Optical properties of polycrystalline CdS films

Optical spectra of CdS films vacuum evaporated onto fused silica substrates were measured in reflection and transmission for photon energies from 0.5 to 6.5 eV. For films of about 2000 Å thickness (determined from the optical measurements), the spectral structure, especially from 4 to 6 eV, differs markedly from that of single crystals and depends on the substrate temperature (up to about 200 °C) during deposition. The changes are correlated with critical points in the joint density of states from band-structure calculations and are attributed to grain-boundary disorder in the CdS on room-temperature substrates, together with some surface roughness on the high-temperature substrates.