Role of carbon in GaN

GaN samples, containing various concentrations of carbon and doped intentionally with silicon, have been grown heteroepitaxially on sapphire using metal-organic chemical-vapor deposition. These samples have been characterized by a variety of electrical and optical techniques, and the resulting experimental data are compared to density-functional-theory calculations of the formation energies and electronic states of substitutional and interstitial carbon in hexagonal GaN. We find that in samples where the silicon concentration exceeds that of carbon, carbon sits in the N substitutional site, acting as an acceptor and partially compensating the material. However, when carbon densities exceed those for Si, GaN becomes semi-insulating due to carbon occupation of both N and Ga substitutional lattice sites, and a new luminescence peak appears at ∼3 eV. Calculated formation energies of carbon in both sites are strong functions of both the Fermi level and growth stoichiometry. The former dependence gives rise to self-compensation when [C]>[Si] because the formation energy of the Ga substitutional configuration (the donor state) becomes equal to that of the N substitutional site, effectively pinning the Fermi level as it approaches midgap. Our results suggest that effective p-type doping of GaN can only be achieved under Ga-rich growth conditions.