Substitutional and interstitial carbon in wurtzite GaN

First-principles theoretical results are presented for substitutional and interstitial carbon in wurtzite GaN. Carbon is found to be a shallow acceptor when substituted for nitrogen (C-N) and a shallow donor when substituted for gallium (C-Ga). Interstitial carbon (C-I) is found to assume different configurations depending on the Fermi level: A site at the center of the c-axis channel is favored when the Fermi level is below 0.9 eV (relative to the valence band maximum) and a split-interstitial configuration is favored otherwise. Both configurations produce partly filled energy levels near the middle of the gap, and C-I should therefore exhibit deep donor behavior in p-type GaN and deep acceptor behavior in n-type GaN. Formation energies for C-N, C-Ga, and C-I are similar, making it likely that C-N acceptors will be compensated by other carbon species. C-Ga is predicted to be the primary compensating species when growth occurs under N-rich conditions while channel C-I is predicted to be the primary compensating species under Ga-rich growth conditions. Self-compensation is predicted to be more significant under Ga-rich growth conditions than under N-rich conditions. Experimental evidence for self-compensation is discussed. Four carbon complexes are discussed. C-N-V-Ga is found to be unstable when the Fermi level is above the middle of the gap due to the high stability of gallium vacancies (V-Ga). The C-N-V-Ga complex was previously suggested as a source of the broad 2.2 eV luminescence peak often observed in n-type GaN. The present results indicate that this is unlikely. The C-I-C-N complex is capable of forming in carbon doped GaN grown under Ga-rich conditions if the mobility of the constituents is high enough. Experimental evidence for its existence is discussed. (C) 2002 American Institute of Physics.