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)]