We have grown two series, ‘SLOW’ and ‘FAST’, of high quality a-Si:H films by the VHF-GD (Very-High-Frequency Glow-Discharge) technique, the first at a low deposition rate of R∼3.6 Å/sec and the other at a high rate of R˜17 Å/sec. Both series show the same bulk deep defect density Nb≈4×1015/cm3 in the annealed state ‘A’ as measured by PDS. From lightinduced degradation experiments (Staebler-Wronski effect, SWE) the following interesting features are observed : a) The SWE is a bulk effect. To demonstrate this result we propose the use of PDS in combination with variable film thicknesses as a powerful and unambiguous method, b) The increase of the deep bulk defect density ΔNb up to the surated state of the SWE (state ‘B’) is the same for series ‘SLOW and ‘FAST’, c) However, the kinetics of the SWE differ considerably.
These results are interpreted with the help of experimentally observed continuous variation of the Urbach energy E0 of the valence bandtail accross the film. The E0 profile is, in turn, related to the deposition rate R. The presence of this inhomofgenei yis also discussed in context with.the recently developed equilibration theories relating deep and shallow (bandtail) states. Preliminary results in this study seem also to support the model involving conversion of weak Si-Si bonds into dangling bond upon light soaking. Further the influence of internal film stress as well as the effect of impurities on the degradation behaviour are addressed.