Effect of post-deposition treatments on the hydrogenation of hot-wire deposited amorphous silicon films

Abstract

The effect of different post-deposition treatments on the structure of Hot-Wire (HW) deposited intrinsic a-Si:H thin films is investigated. These treatments are applied in order to rehydrogenate the top region of the film, which, due to the high deposition temperatures of these films, becomes depleted of hydrogen. Using secondary ion mass spectroscopy (SIMS) for measurement of the hydrogen profiles and Raman spectroscopy measurements to investigate the hydrogen bonding configuration at the surface, we were able to determine the effect of the different treatments on the films. We show that the hydrogen profile in the subsurface region is not a result of the out-diffusion of hydrogen while the sample is cooled down after the deposition, but develops during the growth of the film. The most effective bulk rehydrogenation is achieved with an atomic hydrogen treatment, for which we determined a diffusion coefficient for atomic hydrogen of 7×10−14 cm2 s−1 at 430°C. Rapid cooling and a hydrogen plasma treatment are shown to result in an improved top surface hydrogenation of the intrinsic film. Despite the differences in top surface hydrogenation of the intrinsic films, application of the treatments in real n-i-p solar cell devices did not show an improvement in the cell properties. We attribute this to the existence of defects deeper in the bulk of the intrinsic layer or at the n/i-interface, being the dominating effect that limits the cell performance. Copyright © 1999 John Wiley & Sons, Ltd.