Koichi Murata

High-density G-centers, light-emitting point defects in silicon crystal

We propose a new method of creating light-emitting point defects, or G-centers, by modifying a silicon surface with hexamethyldisilazane followed by laser annealing of the surface region. This laser annealing process has two advantages: creation of highly dense G-centers by incorporating carbon atoms into the silicon during heating; freezing in the created G-centers during rapid cooling. The method provides a surface region of up to 200 nm with highly dense carbon atoms of up to 4 × 1019 cm−3 to create G-centers, above the solubility limit of carbon atoms in silicon crystal (3 × 1017 cm−3). Photoluminescence measurement reveals that the higher-speed laser annealing produces stronger G-center luminescence. We demonstrate electrically-driven emission from the G-centers in samples made using our new method.

Hybrid Laser Activation of Highly Concentrated Bi Donors in Wire-δ-Doped Silicon

Hybrid laser annealing, i.e., a serial combination of laser exposure and furnace annealing, is demonstrated to activate Bi donors that are wire-δ-doped in Si. The photoluminescence reveals that the dense Bi atoms are activated so efficiently that an impurity band develops upon rapid radiation heating of the focused area close to the melting point of Si. The unintentional defects that are created thereby can be totally eliminated by subsequent furnace annealing at 390 °C. As a result, we attained a record concentration of active Bi donors >1018 cm-3 in excess of the predicted solubility limit.

Dopant activation mechanism of Bi wire- δ -doping into Si crystal, investigated with wavelength dispersive fluorescence x-ray absorption fine structure and density functional theory

We successfully characterized the local structures of Bi atoms in a wire-δ-doped layer (1/8 ML) in a Si crystal, using wavelength dispersive fluorescence x-ray absorption fine structure at the beamline BL37XU, in SPring-8, with the help of density functional theory calculations. It was found that the burial of Bi nanolines on the Si(0u20090u20091) surface, via growth of Si capping layer at 400 °C by molecular beam epitaxy, reduced the Bi-Si bond length from [Formula: see text] to [Formula: see text] Å. We infer that following epitaxial growth the Bi-Bi dimers of the nanoline are broken, and the Bi atoms are located at substitutional sites within the Si crystal, leading to the shorter Bi-Si bond lengths.

Development of an MR-compatible configurable brush stimulation device

In order to evaluate sensory disturbance, a subjective method is performed, so that the evaluation result is influenced by subjective factors. fMRI is used for observing brain activity objectively. Therefore the brain response to a stimulation measured by fMRI could become a useful identification tool for the objective evaluation of the sensory disturbance. The purpose of this study is to develop an MR-compatible sensory stimulation device capable of providing brush stimulation to several positions with separate modules, and to confirm the feasibility of the device by a basic operation experiment and an fMRI experiment. The developed device consists of both an MR-compatible stimulator placed inside the MRI room, a tube-rod mechanism and a driver placed outside the MRI room. The tube-rod mechanism is adopted for power transmission from the driver to the stimulator. Also, in order to provide the stimulation to several positions in the limited space, the device consists of the stimulation module and the positioning module that moves the stimulation module. For the basic operation experiment, we measure a variation of the automated and manual brush stimulation period. For the fMRI experiment, the brush stimulation is provided to the middle fingertip and the palm of a subject in a trial using the developed device. As a result, the standard deviations of the automated brush stimulation period is less than 7.0 ms. This result was smaller than that of the manual stimulation period. Also, the brush stimulation to the fingertip and the palm activated the somatosensory areas respectively. In conclusion, we confirmed the feasibility of the developed device through the experiments.

A brain phantom for motion-corrected PROPELLER showing image contrast and construction similar to those of in vivo MRI

PURPOSEnA fast spin-echo sequence based on the Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction (PROPELLER) technique is a magnetic resonance (MR) imaging data acquisition and reconstruction method for correcting motion during scans. Previous studies attempted to verify the in vivo capabilities of motion-corrected PROPELLER in real clinical situations. However, such experiments are limited by repeated, stray head motion by research participants during the prescribed and precise head motion protocol of a PROPELLER acquisition. Therefore, our purpose was to develop a brain phantom set for motion-corrected PROPELLER.nnnMATERIALS AND METHODSnThe profile curves of the signal intensities on the in vivo T2-weighted image (T2WI) and 3-D rapid prototyping technology were used to produce the phantom. In addition, we used a homemade driver system to achieve in-plane motion at the intended timing. We calculated the Pearsons correlation coefficient (R2) between the signal intensities of the in vivo T2WI and the phantom T2WI and clarified the rotation precision of the driver system. In addition, we used the phantom set to perform initial experiments to show the rotational angle and frequency dependences of PROPELLER.nnnRESULTSnThe in vivo and phantom T2WIs were visually congruent, with a significant correlation (R2) of 0.955 (p<.001). The rotational precision of the driver system was within 1 degree of tolerance. The experiment on the rotational angle dependency showed image discrepancies between the rotational angles. The experiment on the rotational frequency dependency showed that the reconstructed images became increasingly blurred by the corruption of the blades as the number of motions increased.nnnCONCLUSIONSnIn this study, we developed a phantom that showed image contrasts and construction similar to the in vivo T2WI. In addition, our homemade driver system achieved precise in-plane motion at the intended timing. Our proposed phantom set could perform systematic experiments with a real clinical MR image, which to date has not been possible in in vivo studies. Further investigation should focus on the improvement of the motion-correction algorithm in PROPELLER using our phantom set for what would traditionally be considered problematic patients (children, emergency patients, elderly, those with dementia, and so on).

Magnetic resonance compatible stimulation device capable of providing passive and active finger movements

The purpose of this study is to develop a magnetic resonance (MR)-compatible stimulation device that can passively and actively provide flexion and extension movements to the two joints of the fingers independently without limiting the hand posture, and confirm the feasibility of the device through an MR-compatibility experiment and fMRI experiment. The developed device is composed of four parts: a control unit, power unit containing non-magnetic ultrasonic motors, wearable exoskeleton hand part, and tube-rod mechanisms. Each tube-rod mechanism is a flexible power transmission that connects the power unit to the hand part, without limiting the users hand posture. The device can move the metacarpal phalangeal (MP) and proximal interphalangeal (PIP) joints independently. For the MR-compatibility experiment, we measure the signal-to-noise ratio (SNR) of MR images and the angles of potentiometers attached to the joints of the hand part. In the fMRI experiment, passive and active movements are provided to the fingers of a healthy participant using the developed device. As a result, the maximum variation of the SNR was 3.0 %, which was low enough to be virtually negligible. Moreover, the developed device functioned properly in the MRI environment. We observed different brain activities with the passive and active movements, and the results agreed with the known phenomena. In conclusion, we could confirm the feasibility of the developed MR-compatible stimulation device.

Autosurfactant of the second kind: Bi enables δ-doping of Bi in Si

Surfactants in heteroepitaxy are catalytic elements that float up to the surface during growth to control the energetics/kinetics of adatoms. “Autosurfactants” are exceptional in that the surfactant action is self-contained without foreign species. So far, autosurfactants as surface smootheners are known. Here, we demonstrate a different class of autosurfactants as surface-segregation quenchers: Bi, a dopant with a strong surface-segregation tendency in Si, is utilized to lock otherwise elusive Bi adatoms themselves to the Si lattice underneath during molecular beam epitaxy. Quasi-1D δ-doping of Bi in Si up to 4u2009×u20091020u2009cm−3 in terms of volume concentration is achieved.

Bionic scope: wearable system for visual extension triggered by bioelectrical signal

Visual extension has been an essential issue because the visual information accounts for a large part of sensory information which human processes. There are some instruments which are used to watch distant, objects or people, such as a monocle, a binocular, and a telescope. When we use these instruments, we firstly take a general view without them and adjust magnification and focus of them. These operations are complicated and occupy the users hands. Therefore, a visual extension device that is capable of being used easily without hands is extremely useful. A system developed in the previous work recognizes the movement of the users eyelid and operating devices by using it [Hideaki et al. 2013]. However, a camera is placed in front of the eye, and that obstructs the field of view. In addition, image recognition needs much calculation cost and it is difficult to be processed in a small computer. When human intends to move his/her muscles, bioelectrical signal (BES) leaks out on the surface of skin. The BES can be measured by small and thin electrodes attached to the surface of the skin. By using the BES, users operational intentions can be detected promptly without obstructing the users field of view. Moreover, using BES sensors can reduce electrical power, and contribute to downsizing systems.

Characterization of Highly Concentrated Bi Donors Wire-δ-Doped in Si

We studied the Bi wire-δ-doping process to achieve a high concentration of Bi donors in Si. Our process has two steps: (i) burial of Bi nanowires in Si by molecular beam epitaxy, and (ii) activation of Bi atoms in the δ-doped layer by laser annealing. The peak concentration of Bi atoms in the δ-doped layer is controlled by two parameters: the coverage of surfactant layer, and the growth temperature during the Si cap-layer growth, whose maximum concentration is larger than 1020 cm-3. Photoluminescence and electrical carrier transport measurements reveal that dense Bi atoms are activated upon heating the area at close to the melting point of Si. As a result, our doping process results in Bi donors in the wire-δ-doped layer with concentration of >1018 cm-3. This will be useful for establishing next-generation, quantum information processing platform.