The light-induced creation of metastable defects in undoped hydrogenated amorphous material was monitored by means of photoconductivity and absorption coefficient measurements. The density of states in the gap was deduced via de-convolution of the sub-gap absorption coefficient, as measured by using the constant photo-current method. It was found that the decay of the photoconductivity, and changes in the integrated density of sub-gap states, obeyed the dependence upon illumination time that was predicted by the so-called bond-breaking model. The density of occupied states which was deduced from the de-convolution procedure revealed the presence of 2 peaks within the gap. After subtracting the valence band-tail contribution, these peaks could be fitted by using two Gaussian curves. The areas under both curves increased as the one-third power of the illumination time, while their positions and widths remained unchanged. According to the energy positions of these peaks, they could be attributed to the neutral and negatively charged Si dangling bonds, D0 and D-. In an intrinsic sample, the density of charged defects exceeded that of neutral defects, with the D-/D0 ratio being approximately 1:6. This ratio was independent of the illumination time.

J.A.Schmidt, R.Arce, R.H.Buitrago, R.R.Koropecki: Physical Review B, 1997, 55[15], 9621-7