Photoluminescence analysis of CdS thin films under phase transition

Using the chemical bath deposition (CBD) method, CdS thin films in the cubic metastable phase were grown on glass substrates. On thermal annealing (TA) in Ar+S-2 atmosphere in the 240-510 degrees C temperature range, the samples transform to the wurtzite phase. This hexagonal crystalline phase is the stable phase for CdS. From X-ray diffraction analysis the transition point from zincblende to wurtzite is found to occur in the 240-300 degrees C region. From photoacoustic absorption spectra the energy band gap E(g) of the semiconductor was calculated. A plot of E(g) vs. TA shows a minimum value (2.28 eV) at 300 degrees C, in the region of the transition point. Photoluminescence measurements, in the 1.40-2.60 eV emission energy range, show the 'well known' green emission band for the as-grown sample and for those annealed at high temperatures (360-510 degrees C). For samples annealed in the intermediate region (240 and 300 degrees C), a second band located at 2.2 eV is present. This band, denominated the yellow band and arising in the region of temperature of the transition point, is ascribed to Cd interstitials. A mechanism relating the Cd interstitials to the phase transformation is proposed.