Thickness dependences of properties of P- and B-doped hydrogenated amorphous silicon

Abstract

The thickness dependences of the dark conductivity and the photoconductivity of glow-discharge amorphous Si: H films doped at various doping ratios with phosphorus and boron have been investigated. For decreasing thickness an increase of the activation energy for dark conductivity was observed for P- and B-doped films, whereas for undoped films the activation energy decreased. A reduction of the photoconductivity for the undoped and B-doped films and an enhancement for the P-doped films with decreasing thickness were also observed. These results are analysed in terms of a two-layer model and, as a result, the existence of a highly conductive layer in undoped films and a highly resistive layer in P- and B-doped films are deduced and interpreted in terms of a high density of donor-like localized states near the surfaces. It is concluded that, for the P-doped films, the photoconductivity near the free surface is higher than in the bulk, but near the interface with the substrate the photoconductivity is lower than in the bulk. On the other hand, for the undoped films the photoconductivity in the bulk is the highest.