Anisotropic dielectric response and surface aging of mercuric iodide crystal studied by variable angle spectroscopic ellipsometry

Abstract

Variable angle spectroscopic ellipsometry (VASE) was employed to study the anisotropic optical properties and surface aging phenomena of mercuric iodide (HgI 2) — a material for room-temperature radiation detectors. In the optical property study, two surface orientations of this tetragonal crystal were selected, i.e. an a-plane and a c-plane sample. Room temperature multiple-angle SE measurements from both samples with three different optical configurations along with polarized transmission measurements were analyzed simultaneously by the VASE analysis through multiple-sample, multiple-model methods. Anisotropic dielectric functions of single crystal HgI 2, ε ⊥(ω) and ε ‖(ω), for optical electric field vectors oriented perpendicular and parallel to the c axis, respectively, were obtained over the range of 1.24–5.1 eV. In the surface aging characterization, three different initial HgI 2 surface conditions were selected, i.e. an as-grown surface, a cleaved surface, and a surface chemically etched by 10% KI solution. A model of the HgI 2 crystal including top surface roughness and subsurface defects was established and characterized, as a function of time, by the VASE analysis. In this model, the surface defects associated with the surface aging were modeled by the Bruggeman effective-medium approximation (EMA) as a HgI 2/void mixed overlayer. The relative 2-dimensional (2D) surface-defect-densities were then monitored as the surface aging proceeded.