E. L. Briggs

Amphoteric properties of substitutional carbon impurity in GaN and AlN

Electronic structure and formation energies of substitutional carbon impurities in hexagonal GaN and AlN were studied by quantum molecular dynamics. Substitutions on both cation and anion sites were considered. Ccation is a shallow donor, while CN is a shallow acceptor. A DX‐like configuration is metastable for CGa and CAl0, and stable for CAl−. The solubility of carbon is excellent, but it is accompanied by efficient self‐compensation of carbon acceptors by carbon donors. This is due to two factors: (i) the large energy gain induced by electron transfer from Ccation to CN, which is close to the band gap energy, and (ii) the large binding energy (∼1 eV) of nearest neighbor Ccation+‐CN− pairs.

Large-Scale Applications of Real-Space Multigrid Methods to Surfaces, Nanotubes, and Quantum Transport

The development and applications of real-space multigrid methods are discussed. Multigrid techniques provide preconditioning and convergence acceleration at all length scales, and therefore lead to particularly efficient algorithms. When using localization regions and optimized, non-orthogonal orbitals, calculations involving over 1000 atoms become practical on massively parallel computers. The applications discussed in this chapter include: (i) dopant incorporation and ordering effects during surface incorporation of boron, which lead to the formation of ordered domains at half-monolayer coverage; (ii) incorporation of Mg into GaN during growth, and in particular the conditions that would lead to maximum p-type doping; (iii) optical fingerprints of surface structures for use in real-time feedback control of growth: and (iv) mechanisms of stress release and quantum transport properties of carbon nanotubes.

Theory of Interfaces and Surfaces of Wide-Gap Nitrides

The results of extensive theoretical studies of the properties of interfaces and surfaces of the wide-gap III-V nitride semiconductors are reviewed. The phenomena that we have investigated include band offsets and transport properties in nitride-based devices, surface reconstruction energetics and adsorbate-substrate interactions. In the modelling of realistic blue-laser multiquantum well structures such as In 0.2 Ga 0.8 N/In 1− x Ga x N ( x > 0.8) superlattices, pyro- and piezo-electric effects are found to be strong enough to reduce the interband recombination rate and limit the efficiency of an actual device. We have also studied the influence of growth conditions and surface polarity upon the morphology of (0001) GaN surfaces, which are the primary growth faces. The charge transfer between the Ga and N atoms in GaN and the very large electronegativity of nitrogen are found to play decisive roles in determining the stable reconstructions. The behavior of Mg at the Ga-terminated (0001) surface has also been investigated. We find that Mg in the Gasubstitutional site is preferred to adatom sites on the surface. Further, our results suggest that Mg prefers to segregate to the surface.

Theory of Interfaces in Wide-Gap Nitrides

The results of theoretical studies of the bulk and interface properties of nitrides are presented. As a test the bulk properties, including phonons of GaN at the Γ-point, are calculated and found to be in excellent agreement with the experimental data. At interfaces, the strain effects on the band offsets range from 20% to 40%, depending on the substrate. The AlN/GaN/InN interfaces are all of type I, while the Al 0.5 Ga 0.5 N on A1N zinc-blende (001) interface is of type II. Further, an interface similar to those used in the recent blue laser diodes is of type I and does not have any electronically active interface states. The valence band-offset in the (0001) GaN on A1N interface is -0.57 eV and the conduction band-offset is 1.87 eV.

Theory of Defects, Doping, Surfaces and Interfaces in Wide Gap Nitrides

The results of extensive theoretical studies of group IV impurities and surface and interface properties of nitrides are presented and compared with available experimental data. Among the impurities, we have considered substitutional C, Si, and Ge. C N is a very shallow acceptor, and thus a promising p-type dopant. Both Si and Ge are excellent donors in GaN. However, in AlGaN alloys the DX configurations are stable for a sufficiently high Al content, which quenches the doping efficiency. At high concentrations, it is energetically favorable for group IV impurities to form nearest-neighbor X cation -X N pairs. Turning to surfaces, AIN is known to exhibit NEA. We find that the NEA property depends sensitively on surface reconstruction and termination. At interfaces, the strain effects on the band offsets range from 20% to 40%, depending on the substrate. The AIN/GaN/InN interfaces are all of type I, while the A1 0.5 Ga 0.5 N/A1N zinc-blende (001) interface may be of type II. Further, the calculated bulk polarizations in wurtzite AIN and GaN are -1.2 and -0.45 μC/cm 2 , respectively, and the interface contribution to the polarization in the GaN/AlN wurtzite multi-quantum-well is small.