Electronic and structural properties of GaN by the full-potential linear muffin-tin orbitals method: The role of the<i>d</i>electrons

The structural and electronic properties of cubic GaN are studied within the local-density approximation by the full-potential linear muffin-tin orbitals method. The Ga 3d electrons are treated as band states, and no shape approximation is made to the potential and charge density. The influence of d electrons on the band structure, charge density, and bonding properties is analyzed. Due to the energy resonance of Ga 3d states with nitrogen 2s states, the cation d bands are not inert, and features unusual for a III-V compound are found in the lower part of the valence band and in the valence charge density and density of states. To clarify the influence of the d states on the cohesive properties, additional full- and frozen-overlapped-core calculations were performed for GaN, cubic ZnS, GaAs, and Si. The results show, in addition to the known importance of core-valence exchange-correlation nonlinearity, that an explicit description of closed-shell interaction has a noticeable effect on the cohesive properties of GaN. Since its band structure and cohesive properties are sensitive to a proper treatment of the cation d bands, GaN appears to be somewhat exceptional among the III-V compounds and reminiscent of II-VI materials.