Midgap recombination centers in GaN induced by keV- to MeV-range electron-beam irradiation

Roles of electron-beam (EB) irradiation in the incorporation of midgap recombination centers (MGRCs) in n-GaN are investigated using a complementary combination of photoluminescence (PL), time-resolved PL, and positron annihilation spectroscopy. The irradiation energies were 137 keV, 675 keV, and 2 MeV, corresponding to the calculated displacements of only N atoms, Ga and N atoms in a 1:1 ratio, and Ga and N atoms in a 3:1 ratio, respectively. The relative PL intensities at 12 K of the yellow and red luminescence bands associated with carbon impurities and nitrogen-vacancy (VN)-related defects, respectively, normalized to the near-band edge emission intensity, varied with irradiation energy in a manner consistent with the calculated atomic displacement ratios. The PL lifetimes for the near-band edge emission at 293 K decreased with increasing fluence, irrespective of the irradiation energy, indicating that higher fluences resulted in higher MGRC concentrations. The most probable candidates for the dominant MGRCs were vacancy clusters comprising a Ga vacancy (VGa) and one or more VNs, namely, VGa(VN)n, which act as nonradiative recombination centers. While lower irradiation energies generate lower total concentrations of VGa and VN at a constant fluence, larger values of n were obtained at lower irradiation energies, reflecting the irradiation energy-dependent atomic displacement ratios. These results indicate that the structure of vacancy complexes is strongly influenced by irradiation energy and provide a basis for controlled manipulation of intrinsic defects via EB irradiation.