Distribution of occupied near-surface band-gap states ina-Si:H

Abstract

We investigate the distribution of occupied band-gap states in undoped, B-doped, and P-doped a-Si:H within the first ∼100 Å of the surface using total-yield photoelectron spectroscopy in combination with the Kelvin probe. In clean, undoped a-Si:H the occupied density of states extracted from the measured yield spectrum consists of a linear valence-band edge, an exponential valence-band tail decreasing into the gap, and a broad band of deep defect states superposed on this tail. The deep-defect density of undoped a-Si:H measured by total yield is 2 orders of magnitude larger than that measured by photothermal deflection spectroscopy (PDS) on simultaneously prepared 1-μm-thick samples, from which we infer the existence of an excess density of deep defect states near the surface corresponding to an equivalent surface-state density of 3×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$.