Wet-Chemical Processing of Tin-Doped Indium Oxide Layers
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ABSTRACT For industrial applications the wet-chemical deposition of tin-doped indium oxide (ITO) layers would be favourable, especially at low temperatures. It is shown that conductive transparent films can be made by spinning solutions which contain alkoxide precursors of indium and tin. The hydrolysis conditions of these alkoxides are varied using different water to indiumalkoxide ratios and different hydrolysis times. It is shown that when processed at low temperatures, the hydrolysis of these alkoxides has a severe influence on the electrical properties of the layers. A decrease in resistance as a function of time is observed for ITO layers which are stored at room temperature in an ambient atmosphere. Annealing at 350'C in forming gas further reduces the resistance of the ITO layers.
INTRODUCTION ITO layers are widely used as transparent conductive layers on glass. Using vacuum deposition techniques such as sputtering, layers with a sheet resistance of 1 to 10 Q/square for a layer thickness of 100 to 50 nm, can be deposited. For industrial applications the wetchemical deposition of ITO layers would be attractive, especially at low temperatures. Wetchemical sol-gel techniques have been used for the deposition of ITO layers"7 . The advantage of the sol-gel process is that the precursor can be hydrolysed before the layer is deposited. No decomposition of the solvent is necessary for the formation of water in order to hydrolyse the indium and tin compound as is the case when using a spray-pyrolysis. Various organometallic indium and tin compounds can be used as precursors. During the sol-gel processing the reactivity with respect to hydrolysis and/or condensation can be controlled by the addition of complexing agents. A well-known complexing agent is acetylacetone. Other complexing agents such as triehanolamine (TEA) or ethylacetoacetate (EAA) can also be very effective 5. Some preliminary results of the investigations into the processing conditions for the deposition of ITO using metalalkoxides are reported in this paper. The dopant level of Sn' in the indium oxide lattice is of importance for the concentration of charge carriers8"9 . To that end the conductivity of ITO layers was measured as a function of the dopant concentration. The hydrolysis conditions can be varied using different water to indium alkoxide ratios, and different hydrolysis times. Finally, the temperature dependency of the conductivity of the ITO layers is studied. Although the atmosphere in which the ITO layers are fired has an enormous effect on the conductivity of these layers, we concentrated in this paper on effects we obtained when the ITO layers were fired in an ambient atmposphere. The results of the annealing experiments in a forming gas atmosphere are briefly reported. 469 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
EXPERIMENTAL Precursors For the wet-chemical preparation of ITO layers it is necessary to start with homogeneous solutions of hydrolysed indium- and tin-alkoxide compounds. Disadvantage of thi
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