ZnO: In Thin Films Prepared with Different Precursor Salts by Spray Pyrolysis and Studied by Electron Microscopy.
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INTRODUCTION Transparency, light trapping and conductivity properties of thin films of semiconductor oxides have motivated numerous applications in gas sensors [1] antireflecting coatings [2],solar cells [3], transparent electrodes[4], among others.In the literature fabrication of zinc oxide and tin oxide thin films for mentioned uses has been reported frequently [5].Zinc oxide films obtaining, has been reported using different techniques like cathodic pulverization [6], reactive evaporation [7],phase vapor deposition [8], chemical bath [9] and spray pyrolysis [l0].In this work we report some results of zinc oxide films obtained by spray pyrolysis; our films present values for resisitivity and transparency very similar to those reported for films obtained with more sophisticated techniques [1l].An increase in light trapping was obtained after surface texture of material was improved. This is achieved modifiying material structure in order to originate a preferential growth in (110) direction [12].One of the ways to obtain better texture is adding Indium;the role of indium atoms in thin films will be to decrease the resitivity.We obtained a better texture using like unpurifier indium acetate.However, after a more extended analysis, we found that besides the starting solution, its acidity plays an important role. EXPERIMENTAL We tried films growth with three differents solutions: 1.- Zinc acetate was dissolved in water,adding alcohol until we reach a water/ alcohol ratio, 3:1 .Precipitation was produced and it was avoided after adding some drops of acetic acid to solution, which we will denominate as solution with high pH. 587 Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society
2.- Alcohol and water were mixed in a 3:1 ratio in volume,then zinc acetate was added until reaching a final molarity of 0.1 M. Also precipitation was observed and a greater quantity of acetic acid than in previous case was neccesary.This solution is identified like the one with low pH. 3.- Zinc acetate was dissolved in methanol.No precipitation was observed; this is a consequence of high disassocitiviy of methanol and it was not necessary to add acetic acid to solution, which we call, methanol solution. Deposition by spray pyrolysis was done on glass substrates kept 0 at 400, 450 and 500 C, respectively with +/5 °C variation.Film growth was stopped when a green color, that means an approximately 600 nm thickness, was detected in all cases. Transport gas used was N2 and a flux rate of 9 ml/min was kept constant.Electrical resistance was measured in darkness condition and thickness was determinated from transmittance spectra in visible range following the method proposed by Manifacier et al.[12].Structural properties were derived from X ray diffraction experiments and transmission electron microscopy observations. Surface details like texture and grain sizes were observed from scanning electron micrographies at several magnifications. RESULTS Figure 1 shows variations of resistivity against substrate temperature.For ZnO:in fi
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