Hiroyuki Okino

In situ resistance measurements of epitaxial cobalt silicide nanowires on Si(110)

We have performed in situ resistance measurements for individual epitaxial CoSi2 nanowires (NWs) (approximately 60 nm wide and 5μm long) formed on a Si(110) surface. Two- and four-point probe measurements were done with a multitip scanning tunneling microscope at room temperature. The NWs were well isolated from the substrate by a Schottky barrier with zero-bias resistance of 107Ω. The resistivity of the NWs was 30μΩcm, which is similar to that for high-quality epitaxial films. The NW resistance was essentially unchanged after exposure to air.

Electrical conduction of Ge nanodot arrays formed on an oxidized Si surface

Carrier transport mechanism on Ge nanodot arrays formed on SiO2 monolayer covering over the Si surface is investigated by microscopic four-point-probe measurements combined with core-level photoemission spectroscopy and scanning tunneling microscopy. Different conduction natures are found depending on whether or not the nanodots and the substrate are directly connected by subnanometer-sized voids penetrating the SiO2 layer. In the presence of the voids, conductivity is regulated by the dot-size through quantum-size effect.

Si(111)-√21×√21-(Ag+Cs) Surface Studied by Scanning Tunneling Microscopy and Angle-Resolved Photoemission Spectroscopy

Scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) were used to study the atomic and electronic structures of the Si(111)-√21×√21-(Ag+Cs) surface (√21-Cs in short), which was induced by depositing caesium atoms on the Si(111)-√3×√3-Ag surface at room temperature (RT). Compared with previously reported STM images of noble-metal induced √21×√21 phases including the Si(111)-√21×√21-(Ag+Ag) and Si(111)-√21×√21-(Ag+Au) surfaces (√21-Ag and √21-Au, respectively), the √21-Cs surface displayed quite different features in STM images. The ARPES data of the √21-Cs surface revealed an intrinsic dispersive surface-state band, together with a non-dispersive one near the Fermi level, which was also different from those of the √21-Ag and √21-Au surfaces. These results strongly suggest different atomic arrangements between Cs- and noble-metal induced √21×√21 phases. Unlike the √21-Ag and √21-Au phases, the framework of the initial Si(111)-√3×√3-Ag substrate is totally broken at the √21-Cs phase.

Analyzing charge distribution in the termination area of 4H-SiC diodes by measuring depletion-layer capacitance

The distribution of positive-charge density at the SiO2/SiC interface of the termination area (Q TM) was analyzed by measuring the depletion-layer capacitance of 4H-SiC PN diodes with different termination structures. A change in Q TM induced by reverse-bias stressing (ΔQ TM) caused a change in the breakdown voltage of the diodes. By comparing the measured depletion-layer capacitance to the simulated value, the initial Q TM () and the distribution of ΔQ TM were clarified. It is concluded from these results that the distribution of ΔQ TM was not uniform but that positive charges mostly accumulated in the termination area under a high applied electric field.

Measuring depletion-layer capacitance to analyze a decrease in breakdown voltage of 4H-SiC diodes

A decrease in breakdown voltage (VBD) in the termination area of 4H-SiC PN diodes after 64 h reverse-bias stressing was analyzed. To analyze the VBD decrease, a novel analysis method based on the results of measuring depletion-layer capacitance of the PN diodes was proposed. The measurement results indicate that the positive-charge density (QTM) at the SiO2/SiC interface of the termination area increased after the reverse-bias stressing. Besides, in the case of QTM of 1012 cm−2 at the SiO2/SiC interface, the decrease in the measured capacitance showed the same tendency as the decrease in the simulated capacitance. By comparing the measured full depletion voltage and simulated full depletion voltage, the amount of QTM was estimated. It is thus concluded that the proposed method for measuring the depletion-layer capacitance is effective for analyzing the QTM change in the termination area.

Analysis of high reverse currents of 4H-SiC Schottky-barrier diodes

Nickel (Ni), titanium (Ti), and molybdenum (Mo) 4H-silicon carbide Schottky-barrier diodes (SiC SBDs) were fabricated and used to investigate the relation between forward and reverse currents. Temperature dependence of reverse current follows a theory that includes tunneling in regard to thermionic emission, namely, temperature dependence is weak at low temperature but strong at high temperatures. On the other hand, the reverse currents of the Ni and Mo SBDs are higher than their respective currents calculated from their Schottky barrier heights (SBHs), whereas the reverse current of the Ti SBD agrees well with that calculated from its SBH. The cause of the high reverse currents was investigated from the viewpoints of low barrier patch, Gaussian distribution of barrier height (GD), thin surface barrier, and electron effective mass. The high reverse current of the Ni and Mo SBDs can be explained not in terms of a low-barrier patch, GD, or thin surface barrier but in terms of small effective masses. Investigation of crystal structures at the Schottky interface revealed a large lattice mismatch between the metals (Ni, Ti, or Mo) and SiC for the Ni and Mo SBDs. The small effective mass is possibly attributed to the large lattice mismatch, which might generate transition layers at the Schottky interface. It is concluded from these results that the lattice constant as well as the work function is an important factor in selecting the metal species as the Schottky metal for wide band-gap SBDs, for which tunneling current dominates reverse current.Nickel (Ni), titanium (Ti), and molybdenum (Mo) 4H-silicon carbide Schottky-barrier diodes (SiC SBDs) were fabricated and used to investigate the relation between forward and reverse currents. Temperature dependence of reverse current follows a theory that includes tunneling in regard to thermionic emission, namely, temperature dependence is weak at low temperature but strong at high temperatures. On the other hand, the reverse currents of the Ni and Mo SBDs are higher than their respective currents calculated from their Schottky barrier heights (SBHs), whereas the reverse current of the Ti SBD agrees well with that calculated from its SBH. The cause of the high reverse currents was investigated from the viewpoints of low barrier patch, Gaussian distribution of barrier height (GD), thin surface barrier, and electron effective mass. The high reverse current of the Ni and Mo SBDs can be explained not in terms of a low-barrier patch, GD, or thin surface barrier but in terms of small effective masses. Investi...