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

• Received 21 December 2001

DOI:https://doi.org/10.1103/PhysRevB.65.245110