Defect control and Si/Ge core-shell heterojunction formation on silicon nanowire surfaces formed using the top-down method

Control of surface defects and impurity doping are important keys to realizing devices that use semiconductor nanowires (NWs). As a structure capable of suppressing impurity scattering, p-Si/i (intrinsic)-Ge core-shell NWs with radial heterojunctions inside the NWs were formed. When forming NWs using a top-down method, the positions of the NWs can be controlled, but their surface is damaged. When heat treatment for repairing surface damage is performed, the surface roughness of the NWs closely depends on the kind of atmospheric gas. Oxidation and chemical etching prior to shell formation removes the surface damaged layer on p-SiNWs and simultaneously achieves a reduction in the diameter of the NWs. Finally, hole gas accumulation, which is important for suppressing impurity scattering, can be observed in the i-Ge layers of p-Si/i-Ge core-shell NWs.

Plasma excited molecular beam epitaxy－proposal and achievements through R&D of compound semiconductor materials and devices－

This paper reviews 35 years of brief history on plasma-excited molecular beam epitaxy, focusing on special values added to conventional Molecular Beam Epitaxy (MBE) through usage of plasma-excited molecular beams. These include low temperature surface cleaning, low temperature growth, selected area re-growth and impurity doping. These technologies are extremely important to realize nano-scale low-dimensional device structures. InN and In-rich InGaN are also highlighted as unique material systems, which plasma-excited MBE process is inevitable to grow. Future prospect of this technology will also be included from the device application viewpoints.

Controlled Lattice Thermal Conductivity of Transparent Conductive Oxide Thin Film via Localized Vibration of Doping Atoms

Amorphization using impurity doping is a promising approach to improve the thermoelectric properties of tin-doped indium oxide (ITO) thin films. However, an abnormal phenomenon has been observed where an excessive concentration of doped atoms increases the lattice thermal conductivity (κl). To elucidate this paradox, we propose two hypotheses: (1) metal hydroxide formation due to the low bond enthalpy energy of O and metal atoms and (2) localized vibration due to excessive impurity doping. To verify these hypotheses, we doped ZnO and CeO2, which have low and high bond enthalpies with oxygen, respectively, into the ITO thin film. Regardless of the bond enthalpy energy, the κl values of the two thin films increased due to excessive doping. Fourier transform infrared spectroscopy was conducted to determine the metal hydroxide formation. There was no significant difference in wave absorbance originating from the OH stretching vibration. Therefore, the increase in κl due to the excessive doping was due to the formation of localized regions in the thin film. These results could be valuable for various applications using other transparent conductive oxides and guide the control of the properties of thin films.