Phase Identification and Ordered Vacancy Imaging in Epitaxial Metallic Ta₂N Thin Films

Epitaxial transition metal nitrides (TMNs) are an emerging class of crystalline thin film metals that can be heteroepitaxially integrated with common group III-nitride semiconductors such as GaN and AlN. Within a binary family of TMN compounds (i.e., TaN), several phases typically exist, many with similar crystal structures that are difficult to distinguish by conventional X-ray diffraction or other bulk characterization means. In this work, we demonstrate the combined power of high-resolution transmission and aberration-corrected scanning transmission electron microscopy for definitive phase identification of tantalum nitrides with different N-sublattice ordering. Analysis of molecular beam epitaxy-grown γ-Ta₂N films on SiC substrates shows that the films are γ phase, threading dislocation-free, and Ta-deficient. The lack of Ta manifests as ordered Ta vacancy planar defects oriented in the plane perpendicular to the [0001] growth direction and accounts for the substoichiometry. Optimization of the growth parameters should reduce the Ta vacancy concentration, and alternatively, exploitation of the attractive nature of the Ta vacancies may enable novel planar structures. These findings serve as an important first step in applying this epitaxial TMN material for new electronic and superconducting device structures.