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

Abstract

We identify the previously unknown mechanism whereby rocksalt ${\mathrm{Ta}}_{x}\mathrm{N}$ 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 ${(V}_{\mathrm{Ta}}).$ The observed enhanced resistivity, transition from electron to hole conduction at $x\ensuremath{\sim}0.6,$ and diminished mid-IR reflectance are consistent with the dominance of the ${V}_{\mathrm{Ta}}$ defect in nitrogen-rich material.