Remigijus Vasiliauskas

Polarization doping of graphene on silicon carbide

The doping of quasi-freestanding graphene (QFG) on H-terminated, Si-face 6H-, 4H-, and 3C-SiC is studied by angle-resolved photoelectron spectroscopy close to the Dirac point. Using semi-insulating as well as n-type doped substrates we shed light on the contributions to the charge carrier density in QFG caused by (i) the spontaneous polarization of the substrate, and (ii) the band alignment between the substrate and the graphene layer. In this way we provide quantitative support for the previously suggested model of polarization doping of graphene on SiC (Ristein et al 2012 Phys. Rev. Lett. 108 246104).

Large-area microfocal spectroscopic ellipsometry mapping of thickness and electronic properties of epitaxial graphene on Si- and C-face of 3C-SiC(111)

Microfocal spectroscopic ellipsometry mapping of the electronic properties and thickness of epitaxial graphene grown by high-temperature sublimation on 3C-SiC (111) substrates is reported. Growth of one monolayer graphene is demonstrated on both Si- and C-polarity of the 3C-SiC substrates and it is shown that large area homogeneous single monolayer graphene can be achieved on the Si-face substrates. Correlations between the number of graphene monolayers on one hand and the main transition associated with an exciton enhanced van Hove singularity at ∼4.5 eV and the free-charge carrier scattering time, on the other are established. It is shown that the interface structure on the Si- and C-polarity of the 3C-SiC(111) differs and has a determining role for the thickness and electronic properties homogeneity of the epitaxial graphene.

Sublimation Growth and Structural Characterization of 3C-SiC on Hexagonal and Cubic SiC Seeds

Epitaxial growth of cubic silicon carbide on 6H-SiC substrates, and 6H-SiC substrates with (111) 3C-SiC buffer layer, deposited by vapour liquid solid mechanism, was compared. The morphological details of the grown layers were studied by optical microscopy and their microstructure by transmission electron microscopy. The influence of the substrate on the nucleation of 3C-SiC, the initial homoepitaxial 6H-SiC nucleation before 3C-SiC as well as the formation of defects, are discussed.

Progress in 3C-SiC growth and novel applications

Recent research efforts in growth of 3C-SiC are reviewed. Sublimation growth is addressed with an emphasis on the enhanced understanding of polytype stability in relation to growth conditions, such as supersaturation and Si/C ratio. It is shown that at low temperature/supersaturation spiral 6H-SiC growth is favored, which prepares the surface for 3C-SiC nucleation. Provided the supersaturation is high enough, 3C-SiC nucleates as two-dimensional islands on terraces of the homoepitaxial 6H-SiC. Effect of different substrate surface preparations is considered. Typical extended defects and their electrical activity is discussed. Finally, possible novel applications are outlined.

Two-Dimensional Nucleation of Cubic and 6H Silicon Carbide

The initial stage of heteroepitaxial growth of 3C-SiC and homoepitaxial growth of 6H-SiC on nominal 6H-SiC on-axis substrates has been studied. Before 3C-SiC starts to nucleate, 6H-SiC grows in a step-flow growth mode due to a slight off-orientation of the substrate surface already at about 1500oC. In the 1650-1700oC temperature interval 3C-SiC nucleates as 2D islands. A distance away from the 3C-SiC island 6H-SiC grows in step-flow mechanism. In the vicinity of the 3C-SiC islands the 6H-SiC growth steps start to change direction and even split into two steps with the equal height of 0.5 nm, which is approaching the unit cell size of cubic SiC. When the supersaturation is lower in comparison with the conditions for 3C-SiC growth, there is only formation of 6H-SiC, i.e. homoepitaxial growth. The growth mode of 6H-SiC is dependent on temperature. At the lowest temperature there is spiral growth while at higher temperature 2D nucleation is preferred.

Impact of extended defects on Hall and magnetoresistivity effects in cubic silicon carbide

From magnetoresistivity effect measurements the carrier mobility at room- temperature is 200 cm2/Vs in heteroepitaxially grown 3C-SiC on 6H-SiC by sublimation epitaxy. The main scattering mechanism ...

Structural Properties of 3C-SiC Grown by Sublimation Epitaxy

The present paper deals with morphological and structural investigation of 3C-SiC layers grown by sublimation epitaxy on on axis 6H-SiC(0001) at source temperature 2000 °C, under vacuum conditions (<10-5 mbar) and different temperature gradients in the range of 5-8 °C/mm. The layer grown at a temperature gradient 6 °C/mm has the largest average domain size of 0.4 mm2 assessed by optical microscope in transmission mode. The rocking curve full width at half maximum (FWHM) of (111) reflection is 43 arcsec which suggests good crystalline quality. The AFM image of the same layer shows steps with height 0.25 nm and 0.75 nm which are characteristic of a stacking fault free 3C-SiC surface and c-axis repeat height, respectively.

Thickness Uniformity and Electron Doping in Epitaxial Graphene on SiC

Large variations have been observed in the uniformity and carrier concentration of epitaxial graphene grown on SiC by sublimation for samples grown under identical conditions and on nominally on-axis hexagonal SiC (0001) substrates. We have previously shown that these issues are both related to the morphology of the graphene-SiC surface after sublimation growth. Here we present a study on how the substrate polytype, substrate surface morphology and surface restructuring during sublimation growth affect the uniformity and carrier concentration in epitaxial graphene on SiC. These issues were investigated employing surface morphology mapping by atomic force microscopy coupled with local surface potential mapping using Scanning Kelvin probe microscopy.

Macrodefects in Cubic Silicon Carbide Crystals

Different sublimation growth conditions of 3C-SiC approaching a bulk process have been investigated with the focus on appearance of macrodefects. The growth rate of 3C-SiC crystals grown on 6H-SiC varied from 380 to 460 μm/h with the thickness of the crystals from 190 to 230 μm, respectively. The formation of macrodefects with void character was revealed at the early stage of 3C-SiC crystal growth. The highest concentration of macrodefects appears in the vicinity of the domain in samples grown under high temperature gradient and fastest temperature ramp up. The formation of macrodefects was related to carbon deficiency which appear due to high Si/C ratio which is used to enable formation of the 3C-SiC polytype.

Sublimation epitaxy of cubic silicon carbide in vacuum

The epitaxial growth was compared of cubic silicon carbide on 6H-SiC substrates prepared in four different ways: (i) as received, (ii) re-polished, (iii) annealed and subsequently covered by a Si layer, (iv) with a (111) 3C-SiC buffer layer. The morphological details of the grown layers were studied by optical microscopy and their structure, by transmission electron microscopy. The influence of the substrate on the nucleation of 3C-SiC, on the homoepitaxial 6H-SiC nucleation before 3C-SiC and on the defect formation, primarily twinning, is discussed.