Photocrystallization in a-Se films with and without As-Se buffer layers

Photo-induced crystallization is studied for temperatures of 250 - 435 K in amorphous Se (a-Se) HARP* imaging targets using Raman spectroscopy to detect the appearance and growth-rate of trigonal Se in these thin-film structures. We observe striking differences between HARP films in which a thin buffer layer of As-Se alloy is, or is not, deposited prior to growth of the principal a-Se photoconductive layer. Films containing an As-Se buffer appear to be much more stable; no photocrystallization is found within the temperature range studied, even after 3.5 hours of laser exposure (0.17W/mm$^{2}$ at 647nm). Whereas for films with no As-Se buffer, photocrystallization readily occurs in two temperature regimes below and above the a-Se glass transition (Tg $\sim $310K), and there is a regime in the neighborhood of Tg where photocrystallization is absent. We discuss these results in terms of a polymerization model under the competing effects of shear-strain at the substrate and As-cross-linking in the buffer layer, which, respectively, tend to promote and inhibit crystallization in the thick Se over-layer. *\textit{High-gain Avalanche Rushing Photodetector}.