Integrating Novel Nitride Barriers and Conventional Nitride Superconductors into Epitaxial Junctions: Early Steps toward New Materials Platforms for Quantum Computing

Although nitrogen is approximately four times more prevalent than oxygen in the Earth’s atmosphere, the stability of dinitrogen leads to an order of magnitude fewer known nitrides than oxides. Despite this, many of the known nitrides are important technological materials. Transition metal nitrides (<i>TM</i>Ns) in the rocksalt crystal structure are one such example: these chemically stable, high-symmetry superconductors are well suited for epitaxial thin film integration with common substrates. To extend the functionality of <i>TM</i>Ns and create epitaxial superconductor–semiconductor heterojunctions, suitable rocksalt-structured semiconductors are required. Our team has discovered a series of new rocksalt-structured nitride semiconductors with band gaps ranging from <i>E</i>_G=0.9-2.4 eV, described as either Mg<i>TM</i>N₂ (<i>TM</i>=group 4 transition metal) or Mg₂<i>TM</i>N₃ (<i>TM</i>=group 5 transition metal). The lattice parameters of these new materials are compatible with <i>TM</i>Ns, suggesting the possibility for integration into epitaxial structures, such as Josephson junctions and other circuitry forming superconducting qubits. Here, we study the epitaxial integration of <i>TM</i>Ns on Al₂O₃ substrates and of Mg<i>TM</i>N₂ and Mg₂<i>TM</i>N₃ compounds onto <i>TM</i>Ns. We optimize <i>TM</i>N growth to repeatably achieve high <i>T</i>_C for epitaxial NbN (15K), TiN (5K), and Nb_0.5Ti_0.5N (13K). Using electron and X-ray probes we show that heteroepitaxy can be readily achieved between <i>TM</i>Ns and the novel semiconductors. Epitaxial NbN-Mg₂NbN₃-NbN trilayer heterojunctions grown on sapphire are demonstrated with barrier layers down to ~3 nm and crystal coherence across the entire heterojunction stack. However, we also find that the inherent polymorphism of NbN complicates it’s use, motivating the eventual use of other <i>TM</i>Ns such as Nb_0.5Ti_0.5N alloys.