Toshitaka Kubo

Surface Phonons, Electronic Structure and Chemical Reactivity of Diamond (100)(2 ×1) Surface

Surface phonons, electronic structure and chemical reactivity of the diamond (100)(2 ×1) surface have been studied using electron energy loss spectroscopy (EELS), thermal desorption spectroscopy (TDS) and low-energy electron diffraction (LEED). Vibrational losses are observed at ~80, 92, 123, 135, 147 and 165 meV for a clean C(100)(2 ×1) surface. The 92 meV loss is assigned to the in-phase bouncing mode of the surface dimers. The origins of the other losses are discussed. Electronic transition is observed at 3.5 eV which is associated with the interband transition between the π and π* surface states. The chemical reactivity of the C(100)(2 ×1) surface towards several gases, H, H2, O, O2, CO, N2O and C2H2, has been investigated at 90 and 300 K. The chemical reactivity of the C(100)(2 ×1) surface towards these gases is compared with that of the Si(100)(2 ×1) surface, and the origin of the difference in the reactivity is discussed.

Investigation on the Surface Electronic States of the Si(001) c(4×2) and c(8×8) Surfaces: An Electron Energy Loss Spectroscopy Study

The surface electronic states of the clean (and CO-covered) Si(001) c(4×2) and c(8×8) surfaces at 90 K have been studied by using high resolution electron energy loss spectroscopy. On the clean c(8×8) surface, the loss peaks are observed at 0.3, 0.7 and 1.2 eV. The 1.2 eV loss is sensitive to the CO adsorption, whereas the 0.3 and 0.7 eV losses are not sensitive. These suggest that the c(8×8) structure contains the defects which are not understood by the dimer vacancy model.

Characterization of Effective Mobility and Its Degradation Mechanism in MoS 2 MOSFETs

Effective mobility in top-gated MoS2 metal-oxide-semiconductor field-effect transistors (MOSFETs) with HfO2/TaN gates was investigated. We propose a model for effective mobility in MoS2 MOSFETs and discuss its usefulness. Parameter fitting of the proposed effective mobility model makes possible a discussion of how the scattering mechanism limits the effective mobility. Charged impurities, phonons, and surface roughness were presumed to be possible causes of scattering, and the effect of contact resistance was also taken into account. In the device exhibiting the better mobility of 17.4 cm2/Vs, phonon-assisted hopping carrier transport probably limited the mobility. In contrast, in the device exhibiting the worse mobility of 2.8 cm2 /Vs, scattering caused by charged impurities dominated the transport characteristics. The proposed model facilitates device-to-device comparisons of carrier transport characteristics of MoS2 transistors at room temperature and accelerates their further development.

The investigation of graphene film as a new electrical contact material

With the advent of nanotechnology research, new nano-materials and nano-structural devices have been invented and fabricated. Particularly low dimensional materials such as fullerene, carbon nanotube and graphene have been investigated intensively in order to realize new functional electronic devices on a smaller scale. Since graphene surface is inactive chemically and electrically conductive, graphene film on copper is thought to be a promising electrical contact material to prevent from oxidation in air and we have started to investigate the electrical contact property with nano-indentation manipulator in scanning electron microscope. Force-resistance characteristics with and without graphene on copper were measured. Compared with copper foil, graphene film showed very stable and conductive force-resistance characteristics even after oxidation process of 180°C for 16 hours in air condition. This indicates that graphene coating is very effective to refrain from surface oxidation on copper substrate.

Surface structures of rutile TiO2(114)

The surface structures of rutile TiO2(114) have been studied using a combination of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Depending on the sample preparation, the surface exhibits many complicated local nanostructures, e.g., dot-like, missing row, row-like (1 × 3), and twin dotted (2 × 2) structures. After several cycles of sputtering and high-temperature annealing, all samples exhibit triangular pyramidal structure. Microfaceted structural models, which are composed of combinations of {111} and (001) microfacets, can explain all experimental results as well as the structural variety. The calculated STM images are in good agreement with the experimental results. The decreasing density of dangling bonds, the increasing coordination number, and the evolution of non-polar structures stabilize the surface energy, which results in the microfaceted reconstructions. The formation of various nanostructures and the surface stoichiometric changes are discussed.

Characterization of effective mobility by split C-V technique in MoS2 MOSFETs with high-k/metal gate

Effective mobility in top-gated MoS2 metal-oxide-semiconductor field-effect transistors (MOSFETs) with HfO2/TaN gate was investigated. We realized C-V measurements of MoS2 MOSFETs with a small gate area. The resultant surface carrier concentration dependence of the effective mobility suggested that surface roughness scattering was responsible for the accumulated electron mobility in the fabricated MoS2 MOSFET. The scanning tunneling microscope image exhibited atomic scale roughness on the cleaved MoS2 surface. This suggested that the surface roughness should be reduced to improve the performance of MoS2 MOSFETs.