We demonstrated a wide range of magnesium (Mg) doping control (1016–1020 cm−3) in a GaN layer grown by metalorganic vapor phase epitaxy on a freestanding GaN substrate and investigated the defect states at low and high Mg concentrations ([Mg]). Hydrogen concentrations ([H]) in as-grown GaN samples showed a one-to-one relationship with [Mg] over the wide Mg doping range of 6 × 1016–3 × 1019 cm−3 due to the formation of Mg-H complexes but exhibited gaps between [Mg] and [H] at the low and the high ends of the doping range. At low [Mg], we found that [H] was in good agreement with the sum of [Mg] and carbon (C) concentrations, indicating the formation of C-H complexes. The acceptor concentration (Na) was significantly decreased for heavily Mg-doped samples with lower [H], while Na values close to [Mg] were obtained for samples having [H] close to [Mg]. These suggest that an Mg atom forming an Mg–H bond in the as-grown samples plays as an acceptor after annealing while an Mg atom not forming an Mg-H complex has other states. In the heavily Mg-doped layers, transmission electron microscopy (TEM) and scanning TEM (STEM) analyses showed that nano-scale defects are formed and that these defects are pyramidal inversion domains (IDs) with Mg segregation at the top (0001) boundary. We estimated the amount of segregated Mg atoms on the basis of our TEM-STEM analyses and concluded that most parts of the Mg atoms not forming Mg-H complexes are segregated at the ID boundaries.

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