Search

A diamond-like carbon (DLC) film was deposited using high-power pulsed magnetron sputtering (HPPMS). The effect of pulse width on the deposition of DLC film was investigated under constant peak power density or average power density to clarify the densification mechanism of DLC film. The maximum hardness of 25 GPa analyzed by nanoindentation was obtained using Ar gas without negative substrate bias voltage at pulse width 30 µs and peak power density of 1.5 kW/cm2. The flux and energy of C+ and Ar+ incident to the DLC film was evaluated by using energy-resolved and time-resolved mass spectrometry to clarify the relation between the input power to the target and behavior of produced ions. The change in hardness is well correlated with ion flux ratio C+/Ar+. This result indicates that a flux and energy of Ar+ as well as C+ is key parameter to characterize the microstructure of DLC film.

We investigated the dominant factors affecting the Al composition of α-(AlGa)2O3 alloy thin films in mist CVD, focusing on the acetylacetonation of the source solutions. The Al–acetylacetonate (acac) complex formation time was evaluated using Fourier-transform IR spectroscopy to examine the effectiveness of heating during stirring of the source solutions. Then, α-(AlGa)2O3 alloy thin films were grown in source solutions in which the state of the acac complex was changed by adding ammonia water and/or hydrochloric acid, and the Al composition increased with increasing the pH value due to the acceleration of acetylacetonation. The growth-temperature dependence of the Al composition was also experimentally investigated. The Al composition decreased with increasing growth temperature. The reasons are due to the strength of the Al–O bond or the dissociation of the Al–acac ligand.

The effect of Mg channeled implantation into epitaxially grown gallium nitride (GaN) was studied using Hall-effect measurements, photoluminescence (PL), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and Rutherford backscattering spectroscopy (RBS). In the channeled implantation, deeper profiles were obtained with lower implantation energy and less damage compared to random implantation. The donor–acceptor pair signal at ∼3.28 eV, suggesting that Mg is activated, was confirmed by PL measurement when the ion dose and implantation energy are 1 × 1014 cm−2 and 20 keV, respectively. However, even with channeled implantation, several types of defects including point defects and oblong defects as seen in the random implantation were observed by TEM/STEM analysis. RBS analysis showed slightly worse crystal qualities in channeled implantation compared to non-implanted samples. Mg channeled implantation is useful to achieve deeper profiles (>1 μm), but further condition tuning of process will be necessary for practical application.

Implantable retinal prostheses are stimulation devices used to compensate for the light sensitivity loss of retinal cells. In this study, we propose and demonstrate a novel method to significantly reduce the setting time for the stimulation conditions of a retinal prosthesis chip capable of multi-electrode stimulation. The efficiency of the control method is increased while using only two wires, as in our previous work. The chip comprises an 8 bit ID and 7 electrodes, and the stimulation current value can be set from 50 to 1550 μA. The fabricated chip requires only 32 pulses to set the stimulation conditions, which is approximately 1/65 of that of our previous chip. Furthermore, it is equipped with a complementary metal–oxide–semiconductor rectifier to enable it to be driven by a rectangular AC power supply. The effectiveness of the chip is demonstrated by setting the stimulation conditions at approximately 18 μs per electrode at a clock frequency of 2.3 MHz.

Effects of the surface modification by O2 plasma exposure on the Al/Ge(111) structure have been investigated in order to get an insight into a layer transfer technique of the ultrathin Ge layer segregated on the Al/Ge(111) structure toward the device fabrication, and then the wafer bonding of the Al/Ge(111) structure to the thermally-grown SiO2/Si structure has been demonstrated. The O2 plasma treatment and the subsequent pure water rinse were found to be effective to form the hydrophilic surface of the Al/Ge(111) structure with a suppression of the segregated Ge layer oxidation. The Al/Ge(111) structure with the hydrophilic surface was then bonded to the SiO2/Si substrate, and its bonding strength was enough to perform Ge thinning by the chemical mechanical polishing and the wet-chemical etching using H2O2-based solutions. Ohmic contact of the ring-type device pattern with the segregated Ge/Al stack was achieved by using the remaining p-type Ge substrate as the contact pads.

Control of diffusion and segregation of Si atoms through a thin metal layer from a stacked Si structure is one of the effective techniques to grow two-dimensional (2D) or ultrathin Si crystals. We have studied the formation of the Al layer with a flat surface and high crystallinity on a wet-cleaned Si(111) wafer by thermal evaporation in order to use it as a growth template for ultrathin Si crystals. Then, the impacts of the annealing in N2 ambient on the surface flatness, the Al crystallinity, and the chemical bonding features for the Al/Si(111) structure were investigated. A formation of a sub-nanometer Si layer on the Al(111) surface using Si segregation with keeping surface flatness was demonstrated by the control of annealing temperature.

In this paper, we propose an improved on-pixel polarizer structure for an image sensor, which was fabricated using the 0.35 μm standard CMOS process with dual metal layers (Metal 1 and Metal 2), i.e. two layers of gratings with metal-wiring layers were stacked. By designing the line/space ratio as 0.7/0.7 μm, the extinction ratio was significantly improved at a wavelength of 780 nm, which was over 1.4 times higher than that of a single-layer grating, and the detection sensitivity of the polarization change was enhanced by more than 1.8 times. We also demonstrated electro-optical imaging at radio frequency using this image sensor.

In this paper, a miniature wireless system for use in conjunction with original lensless CMOS-based imaging devices is developed for in vivo imaging experiments. The system mainly comprises an image sensor, a microcontroller, and a Bluetooth Low Energy module for wireless data transmission. In addition to the hardware suitable for studies imposing freely-moving conditions, image sampling and processing features are implemented. Results demonstrate readiness for imaging in vivo, with adequate data transfer speed for a 12 × 12 pixel region of interest with an area of 180 × 180 μm2.

Formation of an ultra-thin nickel-germanide (Ni-germanide) film on a SiO2 film has been attempted with stacking structures of Ni with various thicknesses formed on Ge films with thicknesses of 4 nm or 20 nm and annealing in an N2 ambient condition. Physical analyses revealed that the ultra-thin Ni-germanide films with smooth surfaces could be formed on the SiO2 film after annealing at 400 °C without depending on the Ni thickness on the 4 nm thick Ge films. In the formation, reductive and oxidative reactions occurred in the films, which are quite important for determining a composition of the Ni-germanide.

We developed implementation technologies for artificial vision devices compatible with suprachoroidal transretinal stimulation. We aimed to develop a device that can be safely implanted in the sclera of the eye for a long period. Using parylene C and bioceramics as biocompatible base materials, we realized a device with high in vivo safety by making the device structure flexible and reducing the wires of control signals. We successfully created a prototype device that combines a flexible wire structure based on a parylene C thin film with a wire-saving CMOS smart electrode structure based on CMOS integrated circuits. We conducted in vitro and in vivo experiments to confirm their performances. The immersion test confirmed that the device could work normally for four days. Furthermore, in the in vivo experiments using rats, we successfully measured evoked potentials in the brain induced by the stimulation current using the device.

In this study, a dual-band hybrid filter that achieves high excitation light rejection performance in a lensless imaging system was fabricated and incorporated into an imaging device. The hybrid filter consisted of interference and absorption filters, and a fiber optic plate (FOP). The interference filters were attached to both sides of the FOP, which was placed on top of the absorption filter to suppress the decrease in spatial resolution. In addition, the lamination order was optimized to achieve a high fluorescence observation performance. The fabricated hybrid filter was mounted on an image sensor and had the ability to indicate the green and red fluorescence components.

Anode-size-independent forward current–density/voltage (V) characteristics of circular GaN p+n diodes were extracted (in the V range of 2.7−3.0 V) from measured ones by an estimation technique of the effective anode radius, followed by the least-squares fitting technique of peripheral-to-area ratio analysis. It was found that the extracted diffusion current of diodes with a higher p-type doping level is lower than that of diodes with a lower one and that both currents have an ideality factor of unity. Since these findings agree with the diffusion theory, the demonstrated method should be useful for theoretical analysis of GaN p+n diodes.

We have studied the impact of Ge substrate heating during ∼25 nm thick Al deposition and post annealing in N2 ambient on the surface flatness of an Al/Ge(111) structure, the crystallographic structure of the deposited Al layer, and formation of a Ge segregated layer. Surface segregation of Ge atoms on a flat metal surface is an effective means of growing two-dimensional Ge crystals as well as an ultrathin Ge crystalline layer. The surface morphology of the Al/Ge(111) structure becomes flat by substrate heating during Al deposition. The crystallinity of the Al layer on Ge(111) can be improved by both substrate heating and post annealing. Ge segregation on a flat Al(111) surface also occurred by post annealing.

The characteristics of p-n junction diodes on heavily Ge-doped substrates were compared with those on conventional Si-doped substrates. A large decrease in on-resistance was observed which greatly exceeded the component due to the lower resistance of the Ge-doped substrate. This result implied an occurrence of a strong conductivity modulation by the heavily doped substrate. Current-density dependence on the anode-electrode diameter could be minimized by the use of the substrate. No significant difference was observed in the breakdown voltage even when the doping concentration in the substrate was increased. It has been clarified that the heavily Ge-doped GaN substrate is essential for low on-resistance and large size GaN power devices.

We have characterized charged states of a self-aligned Si quantum dot (QD) structure with an areal density as high as ∼1013 cm−2 by surface potential measurements by using atomic force microscopy (AFM)/Kelvin probe force microscopy (KFM). By line scanning with the electrically-biased AFM tip with respect to the substrate in a contact mode, the corresponding area was negatively charged caused by electron injection from the tip into QDs. Cross-sectional profiles of the surface potential were clearly changed, which depends on the tip bias voltages. Also, a stepwise shape in a relationship between the change of the surface potential (ΔV) and the tip bias voltages was observed, reflecting electron charging into the discrete energy levels due to the quantum confinement effect. By assuming a simple equivalent circuit model for KFM measurements of the self-aligned Si-QDs, calculated results for ΔV are in good agreement with the measured results. The agreements precisely clarified the charged states in Si-QDs.

Influences of hole traps on large forward current (I F) of GaN p+n diodes on low-dislocation-density (≤4 × 105 cm−2) GaN substrates were numerically investigated. As with the reported simulation of GaAs p+n diodes, hole traps were found to increase the electron concentration in the drift layer when forward voltage V F was increased. To reproduce the measured I F/V F characteristics, however, we had to assume an unrealistic electron mobility increasing with the electric field. We, therefore, concluded that sources other than hole traps were responsible for the reported enhancement of conductivity modulation in GaN p+n diodes.

To confirm the feasibility of a theoretically proposed method of forming free-standing germanene [Araidai et al., J. Appl. Phys. 128, 125301 (2020).], we experimentally investigated the hydrogen desorption properties of hydrogen-terminated germanane (GeH) flakes. Thermal desorption spectroscopy analysis revealed that hydrogen desorption occurred during heating under an ultrahigh vacuum environment corresponding to a mass loss of 1.0 wt%. Moreover, we have found that using an ultrahigh vacuum, ambient and short-time annealing for hydrogen desorption is a key to sustain the crystal structures.