Photoemission study of near-surface band gap states in a-Si:H

Abstract

We use total yield photoelectron spectroscopy in combination with the Kelvin probe to study changes in the work function and the distribution of occupied band gap states upon boron doping and oxidation of amorphous hydrogenated silicon (a-Si:H) surfaces. We observe 3×1011 occupied surface states/cm2 in clean undoped a-Si:H in an 0.4-eV wide band centered 5.0 eV below the vacuum level, which we ascribe to intrinsic surface defects. Incorporation of boron removes these states and exposes the intrinsic valence band tail, which is exponential over four orders of magnitude in the density of states. In contrast to c-Si, where adsorbed oxygen decreases the large intrinsic surface state density (4×1014 states/cm2 ) on the clean, cleaved surface, the relatively low density of intrinsic surface states on a-Si:H allows the observation of oxygen-induced surface band gap states. The origin of these states is discussed.