Niobium nitride grown on III-nitrides by MBE: Epitaxial growth and phase transitions, prospects for hybrid superconductor/metal/semiconductor heterostructures on silicon

Abstract

III–N nitride semiconductor materials have made remarkable breakthroughs in the fields of optoelectronics and electronics using epitaxial heterostructures combining binary and ternary alloys (GaN, AlN, InGaN, and AlGaN). Nowadays, it is interesting to study the epitaxial integration of other nitride materials to implement new functionalities in III-nitride semiconductors. Niobium nitride (NbN), usually deposited by sputtering, is a well-established superconducting metal; therefore, it is interesting to investigate NbN epitaxy and its integration with III-nitrides. Using ammonia-molecular beam epitaxy (MBE-NH3), the epitaxial growth of NbN thin films on aluminum nitride (AlN) grown on silicon is studied. Three different phases, δ-NbN, the superconducting cubic-phase, β-Nb2N, the niobium-rich hexagonal phase, and ε-NbN, the 1:1 stoichiometric hexagonal phase, are stabilized. Phases are characterized ex situ by x-ray diffraction, atomic force microscopy, and scanning transmission electron microscopy. The three phases are also well identified in situ by specific reflection high energy electron diffraction patterns, which allow to study phase transitions during annealing. An experimental phase diagram is established. Various heterostructures combining III-N materials and NbN are epitaxially grown on silicon substrates, demonstrating the potential of this system for fabricating epitaxial hybrid superconductor/metal/semiconductor heterostructures.