Surface aging of HgI2 crystals studied by VASE and AFM

Variable angle spectroscopic ellipsometry (VASE) and atomic force microscopy (AFM) measurements have been employed to characterize the surface aging of HgI2 crystals — a material for room-temperature radiation detectors. Four different initial HgI2 surface conditions were selected, i.e. an as-grown surface, a cleaved surface, a surface chemically etched by 10% KI solution and a parylene coated surface. A model including top surface roughness and subsurface defects of HgI2 crystal, related to surface aging, was established and characterized, as a function of real time, by the VASE analysis. The VASE measurements indicated that high surface aging rates were related to high initial effective two-dimensional (2D) surface-defect densities. The AFM profiles revealed increasing physical surface roughness as surface aging took place. The cleaved HgI2 crystal surface presented a smooth surface and the lowest surface aging rate, while the as-grown HgI2 surface also presented a very low surface aging. A parylene coating on the HgI2 surface can greatly slow down the surface aging.