Hirokazu Fukidome

Epitaxial graphene on silicon substrates

By forming an ultrathin (~100?nm) SiC film on Si substrates and by annealing it at ~1500?K in vacuo, few-layer graphene is formed on Si substrates. Graphene grows on three major low-index surfaces: (1?1?1), (1?0?0) and (1?1?0), allowing us to tune its electronic properties by controlling the crystallographic orientation of the substrate. This graphene on silicon (GOS) technology thus paves the way to industrialization of this new material with inherent excellence. With its feasibility in Si technology, GOS is one of the most promising candidates as a material for Beyond CMOS technology.

Epitaxial Growth Processes of Graphene on Silicon Substrates

Few-layers graphene is epitaxially grown on silicon substrates via SiC thin films inserted in between. We have conducted a detailed structural characterization of this graphene-on-silicon (GOS) material by Raman spectroscopy and transmission-electron microscopy, to obtain insights into the impacts of process parameters on defect formation. Results suggest that defects in graphene preferentially dwell at steps. Future flattening of the SiC surface, prior to graphene growth, is thus expected to contribute to the improvement of GOS quality.

A Very Simple Method of Flattening Si(111) Surface at an Atomic Level Using Oxygen-Free Water

Si(111) surfaces were found to be very easily flattened at an atomic level by immersing the wafers in water, from which dissolved oxygen was removed by the addition of sulfite ion as chemical deoxygenator, at room temperature. After the treatment with this oxygen-free water, the Si(111) surfaces slightly misoriented in the direction showed straight and parallel steps and wide terraces under atomic force microscopy observation. When wafers slightly misoriented in the opposite direction were treated in the same manner, the steps showed a characteristic zigzag pattern with an angle of 60°. The steps that appeared on both surfaces were attributable to monohydride steps generated on the edge of flat terraces.

Electrochemical study of atomically flattening process of silicon surface in 40% NH4F solution

Abstract From AFM observations, we found that atomic flattening of Si(111) surfaces in 40% NH4F solutions was accelerated by removing oxygen using a chemical deoxygenator. Furthermore, the surfaces became flatter by treatment in solutions without oxygen. The dissolution of Si in 40% NH4F solutions can be electrochemically monitored as anodic currents. From the measurements of the anodic currents, we found that the dissolution rate of Si was enhanced by removing oxygen from the solution. These results suggest that oxygen blocks the reactive sites on Si from the attacks of etching species in solution. At low temperature and in the absence of oxygen, although the morphological change was slow, the surface became very flat.

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.

Enhanced etching rate of silicon in fluoride containing solutions at pH 6.4

The etching rate of silicon in fluoride-containing solutions was found to show a remarkable pH dependence, having a sharp peak at about pH 6.4. A very similar pH dependence was also observed for the anodic current of n-type silicon electrodes in the fluoride-containing solutions. The chemical reactions of silicon occurring in the fluoride-containing solutions were attributed to the oxidative breaking of the surface Si-Si bonds by HF 2 + ions on the sites where the fluorine atom is bonded. Under acidic conditions, the surface is very stable because it is terminated with hydrogen atoms. In solutions at pHs higher than 7, the reaction rate becomes low owing to the very low concentrations of HF 2 - ions in these solutions. These factors are concluded to be the reasons for the appearance of peaks at pH about 6.4 for the etching rate and the anodic current.

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.

Precise control of epitaxy of graphene by microfabricating SiC substrate

Epitaxial graphene (EG) on SiC is promising owing to a capability to produce high-quality film on a wafer scale. One of the remaining issues is microscopic thickness variation of EG near surface steps, which induces variations in its electronic properties and device characteristics. We demonstrate here that the variations of layer thickness and electronic properties are minimized by using microfabricated SiC substrates which spatially confines the epitaxy. This technique will contribute to the realization of highly reliable graphene devices.

High-resolution molecular images of rubrene single crystals obtained by frequency modulation atomic force microscopy

Frequency modulation atomic force microscopy (FM-AFM) was employed to study molecular structures of rubrene single crystals in ultrahigh vacuum. Molecularly flat and extraordinarily wide terraces were extended over the width of more than a few micrometers with monomolecular steps. Molecular packing arrangements and internal structures were revealed by FM-AFM. The unit cell determined by FM-AFM was consistent with the lattice parameters of bulk crystal within the experimental error, suggesting that the surface structure of rubrene is not reconstructed.