We demonstrate a simple method for direct observation of the stacking orientation on 4H/6H-SiC {0001} surfaces by low-voltage SEM. The difference in the direction of the stacking orientation is observed as SEM contrast. By utilizing this technique, the bond configuration at {1-10n} steps can be determined by the SEM contrast.
We report a new approach to produce high quality epitaxial graphene based on the concept of controlling Si sublimation rate from SiC surface. By putting a mask substrate to suppress Si sublimation from the SiC surface in ultrahigh vacuum, epitaxial graphene growth at 4H-SiC (0001) was locally controlled. Spatially graded surface graphitization was confirmed in a scanning electron microscopy contrast from the outside unmasked region to the inside masked region. The contrast was discussed with Raman characterization as the increase of graphene thickness and the surface compositional change of SiC. Results indicate two types of growth processes of epitaxial graphene at 4H-SiC (0001) step-terrace structures.
We have prepared epitaxial graphene by a Si sublimation method from 4H-SiC. Single-particle spectroscopy of CdTe quantum dots (QDs) on epitaxial graphene covered with polyvinylpyrrolidone (PVP) or polyethylene glycol (PEG) showed the suppression of luminescence blinking and ∼10 times decreased luminescence intensity as compared with those on a glass. The electronic coupling constant, H01, between CdTe QDs and graphene was calculated to be (3.3 ± 0.4) × 102 cm−1 in PVP and (3.7 ± 0.8) × 102 cm−1 in PEG based on Marcus theory of electron transfer and Tang-Marcus model of blinking with statistical distribution.
The epitaxial graphene growth at the 4H-SiC(0001) surface with intentionally inserted step-free basal plane regions was performed by high temperature annealing in the range of 1600–1900 °C under ultrahigh vacuum. For fabricating inverted-mesa structures with the step-free regions at SiC surfaces, a combined process consisting of a direct laser digging and a Si-vapor etching at 1900 °C was utilized. The graphitized surfaces were characterized by atomic force microscopy, low acceleration voltage (0.1–1.0 kV) scanning electron microscopy and Raman spectroscopy. It was found that the graphene thickness at the SiC step-free surface tends to be suppressed compared with the thickness at background SiC step-terrace surfaces where the steps are intrinsically introduced from intentional/unintentional substrate miscut angles. From the characterization by Raman mapping, 1 ML graphene was obtained at the SiC step-free surface at 1600 °C graphitization in contrast to the case that multilayer graphene was grown at SiC step-terrace surfaces.
A potential of low energy (< 1keV) electron channeling contrast imaging (LE‐ECCI) by scanning electron microscopy (SEM) is demonstrated to characterize the crystallographic stacking sequence of hexagonal 4H‐SiC single crystal within the first unit cell thickness from the topmost surface (< 1nm). It is also revealed that the LE‐SEM signal intensity associated with the crystallographic orientation has enough sensitivity to even the change in Si‐atom and C‐atom (i.e. the change in surface polarity) at a primary electron energy of 0.4keV. The obtained data can be explained by an electron multiple scattering cluster (EMSC) theory where the interference of both incident and diffracted electron waves is considered within a finite size (a few nanometers) of an atomic cluster. So far, for characterizing local crystallographic orientation of bulk crystals using SEM, ECCI with a higher energy (> 10keV) of primary electron beam has been utilized in order to obtain larger interaction volume due to its deeper penetration depth [1]. The electron channeling contrast (ECC) from only a shallower surface region with lower energy has not been applied yet because of the consideration attributed to the lack of enough interaction volume in the lower energy regime. In fact, to our knowledge, the critical smallest volume (specimen minimum thickness) has not been investigated yet associated with ECC. In our previous study [2], a crystallographic orientation contrast from two bilayers thick surface layers of 4H‐SiC single crystal was observed by changing an incident LE (< 1keV) EB direction with respect to the orientation of an atomically flattered 4H‐SiC surface (Figs. 1). This contrast is considered to be the ECC in an extremely low electron energy regime. This result brings about the assumption that even the two bilayers thick interaction volume can contribute to the formation of ECC. In order to verify this assumption, in this study, crystallographic orientation dependence of SEM signal intensity was quantitatively measured on an atomically flat 4H‐SiC surface. Furthermore, in order to exploit the Z ‐information, different polar faces, Si‐terminated (0001) Si‐face, and C‐terminated (000‐1) C‐face were compared, where atomic sites of Si‐ and C‐atoms are switched in the crystal. The obtained results were compared with the EMSC theory in which the concept of short‐range periodicity is included [3]. This theoretical model is more applicable to reproduce the effect of shallower information depth of the LE‐EB, compared with a Bloch‐wave model utilized in the conventional ECCI in the higher electron energy regime. Fig.2 shows the representative SEM signal intensity from 4H‐SiC Si‐face and C‐face as a function of the sample tilting angle obtained at a primary electron energy of 0.4 keV. Although the profile shapes seem to be different at a glance, peak positions are almost the same. Considering the fact that the atomic arrangement below the surface is identical both in Si‐face and C‐face SiC, except Si‐ /C‐atomic sites in the crystal, the difference in the profile shape is ascribed to the contrast of backscattered electron yield [4]. The similar trend was also reproduced by the EMSC calculation.
Step, ridge, and crack submicro/nanostructures of epitaxial graphene on 4H-SiC (0001̅) were characterized using tip-enhanced Raman scattering (TERS) spectroscopy. The nanostructures were created during graphene synthesis due to a difference in the thermal expansion coefficient of graphene and SiC. These structures are a distinctive property of epitaxial graphene, together with other desirable properties, such as large graphene sheet and minimal defects. The results of this study illustrate that the exceptional spatial resolution of TERS allows spectroscopic measurements of individual nanostructures, a feat which normal Raman spectroscopy is not capable of. By analyzing TERS spectra, the change of local strain on the nanoridge and decreased graphene content in the submicrometer crack were detected. Using G′ band positions in the TERS spectra, the strain difference between the ridge center and flat area was calculated to be 1.6 × 10–3 and 5.8 × 10–4 for uniaxial and biaxial strain, respectively. This confirms the proposed mechanism in previous researches that nanoridges on epitaxial graphene form as a relief against compressive strain. With this study, we demonstrate that TERS is a powerful technique for the characterization of individual local nanostructures on epitaxial graphene.
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