Mechanism and control of the metal-to-insulator transition in rocksalt tantalum nitride

We identify the previously unknown mechanism whereby rocksalt TaₓN can be continuously tuned from conducting to insulating through changes in stoichiometry. Experimental measurements on thin films, combined with electronic structure calculations on a host of native defects, show that the tunability arises from changes in the free electron concentration as a result of localization at Ta vacancies (VTa). The observed enhanced resistivity, transition from electron to hole conduction at x~0.6, and diminished mid-IR reflectance are consistent with the dominance of the VTa defect in nitrogen-rich material.