Transparent p-type Cu xS thin films

Abstract

The effect of different mild post-annealing treatments in air, at 270 °C, for 4–6 min, on the optical, electrical, structural and chemical properties of copper sulphide (Cu x S) thin films deposited at room temperature are investigated. Cu x S films, 70 nm thick, are deposited on glass substrates by vacuum thermal evaporation from a Cu 2S:S (50:50 wt.%) sulphur rich powder mixture. The as-deposited highly conductive crystalline CuS (covellite) films show high carrier concentration (∼10 22 cm −3), low electrical resistivity (∼10 −4 Ω cm) and inconclusive p-type conduction. After the mild post-annealing, these films display increasing values of resistivity (∼10 −3 to ∼10 −2 Ω cm) with annealing time and exhibit conclusive p-type conduction. An increase of copper content in Cu x S phases towards the semiconductive Cu 2S (chalcocite) compound with annealing time is reported, due to re-evaporation of sulphur from the films. However, the latter stoichiometry was not obtained, which indicates the presence of vacancies in the Cu lattice. In the most resistive films a Cu 2O phase is also observed, diminishing the amount of available copper to combine with sulphur, and therefore the highest values of optical transmittance are reached (65%). The appearance on the surface of amorphous sulphates with annealing time increase is also detected as a consequence of sulphur oxidation and replacement of sulphur with oxygen. All annealed films are copper deficient in regards to the stoichiometric Cu 2S and exhibit stable p-type conductivity.