Charge transfer of C-related centers in C-doped GaN*

Incorporation of carbon impurities in the concentration range of 10¹⁷-10¹⁹ creates semi-insulating GaN layers as required for electronic power devices. We used electron paramagnetic resonance (EPR) spectroscopy to study the point defects in 2x10¹⁷ -10¹⁹ C-doped GaN substrates grown by hydride vapor phase epitaxy. EPR was performed at 3.5K. An isotropic signal with g=1.987+/- 0.001 was observed in all samples. EPR intensity of the signal increased monotonically with carbon concentration indicating the defect is carbon-related. Under the photo-EPR measurements, the intensity of the signal increased with photon energy greater than 2.75+/- 0.15 eV and the photo-induced signal began to decrease at 0.95+/- 0.05 eV.  Another signal, a well- known shallow donor with g_par=1.951+/-0.001 and g_perp=1.950+/-0.001, also appeared along with the g~1.987 signal in the most lightly doped samples under illumination. The appearance of the donor confirms that the photo-excitation is caused by excitation of an electron from the defect to the conduction band. This implies that the defect level for C-related centers is about 1 eV above the valence band edge, consistent with temperature-dependent Hall measurements.