Junichi Yanagisawa

Enhancement of terahertz electromagnetic wave emission from an undoped GaAs/n-type GaAs epitaxial layer structure

We have investigated the emission of the terahertz electromagnetic wave from an undoped GaAs (200nm)∕n-type GaAs (3μm) epitaxial layer structure (i-GaAs∕n-GaAs structure), where the doping concentration of the n-GaAs layer is 3×1018cm−3. It is found that the first-burst amplitude of terahertz wave of the i-GaAs∕n-GaAs sample is remarkably larger than that of a n-GaAs crystal, which means that the i-GaAs layer enhances the terahertz emission intensity. The first-burst amplitude of the i-GaAs∕n-GaAs sample, by tuning the pump-beam energy to the higher energy side, exceeds that of an i-InAs crystal that is known as one of the most intense terahertz emitters. We, therefore, conclude that the i-GaAs∕n-GaAs structure is useful to obtain intense terahertz emission.

Metastable domain structures of ferromagnetic microstructures observed by soft X-ray magnetic circular dichroism microscopy

The benefit of combining soft X-ray magnetic circular dichroism and photoelectron microscopy is demonstrated by applying this combination to the observation of the magnetic domain structures of rectangular microstructures. The size and aspect-ratio dependence of the transformation of the domain structures by magnetic field pulses is investigated. The switching mechanism, which is very important in the application to magnetic storage, is discussed in terms of transformation between saturated and vortex domain structures.

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.

Estimation of Damage Induced by Focused Ga Ion Beam Irradiation

We have measured the ballistic length lbFIB of a GaAs/AlGaAs sample using the electron focusing effect and the mean free path leFIB of the narrow channel, both formed by focused ion beam (FIB) irradiation, to estimate the damage induced by FIB irradiation. It is observed that scattering centers are induced by FIB irradiation, which exhibit dependence on the electron density, unlike scattering centers due to grown-in defects. The FIB-induced scattering centers distribute far beyond the distance of the FIB spot size. This may be due to the exponential tail distribution of FIB.

Focused ion beam process for a formation of metal/insulator/metal double tunnel junctions

A small conductive island which is connected with source and drain via small tunnel junctions is a fundamental for single electron tunneling (SET) devices. An example of such structures consists of metal/insulator/metal double tunnel junctions and a metal oxide can be used as an insulator. Control of SET by the spin of tunneling electrons is possible by using magnetic metals as the island material, source and drain electrodes. However, microfabrication processes of magnetic metals for SET structures are not well established yet. We have proposed a new in situ process to fabricate small double tunnel junctions using a focused ion beam (FIB). This process is made up of following steps: after deposited metal films (Ni) on an insulator substrate are patterned to have a narrow channel (50 /spl mu/m in the present study) by photolithography was formed, a resist layer is spin-coated for the next lift-off step. The resist used is a double layer resist consisting of nitrocellulose and germanium films. Nitrocellulose can be used as a high-sensitive, selfdeveloping ion beam resist. 100 keV Ga FIB is irradiated across the channel to separate the metal channel and to form source and drain electrodes. To minimize the sputtered groove width which determines the island size, the resistance of the metal channel is monitored during the sputtering step and the FIB irradiation is ended just when the resistance become sharply infinite.

Magnetization process and resistance jumps in a submicron-scale cross-shaped Co wire

The magnetization processes of a submicron-width Co wire with two micron-width crossing probes were investigated by measuring Hall resistance (HR) and magnetoresistance (MR). Three clear jumps and two jumps were observed in the HR and MR, respectively. In order to explain the observed jumps, the magnetization process of the crossing region where the probes cross the wire was calculated by considering the exchange interactions from the current and voltage probes. The calculated HR reproduced the observed three clear jumps. The observed MR can be explained by the anisotropic MR of the crossing region and the contribution of a domain wall. The HR jumps could be related to the discrete changes of the magnetizations in the probes.

Carrier profile of the Si-doped layer in GaAs fabricated by a low-energy focused ion beam/molecular beam epitaxy combined system

Buried Si-doped layers in GaAs were fabricated by low-energy focused Si2+ ion beam (Si FIB) implantation in GaAs grown by molecular beam epitaxy (MBE) and successive overlayer regrowth using an FIB/MBE combined system. Carrier profiles were measured by means of a capacitance–voltage profiling technique. It was found that doped layers with a carrier concentration of 1.2×1012 cm−2 and a width (full width at half maximum) of 23 nm were formed without postannealing for the 200 eV Si implantation at a dose of 7×1012 cm−2. After postannealing, the doping efficiency was improved and the width became narrower. At a dose of 1.4×1013 cm−2, narrower carrier distribution with higher peak density was observed although the total doping efficiency was decreased. In contrast a deeply depleted layer was formed and no carriers were observed for implantations at an energy higher than 400 eV before the postannealing. This may be because damage is significantly reduced for the lower implantation energy.

Low‐energy focused ion beam system and direct deposition of Au and Si

A low‐energy focused ion beam system which is combined with molecular beam epitaxy apparatus was developed. Focusing characteristics were estimated by secondary electron and transmitted ion images. Using this system, the direct deposition of Au and Si was performed, which is important for in situ contact formation. Film purity was measured by Auger electron spectroscopy. Results show that the amount of impurities inside the film depends both on the deposition rate and on the chemical property of the deposition material.

Formation of GaN films by Ga ion direct deposition under nitrogen radical atmosphere

Formation of hydrogen-free gallium nitride (GaN) thin layers by ion beam direct deposition method under nitrogen ambient was investigated. After a Ga ion beam at an energy of 100eV was irradiated on a chip of a Si(111) wafer under a nitrogen gas pressure of 2×10−4Torr using a tungsten hot filament, the composition and the chemical bonding nature of the deposited materials were investigated by x-ray photoelectron spectroscopy (XPS). Although the deposited material using a filament power of 250W showed almost the metallic gallium nature, the XPS spectra of the deposited Ga using the hot filament at a power of 300W was very similar to that of an epitaxially grown GaN reference, indicating the possibility of the formation of GaN thin layer using the present method. Because the pure N2 gas was used as the nitrogen source, no impurity fragments should be incorporated in the deposited materials. As a result, it is shown that the formation of hydrogen-free GaN layers is possible by Ga ion beam direct deposition u...

Direct deposition of silicon and silicon-oxide films using low-energy Si focused ion beams

Abstract Ion beam deposition using low-energy (100 eV) Si2+ focused ion beams (FIB) was performed on gold evaporated substrate at room temperature under various deposition conditions, such as beam scan rates of the FIB and background oxygen pressures of the sample chamber during the deposition. From Auger electron spectroscopy (AES) measurement, it was found that the amount of the oxygen incorporated inside the film can be controlled by the deposition ambient. From the chemical shift in AES spectra and Raman spectroscopy, it was found that deposited Si without oxygen ambient was amorphous and the observed oxygen in the film deposited in high oxygen ambient was chemically bonded with silicon and formed silicon-oxygen bonds.