Ryota Takahashi

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.

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.

Oxygen-Induced Reduction of the Graphitization Temperature of SiC Surface

In the solid–vapor phase equilibria between SiC and O2 system, there exists a region where the reaction (2+x)SiC+O2→(2+x)Si↑+ 2CO↑+ xC↓ takes place [Y. W. Song and F. W. Smith: J. Am. Ceram. Soc. 88 (2005) 1864]. By tuning the temperature and the oxygen pressure used in the graphitization annealing into this region, we have succeeded in the growth of epitaxial graphene on SiC crystals at 1000 °C, which is lower, by 250 °C or more, than the conventional epitaxial graphene method. The method is especially useful to formation of epitaxial graphene on silicon (GOS), which requires a lower graphitization temperature because of the Si substrate as well as of its mission to attain compatibility with Si technology.

Polymer Material as a Gate Dielectric for Graphene Field-Effect-Transistor Applications

Epitaxial-graphene field-effect transistor (EG-FET) with a polymer gate dielectric was fabricated and their electrical characteristics were investigated. The epitaxial graphene layer was formed on a semi-insulating 6H-SiC substrate by a high-temperature annealing in ultrahigh vacuum. The formation of graphene was confirmed by low-energy electron diffraction (LEED), Raman-scattering spectroscopy and X-ray photoelectron spectroscopy (XPS). The polymer gate dielectric (ZEP520a) layer was formed by spin coating, which exhibits good dielectric properties without noticeable structural degradation of the graphene layer. The EG-FETs with this polymer gate dielectric shows an n-type characteristic, with the field-effect mobility of 580 cm2 V-1 s-1.

Investigation of Graphene Field Effect Transistors with Al2O3 Gate Dielectrics Formed by Metal Oxidation

We propose the epitaxial graphene field-effect transistors (EG-FETs) with Al2O3 gate dielectric formed by metal oxidation on semi-insulating 6H-SiC substrate. The Al2O3 gate dielectric layer was formed by thermally oxidizing a thin Al film at 500 °C in an O2 ambient. The electrical characteristics of EG-FETs with Al2O3 gate dielectric have been investigated, which exhibits p-type behavior with the field effect mobility of 120 cm2 V-1 s-1. After the Al2O3 formation, the FWHM of G and D peaks in the Raman-scattering spectra increased, indicating degradation of graphene and thereby accounting for the low field effect mobility of the EG-FETs. This finding suggests that the optimization of post growth heat treatments, such as oxidation and metallization, should play a decisive role in tuning the quality of graphene and the performance of the device made thereof.