Alexander Mattausch

Ab initio study of graphene on SiC.

Employing density-functional calculations we study single and double graphene layers on Si- and C-terminated 1x1-6H-SiC surfaces. We show that, in contrast with earlier assumptions, the first carbon layer is covalently bonded to the substrate and cannot be responsible for the graphene-type electronic spectrum observed experimentally. The characteristic spectrum of freestanding graphene appears with the second carbon layer, which exhibits a weak van der Waals bonding to the underlying structure. For Si-terminated substrate, the interface is metallic, whereas on C face it is semiconducting or semimetallic for single or double graphene coverage, respectively.

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.

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

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.

Structure and vibrational spectra of carbon clusters in SiC

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

The photoluminescence center D I I 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 I I spectrum. The clusters possess very high binding energies which guarantee their thermal stability-a known feature of the D I I center. The dicarbon antisite (C 2 ) S i (two carbon atoms sharing a silicon site) is an important building block of these clusters.

Ab-Initio Study of Dopant Interstitials in 4H-SiC

We investigated the the interstitial configurations of the p-type dopants boron and aluminum and the n-type dopants nitrogen and phosphorus in 4H-SiC by an ab initio method. In particular, the energetics of these defects provides information on the dopant migration mechanisms. The transferability of the earlier results on the boron migration in 3C-SiC to the hexogonal polytype 4H-SiC is verified. Our calculations suggest that for the aluminum migration a kick-out mechanism prevails, whereas nitrogen and phosphorus diffuse via an interstitialcy mechanism.

Thermally stable carbon-related centers in 6H-Sic : Photoluminescence spectra and microscopic models

Recent ab initio calculations [Mattausch et al., Phys. Rev. B 70, 235211 (2004)] of carbon clusters in SiC reveal a possible connection between the tricarbon antisite (C{sub 3}){sub Si} and the U photoluminescence center in 6H-SiC [Evans et al., Phys. Rev. B 66, 35204 (2002)]. Yet, some of the predicted vibrational modes were not observed experimentally. We report experiments that, indeed, confirm the existence of a low-energy mode for the U center (as well as for the HT3 and HT4 centers with spectral details similar to the U center). We calculated the isotope splitting for the (C{sub 3}){sub Si}-defect and found near-perfect agreement with our data. In addition, we discuss the carbon di-interstitial (C{sub 2}){sub Hex} as a model for the Z and HT5 centers. The isotope splitting is also well reproduced, but the absolute values of the local mode energies show a discrepancy of about 10 meV.

Ab Initio Study of the Structural and Electronic Properties of the Graphene/SiC{0001} Interface

Employing density functional theory we investigate the model interface between 1 × 1-6H-SiC{0001} surfaces and graphene layers. We find that the first graphene layer is covalently bonded to the SiC substrate, opposing the earlier assumption of a weak van-der-Waals bonding. The interface at the Si-face is metallic, while on the C-face it remains semiconducting. Further graphene layers are then only weakly bound and the typical graphitic properties of the electronic structure appear.

Kinetic Mechanisms for the Deactivation of Nitrogen in SiC

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.

The Solubility and Defect Equilibrium on the n-Type Dopants Nitrogen and Phosphorus in 4H-SiC: A Theoretical Study

The n-type dopants phosphorus and nitrogen as well as the impurityrelated defects are investigated by an ab initio method. The defect equilibrium and solubility are analysed. The electrical activation of nitrogen is found to saturate above a critical nitrogen concentration, due to the enhanced formation of nitrogenvacancy complexes. The phosphorus activation is not affected by self-compensation or passivation. The calculated phosphorus solubility and implantation experiments suggest that the low value found in PVT-growth is due to the growth kinetics.