Morphological and chemical study of CdTe thin films annealed in CHClF2–O2 gas mixture

Abstract

The effect of annealing in CHClF2–O2 gas mixture on morphological and chemical properties of CdTe thin films deposited by conventional close space sublimation (CSS) has been investigated. Annealing was done at 400°C for 5min in a mixture of CHClF2 and O2 with partial pressures of 50 and 160mbar, respectively, and to reach a total pressure of 800mbar the annealing chamber was filled with Ar. After annealing, residuals at the surface of films were not removed in order to be study. High Resolution Scanning Electron Microscopy (HR-SEM) and X-ray Photoelectron Spectroscopy (XPS) were employed for analysis. The annealing promotes the etching of the CdTe grains at faces and edges; as a consequence, the surface became smooth with a compact structure. The main improvement on morphology by the inclusion of O2 during the annealing was the reduction of the grain boundary width of films. Chemical analysis of samples annealed in CHClF2 with and without mixed oxygen showed that the oxygen in the gas mixture acts as a catalyst in the decomposition of the CHClF2. Annealing with the CHClF2–O2 mixture reduces the oxidized compounds at the surface of films and promotes incorporation of Cl in the bulk. The experimental results give a clear demonstration why CdS/CdTe solar cells fabricated using CHClF2–O2 during chlorination treatments have better performance, as referred in a previous work.