Structural and transport properties of CdS films grown on Si substrates

Abstract

Epitaxial films of CdS have been grown on thermally cleaned single-crystal Si substrates. The films were grown by electron-beam evaporation of CdS powder in a UHV system. X-ray Laue and transmission electron diffraction techniques have been used to examine the structure of the films. It has been found that epitaxial growth occurred on (111) substrates at a deposition temperature of approximately 340 °C. Polycrystalline films were obtained at temperatures below 300 °C on (111) substrates. Films grown on (100) substrates were polycrystalline over the range of deposition temperatures 100–390 °C. The electron transport properties of these films have been studied. The Hall mobility measurements of films grown on (111) substrates gave a value of 320 cm2(V sec)−1 for the epitaxial films, decreasing to approximately 100 cm2(V sec)−1 for polycrystalline films grown at 200 °C. For films grown on 〈100〉 substrates the Hall mobility ranged from 60 cm2(V sec)−1 at 200 °C to approximately 90 cm2(V sec)−1 at 340 °C. The electrical resistivity of the epitaxial films was 20 Ω cm and that of the polycrystalline films ranged from 10 to 300 Ω cm. Photoconductivity measurements of these films showed a relatively high trap density extending to greater than 1 eV above the valence band edge. A peak in the response at 800 nm is thought to be due to that of the Cd vacancies.