Surface states and the exponential valence-band tail in<i>a</i>-Si:H

Abstract

We use total-yield photoelectron spectroscopy to measure the density of occupied states in undoped and lightly boron-doped a-Si:H. We observe a large density of states in the gap of undoped a-Si:H, which we ascribe to near-surface defect states. Incorporation of boron removes these states and allows an unobstructed view of the valence-band tail. This is shown to be exponential over more than 3 orders of magnitude in the density of states with an inverse logarithmic slope of 45 meV, in good agreement with the results of dispersive transport measurements.