Plasma-hydrogenation effects and the thickness dependence of electrical properties and E.S.R. in undoped CVD a-Si

The effects of plasma hydrogénation on the electrical properties, E.S.R. and optical absorption, and the thickness dependences of these various properties have been investigated for undoped CVD a-Si films. Under plasma hydrogenation at 300°C, the dark conductivity and E.S.R. spin density decrease and the photoconductivity increases with a plasma annealing time of up to 30 min. For the hydrogenated samples, large reductions of these properties and a large enhancement of the activation energy in the dark conductivity are found with a decrease in the film thickness. These results for the electrical properties are analysed with a Schottky-barrier model, and as a consequence their thickness dependences are interpreted by a surface band bending due to positively charged localized states near the free surface which act as recombination centres in photoconduction.