Defect equilibration in amorphous silicon films submitted to high intensity illumination

In plasma-deposited a-Si:H films, the increase of the metastable defect density produced by high-intensity illumination usually follows a stretched-exponential time-dependence, with a characteristic time τ_SE and a steady-state value N_SS. For a wide range of deposition conditions, we have observed that both parameters depend on the material properties. The strong correlation between N_SS and the monohydride [SiH] 2000 density, reported previously, has been interpreted as due to the trapping of metastable H atoms at specific sites. In this study of the kinetics of defect equilibration under high-intensity illumination, we find two groups of a-Si:H films with fast and slow kinetics, respectively. These two groups display a very different dependence of the defect creation rate as a function of the optical gap. For the fast kinetics films, we emphasize the critical influence of the Urbach energy E_u deduced from the exponential optical absorption edge (1/τ_SE increases as a function of E_u). The slow kinetics films are characterized by a high nanovoid density evidenced by their SiH_x infrared signature of 2090 cm^-1. The results are discussed in relation to the medium-range H motion.