An explanation for the low-temperature H evolution peak in hydrogenated amorphous silicon films deposited 'on the edge of crystallinity'

The puzzling existence of a sharp low-temperature (T = 400 degrees C) H evo lution peak in compact hydrogenated amorphous silicon (a-Si:H) films deposi ted 'on the edge of crystallinity' is examined. From infrared absorption an d X-ray diffraction (XRD) measurements, we show that none of the standard m ethods used to explain the existence of this peak in a-Si:H materials is ap plicable to the present films. From the Si-H wag-mode peak frequency, we po stulate the existence of very small Si crystallites contained within the am orphous matrix. While the crystallite Volume fraction is too small to be de tected by XRD in the as-grown films, crystallization is observed for this m aterial at anneal temperatures as low as 500 degrees C. It is proposed that these crystallites catalyse the crystallization of the remainder of the am orphous matrix upon moderate annealing, enabling H surface desorption and H z out-diffusion to the sample surface along newly formed grain boundaries a t low anneal temperatures.