Shunsuke Abe

Controls over Structural and Electronic Properties of Epitaxial Graphene on Silicon Using Surface Termination of 3C-SiC(111)/Si

Epitaxial graphene on Si (GOS) using a heteroepitaxy of 3C-SiC/Si has attracted recent attention owing to its capability to fuse graphene with Si-based electronics. We demonstrate that the stacking, interface structure, and hence, electronic properties of GOS can be controlled by tuning the surface termination of 3C-SiC(111)/Si, with a proper choice of Si substrate and SiC growth conditions. On the Si-terminated 3C-SiC(111)/Si(111) surface, GOS is Bernal-stacked with a band splitting, while on the C-terminated 3C-SiC(111)/Si(110) surface, GOS is turbostratically stacked without a band splitting. This work enables us to precisely control the electronic properties of GOS for forthcoming devices.

Surface Chemistry Involved in Epitaxy of Graphene on 3C-SiC(111)/Si(111).

Surface chemistry involved in the epitaxy of graphene by sublimating Si atoms from the surface of epitaxial 3C-SiC(111) thin films on Si(111) has been studied. The change in the surface composition during graphene epitaxy is monitored by in situ temperature-programmed desorption spectroscopy using deuterium as a probe (D2-TPD) and complementarily by ex situ Raman and C1s core-level spectroscopies. The surface of the 3C-SiC(111)/Si(111) is Si-terminated before the graphitization, and it becomes C-terminated via the formation of C-rich (6√3 × 6√3)R30° reconstruction as the graphitization proceeds, in a similar manner as the epitaxy of graphene on Si-terminated 6H-SiC(0001) proceeds.

Low-Energy-Electron-Diffraction and X-ray-Phototelectron-Spectroscopy Studies of Graphitization of 3C-SiC(111) Thin Film on Si(111) Substrate

Epitaxial graphene can be formed on silicon substrates by annealing a 3C-SiC film formed on a silicon substrate in ultrahigh vacuum (G/3C-SiC/Si). In this work, we explore the graphitization process on the 3C-SiC(111)/Si(111) surface by using low-energy electron diffraction and X-ray photoelectron spectroscopy (XPS) and compare them with that on 6H-SiC(0001). Upon annealing at T≥1150 °C, the 3C-SiC(111)/Si(111) surface follows the sequence of (√3×√3)R30°, (6√3×6√3)R30°, and (1×1)graphene in the surface structures. The C 1s core level according to XPS indicates that a buffer layer, identical with that in G/6H-SiC(0001), exists at the G/3C-SiC(111) buffer. These observations strongly suggest that graphitization on the surface of the 3C-SiC(111) face proceeds in a similar manner to that on the Si-terminated hexagonal bulk SiC crystals.

Control of epitaxy of graphene by crystallographic orientation of a Si substrate toward device applications

Graphene is a promising material in next-generation devices. Large-scale epitaxial graphene should be grown on Si substrates to transfer the accumulated technologies to integrated devices. We have for this reason developed epitaxy of graphene on Si (GOS) and device operation of the backgate field-effect transistors (FETs) using GOS has been confirmed. It is demonstrated in this paper that the GOS method enables us to tune the structural and electronic properties of graphene in terms of the crystallographic orientation of the Si substrate. Furthermore, it is shown that the uniformity of the GOS process within a sizable area enables us to reliably fabricate topgate FETs using conventional lithography techniques. GOS can be thus the key material in next-generation devices owing to the tunability of the electronic structure by the crystallographic orientation of the Si substrate.

Transmission Electron Microscopy and Raman-Scattering Spectroscopy Observation on the Interface Structure of Graphene Formed on Si Substrates with Various Orientations

Graphene can be grown on three major low-index substrates of Si(111), (110), and (001) by forming a 3C-SiC thin film and by subliming Si atoms from the top few layers of the SiC film. We have investigated the structure of graphene/3C-SiC interface by cross-sectional transmission electron microscopy (XTEM) and Raman-scattering spectroscopy. While the interface layer quite similar to that on the graphene/6H-SiC(0001) face is found to exist on the 3C-SiC(111)/Si(111) substrate, no such interface structure exists on the (110)- and (001)-oriented faces.

High-Frequency Coherent Phonons in Graphene on Silicon

High-frequency coherent vibrations have been investigated in graphene on silicon substrates using pump-probe anisotropic reflectivity spectroscopy with 7.5 fs laser pulses. The coherent phonons of both the intralayer C=C stretching mode (G-mode) and the disorder-induced mode (D-mode) are clearly observed even in monolayer graphenes. The G-mode shifts to higher frequencies as the number of graphene layers decreases, whereas the D-mode does not exhibit such hardening. Moreover, the D-mode softens upon photoexcitation and then shifts to higher frequencies during the relaxation process. The different frequency shift behaviors in the G- and D-modes are discussed.

Temperature-Programmed Desorption Observation of Graphene-on-Silicon Process

With its industrial adaptability, graphene-on-silicon (GOS), formed by ultrahigh-vacuum annealing of a SiC thin film on a silicon substrate, is attracting recent attention. Little is known, however, about the growth mechanism of GOS. We demonstrate in this paper that temperature-programmed-desorption spectroscopy of deuterium (D2-TPD) can be a powerful in-situ probe to investigate the surface chemistry during formation of epitaxial graphene (EG) on SiC crystals. Using the D2-TPD, the surface stoichiometry and the back-bonds of the surface atoms, including their dependence on the crystallographic orientations [Si(111), Si(100), and Si(110)] can be obtained. Difference in the growth mechanism of GOS among the orientations is discussed based on the results.

Improvement in Film Quality of Epitaxial Graphene on SiC(111)/Si(111) by SiH4 Pretreatment

The epitaxy of graphene on 3C-SiC/Si (GOS) has attracted much attention owing to its viability to fuse graphene with Si-based technologies. It is known that the surface condition of the 3C-SiC thin film before graphitization plays a decisive role in determining the quality of the GOS film. We have investigated the effect of the pretreatment of the 3C-SiC thin film in vacuo at a SiH4 partial pressure of 6.7 ×10-4 Pa on the subsequent formation of graphene. As a result, it is revealed that the SiH4 pretreatment restores the defects on the SiC surface, such as the Si vacancy and point defects formed by the presence of native oxides, and improves the quality of graphene. The effect is found to be highest when the substrate temperature is 1173 K.

A ramsayite-type oxide, Ca2Sn2Al2O9

The title compound, dicalcium nonaoxidodistannate(IV)dialuminate, is the second example which crystallizes in the isotypic structure of a pyroxene silicate, Na2Ti2Si2O9 (ramsayite). ∞ 1[Sn2O8] chains and pyroxene-type ∞ 1[Al2O6] chains are formed along the b axis by sharing O atoms. The Ca atoms are situated in the resulting channels and exhibit a coordination number of 7.