Optical properties of CdxZn1-xSe (0 ≤ x ≤ 1) thin films

Thin films of CdxZn1-xSe( x = 0, 0.5, 0.8, 1) were formed onto chemically and ultrasonically cleaned glass substrates in vacuum (10-5 Torr) thermal evaporation of CdSe and ZnSe powders at various substrate temperatures. Substrate temperatures were varied from 35 to 2000C for each composition of the films. Optical transmittance and reflectance of the films were measured by uv-vis-nir spectrophotometer covering photon wavelength range between 250 and 2500 nm. From the optical transmittance and reflectance, the absorption coefficients of the films were calculated utilizing a standard theory. The variation of absorption coefficient with photon energy was fitted to the existing models to ascertain the nature and extent of band-gap of the samples. The dependence of transmittance, absorption coefficient and band-gap energy of the films on photon wavelength, for various compositions and substrate temperatures, was critically analyzed. Almost all the films exhibit tail absorption below the fundamental absorption edge. The origin of the tail absorption is probably the localized or impurity state in the sample. The films have high absorption coefficient (∼105 cm-1) above the fundamental edge. The energy gap of the samples was found to be direct- allowed. The energy gap of CdxZn1-xSe films varies from 1.7 to 2.0 eV as x varies from 1 to 0. The energy gap for x=1 ( i.e., for CdSe) is in conformity with the literature values but for x=0 (i.e., for ZnSe) in somewhat lower. This discrepancy may be attributed to the existence of some gap states. The band-gap energies of CdxZn1-xSe vary almost inversely with x values. (Author)