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In SiC, mobile point defects may form thermally stable clusters and aggregates, such as di-vacancies or carbon interstitial complexes. Although predicted by theory, experimental evidence of such clusters became available only recently. Combining theoretical and experimental approaches, the unique identification of the di-vacancy, the carbon vacancy-antisite complex with the spin resonance centers P6/P7 and SI5 was recently achieved. In this way also the di-carbon and tri-carbon antisites with the photoluminiscence centers P–T and U, HT3 and HT4, respectively were identified. The two identified vacancy complexes show distinct properties: while the di-vacancy, like the silicon vacancy possesses a high-spin ground state, the carbon vacancy–antisite complex, like the carbon vacancy, is a Jahn–Teller center. These effects consistently explain the complex properties of the spin resonance spectra and are discussed in detail for the isolated vacancies. The aggregation of vacancies proved to be relevant in the explantation of the kinetic deactivation of nitrogen in co-implanted SiC. This and further evidence for defect aggregates underline the relevance of this notion. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Using density functional theory, we investigate the 6H-SiC{0001} surfaces in the unreconstructed 1 × 1 and the H-passivated configuration. The strong correlation effects of the dangling bonds at the surface are treated by spin-polarised calculations including the Hubbard-U parameter. We find that the clean surfaces are semiconducting with surface states in good agreement with experimental data. The impact of the Hubbard-U is stronger on the C-terminated face. For the H-passivated surfaces we find resonances in the valence band. The antibonding C−H state is located in the upper part of the bandgap around the ¯􀀀-point.

Kinetic mechanisms for the deactivation of nitrogen are investigated by ab initio theory. We find that the interaction of nitrogen with self-interstitials can lead to a deactivation of nitrogen, yet it cannot explain the experimentally observed nitrogen deactivation at high temperatures in silicon co-implanted samples. Our analysis suggests the aggregation of vacancies at high temperatures and the subsequent formation of passive nitrogen-vacancy complexes as a likely explanation.

We observe new photoluminescence centers in electron-irradiated 6H-SiC with phonon replicas up to 250 meV and clear threefold isotope splitting of the highest energy mode. Based on ab initio calculations, we discuss the tri-carbon anti-site (C3)Si and the di-interstitial (C2)Hex as models for these centers.

Using an ab initio method we analyze the mechanisms of the boron diffusion with emphasis on the role of the intrinsic interstitials. It is shown that the boron diffusion is dominated by a kick-out mechanism. The different effect of silicon and carbon interstitials gives rise to kinetic effects. A preference for a kick-in of the boron interstitial into the carbon lattice sites is found. Kinetic effects reported in co-implantation experiments and in-diffusion experiments are explained by our findings.

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.

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., 15 pages, 6 figures, accepted by Phys. Rev. B

The interstitial and vacancy mediated boron diffusion in silicon carbide is investigated with an ab initio method. The boron interstitials in $p$-type and $n$-type materials are found to be far more mobile than the boron-vacancy complexes. A kick-out mechanism and an interstitialcy mechanism govern the diffusion in $p$-type/intrinsic and $n$-type material, respectively. A comparison of activation energies demonstrates that the equilibrium diffusion is dominated by the positive hexagonal interstitial for typical experimental conditions. The activation energy and the charge state is in agreement with experimental findings. The analysis of the kick-out reaction demonstrates that silicon and carbon interstitials have different effects on boron acceptors on the silicon and carbon sublattice (${\mathrm{B}}_{\mathrm{Si}}$ and ${\mathrm{B}}_{\mathrm{C}}$). While silicon interstitials mediate the diffusion of both acceptors, carbon interstitials are only relevant for ${\mathrm{B}}_{\mathrm{C}}$. A larger kick-in barrier into silicon sites is found than into carbon sites. This implies a dominant formation of ${\mathrm{B}}_{\mathrm{C}}$ in the extended diffusion tails of boron profiles. The stable boron complexes with carbon or boron interstitials are found that potentially reduce the boron diffusion. This and the characteristics of the kick-out mechanism facilitate an explanation of recent co-implantation experiments.

The annealing kinetics of mobile intrinsic defects in cubic SiC is investigated by an ab initio method based on density-functional theory. The interstitial-vacancy recombination, the diffusion of vacancies, and interstitials to defect sinks (e.g., surfaces or dislocations) as well as the formation of interstitial clusters are considered. The calculated migration and reaction barriers suggest a hierarchical ordering of competing annealing mechanisms. The higher mobility of carbon and silicon interstitials as compared to the vacancies drives the annealing mechanisms at lower temperatures including the vacancy-interstitial recombination and the formation of interstitial carbon clusters. These clusters act as a source of carbon interstials at elevated temperatures. In p-type material the transformation of the silicon vacancy into the more stable vacancy-antisite complex constitutes an annealing mechanism which is activated before the vacancy migration. Recent annealing studies of vacancy-related centers in irradiated 3C-SiC and 4H-SiC and semi-insulating 4H-SiC are interpreted in terms of the proposed hierarchy of annealing mechanisms.

The photoluminescence center ${D}_{\mathrm{II}}$ 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 ${D}_{\mathrm{II}}$ spectrum. The clusters possess very high binding energies which guarantee their thermal stability---a known feature of the ${D}_{\mathrm{II}}$ center. The dicarbon antisite $({\mathrm{C}}_{2}{)}_{\mathrm{Si}}$ (two carbon atoms sharing a silicon site) is an important building block of these clusters.

To reveal the microscopic origin of the spin-resonance centers in 3C- and 4H-SiC, we perform first-principles calculations of the hyperfine tensors for vacancy-related defects and interstitials. The calculations for the silicon vacancy corroborates the earlier experimental identification. The signature of the carbon vacancy in 4H-SiC, in contrast to the silicon vacancy, is found to discriminate between cubic and hexagonal sites. The EI5 center in 4H-SiC can be attributed to a carbon vacancy at the cubic site. The assignment of the T5 center in 3C-SiC to a carbon vacancy or a complex of a carbon vacancy with hydrogen is not supported. The EI6 center in 4H-SiC originally identified as a silicon antisite is not compatible with the HF signature of this defect.