Photocurrent spectroscopy investigations of Mg-related defect levels in p-type GaN

The defect levels associated with the Mg impurity in p-type GaN films were systematically investigated in terms of doping concentration by using photocurrent spectroscopy. Mg-doped GaN samples were grown on sapphire substrates by using metal-organic chemical-vapor deposition (MOCVD) and were annealed in a nitrogen atmosphere at 850 degrees C for 10 min. At room temperature, photocurrent (PC) spectra showed two peaks, one at, 3.31 and the other at 3.15 eV, associated with acceptor levels formed at 300 and 142 meV, respectively, above the valence band in the as-grown samples. However, after thermal annealing, PC spectra exhibited various additional peaks, depending on the Mg concentration. In the GaN samples with a Mg concentration around 6 similar to 7 x 10(17) cm(-3), we observed PC peaks related to Mg at 3.31 and 3.02 eV and to the carbon acceptor at 3.17 eV. For moderately Mg-doped GaN samples, i.e., a hole concentration of p = 3 similar to 4 X 10(17) cm(-3), an additional peak was observed around 0.9 eV, which could be attributed to defects related to the Ga vacancy. For relatively low Mg-doped samples whose hole concentrations are 1 similar to 2 X 10(17) cm(-3), an additional broad peak was observed around 1.3 eV. This peak might be related to the yellow band luminescence. As the Mg concentration was increased, the concentration of Ga vacancies could be reduced because the Mg occupied the substitutional sites of Ga in the GaN lattice. When the hole concentration was above 6 similar to 7 X 10(17) cm(-3), the yellow luminescence and the Ga-vacancy-related peaks disappeared completely.