Raman studies of crystallization and photo-darkening in HARP a-Se targets

The build-up of light-induced defects in the a-Se targets of High-gain Avalanche Rushing Photoconductor (HARP) cameras is important for the robustness of these detectors. Recent experiments show that the photo-induced dark spots in HARP targets exhibit irreversible and reversible components, i.e., spot transparency is only partly restored by heating to 35C, $\sim $1C below the a-Se glass transition.[1] We report Raman studies of laser(2mW HeNe)-induced crystallization in a-Se camera targets. The rate of increase in the trigonal c-Se Raman intensity (233 cm$^{-1}$ peak) is measured \textit{vs.} exposure time for local temperatures in the range 25 -- 65 C, as found from Stokes-to-anti-Stokes ratios. Lateral- and depth- profiling by optical microscopy easily correlate the Raman-detected crystallization regions with visual damage. Further studies \textit{vs.} temperature and HARP-target usage are in progress to unravel the damage mechanisms. [1] A. Reznik \textit{et. al.}, submitted to J. Non-Cryst. Solids