Molecular dynamics study of damage accumulation in GaN during ion beam irradiation

We have used molecular dynamics methods to study the accumulation of damage during ion beam irradiation of GaN. First we analyzed individual recoils between 200 eV and 10 keV. We found that the spatial average of the threshold displacement energy was high and much less damage was produced in GaN than in Si, Ge, or GaAs cascades. Most of the damage was in isolated point defects or small clusters, which enhances the damage recombination probability. Ion beam amorphization was simulated by starting successive 400-eV or 5-keV recoils in an initially perfect crystal. The development of volume, energy, and ring statistics, as well as segregation of compounds, was followed through the process. The simulations show that the amorphization begins with single defects and formation of long weak Ga-Ga bonds in the distorted lattice. We also observe that nitrogen gas is produced during prolonged irradiation, in agreement with experimental observations. We recognize two reasons for the high amorphization dose of GaN, the high threshold displacement energy and different varieties of in-cascade recombination.