Papers by Author: D. Siche

Abstract: We are aiming at understanding the graphene formation mechanism on different SiC polytypes (6H, 4H and 3C) and orientations with the ultimate goal to fabricate large area graphene (up to 2 inch) with controlled number of monolayers and spatial uniformity. To reach the objectives we are using high-temperature atmospheric pressure sublimation process in an inductively heated furnace. The epitaxial graphene is characterized by ARPES, LEEM and Raman spectroscopy. Theoretical studies are employed to get better insight of graphene patterns and stability. Reproducible results of single layer graphene on the Si-face of 6H and 4H-SiC polytypes have been attained. It is demonstrated that thickness uniformity of graphene is very sensitive to the substrate miscut.

Abstract: C-plane substrates with off-orientation to <1120 > may stabilize the grown polytype, but the stacking fault density (SFD) increases from zero in the on-axis sample to 4500 cm-1 (7.7° off). The SF form preferentially at the seed-crystal-interface by a kinetically induced rearrangement of surface ad-atoms on m-facets. Most SF start in bundles with an average distance of 100 .m, which are subdivided in smaller bundles with 8 .m distance. They start preferentially from the upper corner of the vertical non-polar plane of bunched steps, which may be composed of small pyramids with m-facet surfaces. The dislocation density could decrease with increasing SFD by a pinning mechanism.

Abstract: The defect distribution in 4H-SiC single crystals in dependence on the seed polarity and its off-orientation was investigated by KOH-etching, optical microscopy and X-ray topography. Micropipe density, stacking fault density and dislocation density were determined for 2” crystals grown in <000-1> direction 0 - 7° off towards <11-20> and for crystals up to 1” in diameter grown in <11-20> or a- and <1-100> or m-directions and using repeated a-face growth. For the growth in polar directions the micropipe density and dislocation density decrease with increasing offorientation of the seed. A similar behavior was found for the stacking fault density and dislocation density in non-polar directions with off-orientation to c-direction. Nevertheless, while the dislocation density could be reduced up to three orders of magnitude for the growth along non-polar directions, the stacking fault density was continuously increasing. Additionally, the defect distribution after repeated a-face growth will be discussed in terms of growth related and kinetic models.

Abstract: Planar defects have been found in nitrogen doped 2" 4H-SiC crystals grown on off-axis seeds. The doping level was 1×1019cm-3, which is below the critical one for the thermally activated cubic stacking fault formation in the 4H matrix. Planar defects in the doped region are nucleated on the whole seed surface outside the growth facet. They are coexisting 15R and 6H lamellas of unitcell height as revealed by means of luminescence and high resolution transmission electron microscopy. These inclusions are preferably formed at the rim of the growth facet, where polytype change occurs after switching off the nitrogen flow during growth.