  The density of states and the band diagrams were computed for diamond, cubic
boron nitrde (cBN), and hexagonal boron nitride (hBN) using a
Korringa-Kohn-Rostoker (KKR) scheme to investigate the shift of the Fermi level
by impurity-atom doping below 10 at.%. The dopant atoms were B and N for
diamond, Be, Si, and C for cBN, and Be and C for hBN. It was found that the
Fermi level was located at the valence band maximum or the conduction band
minimum in the following seven cases: (i) the B concentration was 0.3 at.% in
B-doped diamond, (ii) the N concentration was 0.4 at.% in N-doped diamond,
(iii) the concentration of Be substituting B was 0.9 at.% in cBN, (iv) the
concentration of Si substituting B was 0.3 at.% in cBN, (v) the concentration
of C substituting B was 0.3 at.% in cBN, (vi) the concentration of C
substituting N was 0.9 at.% in cBN, and (vii) the concentration of Be
substituting B was ~2 at.% in hBN. Each of these values indicates the critical
dopant concentration for semiconductor-to-metal transition. In B-doped diamond,
it serves a measure for the occurrence of superconductivity at low temperature.
