Katsuya Nozawa

Suppression of Inhomogeneous Segregation in Graphene Growth on Epitaxial Metal Films

Large-scale uniform graphene growth was achieved by suppressing inhomogeneous carbon segregation using a single domain Ru film epitaxially grown on a sapphire substrate. An investigation of how the metal thickness affected growth and a comparative study on metals with different crystal structures have revealed that locally enhanced carbon segregation at stacking domain boundaries of metal is the origin of inhomogeneous graphene growth. Single domain Ru film has no stacking domain boundary, and the graphene growth on it is mainly caused not by segregation but by a surface catalytic reaction. Suppression of local segregation is essential for uniform graphene growth on epitaxial metal films.

Quantum Hall charge sensor for single-donor nuclear spin detection in silicon

We propose a novel scheme for detecting the nuclear spin state of a phosphorus impurity in silicon. Selective ionization by optical pumping of the donor-bound exciton transition is detected electrically using the quantum Hall effect.

Preparation of Ge1-yCy Alloys by C Implantation into Ge Crystal and Their Raman Spectra

We attempted to prepare Ge1-yCy alloys by implantation of C atoms into Ge crystals and post-annealing. For samples annealed at temperatures higher than 450°C, X-ray diffraction (XRD) spectra indicated that the Ge1-yCy alloys were successfully prepared. The highest substitutional C content was about 1 at.%. The optimum annealing temperature to incorporate C atoms into the substitutional sites was 450–500°C. We also investigated the dependence of the Raman intensity of the Ge–C local mode on the substitutional C content. The intensity of the Ge–C local mode was found to increase in proportion to the substitutional C content. Furthermore, the relationship between Ge–C peak intensity and substitutional C content was relatively in good agreement with that theoretically predicted.

Bandgap and Strain Engineering in SiGeC Heterojunction Bipolar Transistors

The incorporation of C into Si1-xGex alloys contributes to enlarging the critical layer thickness and to improving the thermal budget. It also realizes a narrower bandgap with compensated strain. These effects would introduce good performance at high frequency in the devices. We fabricate heterojunction bipolar transistors (HBTs) with an Si1-x-yGexCy base layer using ultrahigh-vacuum chemical vapor deposition (UHV-CVD) technology. The bandgaps of Si1-x-yGexCy base layers are measured by the evaluation of the collector current dependence on temperature. The strain of the pseudomorphic Si1-x-yGexCy layer is also extracted by an analysis of the X-ray diffraction spectra. Good flexibility of Si1-x-yGexCy alloy is shown for the bandgap and strain engineering. The devices using the excellent characteristics of Si1-x-yGexCy alloy have numerous applications for wireless telecommunications.

Reduction of neutral base recombination in narrow band-gap SiGeC base heterojunction bipolar transistors

Narrow band-gap SiGeC base HBTs were fabricated by Si compatible processing. By introducing 0.3% carbon into the SiGe layers, lattice strain was decreased and neutral base recombination localized at the collector-base heterojunction was significantly reduced.

Infrared Absorption Spectra of C Local Mode in Si1-x-yGexCy Crystals

The local vibration mode of substitutional C atoms (C-LVM) in high-quality Si1-x-yGexCy crystals was studied by infrared absorption spectroscopy. The peak intensity and full width at half maximum of C-LVM were found to change depending on Ge content as well as substitutional C content. However, the integrated intensity of C-LVM exhibited a linear dependence on the substitutional C content. These results demonstrate that the effective charge of substitutional C atoms in Si1-x-yGexCy crystals is independent of their atomic configurations. Moreover, the present results clearly indicate that the substitutional C content can be estimated from the integrated intensity of C-LVM.

Potential-Energy Surface of Graphene on Transition-Metal Surfaces

The nature of graphene/metal interface is of great interest for understanding graphene growth by chemical vapor deposition. We have calculated the potential-energy surface (PES) of graphene on catalyst transition-metal surfaces. We found that the adsorption state of graphene varies from chemisorption at the minimum of PES to physisorption at the maximum of PES. The minima of PES highly depend on the type of metal, whereas the maxima of PES are nearly independent. The order of PES roughness is Cu < Ni < Co. The level of metal d-band is a key factor for PES roughness and the adsorption state of graphene.