Optical performance of Tin doped Indium Oxide (ITO) thin films prepared by sol gel dip coating techniques using Acrylamide route

: At present various oxide of metal semiconductors play significant role in the field of electronics device. Most of the semiconductor devices exploit the special characteristics of the junction between a p-type and n-type semiconductor, these devices can be made extremely small in size and they are incredibly fast in their response. Generally metal have good reflectivity in the electromagnetic region of infrared and visible radiation. Indium oxide material doped with tin (ITO) are recently used in the substrate material for various applications, because of it has special properties are low resistivity and high optical transmittance in the visible region. In this paper, we prepare ITO films with different tin concentration (5%, 10%, 30%, 50% and 70%) using acrylamide sol gel dip coated method and its results were reported. TCO materials have good electrical conductivity and optical transparency, and also it has n-type semiconductor with a band gap between 3.5 and 4.3 eV. An X-ray study indicates all the prepared samples were bixbyte structure. Optical behaviour of materials can be understood in the near infrared and visible spectrum. Some optical parameters refractive index, extinction coefficient and dielectric constant of ITO films are calculated from the data received from the UV transmission studies. Using W-D model the dispersion of refractive index was calculated. The optical band gap, oscillator energy, dispersion energy and optical conductivity and N/m* ratio were estimated and this material is well suitable for dye sensitized solar cell and sensor application.


Introduction:
Indium oxide doped with tin (ITO) is likely to be very good materials in the field of material science research group, in the room temperature, the material has wide band gap of 3.5 -4.3 eV. Now-a-days ITO films used in the field of optoelectronics such as varistors, gas sensor, biosensors, transparent electrode, solar energy efficient windows, P-N junction emitting light, panel display etc. The film thickness and substrate temperature are major role to maintain the optimum condition in visible region (transparency) and near IR regions (reflectance), and also percentage of dopant, annealing temperature are optimized. Today several techniques was used to prepare ITO films including the sol gel process [1] chemical evaporation [2] pulsed laser deposition [3] and electron beam evaporation [4]. All the methods are very expensive and require for high vacuum so that we choose sol gel dip coating method. These techniques are required minimum tool cost and inexpensive method for depositing films. We know that most of the optoelectronic materials are the function of wavelength and also to predict photo electric behaviour of a device. Therefore an accurate knowledge of the structural and optical properties of ITO is important for the designing optoelectronics device. In this paper we studied some of optical behaviour such as refractive index, extinction coefficient optical band gap and complex dielectric constant and transmittance, absorbance of ITO film deposited on glass substrate.

Experimental procedure:
Using glass substrate the binary compound (Indium oxide) doped with tin (ITO) were deposited on one of the chemical methods such as sol gel techniques. The coating sols were prepared using 10 ml of 0.45 M solution of Indium (InCl3 4 H2O) and tin salts (SnCl4 5H2O) were taken in a clean glass beaker. This mixture was heated to 70 °C and we choose the pH in the range of 9 then by the addition of 0.25gm of N,N,bis methylence acrylamide, 20 minutes after we add 2 mg ammonium persulphate initiator was mixed to the solutions. After few minutes the solutions appear viscous nature. We take glass substrate in the size of breadth is 2.5 cm and 7.5 cm length and removal rate of the substrate is 1 cm/min. After the film formation the film surface was dried in microwave oven for 15 min followed by the heat treatment 350 °to 450 ° for different duration time is 15 min to 90 min. The time duration for 50 min is well optimised. Using surface profilometer to measure the films thickness its vary from 450 nm to 950nm to increase of temperature.

Result and discussion:
The XRD techniques is used to determining the atomic and molecular structure of ITO films using CuKα radiation corresponding to X-ray wavelength is 0.154nm. The transmittance spectrum gives the information of surface Plasmon's resonance indicating the crystallite size in the nanometre range. Transmittance spectra give the information of transmittance of ITO film deposited on glass substrate and also we found that transmittance value increases from 50% to 80%. The transmittance of the films exhibited ripples pattern due to interference of light which may due to the characteristic of interference between light and nanostructured materials [11]. The transmission was found to be maximum for low SnO2 percentages of SnO2 in ITO films. The refractive index (n) was evaluated from the measured transmittance versus wavelength graph [13]. The refractive index was found to decrease with wavelength. It was observed that the refractive indices of the Sn-doped In2O3 thin films were smaller than for the pure In2O3 film in the measured wavelength range [14]. Using W.D model, some of the optical properties such as dispersion and spectral dependence of the refractive index of many semiconductors are calculated from the following equations.
Where n is the refractive index, Eo is the average excitation energy known as the oscillator energy. Ed is the dispersion energy called the oscillator strength and hγ is the incident photon energy. The detailed procedure of dispersion parameter such as average excitation energy and dispersion energy were estimated [15]. When a graph is plotted between (n 2 -1) -1 versus (hγ) 2 for ITO thin films, which yields a straight line for normal behaviour having the slope (E0Ed) -1 and the intercept with the vertical axis equal to Eo/Ed. The values of the parameter E0 and Ed can be estimated from the positive curvature deviation from linearity at longer wavelength is usually observed due to the negative contribution of lattice vibrations on the refractive index.
The value of optical constant reported in the table. Then carrier concentration has been calculated from the plasma frequency whose expression is given below In a semiconductor the carrier concentration denoted as (N) and also it varies according to the square of plasma frequency wp as [16][17] Wp 2 = (4 2) ( * ∝) --------(12) Using equation 12 we calculate carrier concentration and plasma frequency [18 -20] If the carrier concentration is known the effective mass of the charge carrier could be found out from the plasma frequency. The band gap energies increase due to the decrease in crystallite size.

Conclusion:
Using sol gel dip coating method the various concentration of tin doped with indium oxide powder was coated in the clean glass substrate. Structural property and its micro behaviour of prepared films were analysed by XRD techniques which indicate bixbyte structure corresponding (222) peak. From the XRD measurement data we found some microstructure property. The Grain size increases with decrease of tin concentration (5%).
Energy dispersive spectra confirm the presence of tin, indium and oxide are presented in our samples. Surface roughness observed in the atomic force microscopy. Tin concentration increases to 10% the resistivity of the films decreased from 20 ohm cm (Indium oxide at 450 ֯ C) to 0.01 ohm cm .Beyond 10% tin the resistivity increases from 0.1 ohm cm to 250 ohm am at 70% tin concentration. The value of mobility and carrier concentration increases up to 10% tin concentration beyond this value of Tin oxide the carrier density decreases. Finally we have to calculate the single oscillator and dispersion energy were calculated from the data of optical transmission measurements in the ITO thin films. This material well suitable for solar cell application, sensors behaviour and also photo activity degradation of dye and other organic pollutants.