Samples containing various concentrations of C, and doped with Si, were grown hetero-epitaxially onto sapphire by using metal–organic chemical-vapor deposition. These samples were characterized by a variety of electrical and optical techniques, and the resulting experimental data were compared to density-functional-theory calculations of the formation energies and electronic states of substitutional and interstitial C in hexagonal GaN. It was found that, in samples where the Si concentration exceeded that of C, the C sits in the N substitutional site, acting as an acceptor and partially compensating the material. However, when C densities exceeded those for Si, GaN became semi-insulating due to C occupation of both N and Ga substitutional lattice sites, and a new luminescence peak appeared at about 3eV. Calculated formation energies of C in both sites were strong functions of both the Fermi level and growth stoichiometry. The former dependence gave rise to self-compensation when [C] > [Si] because the formation energy of the Ga substitutional configuration (the donor state) became equal to that of the N substitutional site, effectively pinning the Fermi level as it approaches mid-gap. The results suggested that effective p-type doping of GaN could only be achieved under Ga-rich growth conditions.

Role of Carbon in GaN. C.H.Seager, A.F.Wright, J.Yu, W.Götz: Journal of Applied Physics, 2002, 92[11], 6553-60