K. Gamo

Contact resistance of multiwall carbon nanotubes

Multiwall carbon nanotubes (CNTs) with Au/Ti electrodes were fabricated. The zero-bias resistances were measured at room temperature. The resistance was dominated by the contact resistance and was not significantly affected by the intrinsic resistance of the CNTs which should be determined by diameter, chirality, defect and bend of the CNTs. It is shown that the contact resistance can be controlled by the length of part of the CNT which is covered by the electrode. At low temperature, the multiwall CNT behaved as a single Coulomb island without dividing into several islands. In this case, the contact resistance between the CNT and the metal formed the tunnel resistance in Coulomb blockade unlike the case of single-wall CNT.

Dynamic behavior of pulsed-laser annealing in ion-implanted silicon: Measurement of the time dependent optical reflectance

Abstract In order to investigate the dynamic behavior of laser annealing processes, we have measured the time-dependent optical reflectance of ion-implanted silicon under pulsed-laser irradiation. We found that the amorphous to crystalline transition is completed within several hundred ns, depending on the laser energy and the pulse width.

Nanoporous structure formations on germanium surfaces by focused ion beam irradiations

The formation of porous structures of nanometre size (nanoporous structures) on germanium (Ge) surfaces by focused ion beam (FIB) irradiations was investigated using various FIB conditions such as ion species, irradiation energies, total fluences, fluence rates, and incident angles. FIB-irradiated regions were observed using a scanning electron microscope and an atomic force microscope. It is found that, using a focused Ga ion beam (Ga FIB) at an energy of 100 keV, the irradiated Ge surface swelled up to ion fluence of 2 × 1017 cm−2 with nanoporous structures and then was etched for larger fluences. The shape of swollen nanoporous structures depended on the fluence rate and the incident angle of the Ga FIB. However, such porous structures were observed neither for low-energy (15–30 keV) FIB irradiations using Si and Au ions nor for high-energy (200 keV), heavy ion (Au) irradiation. These observations might be helpful in discussing the formation mechanisms of the nanoporous structures on Ge surfaces by ion beam irradiations. Fabrication of patterned structures at selected regions on the Ge surface was demonstrated without using any masks.

Commensurate classical orbits on triangular lattices of anti-dots

Abstract A novel low-field magnetoresistance oscillation is observed in a high- mobility GaAs-AlGaAs heterojunction, where an array of scatterers is introduced artificially by etching. The period of the oscillation is governed by the ratio of the cyclotron diameter at the Fermi energy to the period of the anti-dots. The effect arises from the commensurate cyclotron orbits on the triangular lattice of anti-dots.

Shubnikov-de Haas effect of multi-terminal GaAs/AlGaAs quantum wire in non-local geometry

Abstract The magnetoresistance and Hall resistance in a quasi-ballistic multi-terminal quantum wire of GaAs/AlGaAs heterostructure have been investigated. The Shubnikov-de Haas (SdH.) oscillations in the non-local geometry, where the current probes and voltage probes are separated, are observed at high magnetic fields, in addition to the observation of the last Hall plateau, the negative bend and transfer resistance at low magnetic fields. The amplitudes of SdH. oscillations decrease with increasing nominal separation length between the current and voltage probes. The decay length (Ls) decreases from 3.5μm at B=1T to 2.0μm at B=5T with increasing magnetic field. In all cases, Ls is longer than the ballistic mean free path of electrons. Possible origins about this magnetic field dependence of Ls are discussed in connection with the edge states of the quantum wire.

Quantized resistance in in‐plane gated narrow constriction fabricated by wet etching

A narrow in‐plane gated constriction is defined by technique employing electron beam lithography and wet chemical etching. Two‐dimensional electron gas beside the narrow channel is used to control the number of occupied subbands in the constriction. A ballistic transport through the point contact is manifested by the observation of successive resistance steps when the subbands are depopulated.

Laser annealing of ohmic contacts on GaAs

Backside irradiation through the substrate with a Q‐switched Nd:glass laser (l = 1.06 mm) has been used to produce Au‐Ge ohmic contacts on GaAs. Conventional thermal alloying was done for comparison. The structure of the contacts was investigated by 2‐MeV He‐ion Rutherford backscattering technique, photographs, and I‐V characteristic measurements. The irradiated Au‐Ge contacts exhibit good ohmic contact behavior and good surface morphology. Ohmic contacts with specific contact resistance rc less than 2×10−5 W cm2 have been measured within the laser energy density of 0.3–0.5 J/cm2, while the thermally alloyed contacts yielded rc = 1.3×10−4 W cm2.

Effects of ion‐implanted atoms upon conduction electron spin resonance (CESR) in a Si : P system

The behavior of the conduction electron spin resonance of ion‐implanted (Si : P)  : Sb and (Si : P)  : Te systems is observed to be strongly modified by the presence of Sb or Te substitutional atoms in shallower surface layers; i.e., the ESR signal shows an anomalous line broadening. The origin of the anomalous broadening may be due to the spin‐orbit interaction between the conduction electrons contained within a thin layer (⩽25 μm) and implanted Sb or Te impurities contained within a thin layer (.=.0.1 μm).

Transport properties in artificial lateral superlattice

We investigate commensurate oscillations in ordered and disordered artificial lateral superlattice (ALS) systems, in which the anti-dots are arranged in a square or triangular lattice. With increasing disorder of the anti-dot location, the peaks of the commensurate oscillations fade out. The peak heights are more strongly affected by the disorder along perpendicular direction to the current than by that along the parallel direction. In the square ALS system, the commensurate oscillations seem to be determined principally by the order along the perpendicular direction to the current, while in the triangular ALS system, the commensurate oscillations would be determined by the nearest neighbor distance between anti-dots and the order along the perpendicular direction. In addition, the appearance of each peak is determined by the ratio of the anti-dot diameter to the ALS period. The weak localization effect in very low magnetic field and the strongly temperature dependent conductance in the absence of magnetic field are also observed in the ALS systems.

Control of selective adsorption behavior of Ti-binding ferritin on a SiO2 substrate by atomic-scale modulation of local surface charges

We have systematically investigated the influence of local surface potential distribution on selective adsorption of Ti-binding ferritin (TBF) on SiO2 substrates. We found that the TBF exhibits the adsorption ability onto Ti-decorated SiO2 surface of less than one atomic layer coverage. This experimental finding enabled us to propose a model in which the positively charged sites and their nanometer-scale interval are the dominant factors that determine bond strength between TBF and SiO2 surfaces. Furthermore, on the basis of this model, the adsorption behavior of TBF onto bare SiO2 surfaces was controlled by generation and/or recovery of electrical defects in SiO2.