Influence of temperature on the optical absorption edge in amorphous Zn–P thin films

Abstract

The effect of temperature (in the range 8 to 300 K) on the fundamental absorption edge of amorphous Zn32P68 thin films have been determined from transmission and reflectivity measurements. Both the Tauc gap, EG, and the Urbach energy, EU, describing the absorption edge, have been shown to follow the Einstein expression for the total energy of a solid. Below 150 K, these two parameters are almost independent of temperature, while in the range 150 to 300 K, they change approximately linearly, but in opposite directions (EG decreases from 1.43 to 1.28 eV, while EU increases from 109 to 145 meV). The coordinates of the temperature independent Urbach focus and a linear dependence between the Tauc gap and the Urbach edge have been found. The observed changes of the optical gap and Urbach energy are discussed and compared with the structural study of the amorphous Zn–P films.