Effect of thickness and heat treatment on the electrical and optical properties of (Ge2S3)1(Sb2Se3)1 thin ®lms

An investigation of the electrical and optical properties of (Ge2S3)1(Sb2Se3)1, Sb2Se3, and Ge2S3 thin ®lms prepared by thermal evaporation having different thicknesses and annealing temperatures has been carried out. The structure of synthesised glass and thin ®lms 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 DE for dc conductivity and the density of localised states at the Fermi level N(Ef) 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 ®lms for (Ge2S3)1(Sb2Se3)1, Sb2Se3 and Ge2S3 amorphous thin ®lms. The mechanism of the optical absorption follows the rule of direct forbidden transition. The optical energy gap (Eopt) increased from 1.37 to 1.80 eV with increasing the thickness. Also Eopt 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 Sb2Se3 where Eopt 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. # 2001 Elsevier Science B.V. All rights reserved