We have found a new photoluminescence (PL) band with unusual properties in GaN. The blue band, termed as the BL C band, has a maximum at about 2.9 eV and an extremely short lifetime (shorter than 1 ns for a free electrons concentration of about 10 18 cm-3). The electron-and holecapture coefficients for this defect-related band are estimated as 10-9 and 10-10 cm 3 /s, respectively. The BL C band is observed only in GaN samples with relatively high concentration of carbon impurity, where the yellow luminescence (the YL1 band) with a maximum at 2.2 eV is the dominant defect-related PL. Both the YL1 and BL C bands likely originate from the C N defect, namely from electron transitions via the −/0 and 0/+ thermodynamic transition levels of the C N. BL C band appears only at high excitation intensities in n-type GaN samples co-doped with Si and C, and it can be found in wide range of excitation intensities in semi-insulating (presumably ptype) GaN samples doped with C. The properties and behavior of the YL1 and BL C bands can be explained using phenomenological models and first-principles calculations.
Ammonothermal GaN samples with the concentration of free electrons of 1018 and 1019 cm−3 were annealed in a wide range of temperatures (Tann = 300–1400 °C) under atmospheric N2 pressure and under ultra-high N2 pressure conditions to avoid the GaN decomposition. Photoluminescence (PL) studies reveal the YL2 band with a maximum at 2.3 eV before annealing and two new PL bands after annealing at Tann > 600 °C: the OL3 band with a maximum at 2.1 eV and the RL4 band with a maximum at 1.6–1.7 eV. The ammonothermal GaN samples have high concentrations of complexes containing gallium vacancy (VGa), hydrogen, and oxygen. The first-principles calculations suggest that the VGa-3Hi complex is the origin of the YL2 band, while the VGa-3ON complex is responsible for the RL4 band.
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