Study of annealing-induced changes in CdS thin films using X-ray diffraction and Raman spectroscopy

We have investigated as grown and annealed (300 deg. C, 400 deg. C and 500 deg. C) thin films of CdS grown on GaAs (001) by chemical bath deposition. X-ray diffraction (XRD) shows that the as grown CdS film is polycrystalline and predominantly cubic. A residual compressive stress of the order of 1.45% in the as grown film relaxes on annealing the film at 300 deg. C. Furthermore, CdS film undergoes a structural phase transition from the metastable cubic phase to the stable hexagonal phase, when, annealed at 500 deg. C. This is accompanied by significant improvement in crystalline quality of the film. Line shape analysis of the asymmetry of the longitudinal optical phonon shows a disorder-activated mode, which correlates well with the crystalline quality estimated from XRD and photoluminescence measurements. The additional features observed in the Raman spectra ∼ 254 cm-1 and 309 cm-1 are investigated using temperature dependent Raman spectroscopy and identified as superposition of transverse optical: E1 (TO) and E2 phonons at q = 0 and combination mode (two zone-edge E2 phonons) respectively