Effects of CdCl2 treatment on the recrystallization and electro-optical properties of CdTe thin films

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Abstract
The effects of CdCl 2 processing on the physical and electro-optical properties of CdTe were evaluated for thin films produced by physical vapor deposition and close-spaced sublimation (CSS). Two substrates (CdS and Indium–tin–oxide) were used with the physical vapor deposition(PVD)films specifically to isolate the effects of the Cd ( S x Te 1−x ) alloy formed during the treatment of filmsdeposited on CdS. The samples were analyzed by x-ray diffraction(XRD),atomic force microscopy(AFM), and photoluminescence. The observed changes in microstructure were caused by recrystallization, which consisted of the nucleation and development of a new CdTestructure and subsequent grain growth. Nevertheless, for these processes to take place, it was necessary that enough lattice-strain energy was available in the films. For this reason, PVDfilms did recrystallize, while CSS films did not. For the first time, recrystallization was observed directly in AFM images of CdTefilms and confirmed by XRD analysis, which indicated the existence of two lattice parameters in PVD samples treated at 350 °C. For samples treated at 400 °C, the CdCl 2 treatment improved the minority-carrier lifetime of the films by more than one order of magnitude. This improvement was attributed to the elimination of deep defect levels within the band gap of the CdTefilms as a result of the treatment. The sulfur diffusion into CdTefilmsdeposited on CdS, during the CdCl 2 treatment at 400 °C, strongly affected the defect structure.