Structural, optical and electrical properties of (CdS)1-x (ZnTe)x solid solution thin films prepared by vacuum thermal evaporation method

Abstract The II-VI compounds and their solid solutions are promising source for various types of thin film devices such as thin film transistors, optoelectronic devices and solar cells. The formation of (CdS)1−x(ZnTe)x layer by interdiffusion of CdS and ZnTe during the fabrication of CdS/ZnTe heterojunction thin film was previous reported. This layer is thought to be important because it relieves strain at the CdS/ZnTe interface that would otherwise exist due to the excess 10 % lattice mismatch between the two materials. Therefore, it is essential to have a full understanding of the physical properties of (CdS)1−x(ZnTe)x alloy thin films. In this work, (CdS)1−x(ZnTe)x thin films in the entire composition range (0≤x≤1.0) were prepared by vacuum thermal evaporation on glass substrate using mixed powders of high purity of CdS and ZnTe compounds as the precursor. XRD revealed that the films exhibited a hexagonal structure with the preferred orientation of (002) plane when x ≤ 0.2. However, when x ≥ 0.8, they belonged to a cubic structure with the preferred orientation of (111) plane. For the composition 0.4 ≤ x ≤0.6, the hexagonal and cubic phases coexisted in the system and the films became less preferentially oriented. SEM and EDS were used to study the surface morphology and elemental composition of the samples. The crystallite size of the as-deposited films in the range 88–361 nm was observed by AFM image. The FTIR transmission spectra in the range 400−1,000 cm−1 revealed the characteristics of Cd-S and Zn-Te vibrational modes. Optical properties of the films were performed with UV-Vis spectrophotometer in the wavelength range 400−1,000 nm. The variation of direct energy gap with composition (x) was in good agreement with the quadratic form, giving an upward bowing parameter (b) of 1.23 eV. Electrical properties of the films were evaluated by resistivity and Hall effect measurements in the van der Pauw configuration. From transient photoconductivity measurement, the decay current data were better fitted with multiple exponential functions resulting in the several slow decay times. Density of trap states corresponding to its decay time was also evaluated from the decay current data.