A comparison of the optical properties of crystalline and amorphous CdS

Abstract

Polycrystalline and amorphous films of CdS have been vacuum deposited onto fused sapphire substrates. The polycrystalline samples are obtained using heated substrates and the amorphous samples are grown on cooled substrates. The material is considered to be amorphous if the crystallite size is smaller than 20 Å as determined by X-ray diffraction.Normal incidence reflection and transmission have been measured by the sample-in, sample-out technique over a range of wavelengths (2600 Å < λ < 9000 Å) for both polycrystalline and amorphous materials. The complex index of refraction has been calculated using a smooth curve thickness variation method developed by the authors. The polycrystalline results agree well with previous values except in the ultraviolet portion of the spectrum. In this region, the crystalline films exhibit surface roughness due to the crystallite grains and hence cause the measured reflectivity and calculated index of refraction to be smaller than the bulk values. The amorphous material does not show this decrease at high energies because the grain size is much smaller. In the amorphous material the absorption edge is shifted to lower energies and is no longer a direct transition as in the crystalline case, but is a nondirect transition. The optical data are consistent with a model of an amorphous material which assumes that, at low photon energies, transitions take place from localized to extended states (or vice-versa) while at higher energies they also occur between extended states.