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A new technique for growing large surface area SnO2 thin film (RGTO technique)

A new technique for growing SnO2 thin films with high surface area is described, based on tin rheotaxial growth and its thermal oxidation (RGTO). Tin thin films, when grown, present a surface characterized by spheroidal agglomerates due essentially to the surface tension of the liquid metal; these agglomerates do not seem to have any electrical continuity. By means of a thermal oxidation, both the transformation of the metal into a semiconductor and the thin film continuity are obtained, owing to the volume increase during the above-mentioned phase transformation. After this annealing cycle, SnO2 thin films are slightly oriented in the (101) direction and present an electrical resistivity equal to about 102 Omega cm. So, the surfaces of SnO2 films appear to be formed by spongeous agglomerates, which are in electrical contact with the nearest agglomerates. SnO2 thin films grown by this method show a high sensitivity (defined as the relative per cent conductance variation) to H2; in fact, sensitivity to 200 PPM H2 in synthetic air at ambient pressure is equal to 400%. It seems to be possible to prepare other metal oxide semiconducting thin films with a high surface area using this technique.