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The authors have measured the total hydrogen concentration (H) and the hydrogen concentration in SiH (H1), SiH2 (H2) and other sites (H'). From uniform depth profiles of total hydrogen concentration they conclude that hydrogen concentration is not limited by diffusion. They report here the variations of (H), (H1), (H2) with annealing temperature for substrate temperature Ts=150 degrees C, 190 degrees C and 250 degrees C and deposition rates ' nu ' ranging from 20 to 120 AA min-1. The authors lose hydrogen only above the annealing temperature at which hydrogen saturates the Si matrix; (H) saturates at a value (Hs) which is independent of Ts and nu , at each annealing temperature TA. Annealing induces simultaneous variations of the hydrogen concentration on all the sites. All these results are interpreted by a model according to which the hydrogen is linked to the a-Si matrix defects, filling first the deeper ones and then the more shallow defects. According to this model the total hydrogen concentration is limited by the total Si matrix defect concentration.

The dependence on annealing temperature of the optical absorption coefficient and of the optical gap E0 of sputtered a-Si:H at various deposition rates (20-120 AA min-1) and substrate temperatures is reported. The hydrogen concentrations on various sites for identical films are reported. A previous model, according to which hydrogen links onto Si structural defects, and E0 depends on the deformation of the silicon matrix, is extended to interpret these experimental results. The hydrogen concentration appears to be controlled by the number and size of Si matrix defects, which explains the H saturation behaviour found experimentally. It is suggested that the optical gap, and hence the mobility gap, depend on very shallow defects introduced by stretched Si-Si bonds mainly as satellites of larger defects. This picture allows the interpretation of the experimental results on both virgin and annealed films.