Transition metal nitrides
- Tantalum nitride
- Scandium nitride
Tantalum nitride
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)
Scandium nitride
The ‘Unexplored' Early Transition Metal Nitrides
The early transition metal (refractory) nitrides are relatively unexplored to date, both theoretically and experimentally. They exhibit unique properties such as high hardness, brittleness, high melting point, and in some, superconductivity. They have technological applications in the area of, for example, hard coatings for cutting tools and possible potential for applications in magnetic recording and sensing. Recently there has been an increasing interest in these materials systems with the aim of further exploring their properties and exploiting them in industrial applications.
Recently using the screened-exchange local density approximation, we have shown that in contrast to earlier understanding ScN, YN, and LaN are narrow band-gap semiconductors and not semimetals. [C. Stampfl, W. Mannstadt, R. Asahi, and A. J. Freeman, Phys. Rev. B 63, 155106 (2001)] Therefore, these early transition metal (refractory) nitrides represent a new system of semiconducting materials.
The prospect of additional semiconducting III-N materials to those of technologically important GaN, AlN, and InN (e.g., in relation to optoelectronic (and high temperature) devices such as blue laser diodes, opens up the possibility of their complementary use in such GaN-based technologies. If this be so, then the nature of their surfaces becomes an important consideration; for example, the electronic properties, quality and stability, and the ability to form atomically smooth surfaces and interfaces.
For the ScN(001) surface, our calculations predict that the ideal-relaxed surface has the lowest formation energy for most of the range of the allowed chemical potentials - and is semi-conducting – while N-deficient structures, which are predicted to form for Sc-rich conditions, are metallic in nature. [C.Stampfl and A.J. Freeman, Phys. Rev. B 65, 161204(R) (2002)]
Our calculations predict the detailed atomic structure, which the scanning tunneling microscopy experiments were unable to determine, and are consistent with other observed properties: Below left a surface created under metal-rich conditions which is reported to be of metallic nature, and below right, the surface created under N-rich conditions which is found to be semi-conducting, in accord with our calculations.
Left: STM image created under Sc-rich conditions [A. Smith et al, J. Appl. Phys. 90, 1809 (2001)]
Right: STM image created under N-rich conditions [H. Al-Brithen et al.Appl. Phys. Lett. 77, 2485 (2000)]
