Light-induced defects in hydrogenated amorphous silicon studied by the constant-photocurrent method

The light-induced creation of metastable defects in undoped hydrogenated amorphous silicon has been followed using photoconductivity and absorption coefficient measurements. The density of states in the gap was obtained from the deconvolution of the subgap absorption coefficient measured by the constant-photocurrent method. We found that the decay of the photoconductivity and the evolution of the integrated density of subgap states follow the dependence on illumination time (tᵢₗₗ) predicted by the ``bond-breaking'' model. The density of occupied states obtained from the deconvolution procedure shows the presence of two peaks within the gap. After subtracting the valence band-tail contribution, these peaks can be well fitted with two Gaussians. The areas of both Gaussians increase as tᵢₗₗ1/3 while their positions and widths remain unchanged. According to the energy position of these peaks, they are ascribed to the neutral and negatively charged silicon dangling bonds D⁰ and D^-. We found that in this intrinsic sample the density of charged defects exceeds that of neutral defects, their ratio D^-/D⁰ being approximately 2.6. This ratio is independent of the illumination time.