Defect Level of a C-Related Center in C-Doped GaN

High-power applications of GaN require semi-insulating substrates, which can be obtained by incorporation of impurities like carbon. We used electron paramagnetic resonance (EPR) to investigate point defects in 0.2 mm thick free-standing GaN grown by hydride vapor phase epitaxy doped with 10¹⁷-10¹⁹ cm^-3 carbon. The amplitude of a nearly isotropic signal with g _~ 1.987 increased with carbon concentration suggesting that the defect is related to the impurity. Photo-EPR was performed at 3.5 K on the mostly heavily doped samples. The signal increased with photon energy greater than 2.75 ± 0.15 eV. Subsequent illumination of the photo-excited center revealed that the intensity began to decrease at 0.95 ± 0.05 eV. Considering temperature dependent Hall measurements which indicate a hole activation energy of 1 eV, we interpret the 0.95 eV threshold as the energy required to excite electrons from the valance band to the defect, and conclude that the EPR detected center is responsible for compensation. The dependence of the EPR intensity on carbon concentration further suggests that the defect is C-related, perhaps substitutional carbon, in agreement with the theoretically predicted defect level of 0.9 eV.