Effect of annealing on the optical gap of a-Si:H

The dependence on annealing temperature of the optical absorption coefficient and of the optical gap E 0 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 E 0 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.