Growth, structural and optical properties of Cu3N films

Thin films of Cu3N were grown on (0 0 1) MgO and (0 0 1) Cu substrates by molecular beam epitaxy of copper in the presence of nitrogen obtained from a radio frequency atomic source. The different aspects of the growth and the properties of the copper nitride films were investigated in relation with growth parameters such as: substrate temperature, growth rate and flux of nitrogen. On MgO substrates, despite a mismatch as high as ∼10%, the Cu3N films grow as epitaxial layers even at room temperature. The upper temperature limit for growth was found to be ∼250 °C. It is likely that the growth mechanism proceeds in a layer-by-layer fashion similar to the growth of γ′-Fe4N layers on MgO substrates using the same growth method. This results in the growth of smooth layers of Cu3N. lms, grown as a pure phase, have a behavior typical for an insulator with an optical gap of 1.65 eV. If a low amount of Cu impurities is present in the Cu3N insulating matrix, the films still have the overall behavior of an insulator, but with a reduced optical band gap. For higher amounts of metal impurities, seen as Cu crystallites in the epitaxial Cu3N phase, the optical band gap reduces even further, and additionally, an extra absorption peak due to Cu appears. The films grown on (0 0 1) Cu substrates, despite the better lattice match, did not show a striking improvement of the crystal quality.