Fumihiko Yamada

Electron transport properties of Si nanosheets: Transition from direct tunneling to Fowler-Nordheim tunneling

We have characterized the electron transport properties of n-decylamine-functionalized Si nanosheets (NSs) using atomic force microscopy with a conductive cantilever under vacuum conditions at room temperature. Electrons are transported from the cantilever to the substrate through Si NSs. The Si NSs exhibit nonresonant tunneling; the transport mechanisms are based on direct tunneling at low bias voltages and Fowler–Nordheim tunneling at high bias voltages.

RHEED transients during InAs quantum dot growth by MBE

The growth mechanisms of InAs self-assembled quantum dots (QDs) on GaAs(001) by molecular beam epitaxy are studied by reflection high-energy electron diffraction (RHEED) transients along the two major axes, [110] and [11¯0]. The authors observe anisotropy in the intensity transients and that there are two stages in QD formation, which emerge as different slopes in the RHEED transients. The authors attribute the anisotropy of the RHEED transients to the shape of QDs based on analysis using atomic force microscopy. The difference in the QD formation processes at each slope is investigated together with photoluminescence measurements. The authors observe that the QD density increases during the first slope whereas the QD density remains constant and the QD size increases during the second slope.

Fabrication and coupling investigation of films of PbS quantum dots

The authors report the self-assembly and characterization of monolayer PbS quantum dots (QDs) on GaAs substrates and the effect of thermal annealing on close-packed PbS QD films. The QD monolayer film was prepared using a 1,6-hexanedithiol self-assembled monolayer to link PbS QDs to GaAs substrates. Atomic force microscopy measurement confirmed the formation of PbS QD monolayer on GaAs. The temperature dependence of photoluminescence (PL) intensity of PbS QDs shows a feature typical for close-packed PbS QD films. For investigation of the influence of thermal annealing, three close-packed films of colloidal PbS QDs were prepared by drop coating and treated at different annealing temperatures. By controlling the annealing temperature, the PL intensity of close-packed films at room temperature can be improved. The room temperature PL intensity can be adjusted to be higher than that at 10 K. These results provide insights for future applications of colloidal QDs and the improvement of their performances.

Control of surface morphology and electronic properties of III-V semiconductors using molecular modification

GaAs surface was modified by self-assembled monolayer of organic molecules through wet chemical processes, which include etching and coating. Octadecanethiol (ODT) and benzenethiol (BT) were the choice as modifiers. Etched surface of the GaAs, whose quality greatly affects the morphology and properties of the resulting modified GaAs, was investigated by x-ray photoelectron spectroscopy combined with atomic force microscopy (AFM). Nanometer-scale particlelike structures of Ga2O3 appeared on the surface after etching by etchant with low acid concentration. The particle size was controllable by the acid concentration and etching time. Almost the entire surface of the GaAs substrate was successfully covered with thiol molecules when GaAs etched by the etchant with high acid concentration was used. The conduction of the ODT-coated GaAs measured with conductive prove AFM using Rh coated cantilevers was below the detection limit of the current amplifier. However, current-voltage characteristics of BT-coated GaAs...

Laser-induced local activation of Mg-doped GaN with a high lateral resolution for high power vertical devices

A method for laser-induced local p-type activation of an as-grown Mg-doped GaN sample with a high lateral resolution is developed for realizing high power vertical devices for the first time. As-grown Mg-doped GaN is converted to p-type GaN in a confined local area. The transition from an insulating to a p-type area is realized to take place within about 1–2 μm fine resolution. The results show that the technique can be applied in fabricating the devices such as vertical field effect transistors, vertical bipolar transistors and vertical Schottkey diode so on with a current confinement region using a p-type carrier-blocking layer formed by this technique.

nm-scaled workfunction mapping of the interfaces of silicon heterojunction (SHJ) solar cell using Kelvin probe force microscopy

We performed workfunction mapping of the cleaved interface of silicon heterojunction (SHJ) solar cell. While it has been widely accepted that the efficiency of solar cells depends on their electric contacts at various interfaces, no direct information on the electronic properties had been obtained on nm-scale. In this work, we employed Kelvin probe force microscopy (KFM) and simultaneously measured the workfunction and morphology of the cleaved interfaces on nm-scale for the first time. We show the measured workfunction differences between the surface layers on the cleaved SHJ solar cell, and discuss how such information can be used to improve the quality of the devices.

Self-assembled colloidal PbS quantum dots on GaAs substrates

We report the fabrication and analysis of self-assembled monolayer and bilayer films of colloidal PbS quantum dots (QDs) on GaAs (001) substrates. 1,6-hexanedithiol is used as link molecule between QDs and GaAs substrates. Atomic force microscopy (AFM) and photoluminescence (PL) measurements confirm the formation of PbS QD film on GaAs. For the monolayer PbS QD film, the temperature-dependent PL shows a feature typical of close-packed film. For the bilayer PbS QD film fabricated from two different mean-sized PbS QDs, we find that the stacking sequence of QDs with different size affects the quantum yield and emission wavelength of the film.