Maki Suemitsu

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.

Low-temperature formation of an interfacial buffer layer using monomethylsilane for 3C–SiC/Si(100) heteroepitaxy

By using monomethylsilane (MMS:H3Si–CH3), we have formed a Si1−xCx interfacial buffer layer for 3C–SiC/Si(100) heteroepitaxy at substrate temperature Tf of as low as 450–650 °C, which is compared to the conventional carbonization temperature of 900 °C or higher. The buffer layer allows the subsequent growth of high-quality single-crystalline 3C–SiC films at 900 °C without formation of voids in the Si substrate at the interface. The grown 3C–SiC films degrade for Tf 650 °C. The low processing temperature as well as the suppressed Si outdiffusion can be related to the inclusion of both Si–H and Si–C bonds within the MMS molecule.

High Quality Silicon Epitaxy at 500°C using Silane Gas-Source Molecular Beam Technique

High-quality silicon epitaxial films were successfully obtained at as low as 500°C using a SiH4 molecular beam technique in a UHV system. Perfect selective growth onto SiO2-patterned wafers, which allowed a precise measurement on epitaxial film thickness through the height of the step formed by the selective growth, was also observed. Detailed Arrhenius plots on the growth rate were thus obtained for the temperature range 500–800°C. The plots showed a break around 600°C, separating a lower- and a higher-temperature region with activation energies of 21 kcal/mol and 3.6 kcal/mol, respectively. Growth rate measurements with the silane pressure varied for 2.0, 3.0, and 4.0×10-4 Torr revealed first-order reaction kinetics for the lower-temperature region and a second-order nature for the higher-temperature region.

Structure, Chemical Bonding and These Thermal Stabilities of Diamond-Like Carbon (DLC) Films by RF Magnetron Sputtering

We have deposited diamond-like carbon (DLC) films using RF magnetron sputtering techniques, and investigated structure, chemical bonding of deposited films and these thermal stabilities by Raman spectroscopy and photoelectron spectroscopy. It has been found that the film deposited under typical conditions is amorphous carbon (a-C) with 62% sp2 and 38% sp3 bonds. Ordering of a-C has been observed with an increase in substrate temperature during deposition and similarly observed after postannealing, although the sp3/sp2 ratio in a film does not change even at 900°C. The absence of conversion between sp3 and sp2 bonds indicates that the DLC films have high thermal stabilities.

Effects of mixing germane in silane gas‐source molecular beam epitaxy

Growth of SiGe gas‐source molecular beam epitaxy (GSMBE) using silane/germane mixture has been investigated for the germane content of 0%, 0.8%, and 2.6%. From detailed measurements on the growth rate, a separation into high‐ and low‐temperature regions of the growth rate, as in silane‐GSMBE system, has been found to exist in this silane/germane system. A simultaneous measurement on the surface hydrogen coverage has revealed that the growth in the low‐temperature region is rate limited by the surface hydrogen desorption process, reasoning the enhanced growth rate with germane in terms of the reduced coverage of the surface hydrogen. All the growth rates followed a same fourth power dependence on the free‐adsorption site, which suggests a validity of the four‐site adsorption model, established for silane‐GSMBE, in silane/germane GSMBE.

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.

Low Temperature Silicon Surface Cleaning by HF Etching/Ultraviolet Ozone Cleaning (HF/UVOC) Method (I)—Optimization of the HF Treatment—

Several variations of fluoric acid (HF) treatments of silicon substrates were examined for their adaptability as a pretreatment method for a silicon epitaxy process. Treatments with and without distilled, deionized (DI) water rinse, of different HF concentrations, and of different methods of HF supply were tested and their residual carbonic impurity contents were measured using RHEED. As a result, HF treatments by themselves were found to be insufficient in passivating the surface dangling bonds irrespective of the method of HF supply: dipping into the solution or exposure to the vapor. The optimum procedure of HF treatment thus proposed is a succession of (a) HF dipping, (b) DI-water rinsing, (c) nitrogen-gas blowing, and (d) UV-ozone cleaning.

Silane adsorption on Si(001)2×1

Surface hydrogen coverage and the surface reconstruction of a silane‐saturated Si(001)2×1 surface were investigated using the thermal‐desorption‐spectroscopy (TDS) and the reflection‐high‐energy‐electron‐diffraction measurements. The TDS spectrum mainly presented a single β1 peak around 520 °C, indicating the predominance of a Si monohydride phase on this surface. This observation agreed with the surface hydrogen coverage (H/Si=1.1–1.4) obtained from the integrated peak area of the TDS spectrum. By a repeated silane‐saturation/thermal‐desorption experiment, it was also clarified that all the hydrogen atoms in the silane molecules adsorb at this room temperature exposure. Adsorption mechanisms of silane molecules onto Si(001) surfaces are discussed and a model is presented based on the result.

Formation of quasi-single-domain 3C-SiC on nominally on-axis Si(001) substrate using organosilane buffer layer

Quasi-single-domain 3C-SiC films have been successfully grown on nominally on-axis Si(001) substrate. The starting surface is either of 2×1 quasi-single-domain or of 2×1+1×2 double-domain. The point here is to use dc-resistive heating of the substrate and to form a low-temperature (650 °C) interfacial buffer layer using monomethylsilane (H3 C-SiH3). The dc resistive heating serves to form a single-domain Si(001)-2×1 or 1×2 starting surface or to develop a single-domain 3C-SiC(001)-2×3 or 3×2 surface on a 2×1+1×2 double-domain Si(001) substrate. When a single-domain Si(001) starting surface is utilized, it is not the dc polarity during growth but the surface reconstruction of the starting surface that determines the dominant domain in the 3C-SiC film. The thickness of the single-domain 3C-SiC film is as thin as ∼45–200 nm, which is about three orders of magnitude smaller than that required in a previous study (>5 μm).