Heteroepitaxial Growth, Degenerate State, and Superconductivity of Perovskite-Type LaWN3 Thin Films

Abstract

LaWN3 epitaxial thin films were successfully grown on α-Al2O3 (0001) substrates with a multi-cathode radio frequency magnetron sputtering gun system using metal targets and nitrogen gas. A high substrate temperature (>1000 °C) was a key requirement for heteroepitaxial growth, and the optimum substrate temperature, in terms of crystallinity, was determined to be 1125 °C at the optimized cathode power densities. The epitaxial film did not exhibit ferroelectricity, which has been predicted by first-principles calculations; however, n-type degenerate semiconducting behaviors were observed together with a high electron density of 1.9 × 1022 cm–3 and an enlarged optical band gap of 1.83 eV, owing to band-filling. Furthermore, superconductivity was observed at 0.73 K. The degenerate state and superconductivity originate from unavoidable off-stoichiometry in the films. Thus, alternative growth processes that achieve a higher nitrogen chemical potential are necessary to decrease the carrier density and allow for the emergence of the ferroelectric properties.