Study of the influence of annealing on the properties of CBD-CdS thin films

Abstract CdS and In(S, OH) films deposited by chemical bath deposition (CBD) on SnO2 substrates have been studied by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy (XPS) and optical absorption (infrared, visible and near UV) measurements. It is shown that, even after air annealing half an hour at 623 K, the CdS films are poorly crystallized. More precisely the films are neither cubic nor hexagonal but they crystallize in polytype structures. At the end of the depositing process the films are quite inhomogeneous, large clusters are randomly distributed on the thin films. After sonically cleaning these heaps are removed from the films. The under layer appears very homogeneous with very high coverage efficiency and good optical properties (Eg=2.5 eV). XPS measurements have shown that some oxygen compound contaminants are present in the CdS films such as CdO, Cd(OH)2, CdCO3 and absorbed H2O, while some CdSO4 is present at the surface of the films. Moreover, XPS depth profiles have shown that there is some Cd diffusion in the under layer during the CBD process. During annealing, there is some surface sulfur loss with some SO2 volatile compound formation and a part of the absorbed water escapes from the film. The other information given by the depth XPS profile it that the annealing does not modify the atoms distribution. It is shown that there is no sulfur diffusion in the SnO2 film, while the Cd diffusion does not change during the heating process. It is shown that in the case of In(S, OH), the films are still amorphous after an annealing of half an hour at 623 K. The surface of these amorphous films is quite inhomogeneous in such a way that XPS profiles are difficult to interpret. However, XPS quantitative analysis show that the films are sulfur deficient and that some OH are present in the films, even if after annealing this OH contribution decreases. From this study it can be concluded that the more homogeneous films (CdS) allow to obtain more reproducible results.

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