Effect of light soaking and annealing on the stability of hydrogenated amorphous silicon films deposited using pure and highly helium diluted silane

We present a comparative study of the effects of the light soaking and annealing on the density of deep defect states, the width of valence-band tail (Urbach tail) and the refractive index of two different types of undoped hydrogenated amorphous silicon (a-Si:H) films. The samples were prepared by radio-frequency plasma enhanced chemical vapour deposition (PECVD) from pure silane at low rates (0.7-2 Å s^-1), known as 'standard' samples, and highly helium diluted silane at high rates (11-13 Å s^-1), at two substrate temperatures equal to 250 and 350 °C for each type of films. The results obtained for the as grown samples show that increasing the substrate temperature from 250 to 350 °C leads to an increase in the refractive index for all films, suggesting an increase in their compactness. Moreover, the density of defects and Urbach tail measured for the helium diluted samples also decrease when increasing the deposition temperature, as a result of the elimination of the polyhydride Si-H₂ and (Si-H₂)_n complexes present in the films grown at 250 °C. We obtain values comparable to those measured for optimised standard films. Light soaking induces a decrease in the static refractive index in all films however, the helium diluted samples deposited at 350 °C still exhibit a much higher compactness than the other ones. On the other hand, although the density of deep defect states increases upon illumination for all samples, without any changes either in the Urbach tail or in the related-hydrogen microstructure, the helium diluted films show comparable stability to that observed for the optimised standard ones deposited at low rates. The analysis of the data obtained for the annealed samples at 200 °C indicate that the native defects in the as grown films are of different nature from those created by illumination.