aim of the work

2.1 Review of Literature

Zinc oxide, being a very old material, has been prepared and characterized

thoroughly for their different properties both in undoped and doped form. Consequently

an extensive literature is available on this material. To date, a large number of

conventional as well as novel techniques of thin film deposition have been employed to

deposit thin films of ZnO and doped ZnO. The techniques used are several variants of the

evaporation and sputtering processes, LASER assisted techniques, different types of

chemical vapour deposition, spray pyrolysis, sol-gel, spin coating etc. This chapter

contains a brief survey of literature on different preparation techniques that has been

applied to deposit ZnO and doped ZnO thin films and their properties with particular

reference to structural properties such as preferred orientation, paticle size etc. The

morphological, electrical and optical properties of the synthesized films have also been

addressed in brief.

Almost all the usual preparation techniques used for thin film preparation

produces polycrystalline ZnO films. Earlier, vapour phase and reactive sputtering method

were the most applied techniques for the preparation of thin films of ZnO [1]. Good

25

conducting transparent ZnO was first fabricated in 1979 [2] using an enhanced reactive

evaporation technique for solar cell applications. Since then the technique has been

utilised [3-5] to prepare ZnO films. The films have been prepared by direct evaporation

of zinc oxide source or from a source of metallic zinc by reactive evaporation. Electron

beam evaporation technique has been utilized to deposit conducting and transparent films

of ZnO [6-7]. The resistivity and the rate of deposition have been found to depend on

oxygen partial pressure, input power and substrate temperature.

Sputtering is the most commonly used technique usd to obtain uniform films with

good orientation. Different sputtering techniques have been commonly used for the

deposition of ZnO thin film with substrate temperature ranging from room temperature to

500oC [8-27]. Apart from glass or corning glass substrate, sapphire, silicon (100),

alumina, quartz and metal substrates has been used in these works. Non-stoichiometric

ZnO films can be prepared by sputtering either a metallic zinc target in the presence of an

oxygen-argon atmosphere, or from an oxide target, usually in a gas mixture of hydrogen

and argon. For sputtered films, the structural properties and growth rates are strongly

influenced by various processing conditions, such as the gas phase composition, sputter

gas pressure (high pressure yields more porous layer and low pressure induces columnar

growth), plasma condition, sputter power (which influences actual deposition temperature

and diffusion at the surface during deposition), substrate type (which affects initial

growth and thus determines to a great extent the structure of the film) and finally, the

deposition geometry. Other than the deposition parameters, postdeposition treatments

also greatly affect the properties of the films. In general, crystallinity improves with

increase in substrate temperature. Further, if the distance between the substrate and the

target is less than the mean free path of the zinc atoms, films of better texture are

observed.

ZnO films have been grown using several variants of chemical vapour deposition

technique for piezoelectric, electro-optic and guided wave device applications [28-42]. In

conventional CVD systems hydrogen is normally used as the reducing gas for the ZnO

26

source, but these workers used NH3 in order to slow down the growth rate and improve

the film quality. The parameters governing the film growth rate in CVD process are

source temperature, substrate temperature, flow rate of the carrier gas and the distance

between source and the substrate. Natsume et. al. [34] deposited ZnO film by normal

CVD method with substrate temperature between 500-600oC. To obtain highly oriented

film growth at low substrate temperature, a variety of deposition methods have been

used. Shimizu et. al. [28] grew the film in the temperature range 150-350oC by plasma

enhanced MOCVD (Metal Oxide Chemical Vapour Deposition) using diethylzinc as the

source material. The adhesion of these films on sapphire substrate was stronger than that

on glass substrate. The same group [32] also reported photo-induced MOCVD growth of

ZnO film using a high pressure mercury lamp or xenon-mercury lamp as the light source.

Solanki and Collins [29] adopted a laser assisted MOCVD growth process using excimer

laser source. This method has got certain significant features e. g., lowering of substrate

temperature, selective deposition, high deposition rate and better crystal surface quality.

The properties of these films compared well with those of the films grown by the

MOCVD process. Khan and O’brian [31] used anhydrous zinc acetate for the growth of

ZnO in a low-pressure MOCVD method in the temperature range 350-420oC. The use

of zinc acetate as precursor for ZnO film deposition was first reported by them [31]. The

CVD technique has also been exploited to grow transparent conducting ZnO films. The

growth rate in this process was controlled by a complex multistep oxidation process that

was dominated by chemical reactions among free radicals.

Atomic layer epitaxy (ALE) and molecular beam epitaxy techniques have been

used to deposite the ZnO film [43-44]. Laser assisted techniques [45-50] has been

successfully utilised to deposit ZnO layer at atmospheric pressure. Lanno et. al. [45]

deposited the film by using pulsed laser e.g., a Q-switched Nd: YAG laser and a KrF

excimer laser. PLD (Pulsed Laser Deposition) method does not demand any further post-

deposition annealing treatment of the grown films and have been utilized to grow ZnO

thin film [49-50].

27

A number of cost effective chemical techniques also have been utilized to deposit

ZnO thin film. Transparent conducting films of zinc oxide have been successfully

prepared using spray pyrolysis technique [51 - 65]. This is a very useful and simple

technique for the preparation of different semiconducting thin films. It has been found to

be suitable for the preparation of ZnO films particularly for solar cell and gas sensor

applications. In this technique, an aqueous solution of zinc acetate is usually used as the

spray solution. This precursor is selected due to its high vapour pressure at relatively low

temperatures. The addition of a few drops of acetic acid prohibits the precipitation of zinc

hydroxide thereby making the solution suitable for spraying. This helps in producing

better quality optically transparent films. In this technique, chemicals in the form of

atomized droplets are brought in contact with the pre-heated substrate whose temperature

normally varies between 150-550oC. The deposition parameters that control the quality of

the film in this technique are substrate temperature, solution flow rate, air (carrier gas)

flow rate etc. Rol of substrate temperature on strucrural and morphological properties has

been reported by Gumus et. al. [62]. Wu et. al. [52] applied spray pyrolysis technique

using ultrasonic nebulization of zinc acetate solution. They obtained ZnO film on silicon

and silica substrates at 380oC. The spray pyrolysis process allows the coating of large

surface and it is compatible with mass production systems [61].

Dipping of the substrate in a liquid containing the metal ions followed by heat

treatment is one of the simplest methods of film preparation. The liquid can be either a

simple solution or sol or gel. Sol-gel has been widely used to synthesize ZnO thin films

[66-73]. Later, the technique has been used by many others [69-73] due to its simplicity

and non-requirement of any special apparatus. Ghodsi et. al. [73] used zinc acetate to

prepare ZnO films on glass substrate while Gupta et. al. [71] synthesized films on

conducting glass support (SnO2:F overlayer).

The chemical bath deposition (CBD) technique is an open-bath wet-chemical

method that has been widely employed for the synthesis of metal oxide thin films [74-

78]. A two-step modified CBD technique has been reported by Vijayan et. al. [77] to get

28

good quality adherent thin films of ZnO. On the other hand, a much simpler process is

chemical dipping in which the film is prepared from a simple aqueous solution instead of

a non-aqueous medium used in the sol-gel method. ZnO films by chemical deposition

were first obtained as a byproduct in an attempt to prepare the composite CdxZn1-xS film

by Call et. al. [79]. A precipitate of zinc hydroxide was formed by adding sodium

hydroxide to zinc sulphate. Heating the solution to 80-90oC initiated ZnO deposition.

The name SILAR was ascribed to this method by Nicolau et. al. [80] with the name

SILAR since it involves adsorption of a layer of complex ion on the substrate followed

by reaction of the adsorbed ion layer. Lupan et. al. [81] synthesized ZnO films using

sodium zincate bath as cationic precursor and named the technique succesisive chemical

solution deposition (SCSD). Recently Raidou et.al. [82] reported synthesis of ZnO thin

films by SILAR using ammonium zincate as cationic precursor. Different parameters

governing the film growth by SILAR includes choice of suitable metal salt complex as

precursor, temeprature of deposition, and concentration and pH of the reactant solution

among many others.

Another simple process, electroless deposition technique has also been utilised to

grow ZnO film [83-84]. In this technique, controlled homogeneous precipitate of metal

hydroxide is obtained by slow reaction on the surface of the substrate and the

corresponding oxide is obtained by post deposition heat treatment. Synthesis of ZnO

films at as low as 65oC has been reported using electrodeposition on ITO (In-coated tin

oxide) substrate [85]. Lupan et. al. reported preparation of ZnO thin films by

electrochemical deposition [86].

To conclude the discussion on the film deposition techniques, it may be noted that

various methods, ranging from a simple to a sophisticated one, have been utilised by

researchers to obtain ZnO film. The properties of the resulting films depend markedly on

the deposition parameters of each technique. As ZnO films have a large number of

commercial applications, the requirements of each application guide one to choose a

particular method over others.

29

To characterise the thin films of ZnO with respect to different growth and

processing conditions, X-ray diffraction analysis is normally performed. The most

important planes of ZnO with hexagonal wurtzite structures are (100), (101), (002) and

(110) [79]. It has been observed that with the increase of substrate or growth temperature,

the intensity of (002) peak perpendicular to the substrate increases at the expense of (100)

peak together with the increase of crystallite size i.e. crystallinity of the films. The effect

is almost universal irrespective of the preparation technique used. Such preferred

orientation is one of the most important properties of thin films. Together with the film

thickness and the microstructure, it is very important to control the orientation of the

crystallites in the films particularly for some of the device applications.

Pronounced (002) orientation of the crystallites was observed for ZnO films

prepared by activated reactive evaporation [5]. Other peaks of (101) and (102) were

present with small intensities. For sputtered films, the development of c-axis preferred

orientation was found to depend on the sputtering conditions. Films deposited from pure

metallic Zn and ceramic ZnO targets using dc magnetron sputtering also shows (002)

texture [21]. Films synthesized by rf sputtering on both glass and silicon

[ (001)]Si substrate [22, 25] shows preferred growth along c-axis. Strongly c-axis oriented

ZnO films on either Si/SiO2 or Si substrate has been reported using rf magnetron

sputtering [19]. The films exhibited a sharp fundamental absorption edge with a band gap

width of 3.31 eV [19]. Lee et. al. [8] also reported strong c-axis orientation in rf

magnetron sputtered film. They further reported that the strong c-axis orientation is a

function of film thickness. Atom beam sputtered films has been reported to have

preferred (002) growth [24]. Effect of surface roughness on the c-axis preferred

orientation for ZnO films deposited by rf magnetron sputtering has been discussed by Lee

et. al. [18]. Although Major et. al. [9] observed that orientation is independent of film

thickness, a decrease in preferred orientation was observed with increasing film thickness

for sputtered films [8]. Sputtered films are polycrystalline with grain size depending on

the substrate temperature. Higher substrate temperature results in larger grain sizes. Grain

size distribution in sputtered films is nearly monomodal. Jeong et. al. [20] reported

30

excellent transmittance (~95%) in the visible region for sputtered films. Hezam et. al.

[26] also reported high transmittance of ~85% in the visible region with an optical band

gap of 3.28 eV. The films are highly c-axis oriented with nearly spherical grains [26].

Suchea et. al. reported grain dimension in the range 10-50 nm for dc magnetron sputtered

ZnO film [21].

Natsume et. al. [34] obtained highly c-axis (002) oriented film produced by CVD

at a temperature range of 500-600oC. Maruyama and Shionoya [36] deposited c-axis

oriented ZnO films by a CVD process at 180oC. Shimizu et. al. [32] utilised plasma

enhanced MOCVD (PEMOCVD) technique to produce c-axis oriented ZnO films in the

temperature range 150-300oC on glass substrate and epitaxial film on sapphire

substrate.Different variants of CVD has been reported to grow preferred c-axis oriented

films [39-42]. Films synthesized by PLD [45] demonstrated how the preferential c-axis

oriented growth could be achieved by optimising the laser wavelength, fluence and

substrate temperature. Using PLD, Kotlyarchuk et. al. [49] repoted strongly oriented

grains in the basal plane direction and were grown along c-axis. Films had good

adherence with an optical transparency of ~85%. Average particle size of 55nm was

obtained from measured FWHM (Full width at half maximum intensity) value of 0.16o

[49].

XRD studies of spray pyrolytically deposited films [87] indicates that at

deposition temperatures less than 300oC, (101) and (100) are the most dominant

orientations. The (002) plane is present at these temperatures with significantly less

intensity. However, at an increased temperature (300oC), the (002) orientation becomes

progressively more important and the intensity of the (101) and (100) peaks start

decreasing. Film morphology is very sensitive to substrate temperature and thickness for

spray deposited films [87]. Uniform high quality ZnO thin films with high c-axis

orientation have been reported using spray pyrolysis [53, 58-60]. Highly transparent

polycrystalline ZnO thin film with a direct band gap of 3.27 eV has been reported using

spray pyrolysis [61]. The FWHM value for (002) peak was as low as ~0.23o and the

31

average particle size was 40 nm in this work [61]. Requirement of specific preparation

conditions and working temperature to obtain films having (100) or (002) orientation

using ultrasonic spray pyrolysis (USP) technique has been reported [62]. Ayouchi et. al.

[58] reported a dercease in bandgap from 3.33 eV to 3.31 eV as the substrate temperature

is increased. The resistivity of the films also increases and reaches maximum for 553K

substrate temperature. Selim et. al. [59] reported preferred c-axis orientation and the

polycrystalline film contained with needle like particles.

Although Okamura et. al. [88] reported non-oriented polycrystalline films

obtained using sol-gel, the technique has been successfully utilised to obtain highly c-axis

oriented ZnO films [67-68, 72]. Particle size of 40 nm has been repoted for sol-gel films

by Nirmala et. al. [72]. Using solgel technique, Khan et. al. reported particle size of ~20

nm [89] and Ilican et. al. [70] reported round grains with average particle size of 50 nm.

Crystallite size in the range 20-33 nm has been reported for spray pyrolysed film

[90] while particle size of 19.06 nm has been reported for spin-coated ZnO films [91]. In

all these reports evaluation of particle size was made using Scherrer relation according to

which the grain size is directly related to the full width at half maximum (FWHM)

intensity of X-ray diffraction peaky [92].

Polycrystalline hexagonal nano film with (002) preferred orientation has been

reported with a band gap of 3.24-3.27 eV and refractive index 2.29-2.34 using CBD

technique [77]. Electrodeposited films [85] has been found to give (101) as highest

orientation instead of (002) possibly due to low synthesis temperature. Call et. al. [79]

also obtained of ZnO with a slight preferred orientation toward (100). The film was

synthesized at 80-90oC. It has been observed that the direct band gap energy was

increased from 3.23 to 3.37 eV after annealing at 300oC. Electroless deposition also has

been reporte to give a band gap of 3 eV and transmittance of 85% [84].

32

It appears from the above discussion that c-axis orientation is a common

phenomenon in the ZnO film deposition by both physical and chemical processes. Such

preferred basal orientation is typically observed ZnO film since the surface energy

density of the (002) orientation is the lowest in hexagonal wurtzite ZnO structure [93].

Quantitative information concerning the preferential crystal orientation can be obtained

from the texture coefficient (TC) [94-95]. If TC(hkl)≈1 for all the (hkl) planes considered,

then the films are with a randomly oriented crystallite similar to the JCPDS reference. As

texture coefficient for a particular plane ( )hkl increases, this indicates that the preferential

growth of the crystallites in the direction of ( )hkl plane increases.

Reports on electrical measurements show wide scatter in resistivity value. Highly

pure (Stoichiometric) ZnO have very high resistance as expected. Deviation from

stoichiomtery produces zinc interstial and/or oxygen vacancy as donor states [77].

Electrons formed by the ionization of zinc atoms and/or oxygen vacancies directly

controls the number of charge carriers (electrons) available for conduction [77]. The film

stoichiometry depends on the deposition method used, deposition parameters, post

deposition treatments etc. The amount of nonstoichiometry controls the resistivity of ZnO

[17, 21]. Thus low resistive ZnO can be prepared either by adjusting the film

stoichiomtery or by doping [12]. Doping with appropriate metal atoms, such as, Al , Sn,

Ga, In, etc., the resistivity can be changed from values as high as 1010 Ω-cm to values as

low as 410− Ω-cm [21]. Jeong et. al. [20] reported highest resistivity of the order of 1410

Ω-cm for undoepd ZnO polycrystalline thin film while Al doping reduces it to the order

of 410− Ω-cm. The wide range of conductivities and conductivity changes make ZnO

films suitable for resistive mode gas sensors [21].

The morphology of the deposited films are influenced by many factors such as

deposition technique and deposition parameters, nature and temperature of substrate, post

depsotion treatments etc. Sputtered films have been reported to exhibit columnar

microstructure [8, 19] as well as spherical/nanogranular microstructure [21, 26]

33

depending on the dposition parameters. Morphology of films obtained by chemical routes

generally shows round or spherical shped grains, off spherical grains or bean like grains

[57-58, 72, 121, 128].

The first successful attempt to prepare Cd doped ZnO film by chemical technique,

to the best of our knowledge was made by Yogeeswaran et. al. [96] through combustion

synthesis of cadmium chloride ( )2CdCl in presence of zinc nitrate and urea. Cd-doping

resulted in band gap shrinkage [96] compared to pure ZnO and improved

photoelectrochemical response over the wavelength range ~300 to ~450 nm. Diffuse

reflectance spectroscopy showed the optical band gap of ZnO to shrink from 3.14 to 3.07

eV on Cd doping. Cd-doped ZnO nanowires exhibit a positive temperature coefficient of

resistance effect [97], which is quite abnormal as compared to pure ZnO. Further this can

be an excellent sensor material at room temperature. Although cadmium doped ZnO is

one of the promising candidates in the field of optoelectronics and also for the fabrication

of ZnO based devices [63], the knowledge of the physical properties of Cd doped ZnO

has been very limited until recent times. Cadmium oxide possesses cubic structure and a

narrow direct band gap of 2.3 eV, whereas ZnO possesses is a wide band gap of 3.2 eV

[98-99]. Hence, it is possible to modify the physical properties of ZnO upon mixing with

CdO. In recent times there are reports of Cd doped ZnO either in thin film or

nanostructured form with varying amount of Cd incorporation [63, 96, 100-105].

Substitution of zinc ion by isoelectronic element cadmium has been reported by

complicated physical processes such as pulsed laser deposition (PLD) [101], metal-

organic vapor phase epitaxy (MOVPE) [102], vapor-liquid-solid (VLS) [103]. Reports of

cadmium doped zinc oxide synthesized through chemical routes are relatively rare.

Among the chemical techniques, sol-gel [100, 104-105] and spray pyrolysis [63] has been

employed to deposit Cd doped ZnO films. The only effort to synthesize Cd doped ZnO

films by SILAR turned out to be unsuccessful [106]. The change in fundmental

absorption edge with Cd incorporation and cecrease of band gap with Cd doping has been

reported in these works [63, 105]. The optical constants of the films such as refractive

index, extinction coefficient and dielectric constants also changed with Cd doping and the

34

electrical conductivity of the films was improved by incorporation of Cd in the ZnO film

[105]. Scanning electron microscopy (SEM) images indicated that the films have a

wrinkle network with uniform size distributions [105]. Enhancement of polycrystallinity,

decrease of grain orientation in the (002) axis with an increase in the FWHM value

(indication of smaller grains) has been reported for spray pyrolysed films [63]. Alongwith

grain size, surface roughness value also decreased. The decrease in surface roughness

upon increasing the Cd concentration can be attributed to the polycrystallization of the

films which was confirmed by XRD analysis [63]. Undoped ZnO possesses larger grains

with offspherical shape. A substantial red shift of the band gap value was reported by PL

and optical transmittance measurements which can be interpreted in terms of band gap

modulation due to Cd doping. The electrical resistivity measurements show that the sheet

resistance of the films decreases for higher Cd concentrations, which is attributed to the

low resistance value of CdO [63]. Lowering of particle size due to Cd doping has been

attributed to strain developed in the material [104]. Decrease in band gap was also

reported in this work. The microstructure was found to compose of high density closely

packed nano/submicro rods over a large area [104].

The transition metal Manganese (Mn) has been doped primarily for its coexisting

magnetic, semi-conducting and optical properties [107]. Thus Mn doped ZnO has been

synthesized primarily to study its ferromagnetic behavior [108-109]. Due to its unique

magneto-optical, magneto-electrical, and magneto-transport properties, it is also

considered as a dilute magnetic semiconductor (DMS) material. DMS materials are

essential for future-generation spintronic device applications [110]. Various physical and

chemical techniques that has been used to deposit Mn doped ZnO thin films and

nanofilms includes atomic layer deposition (ALD) [111], sol-gel [72, 112-113], metal

organic chemical vapour deposition (MOCVD) [36], ion implantation [114], pulsed laser

deposition (PLD) [115] and solid state sintering [116] etc. While attempt to synthesize Cd

doped ZnO thin film by SILAR has remained an unsuccessful attempt [106], those on Mn

doped ZnO is nonexisting. Also the microstructural effects of Mn doping in ZnO thin

film are not well established [107].

35

Although Karamat et. al. [117] reported an increase in band gap energy with

increasing Mn incorporation (the reason was attributed to Burstein–Moss shift), most of

the workers have reported a decrease in band gap energy with increasing Mn content [72,

118-120]. The decrease in band gap value with increased Mn doping concentration has

been accounted due to the sp-d exchange interactions and has been theoretically

explained using the second–order perturbation theory [72, 119-120]. A decrease in band

gap energy from 3.27 eV for undoped ZnO to 2.78 eV for 3% Mn doped ZnO has been

reported by Senthilkumar et al [119] and has been attributed to s-d and p-d interactions

giving rise to band gap bowing. A decrease in transmittance in the visible due to Mn

doping has been reported for sol-gel films [72]. The crystalline nature (high c-axis

orientation) was found to be effectd by doping, by which more impurities were included

in the ZnO crystal [72]. The SEM images of ZnO resemble a granular surface. The

incorporation of Mn ions changed surface morphology to a wrinkle network [72]. Mn

doping was found to reduce the particle size from 40 nm to ~20 nm [72]. Similar effect

has been reported in Mn doped ZnO nanoparticles [121]. Microstructure consisting of

spherical nanoparticles and nanorods with wrinkle structure has also been reported by

Srinivasan et al [122]. Iintroduction of impurity level in the bandgap due to Mn

incorporation was reported by Wang et. al. [123] for Mn: ZnO nanocrystals. Hindrance of

grain growth due to Mn incorporation and decrease in band gap has been reported [119].

For transparent conducting oxide (TCO) thin films like ZnO, it is always desired

to improve the electrical conduction without affecting its excellent optical properties. The

interest in Al doped ZnO (AZO) films is primarily to explore the possibility of tailoring

its electrical and optical properties. Al is chosen as dopant material because of its easy

and abundant availability. Accordingly, synthesis of polycrystalline and nanocrystalline

AZO thin films has been widely carried out using different techniques such as ultrasonic

chemical vapor deposition [35], spray pyrolysis [60, 64], pulsed laser deposition [124 -

125], RF magnetron sputtering [8, 21, 23, 126], helicon-wave excited plasma (HWP)

deposition [127], electroless deposition [83], sol-gel [69, 73], pulsed laser ablation [46],

chemical beam deposition [74] and SILAR [128-129] among many others. For SILAR

36

deposited films, sodium zincate bath has been used as cationic precursor which always

introduces the possibility of highly mobile sodium ions in the deposited films that can be

detrimental for their practical applications.

The enhancement of band gap [60, 83, 73, 126], lowering of resistivity [60, 73,

126-130] and increase of optical transparency [60, 127] for Aluminum incorporation has

been reported in these works. The bandgap enhancement is particularly significant for

nanocrystalline thin films. The enhancement of band gap due to Al incorporation has

been explained by Burstein-Moss effect. Normally the optimum incorporation of

aluminium has been reported to be around 1-3 at. % [73, 124, 126]. Reports on the effect

of Al doping on microstructual properties such as particle size, crystallinity, c-axis

orientation, surface roughness etc. are relatively less [73, 126]. While Kim et. al. [126]

reported improved crystallinity; Zhou et al [73] reported enhanced preferred c-axis

orientation for AZO films. Kim et. al. [126] also reported stronger c-axis orientation with

Al incorporation. The orientation gets stronger with temperature. Similar observation of

enhanced c-axis prefree orientation due to Al incorporation has been reported by Jayaraj

et. al. [132]. Shan et. al. [130] reported that at low temperatures, (101) orientation

dominates. However the (002) orientation becomes predominant at high temperatures. At

low temperatures however (101) peak is predominant [130]. They also reported that Al

doping can decrease the refractive index of ZnO. For PLD films [131], samples grown at

low temperature shows amorphous nature, but sample grown at higher than 400oC show

preferred (002) orientation. An enhancement of band gap was also reported in PLD films.

Our initial results also suggested that aluminum incorporation increases grain size and

increases preferred c-axis orientation [129]. Film's resistivity and sheet resistance

significantly decreased as film thickness increased [126]. Higher surface roughness and

irregular surface structure occurred at 200 °C substrate temperature [126]. Films possess

high optical transmittance of approximately 90% and demonstrated an optical band gap

of 3.35 eV [126]. It is well known that the microstructural and optical properties of ZnO

films are very sensitive to the method of preparation, the type and amount of dopant.

Apart form this, the microstructural features of the materials depends on the fraction of

37

Al incorporated into the lattice (by replacing zinc substitutionally) and the fraction going

to the non-crystalline region (grain boundaries). So far, no conclusive effort has been

made in this direction. A detailed microstructural analysis coupled with optical and

electrical measurements is necessary to resolve this issue.

The lowering of particle size due to Al incorporation has been attributed to the

replacement of relatively bigger 2Zn

+ ion (0.074 nm) by the relatively smaller 3Al

+ ion

(0.054 nm) during the formation of AZO [64]. This leads to a decrease in the lattice

constants, which in turn is responsible for the change in the crystallite size. However, it

has been observed that the crystallite size in the doped films does not vary in any regular

pattern with Al -dopant concentration [64], which is attributed to the lattice disorder

produced in the films at higher dopant concentration due to difference in the ionic radii of

2Zn

+ and 3Al

+ . The electrical investigation revealed that with Al -doping the conductivity

of the ZnO film improves upto a certain percentage of Al incorporation, but beyond that

it decreased with further doping [59, 64]. The optical observations on the films indicate a

blue-shift in the absorption edge, improved emission in the UV region and a widening of

the bandgap with increasing Al -dopant concentration [64].

Ni doped ZnO (NZO) is considered as an important II-VI diluted magnetic

semiconductor (DMS) material due to its unique magneto-electrical and magneto-

transport properties [91, 133]. Studies on DMSs have been spurred on by the urge to

develop storage device and spin electronics [134-136]. Certainly, there are reported

results regarding the properties of Ni-doped ZnO thin films and nanofilms obtained by

various deposition techniques such as pulsed laser deposition [137-138], spin coating

[133, 139-140], atom beam sputtering [141], fast atom beam sputtering [24], pulsed

electrodeposition-assisted chemical bath deposition method [78], auto-combustion

method [142], reactive electron beam evaporation [143], etc. They have been prepared

primarily for study of their ferromagnetic properties. However reports describing the

38

synthesis and properties of NZO deposited by aqueous solution techniques at low

temperatures are rare.

A decrease in band gap has been reported for sol-gel NZO [144]. Spin coated

NZO also has been repoted with high preferred orientation [140]. The nanogranular

nature of the films has examined by transmission electron microscopy (TEM) [124].

Marginal increase in particle size from 24nm for undoped ZnO to 26.5 nm for Ni doped

ZnO has been reported for films prepared by SILAR technique [145] For solgel

synthesized NZO films [146], smooth surface with roughness limited to 4nm has been

reported.

High quality Ni doped ZnO with preferred c-axis orientation has ben reported by

PLD technique [138]. Ni doping also has been reported to decrease resistivity by almost

two orders of magnitude [141]. Ni has been reported to be present in divalent state [91,

141]. Transmission in NZO (~83%) was found to be less than that of pure ZnO (~90%).

Two important mechanisms reported in the literature viz. influence of d–d transition

bands and electron scattering from crystallites/grains are discussed as the possible causes

for the increase in conductivity on Ni doping in ZnO. However the mechanism of such

increase in conductivity is still inconclusive [141].

Yildiz et. al. [91] however reported enhanced resistivity for NZO due to reduction

in charge carrier concentration. They also reported that Ni doping reduces particle size.

Decrease in optical transparency and increase in resisitivity has been reported for fast

atom beam sputtered NZO [24]. High electrical conductivity of undoped film is explained

on the basis of presence of oxygen vacancies. Decrease of electrical conductivity due to

nickel doping is explained on the basis of compensation of oxygen vacancies [24].

Decrease in carrier density and mobility has also been reported for NZO films

synthesized by pulsed laser deposition [137]. It appears that a detailed understanding of

microstructure and its correlation with electrical property is needed for further

understanding.

39

2.2 Aims and objectives of the present work

Preparation and characterization of polycrystalline thin films ZnO and doped ZnO

via different techniques have attracted considerable attention due to their wide

application prospects. Consideration of simplicity, economy and input energy suggest

that thin films of these materials be deposited by a low temperature and simpler chemical

route. Therefore, it is necessary to develop a low temperature deposition methodology for

the growth of ZnO and doped ZnO films. The primary aim of the present work was to use

a relatively new and less utilized SILAR technique to prepare ZnO thin films from

different zinc complex solutions and their characterization. Compared to other chemical

techniques, SILAR has remained a relatively less investigated method for ZnO and doped

ZnO and the potential of this technique is yet to be explored in full.

Further since the preparation of thin films by SILAR can be carried out under

mild conditions and at lower processing temperatures, doping of metal atoms at low

temperatures may be particularly suitable by this method. The technique offers a wide

spectrum of deposition parameters to control such as choice of suitable precursors,

concentration and pH of the reacting precursors, temperature of deposition etc. Optimum

synthesis condition that provides regular growth for every particular dopant needs be

determined. The objective of the work therefore includes exploring the possibility of

utilizing SILAR to impurify ZnO thin film with some metals (Cd, Mn, Al and Ni).

Reports on synthesis of Cd and Mn doped ZnO by SILAR is nonexisting to the best of

our knowledge. Only a very few reports are available for Al and Ni doping in ZnO thin

films by SILAR. In almost all these reports, sodium containing cationic precursor was

used which always introduces the possibility of highly mobile sodium ions in the films

which can be detrimental for their practical applications. The aim of the work was to

prepare such doped films from different cationic precursors using the flexibility of

SILAR technique and the influence exterted by the metal dopants on the physical

properties (structural and morphological properties, optical band gap, electrical properties

etc.) of ZnO films.

40

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48

CHAPTER 3

Instrumental techniques and

Theoretical considerations

3.1 Instrumental techniques

This section deals with different experimental techniques which have been used to

characterize the ZnO and doped ZnO thin films. A brief discussion about X-ray

diffraction (XRD), Scanning electron microscope (SEM), Transmission electron

microscope (TEM), Energy dispersive X-rays (EDX) and Ultraviolet-visble (UV-VIS)

spectrophotometer has been included here.

3.1.1 X-ray Diffraction (XRD) Analysis

X-ray diffraction is a versatile and non-destructive analytical method to uniquely

identify the crystalline phases present and to study the structural properties. A given

substance always produces a characteristic diffraction pattern, whether the substance is

present in the pure state or as one constituent of a mixture of substances. This fact is the

basis for the diffraction method of analysis. Qualitative analysis for a particular substance

is accomplished by identification of the pattern of that substance. Quantitative analysis is

49

also possible, because the intensities of diffraction lines due to one constituent of a

mixture depend on the proportion of that constituent in the mixture.

As the spacing between the atomic arrays in a material is in the atomic range, to

look inside these arrays, one needs a light of wavelength in the order of atomic distance

i.e. angstrom (Å). When X-rays of wavelength λ is incident on a crystalline material

(single crystal or polycrystalline), which is mounted in the center of a diffractometer, at

an angle of incidence, they interact with the parallel atomic planes and produce a

diffraction pattern. The schematic of X-ray diffraction by lattice is shown in Figure 3.1.

Figure 3.1: Schematic of X-ray diffraction by lattice

The angle between the diffracted beam and the transmitted beam is always 2θ .

This is known as the diffraction angle and it is this angle, rather thanθ , which is usually

measured experimentally [1]. The condition for diffraction at any observable angle is

given by Bragg law [1]

2 sin (3.1)n dλ θ= →

50

where n is the order of diffraction and d is the interplaner spacing. Bragg law gives the

condition for strong reflection for rays reflected from adjacent atomic planes and it

occurs when the path difference between them is equal to an integral multiple )(n of

wavelength. The angle θ for which equation 3.1 is satisfied is called the Bragg angle.

The diffracted beam is detected in a counter (gas counter or solid state counter)

which moves through the angular range of reflections and the intensity of the diffracted

beam is recorded on a synchronously advancing chart. Comparing this data with the

standard JCPDS (Joint Committee of Powder Diffraction Standards) data file, one can

identify the structure of the substance, the lattice parameters and the planes present. From

X-ray diffraction patterns, the crystallite size or particle size ( )D can be found using the

Scherrer’s formula [1]

(3.2)cos

D

kD

λ

β θ= →

where k is a constant determined by the geometry of the crystallites and it is

approximately 0.9 for spherical particles, D

β is the full width at half maximum (FWHM)

intensity of the observed diffraction peak. The broadening considered in Scherrer

equation is due to particle size alone. The angular width at a point where the intensity has

fallen to half of its maximum value (Full width at half maximum intensity or FWHM) is a

measure of broadening of x-ray peak [1].

The crystal structure and orientation of the ZnO and doped ZnO films were

investigated from the X-ray diffraction (XRD) patterns. The x-ray diffraction (XRD)

profiles of the samples were recorded using −Ni filtered αCuK radiation (λ=1.5418 Å)

from a highly stabilized and automated Philips X-ray generator (PW 1830) operated at 40

kV and 20 mA. The experimental peak positions were compared with the standard

JCPDS files and the Miller indices were indexed to the peaks.

51

3.1.2 Electron Microscopes: SEM and TEM

Scanning Electron Microscope (SEM) is one of the most versatile instruments

available for the characterization of heterogeneous materials and surfaces on a

micrometer and sub-micrometer scale. SEM generates images by scanning the specimen

with a beam of electrons [2-3]. The electron beam from an electron gun is allowed to pass

through electromagnetic lenses before falling on the sample. The electromagnetic lenses

are used to focus the electrons into a very thin beam. The finely focused electron beam is

allowed to sweep rapidly over the surface of the specimen. The molecules in the

specimen are excited to high energy levels in this process and emit secondary electrons.

Apart from secondary electrons, back scattered electrons, characteristic X-rays and

photons of various energies are also produced. Primarily, the electrons so produced are

used to form an image of the specimen surface. The image signal is collected by a

detector [2-3]. The display devices provides for both visual observation and photographic

recording. Thus SEM can extract structural information of a thin film material. Thin films

are usually coated with a conductive material prior to imaging to render the surface

conductive.

Electron microscopes in which electrons are allowed to transmit through the

objects are known as transmission electron microscope (TEM). TEM consists of an

electron gun, central column, electromagnetic lenses and a fluorescent screen. The

electron gun is the source of electrons. The microscope column is an evacuated metal

tube through which the electron travels. The image of a sample is formed by illuminating

the sample with an electron beam and detecting the electrons that are transmitted through

sample [4]. The electromagnetic lenses are used to focus the electrons into a very thin

beam. Depending on the density of the material present, some of the electrons are

scattered and disappear from the beam. At the bottom of the microscope the unscattered

electrons hit a fluorescent screen, which gives rise to a “shadow image” of the specimen

with its different parts displayed in varied darkness according to the density. The image

can be directly studied by the operator or photographed with a camera.

52

3.1.3 Ultraviolet –Visible (UV-VIS) spectroscopy

UV-VIS spectroscopy refers to absorption spectroscopy in the UV-visible spectral

region. Studies by optical absorption explain the band structure of material. There are two

types of optical transitions, which can occur at the fundamental absorption edge of

crystalline as well as non-crystalline semiconducting materials. They are direct and

indirect transitions. When a light beam falls on a thin film semiconducting material, a

part of the beam will be reflected, another part will be transmitted through the film, and

the rest of the beam will be absorbed. Absorption of photons causes transition of the

electrons from valance band to conduction band. The absorption ability is measured by its

absorption coefficient ( )α which is a function of frequency [5-7] and is defined as [7]

(3.3)t

oI I e

α−= →

where I is the intensity of the transmitted beam, o

I is the intensity of the incident light

and t is the thickness of the film. The nature of transition is determined by using the

relation [5]

( ) ( ) (3.4)n

gh A h Eα ν ν= − →

where hν is the photon energy, g

E is the band gap energy, A is a constant and it is

function of index of refraction and hole/electron effective masses [5]. The constant n is

equal to two (2) for direct transition and equal to one (1) for indirect transition [5-6].

Several models of spectrophotometers of varying degree of sophistication are

available. These include single beam, double beam, relecting and multibeam instruments.

Deuterium or hydrogen lamp (for UV light) and tungsten lamp (for visible light) are

generally used as the light sources. In dual beam spectrophotometers, the incident light is

53

split into two beams of equal intensity, one of which passes through the reference cuvette

and the other through the sample cuvette. The instrument records the change in

absorbance in the sample with respect to the reference. The detector records the change in

absorbance in the sample. For our experiments, a dual beam UV-VIS spectrophotometer

(Shimadzu, Model No. UV-1800; Spectral resolution 1 nm) was used.

3.1.4 Energy dispersive X-ray spectroscopy (EDS or EDX)

EDX is an analytical technique used for the elemental analysis or chemical

characterization of a sample. It is one of the variants of X-ray fluorescence spectroscopy

which is based on the investigation of a sample through interactions between

electromagnetic radiation and matter. Quantitative estimation is made by analyzing the

X-rays emitted by the matter when it is bombarded with charged particles. Its

characterization capabilities lie on the fundamental principle that each element has a

unique atomic structure allowing X-rays that are characteristic of an element's atomic

structure to be identified uniquely from one another.

3.2 Theoretical considerations

3.2.1 Preferred orientation

Quantitative information concerning the preferred crystal orientation can be

obtained from the texture coefficient ( )TC , defined as [5, 8-9]

( )( ) / ( )

( ) (3.5)1 ( ) / ( )

o

o

n

I hkl I hklTC hkl

I hkl I hkln

= →

∑

where ( )TC hkl is the texture coefficient for the ( )hkl plane, n is the number of

diffraction peaks considered, ( )I hkl is the measured x-ray intensity of the ( )hkl plane

54

[converted to relative intensity when ( )TC hkl is evaluated, by taking the observed highest

intensity as hundred (100)] and ( )o

I hkl is the corresponding relative intensity according

to JCPDS card for ZnO [10]. ( )o

I hkl represents the x-ray intensities (relative intensities)

from standard ZnO powder with randomly oriented grains and with no preferred

orientation.

If ( ) 1TC hkl ≈ for all the (hkl) planes considered, then the films are with a

randomly oriented crystallite similar to the JCPDS reference, while values higher than 1

indicate the abundance of grains in a given (hkl) direction. Values of ( )TC hkl in the range

0 ( ) 1TC hkl⟨ ⟨ indicate lack of grains oriented in that direction. As ( )TC hkl increases, the

preferential growth of the crystallites along the plane ( )hkl enhances [5, 9].

3.2.2 Particle size estimation

Although Scherrer equation (Eqn. 3.2) is normally utilized to evaluate particle

size, it does not take account of microstrain present in polycrystalline thin films. The

broadening D

β in scherrer equation represents the broadening due to particle size alone

[1, 11]. However, the total contribution to the observed broadening in x-ray diffraction

peaks (called x-ray peak broadening or x-ray line broadening) is due to particle size

broadening ( )Dβ , broadening due to microstrain ( )s

β and instrumental broadening ( )iβ .

The instrumental broadening arises from various factors such as non-parallelism of the

incident x-ray beam; presence of other wavelengths apart from CuKα etc. [1] and it is a

constant for a particular experimental setup. Strain arises in polycrystalline materials due

to various defects (point defects, line defects, planar defects and volume defects) and this

gives rise to a broadening in x-ray diffraction peaks [1]. Thus the experimentally

observed broadening in x-ray diffraction pattern ( )o

β can be written as [1]:

(3.6)o D S i

β β β β= + + →

55

The broadening due to particle size and microstrain is obtained by subtracting

instrumental broadening from the observed or measured broadening and can be written as

(3.6)o i D S

β β β β β= − = + →

The broadening due to microstrain can be written as [1]:

4 tan (3.7)S

β ε θ= →

Equation 3.7 coupled with equations 3.6 and 3.2 gives the Williamson-Hall

equation [1, 11]

cos 4 sin (3.8)k

D

λβ θ ε θ= + →

where ε is the microstrain in the film.

Thus a plot of cosβ θ against 4 sinθ is a straight line and is called the

Williamspn-Hall (or W-H) plot. The slope of the plot represents average strain in the film

whereas the inverse of intercept of the straigt line on cosβ θ axis gives the crystallite

size ( )D according to equation 3.8.

The separation of crystallite size and microstrain can be done if the shapes of their

individual profiles are known. Normally a Gaussian function represents strain broadening

and a Lorentzian function represents crystallite size broadening [1].

56

References

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2. L. Reamer, “Scanning Electron Microscopy - Physics of image formation and

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3. R. E. lee, in “Scanning Electron Microscopy and X-ray Microanalysis”, Prentice Hall

(1993).

4. J. C. H. Spence, in “High-resolution electron microscopy (Monograph on the Physics

and Chemistry of materials)” 3rd

Edition, Oxford Science Publications (2009).

5. J. I. Pankove, in “Optical Processes in Semiconductors” (Dover Publications Inc.,

New York) (1975)

6. C. F. Klingshirn, in “Semiconductor Optics” (Springer-Verlag, Berlin, Heridelberg,

1997).

7. K. H. Kim, R. A. Wibowo and B. Munir, Materials Letters 60 (2006) 1931.

8. S. Ilican, Y. Caglar and M. Caglar, J. of Optoelectronics and Advanced Materials 10

(2008) 2578.

9. C. S. Prajapati and P. P. Sahay, Crystal Research Technology 46 (2011) 1086.

10. B. Post, S. Weissmann and H. F. McMurdie (eds.), Joint Committee on Powder

Diffraction standards, Inorganic Vol., Card No. 36-1451, International Centre for

Diffraction Data, Swarthmore, PA (1990).

11. H. P. Klug and L. E. Alexander, in “X-ray diffraction procedures for polycrystalline

and amorphous materials” (2nd

edition, Wiley-Interscience, 1974).

57

CHAPTER 4

Preparation of ZnO thin films by SILAR

and their characterization

4.1 Introduction

ZnO films by chemical deposition technique were first reported in 1980 by Call

et. al. [1] who obtained it as an accidental by-product in an attempt to deposit the

composite 1x x

Cd Zn S− films. Addition of zinc salt to a CdS depositing solution produced

CdS films containing large amount of ZnO impurities. Further experiments led to

refinement of the method for aqueous deposition of ZnO. The chemical deposition of

ZnO involving multiple dipping of the substrate in an aqueous solution of a zinc complex

was further developed in 1987 by Ristov et. al. [2]. The process was named as chemical

deposition process or chemical dipping process in these works [1-2]. The technique was

given the name SILAR (Successive ion layer adsorption and reaction) by Nicolau et. al.

[3]. Normally sodium zincate or ammonium zincate bath is used as the cationic precursor.

Jimenez-Gonzailez et. al. [4] and Raidou et. al. [5] deposited ZnO film using this

technique from ammonium zincate bath prepared from zinc sulphate as the starting

precursor. Preparation and characterization of ZnO films deposited from sodium zincate

and ammonium zincate complex with zinc sulphate as starting precursor has also been

reported by Chatterjee et. al. [6] and Mitra et. al. [7].

58

In the present investigation ZnO thin films were deposited from ammonium

zincate complex as cationic precursor with zinc acetate ( )3 222Zn CH COO H O as the

starting reagent for the first time. Zinc acetate has a number of distinctive properties. It is

known to be a ‘mono-precursor’ [8]. Also the ammonium acetate formed during its

reaction with ammonia is highly soluble in water which reduces the possibility of

impurity incorporation in the deposited films. Ammonium acetate is also a relatively

unusual example of a salt that melts at low temperatures. For comparison of physical

properties of the films synthesized from ammonium zincate bath, films were also

deposited from sodium zincate and zinc chloride as cationic precursors. This was also

essential since certain metals could not be doped from ammonium zincate bath (discussed

in subsequent chapters) while they could be successfully doped from other zinc complex

baths. Different experimental parameters such as chemical nature of the bath solution

including concentration and pH, temperature of deposition etc. governing the film growth

process have been studied.

4.2 Preparation of ZnO thin films

Figure 4.1 shows the simple process flow sheet for deposition of ZnO film from

cationic and anionic baths. The cationic bath is of ammonium zincate solution

( )4 22NH ZnO kept at room temperature (RT) and the anionic bath is of distilled water

maintained near boiling point (96–98oC). Sodium zincate [ ]2 2Na ZnO and zinc chloride

bath ( )2ZnCl as cationic precursor was also used in separate set of experiments. The

substrate (normally a glass slide) was alternatively dipped in the baths containing the zinc

complex solution and hot water. Although microscope glass substrates cannot withstand

very high temperatures, it is widely used since the electrical and optical measurements

are not disturbed by an underlying layer and are thus easier to interpret. Substrate

cleaning prior to deposition is an important step in thin film prepartion in order to remove

the contaminants that would otherwise affect the properties of the film. The sequence of

59

substrate cleaning steps was: overnight (24 hours) cleaning in chromic acid, rinsing in

double distilled water followed by ultrasonic cleaning in equivolume mixture of acetone

and ethyl alcohol. The cleaned substrate was tightly held in a holder so that only a

requisite area for film deposition is exposed. Thus the film deposition area could be

easily varied by adjusting the holder arrangement. For some specific experiments (Mn

doping in ZnO), quartz substrate was used and the cleaning of such substrate has been

discussed in chapter 6.

Pre-cleaned Substrate

Alternate Dipping

Figure 4.1: Process flow sheet for SILAR deposition of ZnO

( )4 22NH ZnO bath

(Cationic precursor)

Hot water bath

(Anionic precursor)

ZnO Film

Post-Deposition

Air Anneal

Single Phase

ZnO

60

4.2.1 Preparation of bath solutions

The ammonium zincate bath used for deposition of ZnO was prepared by slow

addition of ammonium hydroxide (~25% pure ammonia solution, Merck, Mol. Wt. 17.03

g/mol) to an aqueous solution of analytical grade zinc acetate

dihydrate ( )3 22.2Zn CH COO H O supplied by Merck. Addition of ammonia solution in

zinc acetate solution initially gives rise to a white precipitate of zinc hydroxide

[ ]2( )Zn OH according to the reaction

( )3 2 4 3 42( ) 2 2 (4.1)Zn CH COO NH OH Zn OH CH COONH+ = + →

However, on further addition of ammonia, the precipitate dissolved forming the

( )4 22NH ZnO bath following the reaction

( )2 4 4 2 22( ) 2 2 (4.2)Zn OH NH OH NH ZnO H O+ = + →

Ammonia solution was introduced slowly under continuous stirring until the

solution becomes clear and homogeneous. An excess of alkali is always required to have

a stable ammonium zincate bath [7, 9]. Thus the overall reaction leading to the formation

of ammonium zincate is obtained by adding equations (4.1) and (4.2):

( ) ( )3 4 4 2 3 4 22 24 2 2 (4.3)Zn CH COO NH OH NH ZnO CH COONH H O+ = + + →

The sodium zincate bath was prepared by addition of a solution sodium hydroxide

( NaOH pellets, Merck, mol. wt. 40) to an aqueous solution of zinc sulphate

( )4 2.7ZnSO H O [Merck, Mol. Wt. 287.54]. Similar to the case of ammonium bath,

initially a white precipitate of zinc hydroxide appeared according to the reaction

61

( )4 2 2 42 ( ) (4.4)ZnSO NaOH Zn OH Na SO+ = + →

However, on further addition of NaOH , the precipitate dissolved forming the 2 2Na ZnO

bath following the reaction

2 2 2 2( ) 2 2 (4.5)Zn OH NaOH Na ZnO H O+ = + →

Thus the overall reaction leading to the formation of sodium zincate is obtained

by adding equations (4.4) and (4.5):

4 2 2 2 4 24 2 (4.6)ZnSO NaOH Na ZnO Na SO H O+ → + + →

Similar to the case of ammonium zincate complex, an excess of NaOH is always

required to obtain a stable aqueous solution of sodium zincate [10]. Thus, to get a stable

solution of zinc complex ion ( )2Zn

+ , strong alkalinity is necessary.

In case of sodium zincate bath, only one stable zincate namely 2 2 2.4Na ZnO H O is

possible at room temperature [10]. The compound is highly soluble in water. However, a

measurable amount of ammonium zincate is not present in ammoniacal solution of zinc

oxide and instead, a series of complex cations can result in case of ammonium zincate

solution [9]. These are ( )3 2Zn NH

++ , ( )3 4

Zn NH++

and ( )3 6Zn NH

++ . Jimenez-

Gonzailez et. al. [4] also inferred the presence of tetraamminezinc (II), ( )3 4Zn NH

++ in

the solution. A stock solution of 0.5 molar (0.5 M) concentration was prepared in each

case and it was then diluted with double distilled water to get bath solutions of requisite

concentrations during the experiments. One of the problems encountered with the

ammonium zincate bath was that, due to evaporation of ammonia, the pH of the stock

solution slowly reduced with consequent precipitation of zinc hydroxide [9-10]. In such a

62

situation the pH was restored by adding appropriate amount of ammonia. Sodium zincate

bath was, however, stable for a reasonably long time as there was no possibility of

evaporation of sodium hydroxide.

4.2.2 Optimization of pH and Concentration of the zincate baths

The deposition rate process was studied by monitoring the variation of film

thickness with pH and concentration ( )C of the zincate bath and the number of dipping in

the reacting baths (cationic and anionic). The growth rate and quality of the film was

found to depend on the concentration of the zincate baths and the adherence of the film

on the substarte surface was found to be a stringent function of zincate bath pH. It was

found that above 0.125M concentration of the ammonium zincate bath, the growth

process was erratic and nonuniform. Particle absorption took place on the film surface

making the growth process nonuniform and resulting in poor quality films. For still

higher concentrations (in excess of 0.15M), film detachment from substrate surface took

place and no layer could be deposited. The ammonium zincate bath concentration was

therefore optimized at 0.1M to get good quality film with a reasonable growth rate.

Lower concentrations (less than 0.1 M) reduce the growth rate of the film in a linear

proportion. In case of sodium zincate bath, the growth process was found to be erratic

and nonuniform above 0.15M concentration. The bath concentration in this could be

optimized in the range 0.1-0.125M. For all further experiments the concentration of both

the zinacte bath was kept at 0.1M. For 0.1M solution of ammonium zincate complex, films produced with pH<10.7

was found to consist of non-adherent powder like precipitates indicating poor quality of

deposition. The bath solution also tends to lose stability for pH<10.7 and ammonium

zincate bath was found to be stable for pH values ≥ 10.7. However, above pH value of

10.9, the growth rate decreases abruptly. Also for more basic solutions (pH value in

excess of 10.9), it appears that dissolution of already deposited ZnO film occurs when

reintroduced into the solution. Thus the pH value was optimized in the range 10.75-10.85

63

for all further experiments from ammonium zincate bath with a minimum of ammonia

addition. For 0.1M sodium zincate complex, the pH value was similarly optimized in the

range 13.15-13.25 in order to get adherent films. The pH measurements were carried out

in a systronics pH meter (Model 335). Table 4.1 shows the optimized deposition

parameters for different zinc complex cationic baths used in the present investigation.

Table – 4.1: Optimized deposition parameters for different zinc complexes

Cationic

bath

Anionic

bath

( )C M

(Cationic

bath)

( )C M

(Anionic

bath)

pH

(Cationic

bath)

pH

(Anionic

bath)

T (oC)

(Cationic

bath)

T (oC)

(Anionic

bath)

( )4 22NH ZnO

2H O 0.1 M - 10.80 0.05±

- R. T. 97 1± oC

2 2Na ZnO 2H O 0.1 –

0.125 M

- 13.20 0.05±

- R. T. 97 1± o

C

2ZnCl* NaOH 0.1 M 0.075 M 4.70 0.05± 11.1 0.05±

R. T. 70

oC

* The details of ZnO film deposition from 2ZnCl solution as cationic bath and

NaOH solution as anionic bath has been discussed in section 6.1 of chapter 6.

4.2.3 Deposition of ZnO films

Pure zinc oxide films were deposited on microscope glass slide substrates by

alternate dipping of the substrate into 0.1M ammonium zincate ( )4 2NH ZnO bath kept

at room temperature and hot water maintained at 97 ± 1oC. The pH of the zincate bath

was maintained in the range 10.75-10.85 with appropriate ammonia addition during film

depsotion process.

64

The deposition process consisted of the following steps: (i) immersion of the pre-

cleaned substrate in the zincate solution kept at room temperature; (ii) withdrawal of the

substrate (which carries a thin layer of the complex zinc solution adhered to the surface

of the substrate) and (iii) introduction of the substrate with the zincate solution layer into

hot water bath (which lead to the chemical reaction between adsorbed zinc complex and

hot water on the substrate surface). Thus one complete cycle of dipping involves dipping

into the zincate bath, its withdrawal from the bath followed by dipping into hot water

bath. The dipping time in each bath was two (2) seconds (s). This cycle of dipping was

repeated several times in order to increase the overall film thickness.

The reaction occurring on the substrate surface leading to the formation of ZnO

may be represented as [7]:

( )4 2 2 422 (4.7)NH ZnO H O ZnO NH OH+ → + →

However, the detailed chemical reaction involving the presence of

tetraamminezinc (II) ( )2

3 4Zn NH

+ complex [4] in presence of an excess ammonical

solution may be quite complicated. The reaction involves release of zinc (II) ion in water

bath followed by reaction of this cation with hydroxyl ion ( )OH− present in the water

bath. Thus the possible reaction scheme may be represented as:

( )2

2

3 2 444 4 (4.8)Zn NH H O Zn NH OH

++ + − + → + →

2

22 ( ) (4.9)Zn OH Zn OH+ −+ → →

Since the reaction temperature is close to the boiling point of water, zinc

hydroxide breaks to give zinc oxide:

2 2( ) (4.10)Zn OH ZnO H O→ + →

65

Thus some amount of zinc hydroxide is always expected in the as-deposited film

and this has been experimentally observed [2, 6, 11]. A part of the ZnO so formed was

deposited onto the substrate as a strongly adherent film and another part of it formed a

precipitate in the hot water bath. Thus, only the strongly adherent microcrystals remained

on the surface. These crystals then serve as nuclei for further growth during subsequent

dipping [5, 7]. As a part of the precipitate during each dipping remained dispersed in the

hot water bath, its concentration increased with the number of dipping and to maintain

the uniformity of the film deposition process it is preferable to change the concentrated

hot water bath at regular intervals with a fresh water bath. The zincate solution was also

changed at definite intervals so that the concentration of the bath remained effectively

constant during the entire deposition process. This was particularly required when the

number of deposition cycle was significantly high. Both the hot water bath and zincate

bath was changed after each 25 dipping cycles. The deposited film was subsequently heat

treated in air at 200ºC for 2 hours to get milky white colored ZnO thin film.

In case of sodium zincate bath, the reaction occurring on the substrate surface

leading to the formation of ZnO may be represented as [6]:

2 2 2 2 (4.11)Na ZnO H O ZnO NaOH+ → + →

It can be seen that sodium ( )Na goes as NaOH by hydrolysis and excess Na , if

any, was removed when the film was thoroughly washed with distilled water.

It is to mention that the ammonium zincate bath used for deposition contains

ammonium acetate as well (eqn. 4.3). It appears that while the zincate precipitates as ZnO

in presence of high concentration of water when dipped in the hot water bath, the acetate

goes into the solution due to its high solubility in water. The much higher solubility of

ammonium acetate compared to sodium sulphate (contained alongwith sodium zincate in

eqn. 4.6) [12-13] reduces the possibility of impurity incorporation in the deposited films.

66

Thus use of sodium zincate bath [5] instead of ammonium zincate bath always introduces

the possibility of incorporation of highly mobile sodium ions in the film, which can be

detrimental for their practical applications. The use of sodium hydroxide as anionic

precursor for films deposited from 0.1 M 2ZnCl bath as cationic precursor and 0.075 M

NaOH bath as anionic precursor (discussed in section 6.1, chapter 6) also introduces the

possibility of incorporation of highly mobile sodium ions in the film. Also the higher

solubility of ammonium acetate compared to ammonium sulphate (obtained if zinc

sulphate is used [4-5, 7] instead of zinc acetate as starting reagent) reduces the possibility

of impurity incorporation (sulpher ion as impurity in case of ammonium sulphate

compared to carbon as impurity in case of ammonium acetate).

4.2.4 Film thickness and its measurement

Figure 4.2 shows the variation film thickness ( )t with number of dipping (25-100

dipping cycles) for ammonium zincate bath under optimized conditions. The result

implies a linear increase of ZnO film thickness with number of dippings. There is an

overall variation of ± 5% in the film thickness data (shown as error bars against each data

point of figure 4.2) which reflects a small variability in the deposition process arising

probably from small experimental scatter in the zincate bath concentration values as well

as due to the nonuniformity of the substrate handling procedure as the deposition is

carried out manually. The growth rate was ~0.0162 µm per dipping for 0.1M ammonium

zincate bath. The corresponding value 0.1 M sodium zincate bath was ~0.02 µm per

dipping. In other words the growth rate was ~0.162 µm per dipping per mole for

ammonium zincate bath and it was ~0.20 µm per dipping per mole for sodium zincate

bath. Table 4.2 shows the thickness ( )t values against number of dipping ( )N for

ammonium and sodium zincate bath. Average of three measurements is shown in the

table. The growth rate was ~0.021 µm per dipping for films deposited from 2ZnCl as

cationic precursor (Section 6.1, chapter 6).

67

Figure 4.2: Dependence of film thickness on number of dipping cycle

Table –4.2: Thickness of ZnO films from ammonium and sodium zincate baths

Number of

dipping

( )N

Thickness ( )mµ

[0.1M ( )4 22NH ZnO ]

Thickness ( )mµ

[0.1M 2 2Na ZnO ]

25 0.355 0.49

50 0.81 1.02

75 1.24 1.52

100 1.63 2.03

The film thickness was determined by weight difference-density consideration

method or gravimetry method [7, 14-15] using an electronic high-precision balance. The

gravimetry method measures the change in weight of the substrate due to film deposition

and using the known area of film deposition and utilizing the data of the theoretical

0 25 50 75 100 125

0.0

0.5

1.0

1.5

2.0

Thic

kn

ess (

µm

)

No. of dipping (N)

68

density of ZnO (5.6 gm/cm3) [11]. Thus, if

1' 'W and 2' 'W be the weights of the substrate

before and after film deposition in gm., ' 'A be the area of the deposited film in cm2 and

' 'ρ be the theoretical density of ZnO, then the film thickness can be evaluated using the

equation:

( )2 1 410W W

tAρ

−−= × mµ (4.12)→

A check of film thickness was made by measuring the thickness using cross-

sectional SEM. Figure 4.3 shows the cross-sectional SEM micrograph of ZnO film of

thickness 2.0 µm measured gravimetrically (obtained by 125 dipping from ammonium

zincate bath). An average thickness of 2.64 µm was obtained from SEM micrograph. The

value was an average of several measurements on different portions. This indicates a

porosity of ~ 32% in the deposited films. Similar experiments on films deposited from

sodium zincate bath shows a porosity of ~ 30% (not shown here for brevity). Those from

zinc chloride bath show a porosity of ~ 22% (Section 6.1, chapter 6).

Figure 4.3: Cross-sectional SEM of ZnO film prepared from ammonium zincate bath

Substrate

Film b

b

Thickness

69

The gravimetry method of film thickness determination has some limitations

because of non-uniformity, porosity and edge tapering effects in the chemically deposited

films with porous microstructure. The actual density of the film is always lower than the

theoretical density used in gravimetry technique which does not takes account of

porosity. Thus the actual thickness is always greater than the measure one using

theoretical density. However, this error does not affect the comparative data of measured

film thickness.

4.3 Structural characterization by XRD: Evaluation of particle size

Figure 4.4 shows the x-ray diffraction patterns of ZnO thin film prepared on glass

substrate from ammonium zincate bath. The diffraction patterns were recorded at room

temperature. Figure 4.4 (a) to 4.4 (d) shows the spectra of the samples heat treated at

200oC, 300

oC, 350

oC and 400

oC redpectively. The heat treatment was done in air for 2

hours. The materials were scanned in the range 20-60o. The 2θ variation was employed

with a 0.05 degrees step and a time step of 1 second. Intensity in arbitrary units is plotted

against 2θ in the figure.

It is seen from figure 4.4 (a) that diffraction peaks appear at 31.714o, 34.389o,

36.205o, 47.434

oand 56.576

o. The peak positions do not change significantly due to heat

treatment at different temperatures. The diffraction patterns reveal the formation of

phase-pure polycrystalline ZnO film with hexagonal wurtzite structure and good

crystalline quality without any appreciable changes from pure ZnO. All the peaks are in

good agreement with the Joint committee on powder diffraction standard (JCPDS) data

belonging to hexagonal ZnO structure [16]. The corresponding reflecting planes are

(100), (002), (101), (102) and (110) respectively. The XRD patterns of all the samples

exhibit enhanced intensities for the peaks corresponding to (002) plane, indicating

preferred orientation along c-axis.

70

Figure 4.4: X-ray diffraction pattern of ZnO thin films heat treated at (a) 200oC,

(b) 300oC, (c) 350

oC and (d) 400

oC

Table 4.3 shows the relative intensities for ZnO powder (with no preferred

orientation) [16] and ZnO thin film heat treated at 300oC (observed in this work). The

relative intensities for ZnO film was evaluated by taking the highest intensity of (002)

peak as hundred (100). The actual values of intensitie obtained are shown in the third

column of table 4.3 and the relative intensities are shown in the last column. The (101)

peak appears with maximum intensity in ZnO powder with no preferred orientation [16].

However, the (002) peak appears with maximum intensity in the SILAR deposited ZnO

films. Table shows the relative intensities for the first three peaks since they appeared

20 30 40 50 60

0

500

1000

1500

2000

(110)

(102)

(101)

(002)

(a)

2θ (degree)

20 30 40 50 60

0

500

1000

1500

2000

(b)

Inte

nsity (

a.u

.)

20 30 40 50 60

0

500

1000

1500

2000

(c)

20 30 40 50 60

0

500

1000

1500

2000

(100)

(d)

71

with high intensity in the XRD pattern of the films. Figure 4.5 shows the variations of

( )TC hkl for (002) peak evaluated using eqn. 3.5 (Chapter 3, section 3.2.1).

Table –4.3: Relative intensities of diffraction peaks for ZnO powder and ZnO thin film

Diffraction plane Relative intensities

for ZnO powder

( )o

I hkl [17]

Observed intensities

for ZnO film heat

treated at 3000C

Relative intensities

for ZnO film heat

treated at 3000C

( )I hkl

(100) 57 365 ~23.13

(002) 44 1578 100

(101) 100 445 ~28.20

Figure 4.5: Variation of (002)TC with temperature for ZnO films prepared from

ammonium zincate bath

Since three diffraction peaks were used ((100), (002), (101)), the maximum value

TC(hkl) possible is 3. The texture coefficient for the (002) orientation has been found to

increase from ~2.247 to ~2.291 as the annealing temperature is increased from 200oC to

300oC. No significant change is found with further heat treatment. All subsequent

200 250 300 350 400

2.20

2.25

2.30

2.35

2.40

TC

(0

02

)

T(oC)

72

measurements were made on films heat treated at 350oC. The value of the texture

coefficient indicates the maximum preferred orientation of the films along the diffraction

plane under consideration, meaning that the increase in preferred orientation is associated

with increase in the number of grains along that plane.

Thus with increase in annealing temperature, the crystallinity along (002) plane

improves upto 300oC and finally saturates. Table 4.4 shows the comparison of

(002)TC of ZnO films obtained from three different zinc complexes under optimized

deposition conditions of table 4.1. All the films were heat treated at 350oC for 2 hours

prior to XRD measurements. The x-ray patterns for films deposited from sodium zincate

bath and zinc chloride bath has been discussed in subsequent chapters (section 5.2 of

chapter 5 and section 6.2 of chapter 6). It is quite evident that films deposited from

ammonium zincate bath have highest preferred c-axis orientation.

Table – 4.4: (002)TC values for ZnO films prepared from different zinc complexes

Cationic precursor ( )TC hkl vale for (002) plane

( )4 22NH ZnO solution 2.29 0.01±

2 2Na ZnO solution 1.95 0.01±

2ZnCl solution 1.82 0.01±

Utilizing the X-ray diffraction data, the average particle size was estimated from

Williamson-Hall equation (Eqn. 3.8, section 3.2.2, chapter 3). The broadening due to

particle size and strain taken together i.e. β was obtained from the experimentally

observed broadening ( )oβ using the equation [17-18]:

(4.13)o i

β β β= − →

Diffraction data from standard silicon ( )Si powder was used to measure the

instrumental broadeningi

β . Normally silicon powder with very high particle size is used

73

as a standard [19-20]. Large particle size in the standard ensures that the broadening due

to particle size is negligible in the standard according to Scherre equation (Eqn. 3.2,

Section 3.1.1, chapter 3). Thus broadening observed in the x-ray diffraction pattern of the

standard is due to instrument only. Figure 4.6 shows the XRD pattern of standard silicon

powder. The peaks at 28.48o, 47.3

o, 56.08

o, 69.12

o, 76.22

o, 87.86

o, 94.8

o and 106.56

o

correspond to those from standard silicon.

Figure 4.6: XRD pattern of standard silicon powder

The broadening (in FWHM) against 2θ obtained for standard silicon sample was

plotted in a graph and was used as a reference. Figure 4.7 shows the broadening against

2θ obtained for standard silicon sample. A polynomial fitting of the experimentally

observed values is shown in figure 4.7. The corresponding fitting equation (polynomial

regression) is

2 (4.14)y a bx cx= + + →

40 60 80 100

0

1000

2000

3000

4000

5000

6000

7000

Inte

nsity (

a.

u.)

2θ (degree)

74

where y stands for the broadening in FWHM and x is the angle in 2o θ . a , b and c are

the constants with values 0.109, 47.742 10−− × and 51.263 10−× respectively. The

instrumental broadening at the observed peak positions for ZnO could be evaluated from

the graph as well as from equation 4.14 using the experimental value of 2θ .

Figure 4.7: Instrumental broadening against 2θ for standard Silicon powder.

X-ray line broadening analysis to estimate the observed ( )oβ was carried out

using computer software (MARQ2) [21-22]. The software utilizes Marquardt least-

squares procedure for minimizing the difference between the observed and simulated

diffraction patterns. The peak-shape and intensity of reflection is modeled with a pseudo-

Voigt (pV) analytical function, which is a combination of a Gaussian and a Lorentzian

functions representing lattice strain broadening and crystallite size broadening

respectively. The background intensity is subtracted by fitting the background with a

suitable linear function. A typical plot obtained from MARQ2 analysis for ZnO thin film

20 30 40 50 60 70 80 90 100

0.09

0.10

0.11

0.12

0.13

0.14

0.15

0.16

FW

HM

(d

egre

e)

2θ (degree)

75

synthesized from ammonium zincate bath is shown in Figure 4.8. The dotted curve

represents the experimental intensity data ( )oI and the continuous curve represents the

calculated (simulated) intensity data ( )cI . The difference plot ( )c o

I I− is shown at the

bottom.

Figure 4.8: Observed (dotted) and simulated (continuous) XRD patterns of ZnO film

heated at 350oC

From the values ofo

β obtained using MARQ2 fitting and the corresponding

values of instrumental broadeningi

β , β was calculated using equation 4.13. Figure 4.9

shows the plot of cosβ θ against θsin4 (W-H plots) for 350oC heat treated film. The

slope of the plot represents average strain in the films whereas the inverse of intercept on

cosβ θ axis gives the crystallite size ( )D according to the Williamson-Hall equation.

The particle size was evaluated using 0.9k = , which corresponds to spherical crystallites

and 1.5418λ = Å, the wavelength of αCuK radiation. The average value of particle size

for pure ZnO was ~22.75 nm and the strain in the film was ~ 32.04 10−× .

76

Figure 4.9: Williamson-Hall plots of ZnO film prepared from ammonium zincate bath

The lattice strain in polycrystalline films may arise from various factors. Several

lattice disorders such as point defects (vacancies in ZnO), line defects (dislocations),

planar defects (grain boundaries) and volume defects contribute to the strain. During

transfer and condensation of a liquid layer (zinc complex layer in this case) on a solid

support (glass substrate in this case), stretchning may occur which also may be a possible

source of stress in the films giving rise to lattice strain.

4.4 Electron microscope studies

Figure 4.10 shows the SEM image of ZnO films obtained from ammonium

zincate bath and heat treated at 350oC. Prior to imaging, deposition of thin gold layer was

made in an ion coater [GIKO Engineering ion coater IB-2] to enhance the emission of

secondary electrons for better imaging. The SEM unit (Model S530, Hitachi, Japan) was

operated at 20 kV. SEM investigation at normal incidence revealed polycrystalline

structure with smooth surface for the deposited films. The overall surface morphology

shows grains of nearly spherical shape and more or less uniformly covering the surface

without any cracks.

0.0 0.5 1.0 1.5 2.0 2.5

0.000

0.005

0.010

0.015

βcos

θ

4sinθ

77

Figure 4.10: SEM image of ZnO obtained from ammonium zincate bath

Figure 4.11 shows the TEM micrograph of ZnO powder scratched out from the

substrate. Particle sizes ranging between 23 and 28 nm was observed in the TEM image

with an average value of ~25.8 nm which matches well with x-ray value of 22.75 nm.

Figure 4.11: TEM image of ZnO film obtained from ammonium zincate bath

78

Figure 4.12, on the other hand, shows the TEM image of ZnO film obtained from

sodium zincate bath under optimized condition (Table 4.1). The film was heat treated at

350oC for 2 hours. Particle sizes ranging between 26 to 60 nm was observed in the TEM

image with an average value of ~41 nm. Figure 4.13, on the other hand, shows the

HRSEM (High resolution SEM) image of ZnO film obtained from zinc chloride bath as

cationic precursor under optimized conditions (Table 4.1).

Figure 4.12: TEM image of ZnO obtained from sodium zincate bath

Figure 4.13: HRSEM image of ZnO obtained from zinc chloride bath

79

HRSEM study at normal incidence was undertaken in a FEI FEG Nova 600

Nanolab at 10 kV. The image with magnification 10000× reveals structure consisting of

many spheroid-like nano particles with an average size of ~31.2 nm. The histogram of

particle size distribution is shown in figure 4.14.

Figure 4.14: Histogram of particle size distribution

Table 4.4 shows the particle size values onbtained form different zinc complexes

under optimized conditions of Table 4.1.

Table – 4.4: Particl size of ZnO from different zinc complexes

(Under optimized conditions)

Cationic precursor Particle size [ D (nm)]

( )4 22NH ZnO solution ~25.8 nm (TEM value)

2 2Na ZnO solution ~41 nm (TEM value)

2ZnCl solution ~31.2 nm (HRSEM value)

1 5 2 0 2 5 3 0 3 5 4 0 4 50

1 0

2 0

Particle diameter (nm)

Fre

qu

ency

(p

arti

cles

by

cou

nt)

80

The SEM image of ZnO film obtained from sodium zincate bath under optimized

deposition condition is shown in figure 4.15. The polycrystalline and porous nature is

revealed from the micrograph. The SEM photograph clearly illustrates the formation of

sub-micrometer crystallites distributed more or less uniformly over the surface. Although

no cracks could be detected, some holes indicating porosity is present. Agglomeration of

small crystallites also seems to be present in certain regions on the film surface. The

shape of the particles seems to be off spherical compared to nearly spherical particle for

those prepared from ammonium zincate and zinc chloride bath.

Figure 4.15: SEM image of ZnO film from sodium zincate bath

4.5 EDX and FTIR studies

Figure 4.16 shows EDX spectrum of ZnO film obtained from ammonium zincate

bath. EDX indicates that the products consist of zinc and oxygen elements. The silicon

signal appears from the substrate and the level of silicon contamination detected in the

films deposited is ~0.5 atomic %. No other impurity was detected in the films. Figure

4.17 on the other hand shows the EDX spectrum of ZnO film obtained from sodium

zincate bath. Apart from silicon, sodium was found to be present in approximately 0.7

atomic %. Trace amount of Calcium and Chlorine was also detected in the film.

81

Figure 4.16: EDX spectrum of ZnO obtained from ammonium zincate bath

Figure 4.17: EDX spectrum of ZnO obtained from sodium zincate bath

Fourier Transform Infra Red (FTIR) spectroscopy is a very useful tool to obtain

information about the chemical bonding and for investigating the vibrational properties of

0 5 10 15

0

5000

10000

15000

Zn

Zn

Ca

CaCl

Si

Na

Zn

Zn

O

Ca

Counts

Energy (KeV)

82

synthesized materials. This technique is based on the absorption of infrared radiation by

the material. When a material is irradiated with infrared radiation, absorbed IR radiation

usually excites molecules into a higher vibrational state. The wavelengths that are

absorbed by the sample are characteristic of its molecular structure. The band positions

and absorption peak not only depend on the chemical composition and structure of the

thin films but on the morphology of thin films also. FTIR analysis was performed using

Perkin-Elmer FTIR [FTIR spectrum RX1]. The FTIR spectrum of ZnO prepared from

ammonium zincate bath is shown in Figure 4.18. The FTIR spectra are usually presented

as plots of percent transmission (transmitted intensity) versus wavenumber (in cm-1). The

absorption band observed at 483.4 cm-1

is attributed to the ZnO stretching vibrations [23-

25].

Figure 4.18: FTIR spectrum of ZnO

The band at 1424.4 cm-1

may be attributed to C-O stretching frequencies [26] and

the band at 3443 may be attributed to O-H species in the film [26-27]. The band at 1122.5

could not be exactly assigned. It may be due to weakly bound acetic acid molecule [27].

4800 4200 3600 3000 2400 1800 1200 600 0

4

8

12

16

1122.5

1424.4

3443

483.4

%T

(a. u

.)

Wave number (cm)-1

83

4.6 Discussion of Results on ZnO thin films

An analysis of the results presented here indicates that strongly c-axis oriented

phase pure polycrystalline ZnO films can be prepared by SILAR technique from

ammonium zincate bath with zinc acetate as the staring precursor. The film growth rate is

a very sensitive function of zincate bath pH. Proper optimization of deposition

parameters such as bath concentrations and pH, temperarture of deposition resulted in

fairly uniform, mechanically hard and reproducible films. Clearly, if the deposition

conditions are not optimum, one can get non-adherent films.The pH has been optimized

in the range 10.80 0.05± for ammonium zincate bath and it is optimized the range

13.20 0.05± for sodium zincate bath to get adherent films on glass substrates. The growth

process follows an empirical linear behavior with number of dippoing cycle for both

sodium zincate and ammonium zincate bath. The growth rate is higher for sodium zincate

(~0.2 µm/diiping/mole) compared to ammonium zincate bath (~0.162 µm/diiping/mole).

The average particle size estimated by x-ray line broadening method was found to

be 22.75 nm (~25.8 nm from TEM) for films deposited from ammonium zincate bath. It

is evident from the present investigation that lowest particle size with highest preferred c-

axis orientation [Texture coeffcient value of ~2.29 for (002) plane] is obtained from

ammonium zincate complex. Films produced from sodium zincate bath exhibits highest

particle size (~41 nm from TEM) indicating possibly that sodium promotes grain growth.

SEM investigation shows round shape grains for films deposited from ammonium zincate

complex whereas it is off spherical for those deposited form sodium zincate bath. Grains

are uniformly distributed throughout the surface for films deposited from ammonium

zincate bath exhibiting the superiority of the films obtained from sodium zincate bath.

The porosity in the films deposited from zincate baths is quite high and it ranges between

30 to 32% as estimated from from cross sectional SEM observations. Films prepared

from ammonium zincat bath are phase pure containing no other impurities as revealed

from EDX. FTIR spectrum reveals the presence of ZnO stretching vibration.

84

References

1. R. L. Call, N. K. Jaber, K. Seshan and Jr. J. R. Whyte, Solar Energy Materials 2

(1980) 373.

2. M. Ristov, G. J. Sinadinovski, I. Grozdanov and M. Mitreski, Thin Solid Films 149

(1987) 65.

3. Y. F. Nicolau, M. Dupuy and M. Brunel, J. Electrochem. Soc. 137 (1990) 2915.

4. A. E. Jimenez-Gonzailez and P. K. Nair, Semicond. Sci. Technol. 10 (1995) 1277.

5. A. Raidou, M. Aggoer, A. Qachasu, L. Lanab and M. Fahoume, M. J. Cond. Mat. 12

(2012) 125.

6. A. P. Chatterjee, P. Mitra and A. K. Mukhopadhyay, J. Mat. Sc. 34 (1999) 4225.

7. P. Mitra and J. Khan, Mater. Chem. Phys. 98 (2006) 279.

8. A. Wojcik, M. Godlewski, E. Guziewicz, R. Minikayev and W. Paszkowicz, J.

Crystal Growth 310 (2008) 284.

9. M. C. Sneed and R. C. Brasted, in “Comprehensive Inorganic Chemistry”, Vol. 4

(Princeton, New York), 1955.

10. J. W. Mellor, in “A Comprehensive Treatise on Inorganic and Theoretical

Chemistry”, Vol. 4, (Longman-NY, USA, 1946) p. 521.

11. A. E. Rakhshani, Appl. Phys. A92 (2008) 413.

12. M. H. Lietzke, H. Marshall and L.William, Journal of Solution Chemistry 15 (1986)

903.

13. Flinn Scientific Inc, Material Safety Data sheet (MSDS, 2002).

14. S. S. Kale, R. S.Mane, H. M. Pathan, A. V. Shaikh, O. S. Joo and S. H. Han, Applied

Surface Science 253 (2007) 4335.

15. S. Ilican, Y. Caglar and M. Caglar, J. of Optoelectronics and Advanced Materials 10

(2008) 2578.

16. B. Post, S. Weissmann and H. F. McMurdie (eds.), Joint Committee on Powder

Diffraction standards, Inorganic Vol., Card No. 36-1451, International Centre for

Diffraction Data, Swarthmore, PA (1990).

17. B. E. Warren, in “X-ray diffraction” 2nd

Edition Courier Dover publications 1969.

85

18. H. P. Klug and L. E. Alexander, in “X-ray diffraction procedures for polycrystallime

and amorphous materials” (Wiley, New York, 1974).

19. P. Sharma, A. Gupta, K. V. Rao, F. J. Owens, R. Sharma, R. Ahuja, J. M. O. Gullen,

B. Johansson and G. A. Gehring, Nature Material 2 (2003) 673.

20. S. Patra, P. Mitra and S. K. Pradhan, Mat. Res. 14 (2011) 17.

21. X. Jumin and J. Wang, Mater. Lett. 49 (2001) 318.

22. B. Ghosh, H. Dutta and S. K. Pradhan, Journal of Alloys and Compounds 479 (2009)

193.

23. Z. Yang, Z. Ye, Z. Xu, B. H. Zhao, Physica E 42 (2009)116.

24. T. Ivanova, A. Harizanova, T. Koutzarova, B. Vertruyen, Materials Letters 64 (2010)

1147.

25. Z. R. Khan, M. S. Khan, Zulfequar, M. S. Khan, Materials Sciences and Applications

2 (2011) 340.

26. M. N. Kamalasanan and S. Chandra, Thin Solid Films 288 (1996) 112.

27. A. Pakdel and F. E. Ghodsi, Pramana - J. Phys. 76 (2011) 973.

86

CHAPTER 5

Preparation of Cd doped ZnO thin films by

SILAR and their characterization

5.1 Preparation of Cd doped ZnO (Cd:ZnO) films

Preparation of Cd doped ZnO was carried out from 0.1M sodium zincate

( )2 2Na ZnO bath kept at room temperature and hot water bath. Cadmium doping was

done by adding cadmium chloride (CdCl2.H2O, GR grade, Mol. Wt. 201.32) in sodium

zincate bath. Efforts to prepare Cd:ZnO films from ammonium zincate bath resulted in

instability of the bath as precipitates appear within the bath. The stability of ammonium

zincate bath is very sensitive with respect to pH due to evaporation of ammonia (Scetion

4.2.1, Chapter 4). Cadmium incorporation reduces the bath pH and results in unstable

ammonium zincate bath. The details of preparation of sodium zincate bath have already

been discussed in Chapter 4 section 4.2. The precleaned glass substrate was alternately

dipped in the cationic precursor (sodium zincate solution containing cadmium chloride)

for 2 s and for 2 s in hot water bath. The cadmium concentration was varied upto 10%

(atomic %) in the bath solution. More than 10% cadmium chloride could not be dissolved

in sodium zicate bath and precipitates appear. Fifty (50) dipping cycles were performed

for the present experiment. The thickness for pure ZnO film measured gravimetrically

was ~ 1.0 µm. The growth rate was found to decrease with increasing Cd doping. The

film thickness was ~0.94 µm for 5% Cd:ZnO and 0.85 µm for 10% Cd:ZnO.

87

5.2 Structural characterization: Evaluation of particle size

The X-ray diffraction patterns of undoped and Cd doped ZnO thin films are shown

in figure 5.1. The diffraction pattern for undoped ZnO is shown in figure 5.1 (a), while

figures 5.1(b) and 5.1(c) shows the diffractograms for 5% and 10% Cd:ZnO films

respectively. All the films were heat treated 350oC for 2 hr. prior to structural

characterization. Peaks for undoped ZnO appears at 31.60o, 34.35o, 36.20o, 47.55o and

56.55o corresponding to the reflecting planes (100), (002), (101), (102) and (110) which

are characteristics of hexagonal ZnO (Section 4.3, Chapter 4). The diffraction peaks are

shifted only marginally towards the low angle side due to slightly higher ionic radius of

cadmium (II) ion compared to that of zinc (II) ion [1-2].

Figure 5.1: XRD patterns of (a) ZnO, (b) 5% Cd doped ZnO and (c) 10% Cd doped ZnO

It is evident from figure 5.1(a) that undoped ZnO film prepared from sodium

zincate bath have a polycrystalline structure with strong preferred orientation in the (002)

20 30 40 50 60

0

500

1000

1500

2000

(110)

(102

)

(10

1)

(002)

(100)

(a)

2θ (degree)

0

500

1000

1500

2000

(b)

Inte

nsity (

a.u

.)

0

500

1000

1500

2000

(c)

88

direction. Compared with pure ZnO film, the intensity of (002) peak i.e. preferred c-axis

orientation decreases for Cd:ZnO films. The TC value for (002) plane of undoped ZnO

was ~1.95 and it decreases to ~0.6 for 10% Cd:ZnO film. [Evaluated using eqn. 3.5

(Section 3.2.1 of chapter 3) and following the method discussed in section 4.3 of chapter

4]. MARQ2 analysis (as discussed in section 4.3 of chapter 4) was carried out for

undoped ZnO sample and the fitting curve is shown in Figure 5.2. Intensity in arbitrary

units along y − axis is not shown in the figure.

Figure 5.2: Observed (dotted) and simulated (continuous) x-ray diffraction patterns of

ZnO. The difference plot is shown at the bottom.

Table 5.1 shows the values of peak positions, observed broadeningo

β obtained

using MARQ2 fitting and corresponding values of instrumental broadeningi

β . Using

these values, β was calculated from equation 4.13 (Section 4.3, Chapter 4). These values

of β were converted to radians for particle size estimation using Scherrer equation (Eqn.

3.2, Section 3.1.1, Chapter 3). The average value of particle size for undoped ZnO was

~36.73 nm. The actual particle size will be little higher than this since broadening due to

31.0 32.0 33.0 34.0 35.0 36.0 37.0

89

strain was not taken into account while particle size estimation. The x-ray value is less

than the TEM value of ~ 41 nm (Figure 4.12, Chapter 4).

Table – 5.1: Particle size in undoped ZnO film prepared from sodium zincate bath

Peak position

(2θ) o

β

(in degrees)

iβ

(in degrees)

β

(in degrees)

Particle

size (nm)

Average

particle

size

31.6 0.331 0.0979 0.2331 37.74

34.35 0.305 0.0981 0.2069 41.93 36.73 nm

36.2 0.397 0.0983 0.2988 30.51

The FWHM ( )β values for Cd:ZnO films was found to increase with Cd

incorporation. The average value of β in degrees for 5% Cd:ZnO and 10% Cd:ZnO are

0.2735 and 0.2899 respectively as opposed to 0.2427 for pure ZnO. The average particles

size comes out to be ~32 nm and ~29.9 nm respectively for 5% and 10% Cd:ZnO

respectively using the corresponding values of β . Such broadening of x-ray diffraction

peaks and decrease in particle size for Cd doped films has been reported by Maiti et. al.

[1] and Vijayalakshmi et. al. [3]. These observations along with decrease in relative

intensity of (002) peak confirms that Cd incorporation increases the degree of

polycrystallinity of the films. The decrease in particle size with increasing Cd

incorporation is possibly due to strain developed in the films due to replacement of

2Zn

+ ion by 2Cd

+ ion in ZnO lattice [1]. As explained in some literature, such increase of

strain energy may lead to a loss of preferred orientation and enhancement of random

orientation in polycrystalline ZnO [4-5].

5.3 SEM and EDX studies

Figures 5.3, 5.4 and 5.5 shows the SEM images of pure, 5% and 10% Cd: ZnO

films respectively. The images show a general view of the morphology of pure and Cd

doped ZnO films synthesized on glass substrate. The polycrystalline structure is revealed

90

from the SEM micrographs. The films are porous as evident from absence of close

packed morphology. The formation of sub-micrometer crystallites of varying sizes

indicates agglomeration and such agglomeration in certain regions of the films is evident

from the figures. Such agglomeration makes it difficult to evaluate the grain size form

SEM images. Some difference in surface morphology is observed for Cd:ZnO films

[Figures 5.4 and 5.5] compared to pure ZnO [Figure 5.3]. It appears that the morphology

gets less rougher for Cd:ZnO films. Similar observation of reduced surface roughness has

been reported for spray pyrolysed films [3].

Figure 5.3: SEM image of ZnO Figure 5.4: SEM image of 5% Cd:ZnO

,

Figure 5.5: SEM image of 10% Cd:ZnO

91

The compositional analysis of Cd doped ZnO films carried out by energy

dispersive X-ray (EDX) analysis is shown in figure 5.6 Figure 5.6 (a) shows the EDX

spectrum of 5% Cd:ZnO and 5.6 (b) shows the spectrum of 10% Cd:ZnO. The films were

repeatedly washed in hot water before EDX analysis.

Figure 5.6. EDX pattern of (a) 5% Cd:ZnO and (b) 10% Cd:ZnO

(a)

(b)

92

The EDX spectrum confirmed the presence of Zn, O and Cd elements in the

deposited films. The silicon signal appears from the substrate. Trace amount of carbon

was also detected in the film. Dopant concentration in these two cases was 5% and 10%

in the starting solution. Accordingly the expected Cd/Zn ratio was 0.05 and 0.1 in the

films. We actually obtained the Cd/Zn ratio in the films as 0.0075 and 0.0185

respectively indicating that the amount of Cd incorporation in the film is much less than

the amount of Cd in the starting solution.

5.4 Optical band gap evaluation of Cd:ZnO films

The optical absorption spectrum of the undoped ZnO and Cd doped ZnO thin

films were determined at room temperature in a UV-VIS spectrophotometer (Shimadzu,

Model No. UV-1800) in the wavelength range 200-500nm. The spectra were recorded by

taking a similar glass as a reference on which film deposition was carried out and hence

the absorption spectra obtained was from the films only (Section 3.1.3, Chapter 3). The

band gap of the films has been calculated from the absorption edge of the spectrum. Both

ZnO and CdO are considered as direct band gap materials [6]. The energy gap ( )gE can

thus be estimated by assuming direct transition between conduction band and valance

bands. Thus the direct band gap can be evaluated by putting 2n = in eqn. 3.4 (section

3.1.3, chapter 3) i.e. from the equation

( ) ( )2(5.1)

gh A h Eα ν ν= − →

The direct band gap is determined using this equation when linerar portion of

( )2

hα ν against hν plot is extrapolated to intersect the energy axis at α =0. Plot of

( )2

hα ν against hν for undoped and cadmium doped ZnO films are shown in figure 5.7.

The presence of a single slope in the plot suggests that the films have direct and allowed

93

transition. Figure 5.7 (a) shows the spectrum for pure ZnO while figures 5.7 (b) and 5.7

(c) shows the spectrum for 5% Cd:ZnO and 10% Cd:ZnO respectively.

Figure 5.7: Photon energy (eV) dependence of (a) ZnO, (b) 5% Cd:ZnO and

(c) 10% Cd:ZnO

It is seen that with the increase of cadmium doping level, the fundamental

absorption edge decreases. The value of g

E for undoped ZnO is 3.18 eV. It decreases to

3.14 eV for 5% Cd:ZnO and to 3.11 eV for 10% Cd:ZnO. The measured values are an

average of at least three measurements and are within the error limit of 0.01± eV. This

decrease can be accounted for the large difference in g

E values of ZnO and CdO [3, 7-9].

While Maiti et. al. [1] reported a decrease in band gap value from 3.29 eV for undoped

ZnO to 3.15 ev for 6% Cd doped ZnO, Vijayalakshmi et. al. [3] reported a decrease

from 3.12 eV for undoped to 2.96 eV for 25% Cd doped ZnO.

2.4 2.6 2.8 3.0 3.2 3.4

0

20

40

60

80

100

120

140

160

180

(c)

(a)(b)

(αh

ν)2

hν

94

5.5 Discussion of Results on Cd:ZnO thin films

Cd doped ZnO films could be successfully synthesized from sodium zincate bath

with cadmium chloride as source of Cd by SILAR. The films had good adherence to the

substrate. XRD spectra revealed that the films are polycrystalline with hexagonal ZnO

structure. Particle size evaluated using x-ray line broadening analysis shows a constantly

decreasing trend with increasing Cd incorporation. The average particle size of undoped

ZnO from sodium zincate bath is ~ 36.73 nm evaluated by x-ray line broadening method

neglecting strain broadening. The corresponding value from TEM is ~ 41 nm. The

average particle size reduces to ~32 nm for 5% Cd:ZnO and ~29.9 nm for 10% Cd:ZnO

evaluated by x-ray method. The undoped ZnO film is polycrystalline with strong

preferred c-axis orientation. The preferred orientation is lost and the degree of

polycrystallinity increases with increasing Cd incorporation. SEM shows polycrystalline

and porous nature of the films with surface morphology getting smoother due to Cd

incorporation. With increase of Cd doping, the fundamental absorption edge changes.

The value of fundamental absorption edge ( )gE is 3.18 eV for pure ZnO and it decreases

to 3.11 eV for 10% Cd:ZnO. Upon increasing the Cd concentration in the starting

solution, the amount of Cd in the solid films increases. The low incorporation of Cd into

the films (0.75% in the film against 5% in the starting solution for 5% Cd:ZnO film and

1.85% in the film against 10% in the starting solution for 10% Cd:ZnO film obtained

from EDX measurements) may be due to mild working conditions of SILAR technique.

More than 10% dopant addition in the starting solution was not possible since the starting

solution loses stability and precipitates appear within the bath. These observations along

with EDX observation confirm the replacement of zinc ion by cadmium ions in the ZnO

lattice. Although it is difficult to pedict the exact amount of Cd incorporation from EDX

analysis (which measures the atomic % by measuring the area under the curve in the

spectrum), marginal shift in diffraction peak positions and moderate reduction of optical

band gap energy apart from EDX estimation indicates that Cd incorporation in the films

is less than that in the starting solution.

95

References

1. U. N. Maiti, P. K. Ghosh, S. F. Ahmed, M. K. Mitra and K. K. Chattopadhyay, J.

Sol–Gel Sci. Technol. 41 (2007) 87.

2. F. Z. Wang, Z. Z. Ye, D. W. Ma, L. P. Zhu, F. Zhuge and H. P.He, Appl Phys Lett.

87 (2005) 143101.

3. S. Vijayalakshmi, S. Venkataraj and R. Jayavel, J. Phys. D: Appl. Phys. 41 (2008)

245403.

4. S. C. Seel, R. Carel and C. V. Thompson, in “Polycrystalline thin films: Structures,

textures, properties and applications II” (Mater. Res. Symp. Proc., Pittsburg, PA,

1996).

5. Y. E. Lee, Y. J. Kim and H. J. Kim, J. Mater. Res. 13 (1998) 1260.

6. H. Tabet-Derraz, N. Benramdane, D. Nacer, A. Bouzidi and M. Medles, Sol. Energy

Mater. Solar Cells 73 (2002) 249.

7. L. F. Dong, Z. Cui and Z. K. Zhang, Nanostruct. Mater. 8 (1997) 815.

8. H. Cao, J. Y. Xu, D. Z. Zhang, S. H. Chang, S. T. Ho, E. W. Seeling, X. Liu and R.

P. H. Chang, Phys. Rev. Lett. 84 (2000) 558410.

9. O. Vigil, L. Vaillant, F. Cruz, G. Santana, A. M. Acevedo and G. C. Puente, 2000

Thin Solid Films 361 (2000) 53.

96

CHAPTER 6

Preparation of Mn doped ZnO thin films

by SILAR and their characterization

6.1 Preparation of films and thickness measurements

Preparation of Mn doped ZnO film was carrid out from zinc chloride

( )2ZnCl solution as cationic precursor containing manganese (II) chloride as source of

Mn ion and sodium hydroxide ( )NaOH solution as anionic precursor. Efforts to

synthesize Mn doped ZnO from sodium zincate as well as ammonium zincate bath failed

possibly due to lowering of pH upon dopant addition. Although manganese chloride

could be dissolve in low concentrations in these baths, no film formation took place.

The concentration of the zinc chloride (ZnCl2.2H2O, Merck, Mol. Wt. 136.28)

bath and sodium hydroxide bath was optimized at 0.1 M and 0.075 M respectively for

synthesis of good quality adherent film. For concentrations more than 0.125 M for zinc

chloride bath and for concentrations more than 0.1 M for sodium hydroxide bath, the

growth process nonuniform resulting in poor quality and nonadherent films. The cationic

precursor was at room temperature and the temperature of anionic precursor was

optimized at 70oC.

97

One of the problems with zinc chloride solution is that complete dissolution of the

solute does not occur and precipitate appears on standing. Addition of three drops of

acetic acid (~ 0.3 cc by volume) gives a clear transparent solution. The pH of the

transparent solution was 4.70 ± 0.05. Sodium hydroxide solution (0.075 M) was prepared

by dissolving NaOH pellets (Merck, Mol. Wt. 40) in deionized water. The optimized pH

of the sodium hydroxide bath was 11.10± 0.05. Alongwith bath concentrations, the pH

and temperature of the baths were found to be optimum for getting adherent films on

substrate.

Although adherent films could be obtained on glass substrate, it was found during

the course of the experiment that adhesion of the films on quartz substrate was stronger

compared to glass. The adherence of the ZnO films on glass substrate was found to be

somewhat lesser compared to those deposited from sodium or ammonium zincate baths.

Both microscope glass slides and commercially available quartz substrates were used for

film deposition. For Mn doping, Manganese (II) chloride (MnCl2.4H2O, Merck, Mol.

Wt. 197.9) was dissolved in zinc chloride solution. Addition of manganese chloride

tetrahydrate gave the solution a slightly ash colouration. The resulting mixture was stirred

using a magnetic stirrer for about 10 minutes. After stirring the manganese chloride salt

gets completely dissolved in the solution. The manganese concentration was varied upto

5% in the zinc chloride solution for the preparation of doped films. For 10%≥ dopant

addition, the bath pH of the cationic precursor reduces reulting in slow growth rate and

poor quality of the coated films. Accordingly the dopant addition was restricted to 5

atomic %.

The quartz substrate was cleaned, before deposition, by etching in 1%

hydrofluoric acid )(HF for 24 hours followed by ultrasonic cleaning in equivolume

acetone and alcohol and thorough rinsing in deionized water. The cleaned substrate was

alternatively dipped in zinc chloride solution impurified with Mn (II) chloride and hot

NaOH solution. Dipping for 2 s in each bath constitutes one complete dipping cycle.

98

The film thickness )(t was built up by increasing the number of dipping cycle.

Fifty (50) dipping cycles were performed in the present experimens. The deposited films

were subsequently annealed in air at 350oC for 2 hr. Figure 6.1 shows the dependence of

film thickness (measured gravimetrically) on the number of dipping cycle )(N for

undoped ZnO films on quartz substrate.

It is seen from figure 6.1 that the film thickness follows a linear growth law with

number of dipping cycle and the growth rate was found to be 0.021 µm/dipping. There is

an overall variation of ± 5% in the film thickness data (shown as error bars against each

data point of Fig. 6.1). The growth rate was found to be uniformly low for glass substrate.

Figure 6.1: Dependence of film thickness on number of dipping cycle

The film thickness was verified against cross sectional SEM. Some portion of the

quartz substrate was acid etched to remove film from that area in order to create a step for

thickness measurement. Figure 6.2 shows the cross-sectional SEM micrograph of

undoped ZnO film of thickness 2.1 µm measured gravimetrically (obtained by 100

dipping). An average thickness of 2.56 µm was obtained from SEM micrograph.

0 25 50 75 100 125

0.0

0.5

1.0

1.5

2.0

2.5

Th

ickne

ss (

µm

)

No. of dipping (N)

99

The actual thickness determined from cross-sectional SEM is 22% higher than the

gravimetric value (2.56 µm measured by SEM as opposed to 2.1 µm measured

gravimetrically). The value was an average of several measurements on different

portions. This indicates an average porosity of ~ 22% in the deposited films. The films

are less porous compared to those from ammonium and sodium zincate baths (Section

4.2.4, Chapter 4).

Figure 6.2: Cross-sectional SEM of ZnO film

The growth rate was found to decrease with Mn incorporation. For 50 dipping, the

film thickness for pure ZnO was 1.05 µm. The corresponding thickness for 2% Mn:ZnO

film was 0.94 µm and for 5% Mn:ZnO, the thickness was 0.82 µm. Thus the growth rate

decreases with increasing Mn incorporation. The ZnO film was white in appearance and

Mn doped films were slightly brownish with a good adherence to the substrate.

Table 6.1 shows the thickness and growth rate values for undoped and Mn doped

films for 50 dipping on quartz substrate. The thickness of pure ZnO film was 0.80

mµ and Mn doped film it was 0.71 mµ on glass substrate.

Quartz substrate

Film

100

Table 6.1: Thickness and growth rate for ZnO and Mn:ZnO films

Film Thickness (µm) Growth rate

(µm/dipping)

ZnO 1.05 0.021

2% Mn:ZnO 0.94 0.0188

5% Mn:ZnO 0.82 0.0164

6.2 Structural characterization: Evalaution of particle size and strain

The X-ray diffraction patterns of undoped ZnO and Mn doped ZnO films

deposited on quartz substrate are shown in figure 6.3. The diffraction pattern for undoped

ZnO is shown in figure 6.3 (a). Figure 6.3 (b) and 6.3 (c) shows the diffractograms for

2% and 5% ZnOMn : films respectively. The films were heat treated at 350oC for 2 hr.

prior to structural characterization. The materials were scanned in the range 25-65o. It is

seen from figure 6.3 (a) that peaks appear at 31.708o, 34.397

o, 36.183

o, 47.516

o, 56.551

o

and 62.88o. The corresponding values for 5% Mn:ZnO are 31.69

o, 34.372

o, 36.168

o,

47.5o, 56.542

o and 62.87

o. The diffractogram of the sample reveals that all the peaks are

in good agreement with the JCPDS data belonging to hexagonal ZnO structure. The

corresponding reflecting planes are (100), (002), (101), (102), (110) and (103)

respectively. The (002) peak appears with maximum intensity at 34.397o. Apart from

ZnO characteristic peaks, no extra peaks due to manganese clusters, zinc or their complex

oxides could be detected within the detection limit of XRD. This observation is an

indication of the fact that the films do not have any phase segregation or secondary phase

formation as well as Mn incorporation into ZnO lattice.

It is evident from figure 6.3 (a) that undoped ZnO film have a polycrystalline

structure with preferred orientation along the (002) diffraction plane. Compared to

undoped ZnO film, the intensity of (002) peak decreases for Mn:ZnO films. This results

101

in an increase of relative intensity of (101) peak with respect to (002) peak. The (101)

peak appears with maximum intensity in ZnO powders with no preferred orientation

(JCPDS Card No. 36-1451). Thus crystalline nature of films was affected due to

enhancement of dopant concentration. Such loss of preferred c-axis orientation and

enhancement of polycrystalline nature with Mn incorporation has been reported by

Nirmala et. al [1]. The value of (002)TC for undoped ZnO is ~1.82 and it decreases to

~0.41 for 5% Mn:ZnO film [Evaluated using eqn. 3.5 (Section 3.2.1 of chapter 3) and

following the method discussed in section 4.3 of chapter 4].

Figure 6.3: X-ray diffraction pattern of (a) ZnO, (b) 2% Mn:ZnO and (c) 5% Mn:ZnO.

30 40 50 60

0

500

1000

1500

2000

2500

(103)

(110)

(102)

(101)

(100) (0

02)

(a)

2θ (degree)

30 40 50 60

0

500

1000

1500

2000

2500

(b)

Inte

nsity (

a.u

.)

30 40 50 60

0

500

1000

1500

2000

2500

(c)

102

The peaks of the diffraction pattern of the doped sample are slightly shifted to left

compared to undoped ZnO. This is possibly because the ionic radius of 2Mn

+ (0.83Å) is

larger than that of 2Zn

+ (0.74 Å) [1]. A typical plot of MARQ2 analysis for 5% Mn:ZnO

sample is shown in Figure 6.4.

Figure 6.4: Observed (dotted) and simulated (continuous) x-ray diffraction patterns of

5% Mn:ZnO on quartz substrate

The average value of particle size for undoped ZnO evaluated by x-ray line

broadening method is 29.71 0.01± nm. It decreases to 26.79 0.01± nm for 2% Mn:ZnO

and 23.76 0.01± nm for 5% Mn:ZnO. Figure 6.5 shows the W-H plots of ZnO, 2%

Mn:ZnO and 5% Mn:ZnO. The average microstrain in the films as determined from W-H

plots is 0.0013, 0.00137 and 0.00146 respectively for pure, 2% Mn:ZnO and 5% Mn:ZnO

films respectively. Thus the particle size decreases with increasing Mn incorporation and

the strain increases. The decrease in average particle size with increasing Mn doping i.e.

hindrance of grain growth upon Mn incorporation has been reported by other workers [2-

30.0 31.0 32.0 33.0 34.0 35.0 36.0 37.0 38.0

Inte

nsi

ty (

a. u

.)

2θ

103

4]. The decrease in crystal quality with Mn doping has been reported by Lee et al [5]. The

decrease in average particle size might be due to development of strain because of Mn

incorporation. Such enhancement of average microstrain with Mn incorporation has been

observed in the present work. The enhancement of strain due to Mn incorporation might

be due to larger ionic radius of Mn ion than Zn ion.

Figure 6.5: W-H plots of (a) pure ZnO, (b) 2% Mn:ZnO and (c) 5% Mn:ZnO films The X-ray diffraction patterns of undoped ZnO and 5% Mn doped ZnO films

deposited on glass substrates is shown in figure 6.6. The diffraction pattern for undoped

ZnO is shown in figure 6.6 (a). Figure 6.6 (b) shows the diffractograms for 5%

ZnOMn : film. The films were heat treated at 350oC for 2 hr. prior to structural

characterization. The step-scan data were recorded for the angular range 20o to 70

o.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

0.000

0.005

0.010

0.015

0.020

(a)

4sinθ

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

0.000

0.005

0.010

0.015

0.020(b)

βco

sθ

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

0.000

0.005

0.010

0.015

0.020

(c)

104

30 40 50 60

0

400

800

1200

1600

2000

2400

2800

(a)

2θ (degree)

30 40 50 60

0

400

800

1200

1600

2000

2400

2800

(b)

Inte

nsity (

a.u

.)

Figure 6.6: X-ray diffraction pattern of (a) ZnO and (b) 5% Mn:ZnO deposited on glass

It is seen from figure 6.6 (a) that peaks appears at 31.7o, 34.42o, 36.2o, 47.46o,

56.56o and 62.84

o. Similar observation of loss of preferred orientation along c-axis is

evident from the figure. The preferred orientation of the films is governed by the total

system energy, which is the summation of the strain and surface energies [6]. Thus

increase in strain energy effects the preferred growth along c-axis since it is known that

the driving force towards preferred orientation arises out of total energy minimization of

the system.

6.3 SEM and EDX studies

Figure 6.7 shows the HRSEM micrograph of pure ZnO film prepared on quartz

substrate. HRSEM study was undertaken in a FEI FEG Nova 600 Nanolab at 5 kV. The

SEM image shows structure consisting of many spherical shaped nano particles with an

105

averge size of ~ 31 nm. This is similar to the result obtained on glass substrate (chapter 4,

Figure 4.18). The average particle size of ~31 nm matches well with that obtained using

x-ray line broadening analysis of ~29.71 nm. Figure 6.8 shows the SEM image of 5%

Mn:ZnO film on quartz substrate. Surface morphology of 5% Mn:ZnO film shows

wrinkle structure with formation of nanorods in certain regions.

Figure 6.7: HRSEM image of ZnO on quartz substrate

Figure 6.8: SEM image of 5% Mn:ZnO thin film

106

Similar observation of appearance of wrinkle structure due to Mn incorporation

has been reported by Nirmal et al [1]. Srinivasan et. al. [7] also reported microstructure

consisting of nanorods with wrinkle structure for Mn doped films. Formation of such

nanorods in Mn doped ZnO films have also been reported by Karamat et. al. [8]. Figure

6.9 shows the HRSEM image of 5% Mn:ZnO film on glass substrate with magnification

25000× [10].

Figure 6.9: HRSEM image of 5% Mn:ZnO

Figure 6.10 shows the energy dispersive X-ray spectrum of Mn:ZnO films

prepared on quartz substrate. Figure 6.10 (a) shows the EDX spectrum of 2% Mn:ZnO

and 6.10 (b) shows the spectrum of 5% Mn:ZnO. The EDX spectrum confirmed the

presence of Zn, O and Mn elements in the deposited films i.e. incorporation of Mn in

ZnO lattice. The silicon signal appears from the quartz substrate. Dopant concentration in

these two cases was 2% and 5% in the starting solution. Accordingly the expected Mn/Zn

ratio was 0.02 and 0.05 in the films. We actually obtained the Mn/Zn ratio in the films as

0.0131 and 0.0284 respectively indicating that the amount of Mn incorporation in the

film is less than the amount of Mn in the starting solution. The real Mn content in the

deposited films was 1.31% and 2.84% as obtained from EDX spectrum.

107

Figure 6.10. EDX pattern of (a) 2% Mn:ZnO and (b) 5% Mn:ZnO

(b)

(a)

108

Figure 6.11 reveals the EDX spectrum of 5% Mn:ZnO film on glass substrate.

Trace amount of calcium (Ca) impurity was also detected in the film.

Figure 6.11: EDX pattern of 5% Mn:ZnO

6.4 Evaluation of band gap from Optical absorption

The optical absorption spectra were recorded by using a similar quartz substrate

as a reference and hence the absorption due to the film only was obtained. Figure 6.12

shows the dependence of optical absorbance ( )α on wavelength ( )λ . While figure 6.12

(a) shows the dependence of α on λ for pure ZnO, figures 6.12 (b) and 6.12 (c) shows

dependence of α on λ for 2% Mn:ZnO and 5% Mn:ZnO respectively. Plot of

( )2

hα ν against hν for undoped and Mn doped ZnO films was derived from figure 6.12

and is shown in figure 6.13. Figure 6.13 (a) shows the spectrum of undoped ZnO while

figures 6.13 (b) and 6.13 (c) shows the spectrum of 2% Mn:ZnO and 5% Mn:ZnO

respectively. The direct band gap is determined using this equation when linerar portion

of ( )2

hα ν against hν plot is extrapolated to intersect the energy axis at α =0.

109

Figure 6.12. Plots of absorbance vs wavelength for (a) undoped ZnO; (b) 2% Mn:ZnO

and (c) 5% Mn:ZnO

Figure 6.13: Plots of ( )2ναh vs νh for (a) pure ZnO; (b) 2% Mn:ZnO and (c) 5% Mn:ZnO

360 380 400 420 440 460 480 500

1.0

1.5

2.0

2.5

3.0

3.5

4.0

(c)(b)

(a)

absorb

an

ce

(α

)

λ(nm)

2.4 2.6 2.8 3.0 3.2 3.4 3.6

0

20

40

60

80

100

120

140

160

180

200

(b)

(c)

(a)

(αh

ν)2

hν (eV)

110

It is seen that with the increase of manganese doping level, the fundamental

absorption edge decreases. The value of g

E for undoped ZnO is 3.22 0.01± eV. It

decreases to 3.13 0.01± eV for 2% Mn:ZnO and to 3.06 0.01± eV for 5% Mn:ZnO.

The decrease in band gap value with increased Mn doping concentration has been

accounted due to the sp-d exchange interactions and has been theoretically explained

using the second–order perturbation theory [1, 9-10]. A decrease in band gap energy from

3.27 eV for undoped ZnO to 2.78 eV for 3% Mn doped ZnO has been reported by

Senthilkumar et. al. [4] and has been attributed to s-d and p-d interactions giving rise to

band gap bowing.

Similar results on glass substrate for undoped 5% Mn:ZnO film is shown in figure

6.14. The value of g

E for undoped ZnO is 3.20 eV and it decreases to 3.04 eV for 5%

Mn:ZnO. The data for drawing figure 6.14 was extracted from the data of α versus λ .

Figure 6.14: Plots of ( )2ναh vs νh (in eV) for (a) pure ZnO and (b) 5% Mn:ZnO

2.4 2.6 2.8 3.0 3.2 3.4

0

20

40

60

80

100

120

140

160

(b)

(a)

(αh

ν)2

hν

111

6.5 Discussion of results on Mn:ZnO thin films

The primary aim of this investigation was to explore the possibility of doping or

impurifying ZnO with manganese by SILAR method. Mn doped ZnO films with different

percentage of Mn content (upto 5%) could be successfully synthesized by suitable choice

of cationic and anionic precursors under optimized deposition conditions. Zinc chloride

bath with manganese chloride as source of Mn ion was used as cationic precursor and

sodium hydroxide was used as anionic precursor. The film growth rate was found to

increase linearly with number of dipping cycle. Better adherence of Mn:ZnO films were

obtained on quartz substrate compared to glass substrate. More than 5% dopant addition

was difficult to obtain due to lowering of stability of the cationic bath. Particle size

evaluated using x-ray line broadening analysis shows a constantly decreasing trend with

increasing manganese incorporation. The average particle size of ~29.71 nm for undoped

ZnO evaluated by x-ray line broadening method matches well with HRSEM observation

(~31nm). The average particle size reduces to ~26.69 nm for 2% Mn:ZnO and ~23.76 nm

for 5% Mn:ZnO. The films are polycrystalline with an average porosity of ~22%. The

polycrystallinity of the films as well as the average microstrain (evaluated using

Williamson-Hall equation) increases with increasing Mn incorporation. Mn doping also

influences the morphology of the films. The undoped films contained nearly spherical

grains. On the other hand microstructure consisting of wrinkle structure was observed

due to Mn incorporation. The observation was similar for both quartz and glass

substrates. These observations along with EDX observation confirms the replacement of

zinc ion by manganese ions in the ZnO lattice. The real Mn content in the deposited film

was less than that in the starting solution as evident from EDX measurements. The

oxidation state of Mn in ZnO is controversial and no experiment was taken up in this

direction. This is important for magnetic properties on Mn:ZnO. Mn doping reduces the

value of fundamental absorption edge from ~3.22 eV for pure ZnO to ~3.06 eV for 5%

Mn:ZnO for films deposited on quartz substrate. Corresponding values on glass were

3.20 eV and 3.04 eV respectively. Incorporation of Mn has a strong effect on the

structural, morphological and optical properties of ZnO.

112

References

1. M. Nirmala and A. Anukaliani, Photonics Letters of Poland 2 (2010) 189.

2. J. Luo, J. K. Liang, Q. L. Liu, F. S. Liu, Y. Zhang, B. J. Sun and G. H. Rao, J. Appl.

Phys 97 (2005) 086106.

3. S. Deka and P. A. Roy, Solid State Communications 142 (2007) 190.

4. S. Senthilkumar, K. Rajendran, S. Banerjee, T. K. Chini and V. Sengodan, Materials

Science in Semiconductor Processing 11 (2008) 6.

5. J. H. Lee and B. O. Park, Thin Solid Films 426 (2003) 94.

6. U. C. Oh and J. H. Je, J. Appl. Phys. 74 (1993) 1692.

7. G. Srinivasan and J. Kumar, Applied Surface Science 254 (2008) 7285.

8. S. Karamat, S. Mahmood, J. J. Lin, Z. Y. Pan, P. Lee, T. L. Tan, S. V. Springham, R.

V. Ramanujan and R. S. Rawat, Applied Surface Science 254 (2008) 7285.

9. R. B. Bylsma, W. M. Becker, J. Kossut, U. Debska and D. Y. Short, Phys Rev B 33

(1986) 8207.

10. P. Singh, A. Kaushal and D. Kaur, J. Alloys and Compounds 471 (2009) 11.

113

CHAPTER 7

Preparation of Al doped ZnO (AZO) thin

films by SILAR and their characterization

7.1. Preparation of AZO films

Al doped ZnO films could be deposited from both sodium zincate and ammonium

zincate complex on glass substrate. The results presented here are for ammonium zincate

bath. Aluminium doping was carried out by adding hexahydrate aluminium chloride

(3 2.6AlCl H O , Merck) as the source of dopant and was added in requisite amount in the

ammonim zincate bath. Fifty (50) dipping cycles were performed for ZnO and AZO films

in the present experiment.

The aluminium concentration was varied upto 2% since optimum incorporation of

Al in ZnO has been reported to be around 1-3 atomic% [1-3]. All the deposited films

were white in color and homogeneous. Coated films were well adherent on glass

substrate. The thickness for pure ZnO film measured gravimetrically was ~ 0.8 µm (0.162

µm per dipping). Growth rate of the films was found to increase due to Al incorporation.

The thickness for 1% Al doped film was found to be ~0.96 µm (0.192 µm per dipping)

indicating higher growth rate due to Al incorporation.

114

7.2 Structural characterization by XRD: Evaluation of TC (002)

The X-ray diffraction patterns of undoped and Al doped ZnO films with different

Al content prepared from ammonium zincate bath are presented in figure 7.1. The films

were annealed at 350oC for 2 hr. in air prior to structural characterization. The diffraction

angle 2θ was scanned in the range 20o to 70

o.

.

Figure 7.1: X-ray diffraction pattern of (a) ZnO, (b) 0.5% AZO, (c) 0.75% AZO,

(d) 1% AZO, (e) 1.5% AZO and (f) 2% AZO

20 30 40 50 60 70

0

1150

2300

(103)

(110)

(102)

(101)

(100) (a)

2θ (degree)

0

1150

2300

(b)

0

1150

2300

(002)

(c)

0

1150

2300

(d)

Inte

nsity

(a. u.)

0

1150

2300

(e)

0

1150

2300

(f)

115

Figure 7.1 (a) shows the diffraction pattern for undoped ZnO while figures 7.1(b),

7.1 (c), 7.1 (d), 7.1(e) and 7.1 (f) shows the diffraction patterns for 0.5% AZO, 0.75%

AZO, 1% AZO, 1.5% AZO and 2% AZO respectively. It is seen from figure 7.1 (a) that

peaks appear at 31.75o, 34.389

o, 36.205

o, 47.434

o, 56.576

o and 62.855

o. The

corresponding reflecting planes are (100), (002), (101), (102), (110) and (103)

respectively. The (002) peak appears with maximum intensity in pure and all Al doped

films indicating all the samples have strong preferred c-axis orientation i.e. preferred

orientation of the crystals with c-axis perpendicular to the substrate. The other peaks

corresponding to (100), (101), (102), (110) and (103) are present with low relative

intensities. No measurable change in diffraction angles were found due to Al doping.

For films with high Al content (more than 1% AZO films), the relative intensity

of (100) and (101) peak increases indicating some loss of preferred c-axis orientation for

heavily doped films.

Table 7.1 shows the values of TC (002) for undoped and Al doped films. The

value increases initially and does not change much for films upto 1% doping. It then

starts decreasing sharply.

Table 7.1: TC(002) values for undoped and Al doped ZnO films

Film TC (002)

ZnO ~2.29

0.5% AZO ~2.32

0.75% AZO ~2.32

1% AZO ~2.33

1.5% AZO ~1.98

2% AZO ~1.83

116

The MARQ2 plot for 0.5% AZO sample is shown in Figure 7.2.

Figure 7.2: Observed (dotted) and simulated (continuous) XRD patterns of 0.5% AZO

Figure 7.3 shows the Williamson-Hall plots of pure ZnO, 1% AZO and 2% AZO

films. The average value of particle size estimated using 0.9k = for undoped ZnO is

~22.75 nm. It increases marginally to ~24.26 nm for 1% Al :ZnO and to ~25.13 nm for

2% Al :ZnO.

Thus with increasing doping concentration the particle size shows a slightly

increasing trend. While majority of the researchers have reported a marginal decrease in

grain size due to Al incorporation [4-6], Rakhshani [7] have reported that Al -doping

does not modify the size of the grains. Tewari et al. [4] however concluded that the

crystallite size does not vary in any regular pattern with Al incorporation. In all these

works, Scherrer equation (Eqn 3.2, Chapter 3) was applied to evaluate the grain size

which only takes account of particle size broadening. In our present work we have

117

utilized the Williamson-Hall equation which is more accurate since it takes account of

both instrumental broadening and strain broadening to estimate the particle size and

accordingly gives much more reliable results compared to Scherrer equation. Such

increase in particle size may be due to enhanced thickness of AZO films observed in our

present work. It seems that the film tends to lower its surface energy as it becomes

thicker during deposition. During the process the lower-surface-energy grains may

become larger as film thickness increases [8-9]. This is achieved by diffusion within a

thin surface layer of atoms from a particular crystallite to one having a lower surface

energy. The strain in the films was found to reduce from ~0.00204 for ZnO (section 4.3,

Chapter 4) to ~0.002 for 1% AZO film and ~0.00195 for 2% AZO film.

Figure 7.3: Williamson-Hall plots of (a) pure ZnO, (b) 1% Al :ZnO and (c) 2% Al :ZnO.

0.0 0.5 1.0 1.5 2.0 2.5

0.000

0.005

0.010

0.015

(a)

4sinθ

0.0 0.5 1.0 1.5 2.0 2.5

0.000

0.005

0.010

0.015

(b)

βco

sθ

0.0 0.5 1.0 1.5 2.0 2.5

0.000

0.005

0.010

0.015

(c)

118

Figure 7.4 shows the SEM micrograph of undoped ZnO film while figure 7.5

shows the same for 1% AZO film. The incorporation of Al into the lattice affects the

morphology as can be seen from the micrographs. Figure 7.4 (a) shows the SEM image at

normal incidence with magnification ×12000. It is evident that the microstructure

consists of many round shaped clearly defined grains (crystalline particles) covering the

substrate surface more or less uniformly. However there is agglomeration in certain

regions of the film which is clearly visible in the SEM image with magnification ×35000

[figure 7.4 (b)] of the same film].

The AZO film on the other hand shows particles with off spherical shape. Thus

Al doping seems to have modified the shape of the grains. Figure 7.5 (a) shows the SEM

image of 1% AZO film with magnification ×12000 while 7.5 (b) shows the SEM image

of the same film with magnification ×40000. The microstructure is composed of uniform

and compact interconnected grains. Also the film appears to have less porosity and more

rough than ZnO film indicating that the microstructure became denser with

Al incorporation. Similar observation of higher surface roughness due to Al doping has

been reported by Kim et. al. [3]. Thus incorporation of Al leads to a more continuous

film having higher density and less smooth surface compared to undoped ZnO.

Fig. 7.4 (a) Fig. 7.4 (b)

Figure 7.4: SEM image of ZnO (a) with magnification ×12000 and (b) with magnification ×35000

119

Fig. 7.5 (a) Fig. 7.5 (b)

Figure 7.5: SEM image of 1% AZO film (a) with magnification ×12000

and (b) with magnification ×40000

Figure 7.6 shows the energy dispersive X-ray spectrum of 1% Al :ZnO film.

Figure 7.6: EDX pattern of 1% Al :ZnO

120

Although no compositional analysis was attempted in the present study, the

incorporation of Al in the films was verified by the EDX result. The spectrum reveals

the presence of Zn, O and Al elements in the deposited films. The silicon signal appears

from the substrate. Trace amount of C and S impurities was also detected in the film.

7.3 Band gap evaluation from optical absorption

The optical absorbance spectrum was measured within the wavelength range of

200–500 nm. Plots of ( )2

hα ν against hν for undoped and Al doped ZnO films are

shown in figures 7.7. Figure 7.7 (a) shows the spectrum of pure ZnO while figures 7.7

(b) and 7.7 (c) shows the spectrum of 1% Al :ZnO and 2% Al :ZnO respectively.

Figure 7.7: Plots of ( )2

hα ν vs photon energy (in eV) of (a) pure ZnO, (b) 1% Al :ZnO

and (c) 2% Al :ZnO

2.4 2.6 2.8 3.0 3.2 3.4

0

20

40

60

80

100

120

140

160

180

(c)

(b)

(a)

(αh

ν)2

hν

121

The optical band gap shows an increase for 1% AZO compared to pure ZnO. For

pure ZnO the band gap is 3.23 0.01± eV and for 1% AZO, it increases to 3.29 0.01± eV.

Such widening of optical band gap with Al doping is well described by Burstein-Moss

effect [10-14]. For AZO films, compared with undoped ZnO films, the contribution from

3Al

+ ions on substitutional sites of 2Zn

+ ions and Al interstitial atoms determines the

widening of the band gap caused by increase in carrier concentration. This is the well

known Burstein–Moss effect and is due to the Fermi level moving into the conduction

band. Since Al -doping increases the carrier concentration in the conduction band, the

optical bandgap energy increases. Enhancement of band gap thus also ensures that Al

was successfully doped in the ZnO thin films. It is further observed in our present work

that a decrease in band gap occurs for 2% AZO film. The value of band gap for 2% AZO

is 3.18 0.01± . Such unusual red shift of fundamental absorption edge has been reported

by Mohanty et. al. [15] and has been explained in terms of stress relaxation mechanism.

The reduction in slope of the linear portion of the plot [figure 7.7 (c)] observed in our

present work suggest introduction of defect states within the band gap. Thus we interpret

this shift due to merging of an impurity band into the conduction band, thereby shrinking

the band gap. Formation of such impurity band giving rise to new donor electronic states

just below the conduction band is possible and this arises due to hybridization between

states of the ZnO matrix and of the Al dopant [16]. It seems that such formation of donor

levels compensates the Burstein–Moss effect and results in narrowing of the effective

band gap of AZO. The reduction of stress due to enhanced thickness [15] of AZO films

compared to pure ZnO may also have some contribution to the observed red shift. Our

present observation also suggests an enhanced growth rate giving rise to increased

thickness and lowering of strain due to Al incorporation.

7.4 Electrical resistance measurements

.

The electrical resistance measurement of pure and Al doped ZnO films was

carried out in the surface mode using the conventional DC two-point probe technique.

122

The resistance measurement was made in a Keithley 6514 system electrometer. The film

was kept in the dark inside a quartz tube furnace. Ohmic contacts using high conducting

silver paste (curing temperature 200oC) was made onto the surface of the film.

Approximately 20 mm long silver (Ag) contacts, separated by 5 mm, were made on the

films (30 mm× 25 mm) for electrical measurements. The width of the electrodes was

approximately 1 mm. The electrical resistance was measured at 100oC. The films were

heat treated at 200oC for 2 hr. prior to resistance measurements.

Electrical resistance measurement at 100oC is shown in figure 7.8. The figure

shows a marked decrease in resistance due to Al incorporation initially. While undoped

ZnO shows a resistance of ~2.28 MΩ, 1% AZO shows a resistance value of ~0.189 MΩ.

Such decrease in resistance confirms the substitutional replacement of 2Zn

+ ions by 3Al

+

and subsequent enhancement of carrier density. With further increase in Al concentration

the resistance value started to increase. The initial decrease of resistance with increase in

Al concentration has been attributed to increase in carrier concentration and also due to

increase in mobility [17]. Distribution of Al in the grains leading to interconnected

grains and a more continuous film as has been observed in our present work also might

contribute to the lowering of resistance by enhancing the mobility of charge carriers.

Beyond a certain doping concentration, a decrease in mobility but a small change

in carrier concentration has been reported [4, 18]. Our present observation suggests that

beyond a certain doping concentration, the doping atoms do not occupy the lattice sites

but possibly result in some kind of defects. Such limited incorporation of Al into ZnO

lattice is consistent with the results reported by researchers [1-3, 17, 19]. The defects

produced beyond this optimum level of doping gives rise to states within the band gap

reducing the effective band gap, observed in the present work. However their

contribution to enhanced carrier density is neutralized by drastic decrease in mobility and

thereby effectively reducing the resistance. Such drastic reduction of mobility due to

segregation of dopants at grain boundaries has been reported by Shrestha et. al. [5]. It

seems that the micro-mechanism of the influence of dopants is quite complicated. It

123

appears that beyond certain doping concentration there is segregation of dopant atoms at

the noncrystalline regions which produces disorder in the lattice. These defects act as

scattering centers giving rise to various scattering mechanisms and resulting in a sharp

decrease in the mobility.

Figure 7.8: Variation of electrical resistance at 100oC with Al concentration.

Figure 7.9 shows the data of the variation of the electrical conductance with

reciprocal temperature (1000/T) in the temperature range 300-400K with a control

accuracy of 1± K. The electrical conductance ( )Σ was directly evaluated from the

measured value of electrical resistance ( )R . The decrease in resistance with increasing

temperature following semiconducting behaviour of ZnO is observed.

The conduction process may be described by the following equation:

( )exp (7.1)o

EkT

∑ = ∑ − →

0.0 0.5 1.0 1.5 2.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

R (

M-o

hm

)

Al concentration (at %)

124

Figure 7.9: Temperature dependence of electrical conductance in the 300-400K range for

(a) undoped ZnO and (b) 1% AZO film

In equation 7.1, 1

R

∑ =

is the conductance at temperatureT , o

∑ is the pre-

exponential factor for the temperature range 300-400K, k is the Boltzmann’s constant,

T is the absolute temperature and E is the activation energy barrier value. The

experimentally obtained value of E is ~0.26 eV for pure ZnO and it seems to have

remain unaffected due to Al doping. Thus there is no change in activation barrier value

due to doping. An activation barrier value of 0.24-0.28 eV is normally associated with

neutral oxygen vacancy acting as donor state [20-21].

The decrease of resistance by approximately one order in the entire temperature

range studied shows that Al atoms are incorporated into the ZnO lattice and contributes

conduction electrons according to the equation

3 2 (7.2)Al Al e+ +→ + →

2Al

+ replaces 2Zn

+ and the electron released is free to contribute to electrical conduction.

2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.310

-8

10-7

10-6

10-5

(b)

(a)

Co

nd

ucta

nce

(o

hm

)-1

103/T (

oK)

-1

125

The value of effective density of conduction electrons (eff

n ) can be calculated

from the equation [22]:

(7.3)eff

n eσ µ= →

where 1

σρ

= is the electrical conductivity, e is the electronic charge and µ is the

mobility. The resistivity ρ was determined from the relationRL

Aρ = , where L is the

distance between the two silver electrodes and A is the area, which is the product of

length of the electrodes and thickness of the film. Here R is the measured resistance,

L ≅ 5 mm. The area A is the product of length of the electrodes ( ≅ 20 mm) and

thickness of the film ( ≅ 5 µ m). From our present results, the value of eff

n comes out to

be of the order of 1013

/cm3 at around 380K for ZnO film. For AZO film it is of the order

of 1014

/cm3 at 380K. While calculating

effn , the mobility value was assumed to be

constant and equal to 20 cm2 Vs

-1 i e. temperature dependence of mobility was not taken

into account [23]. This will introduce only a small error in the calculation and will not

affect the order of carrier density.

7.5 Electrical resistance measurements in presence of LPG

Some additional electrical measurements for AZO films were carried out in

presence of LPG (Liquefied Petroleum gas) since Al doped ZnO is a promising gas

sensor material [24-25]. However, only a few publications describing the sensing

behavior of Al doped ZnO thin films deposited by aqueous solution techniques at low

temperatures are available [26-27].

The electrical resistance of the films was measured before and after exposure to

LPG. The sensitivity of the film was determined at different operating temperatures in the

126

range 250-375oC in presence of LPG in air. Commercially available calibrated mixtures

of LPG were used for this purpose. Before exposing to LPG, the film was allowed to

equilibrate at each operating temperature for 30 minutes. The percent sensitivity was

estimated by measuring the percent reduction of resistance in presence of the target gas.

Thus if air

R and gas

R represents the equilibrium sample resistance in ambient air and under

test gas respectively, the percent sensitivity ( %)S can be expressed as [28]

% 100 (7.4)air gas

air

R RS

R

−= × →

Figure 7.10 shows the percent sensitivity ( %)S as a function of operating

temperature in presence of 1.6 vol% LPG in air. This value corresponds to 80% LEL

(Lower Explosive Limit) of LPG in air. Butane ( )4 10C H has a LEL of 1.6 vol% and

propane ( )3 8C H has a LEL of 2.1 vol%. Thus the LEL of LPG is generally taken to be 2

vol%. The exposure time to the target gas was 15 minutes.

Figure 7.10: Sensitivity vs. operating temperature for (a) ZnO and (b) 1% AZO film

200 250 300 350 400

0

20

40

60

80

100

(b)

(a)

S (

%)

T (oC)

127

It is observed that compared to undoped ZnO film, Al -doping enhances the

sensitivity of the films at all temperatures. The sensitivity increased with increasing

temperature of the sensor element, reaches a peak value and then decreases again. The

peak sensitivity for AZO film was observed at 325oC and the value of maximum

sensitivity was 87%. Similar characteristic has been reported by Sahay et. al. [29] with

peak sensitivity of ~89% at 325oC in presence of 1 vol% LPG.

Figure 7.11 shows sensing characteristics of the undoped and Al -doped film in

presence of 1.6 vol% LPG in air at 325oC. The plot of resistance ratio gas

air

R

Ragainst

time is shown in the figure 7.11. Faster response (short response time) is given by Al -

doped ZnO compared to undoped ZnO film.

Figure 7.11: Sensing characteristics in presence of 1.6 vol% LPG in air at 325oC for

(a) undoped ZnO and (b) 1% AZO film

0 2 4 6 8 10 12 14 16 18 20

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Gas on

(b)

(a)

Rgas/R

air

Time (Minutes)

128

The gas sensing mechanisms normally accepted for semiconductor sensors

assume that the oxygen adsorbed on the surface of the oxide traps some of the conduction

electrons and thus decreases the material’s conductivity [30]. The surface adsorbed

oxygen species thus becomes negatively charged chemisorbed species ( 2O− , O

− or

)2O

− and acts as reaction centers for gas molecules. When reduction gas molecules come

into contact with this surface, they may interact with this chemisorbed oxygen species,

leading to an inverse charge transference [31]. Upon the return of the electrons to the

conduction band, conductivity increases. The reaction mechanism for LPG (containing

the hydrocarbons propane and butane) with surface adsorbed species leading to the final

products CO2 and H2O is quite complicated and proceeds through several intermediate

steps [29, 32].

Enhancement of sensitivity with operating temperature has been attributed to

increased speed of chemical reaction between the gas molecules and chemisorbed oxygen

species [29]. At high temperatures, gas molecules have enough thermal energy to react

with the chemisorbed species. However the appearance of peak sensitivity at 325oC has

not been explained. It has been established that the nature of the chemisorbed species is a

function of temperature [23]. While 2O

− is considered to be the prominent chemisorbed

species upto 500K, for temperatures higher than 500K, O− is the predominant species. As

is evident from experimental observation, reasonable sensitivity appears for temperatures

higher than 500K where the predominant species isO− . With increase in temperature, the

reaction 2 2O e O− −+ → , leading to the formation of O

− species proceeds at a faster rate

and thereby increasing the density of O− species. Since these are the active sites for

reaction with gas molecules, the sensitivity increases with temperature. This process must

accompany the process of enhancement of thermal energy of the gas molecules with

temperature in order to explain the reduction of sensitivity above a particular

temperature. With further increase in operating temperature a gradual change of the

adsorbed species from O− to 2

O− takes place according to the reaction 2

O e O− −+ → . The

129

latter species, although have high reactivity, is unstable and can go into the lattice as

lattice oxygen [23, 33]. Thus we can presume that at temperatures above 325oC, the

number of chemisorbed species available for surface activity is lowered. This lack in

number of chemisorbed species can slow down the catalytic oxidation reaction and leads

to a decrease in net yield of conduction electrons and hence sensitivity.

The enhancement of sensitivity due to Al incorporation might be due to increase

in electron concentration (carrier density) as has been observed in resistance

measurement as well as less porosity in the film as has been observed in SEM

measurements. Such increase in conduction electron concentration leads to an increased

density of surface active chemisorbed species which are the reaction centers for gas-

surface reaction. Also more porous film allows gas molecules to penetrate inside the film

and the resistance reduction process continues for a longer time. This delay the

attainment of equilibrium resistance value in presence of target gas and increase of

response time as has been observed for undoped film in the present work.

130

7.6 Discussion of Results on AZO thin films

Al -doped ZnO thin film could be successfully synthesized from ammonium

zincate complex with zinc acetate as the starting precursor by SILAR technique. Al

incorporation increases the growth rate of the film. Films had exceelllent adhesion on

glass slides. XRD spectra showed that the films have hexagonal structure with strong

preferred c-axis orientation. Texture coefficient of (002) plane increases due to Al

incorporation indicating improved crystallinity along c-axis. Particle size evaluated using

x-ray line broadening analysis and Williamson-Hall method shows a slightly increasing

trend with increasing Al incorporation alongwith a reduction in strain. The average

particle size for pure ZnO is ~22.75 nm. It increases to ~24.26 nm for 1% AZO film and

~25.13 nm for 2% AZO film. SEM micrograph shows round shaped particles for pure

ZnO. Al doping also seems to influence shape of the grains. AZO films show particles

with off spherical shape with compact interconnected grains. The morphology slaso

exhibits rougher surface compared to undoped ZnO. AZO film also appears to have

higher density (i. e. less porosity) compared to undoped ZnO and consist of compact

interconnected grains leading to a more continuous film. This fact along with

substituional replacement of divalent 2Zn

+ by trivalent 3Al

+ decreases the film resistance.

EDX analysis was carried out to check the incorporation of Al in the doped film.

The band gap of the film increases upto a certain level of doping due to increase

of carrier density. Beyond this limit, there is a narrowing of band gap possibly indicating

merging of an impurity band into the conduction band. The value of band gap for pure

ZnO is ~3.23 eV and it increases to ~3.29 eV for 1% AZO indicating a blue shift for 1%

AZO film. However for 2% AZO film, a decrease in band gap compared to pure ZnO is

observed indicating a red shift of fundamental absorption edge. Relaxation of strain due

to enhance growth and subsequent thickness of AZO films may also contribute to the

observed red shift.

131

The electrical resistance shows an initial decrease with increasing Al content.

With further enhancement of Al incorporation, the resistance increases. The electrical

resistance also decreases initially due to replacement of 2Zn

+ ions by 3Al

+ ions. Beyond a

certain level of doping, the electrical resistance increases due to drastic fall in mobility

arising out of segregation of dopants at grain boundaries. Al incorporation increases the

effective carrier density by approximately one order of magnitude in the temperature

range studied. However, it does not affect the value of activation barrier. The activation

barrier value of ~0.26 eVcan be attributed to oxygen vacancies acting as donor defect.

The sensitivity of the films in presence of LPG increases with temperature,

reaches a maximum and then decreases. The phenomena can be explained by the change

in nature of chemisorbed species with temperature as well as their number density.

Significantly high sensitivity of 87% with a reasonably fast response is observed for AZO

film in presence of 1.6 vol% LPG in air. The desired characteristics of a sensor material

need to be balanced with the processing costs for practical applications. Accordingly flms

synthesized by such a low temperature technique can be useful for sensor applications.

132

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133

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134

CHAPTER 8

Preparation of Ni doped ZnO thin films by

SILAR and their characterization

8.1 Preparation of Ni doped ZnO (NZO) films

Ni doped ZnO thin films were deposited on glass substrates from ammonium

zincate bath. Nickel doping in ammonium zincate bath was carried out by adding nickel

chloride (2 2, 2NiCl H O , Merck) in ammonium zincate bath. The pH of the ammonium

zincate bath was adjusted to ~10.80. Fifty (50) dipping was performed for undoped ZnO

films. Addition of nickel chloride was found to reduce the pH of the zincate bath and

reduces the growth rate. The nickel concentration could be varied upto 10% in the bath

solution and more than 10% dopant addition, the pH reduces to less than 10.70 (the lower

limit for getting stable ammonium zincate bath). This makes the bath unstable and

unsuitable for film deposition as the growth process is a stringent function of pH. The

thickness for pure ZnO film measured gravimetrically was ~ 0.8 µm and the growth rate

of the films reduces with increasing Ni doping. The number of dipping in case of doped

films was adjusted to give more or less identical thickness as that of pure ZnO. Almost

100 dipping was required to get ~ 0.8 µm thick 10% Ni doped ZnO film for which the

bath pH was ~10.72.

135

8.2 Structural Characterization by XRD: Evaluation of particle size

Figure 8.1 shows the X-ray diffraction patterns of undoped and Ni doped ZnO

films. The films were annealed at 350oC for 2 hr. in air prior to structural

characterization. The diffraction pattern for undoepd ZnO is shown in figure 8.1 (a).

Figures 8.1 (b), 8.1 (c) and 8.1 (d) shows the diffractograms for 3% Ni:ZnO, 5% Ni:ZnO

and 10% Ni:ZnO respectively.

Figure 8.1: X-ray diffraction pattern of (a) ZnO, (b) 3% Ni:ZnO, (c) 5% Ni:ZnO and

(d) 10% Ni:ZnO

20 30 40 50 60 70

0

1000

2000

(103)

(110)

(102)

(101)

(002)

(100)

(a)

2θ (degree)

0

1000

2000

(b)

Inte

nsity

(a. u.)

0

1000

2000

(c)

0

1000

2000(d)

136

The appearance of peaks at 31.714o, 34.389

o, 36.205

o, 47.434

o, 56.576

o and

62.855o and the corresponding reflecting planes are shown in the figue. The (002) peak

appears with maximum intensity in pure and Ni doped films indicating all the samples

have high preferred c-axis orientation. Apart from ZnO characteristic peaks, no peaks

that correspond to either nickel, zinc or their complex oxides could be detected within the

detection limit of XRD suggesting the films do not have any phase segregation or

secondary phase formation and also indicating possible incorporation of Ni in ZnO

lattice. The diffraction angles are marginally shifted towards the right side. The ionic

radius of 2Zn

+ is 0.074 nm and of 2Ni

+ is 0.069 nm [1]. Ni has been reported to be present

in a divalent state in ZnO lattice [1-2]. The plot of MARQ2 analysis for 3% Ni:ZnO

sample is shown in Figure 8.2.

Figure 8.2: Observed (dotted) and simulated (continuous) x-ray diffraction patterns of

3% Ni: ZnO

137

From the values ofo

β obtained using MARQ2 fitting and the corresponding

values of instrumental broadeningi

β , β (FWHM) was calculated using equation 4.13

(Section 4.3 of chapter 4). Figure 8.3 shows the Williamson-Hall plots of

cosβ θ against θsin4 . The particle size for pure ZnO was ~22.75 nm (Section 4.3.1,

Chapter 4; TEM value ~25.8 nm). With increasing doping concentration the particle size

shows a slightly decreasing trend. It decreases to ~21.5 nm for 5% Ni:ZnO and further

decreases to ~20.5 nm for 10% Ni:ZnO.

Lupan et. al. [3] however reported a marginal increase in particle size from~ 24

nm for undoped ZnO to 26.5 nm for Ni doped ZnO for films synthesized by SILAR.

However they applied Scherrer equation to evaluate particle size. Instrumental and strain

broadening was not taken into account.

Figure 8.3: Williamson-Hall plots of (a) pure ZnO, (b) 3% Ni:ZnO, (c) 5% Ni:ZnO and (d) 10%

Ni:ZnO

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

0.000

0.005

0.010

0.015

(a)

βco

sθ

4sinθ

0.000

0.005

0.010

0.015

(b)

0.000

0.005

0.010

0.015

(c)

0.000

0.005

0.010

0.015

(d)

138

8.3 SEM and EDX studies

Figure 8.4 shows the SEM image of undoped and Ni doped ZnO films.

Figure 8.4: SEM image of (a) ZnO and (b) 10% Ni:ZnO thin film

(a)

(b)

139

Figure 8.4 (a) shows the SEM image of undoped ZnO film (Figure 7.4 (a),

Chapter 7; reproduced here for convenience) while figure 8.4 (b) shows the SEM image

of 10% Ni:ZnO films. SEM investigation at normal incidence revealed polycrystalline

structure with smooth surface for the deposited films. The overall surface structure shows

grains of nearly spherical shape and more or less uniformly covering the surface. The

grain size and grain shape appears not to be effected due to Ni incorporation; however Ni

doping seems to produce smoother and denser surface. Juan et. al. reported [4] smooth

surface with roughness limited to 4 nm for Ni doped ZnO films.

The compositional analysis of Ni doped ZnO film carried out by EDX analysis is

shown in figure 8.5. Figure 8.5 (a) shows the EDX spectrum of 5% Ni:ZnO and 8.5 (b)

shows the spectrum of 10% Mn:ZnO.

Figure 8.5: EDX pattern of (a) 5% Ni:ZnO and (b) 10% Ni:ZnO

200 400 600 800 1000

0

500

1000

1500

2000

2500

3000

3500

NiLa

O Ka

Zn La

SiKa

NiKbNiKa ZnKb

ZnKa

(a)

keV

200 400 600 800 1000

0

500

1000

1500

2000

2500

3000

3500

NiLa

O Ka

Zn La

SiKa

NiKbNiKa ZnKb

ZnKa

(b)

keV

140

The EDX spectrum confirmed the presence of Zn, O and Ni elements in the

deposited films. The silicon signal appears from glass substrate. Dopant concentration in

these two cases was 5% and 10% in the starting solution. Accordingly the expected Ni/Zn

ratio was 0.05 and 0.1 in the films. The values of Ni/Zn ratio actually obtained in the

films was 0.0303 and 0.0593 respectively indicating that the amount of Ni incorporation

in the film is less than the amount of Ni in the starting solution. The real Ni content in the

deposited films was 3.03% and 5.93% as obtained from EDX spectrum.

8.4 Band gap evaluation from Optical absorption

Plots of ( )2

hα ν against photon energy are shown in figure 8.6. The data for

plotting these graphs was obtained from optical absorbance measurement of α versus λ .

Figure 8.6 (a) shows the spectrum of pure ZnO while figures 8.6 (b) and 8.6 (c) shows the

spectrum of 5% Ni:ZnO and 10% Ni:ZnO respectively.

Figure 8.6: Plots of ( )2

hα ν vs photon energy (in eV) of (a) ZnO, (b) 5% Ni:ZnO and

(c) 10% Ni:ZnO

2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3

0

20

40

60

80

100

120

140

160

180

200

(c)(b)

(a)

(αh

ν)2

hν

141

It is seen that with the increase of nickel doping level, the fundamental absorption

edge decreases. The value of g

E for undoped ZnO is ~3.23eV. It decreases to ~3.21 eV

for 5% Ni:ZnO and to ~3.19eV for 10% Ni:ZnO. A decrease in band gap from 3.28 eV

for pure ZnO to 3.26 eV for Ni doped ZnO has been reported by Xiaolu et. al. [5]. The

decrease in band gap might be due to introduction of defect states within the band gap by

the Ni dopant ions. The decrease in the slope of the linear portion of the curve with

increasing Ni content observed in the present work supports the fact.

8.5 Electrical characterization

Figure 8.7 shows the data on the variation of the electrical conductance with

reciprocal temperature (1000/T) in the temperature range 300-400K for ZnO and Ni:ZnO.

The electrical conductance ( )Σ was directly evaluated from the measured value of

electrical resistance ( )R .

Figure 8.7: Temperature dependence of electrical conductance for (a) ZnO & (b) Ni:ZnO

2.4 2.6 2.8 3.0 3.2 3.410

-8

10-7

10-6

(b)

(a)

Conducta

nce (

ohm

−1 )

1000/T(oK

-1)

142

Figure 8.7(a) shows the variation of conductance for pure ZnO while figure 8.7(b)

shows the same for 10% Ni:ZnO. The conduction process may be described by the

following equation:

( )exp (8.1)o

EkT

∑ = ∑ − →

In this equation, o

∑ is the pre-exponential factor for the temperature range 300-

400K, k is the Boltzmann’s constant, T is the absolute temperature and E is the thermal

activation barrier value. The experimentally obtained value of E is ~0.261 eV for pure

ZnO and ~0.293 eV for Ni:ZnO. An activation barrier value of 0.24-0.28 eV is normally

associated with oxygen vacancy acting as donor state (Section 7.4, Chapter 7). The films

have high resistivity ( )ρ of the order of 104 Ω-cm and low effective donor density of the

order of 1013

/cm3 at room temperature. The effective donor density is governed by donor

defect states (oxygen vacancies in this case) as well as density of chemisorbed species on

the surface which acts as trap state for conduction electrons. It is evident from figure 8.7

that the electrical conductivity decreases with Ni doping. Such decrease in electrical

conductivity due to Ni incorporation has been explained on the basis of compensation of

oxygen vacancies and such compensation may also increase the activation energy value.

[2, 6]. The activation energy barrier value may also increase due to enhanced grain

boundary scattering [2]. Reduced particle size resulting in enhance grain boundary

scattering effect may also have some contribution in increasing the activation energy

barrier value.

143

8.6 Discussion of results on Ni:ZnO thin films The primary aim of the present investigation was to explore the possibility of

doping ZnO with nickel by SILAR technique. Ni doped ZnO films with different

percentage (3%, 5% and 10%) of Ni content could be successfully synthesized through

this technique. The films had good adherence to the substrate. Ni doping reduces the

growth rate. Characterization techniques of XRD, SEM and EDX were utilized to

investigate the effect of Ni doping on the microstructure of Ni:ZnO thin films. XRD

spectra showed that the films are of hexagonal structure with preferred c-axis orientation.

Structural characterization by x-ray diffraction reveals the polycrystalline nature of the

films. Particle size evaluated using x-ray line broadening analysis and Williamson-Hall

method shows a marginally decreasing trend with increasing nickel incorporation. The

average particle size of ~22.75 nm for undoped ZnO reduces to ~20.51 nm for 10%

Ni:ZnO. Surface morphology using SEM shows polycrystalline and porous structure with

grains distributed more or less uniformly over the substrate surface. These observations

along with EDX observation confirm the incorporation of Ni in ZnO. The real Ni content

as obtained from EDX spectrum in the deposited film was 3.03% and 5.93% respectively

as opposed to 5% and 10% in the starting solution indicating that the amount of dopant

incorporation in the films is less than the amount in the starting solution. With increase

of Ni doping, the fundamental absorption edge reduces moderately. It decreases to ~3.19

eV for 10% Ni:ZnO from ~3.23 eV for pure ZnO. The electrical conductance decreases

and the activation barrier value for electrical conduction increases for Ni doping. For pure

ZnO film, the value of activation barrier is ~0.261 eV and for 10% Ni:ZnO it is ~0.293

eV. The values are reproducible within an error limit of 0.005± eV. Compensation of

oxygen vacancies as well as grain boundary scattering effects accounts for this.

144

References

1. S. Ghosh, P. Srivastava, B. Pandey, M. Saurav, P. Bharadwaj, D. K. Avasthi,

D. Kabiraj and S. M. Shivaprasad, Appl. Phys. A: Materials Science & Processing 90

(2008) 765.

2. A. Yildiz, B. Kayhan, B. Turuguzel, A. P. Rambu, F. Iacomi and S. Simon, J. Mater.

Sci: Mater Electron 22 (2011) 1473.

3. O. Lupan, S. Shishiyanu, L. Chow and T. Shishiyanu, Thin Solid Films 516 (2008)

3338.

4. Y. E. X. Juan, S. H. An, Z. Wei, Q. X. Si, X. M. Hua, J. C. Qing, Y. Z. Xin, A. C.

Tong and D. Y. Wei, Scinence China: Technological Sciences 53 (2010) 293.

5. Y. Xiaolu, H. Dan, L. Hangshi, L. Linxiao, C. Xiaoyu and W. Yude Physica B:

Condensed Matter 406 (2011) 3956.

6. B. Pandey, S. Ghosh, P. Srivastava, D. Kabiraj, T. Shripati and N. P. Lalla, Phys. E 41

(2009) 1164.

145

CHAPTER 9

Summary, Conclusions and Scope of

future work

9.1 Summary and Conclusions

The research work embodied in this dissertation was undertaken primarily to

explore the possibility of using a relatively new and less utilized yet economic technique

to prepare ZnO and doped ZnO thin films and their characterization. Compared to other

chemical techniques, successive ionic layer adsorption and reaction (SILAR) technique

involving multiple dipping of a substrate in cationic and anionic precursors has remained

a relatively less investigated method for preparation and characterization ZnO and doped

ZnO thin films. Although a few researchers have earlier employed this technique for the

preparation of ZnO films, the potential of this technique is yet to be explored in full

particularly for doped ZnO films and their characteriztion.

By proper optimization of deposition parameters such as concentration and pH of

cationic and anionic precursors and temperature of deposition, it was possible to get

reproducible, good quality and strongly adherent ZnO films. Ammonium zincate, sodium

zincate and zinc chloride has been used as cationic precursors. For the zincate baths, hot

water was the anionic precursor. The concentration values of all the zincate baths have

been optimized to 0.1 M and the pH values have been optimized in the range

146

10.80 0.05± for ammonium zincate bath and in the range 13.20 0.05± for sodium zincate

bath to get adherent films. In both cases the anionic precursor was hot water maintained

near boiling point. For zinc chloride solution the concentratio and pH values have been

optimized at 0.1M and 4.70 0.05± respectively. The optimized concentration and pH

values of the corresponding anionic precursor ( NaOH solution) were 0.075 M and

11.10 0.05± respectively. The growth process follows an empirical linear behavior with

number of dipping cycle for both the zincate baths. The growth rates in terms of µm per

dipping per mole were ~0.2 and ~0.162 for sodium and ammonium zincate baths

respectively.

X-ray diffraction studies reveal the films are polycrystalline with a preferred c-

axis orientation. The temperature of heat treatment of the deposted films was optimized at

350oC as the texture coefficient (TC) value for (002) preferred orientation almost

saturates after 300oC. Polycrystalline thin films with lowest particle size of ~22.75 nm

estimated using x-ray line broadening analysis (~25.8 nm from TEM observation) and

with highest TC value for (002) plane (~2.29) was obtained from ammonium zincate

bath. Both instrumental broadening and strain broadening was taken into account while

particle size evaluation. All subsequent experiments were carried out for films heat

treated at 350oC. Films produced from sodium zincate bath exhibits highest particle size

(~41 nm from TEM measurements). Films deposited from ammonium zincate bath were

round shaped compared to off spherical shape obtained from sodium zincate bath. Films

from zinc chloride complex were also nearly spherical in shape. Morphology of films

prepared from ammonium zincate bath exhibits superiority over films obtained from

other zinc complexes. The use of zinc acetate as the staring reagent to prepare ammonium

zincate complex was attempted for the first time which reduces possibility of impurity

incorporation compared to other zinc complexes used so far by other researchers. The

porosity in the films deposited from zincate baths is quite high and it ranges between 30

to 32% as observed from cross sectional SEM. FTIR spectrum reveals the presence of

ZnO stretching vibration. The high resistivity of films alongwith high porosity may be

useful for applications in resistive mode gas sensors.

147

Taking into account the optimized parameters of film deposition and with proper

selection of cationic precursor, Cd doped ZnO thin films were successfully deposited

from sodium zincate bath by SILAR for the first time to the best of our knowledge. The

films had good adherence to the substrate. Particle size evaluated using x-ray line

broadening analysis shows a constantly decreasing trend with increasing cadmium

incorporation. The average particle size of undoped ZnO from sodium zincate bath is

~36.73 nm evaluated by x-ray line broadening method neglecting strain broadening. The

corresponding value evaluated from TEM is ~41 nm. The average particle size reduces to

~32 nm for 5% Cd:ZnO and ~29.9 nm for 10% Cd:ZnO evaluated by x-ray method. The

undoped ZnO film is polycrystalline with a preferred c-axis orientation. The preferred

orientation is lost and the degree of polycrystallinity increases with increasing Cd

incorporation. These observations along with EDX observation confirm the incorporation

of Cd in ZnO lattice. SEM shows polycrystalline and porous nature of the films with

surface morphology getting less rough due to Cd incorporation. With increase of Cd

doping, the fundamental absorption edge changes decreases. The value of fundamental

absorption edge is ~3.18 eV for pure ZnO and it decreases to ~3.11 eV for 10% Cd:ZnO.

The small shift in diffraction peak positions, moderate reduction of optical band gap as

well as EDX investigations indicates that Cd incorporation in the films is much less than

that in the starting solution possible due to low deposition temperature characteristics of

SILAR process. Our primary aim was however to explore the possibility of Cd

incoporporation in ZnO by SILAR. Cd doped ZnO films could be successfully

synthesized through this technique and from our experiments we have demonstrated that

the physical properties of ZnO can be well modified by cadmium doping.

Mn doped ZnO films with different percentage of Mn content (upto 5%) could be

successfully synthesized by suitable choice of cationic and anionic precursors under

optimized deposition conditions. Zinc chloride bath with manganese chloride as source of

Mn was used as cationic precursor and sodium hydroxide was used as anionic precursor.

Better adherence on quartz substrate was observed compared to glass. More than 5%

dopant addition is difficult since the cationic bath loses stability possibly due to lowering

148

of pH. Mn incorporation strongly affects the structural, morphological and optical

properties of ZnO. Enhancement of polycrystallinity, decrease of preferred c-axis

orientation, enhancement of microstrain and lowering of particle size was observed for

Mn doping. The average particle size of ~29.71 nm for undoped ZnO evaluated by x-ray

line broadening method (~ 31 nm from HRSEM measurement) reduces to ~23.76 nm for

5% Mn doping. The undoped films contained nearly spherical grains while Mn

incorporation gives wrinkle structure. These observations along with EDX observation

confirms the replacement of zinc ion by manganese ions in the ZnO lattice. The real Mn

content in the deposited film was less than that in the starting solution as obtained from

EDX measurements. Mn doping reduces the value of fundamental absorption edge from

~3.22 eV for pure ZnO to ~3.06 eV for 5% Mn:ZnO for films deposited on quartz

substrate.

Al -doped ZnO thin film could be successfully synthesized from ammonium

zincate complex with hexahydrate aluminium chloride as the source of dopant. Al

incorporation increases the growth rate of the film. The texture coefficient for (002) plane

increases upto a cerain doping percent indicating improved crystallinity along c-axis.

Average particle size increases marginally due to Al incorporation. AZO films show off

spherical and compact interconnected grains leading to a more continuous film. This fact

along with substituional replacement of divalent 2Zn

+ by trivalent 3Al

+ decreases the film

resistance upto a certain dopin level (~1 atomic %). The band gap of the film increases

upto a certain level of doping (~ 1 at.%) due to increase of carrier density. Beyond this

limit, there is a narrowing of band gap possibly indicating merging of an impurity band

into the conduction band. The value of band gap for pure ZnO is ~3.23 eV and it

increases to ~3.29 eV for 1% AZO indicating a blue shift for 1% AZO film. However for

2% AZO film, a decrease in band gap compared to undoped ZnO is observed indicating a

red shift of fundamental absorption edge. This may be due to enhancement of strain

(observed in the present experiment) as a consequence of increased growth rate of AZO

films and/or narrowing of band gap indicating merging of an impurity band into the

conduction band.

149

The electrical resistance shows a decrease with increasing Al content upto a

certain doping (~ 1 at.% doping level) due to replacement of 2Zn

+ ion by 3Al

+ ion. Al

incorporation decreases the resistance by approximately one order of magnitude in the

temperature range 300-400K. However, it does not affect the value of activation barrier

of ~0.26 eV which arises due to oxygen vacancies acting donor state for conduction

electrons. With further enhancement of Al incorporation, the resistance increases

possibly due to drastic fall in mobility

.

As AZO is an important gas sensing material, LPG sensing characteristics of the

films were studied as an immediate conceivable application of the AZO films prepared

by SILAR. Significantly high sensitivity of ~87% with a reasonably fast response is

observed for AZO film in presence of 1.6 vol% LPG in air at 325oC operating

temperature.

Ni doped ZnO films with different percentage (3%, 5% and 10%) of Ni content

could be successfully synthesized by SILAR. XRD studies revealed polycrystalline

structure with preferred c-axis orientation for Ni:ZnO films. Particle size shows a

marginally decreasing trend with increasing nickel incorporation. Surface morphology

using SEM shows polycrystalline and porous structure with round shaped grains

distributed more or less uniformly over the substrate surface. Ni incorporation does not

modify the shape of the particles. However a smother and denser surface is observed due

to Ni incorporation. These observations along with EDX observation confirm the

incorporation of Ni in ZnO. The real Ni content as obtained from EDX spectrum in the

deposited film was 3.03% and 5.93% respectively as opposed to 5% and 10% in the

starting solution indicating that the amount of dopant incorporation in the films is less

than the amount in the starting solution. With increase of Ni doping, the fundamental

absorption edge changes. It decreases to ~3.19 eV for 10% Ni:ZnO from ~3.23 eV for

pure ZnO. The electrical conductance decreases and the activation barrier value for

electrical conduction increases for Ni doping. For pure ZnO film, the value of activation

barrier is ~0.261 eV and for 10% Ni:ZnO it is ~0.293 eV.

150

9.2 Scope of future work

i) The technique of SILAR has been optimized for ZnO and doped ZnO films of

Cd , Mn , Al and Ni . The technique can be extended to prepare and characterize other

metal doped ZnO films. Apart from being a cost-effective and simple technique, the

method uses milder reaction conditions than those employed by most chemical methods

proposed in the literature. Doping of different metals in ZnO may be particularly suitable

by this method.

ii) Microstructural characterization for Cd doped ZnO films prepared successfully

by SILAR has been made. Cd:ZnO (in nanowire form) is an excellet humidity sensor

material as well as a promising PTC material. The technique may be suitable modify to

prepare materials in other physical forms suitable for application purpose. Electrical and

gas sensing characterization of Cd:ZnO films may be studied in future.

iii) Synthesis and characterization of Mn and Ni doped ZnO thin films has been made

in this work. Thes materials are important for their ferromagnetic properties. Magnetic

properties of these materials may be studied in future. With Cd and Mn doping, the

fundamental absorption edge changes. The materials are therefore also useful for

configurations that involved bandgap engineering.

iv) Preparation of Al doped ZnO and their structural, optical and electrical properties

has been studied. The AZO films also show significantly high sensitivity to LPG. AZO is

a promising gas sensor material apart from its application in many other areas such as

transparent conducting oxide (TCO). Since films synthesized by SILAR are highly

resistive and porous, they can be useful for resistive mode gas sensor applications. From

a practical point of view, the desired characteristics of a sensor material (or any other

material for industrial application) need to be balanced with the processing costs. The

SILAR technique offers the desirable cheapness and can be easlity scalable to industrial

level. Thus further investigation may be made on the gas sensing properties of AZO

films.

151

List of Publications:

1. “Preparation of Al-doped ZnO (AZO) Thin Film by SILAR” - S. Mondal, K. P.

Kanta and P. Mitra, Journal of Physical Sciences 12 (2008) 221.

2. “Hydrogen and LPG sensing properties of SnO2 films obtained by direct

oxidation of SILAR deposited SnS” - P. Mitra and S. Mondal, Bulletin of the

Polish Academy of Sciences: Technical Sciences 56 (2008) 295.

3. “Preparation of ZnS and SnS Nanopowders by Modified SILAR Technique”- S.

Patra, S. Mondal, and P. Mitra, Journal of Physical Sciences 13 (2009) 229.

4. “Effect of Manganese Incorporation in ZnO Thin Films Prepared by SILAR”- S.

Mondal and P. Mitra, Science and Society 10 (2012) 139.

5. “Preparation of Cadmium - doped ZnO thin films by SILAR and their

characterization” – S. Mondal and P. Mitra, Bull. Mater. Sci. 35 (2012) 751.

6. “Preparation of Ni doped ZnO thin films by SILAR and their Characterization”-S

Mondal and P Mitra, Indian Journal of Physics, “Published online” doi

10.1007/s12648-012-0198-8.

7. “Preparation of ZnO film on p-Si and I-V Characteristics of p-Si/n-ZnO” - S.

Mondal, K. P. Kanta and P. Mitra, Materials Research, “Published online”

doi.org/10.1590/S1516-14392012005000149.

8. “Preparation of manganese doped ZnO thin films by SILAR and their

characterization”- S. Mondal, S. R. Bhattacharyya and P. Mitra, Bull. Mater.

Sci., “Article in press” [Manuscript ID D-12-00036].

9. “Effect of Al doping on microstructure and optical band gap of ZnO thin film

synthesized by SILAR” - S. Mondal, S. R. Bhattacharyya and P. Mitra, Pramana

– Journal of Physics, “Article in press”.

10. “Structural and morphological characterization of ZnO film synthesized from

different zinc complexes as cationic precursor” - P. Mitra and S. Mondal,

Progress in Theoretical and Applied Physics, “Article in press”.