Single-crystal GaN epilayers were irradiated with heavy inert gas ions (2.3-MeV \(\hbox {Ne}^{8+}\) , 5.3-MeV \(\hbox {Kr}^{19+}\) ) to fluences ranging from \(1.0\times 1.0^{11}\,\hbox {to}\,1.0\times 1.0^{15}\,\hbox {ions}/\hbox {cm}^{2}\) . The strain-related damage accumulation versus ion fluences was studied using high-resolution X-ray diffraction (HRXRD) and ultraviolet–visible (UV–Vis) spectroscopy. The results showed that the damage accumulation was mainly dominated by nuclear energy loss. When the ion fluence was less than \(\sim 0.055\) displacement per atom (dpa), the lattice expansions and lattice strains markedly increased linearly with increasing ion fluences, accompanied by a slow enhancement in the dislocation densities, distortion parameters, and Urbach energy for both ion irradiations. Above this fluence ( \(\sim 0.055\,\hbox {dpa}\) ), the lattice strains presented a slight increase, whereas a remarkable increase was observed in the dislocation densities, distortion parameters, and Urbach energy with the ion fluences after both ion irradiations. \(\sim 0.055\,\hbox {dpa}\) is the threshold ion fluence for defect evolution and lattice damage related to strain. The mechanisms underlying the damage accumulation are discussed in detail.
