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The homogeneous formation of high-mobility oxide semiconductor thin films over large areas at low temperatures was accomplished by optimizing both the film formation process and the low-temperature post-processing via plasma annealing. Increasing the substrate-to-target distance (D ST) and the applied target voltage was found to produce more uniform deposition. The results of the field-effect mobility distributions of IGZO TFTs processed using plasma annealing were founded that plasma annealing generated essentially uniform distributions with μ FE values in the range of 32–35 cm2 V−1 s−1.

Improvement of carrier injection efficiency is essential to achieve lower threshold and shorter wavelength in deep-ultraviolet laser diodes. We have confirmed that the introduction of electron blocking layer and Mg doping layer into the core layer significantly improves the injection efficiency. In this study, we show that optimizing the Mg doping level in the core layer improves the external quantum efficiency by a factor of about 10 compared to the non-doped sample. The dependence of the external quantum efficiency on Mg flow rate can be interpreted in terms of a reduction of the dip at the p-side core/cladding interface in the conduction band. The dip is expected to be suppressed by ionized Mg activated by the Poole–Frenkel effect, resulting in improved carrier injection efficiency. A remarkable improvement in efficiency is also observed when the Mg doping region is limited to the vicinity of the core/cladding layer interface.

This study presents an experimental demonstration of the crucial role of compositional interdiffusion at interfaces in GaAs/AlGaAs alternating superlattices for developing the terahertz quantum cascade lasers operating at high temperatures. By growing GaAs/Al0.3Ga0.7As superlattices using the molecular beam epitaxy technique, an aluminum interdiffusion width of 0.95 nm (equivalent to ∼3.4 monolayers) is estimated. Incorporating this interdiffusion width as an additional design parameter has resulted in a 20 K improvement in the maximum operating temperature of the laser. To quantify the magnitude of the interdiffusion scattering effects on electron dynamics, an independent self-energy that functions based on the axial correlation length is introduced in the non-equilibrium Green’s function model. It clarifies that changes in the lifetime of quantum states and also the population fractions are primarily influenced by the deformation of wave functions at the injector when the interfaces become interdiffused.

This paper presents the sensitivity characteristics of a microelectromechanical systems (MEMS) piezoresistive cantilever and nanohole array used in a waterproof airflow sensor. Previously, a Pitot tube-type waterproof airflow sensor was developed for seabird biologging. Built-in MEMS piezoresistive cantilevers are used as the differential pressure sensing elements. The waterproof function is achieved using nanohole arrays via Laplace pressure. However, the mechanism underlying sensitivity reduction when nanohole arrays are attached is unclear. Here, we experimentally and theoretically verified that the specific constant, which determines the airflow rate through the cantilever and nanohole array, affects sensitivity reduction. An airflow sensor with a small sensitivity reduction was achieved based on the calculated results using appropriate cantilever and nanohole arrays. We demonstrated that the proposed method for estimating sensitivity reduction is useful for designing waterproof airflow sensors using cantilever-type differential pressure sensor elements.

BiSb is a topological insulator with both giant spin Hall effect and high electrical conductivity, which are important for ultralow power spin–orbit-torque magnetoresistive random access memory. However, when a ferromagnetic (FM) thin film is deposited on top of BiSb, large surface roughness and diffusion of Sb out of the BiSb layer into the FM layer results in a much smaller effective spin Hall angle θ SH eff . In this work, we show that by inserting a NiO layer between BiSb and Co, we can significantly improve θ SH eff of BiSb and induce perpendicular magnetic anisotropy at the NiO/Co interface. We obtained a large θ SH eff = 10.3 for the BiSb bottom layer at the NiO layer thickness of 3 nm, which is comparable to the best of BiSb top layer studied so far. Our study shows that BiSb/NiO bilayers are promising for integration of BiSb to SOT-MRAM.

Previous theoretical studies have suggested novel approaches for enhancing the thermoelectric performance of Mg2Si, i.e. electronic band engineering based on tensile strain to enhance the Seebeck coefficient and electrical conductivity simultaneously. To realise this approach, we investigated the doping with isoelectronic impurities (Ca, Sr, and Ba) substituted at Mg sites. We performed variable-cell relaxation calculations for Ca-doped, Sr-doped, and Ba-doped systems using first-principles calculations. The results showed that doping with Ca made the lowest and next-lowest conduction bands approach each other, which is advantageous for enhancing thermoelectric properties. Thermoelectric calculations showed that the distribution of Ca atoms would have a crucial effect on thermoelectric properties. Although Ca doping did not improve the n-type power factor for any structures examined in the present study, the Seebeck coefficient increased for the p-type, especially for a certain structure at 300 and 600 K, resulting in an improved power factor.

The Gd thickness dependence of spin–orbit torque (SOT) and critical current density of SOT switching in ferrimagnetic Gd/FeCo multilayers grown on a Ta seed layer was investigated and compared with the results on GdFeCo alloy/Ta bilayers. Critical current density of SOT switching J sw of Gd/FeCo multilayers was almost the same as those of the GdFeCo alloy films. The magnitudes of damping-like and field-like SOT effective fields, H DL and H FL, respectively, in Gd/FeCo multilayers were confirmed to increase near the compensation point, and the sign of H FL in Gd/FeCo multilayers was confirmed to change across the compensation point as well as GdFeCo alloy. Damping-like and field-like SOTs, estimated by multiplying the SOT effective fields by net magnetization, exhibited slightly smaller values for TM-dominant Gd/FeCo than for TM-dominant GdFeCo, while RE-dominant Gd/FeCo shows almost the same SOTs as RE-dominant GdFeCo.

The fundamental challenges faced by terahertz quantum cascade lasers are the operations below room temperature, and the recent improved operating temperatures are actually based on the narrow-period designs (2-well resonant-phonon scheme). In this study, we address the critical effect of nonrelevant high-confined quantum levels on the laser threshold current in such designs, i.e. at high temperatures, the resonant-tunneling channels passing through neighboring periods via those levels can dominantly increase the laser threshold current. As a result, the laser dynamics significantly shrink to zero even at 270 K. The channels cannot be easily eliminated as they are derived from the specific configuration of a two-quantum-well periodic structure, in which the width of the period is narrow, and the nonrelevant levels from the lower-well are confined down in energy. It is reasonable to suppress the current flowing across those levels by narrowing the lower-well further, but an appropriate balance of its thickness is essential.

To simulate the expansion of the matter created in relativistic nuclear collisions, codes in 3+1 dimensions are used and we are developing a new one. To benchmark such codes, the Sod’s shock tube is often used. A closely related problem is the one-dimensional expansion of a gas into vacuum. In this paper, we study this problem classically and relativistically with the Smoothed Particle Method and test various techniques to improve the precision and speed of the solution.

We have investigated an Al modulation epitaxy (AME) method to obtain step-flow growth of Al droplet free AlN layers by plasma assisted molecular beam epitaxy (MBE). At the usual growth temperature of (Al)GaN/AlN heterostructures, Al atom migration and desorption rate are very low and consequently it is very difficult to avoid the formation of Al droplets on AlN growth front by conventional MBE growth method. By adopting the AME growth method, such a difficulty has been effectively overcome and step flow growth mode of AlN has been clearly observed. By optimizing the AME growth time sequence, namely, AlN growth time and N radical beam treatment time, Al droplet free AlN layers with step flow growth characteristics have been obtained, with atomic flat surfaces and an average atomic step width of ∼118 nm at 970 °C–1000 °C, which is still suitable to grow (Al)GaN/AlN heterostructures by MBE.

Herein, we investigate surface-emitting photonic crystal terahertz quantum cascade lasers adopting double metal waveguide for high light extraction efficiency by utilizing finite-difference time-domain and filter diagonalization method. For the ease of fabrication and scalability of beam power, only a typical uniform 2D unit cell with triangular lattice and connected metal electrodes are considered in this study. Cylinder or uniform triangular prism photonic crystals are adopted as resonators within the selected unit cell structure. Candidate Γ -point modes for efficient surface-normal emission are searched based on the unit cell analysis as a first step. After that, far-field, field distribution, and light-extraction efficiency are calculated to verify the candidate modes’ suitability by deploying 15(16) × 15 unit cells. Extracted results show that the surface-emitting quantum cascade lasers with triangular prism photonic crystal slabs have the possibility to attain high light-extraction efficiency with Gaussian beam shape even with compact chip size and high active region occupancy.

To maximize the population inversion in terahertz quantum cascade lasers for high-temperature operation, the scheme of electron injection following direct-phonon emission is more attractive than the common way via resonant tunneling. If this direct-phonon injection can be realized within a simple quantum structure (only three quantum states), much higher optical gain is expected even at room temperature. However, the attempts of this design in experiments are full of challenges. In this work, the non-equilibrium Green’s function method is introduced to draw a full picture of the quantum transport in this design. It demonstrates that the optical gain is strictly limited by a parasitic absorption from one specific high-lying state. Moreover, a strategy is proposed to avoid this limitation.

In ocean-based renewable energies, such as wave motion, ocean current, tidal current and ocean thermal energy, many studies have been conducted on the wave energy conversion system and they have been tested in many countries. In this study, a tidal current energy is focused. Collecting a tidal flow is very important because it is generally known that a power available from a stream of water is proportional to the cube of the free stream velocity of the current. In this research, a bi-directional turbine system is investigated for a tidal flow that changes the direction periodically. Numerical investigations are conducted on a bi-directional flow collector with guide vane to collect the flow, in which the effects of vane skew angle and blade number are investigated. Calculated results showed that the collector with eight guide vanes was the highest angular moment at turbine inlet. The axial velocity at turbine inlet was larger for smaller value of the vane skew angle, whereas the tangential velocity at the turbine inlet was larger for larger value of the vane skew angle. As the results of both the profiles of axial and tangential velocity, the angular moment took the local maximum value at around the angle of 180 deg. in this study.

A new High-Temperature Superconducting ECR (HTS-ECR) ion source is under development in Research Center for Nuclear Physics (RCNP), Osaka University. This ion source will be used for production of high intensity proton, deuteron and He ion beams. The HTS-ECR magnets are composed of three solenoid coils and a set of sextupole coils made of REBCO tapes, a high-temperature superconductor. The HTS-ECR ion source is designed to operate at frequency of 2.45 GHz and 10 GHz. Performance test of the HTS coils had been carried out at 31 K and 77 K. This HTS coil technology will be applied to development of a meter-size HTS coil system of a high intensity compact AVF cyclotron. This paper introduces the basic design of the HTS-ECR ion source. The performance test results showed that REBCO solenoids remain superconducting state with a current up to 400 A. Simulation results of the magnetic field and electromagnetic field distributions in a plasma chamber fulfilled the requirements of electron cyclotron resonance conditions at 2.45 GHz and 10 GHz. Simulation result of mirror ratios and electromagnetic field amplitudes are also presented in this paper.