Competing effects in the photo-crystallization of a-Se HARP films

Photo-crystallization of amorphous (a-) Se is investigated as a function of temperature (T = 250--340 K) and exposure time by Raman scattering in photo-active films used in high-gain avalanche rushing photoconductor (HARP) cameras. We observe different temperature regimes that reflect the competition of viscosity-driven and strain-driven processes. No photo-crystallization is seen below 260K, and above 318K the crystallization rate is thermally enhanced. For 260--302 K the onset time for photo-crystallization decreases with increasing temperature, but the crystallization rate is T-independent. \textit{Surprisingly}, photo-crystallization is strongly suppressed in a 15 degree range around the glass transition at T$_{g} \quad \sim $ 310K. Our results are in qualitative accord with a theory by Stephens[1] that treats the effects of local strain on the growth of crystalline-Se domains within an a-Se matrix -- the strain arising from the density difference between the crystalline and amorphous phases. [1] R.B. Stephens, J. Appl. Phys. \underline {\textbf{51}}, 6197 (1981).