Comparison of electron and hole initiated impact ionization in zincblende and wurtzite phase gallium nitride

In this paper, we present the first calculations of the electron and hole impact ionization coefficients for both wurtzite and zincblende phase GaN as a function of the applied electric field. The calculations are made using an ensemble Monte Carlo simulator including the full details of the conduction and valence bands derived from an empirical pseudopotential calculation. The interband impact ionization transition rates for both carrier species are determined by direct numerical integration including a wavevector dependent dielectric function. It is found that the electron and hole ionization coefficients are comparable in zincblende GaN at an applied field of similar to 3 MV/cm, yet vary to a slight degree at both higher and lower applied field strengths. In the wurtzite phase, the electron and hole coefficients are comparable at high fields bur diverge at lower applied fields. The most striking result is that the ionization rates are predicted to be substantially different for both carrier species between the two phases. It is predicted that the ionization rates for both carrier species in the zincblende phase are significantly higher than in the wurtzite phase over the full range of applied fields examined.