EFFECT OF (Al, Zn, Cu, AND Sr) DOPING ON STRUCTURAL, OPTICAL AND ELECTRICAL PROPERTIES OF SPRAYED SnO 2 THIN FILMS

Tin dioxide thin films deposited onto a glass substrate were prepared by spray pyrolysis technique, and then doped with different elements which are: Al, Zn, Cu, and Sr by electroplating method, these elements were chosen for their different atomic radii. XRD illustrate that all the films were polycrystalline with a tetragonal rutile structure and a strong preferred orientation of (200) plane. Uv-vis spectrophotometer specters showed that the highest average transmittance of Al/SnO 2 film was about 86.77% in the visible region and the Sr/SnO 2 film had the highest band gap of 3.95 eV. From the MEB images, the morphological characteristics improved when the SnO 2 thin films doped with Al and Zn but the opposite happened when it doped with Cu and Sr. The four-point probe showed that the best sample was for Al/SnO 2 because it had the highest electrical conductivity around 692.306 (Ω.cm) -1 .


INTRODUCTION
Thin films of transparent conductive oxide such as ZnO [1], TiO2 [2], In2O3 [3], and SnO2 [4], are the most scientific research subjects in current applications, and the most common is SnO2 due to its large band gap (3.65 V at 300 K) [5] making it the most widely used in many applications such as a transparent electrode in photovoltaic transformers, amorphous silicon solar cells, liquid crystal display and gas-discharge display [6]. It can be deposited by a number of mechanisms such as spraying pyrolysis [7], L-CVD [8] spin coating [9], theoretically the SnO2 thin film has a low electrical conductivity because its charge carriers have low mobility as well as its low-density charge carrier [5], which leads us to dope it with many elements to improve the most important physical properties, namely the electrical and optical properties, we can mention from these elements: Zr [10], Cu [11],Sb [12]…..etc. The objective of this study is to investigate, for the first time, the influence of these four elements which have various ionic radii (r "Al 3+ "=0.39A°, r "Zn 2+ "=0.40A°,r "Cu 2+ "=0.93A°, and r "Sr 2+ "=1.16A° [13], where r "Sn 4+ "=0.71A° [14]) on the optical and electrical properties of SnO2 thin films, and shed more light on the structure and properties of Al, Zn, Cu and Sr-doped SnO2 thin films. The approach is to: (i) use a simple and low-cost method (spray pyrolysis technique) to deposit SnO2 films onto a glass substrate heated to 450 C° then doped using electroplating method, (ii) characterize their microstructural features, and (iii) measure their physical properties and correlate them with the microstructure.

MATERIAL AND METHODS
SnO2 thin films were prepared using spray pyrolysis method, tin chloride (SnCl2, 2H2O) dissolved in bi-distilled water and methanol (1/1) with the addition of a few drops of hydrochloride (HCl) to obtain more homogenous starting solution. Then it doped choosing electroplating method at 65C° for 15 minute and in electric current between 1 and 5 mA with 0.15M of different sources for Cu, Al, Sr, and Zn, which are CuSo4 .5H2O, Al2So4.18H2O, SrCl2.6H2O, and ZnCl2.7H2O respectively.

Structural properties:
The structural properties of doped and undoped samples were determined using the XRD. Fig. 1 shows that all the samples have the same preferential orientation along c-axis as pure SnO2 it was (200) plane which determines low energy (which means the stability) because it had the highest intensity for all the films, also it had the same structure which was the tetragonal rutile [15], and there are some peaks with low intensity such as : (110)  From XRD data crystalline size was calculated according to the Scherrer formula given by D as follows [16]: Where λ is the X-ray wavelength (λ Kα (Cu)=1.5418 A°), β is the full width at half maximum (FWHM) and Ɵ is the diffraction angle, after that Williamson and Smallman's relation [17] was used to find the dislocation density: From Table 1 we can observe that Al/SnO2 has the highest value of the crystalline size that's mean that the crystallization into the growth axe of SnO2 thin films was improved [18], while the Sr/SnO2 has the lowest value because the crystallization into caxis was deterioration.  Firstly, Fig. 2.b shows Cu/SnO2 surface morphology, we observed a spheric shape particles of negative potentials which transform copper oxide into copper was created may be because the oxide copper is placed with Cu that is mean the Cu 2+ changed to Cu, moreover, the grain size was decreased, this consists too many studies mentioned [17,18] so adding of Cu to SnO2 did not improve its morphology at all. The opposite was happened with the addition of Al and Zn to SnO2 as Fig. 2.c and Fig. 2.d respectively demonstrate; this is due to the increase of grain size, which leads to the reduction of the defects in the obtained thin films. Finally, the Fig. 2.e (Sr/SnO2) shows that there are black dots while for pure SnO2 there are a few as shown in Fig. 2.a which mean that the defects have increased with the addition of Sr, which leads to deterioration of SnO2 morphology.

Optical properties:
The optical properties of doped and undoped SnO2 thin films were characterized using a UV-vis spectrophotometer in the range of [300nm-1100nm]. From Fig. 3 The electrical properties of our films were investigated using four-point probe, the results are shown in Table 2: The following relations was applied [16] to determine the sheet resistance (Rs), the electrical resistivity (ρ), and the electrical conductivity (σ).
As is seeing, the electrical conductivity of Sr/SnO2 and Cu/SnO2 was decreased compared to pure SnO2, this result can be interpreted by the decrease in the number of charge carriers [24], so the so existence of a difference in the structure of Cu and SnO2 or Sr and SnO2 abstract the flow of the electrons [25], also the grain size decreased as we found in Table.1 hence to that there are more grain boundaries which are limiting the mobility of electrons (acts as traps for free carriers and as barriers against transport) which may be responsible for the increase in the electrical resistivity, Sudip Kumarsinla study revealed the same result [26]. The opposite happened for Zn/SnO2 and Al/SnO2, the decrease in the resistivity which can be attributed to the increase in the number of free charge carrier from the donor Zn 2+ or Al 3+ ions that incorporated into the substitutional or interstitial cation location of Sn 4+ [15]. From Fig.5, Al/SnO2 has the highest value of the electrical conductivity (6,923*10 2 (Ω.cm) -1 ).

CONCLUSIONS
Smooth, dense, continuous, and homogenous undoped and (Al, Zn, Cu and Sr) doped SnO2 thin films were successfully deposited onto a glass substrate heated to 500 C° using a simple and low-cost spray pyrolysis technique. The films were polycrystalline and had a tetragonal rutile structure with preferred growth orientation along <200> direction. The band gap of pure SnO2 (3.84 eV) decreased to 3.82 and 3.77 eV as a result of doping with Zn and Al, respectively; and increased to 3.94 eV and 3.95 eV because of doping with Cu and Sr, respectively. Doping SnO2 with Al and Zn increased its crystalline size and improved its optical transmittance, but the opposite was happened when it doped with Cu and Sr. Aluminium and zinc increased the electrical conductivity of SnO2 from 5.41*10 2 (Ω.cm) -1 to 6.92*10 2 (Ω.cm) -1 and 6.28*10 2 (Ω.cm) -1 , respectively, but Strontium and Copper decreased it to 3.04*10 2 and 4.22*10 2 (Ω.cm) -1 respectively.