Table of contents

Halide vapor phase epitaxy of thick GaN films was demonstrated on ScAlMgO₄ (SCAM) substrates, and their self-separation was achieved. The 320-µm-thick GaN film was self-separated from the SCAM substrate during the cooling process after the growth. This separation phenomenon occurred because of both the c-plane cleavability of SCAM and the difference in the thermal-expansion coefficients between GaN and SCAM. The dark-spot densities for the GaN films on the SCAM substrates were approximately 30% lower than those on sapphire substrates. These results indicate that SCAM substrates are promising for fabricating a high-quality freestanding GaN wafer at a low cost.

Highly conductive Ge-doped GaN epitaxial layers were grown by low-temperature pulsed sputtering, and their fundamental structural and electrical properties were investigated. The room-temperature (RT) electron concentration was increased to 5.1 × 10²⁰ cm⁻³ by the Ge doping, and the atomically flat stepped and terraced surface and the crystalline quality of the layers were maintained. Consequently, the RT resistivity was reduced to 0.20 mΩ·cm, which is comparable to that for typical transparent conductive oxides such as indium tin oxide. The contact resistance of Ge-doped GaN with a Ti/Al/Ti/Au metal stack prepared without annealing was as low as 0.087 Ω·mm. Furthermore, the selective formation of a Ge-doped region using an SiO₂ mask was demonstrated. The results clearly indicate the strong potential of pulsed sputtering Ge-doped GaN growth for forming low-parasitic-resistance contact layers of various electrical and optical devices.

We fabricated planar-orientation crystalline thin films of organic semiconductors, in which molecules sit parallel, i.e., "face-on", on the substrate and favor vertical charge transport. Thanks to molecular orientation that is sensitive to surface properties and the self-organization of liquid crystals, planar-orientation crystalline thin films can be prepared by simply cooling a smectic liquid-crystalline organic semiconductor from isotropic temperature with the aid of a poly(vinyl alcohol) (PVA) microtemplate. The molecular orientation of crystalline thin films was investigated by polarized optical microscopy (POM) and X-ray diffraction (XRD) analysis, and the current–voltage characteristics of the films were studied in a diode configuration. The results showed high potential for device applications.

We theoretically and numerically investigate the phase-coupled plasmon-induced transparency effect in graphene plasmonic systems that consist of multiple cascade graphene nanoribbon resonators side-coupled to the bus waveguide. The formation of a transparency window is attributed to the superposition of the detuned resonances in graphene nanoribbons. The gap width between the metal and the graphene provides a new degree of freedom for controlling the round-trip phase accumulated in the bus waveguide, which determines the evolution of the transparency window in terms of intensity and symmetry. Our ultracompact configuration can realize slow light with group indices over 250 as well as retain high transmission intensity.

Time-resolved electric-field-induced sum-frequency generation (EFI-SFG) spectroscopy was employed to study the charge behavior of multilayer organic light-emitting diodes (OLEDs). Through application of the square wave pulse bias to the OLEDs, compensation for the polarization charges in the electron transport layer and the generation of 4,4'-bis[N-(1-naphthyl-N-phenylamino)-biphenyl] (α-NPD) cations were observed. When the pulse voltage was turned off, the α-NPD cations immediately disappeared, confirming that charge recombination occurs at the interfaces. We therefore concluded that time-resolved EFI-SFG is useful for directly probing the carrier behavior in OLEDs in addition to identifying the origin of the charge carriers present in OLEDs.

We propose a planar electro-optic deflector using KTa_1−xNb_xO₃ (KTN) crystals in order to reduce power consumption by reducing its capacitance. We reduced its capacitance by reducing its thickness to maintain the deflection angle. The power consumption was 1/34 that of the bulk-type KTN deflector at 200 kHz. The deflection angles were in good agreement with theoretical values when the dielectric constant was over 10000.

An ion beam technique is used to lightly treat semi-insulating GaAs terahertz (THz) photoconductive antennas (PCAs), and a novel structure with two layers, a carrier acceleration layer and a carrier trapping layer, is demonstrated to afford high-efficiency, high-power THz emitters. The key roles of vacancy defects produced by the ion beam in efficient THz generation are systematically described. The peak distribution of defects at approximately 2.5 µm provides an effective trapping layer for photocarriers during THz generation. Hydrogen ion implantation under reasonable conditions (300 keV, 1 × 10¹⁵ cm⁻²) for fabrication of efficient GaAs PCAs is found to be reproducible.

Mixed-halide perovskites, whose bandgap energies can be widely controlled through choice of composition, are promising for various optoelectronic applications. Herein, we report the photocarrier recombination dynamics in mixed-halide CH₃NH₃Pb(I_1−xBr_x)₃ perovskite films with different Br contents. We observed small changes in the single-carrier trapping rate with respect to the Br content. In contrast, the two-carrier radiative and three-carrier Auger recombination rates increased significantly with the Br content. These increases in the multicarrier recombination rates likely originated from the enhancement of the Coulomb interactions between electrons and holes caused by incorporating Br. Our findings are useful for designing mixed-halide perovskite-based optoelectronic devices.

We propose a particle model for investigating the optical noisy image recovery via stochastic resonance. The light propagating in nonlinear media is regarded as moving particles, which are used for analyzing the nonlinear coupling of signal and noise. Owing to nonlinearity, a signal seeds a potential to reinforce itself at the expense of noise. The applied electric field, noise intensity, and correlation length are important parameters that influence the recovery effects. The noise-hidden image with the signal-to-noise intensity ratio of $1:30$ is successfully restored and an optimal cross-correlation gain of 6.1 is theoretically obtained.

We have developed a method to generate sub-nanosecond 650-nm-band optical pulses. These pulses have a peak power of 40 W and a pulse energy of 13 nJ at a 1-MHz repetition rate. This technology is intended for application in stimulated-emission-depletion microscopy. Our method is based on the pulsed operation of a 1.3-µm-band semiconductor-laser optical amplifier and the second-harmonic generation of the optical pulses after amplification by a Pr-doped fiber amplifier. The resultant peak power and pulse energy of the 650-nm-band optical pulses are two orders of magnitude higher than those directly obtained from a laser diode.

Recently, several petawatt (PW, 10¹⁵ W) lasers with pulse duration of ∼20–30 fs have been introduced throughout the world, pushing the upper limit on laser peak power. However, besides well-known spatio-temporal coupling effects, such as residual spatial/angular chirps and pulsefront tilt/curvature, the spatio-temporal/spectral coupling in compressors induced by wavefront errors of gratings, which could dramatically distort ultra-intense pulses, has been neglected. In this work, for the first time we analyzed this phenomenon and the peak power/intensity degradation induced by it. Our results suggest that the actual performance of femtosecond PW lasers may be worse than previously estimated.

We report on the molecular beam epitaxy and properties of a magnetic topological insulator (TI): Cr-doped Sb₂Te₃. Composition analysis reveals that Cr replaces Sb, and X-ray diffraction confirms that a single-phase textured crystal structure can be obtained for (Cr_xSb_1−x)₂Te₃ with x up to 0.44. A further increase in x results in phase separation or precipitates in the material. The Curie temperature T_C increases with x up to 0.44 and reaches 250 K, which is the highest T_C observed thus far in magnetically doped TIs.

Perpendicularly magnetized ferrimagnetic Gd–Fe–Co thin films with different compositions and multilayer arrangements were subjected to femtosecond laser pulses. The pulses triggered different magnetization dynamics in the various thin films. In the Gd₂₆Fe₆₆Co₈ film, which has an angular-momentum-compensation temperature (T_A) well above ambient temperature (T_exp), monotonic magnetization reversal occurred, whereas the Gd₂₂Fe₇₀Co₈ film (where T_A is well below T_exp) exhibited remarkable wavelike spin modulation with spatial inhomogeneity during relaxation of the laser-induced nonequilibrium state. These findings can enable broad-range tuning of the magneto-optical responses of Gd–Fe–Co alloys, facilitating advances in materials engineering.

The interfacial Dzyaloshinskii–Moriya interaction (iDMI) and the interfacial perpendicular magnetic anisotropy (iPMA) between a heavy metal and ferromagnet are investigated by employing Brillouin light scattering. With increasing thickness of the heavy-metal (Pt) layer, the iDMI and iPMA energy densities are rapidly enhanced and they saturate for a Pt thickness of 2.4 nm. Since these two individual magnetic properties show the same Pt thickness dependence, this is evidence that the iDMI and iPMA at the interface between the heavy metal and ferromagnet, the physical origin of these phenomena, are effectively enhanced upon increasing the thickness of the heavy-metal layer.

We make use of room-temperature magnetostatic surface spin waves (MSSWs) to mediate the interaction between the microwave field from an antenna and the spin of nitrogen-vacancy (NV) centers in diamond. We show that this transport spans distances exceeding 3 mm, a manifestation of the MSSW robustness and large diffusion length. Using the NV spins as a local sensor, we find that the MSSW couples resonantly, and the NV spins amplitude grow linearly with the applied microwave power, suggesting that this approach could be extended to amplify the signal from neighboring spin qubits by several orders of magnitude.

A high flux pinning performance was obtained for a 3.8 vol % BaHfO₃-doped SmBa₂Cu₃O_y superconducting film on a metallic substrate. At a temperature of 77.3 K, an irreversibility field of 16.8 T and the maximum flux pinning force density of 32.5 GN/m³ in fields applied parallel to the c-axis of the film were achieved, which are the highest values reported thus far for REBa₂Cu₃O_y films, to our knowledge. The introduction of well-aligned BaHfO₃ nanorods with a high number density into REBa₂Cu₃O_y films with little or no degradation of the critical temperature is an effective method for improving the flux pinning performance.

In this work, an extraordinary acoustic transmission transparency window with an angle-insensitive acoustic metamaterial plane is proposed and investigated numerically and experimentally. The cell of the metamaterial plane consists of two embedded and coaxially split spherical shells arranged in a square lattice, and the extraordinary acoustic transmission transparency window is caused by the resonance coupling of two embedded split spherical shells. The simulation results reveal that the designed plane has a frequency-selective transparent window with angle insensitive to incident waves. To obtain experimental evidence, the designed samples are fabricated by three-dimensional (3D) printing technology and measured using an acoustic impedance tube testing system. The transmission exported from the experiment coincides with the simulation predictions, which further proves the existence of the acoustic-frequency-selective transparent window caused by the resonance coupling. This phenomenon revealed in the present contribution is widespread and is expected to be utilized for fabricating and designing novel acoustic devices.

Radial deformation of boron nitride nanotubes (BNNTs) plays a significant role in the performances of BNNT-based applications. By performing molecular dynamics simulations, the radial deformation of armchair single-walled BNNTs was investigated. The deformation energy barrier was found to follow a decreasing trend with increasing tube diameter. Two threshold diameters were identified that demarcate three stability regimes for the deformed single-walled BNNTs. Whereas the van der Waals interaction was simply favorable for radial deformation, the electrostatic interaction had a complex effect; it prevented deformation from the initial cylindrical shape but promoted collapse when the opposing tube wall came into proximity.

Nanoimprinting can be used to fabricate various nanostructures on materials without etching processes. We previously demonstrated that the molecular orientation of a photocrosslinkable liquid crystalline polymer (PLCP) was induced by nanoimprinting, using a process called nanoimprint graphoepitaxy. In the current study, we performed multiple nanoimprint graphoepitaxy steps on PLCP films and investigated the resulting molecular orientations. Double nanoimprint graphoepitaxy on the PLCP produced a simple 2-µm line-and-space pattern with a dot molecular orientation pattern.

Glass transitions of Te-based phase-change materials (PCMs) were studied by modulated differential scanning calorimetry. It was found that both Ge₂Sb₂Te₅ and GeTe are marginal glass formers with ΔT (= T_x − T_g) less than 2.1 °C when the heating rate is below 3 °C min⁻¹. The fragilities of Ge₂Sb₂Te₅ and GeTe can be estimated as 46.0 and 39.7, respectively, around the glass transition temperature, implying that a fragile-to-strong transition would be presented in such Te-based PCMs. The above results provide direct experimental evidence to support the investigation of crystallization kinetics in supercooled liquid PCMs.

Efficient InGaN-based 444 nm blue light-emitting diodes (LEDs) were fabricated on low-defect-density $(11\bar{2}2)$ semipolar GaN templates grown on patterned r-sapphire. At 20 A/cm², the packaged $(11\bar{2}2)$ LEDs exhibited a light output power of 2.9 mW (17.8 mW at 100 A/cm²) and a record peak external quantum efficiency of 6.4% showing a negligible efficiency droop and blue shift with drive currents up to 100 A/cm². In addition, we demonstrated light extraction simulations for the $(11\bar{2}2)$ template, which showed that the structured pattern is not only beneficial for limiting the defect propagation but also increases the light extraction by 29% compared with GaN layers grown on planar substrates.

By a self-terminating gate recess etching technique, a normally-off fully recess-gated GaN metal–insulator–semiconductor field-effect transistor (MISFET) was fabricated using Al₂O₃/Si₃N₄ bilayer as gate dielectrics. Owing to the high breakdown electric field (∼10 MV/cm) of the gate dielectrics, the device exhibits a large gate swing of 18 V, a high threshold voltage of 1.7 V (at I_D = 100 µA/mm), a large maximum drain current of 534 mA/mm, a gate leakage current lower than 20 nA/mm in the whole gate swing, and a high OFF-state breakdown voltage of 1282 V. Furthermore, owing to the high gate overdrive (V_GS − V_TH), the on-resistance of the device only increases by 5.4% under a constant stress of V_GS/V_DS = 18 V/1 V.