Table of contents

We evaluated the DC characteristics of InAlN/GaN metal–oxide–semiconductor high-electron-mobility transistors (MOS-HEMTs) using atomic-layer-deposited (ALD-)Al₂O₃ by focusing on air annealing to control defect levels in Al₂O₃ and electronic states at the Al₂O₃/InAlN interface. We clarified that the transconductance linearity and subthreshold slope were improved by air annealing, indicating a reduction in the number of electronic states at the Al₂O₃/InAlN interface. Furthermore, the cathodoluminescence study demonstrated that the oxygen-related defects in ALD-Al₂O₃ were decreased by air annealing. Consequently, we could successfully reduce the threshold voltage shift of InAlN/GaN MOS-HEMTs by using air-annealed ALD-Al₂O₃.

The photoluminescences of ion-implanted (I/I) and epitaxial Mg-doped GaN (GaN:Mg) are compared. The intensities and lifetimes of the near-band-edge and ultraviolet luminescences associated with a Mg_Ga acceptor of I/I GaN:Mg were significantly lower and shorter than those of the epilayers, respectively. Simultaneously, the green luminescence (GL) became dominant. These emissions were quenched far below room temperature. The results indicate the generation of point defects common to GL and nonradiative recombination centers (NRCs) by I/I. Taking the results of positron annihilation measurement into account, N vacancies are the prime candidate to emit GL and create NRCs with Ga vacancies, (V_Ga)_m(V_N)_n, as well as to inhibit p-type conductivity.

In this study, we successfully fabricated vertical GaN merged PiN Schottky (MPS) diodes and comparatively investigated the cyclic p-GaN width (W_p) dependence of their electrical characteristics, including turn-on voltage and reverse leakage current. The MPS diodes with W_p of more than 6 µm can turn on at around 3 V. Increasing W_p can suppress the reverse leakage current. Moreover, the vertical GaN MPS diode with the breakdown voltage of 2 kV was realized for the first time.

The nominal internal quantum efficiency of InGaN/GaN multiple quantum wells significantly increases from 5.6 to 26.8%, as a low-temperature GaN cap layer is grown in N₂/H₂ mixture gas. Meanwhile, the room-temperature photoluminescence (PL) peak energy shows a merely 73 meV blue shift. On the basis of temperature-dependent PL characteristics analysis, the huge improvement in PL efficiency arises mainly from the "etching effect" of hydrogen, which reduces the defect density and indium segregation at the upper well/barrier interface, and consequently contributes to the decrease in the number of nonradiative recombination centers and the enhancement of carrier localization.

We demonstrate the thermally actuated phase change of VO₂ films formed by atomic layer deposition and subsequent thermal annealing on InAlN/AlN/GaN heterostructures. To locally raise the device temperature above the VO₂ semiconductor–metal transition temperature, a two-dimensional electron gas formed within the InAlN/AlN/GaN heterostructure was used as an integrated resistive heater. An ON/OFF resistance ratio of nearly 10³ was achieved for 50 nm VO₂ films over a temperature range of 25 to 105 °C. The time required to switch the VO₂ film from high- to low-resistance states was shown to depend on the applied heater power, with sub-microsecond transition times achieved.

Li-ion conductivity in a solid-state electrolyte has so far been measured by impedance spectroscopy. In this method, however, it is difficult to obtain microstructural information because of the absence of spatial resolution. Here, we show the relationship between the Li-ion mobility and the crystal orientation in Li_0.33La_0.56TiO₃ polycrystalline by electrochemical strain microscopy combined with electron backscatter diffraction. On the experimentally constructed multivariable regression model, we obtained a qualitative Li-ion mobility map of sub-millimeter width with a 100 nm spatial resolution, which is impossible to achieve by only atomic force microscopy. The proposed method must be useful for identifying the Li-ion diffusion pathway in three dimensions.

We reveal two routes of optical carrier injection in pure silicon by means of high-resolution excitation spectroscopy on nanosecond cyclotron resonances. Free carriers are generated either by the band-to-band transition assisted by phonon emission, or via two-body collisions of excitons. The first route was previously masked by a strong excitonic response in steady-state optical spectra at low temperatures. Furthermore, valley polarization is achieved for the cold carriers created by the second route with optimized excess energy. These optical carrier injection routes are crucial to initialize the momentum and valley degrees of freedom of carriers in order to enable versatile applications of indirect-bandgap semiconductors.

Perfect electromagnetic metamaterial absorbers based on three-dimensional (3D) vertical split-ring resonators for an infrared spectral range were fabricated using a combination of high-resolution direct laser write lithography and a simple metalization by sputtering. In accordance with theoretical predictions, the fabricated samples exhibit perfect absorption resonances tunable in the wavelength range of 4.5–9.2 µm by changing the dimensions and spacing of the resonators. The structures exhibit polarization and incidence angle-invariant operation with absorbance in excess of 0.85 for incidence angles up to 30°. In the future, they may find applications as narrow-band thermal emitters and for signal enhancement in mid-IR photodetectors.

We demonstrate large optical Stark shifts in a single quantum dot embedded in a modified H1 photonic crystal nanocavity. We designed the nanocavity to simultaneously possess a high Q factor, a small mode volume, and a high coupling efficiency to the external laser field. This nanocavity enabled the observation of a large Stark shift of 70 µeV even under very weak laser irradiation. The large shift was sustained by only four intracavity photons on average, paving the way for the development of ultralow-power quantum/classical optical devices.

We employ picosecond laser multiple scribing to fabricate oblique sidewalls of AlGaN-based deep ultraviolet light-emitting diode (LED) chips for enhanced light extraction. The multiple scribing lines in the sapphire substrate are intentionally aligned to guide the wafer to be diced along oblique sidewalls with designed angles. Compared with a conventional LED chip having vertical sidewalls, one with sidewalls inclined at 60° exhibits a 13.8% higher light output power at 50 mA without any deterioration of the laser-induced electrical characteristics. Finite-difference time-domain simulation reveals the optimized sidewall inclination angle and indicates that the inclination of the sidewall is effective for TM-polarized light extraction.

We focused on fluorine tin oxide (FTO)-coated glass substrates for perovskite solar cells (PVSCs) and studied the effects of the optical properties and surface morphology on the short-circuit current density (J_sc). The PVSC on our FTO substrate demonstrated a gain in J_sc by 1.4–1.6 mA/cm², compared with the PVSCs on commercial FTO substrates. This is attributed not only to the low absorption of the FTO substrate but also to the suppression of reflection loss, caused by the light trapping effect on the textured surface. Finally, the power conversion efficiency of our PVSC reached >21% with less hysteresis.

We demonstrate a subwavelength position determination method for the terahertz region. Previously, we reported that an off-axis parabolic mirror generates a peculiar transient rotational distribution around the focus on the subwavelength scale. In the method proposed herein, the position is determined by measuring the electric field scattered by a sample placed at this rotational distribution. We perform a realistic numerical calculation and show that this method is feasible for a sample on the wavelength scale and can distinguish a displacement of the order of 0.01 wavelengths. This method can be easily implemented for micro and nanoscale measurement and processing in the terahertz region.

We perform a pilot trial of highly convenient taper fabrication for polymer optical fibers (POFs) using hot water. A ∼380-mm-long POF taper is successfully fabricated, and its ∼150-mm-long waist has a uniform outer diameter of ∼230 µm. The shape is in good agreement with the theoretical prediction. The optical loss dependence on the strain applied to the waist shows an interesting behavior exhibiting three regimes, the origins of which are inferred by microscopic observations. We then discuss the controllability of the taper length.

All-polarization-maintaining, single-port Er:fiber combs offer long-term robust operation as well as high stability. We have built two such combs and evaluated the transfer noise for linking optical clocks. A uniformly broadened spectrum over 135–285 THz with a high signal-to-noise ratio enables the optical frequency measurement of the subharmonics of strontium, ytterbium, and mercury optical lattice clocks with the fractional frequency-noise power spectral density of (1–2) × 10⁻¹⁷ Hz^−1/2 at 1 Hz. By applying a synchronous clock comparison, the comb enables clock ratio measurements with 10⁻¹⁷ instability at 1 s, which is one order of magnitude smaller than the best instability of the frequency ratio of optical lattice clocks.

We propose measuring the change in Brillouin frequency shift (BFS) in an optical fiber with virtually controlled sensitivity based on the Vernier effect using a multimode pump and probe light. We measure BFS from the envelope curve of the spectra for multimode Stokes light without scanning the probe frequency. The sensitivity of BFS is virtually controlled by changing the difference in the mode spacing between the pump and the probe. It is experimentally demonstrated that the virtual sensitivity is controlled from −1.1 to 1.1 GHz/K, the maximum value of which is 1,100 times higher than the standard BFS of 1 MHz/K.

This is the first report of single-cycle terahertz (THz)-wave pulse generation with a wide band (over 6 THz) and a high dynamic range (over 70 dB) using an organic nonlinear optical crystal, 4-dimethylamino-N'-methyl-4'-stilbazolium tosylate (DAST), and the prism-coupled Cherenkov phase-matching method. The prism-coupled approach allowed the use of a DAST crystal as a Cherenkov-type emitter for reduced light absorption by the crystal, resulting in the generation of single- and half-cycle THz pulse shapes, which were detected using dipole and bow-tie antennas, respectively. The resulting THz pulse generation is suitable for various THz-wave application technologies, such as tomography.

The high-power broad-area (BA) photonic bandgap crystal (PBC) diode laser is promising as a high-brightness laser source, however, it suffers from poor lateral beam quality owing to the intrinsic drawback of BA lasers. In this paper, a ladderlike groove structure (LLGS) was proposed to improve both the lateral beam quality and emission power of BA PBC lasers. An approximately 15.4% improvement in output power and 25.2% decrease in the lateral beam parameter product (BPP) were realized and the underlying mechanism was discussed. On the basis of the one-dimensional PBC epitaxial structure, a stable vertical far field was demonstrated.

The high-external differential quantum efficiency operation of a GaInAsP/InP membrane distributed-reflector laser bonded on a Si substrate was achieved by adopting a short cavity design and reducing the waveguide loss and differential resistance. A threshold current of 0.21 mA, an external differential quantum efficiency of 32% for the front-side output, and a power-conversion efficiency of 12% were obtained with a 32-µm distributed feedback section length, a 50-µm distributed-Bragg-reflector section, and a 0.8-µm stripe width. A side-mode suppression ratio of 41 dB was obtained at a bias current of 1 mA.

We demonstrate a very high coincidence-to-accidental ratio of 673 using continuous-wave photon-pair generation in a silicon strip waveguide through spontaneous four-wave mixing. This result is obtained by employing on-chip photonic-crystal-based grating couplers for both low-loss fiber-to-chip coupling and on-chip suppression of generated spontaneous Raman scattering noise. We measure a minimum heralded second-order correlation of $g_{\text{H}}^{(2)}(0) = 0.12$, demonstrating that our source operates in the single-photon regime with low noise.

We theoretically investigate the spin injection in different ferromagnet/insulator/n-Si tunnel contacts by using the lattice non-equilibrium Green's function method. We find that the tunnel contacts with low-barrier materials such as TiO₂ and Ta₂O₅ have far lower resistances than the conventional-barrier materials, resulting in a wider and attainable optimum parameters window for improving the spin-injection efficiency and magnetoresistance ratio of a vertical-spin metal–oxide–semiconductor field-effect transistor. Additionally, we find that the spin-asymmetry coefficient of the TiO₂ tunnel contact has a negative value, while that of the Ta₂O₅ contact can be tuned between positive and negative values by changing the parameters.

Three-dimensional integration processes (based on direct wafer bonding and back-surface silicon removal) for magnetic tunnel junctions with perpendicular magnetization (p-MTJs) were developed. Perfect wafer bonding, namely, bonding without interfacial voids, and damageless silicon removal were successfully demonstrated by using very flat tantalum cap layers. Moreover, p-MTJ nanopillars subjected to these processes exhibited no degradation in magnetoresistance or spin-transfer-torque (STT) switching. Magnetoresistive random access memory (MRAM) technology incorporating these processes (direct wafer bonding and back-surface silicon removal) will make it possible to integrate epitaxial MTJs (with a single-crystal tunnel barrier) and ferromagnetic electrode layers (based on new materials).

The anomalous Hall effect (AHE) is studied in Ta/CoFeB/MgAl₂O₄/Ta multilayers with different thicknesses of MgAl₂O₄ (t), which causes in-plane magnetic anisotropy (IMA) for t = 1.0 nm and perpendicular magnetic anisotropy (PMA) for t ≥ 1.2 nm. Conventional scaling was demonstrated to be not inadequate in our case. The origin of the AHE in Ta/CoFeB/MgAl₂O₄/Ta multilayers is mainly an extrinsic mechanism. The contribution of skew scattering (SS) is unneglectable, and both the SS and side jump are enhanced when the magnetic anisotropy changes from IMA to PMA, indicating that the oxidation at the interface of CoFeB/MgAl₂O₄ has a dominant influence on the AHE.

We demonstrate that the background signal in the nonlocal spin-valve measurement can be sufficiently suppressed by optimizing the electrode design of the lateral spin valve. A relatively long length scale of heat propagation produces spin-independent thermoelectric signals under the combination of the Peltier and Seebeck effects. These unfavorable signals can be reduced by mixing the Peltier effects in two transparent ferromagnetic/nonmagnetic junctions. Proper understanding of the contribution from the heat current in no spin-current area is a key for effective reduction of the spin-independent background signal.

First-principles calculation predicts a large negative to positive change in the intrinsic perpendicular magnetic anisotropy (PMA) on the order of 10⁷ erg/cm³ of a W/Fe(001) multilayer upon reducing the in-plane lattice constant. This PMA arises at the W/Fe interface, and the second interfacial W site plays an important role. To experimentally verify this theoretical prediction, we have grown W/Fe/W(001) epitaxial trilayers on Cr(001) underlayers. By varying the W layer thickness, the in-plane lattice constant of W can be widely controlled, and the large change in PMA from negative to positive is successfully demonstrated.

The voltage-controlled magnetic anisotropy of ferromagnetic metals may offer potential applications of nonvolatile memories with ultralow power consumption. For achieving ultrafast recording and long-time endurance, voltage-induced effects without undesirable lattice distortions should be ensured. In this study, in-situ extended X-ray absorption fine structure analysis of an Fe/Pt/MgO junction demonstrated the unaltered interfacial atomic structure, in which the radial distances between the Pt and the neighboring Fe, Pt, O, and Mg atoms changed by less than ±0.01 Å under electric fields of ±0.18 V/nm. Therefore, the anisotropy change is driven by a purely electronic mechanism without lattice deformation or atomic relaxation.

The incorporation of a symmetric electrostatic potential into quantum wells (QWs) is proposed as a method for modifying the coefficient α of the Rashba spin–orbit interaction. In a symmetric QW for which α = a_soE_z, where E_z is the perpendicular electric field, the constant a_so can be controlled by the symmetric potential. The sign reversal of a_so with the increasing strength of the symmetric potential is demonstrated in (001)-oriented GaAs/AlGaAs QWs via a tight-binding model. The present findings can be used to realize structures with vanishing α in nonzero E_z.

The successful recovery of resistive switching random access memory (RRAM) devices that have undergone switching failure is achieved by introducing a low-temperature supercritical-fluid process that passivates the switching layer. These failed RRAM devices, which are incapable of switching between high- and low-resistance states, were treated with supercritical carbon dioxide with pure water at 120 °C for 1 h. After the treatment, the devices became operational again and showed excellent current–voltage (I–V) characteristics and reliability as before. On the basis of the current conduction mechanism fitting results, we propose a model to explain the phenomenon.

We characterized the near-interface traps (NITs) in SiO₂/4H-SiC structures according to the distributed circuit model, which was originally proposed for Al₂O₃/InGaAs interface structures. We assumed that the NITs had an exponentially decaying distribution from the SiO₂/4H-SiC interface into the oxide, rather than the uniform trap distribution of the conventional model. Using this model with the exponential NIT distribution as a basis, we successfully explained the frequency-dependent characteristics of both the capacitance and conductance in the strong-accumulation condition with reasonable physical parameters. We also observed that nitridation annealing over a long period of time significantly reduced the NIT density distribution.

A single memory cell having both volatile memory (VM) and nonvolatile memory (NVM) functions with an independent asymmetric dual-gate structure is reported, as well as its programming methods. In the case of operating the device as a VM cell, a higher sensing margin is obtained, and an undesirable soft-programming issue is suppressed when a gate-induced drain leakage programming method is used. Additionally, the sensing margin and hold retention time of the VM operation are improved in a programmed state of the NVM function. These results indicate that the proposed device has potential for high-density embedded-memory applications.

Highly luminescent ZnSe-based quantum dots (QDs) were synthesized by a microwave-assisted hydrothermal method. The characteristics of the ZnSe precursor solution strongly influenced the photoluminescence (PL) quantum yields (QYs) of the QDs. The PL QY of ZnSe-core QDs synthesized under the optimum conditions reached 60%. Furthermore, the PL QY further increased to higher than 90% when a ZnS shell was applied to prepare ZnSe/ZnS-core/shell QDs.

Fabricating flexible sensors on paper is intriguing. Here, we exploited chitosan as a buffer layer to facilitate the fabrication of silver nanowire (AgNW) networks and flexible devices on commercial paper. We found that the AgNW networks exhibited uniform distribution, smooth surface, and strong adhesion. The enhanced adhesion of AgNWs was attributed to the intermolecular hydrogen bonding between chitosan and hydroxypropyl methylcellulose (HPMC), which can be tailored by tuning the pH of the chitosan aqueous solution. This facile fabrication method utilizing biodegradable polymers and cost-effective AgNW ink holds great promise for portable, wearable, and disposable paper-based electronics.

In this study, MoS₂ flakes with the configuration of multilayer MoS₂ stacked on monolayer MoS₂ were synthesized by chemical vapor deposition (CVD). The morphology of the MoS₂ flakes transformed from that of a truncated triangle to that of a fishbone-like shape with increasing growth temperature. A novel phenomenon of shape consistency between the upper multilayer MoS₂ and the basal monolayer was found in MoS₂ flakes of triangular, star-like, and fishbone-like shape. The mechanism of this shape consistency is attributed to the active border of the pre-grown monolayer MoS₂ flake. This work enriches the understanding of the growth mechanism of MoS₂ flakes fabricated by CVD.

Monolithically integrating BaTiO₃ on silicon substrates has attracted attention because of the wide spectrum of potential novel applications ranging from electronics to photonics. For optimal device performance, it is important to control the BaTiO₃ domain orientation during thin film preparation. Here, we use molecular beam epitaxy to prepare crystalline BaTiO₃ on Si(001) substrates using a SrTiO₃ buffer layer. A systematic investigation is performed to understand how to control the BaTiO₃ domain orientation through the thickness engineering of the SrTiO₃ buffer layer and the BaTiO₃ layer itself. This provides different possibilities for obtaining a given BaTiO₃ orientation as desired for a specific device application.

The spatial arrangement of molecule adsorbates on a metal surface is very difficult to predict via first-principles calculations and standard optimizing algorithms. In this Letter, we show that a machine learning technique called Bayesian optimization can optimize the arrangement of two medium-sized aromatic adsorbates on a copper (111) surface within tens of density functional theory energy evaluations. The methodology reported here is therefore a step toward first-principles structure predictions for chemically modified surfaces, without the need to first specify the arrangement of molecule adsorbates from experimental data.

We theoretically realized the one-way self-collimation effect of acoustic beams in two-dimensional sonic crystals (SCs), which are composed of irregular rigid rods surrounded by circulating fluids. The parity symmetry (P symmetry) and time-reversal symmetry (T symmetry) of the circulating-fluid SCs (CFSCs) are broken by the asymmetric crystal lattice and the circulating fluids. A large isolation of >30 dB with a maintained wavefront and frequency is realized for output acoustic beams launched in opposite directions. By applying the gradient angular velocities of the circulating fluids, the one-way self-collimated acoustic beams can be bent gradually, yielding an acoustic mirage effect. The one-way self-collimation in CFSCs provides an avenue for manipulating acoustic beams independently of the nonlinearity and mode conversion.