Study of carbon-related point defects in C-doped GaN using photo-induced electron paramagnetic resonance spectroscopy

Incorporation of carbon impurities in GaN creates semi-insulating substrates. However, the role of carbon has not been fully understood. We used photo-induced electron paramagnetic resonance (EPR) to study point defects in mm thick free-standing 10¹⁷-10¹⁹ cm^-3 C-doped GaN. An isotropic signal at g ~1.987 was observed at 3.5 K with an intensity which increases with carbon concentration, indicating the signal represents a C-related defect. In [C]< 6x10¹⁷ cm^-3 samples, additional signals, a neutral donor with g_par=1.951 and g_per=1.950, and an anisotropic signal with g_par~2.121, were observed. The intensity of the C-related and the donor signal start to increase at 2.75 eV and decrease at 0.95 eV. The same quenching threshold for C-related and donor signals suggests that 0.95 eV represents the energy required to excite an electron to C-related defects. The ionization threshold, 0.95 eV, is consistent with the predicted value for (-/0) transition level of C_N. The decrease in donors during quenching is one order of magnitude less than the decrease in C-related defects, which suggests the change in C-related defects is not solely controlled by the donor but also by other defects such as g_par~2.121 defect.