Photoluminescence in cubic and hexagonal CdS films

Cadmium sulfide (CdS) thin films were prepared in cubic structural phase by chemical bath. A set of 25 samples were subjected to thermal annealing (TA) in Ar+S2 atmosphere in the range 100–550°C. Cubic sphalerite (S) CdS is transformed in hexagonal wurtzite (W) with annealing, where S is the metastable modification and W the stable phase. The gradual transformation from S to W can be observed by X-ray diffraction (XRD) measurements. Photoluminescence (PL) spectra in the interval 1.2–2.6 eV of photon energy, display the green emission (GE) band centered at 2.4 eV. This band seems to shift to lower energies for low temperature (T) values of TA (100–300°C), and to move back for high T values (300–550°C). This phenomenon is explained by the arising of a yellow band emission centered at 2.2 eV. The yellow band is originated from the formation of Cd-vacancies (VCd) and Cd interstitials (ICd) as effect of the phase transformation. The density of VCd and ICd was associated with the area (A) under the yellow emission (YE) band. A is maximum at T=300°C, the critical point of the phase transformation.