Nitrogen-Vacancy Engineering for Controlled Phase Transitions in CrN(111) Epitaxial Films

The phase transition in CrN epitaxial films is substantially suppressed by epitaxial constraint. Here, we propose that nitrogen (N) vacancies can be taken as a knob to regulate the phase transition of CrN(111) epitaxial films. To validate this concept, a series of CrN(111) films with controlled N concentrations (approximately from 0.0 to 5.0 at.%) were epitaxially grown on Al2O3(0001) substrates. Experimental characterization reveals that higher N vacancy concentrations significantly facilitate the out-of-plane contraction of the films at 273 K (0.8%), reaching up to 60% of the contraction magnitude of CrN powders (1.2%) without compromising the stability and reproducibility of the phase transition. Reducing N vacancy concentrations diminishes the lattice contraction, lowers the phase transition temperature to 193 K, and triggers a metallic to insulator transition in electrical behavior. First-principles calculations corroborate these findings, showing that N vacancies decrease the internal tensile stress within triangular Cr atomic layers, which enhances the out-of-plane contraction, elevates phase transition temperatures, and promotes bandgap closure. These results establish N vacancies as a critical factor governing phase transition dynamics in CrN systems and provide a practical strategy for successively engineering thermally responsive phase transitions in CrN films, advancing their potential for functional device applications.