Photo-crystallization in a-Se imaging targets: Raman studies of competing effects

Photo-induced crystallization of a-Se is investigated by Raman spectroscopy as a function of temperature (250–340K) and exposure time in thin-film structures used as targets in high-gain avalanche rushing photoconductor (HARP) video cameras. The Stokes-to-Antistokes ratio is monitored to obtain the local temperature Tloc at the laser spot; fluxes (632nm) of 17 and 10W/cm2 are used. We find a rich temperature behavior that reflects the competition of changes in viscosity and strain, and defines four distinct regimes. No photo-crystallization is seen for Tloc below 260K, nor in a 15K range around Tg∼310K. For Tloc in the regime 260–302K the initial rate of crystal growth after onset of photo-crystallization is temperature independent, whereas for Tloc>318K the growth rate is thermally enhanced. Our results are in qualitative accord with a theory by Stephens treating the effects of local strain on the secondary growth of crystalline nuclei in a-Se. We conclude that the observed growth rate between 260 and 302K is driven by local strain, and that relaxation of this strain near Tg suppresses crystal growth until thermally assisted processes accelerate the photo-crystallization at higher temperatures.