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This paper reports an enhancement of the nonvolatile memory characteristics of GaN/AlN resonant tunneling diodes (RTDs) by reducing the crystal defects in the quantum well structure. Pit-shaped crystal defects are strongly suppressed when pure N2, instead of a N2/H2 mixture, is used as a carrier gas and trimethylindium is introduced as a surfactant for metalorganic vapor phase epitaxy of the quantum well structure. In addition, the density of dislocations is lowered by controlling the growth conditions and structure of the buffer layer between a GaN/AlN RTD and a sapphire (0001) substrate. The leakage current through the quantum well structure is lowered, and an extremely high ON/OFF of >1300, which is 20 times higher than the values obtained in previous studies, is induced. Theoretical calculations based on Poisson’s equation and the Tsu–Esaki formula indicate that a high ON/OFF ratio of >103 can be enhanced by increasing the density of electrons accumulating in the quantum well to a level on the order of 1018 cm–3. Furthermore, nonvolatile memory operations were performed by inputting the sequential pulse voltages with a speed of nanosecond time scale which is faster than speeds of electron releases from the crystal defects. These results strongly indicate that the nonvolatile memory characteristics of GaN/AlN RTDs are due to intersubband transitions and electron accumulation in the quantum well and are not attributed to electron trapping by the crystal defects.

High-resolution video has rapidly integrated into our daily life in the context of progress in camera, display, signal processing, and communication technologies. The uppermost video parameters standardized at this moment include 8K, 120-fps, 12b RGB, wide-color-gamut, and HDR. Although a camera that fulfills all these parameters has been reported based on 1.7-inch 33-Mpixel CMOS imagers [1], achieving a smaller form factor while also maintaining image quality is required from the standpoint of mobility, lens design, and depth of focus. In general, miniaturization of the imager causes degradation of the image quality metrics such as sensitivity, dynamic range, and resolution. We deliberated on these difficulties, and set a target optical format of 1.25 inch.

Diamond Schottky barrier diodes (SBDs) with vertical channel structures are fabricated on an insulating diamond substrate. The deep etching technique is used to make a vertical channel structure in the diamond stacking layers of the p+/p-/insulating substrate. Mo Schottky electrodes with the Al2O3 field plate structure are also fabricated to realize a high breakdown voltage. As a result, I-V characteristics with a high forward current density >103 A/cm2 and a high breakdown voltage >700 V are obtained at 25°C and 250 °C. Baliga's figure of merit is 332 MW/cm2, which is equivalent to the high value previously reported for diamond SBDs with lateral channel or pseudovertical channel structures, and is also ten times higher than the limit of Si SBDs. High-power diamond SBDs are expected to be realized on a large area insulating diamond substrate using this technique.

Diamond is a promising material for future high power devices due to high breakdown field, low dielectric constant and high carrier mobility, respectively. From one-dimensional device model, 90% reduction of the power loss is expected by using diamond Schottky barrier diode instead of SiC diode for high temperature power device applications. The high breakdown field of more than 3 MV/cm has been realized by utilizing high Schottky barrier height. The diodes show low leakage current and low on-resistance even at high temperature conditions.

A new nonvolatile memory based on intersubband transitions and electron accumulation in GaN/AlN resonant tunneling diodes (RTDs) was investigated toward the realization of a high-speed nonvolatile random-access memory (RAM) operating on picosecond time scales. It was shown that the endurance of write/erase memory operations can be enhanced by reducing the structural inhomogeneity of GaN/AlN RTDs. Neither mechanical damage nor significant degradation of current–voltage characteristics was observed after repeated write/erase memory operations using GaN/AlN RTDs with small degrees of inhomogeneity. High-endurance nonvolatile RAMs for computing systems are expected to be realized by further improvement in the crystal quality of GaN/AlN RTDs.

We demonstrated all-optical demultiplexing of 160-Gb/s signal to 40- and 80-Gb/s by a Mach-Zehnder Interferometric all-optical switch, where the picosecond cross-phase modulation (XPM) induced by intersubband excitation in InGaAs/AlAsSb coupled double quantum wells is utilized. A bi-directional pump configuration, i.e., two control pulses are injected from both sides of a waveguide chip simultaneously, increases a nonlinear phase shift twice in comparison with injection of single pump beam with forward- and backward direction. The bi-directional pump configuration is the effective way to avoid damaging waveguide facets in the case where high optical power of control pulse is necessary to be injected for optical gating at repetition rate of 40/80GHz. Bit error rate (BER) measurements on 40-Gb/s demultiplexed signal show that the power penalty is decreased slightly for the bi-directional pump case in the BER range less than ∼10-6. The power penalty is 1.3dB at BER of 10-9 for the bi-directional pump case, while it increases by 0.3-0.6dB for single pump cases. A power penalty is influenced mainly by signal attenuation at “off” state due to the insufficient nonlinear phase shift, upper limit of which is constrained by the current low XPM efficiency of ∼0.1rad/pJ and the damage threshold power of ∼100mW in a waveguide facet.

Room temperature photoreflectance (PR) has been performed on InGaAs/AlAsSb quantum wells (QWs) grown on InP substrates. Using a differential reflectance setup, we observed clear and well-resolved structures, which could be attributed to the interband optical transitions originating in both the QWs and InP substrate. The transition energies measured from the PR spectra agree with those theoretically calculated from the confined levels in the QWs.

A new strain compensation method for the InGaAs/AlAs/AlAsSb coupled double quantum wells (CDQWs) has been proposed. The proposed structure is composed of In 0.53 Ga 0.47 As wells, AlAs central barriers, AlAs diffusion-stopping-layers, and AlAsSb external barriers with a high Sb composition which compensate the lattice mismatch produced by the AlAs layers. In the absorption spectra of the fabricated CDQWs, peaks originating from the intersubband transitions (ISBTs) were observed clearly. Furthermore, the intensity and wavelength of the peaks were varied by varying the doping density of the InGaAs wells and the thickness of the AlAs central barriers. The proposed method provides additional latitude in designing all-optical switches based on ISBTs that use the lattice mismatched systems.

Large, very fast phase modulation was observed in transverse electric (TE) probe light when an intersubband transition (ISBT) switch module using an InGaAs/AlAs/AlAsSb quantum well was pumped by transverse magnetic (TM) light. The phase shift amounted to 1.88 rad for a pump pulse energy of 4 pJ (fiber input), with very fast response. This phenomenon is explained by the change in plasma dispersion caused by the redistribution of electrons among subbands having different effective masses. This phase modulation will enable us to realize various novel ultrafast all-optical devices for signal processing.

A field-plate structure is applied to vertical diamond Schottky barrier diode. A sputtered Al2O3 with 0.2 µm thickness is utilized for field-plate insulator. Fabricated diamond VSBD shows low leakage characteristics. Accordingly, the breakdown voltage is improved from 900V to 1,800V.

At sufficiently low temperatures, e.g., below 10 K, current in current–voltage (I–V) characteristics of double-barrier resonant-tunneling diodes (DBRTDs) in the low-current and low-voltage region is inversely proportional to the phase coherence length. However, with increasing temperature, this proportional behavior no longer holds due to thermal energy spread around the Fermi energy in the emitter. In this study, the temperature range in which current amplitude is inversely proportional to the phase coherence length is theoretically derived. The temperature range is dependent on the structural parameters of DBRTDs. By choosing appropriate device parameters, we can make the current amplitude inversely proportional to the phase coherence length even at relatively high temperatures, e.g., 77 K. These results are highly useful for estimating electron phase coherence length over a wide temperature range and for studying the temperature dependence of the phase coherence length using DBRTDs.

The direct butt-joint coupling between the Si-wire-based Michelson interferometer (MI) and the InGaAs/AlAsSb quantum-well waveguide was performed to realize the hybrid integrated ultrafast intersubband transition switch. By attaining the narrow gap between the waveguides, we have observed the Michelson interference spectra between the two signal lights: one is the phase modulated light from the InGaAs/AlAsSb quantum-well waveguide and the other is the light returned from the Si loop mirror of MI. Furthermore, by controlling the phases and powers of two signal lights with the thermooptic phase shifter and variable coupler in MI, we have demonstrated the ultrafast all-optical gating operation with a response time of 2 ps. The all-optical gating switch with a small size of 0.3 mm 2.35 mm was successfully fabricated by the hybrid integration technologies of III-V and Si materials.

Ultrafast all-optical cross phase modulation (XPM) was enhanced in the InGaAs/AlAsSb coupled quantum wells using new InAlAs coupling barrier. Furthermore, a high XPM efficiency of 0.15 rad/pJ, which is approximately three times as large as that of a previous sample with the AlAs coupling barrier, was obtained by realizing the strong optical confinement in a narrower waveguide. From the analysis of the quantum levels and the measurement of the absorption spectra, the enhanced XPM efficiency was suggested to be contributed by the refractive index dispersion of the interband transition that was modulated by the intersubband transitions (ISBTs). This large XPM efficiency is expected to give a higher performance in Mach-Zehnder interferometer-type ultrafast all-optical switch.

Resistance switching memory operations using the bistability in the current–voltage (I–V) characteristics of GaN/AlN resonant tunneling diodes (RTDs) were investigated to realize an ultrafast nonvolatile memory operating at a picosecond time scale. Resistance switching memory operations based on electron accumulation due to intersubband transitions and electron release due to tunneling current were demonstrated with high reproducibility at room temperature when the leakage of electrons accumulating in the quantum well from the deep level in the AlN barrier was suppressed. A nonvolatile memory for the processor core in a normally off computing system is expected to be realized using the bistability in the I–V characteristics of GaN/AlN RTDs.

Ultrafast cross-phase modulation in InGaAs/AlAsSb coupled double quantum wells, where interband dispersion is modulated by intersubband excitation, is applied to a MZI switch demonstrating error-free all-optical demultiplexing of 160-Gb/s OTDM signal to 40-Gb/s.

Ultrafast all-optical switch based on intersubband transitions in InGaAs/AlAs/AlAsSb quantum well is described. Because of very fast intra-band relaxation in conduction band, we can obtain very fast response of around 1ps. The operation principles and characteristics as an absorption saturation type device are described. Also described is the operation as an all-optical phase modulator. With Mach-Zehnder interferometer configuration, error free all-optical demultiplexing operation from 160-Gb/s to 40-Gb/s was achieved.

Ultrafast index change induced by intersubband excitation in ${\text{In}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{As}/{\text{AlAs}}_{y}{\text{Sb}}_{1\ensuremath{-}y}$ quantum well waveguides was evaluated by measuring the phase shift $\ensuremath{\Delta}\ensuremath{\phi}$ in a wide spectral range of $1.2--1.7\text{ }\ensuremath{\mu}\text{m}$ based on a spectral analysis. We obtained a large enhancement in $\ensuremath{\Delta}\ensuremath{\phi}$ by tuning the probe photon energy to interband absorption edge. The measured induced absorption $\ensuremath{\Delta}A$ also increased with reduction in the wavelength. The spectra of $\ensuremath{\Delta}\ensuremath{\phi}$ and $\ensuremath{\Delta}A$ were well explained by the interband dispersion model under intersubband excitation while the intersubband plasma dispersion previously reported has a negligible contribution.

All-optical switch based on the intersubband transition in InGaAs/AlAsSb quantum wells with DFB structure was fabricated using deep-etching technology. The sharp transmission spectrum with a difference of more than 13 dB between the stopband and the off-resonant regions was successfully obtained. Furthermore, the applicability of this device to ultrafast optical switches with a switching speed greater than 160 GHz was investigated by considering the relation between the bandwidth of the spectrum and the operating frequency.

The InGaAs/InAlAs/AlAsSb coupled quantum well is developed to use in Mach-Zehnder interferometric switch utilizing intersubband transition. Strong well-well coupling produced by this structure enhances cross phase modulation, which is effective in reducing the switching energy.

We demonstrated all optical demultiplexing from 160 to 40-Gb/s by intersubband transition switch. This was enabled by quantum well with high XPM efficiency and bi-directional pump configuration.

Ultrafast cross-phase modulation induced by intersubband excitation was ~2-3 times enhanced when tuning probe wavelength from 1640 to 1360 nm, showing a ~300 nm broadband. Interband dispersion model explains wavelength dependence.

We have fabricated a novel InGaAs/AlAsSb coupled double quantum well (C-DQW) with InAlAs coupling barrier for use in all-optical switches based on intersubband transitions (ISBTs). Strong well -well coupling produced by the low potential of the InAlAs coupling barrier shortens the ISB-T wavelength. The FTIR measurements show that the wavelength of ISB absorption decreases to 1.53 mum when the InGaAs wells and InAlAs coupling barrier in the C-DQW consist of 9 and 4 monolayers (MLs), respectively. Moreover, the absorption coefficients at 1.55 mum increase significantly above 500 cm-1 and are therefore equivalent to those of the conventional C-DQW that consists of AlAsSb and AlAs coupling barriers. It is confirmed that the ISB-T wavelength can be reduced effectively by reducing the potential height of the coupling barriers as well as the thicknesses of the wells and coupling barriers.

We report an ultrafast cross phase modulation (XPM) effect in intersubband transition (ISBT) of InGaAs/AlAs/AlAsSb coupled quantum wells, where the ISBT absorption of a transverse-magnetic mode pump signal induces phase modulation of a transverse-electric mode probe signal. Using waveguide-type ISBT devices, we have achieved XPM-based 10 Gbit/s wavelength conversion with a power penalty of 2.53 dB. Also, we propose XPM-based signal processing circuits for gate switching and modulation format conversion.

InGaAs/AlAs/AlAsSb coupled double quantum wells (CDQWs) have been improved for use in intersubband transition switches. The In content of the well layers was reduced in order to investigate the two-photon absorption in our switches. The crystal quality for In content of 0.53 was deteriorated when compared with the case for In composition of 0.8 due to uncompensated strain by AlAs layers. A new strain-compensation for InGaAs/AlAs/AlAsSb CDQWs was achieved by adjusting the Sb composition of the barrier layers.

The bistability characteristics of GaN/AlN resonant tunneling diodes (RTDs) grown on a sapphire substrate by metalorganic vapor phase epitaxy (MOVPE) were investigated to better understand their physical origin and explore their use in nonvolatile memories. The bistability current–voltage (I–V) characteristics of GaN/AlN RTDs, which were due to intersubband transitions and electron accumulation in the quantum well, were clearly observed over a wide temperature range between 50 and 300 K. However, the I–V characteristics sometimes degraded at temperatures above 250 K. Complex staircase structures were observed in the voltage region showing a negative differential resistance in the I–V curve, and the forward current increased or decreased rapidly as the forward-bias voltage increased. Repeated measurements of the I–V characteristics over the wide temperature range between 50 and 300 K revealed that the bistability characteristics of GaN/AlN RTDs degraded owing to the leakage of electrons accumulating in the quantum well through a deep level in the AlN barrier associated with crystal defects such as dislocations and impurities. Therefore, reduction in crystal defect and impurity densities in the AlN barrier, and a careful design that considers deep levels are important for realizing realize ultrafast nonvolatile memories based on the bistability characteristics of GaN/AlN RTDs.

Selective-area growth of diamond films in microwave-plasma chemical vapor deposition was performed using newly developed masks. By forming chemically stable masks made of Ru/Au or Mo/Au, which have high melting points, good adhesion to diamond, and difficulty in forming carbide compounds, patterned diamond films with a large thickness of 50 µm, a large area of 5 mm2, and a high orientation in the [001] direction were successfully grown on (001) diamond substrates without degradation of the crystal quality of masked areas.

Objective The objective of the study was to describe a clinically useful factors index predicting long-term efficacy of uterine artery embolization (UAE). Study Design Newly diagnosed patients with uterine leiomyoma wishing to retain their uterus underwent UAE at our institution. Clinical demographics and 4 prognostic factors were recovered from the medical record. A regrowth-free interval (RFI) was calculated for all patients based on leiomyoma regrowth or recurrence of any previously reported symptoms. RFI by prognostic factor was analyzed by the Kaplan-Meier method. Results Forty-three patients were identified. Two prognostic factors were identified by multivariate analysis: vascularity (dichotomized as hypervascular vs hypovascular; RFI at 2 years, 80% vs 20%, P = .001) and number of nodules (solitary vs multiple; RFI at 2 years, 72% vs 25% at 2 years, P = .001) Conclusion UAE success may be predicted by 2 preoperative parameters. Further investigation is warranted.

We have developed a Mach-Zehnder interferometric all-optical switch employing intersubband transition in an InGaAs∕AlAs∕AlAsSb-coupled double quantum well waveguide. The recently discovered cross-phase modulation phenomenon was utilized as the switching mechanism; the nonlinear index of refraction for transverse electric polarized light is induced by intersubband optical excitation using transverse magnetic pump light. We demonstrate the demultiplexing operation of 160Gbit∕s data signals to 10Gbit∕s using this switch. At the input control pulse energy of 8pJ, the demultiplexed signals showed an extinction ratio better than 10dB, and an error-free demultiplexing was achieved.

We studied peak characteristics in the current–voltage (I–V) curve of triple-barrier resonant-tunneling diodes (TBRTDs). The current peak broadening caused by the well–well and well-electrode couplings is revealed. The electron phase breaking effect is incorporated into the analysis to obtain the I–V curve of TBRTD using the phase correlation method. For strong well–well coupling, the peak current is independent of the phase breaking, while for weak coupling it is dependent of it. The electron coherence can be estimated using TBRTDs designed appropriately.

We analyzed current–voltage (I–V) characteristics of double-barrier resonant tunneling diodes (DBRTDs), taking into account the phase-breaking effect and the structural inhomogeneity by using the correlation function theory of phase breaking to fit the experimental data over four orders of magnitude. The phase-breaking effect appears in the current region of less than one-hundredth of the peak in I–V characteristics, in addition to the peak of the second derivative curve. The well width fluctuation, if any, influences the latter but not the former. Using the theory, the phase-coherent length and the well width fluctuation are estimated separately.

We studied the peak width of current vs voltage (I–V) characteristics of triple-barrier resonant tunneling diodes (TBRTDs) experimentally and theoretically. A GaInAs/InP TBRTD was fabricated by organo metallic vapor phase epitaxy (OMVPE). A theory of I–V characteristics of TBRTDs was developed by taking the structural inhomogeneity into account to explain the experimental peak width. The fluctuation of the well width in a TBRTD grown by OMVPE was estimated as two atomic layers.

We recently encountered a patient with SHP in whom the lesion of the digestive system was identified by ultrasonography and in whom the healing process of the lesions could be observed

The intersubband transition (ISBT) all-optical switches were developed using two different material systems, InGaAs/AlAs/AlAsSb system, and CdS/ZnSe/BeTe system. Fiber modules were fabricated for both material switches. InGaAs/AlAs/AlAsSb switch showed 10dB extinction at fiber input of 15pJ for 500fs pulse and quasi-DEMUX operation was demonstrated at repetition rate of 10GHz for 40GHz pulses train with pulse width of 2ps. CdS/ZnSe/BeTe switch showed 10dB extinction at 13.1pJ fiber input and very fast response of below 560fs. I. Background