Phase-selective sputter epitaxy of <i>δ</i> -NbN, <i>γ</i> -Nb4N3, and <i>β</i> -Nb2N on atomically flat AlN

We report the phase-selective epitaxy of δ-NbN, γ-Nb4N3, and β-Nb2N thin films on AlN/sapphire templates by sputtering. Structural analysis revealed coherent epitaxial growth with sharp x-ray diffraction fringes, phase-dependent surface reconstructions observed by reflection high-energy electron diffraction, and atomically flat morphologies with a root-mean-square roughness below 1 nm. Reciprocal space mapping confirmed lattice coherence, while rocking curves exhibited narrow full width at half maximum values for δ-NbN and β-Nb2N, with slightly broader peaks for γ-Nb4N3. Transport measurements showed distinct superconducting behaviors: δ-NbN displayed an insulating-like resistivity upturn with Tc ∼ 12 K, γ-Nb4N3 exhibited metallic behavior with Tc ∼ 10 K, and β-Nb2N showed metallic conduction with Tc as low as 0.4 K, indicating high phase purity. The retention of high Tc values in ultrathin films underscores the robustness of sputter-grown NbNx on AlN. These results define the growth windows for multiple NbNx phases and establish AlN as a versatile platform for integrating superconductivity with III-nitride semiconductors in future hybrid devices.