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

Abstract

The thickness dependences of the ESR, hydrogen vibrational spectra, and optical absorption have been investigated for glow-discharge amorphous Si: H films doped at various doping ratios with phosphorus and boron. The incorporated hydrogen concentration, the spin density at g = 2.0055 associated with dangling bonds, and the optical gap were observed to increase with decreasing film thickness. The thickness dependence of the spin density could not be observed for a film annealed in a hydrogen plasma. Signals with g = 2·0043 and 2·011, respectively, observed for heavily P- and B-doped films, showed little dependence on film thickness, and were not affected by post-hydrogenation. It is suggested that in neither energy band are the localized tail states affected by post-hydrogenation, but the density of localized states arising from dangling bonds, near the middle of the band gap, decreases during post-hydrogenation. These results are analysed in terms of a two layer-model. It is concluded that the increase in spin density arising from dangling bonds and the increase in hydrogen concentration with decreasing film thickness have their origin in an enhanced spin density and hydrogen concentration near the free surface. The relationship between these properties and electrical conductivity is discussed.