J. Neugebauer

Density-functional theory calculations for poly-atomic systems: electronic structure, static and elastic properties and ab initio molecular dynamics

Abstract The package fhi96md is an efficient code to perform density-functional theory total-energy calculations for materials ranging from insulators to transition metals. The package employs first-principles pseudopotentials, and a plane-wave basis-set. For exchange and correlation both the local density and generalized gradient approximations are implemented. The code has a low storage demand and performs efficiently on low budget personal computers as well as high performance computers.

Determination of wurtzite GaN lattice polarity based on surface reconstruction

We identify two categories of reconstructions occurring on wurtzite GaN surfaces, the first associated with the N face, (0001), and the second associated with the Ga face, (0001). Not only do these two categories of reconstructions have completely different symmetries, but they also have different temperature dependence. It is thus demonstrated that surface reconstructions can be used to identify lattice polarity. Confirmation of the polarity assignment is provided by polarity-selective wet chemical etching of these surfaces.

Electrostatic interactions between charged defects in supercells

Most theoretical calculations for point defects employ the supercell approach. The supercell consists of a few dozen or 100 atoms of the bulk material with a single defect, and is subject to periodic boundary conditions. However, the large density and periodic arrangement of the defects introduce artifacts. They need to be corrected for to extrapolate to the isolated-defect limit. This is particularly important for electrostatic interactions between charged defects, which decay only very slowly (asymptotically like L ―1 ) with increasing supercell lattice constant L. In this paper, we summarize the underlying electrostatics in condensed matter. A novel defect scheme is derived from this analysis. It overcomes limitations of previous schemes with respect to applicability, systematic improvement, and formal justification. Good performance is demonstrated for vacancies in diamond and GaAs.

Theory of doping and defects in III-V nitrides

Doping problems in GaN and in AlGaN alloys are addressed on the basis of state-of-the-art first-principles calculations. For n-type doping we find that nitrogen vacancies are too high in energy to be incorporated during growth, but silicon and oxygen readily form donors. The properties of oxygen, including DX-center formation, support it as the main cause of unintentional n-type conductivity. For p-type doping we find that the solubility of Mg is the main factor limiting the hole concentration in GaN. We discuss the beneficial e⁄ects of hydrogen during acceptor doping. Compensation of acceptors by nitrogen vacancies may occur, becoming increasingly severe as x increases in Al x Ga 1~x N alloys. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 61.72.Ji; 71.55.Eq

Combining GW calculations with exact-exchange density-functional theory: an analysis of valence-band photoemission for compound semiconductors

We report quasi-particle energy calculations of the electronic bandstructure as measured by valence-band photoemission for selected II–VI compounds and group III nitrides. By applying GW as perturbation to the ground state of the fictitious, non-interacting Kohn–Sham electrons of density-functional theory (DFT), we systematically study the electronic structure of zinc-blende GaN, ZnO, ZnS and CdS. Special emphasis is put on analysing the role played by the cation semicore d-electrons that are explicitly included as valence electrons in our pseudo-potential approach. Unlike in the majority of previous GW studies, which are almost exclusively based on ground state calculations in the local-density approximation (LDA), we combine GW with exact-exchange DFT calculations in the optimized-effective potential approach (OEPx). This is a much more elaborate and computationally expensive approach. However, we show that applying the OEPx approach leads to an improved description of the d-electron hybridization compared to the LDA. Moreover, we find that it is essential to use OEPx pseudo-potentials in order to treat core–valence exchange consistently. Our OEPx-based quasi-particle valence bandstructures are in good agreement with available photoemission data in contrast to the ones based on the LDA. We therefore conclude that for these materials, OEPx constitutes the better starting point for subsequent GW calculations.

Theory of the adatom-induced reconstruction of the SiC(0001)√3×√3 surface

Based on first-principles total-energy calculations we have determined the chemical identity and adsorption site of the adatoms observed recently in scanning tunneling microscopy on the SiC(0001)\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3 surface. The calculations indicate that Si adatoms are preferred over C adatoms for the entire allowed range of Si and C chemical potentials. In addition, we find that the adatoms prefer the

Robustness and optimal use of design principles of arthropod exoskeletons studied by ab initio-based multiscale simulations

{\mathit{T}}_{4}

Ab initio and atomistic study of generalized stacking fault energies in Mg and Mg-Y alloys

site over the

Wurtzite GaN Surface Structures Studied by Scanning Tunneling Microscopy and Reflection High Energy Electron Diffraction

{\mathit{H}}_{3}

Exact-exchange-based quasiparticle energy calculations for the band gap, effective masses, and deformation potentials of ScN

site for both Si and C. Based on these results we propose a model for the C-rich 6\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}6\ensuremath{\surd}3 reconstruction.