First-principles pseudopotential calculations have been performed on GaN and AlN in the wurtzite and zinc-blende structures. The mixed-basis approach is employed due to the localized nature of the valence charge density in these materials. In the stress calculation within the mixed-basis set, a correction term is introduced to the stress expression in order to make it consistent with the pressure given by the total-energy calculations. The lattice constants in the wurtzite structure are in good agreement with the experimental data. The band gap appears to be direct except for zinc-blende AlN, which has the conduction-band minimum at the X point. The effective mass of the electron is found to be nearly isotropic for both wurtzite GaN and AlN. The agreement of the optical Γ-phonon frequencies with the Raman experimental data is excellent for wurtzite GaN and good for wurtzite AlN, except for ${\mathit{A}}_1$–transverse-optical (${\mathit{A}}_1$-TO) mode. The calculated ${\mathit{A}}_1$-TO mode frequency of AlN is 11% smaller than the experimental value. Both GaN and AlN are found to have the wurtzite structure in the ground state.

• Received 19 March 1993

DOI:https://doi.org/10.1103/PhysRevB.48.7897