Epitaxial AlBN/β‐Nb2N Ferroelectric/Superconductor Heterostructures

We report the growth of AlBN/β‐Nb2N nitride epitaxial heterostructures in which the AlBN is ferroelectric, and β‐Nb2N with metallic resistivity ≈40 μ at 300 K becomes superconducting below TC ≈ 0.5 K. Using nitrogen plasma molecular beam epitaxy, we grow hexagonal β‐Nb2N films on c‐plane Al2O3 substrates, followed by wurtzite AlBN. The AlBN is in epitaxial registry and rotationally aligned with the β‐Nb2N, and the hexagonal lattices of both nitride layers make angles of 30° with the hexagonal lattice of the Al2O3 substrate. The B composition of the AlBN layer is varied from 0 to 14.7%. It is found to depend weakly on the B flux, but increases strongly with decreasing growth temperature, indicating a reaction rate‐controlled growth. The increase in B content causes a non‐monotonic change in the a‐lattice constant and a monotonic decrease in the c‐lattice constant of AlBN. Sharp, abrupt epitaxial AlBN/β‐Nb2N/Al2O3 heterojunction interfaces and close symmetry matching are observed by transmission electron microscopy. The observation of ferroelectricity and superconductivity in epitaxial nitride heterostructures opens avenues for novel electronic and quantum devices.