Effect of processing conditions on structural and physical properties of Cd-rich thin films of CdTe

Abstract

The physical properties of CdTe thin films formed by vacuum evaporation techniques onto glass substrates were studied as a function of the processing conditions. The structure of CdTe films was examined using an X-ray diffraction (XRD) technique. The films were polycrystalline and cubic; they were grown with the (1 1 1) plane parallel to the substrate. The lattice constant (a = 0.6470 nm) was found to be smaller than that for a stoichiometric samples. This indicates the presence of residual compression stresses or excess Cd in substitutional sites. The crystallite size (10–72 nm) was found to increase with increasing substrate temperature (300–520 K). The films contained free Te which was varied with the substrate temperature. The variation of transmission within the wavelength range from 500 to 1000 nm was also investigated. It showed a band edge at about 1.5 eV. The variations of conductivity and carrier concentration with temperature were investigated. The conductivity data in the low-temperature region were analysed for variable range-hopping conduction on the basis of Mott's model. The analysis of the high-temperature conductivity data was based on Seto's model of thermionic emission. On the other hand, it was found that excess Cd increases the conductivity. However, the density of trap states near the Fermi level N(EF), was proportional to the Cd content.