Yoshiaki Nakamura

Scanning tunneling spectroscopic studies revealed the quantum-confinement effects in Ge nanocrystals formed with ultrahigh density (>1012cm−2) by Ge deposition on ultrathin Si oxide films. With decreasing crystal size, the conduction band maximum upshifted and the valence band minimum downshifted. The energy shift in both cases was about 0.7 eV with the size change from 7 to 2 nm. This shows that the energy band gaps of Ge nanocrystals increased to ∼1.4eV with decreasing size. This size d

The design and fabrication of nanostructured materials to control both thermal and electrical properties are demonstrated for high-performance thermoelectric conversion. We have focused on silicon (Si) because it is an environmentally friendly and ubiquitous element. High bulk thermal conductivity of Si limits its potential as a thermoelectric material. The thermal conductivity of Si has been reduced by introducing grains, or wires, yet a further reduction is required while retaining a high elec

The authors observed a quantum-confinement effect in individual Ge1−xSnx quantum dots (QDs) on Si (111) substrates covered with ultrathin SiO2 films using scanning tunneling spectroscopy at room temperature. The quantum-confinement effect was featured by an increase in the energy band gap of ∼1.5eV with a decrease in QD diameter from 35to4nm. The peaks for quantum levels of QDs became broader with a decrease in the height-diameter aspect ratio of QDs, demonstrating the gradual emergence of two d

Two-dimensional nanoarrays of Ge quantum dots (QDs) with the ability to self-repair were epitaxially grown by self-organization on Si substrates using an ultrathin SiO(2) film technique. Nanometer-sized voids were patterned on ultrathin SiO(2) films by transcription of the pattern of block copolymer films using a selective etching method and worked as nucleation sites for QD growth. The epitaxial QDs were elastically strain-relaxed without misfit dislocations and of uniform size. The epitaxial s

The electrochemical etching method by Ibe et al. [J. Vac. Sci. Technol. A 8, 3570 (1990)] to fabricate sharp tips for scanning tunneling microscopy was modified by shortening the cutoff time of the etching current after the material wire drops off at the air-electrolyte interface. The tip radius measured by field ion microscopy was successfully reduced to 8 nm when the cutoff time was shortened to 50 ns. The dependence of the field-emitting electron current from the sharpest tips was close to on

A Si-based superlattice is one of the promising thermoelectric films for realizing a stand-alone single-chip power supply. Unlike a p-type superlattice (SL) achieving a higher power factor due to strain-induced high hole mobility, in the n-type SL, the strain can degrade the power factor due to lifting conduction band degeneracy. Here, we propose epitaxial Si-rich SiGe/Si SLs with ultrathin Ge segregation interface layers. The ultrathin interface layers are designed to be sufficiently strained,

Abstract Layered silicene with deformed buckled structure attracts great interest as a next generation 2D Dirac thermoelectric material beyond conventional layered materials. However, the difficulty of modulating atomic positions in silicene prevents its realization. This study proposes a method to deform buckled structure in layered silicene by controlling the intercalated atoms, which can dramatically enhance its thermoelectric properties. Silicene buckled structure is deformed in epitaxial Ca

We have developed a novel epitaxial growth technique called nanocontact epitaxy that allows high quality Ge films to be epitaxially grown on Si(001) substrates using spherical nanodots as seed crystals. The nanodots only make contact with the Si substrate through nanowindows in an intermediate ultrathin SiO2 film, and they were elastically strain-relaxed without misfit dislocations. Ge films with a thickness of 130 nm were epitaxially grown on the nanodot seed crystals by solid source molecular

A method to form epitaxial Ge1−xSnx quantum dots (QDs) on Si (111) substrates has been developed by codeposition of Ge and Sn on ultrathin SiO2 films with predeposited Ge nuclei. Hemispherical Ge1−xSnx QDs with an ultrahigh density (∼1012 cm−2) were epitaxially grown in the nanometer-size range. The QD size was controlled by changing the GeSn deposition amount. High-resolution transmission electron microscopy observations revealed that the main formed Ge1−xSnx QDs had less strain and no misfit d

The hopping movements of Cl atoms on a Si(111)-(7 x 7) surface that are enhanced by an electron injection from tips of a scanning tunneling microscope (STM) exhibit a spatial spread from the electron injection point with an anisotropic distribution. The enhanced hopping effect becomes greatest at a sample bias voltage being resonant with the Si-Cl antibonding states and also exhibits an oscillatory decay with the distance from the injection point characterized by the wavelength depending on the

A scanning tunneling microscopic study revealed quantum fluctuation of tunneling currents in individual Ge quantum dots (QDs) on SiO2∕Si. This was due to the charging energy change in the QDs caused by single-electron transfer from or into the QDs. The observed electron discharging time of approximately milliseconds agreed with the propagation model of the electron wave packets from the QDs to the Si substrates by a tunneling effect rather than by passing through voids in the SiO2 smaller than e

We investigated thermal conductivity of epitaxial germanane films: stacked structure of hydrogenated germanenes. It was confirmed that single crystalline germanane films were epitaxially grown on Ge(111). The films exhibited low out-of-plane thermal conductivity of 1.1 ± 0.3 W m−1 K−1 which is lower than other layered materials composed of heavy atoms. This came from weak van der Waals interlayer interaction related to weak polarization in germanane composed of smaller atoms. This demonstrates t

We studied the formation of β-FeSi2 nanodots by codeposition at disilicide stoichiometric deposition rates of Fe and Si on Si (111) substrates covered with ultrathin SiO2 films. Hemispherical β-FeSi2 nanodots with an ultrahigh density (>1012cm−2) and with a narrow size distribution at the average size of ∼5nm diameter were epitaxially grown by the codeposition at 500°C on ultrathin SiO2 films with predeposited Si. High-resolution transmission electron microscope observations showed that t