Acceptor binding energies in GaN and AlN

We employ effective-mass theory for degenerate hole bands to calculate the acceptor binding energies for Be, Mg, Zn, Ca, C, and Si substitutional accepters in GaN and ALN. The calculations are performed through the 6 x 6 Rashba-Sheka-Pikus and the Luttinger-Kohn matrix Hamiltonians for wurtzite (WZ) and zinc-blende (ZB) crystal phases, respectively. An analytic representation for the acceptor pseudopotential is used to introduce the specific nature of the impurity atoms. The energy shift due to polaron effects is also considered in this approach. The ionization energy estimates are in very good agreement with those reported experimentally in WZ GaN. The binding energies for ZB GaN accepters are all predicted to be shallower than the corresponding impurities in the WZ phase. The binding-energy dependence upon the crystal-field splitting in WZ GaN is analyzed. Ionization levels in AlN are found to have similar "shallow" values to those in GaN, but with some important differences which depend on the band structure parametrizations, especially the value of the crystal-field splitting used.