STUDIES OF ELECTRICAL AND OPTICAL PROPERTIES OF Sn DOPED AND UNDOPED ZnO THIN FILMS BY THE SPRAY PYROLYSIS METHOD

Undoped and tin (Sn) doped zinc oxide (ZnO) thin film have been deposited by spray pyrolysis method of zinc acetate and tin chloride. Toe concentration ratio of[Sn ]/ [Zn] was varied from 0-5 at %. Toe effect of doping Sn on physical and optical properties was studied by different techniques such as X-ray diffraction for structural characterization, SEM for thickness and morphology, UV-Vis spectroscopy for optical properties, Hall effect and photoconductivity for electrical characterization. X-ray diffraction (XRD) patterns show that the films deposited are polycrystalline with (002) plane as the preferential orientation. According to Scherrer's equation, grain size values on the films are found between 30 45 nm with spherical shape. Optical transmittance was about 92% in visible range for the optimum film and shows that the band gap decreases from 3 .29 to 3.17 e V. l. Introduction Zinc oxide is a transparent conducting oxide (TCO), a promising material for many different applications such as solar cells, gas sensors, ultrasonic oscillators and transducers [l], an inexpensive n-type semiconductor with a band gap of 3.3 eV which has a hexagonal wurtzite structure [2] and furthermore, it is a piezoelectric material. lt founds its applications in surface acoustic wave devices [3]. Dueto its high optical transparency in the visible region and its electrical conductivity, ZnO thin films have been used as window layers and anti reflecting coating in solar cells [4]. Also it can be used in gas sensors [5]. ZnO thin films have been deposited by many techniques such as sputtering [6], chemical vapor deposition [7], sol-gel [8], laser ablation [9], and spray pyrolysis [l O]. The last technique has been used, because of its simplicity, reproducibility, lack of toxicity and cost effectiveness. Moreover, incorporation of dopants is easier by this technique. Severa! doping elements for ZnO have been studied [11 ], and tin has shown to be a potential agent doping to enhance the physical properties ofZnO films. Prasada et al studied the effect of ethanol as solvent on thin films and showed to have an important effect on thickness and sheetresistivity [12]. In present work Sn doped and undoped ZnO thin films were prepared by the spray pyrolysis method on glass substrate, the structural, optical and electrical properties ofZnO thin films was studied. 2. Experiment ZnO thin films were deposited on glass substrates by spray pyrolysis method. The precursor materials used was zinc acetate dihydrated (Zn(C2H3Qi)2·2H20), prepared a stock solution and was IM zinc acetate dehydrated, prepared by dissolving deionized water. The concentration of zinc aceta te was O. 05 to O .1 M and the Sn/Zn ratio in the solution was 5 at % in the starting solution. Toe precursor solutions were prepared by 10, 16 and 20 mLof lM Zn(C2H3Q2) 2 and 0.062, 0.098 and 0.124 g SnCl2 and dissolved in ethanol at 200 mL. A few drops of acetic acid were added to aqueous solutions to prevent the formation ofhydroxides. The nozzle was kept ata distance of 4 7 cm from the substrate during deposition. Air was used as the carrier gas, at the pressure of 65 psi. The deposition temperature was kept at 400 oC. Thin films were then annealed in a vacuum at 300 oC during 1 h. When aerosol droplets come close to the substrates, a pyrolytic process occurs and ZnO films are produced. The possible reaction proposed by Paraguay et al. [13] is as follows: Zn(C~COO)i 1::-~,.,,·,~=tm,1 ~ Zn(CH,COO)z ll?> ••txt-1 +H20 '-~·pór Zn,O(CH,COO)ó """' ,;.,, •.h:rat. +CH-COOH ;as -·s.1>sm I and Zn,iO(CH-COOH) 6 ~1z4'..b>t·, +3HzO4ZnOi,T =-.r.•J +16CH;.COOH i_..1 The thickness of the films was measured by Perfilometer. Toe film thicknesses were found as -150 mn. The structural analysis of all the thin films was performed with a X-ray diffractometer with Cu Ka().,= 1.54059 Ao) radiation. The diffractometer reflections were taken at room temperature and the value of 20 were swapped between 20° and 60°. The UV-Vis spectra of the films were recorded from 300 mn to 900 nm wavelength using Perkin Elmer Lamda 35 UV-Vis spectrophotometer at room temperature. Surface morphology and microstructure of each film were observed using a field-emission scanning electron microscope (FE-SEM). Electrical measurement was carried out by four pro be method using Keithley Electrometer. 3. Results and discussion X-ray diffraction pattems ofundoped ZnO and Sn-doped ZnO films annealed in a vacuum at 300 oC during 1 h are presented in figure l. These pattems show the peak of crystallized ZnO, (O O 2) preferred orientation, confirming the polycrystalline films with a hexagonal wurtzite structure (Zincite, JCPDS 361451 ). 10 20 30 lAldoped -0,06M -0.08M -0.1M 5%Sn -Q.OSM -0.08M -0.1M .. ,. 2q(•) 10 20 30 40 so 60 70 2q(') Figure l. X-ray diffraction pattern of Znü films undoped and Sn doped deposited on glass substrate has an enhanced (O O 2) orientation. These diffractographs show that the intensities of diffraction peaks declined as ZnO concentrations decreased, Sn doping within ZnO films also caused the crystallinity to detract. The crystalline size of the deposited thin films is deterrnined by using the Debye-Scherrer formula,


l. Introduction
Zinc oxide is a transparent conducting oxide (TCO), a promising material for many different applications such as solar cells, gas sensors, ultrasonic oscillators and transducers [l], an inexpensive n-type semiconductor with a band gap of 3.3 eV which has a hexagonal wurtzite structure [2] and furthermore, it is a piezoelectric material. lt founds its applications in surface acoustic wave devices [3]. Dueto its high optical transparency in the visible region and its electrical conductivity, ZnO thin films have been used as window layers and anti reflecting coating in solar cells [4]. Also it can be used in gas sensors [5]. ZnO thin films have been deposited by many techniques such as sputtering [6], chemical vapor deposition [7], sol-gel [8], laser ablation [9], and spray pyrolysis [l O]. The last technique has been used, because of its simplicity, reproducibility, lack of toxicity and cost effectiveness. Moreover, incorporation of dopants is easier by this technique. Severa! doping elements for ZnO have been studied [11 ], and tin has shown to be a potential agent doping to enhance the physical properties ofZnO films. Prasada et al studied the effect of ethanol as solvent on thin films and showed to have an important effect on thickness and sheetresistivity [12].
In present work Sn doped and undoped ZnO thin films were prepared by the spray pyrolysis method on glass substrate, the structural, optical and electrical properties ofZnO thin films was studied.

Experiment
ZnO thin films were deposited on glass substrates by spray pyrolysis method. The precursor materials used was zinc acetate dihydrated (Zn(C2H3Qi)2·2H20), prepared a stock solution and was IM zinc acetate dehydrated, prepared by dissolving deionized water. The concentration of zinc aceta te was O. 05 to O .1 M and the Sn/Zn ratio in the solution was 5 at % in the starting solution. Toe precursor solutions were prepared by 10, 16 and 20 mLof lM Zn(C2H3Q2) 2 and 0.062, 0.098 and 0.124 g SnCl2 and dissolved in ethanol at 200 mL. A few drops of acetic acid were added to aqueous solutions to prevent the formation ofhydroxides. The nozzle was kept ata distance of 4 7 cm from the substrate during deposition. Air was used as the carrier gas, at the pressure of 65 psi. The deposition temperature was kept at 400 ºC. Thin films were then annealed in a vacuum at 300 ºC during 1 h.
When aerosol droplets come close to the substrates, a pyrolytic process occurs and ZnO films are produced. The possible reaction proposed by Paraguay et al. [13] is as follows: Zn(C~COO)i 1::-~,.,,·,~=tm,1 ~ Zn(CH,COO)z ll?> ••txt--1 +H20 '-~·pór Zn,O(CH,COO)ó """' ,;.,, •. h:rat. +CH-COOH ;as -·s.1>sm I and Zn,iO(CH-COOH) 6~1z4'..b>t·, +3HzO-4ZnOi,T =-.r.•J +16CH;.COOH i_..1 The thickness of the films was measured by Perfilometer. Toe film thicknesses were found as -150 mn. The structural analysis of all the thin films was performed with a X-ray diffractometer with Cu Ka().,= 1.54059 Aº) radiation. The diffractometer reflections were taken at room temperature and the value of 20 were swapped between 20° and 60°. The UV-Vis spectra of the films were recorded from 300 mn to 900 nm wavelength using Perkin Elmer Lamda 35 UV-Vis spectrophotometer at room temperature. Surface morphology and microstructure of each film were observed using a field-emission scanning electron microscope (FE-SEM). Electrical measurement was carried out by four pro be method using Keithley Electrometer. These diffractographs show that the intensities of diffraction peaks declined as ZnO concentrations decreased, Sn doping within ZnO films also caused the crystallinity to detract.

Results and discussion
The crystalline size of the deposited thin films is deterrnined by using the Debye-Scherrer formula, where A is the X-ray wavelength equal to 1.54 A, 0 is Bragg diffraction angle and B (radians) is the full-width at half-maximum. The average crystallite grain size is estimated to be 30 mn for undoped films and 45 mn for Sn doped films. Optical transmission spectra of Sn doped and undoped ZnO thin films deposited on glass substrates are recorded as a function of wavelength in the range of 300-900 nm and shown in figure 2. In order to obtain the band gap, the absorption coefficient (a) is calculated from the transmission and reflection data using the following relation a=.!. In (1-R) t T where t is the film thickness, R is the reflectance and T is the transmittance. For the direct transition, the optical band gap energy ofZnO film is determined using the equation: ahv= A (hv-Eg)1 12 (2) where hv is the photon energy and Eg is the optical band gap andA is a constant.  Figure 3 shows the plot of(ahv)2 vs. hv for ZnO thin films. lt has been observed that the plot of(ahv)2 vs. hv is linear over a wide range of photon energies, indicating a direct band to band transition. The intercepts ( extrapolations) ofthese plots on the energy axis reflect the energy band gaps.
6.0x10 7 6.0x10 7   The surface morphology of the undoped ( a,c,e) and the Sn doped (b,d,f) ZnO thin films observed with SEM are shown in figure 4. Comparatively, tin doped films show the dense and less porous structure than undoped films.
Resistivity at room temperature measured on Sn doped and undoped ZnO films prepared at different doping ratio is given in figure 5, curve behavior showed a reduction in the resistivity value in 0.05M undoped to doped 5% at by five orders of magnitude (10 2 Qcm to 10-2 ncm). The decrease in the resistivity ofthe films by Sn doping an be explained by the substitution of Sn 4 + ions at the Zn 2 + sites leading two free carriers. As the doping levels creased, more dopant atoms occupy the zinc lattices sites, which results in more charge carriers.
Electrical Hall-effect measurements were performed in order to investigate the electrical properties of the ZnO thin films, electron concentration and mobility are 6.609 x 10 19 cm-3 and 2.025 respectively.  ..,,.

Conclusions
Sn doped and undoped ZnO thin films were prepared on glass substrates by spray pyrolysis. The doping concentration of 5 at % is proved to be optimum for tin doped zinc oxide thin films. According to X-rays diffraction, the ZnO films grow preferentially in the direction (002). The average grains size is about 37.5 nm. The electrical resistivity decreases with level doping after heat treatment in a vacuum. The electron concentration is equal to 6.609x 10 19 cm-3 • Heat treatment in a vacuum improves significantly the resistivity, as well as optical and structural properties ofSn doped ZnO films. Small electron mobility can be attributed to the electrons trapped at grain boundaries of crystallites form potential barriers which reduces the mobility.