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.

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 (CN) and a shallow donor when substituted for gallium (CGa). Interstitial carbon (CI) 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 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 are similar, making it likely that CN acceptors will be compensated by other carbon species. CGa is predicted to be the primary compensating species when growth occurs under N-rich conditions while channel CI 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. CN–VGa is found to be unstable when the Fermi level is above the middle of the gap due to the high stability of gallium vacancies (VGa). The CN–VGa 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 CI–CN 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.

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The band gap of AlxGa1−xN is measured for the composition range 0⩽x<0.45; the resulting bowing parameter, b=+0.69 eV, is compared to 20 previous works. A correlation is found between the measured band gaps and the methods used for epitaxial growth of the AlxGa1−xN: directly nucleated or buffered growths of AlxGa1−xN initiated on sapphire at temperatures T>800 °C usually lead to stronger apparent bowing (b>+1.3 eV); while growths initiated using low-temperature buffers on sapphire, followed by high-temperature growth, lead to weaker bowing (b<+1.3 eV). Extant data suggest that the intrinsic band-gap bowing parameter for AlGaN alloys is b=+0.62(±0.45) eV.

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We have performed Monte Carlo calculations t.o determine the charge accumulation on &reading edge dislocations in GaN as a function of the dislocation density and background dopant density. Four possible core structures have been examined, each of which produces defect levels in the gap and may therefore act as electron or hole traps. Our results indicate that charge accumulation, and the resulting electrostatic interactions, can change the relative stabilities of the different. core structures. Structures having Ga and N vacancies at the dislocation core are predicted to be st.able under nitrogen-rich and gallium-rich growth conditions, respectively. Due to dopant depletion at. high dislocat.ion density and the rnult.itude of charge states, the line charge exhibits complex crossover behaxior as the dopant. and dislocat.ion densities vary.Gallium nit,ride films grown on sapphire substrates t y p ically contain bet.ween lo8 and 1O'O threading dislocations per cm2 as a result* of the subst,ant,ial film-substrate chemical and 1att.ice mismatch.l Nevertheless, it. has been possible t.0 fabricate bright and efficient light,-emitt.ing diodes from films composed of GaN alloyed wit.h InN and A1N.2 This success led several researchers t.o specu1at.e early on t.hat t.hreading dislocat.ions in GaN might not act. as efficient. minority-carrier recombination sit.es.' Recent. experiment.al studies, however, have confirmed that there is significant opt.ica13 and elect.rica14 activity associat,ed with these defects. In particular, results from a recent. scanning-capacit.ance microscopy study5 suggest that dislocations are negatively charged in n-type GaN, and studies of transverse mobility in n-type GaN indicate that elect,rons are scattered from these negatively charged dislocations.Recent, charge-st,ate calculations for AlN and GaN' indicate that threading edge disloqations produce defect levels in the forbidden energy gap. These calculations provide estimat.es for these defect. levels. In agreement wit.h experiment.al studies, edge dislocations are predicted to be negatively charged in n-type GaN. However, the amount of charge accumulation could not be quantified because electrostatic interactions between charged defect sites and between defect. sites and ionized dopants were not included in the calculations. In this study, we treat coulomb interactions explicitly and therefore are able t.o predict. the amount. of charge accumulation on an edge dislocation under a variety of doping condit.ions.We examine t.he zero-temperature behavior of threading dis1ocat.ions using simulat,ed annealing.' The simulat.ion cell conbains one dislocation and is of lateral dimension u::' 2 , where u& is t.he dislocation densit.y. Elect.rons, modelled as point charges, can t.ransfer bet,ween dopants (donors or accept,orsf and defect. leveis at. t.he dislocat.ion cores, and among the dopants themselves. The dislocat,ion consists of 50-1000 defect sit.es situated 5.185A apart. We have examined the four dislocat.ion core st,ruct.ures previously cons...

First-principles calculations have been used to determine bowing parameters for disordered zinc-blende Al1−xGaxN and Ga1−xInxN. The direct transition at Γ is found to bow downward for both materials with parameters +0.53 and +1.02 eV, respectively, while the Γ-to-X transition bows upward for Al1−xGaxN (parameter −0.10 eV) and downward for Ga1−xInxN (parameter +0.38 eV). The similarity of the calculated bulk zinc-blende and wurtzite Γ-point transitions also allows estimates to be made of the energy gap versus composition for wurtzite alloys.

The structures and formation energies of neutral and charged edge dislocations in AlN are investigated via density-functional-theory calculations. Stoichiometric structures having full and open cores are considered as well as nonstoichiometric structures having aluminum or nitrogen vacancies along the dislocation core. Formation energies are found to depend strongly on the Fermi level, due to the presence of defect levels in the band gap, and on growth conditions for the case of the nonstoichiometric structures. A structure having aluminum vacancies along the dislocation core is predicted to be most stable in n-type material grown under nitrogen-rich conditions, whereas a nitrogen-vacancy structure is most stable in p-type material grown under aluminum-rich conditions. Estimates are also given for defect energy levels in the gap.