The effects of incorporating water into vacuum-deposited amorphous silicon films

Abstract

This paper describes the effects on some of the electrical and optical properties of a-Si: C films prepared by vacuum deposition in controlled ambient gases of N2, O2, H2, and water vapour. This stidy was initiated in an attempt to determine the role of a low-energy electron beam irradiating the substrate and film as the layer grows. We find that depositing an a-Si:C film in a water-vapour environment, at a partial pressure of only 4 × 10−8 Torr, will result in the film having a room-temperature resistivity two thousand times higher than that for "clean" films deposited in a good vacuum with a partial pressure of water vapour near 4 × 10−9 Torr (1 Torr = 133.3 Pa). If the partial pressure of the water vapour is increased, the film resistivity increases and it becomes photoconducting. By comparison, the other gases tested had no measurable effect on the properties of the films, with the exception of oxygen which increases the film resistance. None of these latter films were photoconducting. We conclude that the improvement observed in the film properties when samples are irradiated with low-energy electrons results more from the effects of water vapour and (or) oxygen which can be freed from cold surfaces near the substrate than from a possible restructuring of the film as postulated earlier (1).