The Metallic to Insulating
Transition in Tantalum Nitride
Through detailed experiments and density functional theory
calculations, we identify the previously unknown mechanism whereby rocksalt
TaxN can be continuously tuned from conducting to insulating
through changes in stoichiometry, as controlled experimentally by the gas
pressure and temperature. The tunability arises from changes in free
electron concentration as a result of localization at Ta-vacancies. 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 Ta-vacancy defect in nitrogen-rich material which we find from our
first-principles calculations has the lowest formation energy. Conversely,
our calculations predict that for more tantalum-rich conditions, nitrogen
vacancies, and other Ta-rich phases, will form [L. Yu, C. Stampfl, D. Marshall,
T. Eshrich, V. Narayanan, J. M. Rowell, N. Newman, and A. J. Freeman,Phys.
Rev. B 65, 245110 (2002); C. Stampfl and A.J. Freeman, Phys. Rev. B, 67,
045408 (2003)]
For increasing concentrations of
Ta-vacancies, the density of states at the Fermi level is reduced, leading
to a decrease in conductivity and an increase in resistivity, as observed
experimentally.
Below is a purely insulating
phase, tritantalum pentanitride which is predicted to form for strongly
N-rich conditions (C. Stampfl and A.J. Freeman, in preparation)
