Michel Bockstedte

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.

Ab initiostudy of the migration of intrinsic defects in3C−SiC

The diffusion of intrinsic defects in 3C-SiC is studied using an ab initio method based on density functional theory. The vacancies are shown to migrate on their own sublattice. The carbon split-interstitials and the two relevant silicon interstitials. namely the tetrahedrally carbon-coordinated interstitial and the (110)-oriented split interstitial, are found to be by far more mobile than the vacancies. The metastability of the silicon vacancy, which transforms into a vacancy-antisite complex in p-type and compensated material, kinetically suppresses its contribution to diffusion processes. The role of interstitials and vacancies in the self-diffusion is analyzed. Consequences for the dopant diffusion are qualitatively discussed. Our analysis emphasizes the relevance of mechanisms based on silicon and carbon interstitials.

Pseudopotential study of binding properties of solids within generalized gradient approximations: The role of core-valence exchange-correlation.

In ab initio pseudopotential calculations within density-functional theory, the nonlinear exchange-correlation interaction between valence and core electrons is often treated linearly through the pseudopotential. We discuss the accuracy and limitations of this approximation regarding a comparison of the local-density approximation ~LDA! and generalized gradient approximations ~GGA’s !, which we find to describe core-valence exchangecorrelation markedly different. ~1! Evaluating the binding properties of a number of typical solids, we demonstrate that the pseudopotential approach and the linearization of core-valence exchange correlation are both accurate and limited in the same way in the GGA as in the LDA. ~2! Examining the practice to carry out GGA calculations using pseudopotentials derived within the LDA, we show that the ensuing results differ significantly from those obtained using pseudopotentials derived within the GGA. As principal source of these differences we identify the distinct behavior of core-valence exchange correlation in the LDA and GGA which, accordingly, contributes substantially to the GGA-induced changes of calculated binding properties. @S0163-1829~98!07204-X#

Coupling of excitons and defect states in boron-nitride nanostructures

The signature of defects in the optical spectra of hexagonal boron nitride (BN) is investigated using many-body perturbation theory. A single BN-sheet serves as a model for different layered BN nanostructures and crystals. In the sheet we embed prototypical defects such as a substitutional impurity, isolated boron and nitrogen vacancies, and the divacancy. Transitions between the deep defect levels and extended states produce characteristic excitation bands that should be responsible for the emission band around 4 eV, observed in luminescence experiments. In addition, defect bound excitons occur that are consistently treated in our ab initio approach along with the “free” exciton. For defects in strong concentration, the coexistence of both bound and free excitons adds substructure to the main exciton peak and provides an explanation for the corresponding feature in cathodo- and photoluminescence spectra.

Ab initio molecular dynamics study of the desorption of D_2 from Si(100)

Ab initio molecular dynamics calculations of deuterium desorbing from Si(100) have been performed in order to monitor the energy redistribution among the various D

Solubility of nitrogen and phosphorus in 4H-SiC: A theoretical study

_2

Structure and vibrational spectra of carbon clusters in SiC

and silicon degrees of freedom during the desorption process. The calculations show that a considerable part of the potential energy at the transition state to desorption is transferred to the silicon lattice. The deuterium molecules leave the surface vibrationally hot and rotationally cold, in agreement with thermal desorption experiments; the mean kinetic energy, however, is larger than found in a laser-induced desorption experiment. We discuss possible reasons for this discrepancy.

Many-Body Effects in the Excitation Spectrum of a Defect in SiC

The n-type dopants phosphorus and nitrogen, and their complexes with intrinsic point defects are investigated in 4H-SiC by first-principles theory. The solubility and electrical activation of the dopants in thermodynamic equilibrium are calculated. For nitrogen, a saturation of the electrical activation above a certain critical concentration is found that is driven by a preferential incorporation of nitrogen into electrically passive nitrogen-vacancy complexes. This explains the observations of recent experiments. An almost complete phosphorus activation is found up to the solubility limit. We suggest that the low phosphorus doping achieved by sublimation growth is related to the growth kinetics.

Orbital-Symmetry-Dependent Electron Transfer through Molecules Assembled on Metal Substrates.

high-frequency LVM’s up to 250meV. The isotope shifts resulting from a 13 C enrichment are analyzed. In the light of these results, the photoluminescence centers DII and P U are discussed. The dicarbon antisite is identified as a plausible key ingredient of the DII-center, whereas the carbon split-interstitial is a likely origin of the P T centers. The comparison of the calculated and observed high-frequency modes suggests that the U-center is also a carbon-antisite based defect.

Carbon antisite clusters in SiC: A possible pathway to the D II center

We show that electron correlations control the photophysics of defects in SiC through both renormalization of the quasiparticle band structure and excitonic effects. We consider the carbon vacancy with two possible excitation channels that involve conduction and valence bands. Corrections to the Kohn-Sham ionization levels strongly depend on the defect charge state. Excitonic effects introduce a redshift of 0.23 eV. The analysis reassigns excitation mechanism at the thresholds in photoinduced paramagnetic resonance measurements [J. Dashdorj, J. Appl. Phys. 104, 113707 (2008)].