Many body effects in the excitation spectrum of a defect in SiC

We show that electron correlations control the photophysics of defects in SiC through both renormalization of the quasiparticle bandstructure and exciton effects. We consider the carbon vacancy, which is a well-identified defect with two possible excitation channels that involve conduction and valence band states. Corrections to the Kohn-Sham ionization levels are found to strongly depend on the occupation of the defect state. Excitonic effects introduce a red shift of 0.23 eV. The analysis unambigiously re-assigns excitation mechanism at the thresholds in photo-induced paramagnetic resonance measurements [J. Dashdorj \emph{et al.}, J. Appl. Phys. \textbf{104}, 113707 (2008)]

Carbon antisite clusters in SiC: a possible pathway to the D_{II} center

The photoluminescence center D_{II} is a persistent intrinsic defect which is common in all SiC polytypes. Its fingerprints are the characteristic phonon replicas in luminescence spectra. We perform ab-initio calculations of vibrational spectra for various defect complexes and find that carbon antisite clusters exhibit vibrational modes in the frequency range of the D_{II} spectrum. The clusters possess very high binding energies which guarantee their thermal stability--a known feature of the D_{II} center. The di-carbon antisite (C_{2})_{Si} (two carbon atoms sharing a silicon site) is an important building block of these clusters.Comment: RevTeX 4, 6 pages, 3 figures Changes in version 2: Section headings, footnote included in text, vibrational data now given for neutral split-interstitial, extended discussion of the [(C_2)_Si]_2 defect incl. figure Changes version 3: Correction of binding energy for 3rd and 4th carbon atom at antisite; correction of typo

Self-vacancies in Gallium Arsenide: an ab initio calculation

We report here a reexamination of the static properties of vacancies in GaAs by means of first-principles density-functional calculations using localized basis sets. Our calculated formation energies yields results that are in good agreement with recent experimental and {\it ab-initio} calculation and provide a complete description of the relaxation geometry and energetic for various charge state of vacancies from both sublattices. Gallium vacancies are stable in the 0, -, -2, -3 charge state, but V_Ga^-3 remains the dominant charge state for intrinsic and n-type GaAs, confirming results from positron annihilation. Interestingly, Arsenic vacancies show two successive negative-U transitions making only +1, -1 and -3 charge states stable, while the intermediate defects are metastable. The second transition (-/-3) brings a resonant bond relaxation for V_As^-3 similar to the one identified for silicon and GaAs divacancies.Comment: 14 page

Structure and vibrational spectra of carbon clusters in SiC

The electronic, structural and vibrational properties of small carbon interstitial and antisite clusters are investigated by ab initio methods in 3C and 4H-SiC. The defects possess sizable dissociation energies and may be formed via condensation of carbon interstitials, e.g. generated in the course of ion implantation. All considered defect complexes possess localized vibrational modes (LVM's) well above the SiC bulk phonon spectrum. In particular, the compact antisite clusters exhibit 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 D_{II} and P-U are discussed. The dicarbon antisite is identified as a plausible key ingredient of the D_{II}-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.Comment: 15 pages, 6 figures, accepted by Phys. Rev.

Modeling sublimation by computer simulation: morphology dependent effective energies

Solid-On-Solid (SOS) computer simulations are employed to investigate the sublimation of surfaces. We distinguish three sublimation regimes: layer-by-layer sublimation, free step flow and hindered step flow. The sublimation regime is selected by the morphology i.e. the terrace width. To each regime corresponds another effective energy. We propose a systematic way to derive microscopic parameters from effective energies and apply this microscopical analysis to the layer-by-layer and the free step flow regime. We adopt analytical calculations from Pimpinelli and Villain and apply them to our model. Key-Words: Computer simulations; Models of surface kinetics; Evaporation and Sublimation; Growth; Surface Diffusion; Surface structure, morphology, roughness, and topography; Cadmium tellurideComment: 12 pages, 6 Postscript figures, uses psfig.st