Papers by Author: Ulrich Starke

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Authors: Christopher L. Frewin, Camilla Coletti, Christian Riedl, Ulrich Starke, Stephen E. Saddow
Abstract: A comprehensive study on the hydrogen etching of numerous SiC polytype surfaces and orientations has been performed in a hot wall CVD reactor under both atmospheric and low pressure conditions. The polytypes studied were 4H and 6H-SiC as well as 3C-SiC grown on Si substrates. For the hexagonal polytypes the wafer surface orientation was both on- and off-axis, i.e. C and Si face. The investigation includes the influence of the prior surface polishing method on the required etching process parameters. 3C-SiC was also studied grown in both the (100) and (111) orientations. After etching, the samples were analyzed via atomic force microscopy (AFM) to determine the surface morphology and the height of the steps formed. For all cases the process conditions necessary to realize a well-ordered surface consisting of unit cell and sub-unit cell height steps were determined. The results of these experiments are summarized and samples of the corresponding AFM analysis presented.
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Authors: Charíya Virojanadara, M. Hetzel, Leif I. Johansson, Wolfgang J. Choyke, Ulrich Starke
Abstract: The atomic and electronic structure of 4H-SiC(1 1 02) surfaces were investigated using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) and photoemission (PES). Two well ordered phases existing on this surface, i.e. (2×1) and c(2×2) are discussed. The (2×1) phase consists of a Si adlayer which is topped by an array of ordered Si-nanowires with electronic states confined to one dimension. For the c(2×2) phase STM indicates the presence of adatoms and PES a surface composition close to bulk SiC stoichiometry. A detailed atomic model for this c(2×2) phase is proposed.
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Authors: J. Bernhardt, A. Seubert, M. Nerding, Ulrich Starke, A. Heinz
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Authors: A. Oliveros, Camilla Coletti, Christopher L. Frewin, Christopher Locke, Ulrich Starke, Stephen E. Saddow
Abstract: An ever-increasing demand for biocompatible materials provides motivation for the development of advanced materials for challenging applications ranging from disease detection to organ function restoration. Carbon-based materials are considered promising candidates because they combine good biocompatibility with high chemical resistance. In this work we present an initial assessment of the biocompatibility of epitaxial graphene on 6H-SiC(0001). We have analyzed the interaction of HaCaT (human keratinocyte) cells on epitaxial graphene and compared it with that on bare 6H-SiC(0001). We have found that for both graphene and 6H-SiC there is evidence of cell-cell and cell substrate interaction which is normally an indication of the biocompatibility of the material.
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Authors: T. Acartürk, Kurt Semmelroth, Gerhard Pensl, Stephen E. Saddow, Ulrich Starke
Abstract: The concentration of nitrogen and phosphorous in SiC bulk material and epitaxial layers was investigated using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The advantage of TOF-SIMS of acquiring a complete mass spectrum in a single run was used to identify the most sensitive atomic ion or ionic cluster for the selected element to be monitored. For the investigation of N with its intrinsic low ionization yield the use of a Cs containing cluster ion is necessary. Selection of a CNCs2 + cluster allows to reach a detection limit of about cN,min » 5×1016 cm-3. In the case of P the elemental ion was used. However, the adjacent mass of 30SiH influences the P peak as well as its background and has to be suppressed. This can be achieved by limiting the residual gas re-adsorption during the measurement resulting in a detection limit of about cP,min » 5×1015 cm-3. These measurement parameters were used to investigate a single crystal SiC bulk sample grown by the modified Lely method with intentional P doping and an N doped epitaxial SiC layer sample.
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Authors: Roland Püsche, Martin Hundhausen, Lothar Ley, Kurt Semmelroth, Gerhard Pensl, Patrick Desperrier, Peter J. Wellmann, Eugene E. Haller, J.W. Ager, Ulrich Starke
Abstract: We study electronic Raman scattering of phosphorus and nitrogen doped silicon carbide (SiC) as a function of temperature in the range 7K < T < 300K. We observe a series of peaks in the Raman spectra which we assign to electronic transitions at nitrogen and phosphorus donors on different lattice sites. These transitions are identified as valley orbit transitions of the 1s donor ground state. From the polarization dependence of the observed peaks, we find that all electronic Raman signals have E2-symmetry of C6v for the hexagonal polytypes (6H-SiC and 4H-SiC) and E-symmetry of C3v for 15R-SiC. We find a reduction of the intensities of all valley-orbit Raman signals with increasing temperature and ascribe this reduction to the decreasing occupation of donor states.
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Authors: Christian Riedl, J. Bernardt, K. Heinz, Ulrich Starke
Abstract: The evolution and structure of graphene layers on 4H-SiC(0001) and the corresponding interface are investigated by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). The surface is characterized by the so-called (6p3£6p3)R30± reconstruction, whose structural properties are still unclear but at the same time are crucial for the controlled growth of homogeneous high-quality large-terrace graphene surfaces. We analyse the properties of three phases in this reconstruction with periodicities (6p3£6p3)R30±, (6£6) and (5£5). Their LEED intensities strongly depend on the surface preparation procedure applied. The graphitization process imprints distinct features in the STM images as well as in the LEED spectra. An easy and practicable determination of the number of graphene layers is outlined by means of LEED intensities.
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Authors: Ameer Al-Temimy, Christian Riedl, Ulrich Starke
Abstract: By carbon evaporation under ultrahigh vacuum (UHV) conditions, epitaxial graphene can be grown on SiC(0001) at significantly lower temperatures than with conventional Si sublimation. Therefore, the degradation of the initial SiC surface morphology can be avoided. The layers of graphene are characterized by low energy electron diffraction (LEED), angle resolved ultraviolet photoelectron spectroscopy (ARUPS), and atomic force microscopy (AFM). On SiC the graphene lattice is rotated by 30o in comparison to preparation by annealing in UHV alone.
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Authors: S. Soubatch, Ulrich Starke
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Authors: Christian Riedl, Camilla Coletti, Takayuki Iwasaki, Ulrich Starke
Abstract: In this report we review how intrinsic drawbacks of epitaxial graphene on SiC(0001) such as n-doping and strong electronic influence of the substrate can be overcome. Besides surface transfer doping from a strong electron acceptor and transfer of epitaxial graphene from SiC(0001) to SiO2 the most promising route is to generate quasi-free standing epitaxial graphene by means of hydrogen intercalation. The hydrogen moves between the (6p3×6p3)R30◦ reconstructed initial carbon (so-called buffer) layer and the SiC substrate. The topmost Si atoms which for epitaxial graphene are covalently bound to this buffer layer, are now saturated by hydrogen bonds. The buffer layer is turned into a quasi-free standing graphene monolayer, epitaxial monolayer graphene turns into a decoupled bilayer. The intercalation is stable in air and can be reversed by annealing to around 900 °C. This technique offers significant advances in epitaxial graphene based nanoelectronics.
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