Preparation and Characterization of Tin Oxide Thin Films by using Spray Pyrolysis Technique

. In this work tin oxide thin films have been prepared by Spray pyrolysis technique, using tin chloride over soda – lime glass. Structural and optical properties were studied under different condition like substrate temperatures of 300, 350 and 400°C, and annealing in air at temperature 300°C for 1 h . The films were characterized by XRD,SEM, AFM, and UV-VIS analysis . The optical spectra of the films were measured in the wavelength range of 300 –900 nm by UV-VIS Spectrometer device. The X-ray diffraction studies confirmed the films have the orthogonal structure at low substrate temperature change to a tetragonal structure at high substrate temperature. Films prepared with different substrate temperatures were correspond to preferential orientation along the plane (132) and (110) planes for structural orthogonal, tetragonal respectively . The grain size founded( 25.2-9.77nm) for different substrate temperatures range ( 300-400°C) after annealing , from the (SEM) images the grain size values of the SnO 2 thin films are found to be in the range of (32.3-24.2 nm) corresponding to the substrate temperature (350 and 400°C) after annealing and from the AFM the root mean square (RMS) values for the thin film at substrate temperature 350C was found to be 10.4 nm, the surface roughness 8.86 nm. The maximum value of transmittance was found to be 78 % around the wavelength of 900nm at substrate temperatures 300°C also have energy gap (2.9-3.4eV) at substrate temperatures range ( 300- 400°C) after annealing in air at temperature 300°C for 1 h.


INTRODUCTIONT
in oxide (SnO2) is one of the semiconducting oxides that has unique optical, electrical, transparent, conductive properties [1]. Tin oxide (SnO2) is wide band gap an n-type semiconductor of tetragonal structure [2] and can resist high temperature [3] . It has been widely used for various catalytic applications such as gas sensors [4] ,photovoltic cells [5],transistors [6] ,lithium batteries [7] , and solar cells [8] ...etc . SnO2 films can be prepared by different techniques such as sputtering [9] ,evaporating tin grains in air [10] ,chemical vapor deposition [11] , thermal evaporation of oxide powders [12] , rapid oxidation of elemental tin [13],the sol-gel method [14],ionbeam assisted deposition [15] and spray pyrolysis [16 . [ The Spray pyrolysis is one of the most cost-effective methods to prepare SnO2 films due to its ability to deposit large uniform area with low fabrication cost, simplicity and low deposition temperature.
In this work we study the effect of substrate temperature on the Structural and optical. properties of SnO 2 thin films .

Experimental
Figure(1) shows a typical spraying system. In this technique the films produced by using • The SnO 2 were prepared by means of dissolving of (0.1M) tin Chloride prepared by using the dissolving in distilled water. • The solution was made to 100ml with distilled water .

•
The nozzle was kept at a distance of 30cm from the substrate during deposition .

•
The glass substrate were cleaned with ethanol in ultrasonically cleaner and then dried. • The solution was sprayed by means of a nozzle, assisted by a carrier gas, over a hot substrate . When aerosol droplets came close to the substrates, the compounds reacted to become a new chemical compound. SnO 2 formulation can be represented as: Table( 1) process parameters for the spray deposition of SnO 2 thin films.

Figure (1) Schematic set-up for spray pyrolysis technique.
The X-ray diffraction (XRD) data of the prepared films were taken using : ( Source CuKα with radiation of wavelength λ =1.5406Ǻ , target: Cu, Current =30mA , Voltage = 40 kV, scanning speed = 5 deg /min) over the diffraction angle range 2θ=20-60° at room temperature .The average crystallite size (D) was estimated using the Scherrer equation as follows [ Distilled water λ, β, and θ are the x-ray wavelength, the full width at half maximum (FWHM) of the diffraction peak, and Bragg's diffraction angle, respectively. Morphology studies of the SnO 2 films on the glass substrate were carried out with scanning electron microscope (SEM) type Inspect S50, and magnification over 100000. The surface distribution of SnO 2 thin films were measured using atomic force microscopy (AFM) using a scanning probe microscopy (CSPM-5000) instrument . UV-VIS, Phoenix-2000V device was used to record the optical transmission for SnO 2 thin films after annealing in air at temperatures 300°C for 1 h. Using furnace type S302AU England .

Results and Discussion Structural Properties X -Ray Diffraction ( XRD ) for SnO 2 thin films
X-ray diffraction( XRD ) of SnO 2 thin film prepared by spray pyrolysis technique at substrate temperatures of ( 300,350 and 400°C) after annealing in air at temperature 300°C for 1h, have been shown in Figure (2). The peaks of the XRD were observed in the range of 2ɵ ( 20° -60°) .
The presence of diffraction peaks indicates that the film is have the orthogonal structure in the low substrate temperature and change to a tetragonal structure in the high substrate temperature . It is revealed that the sprayed film has peaks corresponding to (132) and (110) which is corresponding to the positions θ 2 = 28.24° and 26.57° for orthogonal and tetragonal structure respectively , for the films which depends on the substrate temperature after annealing , the intensity increases as the substrate temperature increases after annealing, due to the improvement of the films crystallinity, while decreases at substrate temperature 400°C .
The crystallite size of SnO 2 films prepared at different substrate temperature was calculated using the full width at half maximum (FWHM) for SnO 2 films using Scherrer formula . The highest crystallite size of (25.2nm) and this value decreases with the increases substrate temperature and after annealing in air at temperatures 300°C for 1 h as shown in Table (2). The values of full width at half maximum (FWHM) are increase with increasing substrate temperature of the films , this indicates that the grain size decreases with increasing substrate temperature . The main calculations of the structural properties have been listed in the Table( 2) .

Scanning Electron Microscope (SEM) for SnO 2 thin films
The surface morphology of the SnO 2 thin films deposited on a glass substrate at substrate temperatures (350 and 400°C) by spray pyrolysis technique was examined by Scanning Electron Microscope (SEM) are shown in Figures [(3-a)-(3-b)] for different magnification after annealing in air at temperatures 300°C for 1 h . As can be seen from the Figures the SnO 2 thin films at different substrate temperature are quite similar except for a small decreasing in particle size, besides that, there are some crystal shapes structures , agglomeration of grains. Clearly, we found the films deposited at higher substrate temperature have the grain size less of the films deposited at low substrate temperature. From the (SEM) images the grain size values of the SnO 2 thin films are found to be in the range of ( 32.3-24.2) nm corresponding to the substrate temperature (350 and 400°C) after annealing respectively. These values are comparable with that estimated from XRD results as shown in Table (3).

Table (3) comparison between the grain size estimated from (XRD) and (SEM)
analysis.

Figure ( 3 ) SEM of SnO 2 films for substrate temperatures( A-350°C ) , (B-400°C ) after annealing Atomic force microscopes (AFM) for SnO 2 thin film
The surface morphology of the SnO 2 thin film was studied also by AFM . Figure (4) shows the typical two-dimensional and three -dimensional AFM image of SnO 2 thin film deposited on a glass substrate at substrate temperature 350°C after annealing in air at temperature 300°C for 1 h using spray pyrolysis technique. AFM surface 3D (threedimensional) it was observed that the distribution of grains is uniformly on the substrate surface. The root mean square (RMS) values of surface roughness are found to be 10.4 nm ,the surface roughness was observed to 8.86 nm.

Optical characterization
The knowledge of optical constants of the thin films are very significant since they can determine the exact application of the films. The optical properties of SnO 2 thin films prepared by spray pyrolysis technique on soda -lime glass substrate with different substrate temperature of 300, 350 and 400°C after annealing have been investigated . The absorption and transmittance spectra were measured in the wavelength range of 300-900nm by UV-VIS Spectrometer device .The film thickness was determined by using the following formula:- Where ( x ) is the fringe width,( ∆x) is the distance between two fringes and ( λ ) is the wavelength of laser light.

3D 2D
Figure ( 5 ) shows the thickness of SnO 2 thin films with different substrate temperatures and after annealing in air at temperature 300°C for 1 h . It is seen that the film thickness increased with increasing substrate temperature from 300°C to 350°C and then decreased with increasing substrate temperature ( 400°C ) . Initially, at lower substrate temperatures 300°C,the temperature may not be sufficient to decompose the sprayed droplets from the solution and this results in a low thickness. At substrate temperature of 350°C, decomposition occurs at the optimum rate resulting in the terminal thickness being attained. A noticeable decrease of the film thickness with increasing the substrate temperature is observed after substrate temperature 350°C. This decrease may be attributed to re-evaporation of film material after deposition or to thermal convection of the sprayed droplet during the deposition process or both [18] .

Figure (5) SnO2 film thickness with substrate temperature
Transmittance for SnO 2 thin film Figure (6) shows the optical transmittance spectra obtained for the SnO 2 thin films in the wavelength range 300-900nm. The substrate temperature and the annealing plays an important role in the films formation. The transmission improves as the substrate temperature increases .The transmittance value of the films found increased to 75 % after annealing in air at temperatures 300°C for 1 h , maximum transmittance in case of the film having a thickness of 120 nm is slightly more than the film with thickness 145 nm . The Absorption Coefficient (α) for SnO 2 films , calculated according to the following equation: ... (4) Where ( d ) is the thickness of thin film and (T) is the transmission. Figure (7) shows the optical absorption spectra for the SnO 2 thin films with different substrate temperature ( 300 -400°C) after annealing in 300ºC for 1h .The absorption coefficient decreases with increasing substrate temperature and a weak change in the absorption edge is observed towards lower wavelengths as substrate temperature increases, this attributed to the effect of the thickness which was thicker for higher substrate temperature .

Eng. &Tech.Journal, Vol. 33,Part (B), No.3,2015 Preparation and Characterization of Tin Oxide Thin Films by using Spray Pyrolysis Technique
The optical Energy Gap (Eg) of the thin films was calculated from the allowed direct transition given by [19] : Where α (cm-1) is the absorption coefficient, h (J.s) is Planck's constant, ʋ(Hz) is the photon frequency, A is constant , and Eg (eV) is the band gap energy. Figure (8) shows the energy gap (Eg) for the SnO 2 thin film with different substrate temperature of 300, 350 and 400°C (after annealing in 300ºC for 1 h) . The results show that the substrate temperature and the annealing influence is very important in the band gap assignment , it found the energy gap increase with increase the substrate temperature after annealing . The optical energy gap of the films varied significantly with the increase of substrate temperature , which was attributed to the presence of various phases in the films . the films formed at substrate temperature 300, 350ºC showed orthogonal structure with an energy band gap of ( 2.9 , 3.4eV) and the films formed at substrate temperature 400ºC showed tetragonal structure with an energy gap of ( 3.1eV) , these results imply that the deposited films can be used in photovoltaic devices.

Conclusions
SnO 2 thin film deposited by spray pyrolysis technique at different substrate temperatures . The XRD studies showed that have orthogonal structure in low substrate temperature and change to a tetragonal structure in the high substrate temperature after annealing the films . The grain size are decreases with increasing substrate temperature after annealing. The film thickness and the values of energy gap , are increasing with increasing substrate temperature to 350ºC and decreases in the 400ºC substrate temperature , while the transmittance decreases with increasing substrate temperature . (SEM) images are found the grain size values to be in the range of (32.3-24.2 nm) corresponding to the substrate temperature (350 and 400°C) after annealing and AFM the root mean square (RMS) values for the thin film at substrate temperature 350C was found to be 10.4 nm, the surface roughness 8.86 nm.