SURFACE AGING OF HGI2 CRYSTALS STUDIED BY VASE AND AFM

Variable angle spectroscopic ellipsometry (VASE) and atomic force micr oscopy (AFM) measurements have been employed to characterize the surfa ce aging of HgI2 crystals - a material for room-temperature radiation detectors. Four different initial HgI2 surface conditions were selecte d, i.e. an as-grown surface, a cleaved surface, a surface chemically e tched by 10% KI solution and a parylene coated surface. A model includ ing top surface roughness and subsurface defects of HgI2 crystal, rela ted to surface aging, was established and characterized, as a function of real time, by the VASE analysis. The VASE measurements indicated t hat high surface aging rates were related to high initial effective tw o-dimensional (2D) surface-defect densities. The AFM profiles revealed increasing physical surface roughness as surface aging took place. Th e cleaved HgI2 crystal surface presented a smooth surface and the lowe st surface aging rate, while the as-grown HgI2 surface also presented a very low surface aging. A parylene coating on the HgI2 surface can g reatly slow down the surface aging.