Electronic structure of defects and impurities in III-V nitrides. II. Be, Mg, and Si in cubic boron nitride

Abstract

The electronic structure of beryllium (Be), magnesium (Mg), and silicon (Si) impurities in zinc-blende boron nitride $(c\ensuremath{-}\mathrm{BN})$ were studied by using the tight-binding linearized muffin-tin-orbitals technique. Calculations were performed using 64-atom supercells centered on either a boron (B) or a nitrogen (N) lattice site. While Be and Mg impurities were substituted only to the B sublattice, substitutions for both B and N were considered in the case of the Si impurity. In each case, total-energy minimization was used to examine lattice relaxation near the impurity site, and the nature of chemical bonding between the impurity and the neighboring atoms of the host crystal was examined in detail. We find that Be and Mg substituted for B each create delocalized levels merged to the states at the valence-band edge. These partially occupied levels can result in $p$-type conductivity such as that which has been observed experimentally. In contrast to the behavior of isolated Be and Mg impurities in $c$-BN, we find that Si substituted at a B site induces delocalized impurity states that overlap with the conduction-band edge of the host. These levels can contribute to the $n$-type conductivity of Si-doped $c$-BN. Si substituted to the anion sublattice induces sharp, partially occupied, and highly localized levels within the forbidden gap (``deep acceptor levels''). The experimentally observed mixture of $n$- and $p$-type $c$-BN can therefore be accounted for by the presence of impurities of each type. The relaxation of the host lattice near the Be and Mg impurities is outward, as is the relaxation near the Si impurity substituted at a N site. Inward relaxation is predicted in the case of Si substituted for B. The total, orbital, and shell-projected densities of states for the impurity and up to six coordinational shells nearest the impurity are analyzed in detail. Charge-density plots relevant to the impurities are also presented.