Simultaneous observation of strong and weak quantum confinement effect in chemically deposited CdSe thin films: A spectro-structural study

CdSe thin films deposited chemically on glass substrates for 4, 8, and 16 h, and subsequently annealed at 400 °C for 1 h, have been studied by a combination of spectroscopic (photoluminescence and Raman scattering) and structure-determining (x-ray diffraction and atomic force microscopy) techniques. Due to a size distribution of constituent grains, photoluminescence spectra of the as-deposited films show weak but broad bands at ∼2.2 eV (strongly confined band) and ∼1.73 eV (weakly confined band). On annealing, intensity of the weakly confined band, at ∼1.7 eV increases as a result of an improvement in the crystalline quality of CdSe nanoclusters. A surface-optic Raman mode at ∼250 cm−1 in as-deposited samples has been observed for the first time. The x-ray diffraction studies of annealed samples show a diffraction peak at 2θ=13° from the (001) plane. The improvement in crystallinity of the films as observed by atomic force microscopy and photoluminescence techniques, the appearance of (001) reflection in the x-ray diffraction pattern, the disappearance of surface-optic Raman mode, and the enhancement of weakly confined band—all as a consequence of annealing—have been discussed and correlated with each other. A film deposition mechanism has been described, which explains the origin of the simultaneous existence of strong and weak quantum confinement effects; the significance this observation in the development of high efficiency photovoltaic solar cells has been emphasized.