Light-induced, reversible, above-gap, optical changes in hydrogenated amorphous silicon films

The purpose of this work is to determine if there are reversible. light-induced changes in the hydrogenated amorphous silicon optical properties in the higher than band gap photon energy region. For that purpose, we propose a new technique in which spectroscopic ellipsometry (SE) measurement is made through a transparent substrate to avoid effects related to changes in surface oxidation or contamination. Decrease in the imaginary part of the pseudo-dielectric function <epsilon(2)> was observed in the higher than band gap photon-energy range after the sample was illuminated with visible light, The change was reversible by thermal annealing at 220 degrees C for one hour. Optical simulations show that the origin of the observed change may be localized to a region within a few nanometers of the interface with the transparent substrate. (C) 2000 Elsevier Science B.V. All rights reserved.