Transition temperatures and crystal structures of singlehyphen;crystal and polycrystalline NbNxfilms

摘要

Singlehyphen;crystal cubic NbNxfilms with thicknesses of a few thousand angstroms were epitaxially grown on cleaved (100) planes of singlehyphen;crystal MgO plates by the vapor phase growth technique. By heat treatment of some of these films in NH3plus;H2or in H2alone, films having the superlattice structures of Nb4N3, NbN, or Nb4N5were prepared. The maximumTcwas observed in cubic NbNx. Both Nb4N3and Nb4N5, the tetragonal phases with longhyphen;rangehyphen;ordered arrangement of vacancies, exhibited superconductivity. NbN1.0and Nb5N6, both with hexagonal structure, did not exhibit superconductivity down to 1.77 K. As the composition of cubic NbNxbecame close to stoichiometric, itsTcincreased. However, the maximumTcof cubic NbNxwith nearly stoichiometric composition was limited by the lattice instability of cubic NbNxand by the resulting cubic NbNxto hexagonal NbN transformation. It might be expected that a higherTcfor compounds of the NbN family could be obtained if a NaClhyphen;type crystal with less vacancies could exist as a stable or metastable phase at ordinary temperatures.