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 al 400 degrees 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 similar to 2.2 eV (strongly confined band) and similar to 1.73 eV (weakly confined band). On annealing, intensity of the weakly confined band, at similar to 1.7 eV increases as a result of an improvement in the crystalline quality of CdSe nanoclusters. A surface-optic Raman mode at similar to 250 cm(-1) in as-deposited samples has been observed for the first time. The x-ray diffraction studies of annealed samples show II diffraction peak at 2 theta= 13 degrees 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 made, and the enhancement of weakly confined band-all as a consequence of annealing-have been discussed and correlated with 1 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. (C) 1997 American Institute of Physics.