First-principles theoretical results were presented for substitutional and interstitial C in wurtzite GaN. The C was found to be a shallow acceptor when substituted for N (CN) and a shallow donor when substituted for Ga (CGa). Interstitial C (CI) was found to assume different configurations depending upon the Fermi level: A site at the center of the c-axis channel was favored when the Fermi level was below 0.9eV (relative to the valence band maximum) and a split-interstitial configuration was favored otherwise. Both configurations produced partly filled energy levels near to the middle of the gap, and CI should therefore exhibit deep donor behavior in p-type GaN and deep acceptor behavior in n-type GaN. Formation energies for CN, CGa, and CI were similar, making it likely that CN acceptors would be compensated by other C species. The CGa was predicted to be the primary compensating species when growth occurred under N-rich conditions while channel CI was predicted to be the primary compensating species under Ga-rich growth conditions. Self-compensation was predicted to be more significant under Ga-rich growth conditions than under N-rich conditions. Experimental evidence for self-compensation was considered. Four C complexes were examined. CN–VGa was found to be unstable when the Fermi level was above the middle of the gap due to the high stability of Ga vacancies (VGa). The CN–VGa complex was previously suggested as a source of the broad 2.2eV luminescence peak often observed in n-type GaN. The present results indicated that this was unlikely. The CI–CN complex was capable of forming in C-doped GaN grown under Ga-rich conditions if the mobility of the constituents was high enough.

Substitutional and Interstitial Carbon in Wurtzite GaN. A.F.Wright: Journal of Applied Physics, 2002, 92[5], 2575-85