Transparent p-type CuxS thin films

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 (CuxS) thin films deposited at room temperature are investigated. CuxS films, 70 nm thick, are deposited on glass substrates by vacuum thermal evaporation from a Cu2S:S (50:50 wt.%) sulphur rich powder mixture. The as-deposited highly conductive crystalline CuS (covellite) films show high carrier concentration (∼1022 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 CuxS phases towards the semiconductive Cu2S (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 Cu2O 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 Cu2S and exhibit stable p-type conductivity.