A. F. Wright

Elastic properties of zinc-blende and wurtzite AlN, GaN, and InN

Elastic constants for zinc-blende and wurtzite AlN, GaN, and InN are obtained from density-functional-theory calculations utilizing ab initio pseudopotentials and plane-wave expansions. Detailed comparisons are made with the available measured values and with results obtained in previous theoretical studies. These comparisons reveal clear discrepancies between the different sets of elastic constants which are further highlighted by examining derived quantities such as the perpendicular strain in a lattice-mismatched epitaxial film and the change in the wurtzite c/a ratio under hydrostatic pressure. Trends among results for the three compounds are also examined as well as differences between results for the zinc-blende and wurtzite phases.

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 ...

The effect of doping and growth stoichiometry on the core structure of a threading edge dislocation in GaN

Density-functional-theory calculations have been performed to study the effect of doping and growth stoichiometry on the core structure of a threading edge dislocation in GaN. Four candidate structures were examined and their formation energies were found to depend strongly on Fermi level and growth stoichiometry. A structure having gallium vacancies at the dislocation core is predicted to be most stable in n-type material grown under nitrogen-rich conditions, while a structure without vacancies is most stable in p-type material grown under these conditions. In material grown under gallium-rich conditions, a structure having nitrogen vacancies at the dislocation core is predicted to be most stable in p-type material, whereas a variety of core structures should be present in n-type material. Edge dislocations are predicted to behave as electron traps in n-type material and may act as hole traps in p-type material depending on the growth conditions.

The band-gap bowing of AlxGa1−xN alloys

The band gap of AlxGa1−xN is measured for the composition range 0⩽x 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.

Charge accumulation at a threading edge dislocation in gallium nitride

We have performed Monte Carlo calculations to determine the charge accumulation on threading 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 stable under nitrogen-rich and gallium-rich growth conditions, respectively. Due to dopant depletion at high dislocation density and the multitude of charge states, the line charge exhibits complex crossover behavior as the dopant and dislocation densities vary.

Substitutional and interstitial oxygen 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 (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 spe...

BOWING PARAMETERS FOR ZINC-BLENDE AL1-XGAXN AND GA1-XINXN

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.

Equilibrium state of hydrogen in gallium nitride: Theory and experiment

Formation energies and vibration frequencies for H in wurtzite GaN were calculated from density-functional theory and used to predict equilibrium state occupancies and solid solubilities at elevated temperatures for p-type, intrinsic, and n-type material. The solubility of deuterium (D) was measured in p-type, Mg-doped GaN at 600, 700, and 800 °C as a function of D2 pressure and compared with theory. Agreement was obtained by reducing the H formation energies 0.22 eV from ab initio theoretical values. The predicted stretch-mode frequency for H bound to the Mg acceptor lies 5% above an observed infrared absorption attributed to this complex. More limited solubility measurements were carried out for nominally undoped material rendered n-type by donors provisionally identified as O impurities, and results agree well with theory after the aforementioned adjustment of formation energies. It is concluded that currently recognized H states and physical processes can account for the equilibrium, elevated-temperat...

Interaction of hydrogen with gallium vacancies in wurtzite GaN

First-principles techniques are used to investigate the interaction of hydrogen with gallium vacancies in wurtzite GaN. The calculations reveal that hydrogen can either compensate a vacancy by donating an electron to a vacancy acceptor level, or passivate the vacancy by forming a hydrogen-vacancy complex. A gallium vacancy can bind up to four hydrogen atoms, and hydrogen removal energies are computed as a function of the number of hydrogen atoms. Removal energies are found to depend strongly on Fermi level and complexes containing more than two hydrogen atoms are predicted to be unstable in n-type GaN. Hydrogen vibration frequencies are computed and compared with previously reported infrared absorption measurements for hydrogen-implanted GaN.

Theoretical study of room temperature optical gain in GaN strained quantum wells

The determination of gain properties in group III nitride quantum wells is complicated by the incomplete knowledge of band structure properties, and the need for a consistent treatment of many‐body Coulomb effects. This letter describes an approach that involves a first‐principles band structure calculation, the results of which are incorporated into a microscopic laser theory where many‐body Coulomb effects are treated in a consistent manner. Using this approach, we investigate quantum well structures composed of alloys of GaN, AlN, and InN, in particular, GaN–AlInN, which has high confinement potentials in both strained and unstrained configurations.