Effect of heat and CdCl₂ treatment on the structure and photoluminescence of CdTe/CdS thin films

This is to report on the structural and Photoluminescence (PL) studies of thin films of CdTe on glass or CdS (0.25 μm)/glass substrates which were prepared by the physical evaporation method. The CdS/glass substrate temperature was the variable parameter in this experiment. The effect of chemical and heat processing and substrate temperature on the structural and the electro-optical properties were analyzed. X-ray diffraction results showed a preferential growth in the (III) direction of treated and untreated films. The Scanning Electron Microscopy results showed considerable grain growth with chemical treatment, and little effect on grain size with heat treatment only. The cycling of the chemical and heat treatment resulted in more growth in grain size. Photoluminescence (PL) spectra, in the region 1.6- 1.2 eV, were taken for untreated and treated samples at 12K. All untreated samples showed undetectable PL spectra. The heat treated samples showed very weak PL relative to the chemically treated samples. Cycling the chemical and heat processing resulted in more intense PL spectra and resolved longitudinal-optical phonon replicas indicative of an improvement in film crystallinity. The PL spectra of samples with increasing substrate temperature showed a substantial decrease in intensity with a shift of the maximum emission towards lower energy. This was attributed to diffusion of sulfur in CdTe. The PL spectra, taken for chemically treated samples from the front excited surface and the junction excited surface are of similar intensity and of approximately similar structure. This was evidence of the penetration of the chemical/heat treatment deep in the whole film.