Energy spike effects in ion-bombarded GaN

We study structural disorder in GaN bombarded at room temperature with 1.3 keV amu(-1) PF(n) (n = 0, 2 and 4) cluster ions. Results are compared with our previous studies of irradiation with atomic ions of different masses. An algorithm for cascade density calculations that take into account the formation of subcascades is presented. Quantitative analysis of both new and previous data shows that an increase in the cascade density above a certain critical value results in a rapid increase in the rate of planar amorphization and the rate of damage buildup in the crystal bulk. Both such rates increase with decreasing sample temperature. This threshold-like behaviour suggests an important role of nonlinear energy spikes in the formation of stable implantation disorder in GaN. We also discuss the striking difference between cascade density effects in damage buildup in different semiconductors, including GaN, ZnO and Si.