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Spectrochimica Acta Part A 74 (2009) 839–842

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

journa l homepage: www.e lsev ier .com/ locate /saa

hort communication

ptical, dielectric and morphological studies of sol–gel derived nanocrystallineiO2 films

. Vishwasa,∗,1, Sudhir Kumar Sharmab, K. Narasimha Raob, S. Mohanb, K.V. Arjuna Gowdac,.P.S. Chakradhard,∗

Department of Physics, M.V.J. College of Engineering, Bangalore 560067, IndiaDepartment of Instrumentation, Indian Institute of Science, Bangalore 560012, IndiaDepartment of Physics, Govt. First Grade College, K.R. Pura, Bangalore 560036, IndiaGlass Technology Laboratory, Central Glass and Ceramic Research Institute (CSIR), Kolkata 700032, India

r t i c l e i n f o

rticle history:eceived 26 March 2009eceived in revised form 3 July 2009ccepted 29 July 2009

ACS:2.79 Wc8.66 Li

a b s t r a c t

Nanocrystalline TiO2 films have been synthesized on glass and silicon substrates by sol–gel technique.The films have been characterized with optical reflectance/transmittance in the wavelength range300–1000 nm and the optical constants (n, k) were estimated by using envelope technique as wellas spectroscopic ellipsometry. Morphological studies have been carried out using atomic force micro-scope (AFM). Metal-Oxide-Silicon (MOS) capacitor was fabricated using conducting coating on TiO2 filmdeposited on silicon. The C–V measurements show that the film annealed at 300 ◦C has a dielectric con-stant of 19.80. The high percentage of transmittance, low surface roughness and high dielectric constant

1.05 Cp

eywords:hin filmshemical synthesisicroscopy

suggests that it can be used as an efficient anti-reflection coating on silicon and other optical coatingapplications and also as a MOS capacitor.

© 2009 Elsevier B.V. All rights reserved.

ptical propertiesielectric response

. Introduction

TiO2 is a metal oxide semiconducting material and has a varietyf applications such as photocatalysts [1], electro-chromic displays2], dye-sensitized solar cells [3], gas sensor [4], wave guide [5]nd optical filters [6]. TiO2 is also used as a dielectric material forlectronic devices. TiO2 has higher dielectric constant than conven-ional dielectric materials such as silicon dioxide or silicon nitride.herefore, it is a promising material for gate insulator of metal oxideemiconductor transistor for the next generation. TiO2 thin filmsan be deposited on the substrate by various techniques out of

hich sol–gel technique has the advantage of relatively low cost

nd ease to control the deposition parameters, uniform large areaeposition and including chemical composition. Thus, it is offeringpromising alternative over to the vacuum deposition techniques

∗ Corresponding author. Tel.: +91 33 2473 0957.E-mail addresses: vishu 792005@yahoo.co.in (M. Vishwas),

reechakra72@yahoo.com (R.P.S. Chakradhar).1 Present address: Department of Physics, Govt. Science College, Tumkur 572103,

arnataka, India. Tel.: +09916325227.

386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2009.07.018

[7–9]. Sol–gel process can be performed mainly in two methods asspin coating and dip coating techniques. TiO2 exhibits three differ-ent phases namely anatase, rutile and brookite. Anatase is highlyunstable and converts into rutile when annealed at temperaturesabove 800 ◦C [10].

In the present study, TiO2 thin films have been deposited on sil-icon and ITO coated glass substrates by spin coating method. Theoptical properties like reflectance, transmittance, refractive indexand extinction coefficient has been studied. Morphological studyhas been carried out using AFM. Metal-Oxide-Silicon (MOS) capac-itor was fabricated using conducting coating on TiO2 film depositedon silicon and their C–V characteristics have also been studied anddiscussed.

2. Experimental

In the present study, titanium (IV) isopropoxide [Ti(OC3H7)4]

is used as an initial organic precursor for sol formation. The tita-nium isopropoxide was dissolved in ethyl alcohol (99.8%) in thevolume ratio 1:10. The optimized volume of concentrated hydro-gen chloride was added as catalyst to precede hydrolysis reaction.The solution was stirred vigorously for 2 h over magnetic stirrer.

8 ica Acta Part A 74 (2009) 839–842

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hen the solution was kept in airtight beaker about 4 h for gel for-ation. After the gel formation, few drops of gel were spin coated

n the silicon and glass substrates. The substrates were initiallyreheated at 60 ◦C for 6 h. After pre-heating, the films have beennnealed at different temperatures up to 300 ◦C.

The optical studies viz. transmittance and reflectance have beenarried out using spectrophotometer (HR 4000-Ocean optics, USA)n the spectral range of 300–1000 nm with a resolution of 0.27 nm.he film thickness has been estimated by envelope technique.he structural investigations were performed with Philips (modelW1710) diffractometer by using Cu K� radiation. The surface mor-hology of films was studied by atomic force microscopy (AFM)ith Shimatzu SPM-6000 in contact mode. The C–V, I–V measure-ents have been carried out by using Agilent 4155C and 4284 LCReter.

. Results and discussion

.1. Transmittance and reflectance studies

The thickness of the films deposited on different substrates wasstimated by envelope technique [11] and was found to be in theange of 80–250 nm. Films deposited with spinning speed of 2K rpmor 30 s results the film thickness 187.46 nm. Film thickness wasound to be decreased with increase in the spinning speed andpinning duration. The estimated thickness values are strongly sup-orted by ellipsometric studies.

Fig. 1 shows the transmittance spectra with wavelength range00–1000 nm for the films deposited at ambient conditionsnd annealed at 100 ◦C (A1) and 200 ◦C (A2) on glass sub-trate. The percentage transmittance maxima (Tmax %) was ∼90%or glass substrate in the visible range. The transmittance hasecreased with increasing temperature due to increase of den-ity of the film. Kityk et al. have reported that the absorptionoefficient drastically increases at lower temperatures (T = 40 K)or low sized nano-crystals (<24 nm) [12]. However, for largeized grains composed of many nano-crystals at higher tem-eratures, the absorption coefficient increases with annealingemperature. Therefore, the transmittance decreases with tem-erature. The reflectance spectra recorded for the films annealed

t different temperatures deposited on silicon is shown in Fig. 2.lmost 30% reflectance percentage has been observed in the vis-

ble region. Increase in the annealing temperature causes theespective decrease in the thickness and increase of reflectance per-entage. It has been previously observed that the film thickness

ig. 1. The change of optical transmittance with wavelength in TiO2 films preparedn glass substrate annealed at 100 ◦C (A1) and 200 ◦C (A2).

Fig. 2. Reflectance spectra of TiO2 films deposited on silicon substrate annealed at100 ◦C (A1) and 200 ◦C (A2).

decreases with increasing annealing temperature and spinningspeed [13].

3.2. Refractive index and extinction coefficient estimations

The refractive index has been estimated by ellipsometric tech-nique as well as envelope technique. The observed refractive indexvalues for TiO2 films annealed at 300 ◦C are found to be 2.00 and2.08 at 550 nm respectively and the refractive index values werefound to be decreased with increase in wavelength as shown inFig. 3. It is also observed that the refractive index increases withincreasing thickness and annealing temperature of the film. This isdue to increase in density and absorbance of the film. The extinctioncoefficient of TiO2 film annealed at 100 ◦C and 200 ◦C is shown inFig. 4. It is observed that the absorbance in the film is negligible at100 ◦C (A1) and it has slightly increased with increasing annealingtemperature 200 ◦C (A2).

3.3. Atomic force microscopy (AFM)

The surface morphology of the TiO2 films deposited on glasssubstrate has been investigated by atomic force microscopy and isshown in Fig. 5. The film surface was highly porous formed due to

Fig. 3. Refractive index of TiO2 films deposited on glass substrate annealed at 300 ◦Cestimated by envelope technique (X) and ellipsometric technique (Y).

M. Vishwas et al. / Spectrochimica Acta Part A 74 (2009) 839–842 841

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ig. 4. The plot of extinction coefficient of the TiO2 film with wavelength for glassubstrates annealed at 100 ◦C (A1) and 200 ◦C (A2).

he agglomerated nanocrystalline particles and their size stronglyepends on the annealing temperature. Films annealed at 300 ◦Cor 4 h showed the average grain size of 235 nm approximately.his may be due to the bigger clusters formed by the coalescencef two or more grains. An increase in the annealing temperatureesults the particle size increase with more uniformity and higherensity. Annealing above the crystallization temperature revealsor the larger and larger agglomeration of grains [14]. Hence well-rystallized anatase phase has been observed after annealing at00 ◦C for 4 h, which is also supported by the XRD results [13].

.4. Dielectric studies

The capacitance–voltage (C–V) measurements performed at anperating frequency of 100 kHz for TiO2 films annealed at 300 ◦Cs shown in Fig. 6. The experimental characteristics are similar toormal C–V dependences, as observed in the case of classical MOStructures with three typical ranges, i.e. accumulation, depletion

nd inversion could be easily recognized (Fig. 6).

For negatively biased structure, negative electron charge athe gate is balanced by positive hole charges accumulated nearhe surface of the semiconductor (p-type). In an ideal MOS sys-em capacitance C measured in this accumulation state is equal to

ig. 5. AFM image of TiO2 film deposited on glass substrate annealed at 300 ◦C.

Fig. 6. The plot of capacitance versus voltage (C–V) of TiO2 film annealed at 300 ◦Cwith operating frequency of 100 kHz.

oxide capacitance Cox = 74.15 pF. It is also observed that the leak-age current of TiO2 films can be improved by annealing at highertemperatures [15]. The dielectric constant (ε) and the oxide layerthickness (EOT) of annealed TiO2 thin film were derived from themaximum capacitance. The oxide layer thickness is calculated byusing the following expression:

EOT = εSiO2

εTiO2

dTiO2(1)

where εSiO2is the dielectric constant of silicon dioxide, εTiO2

isthe dielectric constant of titanium oxide and dTiO2

is the thicknessof titanium oxide. The oxide layer thickness (EOT) is found to be32.89 nm. Switching the bias voltage into positive direction makessemiconductor surface depleted from the holes, so that the addi-tional capacitance of space charge layer Cs, is serially connectedto COX. Moreover, the positive gate biasing makes that at low fre-quency conditions inversion of the conduction type occurs.

In strong inversion conditions, potential changes at the gateare determined by the presence of electrons at the semiconduc-tor metal interface. Thus, the total measured capacitance, similarto the accumulation state, approach to the COX (as indicated thesolid straight line in Fig. 6). A flat band (VFB) voltage of ∼1.956 Vwas obtained for the TiO2/Si at 100 kHz operating frequency [16].Marius and Stamate [17] have reported the frequency dependenceof electric capacitance and dielectric loss and the thickness depen-dence of dielectric constant of TiO2 thin films deposited by dcmagnetron sputtering. They observed that TiO2 films with lowerthickness have a larger value for dielectric constant and dielectricloss is also thickness dependent. In our experiment the value of thedielectric constant, εTiO2

calculated for above mentioned TiO2 filmwas found to be 19.80.

4. Conclusions

The nanocrystalline titanium dioxide (TiO2) thin films have beenprepared by sol–gel technique at ambient room temperature. Thedependence of reflectance, transmittance, optical constants andthickness of the film at different annealing temperatures has been

studied. The extinction coefficient values are found to be negligi-bly small. AFM results confirm the presence of crystalline phasewith grain size of 235 nm. The refractive index values of TiO2films heated at 300 ◦C as measured by envelope technique andellipsometric method are found to be 2.00 and 2.08 at 550 nm

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espectively. The capacitance voltage (C–V) measurements resultedut the value of dielectric constant 19.80. Thus, TiO2 thin filmsxhibiting high percentage transmittance and low surface rough-ess and high dielectric constant. Hence it can be used as an efficientnti-reflection coating on silicon and other optical coating applica-ions and also as MOS capacitor.

cknowledgements

Dr. RPSC thanks Dr. H.S Maiti, Director, CGCRI and Dr. Ranjanen, Head, GTL laboratory, CGCRI for their constant support andncouragement.

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