Effect of thickness and heat treatment on the electrical and optical properties of (Ge 2S 3) 1(Sb 2Se 3) 1 thin films

Abstract

An investigation of the electrical and optical properties of (Ge 2S 3) 1(Sb 2Se 3) 1, Sb 2Se 3, and Ge 2S 3 thin films prepared by thermal evaporation having different thicknesses and annealing temperatures has been carried out. The structure of synthesised glass and thin films were characterised by X-ray diffraction (XRD). The electrical conductivity was measured in the temperature range 300–498 K and thickness range 48.1–401.4 nm. The effect of the thickness and heat treatment on the activation energy Δ E for dc conductivity and the density of localised states at the Fermi level N( E f) were studied. The electrical conductivity measurements depend on the thickness and annealing temperature and exhibit two types of conduction channels that contribute to two conduction mechanisms. Optical absorption measurements have been made on as-deposited and annealed films for (Ge 2S 3) 1(Sb 2Se 3) 1, Sb 2Se 3 and Ge 2S 3 amorphous thin films. The mechanism of the optical absorption follows the rule of direct forbidden transition. The optical energy gap ( E opt) increased from 1.37 to 1.80 eV with increasing the thickness. Also E opt increases with increasing the annealing temperature up to 423 K, then decreases with increasing the annealing temperature. This behaviour is similar to the optical measurements of the system Sb 2Se 3 where E opt decreases rapidly with increasing the annealing temperature above the glass transition. This effect is interpreted in terms of the density of state model proposed by Mott and Davis.