Annealing Temperature, Time and Thickness Dependencies in (TCO) SnO 2 Thin Films Grown by Spray Pyrolysis Technique
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Annealing Temperature, Time and Thickness Dependencies in (TCO) SnO2 Thin Films Grown by Spray Pyrolysis Technique Alfredo Campos, Amanda Watson, Ildemán Abrego and E. Ching-Prado* Natural Science Department, Faculty of Science and Technology, Technological University of Panama, Panama. ABSTRACT Tin oxide thin films were prepared by spray pyrolysis method using SnCl2.2H2O as starting precursor and deposited on glass substrate. Three groups of samples with different preparation conditions (temperature, time and thickness) were synthetized. The samples were characterized using Scanning Electron Microscope (SEM), X-Ray Diffraction (XRD), UVVisible Spectroscopy and Van der Pauw four-point electrical measurements. The grain size in the samples changes from 80 to 500 nm. Optical and electrical parameters were measured or calculated, such as: band gap, refractive index, sheet resistance, transmittance spectrum and figure of merit. Film thicknesses were obtained from fringes features in the transmittance spectra with a variation from 76 to 761 nm. A mechanism of transformation from tin dichloride to tin oxide is proposed and discussed; additionally the visual yellow color of some samples, related with a low transparency, is associated to the amount of abhurite or tin hydroxide complex coexisting with tin oxide. The figure of merit showed that 500 ºC, 42 sprays and 1 hour of annealing time were the best conditions in the preparation of SnO2 with TCO properties. INTRODUCTION Among the wide range of materials with technological applications, deserve special attention materials that are good electrical conductors and transparent. These materials are known as TCOs (transparent and conductive oxides). The interesting properties of these materials allow their use in liquid crystal displays (LCD), light emitting diodes, solar cells, electrical contacts, touch screens and others applications [1]. Different semiconductors have been used as TCOs, among these: SnO2, CdO, ZnO, Cd2SnO4, CdIn2O4 and In2O3 [2]. The most widely used of all these compounds in technological applications is the indium oxide doped with tin (In2O3: Sn), known as ITO. The easy deposition, high conductivity and high transparency of the ITO make it stand out above all other semiconductors. Being the Indium expensive and scarce, recent researches have focused on search new alternatives. One of the most promising candidates is the SnO2 which has good chemical stability and mechanical strength at high temperatures. The SnO2 has a rutile tetragonal structure with lattice parameters a = b = 4.7382 Å, c = 3.1871 Å. The big band gap of tin dioxide (3.62 eV at 27 ° C) does not allow transitions between bands when it is exposed to electromagnetic radiation in the visible spectrum, making it transparent. Oxygen vacancies are intrinsic defects that usually appear in this semiconductor. These defects play the role of electron donor impurities, thus increasing the electrical conductivity [3].
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SnO2 thin films have been prepared by different techniques, such as: spray pyrol
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