ISSN 2319-8133 (Online (An International Research Journal), www.physics-journal.org
Structural Analysis of TiO
2Thin Films Deposited
by Sol-Gel Spin Coating System
R.K. Pandey, Swati Mishra, Avik Karmakar, M.P. Sharma and P.K. Bajpai
Department of Pure and Applied Physics, Guru Ghasidas Vishwavidalaya, Bilaspur, C. G., INDIA
.
(Received on: October 10, 2015)
ABSTRACT
Thin films of TiO2 were successfully deposited on glass substrate using
sol-gel spin coating method and characterized for its structural and optical properties using XRD and UV-VIS spectroscopy. The deposited TiO2 thin films were transparent
in visible range and opaque in UV region. The XRD spectroscopy analysis reveals that the prepared TiO2 thin films were crystallizes in anatase phase. The lattice
constants obtained were a=b = 3.78639Å and c = 9.5181Å axes, respectively. The crystalline size was found to be 36.186 nm. The refractive index and density of deposited TiO2 thin films increases whereas, the porosity decreases with increasing
film thickness.
Keywords: Optical Material, Thin films, Optoelectronic Devices, Semiconductor Oxide.
1. INTRODUCTION
Recently TiO2 based external optical waveguides have been becoming very attractive
as a promising candidate to realize next-generation ultra-dense and ultra- fast optical integrated circuits because of its capability to light confinement up-to nanoscale level and long distance propagation. In addition, these waveguides also offer facility to high-speed signal generation, processing as well as detection1-2. TiO
2 is a n-type semiconductor with a wide indirect energy
band gap. Its thin films have attracted much attention due to its optical, physical, chemical, and electronic properties. Mainly it is used due to its excellent transmittance of visible light, photocatalytic activity, high dielectric constant, high refractive index (at λ = 550 nm, n = 2.54 for anatase or 2.75 for rutile), and high chemical stability3–8. Titanium oxide thin films are
widely used as optical and also protective coatings because of its excellent chemical stability, mechanical hardness and optical transmittance. Sol-gel derived TiO2 films are widely used as
cells. Other potential applications are supposed in optical and electrochromic devices and electrical batteries. Many attempts have been made to modify the physical, chemical and optical properties by mixing TiO2 with oxides of other elements9,10.
Commonly TiO2 crystallizes in three different structures such as tetrahedral anatase
structure, the rutile structure and the orthorhombic brookite structure. The anatase and rutile structures belong to different space groups but both have a tetragonal crystal lattice. Rutile is the most stable form of TiO2, whereas, anatase and brookite are metastable and transform to
the rutile phase upon heating11–15. The phase transformation from anatase to rutile structure
mainly depends on the growth process (film deposition process) of the films. It is significantly affected by defect density, grain boundary concentration, grain size & orientation and particle packing density. The significant characteristics of TiO2 films used for optical and electronic
applications depend on their crystal structure, orientation, and morphology. Therefore, control of the phase structure of the TiO2 thin films during the film growth is important.
Various techniques have been used for the deposition of TiO2 films, such as sol–gel
deposition, spray pyrolysis, pulsed laser deposition, e-beam evaporation, chemical vapor deposition and reactive magnetron sputtering16-18,4,6,8,9. The sol–gel process has many
advantages, such as large-area coating, low-temperature processing, controllable film morphology (porous or dense film), composition control and low cost. Therefore, it is one of the most important techniques for the preparation of functional TiO2 thin films. It is remarkable
that a TiO2 film, obtained by annealing at 400 – 700 °C, showed a predominantly anatase phase
structure, which could change with the number of coatings. The aim of this study is to investigate the structural and optical properties of titanium dioxide (TiO2) thin films. The films
were synthesized by the sol–gel spin coating method and post-annealed at 500 0C in air for 01
hour. We evaluate the effect of the film thickness on their structural, physical and optical properties of TiO2 thin films for its cutting edge electronic and optical applications.
2. EXPERIMENTAL
TiO2 thin films were deposited on glass substrate using sol-gel spin coating method.
The sol solution was prepared by dissolving 1.0 gm TiO2 powder in to 20 ml methanol at room
temperature and 4 ml acetic acid was added as catalyst. In order to complete the reaction process between the materials in the solution, the prepared solution was vigorously stirred for 2h at room temperature. After stirring, solution was left at room temperature for 24h. The glass substrate was used as substrates. Prior to the deposition, the glass substrates were cleaned using acetone, methanol, deionized water and finally, it was sonicated in an ultrasonicator. After cleaning substrate the precursor solution was spin coated on the glass substrate at 3000 rpm for 35 sec using the spin coater system (Apex Instrument). After spinning the deposited thin films were preheated for 5 min at 80 0C and repeated this process again and again for making
the film of 5, 10, 15 and 20 layers. All the chemicals used were of AR grade (Sigma Aldrich) and were used as received without further purification. After successfully deposition, all the deposited TiO2 thin films were placed in a high temperature furnace and annealed at 500 0C
The structural properties of deposited TiO2 thin films were studied using X-ray
diffractometer (Rigaku-Smart lab 9KW) with Cu-Kα operating at wavelength 1.54 Å, operating
voltage 40KV, current 75mA. The dual beam spectrometer (Shimadzu-1700, 20 W halogen lamp and Deuterium lamp) were used to record the transmittance spectra for TiO2 deposited
thin films which is having resolution 1.0nm. The percentage transmittance was recorded in the wavelength range 300 nm to 800 nm.
3. RESULTS AND DISCUSSION
Fig. 1 shows the XRD pattern of TiO2 thin films annealed at 500 0C for 1 hour. The
TiO2 thin film was coated for 10 layers. All the XRD peaks shown at (101), (004), (200), (204),
and (215) are correspond to the anatase crystal planes of TiO2 thin film, respectively. All the
above peaks correspond to the tetragonal anatase phase of TiO2 and a preferred grain
orientation in the (101) direction. A few low intensity peaks corresponding to (103), (105), (116), and (110) planes were also recorded in the XRD spectra which is consistent with JCPDS file no.78-248619.
Table 1 shows the calculated structural parameters of prepared TiO2 thin films. It is
obvious from the values shown in Table 1 that the as prepared thin films have stabilized in a tetragonal anatase TiO2 phase. We have done full profile Reitveld refinement using the
software package PDXL and calculated the values of Rwp = 23.63%, S=1.1453, Rp= 16.34%, Rwp=23.63% and goodness of fitting (χ2) =1.3118. The lattice constants obtained were a=b =
3.78639Å and c = 9.5181Å axes, respectively20,21. Fig: 2 illustrate the Rietveld refinement
fitting of TiO2 Thin Films. It is obvious from the figure that the refinement is well matched
with the original XRD spectra of the deposited TiO2 thin films.
The structural parameters of deposited TiO2 thin films corresponding to prominent
(101) planes were calculated. The strong preferential growth is observed along (101) plane which indicates that the prepared TiO2 thin films are oriented along c-axes and having anatase
crystal structure22,23. The average crystallite size of the films was determined using the
well-known Debye-Scherrer’s formula as given in equation (1) along (101) plane23.
0.9λ D/ β cos θ (nm) (1)
Where λ is the wavelength of X ray (1.5406 Å), θ is the Bragg diffraction angle, β (in radian) is the full width at half maxima and 0.9 is a typical value of a dimensionless shape factor which varies from 0.87 to 1.0 depending upon the shape of crystallite. The average crystalline size of the deposited TiO2 thin films was found to be 36.186nm.
The refractive index as well as the thickness of a deposited thin film deposited onto a substrateis fundamental properties of a material, because it is closely related to the electronic polarizability of ions and the local field inside the material. The estimation of refractive indices of optical materials is of substantial importance for applications in integrated optic devices such as optical waveguides, photonic crystals, optical switches, filters, optical amplifiers and modulators, etc., where the refractive index of a material is the key parameter for device design24,25. The refractive index of deposited TiO
N = [N+ (N2 – n
s2)½]1/2 (2)
Where
N = (ns2+1) / 2 +2ns (Tmax- Tmin) / TmaxTmin (3)
Where ns is the refractive index of the substrate (for glass s =1.52) and Tmax and Tmin represent
the maximum and minimum transmittances at the same wavelength in the fitted envelope curves on the transmittance spectrum. The variation in refractive index with corresponding increase in thickness (number of layers) of depositing thin films is illustrated in Fig.3. It is obvious from the figure that the refractive index increases with increasing film thickness. The increase in refractive index with corresponding increase in thickness may be due to the densification of deposited thin films, reducing the porosity and decreasing the defect density with increasing film thickness. We have observed that annealing shows a slight decrease in transmittance with the increasing annealing temperature. The thin films annealed above 500°C shows a slightly decrease in visible light transmittance. It can be attributed to the densification and the crystalline transformation that increase the refractive index of the deposited TiO2 thin
films.
The porosity of the TiO2 thin films was calculated using the following equation27.
Porosity = (1 – n2-1/n
d2-1)* 100% (4)
Where nd is refractive index of pore-free anatase (nd = 2.52), and n is refractive index of porous
thin films. The variation in porosity with respect to thickness of deposited TiO2 thin films is
shown in Fig.3. It is obvious from the figure that the porosity decreases with increasing film thickness. This may be due to the densification of the TiO2 thin films due pore filling in films
and cross linking reaction of the Ti-O-Ti bonds and electrostatic interaction between O-Ti-O bonds with increasing thickness.
The density and the refractive index of a TiO2 thin film can be expressed as follows: 28
= nf – 0.91933 / 0.42751 (5)
Where nf is the refractive index and ρ is the density of a TiO2 thin film (g/cm3). It is also
obvious from Fig.3 that the density of the deposited TiO2 thin films increases with increasing
film thickness. It is noted that the refractive index and density of thin films of TiO2 increases
with increasing number of layers and/or the thickness of deposited films. This phenomenon may be related to pores destruction and further densification of the films as well as elimination of organic compound. In addition to it is also reflects the degree of crystallization of thin films. However, porosity (the volume of pores per volume of film) decreases with increasing number of layers of the films. In this case we can say that density and porosity is varies inversely and hence, denser film is less porous. The decrease in porosity and increase in refractive index indicates that the crystalline quality of the TiO2 thin films gradually improves with increasing
Fig:1 XRD Spectrum of TiO2Thin Films
Fig:2: Rietveld Refinement fitting of TiO2Thin Films
Fig: 3: Variation in refractive index, density and porosity of TiO2 thin films with corresponding increasing
number of layers
20 30 40 50 60 70 80
0 200 400 600 800 1000
Int
en
sit
y (a
.u.
)
Angle (2)
101
103 004
112 200
105 211
213 204
116 220215
301
Int
en
si
ty
(cp
s)
0.0e+000 2.0e+003 4.0e+003 6.0e+003 8.0e+003
2-theta (deg)
Int
en
si
ty
(cp
s)
20 30 40 50 60 70 80
Table 1:Rietveld refinement fitting (lattice parameter & structure) with141: I41/amd, choice-2 space group of TiO2
Table 2: Variation of Refractive index, Density and Porosity
Film of TiO2 with Refractive index (n) Density (g/cm3) Porosity (%)
5 layers 1.768 1.985 61
10 layers 1.89 2.270 51
15 layers 2.05 2.645 40
20 layers 2.34 2.90 34
4. CONCLUSION
The structural and optical properties of the deposited TiO2 thin films were studied as
a function of the film thickness. The X-ray diffraction patterns of the TiO2 films reveal the
existence of a TiO2 single-phase with the anatase crystal structure. The XRD patterns consist
of a (101) main peak with c-axis orientation. The crystallite size increases with increasing film thickness which attributed to the improve degree of crystallization and consistency in nucleation center densities in films with increasing film thicknesses. The refractive index and density of deposited films increase with corresponding increase in film thickness may be due to the densification, porosity reduction and decreasing the defect density with increasing film thickness. The morphological properties of the films shows a porous nature of the deposited films which decreases with increasing film thickness.
ACKNOWLEDGEMENT
The authors are thankful to UGC, New Delhi, for providing the financially support through Major Research Project No: 41-930/2012/(SR), DST-FIST and UGC –SAP program to provide the characterization equipment’s to carry out the present investigation.
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