Electron drift velocity in high electric fields in zincblende, wurtzite and rocksalt GaN

The influence of GaN crystal symmetry (zincblende, wurtzite, and rocksalt) on the drift velocity of electrons in high electric fields is studied employing Monte Carlo method. The highest drift velocity as well as the most pronounced negative differential mobility is predicted in the zincblende phase. Wurtzite-phase crystals turn out to be inferior in these respects due to the complications of electron and phonon dispersion brought by the lowering symmetry. Experimental data on wurtzite phase reported by Wraback et al. are in excellent agreement with the data of modelling taking into account intense (low-energy phonon-mediated) scattering of electrons between the lowest- and the closely-located upper gamma-valleys. Barker et al. revealed experimentally the velocity-field dependence in cubic GaN to be higher than that in wurtzite phase, however, the experimental drift velocity turned out to be lower than the theoretical one. This might be the signature of coexisting different phases in experimental samples. The rocksalt GaN (still expecting experimental verification) is predicted to manifest the lowest drift velocity of electrons and no negative differential mobility, as a result of the fast exchange of electrons between the lowest-energy X-valleys preventing electron runaway to the upper gamma-valley.