Papers by Author: Christian Riedl

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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.
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.
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.
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.
Authors: Christian Riedl, Ulrich Starke
Abstract: The structural and electronic properties of epitaxial graphene on SiC(0001) are investigated by low energy electron diffraction (LEED) and angle resolved ultraviolet photoelectron spectroscopy (ARUPS). Fingerprints in the spot intensity spectra in LEED allow for the exact determination of the number of layers for the first three graphene layers after being correlated with the electronic bandstructure obtained from ARUPS using He II excitation. Our analysis includes the consideration of samples with different doping levels. A possible influence of the polytype 4H- or 6H-SiC is discussed. LEED by itself turns out to be an easy and practical method for the thickness analysis of epitaxial graphene on SiC(0001).
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