Compositional Trends in the Optical Properties of Amorphous Lone-Pair Semiconductors

Abstract

The pressure and temperature dependence of the refractive index and absorption edge have been measured for a group of amorphous semiconductors with average coordination number between two and four, all containing group-VI elements in twofold coordination (lone-pair semiconductors). Clear compositional trends are observed in the pressure dependence of the optical properties when these measurements are compared with the values for tetrahedral semiconductors. The chemical-bond approach is extended by the introduction of a new material parameter, the average bond-free solid angle (BFSA), in order to explain the large positive pressure coefficient of refractive index ${\left(\frac{\partial n}{\partial P}\right)}_T$ observed for chalcogenide-rich materials and the small negative values observed for tetrahedral semiconductors. BFSA is the solid angle, associated with each atom, which is free of bond charge. When BFSA is large, atoms can move closer together under stress without compressing bonds. In this case local-field corrections cause positive ${\left(\frac{\partial n}{\partial P}\right)}_T$. When BFSA is small, local-field corrections are smaller because the charge density is more uniformly distributed. Strain results in bond compression which gives negative ${\left(\frac{\partial n}{\partial P}\right)}_T$. It is found that BFSA can also explain compositional trends in the pressure coefficient of the absorption edge as well as trends in apparently unrelated material properties such as melting temperature.