Epitaxial AlBN/β‐Nb2N Ferroelectric/Superconductor Heterostructures

We report the growth of AlBN/β‐Nb 2 N nitride epitaxial heterostructures in which the AlBN is ferroelectric, and β‐Nb 2 N with metallic resistivity ≈40 μ at 300 K becomes superconducting below T C ≈ 0.5 K. Using nitrogen plasma molecular beam epitaxy, we grow hexagonal β‐Nb 2 N films on c‐plane Al 2 O 3 substrates, followed by wurtzite AlBN. The AlBN is in epitaxial registry and rotationally aligned with the β‐Nb 2 N, and the hexagonal lattices of both nitride layers make angles of 30° with the hexagonal lattice of the Al 2 O 3 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‐Nb 2 N/Al 2 O 3 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.