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 evolution peak in compact hydrogenated amorphous silicon (a-Si:H) films deposited 'on the edge of crystallinity' is examined. From infrared absorption and X-ray diffraction (XRD) measurements, we show that none of the standard methods used to explain the existence of this peak in a-Si:H materials is applicable to the present films. From the Si-H wag-mode peak frequency, we postulate the existence of very small Si crystallites contained within the amorphous matrix. While the crystallite Volume fraction is too small to be detected by XRD in the as-grown films, crystallization is observed for this material at anneal temperatures as low as 500 degrees C. It is proposed that these crystallites catalyse the crystallization of the remainder of the amorphous matrix upon moderate annealing, enabling H surface desorption and Hz out-diffusion to the sample surface along newly formed grain boundaries at low anneal temperatures.