Electrical conductivity and optical absorption of (Ge 2S 3) 1(Sb 2Te 3) 1 amorphous thin films

An investigation of the electrical and optical properties of vacuum evaporated (Ge 2S 3) 1(Sb 2Te 3) 1 thin films has been carried out as a function of the film thickness (118.9–200 nm) and annealing temperatures (373, 423, and 473 K). The structure of the films was characterized by X-ray diffraction (XRD) which revealed the formation of amorphous films. The electrical conductivity was measured in the temperature range (290–383 K). The effect of the thickness and heat treatment on the activation energy (Δ E) for the dc conductivity and density of the localized states at Fermi level N( E F) were studied. The electrical conductivity measurements were found to be dependant on the film thickness and annealing temperature, and exhibit two types of conduction channels that contribute two conduction mechanisms. Optical absorption measurements showed that the fundamental absorption edge is a function of annealing temperatures and the optical absorption is due to the allowed indirect transition. The optical energy gap ( E opt) decreases with increasing thickness and annealing temperatures below T g. The results are interpreted in terms of the density of state model proposed by Mott and Davis.