Potential gas sensor applications of semiconductor thin films based on changes in photoresponse

A method for improving the discrimination of semiconductor thin-film gas sensors, which utilises the amplitude and phase of the photocurrent response to a modulated light source, is proposed. The photoconductivity of microcrystalline silicon films is examined before and after exposure to the vapour of an iodine solution in ethanol. Preliminary results are presented which suggest that the density and/or capture properties of localised states may be reversibly modified. For cadmium selenide however, a small but systematic temperature-dependent phase lead is observed in this material at low frequencies, even under vacuum-annealed conditions. This is in contrast to present theory, which predicts that for an arbitrary localised state distribution the photocurrent response must lag the applied excitation.