Papers by Author: Uwe Gerstmann

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Authors: E. Rauls, Uwe Gerstmann, M.V.B. Pinheiro, Siegmund Greulich-Weber, Johann Martin Spaeth
Abstract: In a combined theoretical and experimental work, we have investigated the common DI photoluminescence in 6H-SiC material. We present an atomistic model which is able to explain the annealing behavior, i. e. the correlation with the silicon vacancy, the local vibrational modes and the excitonic-like character observed.
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Authors: Peter Deák, B. Aradi, Adam Gali, Uwe Gerstmann, Wolfgang J. Choyke
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Authors: Uwe Gerstmann, E. Rauls, Harald Overhof, Thomas Frauenheim
Abstract: The conclusion which is drawn from the EPR line broadening and narrowing of the N shallow donor in an isotope enriched and non-enriched 4H-SiC and 6H-SiC crystals along with previous ENDOR results shows that the spin-density distribution over the C and Si nuclei differs between the 4H-SiC and 6H-SiC polytypes. The main part of the spin density in 4H-SiC is located on the Si sublattice. In contrast, in 6H-SiC the main part of the spin density is located on the C sublattice. An explanation for the difference in the electronic wave function of the N donor in 4HSiC and 6H-SiC can be found in the large difference in the band structure of two polytypes and in the position of the minima in the Brillouin zone.
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Authors: M.V.B. Pinheiro, E. Rauls, Uwe Gerstmann, Siegmund Greulich-Weber, Johann Martin Spaeth, Harald Overhof
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Authors: Uwe Gerstmann, E. Rauls, S. Sanna, Thomas Frauenheim, Harald Overhof
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Authors: Ekaterina N. Kalabukhova, S.N. Lukin, D.V. Savchenko, W.C. Mitchel, Siegmund Greulich-Weber, Uwe Gerstmann, Andreas Pöppl, J. Hoentsch, E. Rauls, Yurii Rozentzveig, E.N. Mokhov, Mikael Syväjärvi, Rositza Yakimova
Abstract: D-band electron paramagnetic resonance (EPR) measurements as well as X and Q-band field-swept Electron Spin Echo (ESE) and pulsed Electron Nuclear Double Resonance (ENDOR) studies were performed on a series of n-type 4H-SiC wafers grown by different techniques including sublimation sandwich method (SSM), physical vapor transport (PVT) and modified Lely method. Depending on the C/Si ratio and the growth temperature the n-type 4H-SiC wafers revealed, besides a triplet due to nitrogen residing on the cubic site (Nc), two nitrogen (N) related EPR spectra with g||=2.0055, g⊥=2.0010 and g||=2.0063, g⊥=2.0005 with different intensities. In the samples with low C/Si ratio the EPR spectrum with g|| =2.0055, g⊥=2.0010 consists of a triplet with low intensity which is tentatively explained as a N-related complex, while in the samples with high C/Si ratio the triplet is transformed into one structureless line of high intensity, which is explained as being due to an exchange interaction between N donors. In the samples grown at low temperature with enhanced carbon concentration the EPR line with g||=2.0063, g⊥=2.0005 and a small hyperfine (hf) interaction dominates the EPR spectrum. It is attributed to N on the hexagonal lattice site. The interpretation of the EPR data is supported by activation energies and donor concentrations obtained from Hall effect measurements for three donor levels in this series of 4H-SiC samples.
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Authors: Andreas Scholle, Siegmund Greulich-Weber, Eva Rauls, Wolf Gero Schmidt, Uwe Gerstmann
Abstract: In non-annealed 6H-SiC samples that were electron irradiated at room temperature, a new EPR signal due to a S=1 defect center with exceptionally large zero-field splitting (D = +652•10-4 cm-1) has been observed under illumination. A positive sign of D demonstrates that the spin-orbit contribution to the zero-field splitting exceeds by far that of the spin-spin interaction. A principal axis of the fine structure tilted by 59° against the crystal c-axis as well as the exceptionally high zero-field splitting D can be qualitatively understood by the occurrence of additional close-lying defect levels in defect clusters resulting in comparatively large second-order spin-orbit coup¬ling. A tentative assignment to vacancy clusters is supported by the observed annealing behavior.
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Authors: E. Rauls, Uwe Gerstmann, Siegmund Greulich-Weber, Kurt Semmelroth, Gerhard Pensl, Eugene E. Haller
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Authors: Uwe Gerstmann, Siegmund Greulich-Weber, E. Rauls, Johann Martin Spaeth, Ekaterina N. Kalabukhova, E.N. Mokhov, Francesco Mauri
Abstract: Scandium can be used to influence the stoichiometry of SiC during growth of the hexagonal polytypes. Using PL-EPR and total energy calculations in the framework of density functional theory, scandium is predicted to be built in predominantly at the Si-sublattice in form of ScSi acceptors with acceptor levels at 0.55 eV (6H-SiC) and 0.48 eV (4H-SiC). In addition, new PL-EPR spectra are found with a large anisotropy in the g-tensor suggesting defect pairs as an origin.
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Authors: Uwe Gerstmann, E. Rauls, Siegmund Greulich-Weber, Ekaterina N. Kalabukhova, D.V. Savchenko, Andreas Pöppl, Francesco Mauri
Abstract: The microscopic origin of the Nx EPR-lines observed in heavily nitrogen doped 4H-SiC and 6H-SiC is discussed with the help of EPR parameters calculated from first principles. Based on the symmetry of the g-tensors we propose a model with distant NC donor pairs on inequivalent lattice sites which are coupled to S=1 centers but with nearly vanishing zero-field splittings, giving rise to an essentially S=1/2 like spectrum. The proposed aggregation in neutral donor pairs can contribute to the saturation of the free concentration observed in heavily nitrogen doped SiC.
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