Investigation on microstructural and optical properties of nano-crystalline CdSe thin films

In this work, nano-crystalline CdSe thin films were deposited by thermal evaporation technique from CdSe nano-powders on glass substrates. The deposited films were characterized by using optical transmission measurements, and X-ray diffraction (XRD). X-ray line profile analysis revealed that the CdSe films are nano-crystalline and having a wurtzite (hexagonal) structure. These nano-crystalline CdSe films were preferentially oriented along the (002) plane with a c-axis perpendicular to the substrate surface. The crystallite size of these films varied from 16 to 36 nm as measured from X-ray line broadening. The variation of film thickness had a great influence on the microstructural parameters, such as micro-strain, stacking-fault probabilities, and dislocation densities. The obtained results demonstrate that the micro-strain, dislocation density, and stacking fault probabilities decrease by increasing the film thickness. The estimated refractive indices of the films fitted well with Sellmeier dispersion formula over a spectral range, 200 nm and 2500 nm. The effective resonance absorption wavelengths were between 610 and 723 nm. Optical parameters of nano-crystalline CdSe films were investigated extensively, and interpreted in the frame of Wemple-DiDomenico single oscillator model. These parameters provided valuable physical insights, such as the relationship between the ionicity of the CdSe films and microstructural parameters. The studied samples obviously had two distinct regions in Tauc plot, except for that sample CS04, due to the strong influence of strain and dislocations.