中岡 俊裕

Abstract The shape of conductive filaments in CBRAM is important for resistance switching and conductance modulation, especially in applications like neuromorphic and reservoir computing that use conductance as weight. We report on RF-induced modulation of CBRAM using Ge₂Sb_3.4Te_6.2 with sheet-like filaments and compared it to those with dendritic filaments. RF input below 100 MHz reduced SET and RESET voltages, similar to CBRAM with dendritic filaments, but showed significantly different resistance changes. Repeated RF on/off input gradually increased the resistance of low-resistance state, unlike the dendritic filament CBRAM, where the resistance decreased. The increased resistance suggests RF-induced denser sheet-like filaments. Furthermore, the resistance of the high-resistance state showed a peculiar RF-induced resistance change not observed in dendritic filaments. The resistance decreased during RF input and increased to nine times the initial value when RF was switched off. The results show that the conductance modulation by RF input strongly depends on the filament type.

The electronic spins of rare-earth materials are attractive candidates for spin qubits and quantum memories. To access individual spins, tuning of the g-factor is desirable. Here, we report on local strain-dependent g-factors of the ⁵D₀–⁷F₂ transitions of Eu³⁺ centers in GaN:Eu thin films. We have found a clear correlation between the effective g-factor and the emission energy shift induced by the local strain. The combination of micro-photoluminescence and scanning electron microscope/electron backscattering diffraction measurements has revealed that the compressive strain of 0.2%–0.4%, relative to a surrounding reference point, induces an energy shift of about 3 meV. The strain decreases the g-factor of the emission at 1.991 eV from 2.5 to 1.5, while the strain increases the g-factor of the emission at 1.994 eV from 1.1 to 1.7. The result suggests that the g-factor can be tuned by the local strain. On the basis of the strain-induced energy shift and the g-factor, we have identified the optical sites. The ⁵D₀–⁷F₂ transitions observed in this study consist of three optical sites with C_3v symmetry and one site with C_1h symmetry.

© 2020 IEEE. The 24 GHz communication system was demonstrated with wireless multi-sensors by wirelessly powering at 5.8 GHz. The wireless sensors for health monitoring in a space vehicle work as the sensors of temperature, humidity, acceleration, illuminance, battery voltage and sound in this study. Under output power of 30W from a GaN transmitter, a GaAs rectifier of energy harvester was operated with more than 80% RF-DC conversion efficiency.

© 2018 IEEE. In this paper, novel ICs made by different types of semi-conductors as the hybrid semiconductor integrated circuit, HySIC, are introduced. A C-band compact full Si rectifier and a HySIC RF rectifier operating at 5.8 GHz were described using 0.18μm Si CMOS process and chip-to-wafer bonding. From them, RF -DC conversion efficiencies were measured as about 25% and 10%, respectively. Further, a wide power-range HySIC RF energy harvester by combining the rectifiers implemented on the Si substrate was made and the fundamental data were successfully obtained.

A compact C-band energy harvester is designed, test fabricated, and measured. An RF hybrid semiconductor integrated circuit (RF-HySIC) rectifier was fabricated by comprising a Si RFIC matching circuit and a GaN diode. The energy harvester at 5.8-GHz was made in a multi-circuit fashion under a wide input power range and input power control by field-effect transistor switch. The maximum RF-DC conversion efficiencies from a proto type of RF-HySIC rectifier with 50% and 13% were achieved at 5.1 and 5.8 GHz, respectively.

This paper reports on a rectifier with 100 mW class dc output at 5.8 GHz using a gallium nitride (GaN) shottly barrier diode and silicon (Si) matching circuit. The originality of the study lies in its adaptation of the hybrid semiconductor integrated circuit (HySIC) technology that utilizes different types of semiconductors. We have completed the basic steps for developing a rectifier based on the HySIC technology. A nonlinear measurement system of GaN diodes with a jig was constructed and an equivalent circuit model of the diode was established. Based on the diode model, a Si matching circuit was designed. The size of the HySIC rectifier was 3.9 mmx9.5 mm. The RF-dc conversion efficiency of the HySIC rectifier was 10.3% at 5.8 GHz.

We report on the direct observation of Ag filament growth and a peculiar resistance switching in amorphous GeTe films with a lateral electrode geometry. The Ag filament growth was monitored by in-situ optical microscopy. The resistance switching was studied in three electrode pairs, Ag-Ag, Pt-Ag, and Pt-Ag/Pt (Ag electrode covered with Pt). In all the three electrode pairs, similar dendritic Ag filaments were clearly observed growing along both directions from one electrode to the other, according to the applied bias polarity. However, the SET and RESET processes are quite different. The Ag-Ag pair produces a unipolar clockwise switching. The Pt-Ag pair shows a bipolar counter-clockwise switching, as predicted in the basic electrochemical metallization theory, but the observed switching polarity is exactly opposite to the basic theory prediction. The Pt-Ag/Pt pair produces a unipolar counter-clockwise switching. The peculiar SET/RESET processes are explained on the basis of strong Ag diffusion into GeTe matrix resulting in an asymmetric effective electrode pair. The findings suggest that the SET/RESET processes are controlled by the amount of Ag and the electrode geometry. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

We report on a linearly polarized non-classical light emission from a single InGaN/GaN nanocolumn, which is a site-controlled nanostructure allowing for pixel-like large-scale integration. We have developed a shadow mask technique to reduce background emissions arising from nitride deposits around single nanocolumns and defect states of GaN. The signal to background ratio is improved from 0: 5 : 1 to 10 : 1, which allows for detailed polarization-dependent measurement and photon-correlation measurements. Polarization-dependent measurements show that linearly polarized emissions arise from excitonic recombination involving a heavy-hole-like electronic state, corresponding to the bulk exciton of an in-plane polarized A exciton. The second-order coherence function at time zero g((2))(0) is 0.52 at 20 K without background correction. This value is explained in terms of a statistical mixture of a single-photon emission with residual weak background emissions, as well as efficient carrier injection from other localized states. (C) 2016 The Japan Society of Applied Physics

We report on simple fabrication of silver (Ag) nanoparticles and room-temperature growth of silver telluride (Ag-Te) nanowires and novel nanostructures using the Ag nanoparticles as the seed. Size-controlled Ag nanoparticles are formed by thermal annealing of a thin Ag film. For example, a 4 nm thick Ag film is transformed into small and uniform Ag nanoparticles with average diameter of 7 nm and the standard deviation of 2 nm. Room temperature deposition of tellurium onto the Ag nanoparticles produces Ag-Te nanoparticles, nanowires and snail-like nanostructures, depending on the diameter of the Ag nanoparticles. We find that the diameter of seed Ag nanoparticles is the critical growth parameter for the formation of different Ag-Te nanostructures.

We have measured and analyzed the carrier-density dependence of photoluminescence (PL) spectra and the PL efficiency of InGaN/GaN multiple quantum wells in nanocolumns and in a thin film over a wide excitation range. The localized states parameters, such as the tailing parameter, density and size of the localized states, and the mobility edge density are estimated. The spectral change and reduction of PL efficiency are explained by filling of the localized states and population into the extended states around the mobility edge density. We have also found that the nanocolumns have a narrower distribution of the localized states and a higher PL efficiency than those of the film sample although the In composition of the nanocolumns is higher than that of the film. (C) 2015 AIP Publishing LLC.

We have calculated the transition energies of neutral and charged excitons in In0.5Ga0.5As/GaAs quantum dots (QDs) under a lateral electric field up to 40 kV/cm. First, the single-particle electron and hole states under the lateral field are calculated using the 8-band k . p theory. The linear and quadratic piezoelectricity is included. Next, the transition energies are calculated from the electron-hole, electron-electron, and hole-hole Coulomb energies. For a QD with 15-nm base length under a lateral electric field along the [100] direction, the transition energy of the positively charged exciton exhibits a blueshift with increasing field up to 28 kV/cm, followed by a redshift under higher field. In contrast, those of the neutral and negatively charged excitons exhibit only redshifts accompanied by a crossing of the two exciton levels. The calculated result for the positively charged exciton reproduces the unconventional "M"-shaped exciton energy shift observed in our experiment by Nakaoka et al. [Appl. Phys. Lett. 99, 181109 (2011)]. The origin of the blueshift calculated for the positively charged exciton is the enhanced hole density in the QD base corner due to the modification of the piezoelectric potential by the lateral electric field, which causes an increase in the hole-hole Coulomb energy. We found that the amount of the blueshift increases with the QD size. In order to understand the effect of the lateral field direction, we calculate the transition energies under a lateral electric field of 20 kV/cm along the [1 (1) over bar0], [100], and [110] directions. For the positively charged exciton, the transition energy exhibits a redshift for the [1 (1) over bar0] direction, and blueshifts of different amounts for the [100] and [110] directions, indicating that the effects of the lateral field are not equivalent for the [1 (1) over bar0] and [110] directions. It is demonstrated that the direction dependence of the transition energy reflects the symmetry of the confinement potential in the QDs due to piezoelectricity.

We report on the spectroscopy of single InGaN/GaN nanocolumns that are site-controlled nanostructures allowing for pixel-like large-scale integration. A single nanocolumn shows several narrow photoluminescence peaks at around 600 nm at 20–90 K, the linewidth of which ranges between 0.3 and 10 meV. We have observed an interesting temperature dependence of the integrated intensity, the maximum being around 40–70 K, which suggests the existence of efficient carrier injection channels from localized states. The results show that the optical properties of single nanocolumns are favorable for the future large-scale integration of single-photon emitters and qubits.

We report unipolar and bipolar resistive switchings in naturally oxidized AlxO1-x thin films. We find a relationship between the switching behavior and the electrode gap distance. The macro-gap device with the electrode separation of 20 μm shows a unipolar switching behavior while the nanogap device with the separation of 40nm shows a bipolar behavior. The result is explained by a model in which the unipolar or the bipolar switching is governed by the way of the carrier injection into oxygen vacancies. © 2013 The Japan Society of Applied Physics.

We present a simple method that enables both single electron transport through a self-assembled quantum dot and photon emission from the dot. The quantum dot buried in a semiconductor matrix is electrically connected with nanogap electrodes through tunneling junctions formed by a localized diffusion of the nanogap electrode metals. Coulomb blockade stability diagrams for the optically-active dot are clearly resolved at 4.2 K. The position of the quantum dot energy levels with respect to the contact Fermi level is controlled by the kind of metal atoms diffused from the nanogap electrodes. Copyright 2012 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4738368]

We demonstrate photon antibunching from a wavelength controlled quantum dot single-photon source with a side gate. The photoluminescence peaks from the quantum dots embedded in the side-gate structure are clearly identified as a neutral exciton and a neutral biexciton by the studies of excitation-power dependence, polarization dependence, and photon correlation. The neutral exciton energy is controlled by the side gate via the quantum confined Stark effect. Measurement of the second-order autocorrelation function indicates g((2))(0) = 0.07 at 0 V, 0.14 at 0.8 V, and 0.24 at 0.9 V. The results show g((2))(0)s are below the 0.5 limit necessary for classification as a single photon source even under applied gate voltage. We also show a biexciton-exciton cascade which can be used to create entangled photon pairs. (C) 2012 The Japan Society of Applied Physics

We report on the fabrication of a side-gate structure which enables a purely lateral electric field to be applied onto a self-assembled quantum dot. The lateral field produces an unconventional "M'-shaped exciton energy shift-a blueshift followed by a redshift. The unconventional energy shift is reproduced by calculation. The calculation shows that only the positively charged exciton shows the unconventional shift. The origin is attributed to the field-induced hole-concentration in the bottom-corner of the dot, which strongly enhances the repulsive direct Coulomb interaction and reduces the exciton binding energy. (C) 2011 American Institute of Physics. [doi:10.1063/1.3658639]

We study the spin-splitting energies in low-potential-barrier quantum dots, finding splitting energies that are orbital state dependent. The theoretical analysis is done with a generalization of the Fock-Darwin states in the presence of spin-orbit interactions. We discuss experimental evidence indicating that the Rashba interaction strength in vertical InxGa1-xAs/GaAs quantum dots is in the range 80 meV angstrom <= lambda(R) <= 120 meV angstrom. This enhanced spin-orbit interaction can be understood from the high penetration of the electron wave function into the quantum well with low-potential barrier.

We investigated the electronic structures of so-called 'double-capped' InAs/InP quantum dot suitable for a 1.55-mu m band single-photon emitter. From a transmission-electron-microscope image of the quantum dots, we extracted accurate information of the size and shape. We calculated the electron and hole eigenenergies by applying an 8-band k.p perturbation method into the quantum dot model based on the obtained size and shape. The calculated energies of the excitonic excited states agreed well with the resonant absorption peaks in a previously measured photoluminescence excitation spectrum with peaks that had been selected for quasi-resonant excitation to improve single-photon purity. This result suggests that the direct exciton creation into these states contributes to the suppression of excess carriers in the vicinity of the quantum dots, thus leading to a decrease in unwanted multiple-photon emission probabilities. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We fabricate and characterize a novel vertical pillar structure including a self-assembled InAs quantum dot (QD) and an InGaAs quantum well (QW). The vertical current through both the InAs QD and an electrostatically defined QD made in the InGaAs QW can be measured by adjusting the position of the InGaAs QD in the QW plane relative to the InAs QD with two side-gate voltages applied independently. We study optical response of the current through the vertical double QD by irradiating light, which is assumed to be mainly absorbed in the InAs QDs. We successfully probe a time-dependent energy level shift due to the Coulomb interaction from holes trapped in the vicinity of the pillar. (C) 2010 Elsevier B.V. All rights reserved.

We have calculated the energies of the lowest four excitons (two bright states and two dark states) in an InAs/GaAs quantum dot under the longitudinal magnetic field, B. The calculations were carried out using the strain-dependent 8-band k . p theory and the configuration interaction method. At B = 0, the electron-hole exchange interaction determines the splitting between the bright-state energies and that between the dark-state ones. The fine-structure splitting (bright-state splitting) is 30.7 mu eV. At B >= 1T, the Zeeman effect determines both splittings which change linearly with B with different exciton g-factors for the bright and dark states. The transition from the exchange-dominant to Zeeman-dominant splittings occurs at 0 < B < 1T, where the bright-state splitting deviates from the linear change near B = 0. Due to the transition, the degree of circular polarization for the lower (upper) bright state changes from 0% to approximate to 100% (-100%). (C) 2009 Elsevier B.V. All rights reserved.

We report on direct observation of spatial and optical luminescence switching (on and off condition) due to collapse of the coupling conditions of quantum dots (QDs) pair. In particular, one could observe that the relative intensities between two adjacent luminescence peaks and spatial images of individual QDs located closely nearby in a high-density self-assembled QD structure underwent a reversible change with respect to an external electric field.

Rabi oscillations of telecommunication-band excitons and biexcitons have successfully been observed by the photocurrent spectroscopy in a single InAs/GaAs quantum dot. We show the excitonic Rabi oscillations up to the rotation angle of 8 pi as well as the biexciton Rabi oscillation up to 2 pi. The decoherence time of both the telecommunication-band exciton and biexciton is much longer than the excitaion pulse-duration of 40 ps. The results demonstrate that the telecommunication-band exciton and biexciton system is promising for exciton-based-quantum information devices compatible with optical fiber networks. (C) 2010 The Japan Society of Applied Physics

Anisotropic Rabi oscillation in the exciton state in a single InAs/GaAs quantum dot (QD) was demonstrated in the telecommunication-band by selecting two orthogonal polarization angles of the excitation laser. Our InAs QDs were embedded in an intrinsic layer of an n-i-Schottky diode, which provides an electric field to extract photoexcited carriers from QDs. Owing to the potential anisotropy of QDs, the fine structure splitting (FSS) energy in the exciton state in single InAs QDs was similar to 110 mu eV, measured by polarization-resolved photocurrent spectroscopy. The ratio between two different Rabi frequencies, which reflect anisotropic dipole moments of two orthogonal exciton states, was estimated to be similar to 1.2. This demonstrates that the selective control of two orthogonal polarized exciton states is a promising technique for exciton-based-quantum information devices compatible with fiber optics. (C) 2010 The Japan Society of Applied Physics

We investigated the effect of acoustic phonon mediated damping on a telecommunication-band excitonic Rabi oscillation in a single quantum dot (QD). The InAs/GaAs QDs were capped with an InGaAs strain-reducing layer (SRL) to shift the exciton energy to the O-band (the lowest dispersion band; 1.26-1.36 mu m). They were also embedded in an n-i-Schottky diode in order to prove the excitonic state in a single photo-excited QD by photocurrent spectroscopy. The observed exciton linewidth increased linearly at the low temperature due to the exciton-acoustic phonon coupling, and increased drastically above 40 K due to the nonlinear increment of the exciton-LO phonon coupling. We successfully observed excitonic Rabi oscillation up to 40 K even though the acoustic phonon-mediated damping increased with temperature. This result suggests that the telecommunication-band QDs are useful in constructing the exciton-based qubits under the influence of the coupling between excitons and acoustic-phonons. (c) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We demonstrated sub-GHz operation of a single-photon-emitting diode at 1.55 μm using 80-ps-wide electrical pulses. A light-emitting diode (LED) with a quantum dot (QD) layer was fabricated into a nanoscale mesa structure with electrodes. The electroluminescence (EL) and radiative lifetime of a single exciton in the QD was directly determined to be 1.59 ns by time-resolved EL measurement. The single-exciton recombination time agrees with the radiative lifetime calculated with an eight-band kp model. The antibunching behavior of exciton radiative recombination in a currentinjected quantum dot was demonstrated at 1551.2 nm by Hanbury-Brown and Twiss-type photon correlation measurements. Device examination at a high drive rate by changing the delay time between two electrical pulses demonstrated that a QD LED can be used as source of sub-GHz single photons in the C-band triggered by current injection. © 2009 SPIE.

We report on photocurrent (PC) measurements of biexciton in a single self-assembled InAs quantum dot (QD) at a telecommunication wavelength of 1.3 mu m. We use shadow mask technique on an n-i Schottky photodiode structure with QDs to excite a single QD resonantly. Coherent pulse excitation is realized in two types of setups utilizing (i) an optical parametric oscillator and (ii) a stable semiconductor laser diode. In both setups we observe the biexcitonic PC peaks induced by a coherent two-photon process. Especially in the latter setups, the narrower pulse linewidth in energy provides a clearer biexcitonic PC peak because of reduced unwanted excitation. We estimate the binding energy Delta E-B of our telecom-band biexciton to be 0.9 meV from the splitting between excitonic and biexcitonic resonances. The result suggests our telecom-band exciton-biexciton system is a good candidate for the building block of fiber-based control led-rotation quantum logic operation. (C) 2009 WILEYNCH Verlag GmbH & Co. KGaA, Weinheim

Almost zero fine-structure splitting or degeneracy of the exciton spin state is a prerequisite for the production of a polarization-entangled photon pairs from the biexciton decay of a quantum dot, which is necessary for some schemes of quantum cryptography and quantum information processing. We have studied fine-structure splittings in single self-assembled quantum dots by using a photocurrent spectroscopy, whose resolution is higher than 1 mu eV. We show the photocurrent spectroscopy technique as a good tool to characterize the fine-structure splittings with high accuracy for the entanglement generation. Using the technique, we have found a very narrow splitting of less than 3 mu eV, which is limited by a time-dependent fluctuation of the photocurrent peak. (C) 2007 Elsevier B.V. All rights reserved.

We demonstrate a coherent manipulation of both exciton and biexciton in an InAs quantum dot operating at a telecommunication band. Telecommunication laser diode successfully drives the coherent system and enables to probe the carrier dynamics. ©2008 IEEE.