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 + S-2 atmosphere in the range 100-550 degreesC. 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 degreesC), and to move back for high T values (300-550 degreesC). 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 (V-Cd) and Cd interstitials (I-Cd) as effect of the phase transformation. The density of V-Cd and I-Cd was associated with the area (A) under the yellow emission (YE) band. A is maximum at T = 300 degreesC, the critical point of the phase transformation. (C) 2001 Elsevier Science B.V. All rights reserved.