Applied Physics Express

Engineered quantum systems enabling novel capabilities for computation and sensing have blossomed in the last decade. Architectures benefiting from combining complementary physical systems have emerged as promising approaches for quantum technologies. A new class of hybrid quantum systems based on collective spin excitations in ferromagnetic materials has led to the diverse set of platforms outlined in this review article. The coherent interaction between microwave cavity modes and spin-wave modes is presented as a key ingredient for the development of more complex hybrid systems. Indeed, quanta of excitation of the spin-wave modes, called magnons, can also interact coherently with optical photons, phonons, and superconducting qubits in the fields of cavity optomagnonics, cavity magnomechanics, and quantum magnonics, respectively. Notably, quantum optics experiments in magnetically-ordered solid-state systems are within reach thanks to quantum magnonics. Applications of hybrid quantum systems based on magnonics for quantum information processing and quantum sensing are briefly outlined.

In this work, degenerate n-type GaN thin films prepared by co-sputtering from a liquid Ga-target were demonstrated and their low-field scattering mechanisms are described. Extremely high donor concentrations above 3 × 10²⁰ cm⁻³ at low process temperatures (<800 °C) with specific resistivities below 0.5 mΩcm were achieved. The degenerate nature of the sputtered films was verified via temperature-dependent Hall measurements (300–550 K) revealing negligible change in electron mobility and donor concentration. Scattering at ionized impurities was determined to be the major limiting factor with a minor contribution of polar optical-phonon scattering at high temperatures.

Major features of silicon carbide (SiC) power devices include high blocking voltage, low on-state loss, and fast switching, compared with those of the Si counterparts. Through recent progress in the material and device technologies of SiC, production of 600–3300 V class SiC unipolar devices such as power metal-oxide-semiconductor field-effect transistors (MOSFETs) and Schottky barrier diodes has started, and the adoption of SiC devices has been demonstrated to greatly reduce power loss in real systems. However, the interface defects and bulk defects in SiC power MOSFETs severely limit the device performance and reliability. In this review, the advantages and present status of SiC devices are introduced and then defect engineering in SiC power devices is presented. In particular, two critical issues, namely defects near the oxide/SiC interface and the expansion of single Shockley-type stacking faults, are discussed. The current physical understanding as well as attempts to reduce these defects and to minimize defect-associated problems are reviewed.

Visible-wavelength GaN-based photonic-crystal surface-emitting lasers (PCSELs) have attracted attention for various applications, such as materials processing, high-brightness illuminations, and displays. In this letter, we demonstrate GaN-based PCSELs at green wavelengths. We formed a photonic crystal (PC) in p-GaN and filled holes of the PC with SiO₂ to ensure device stability. Through a current injection test under pulsed conditions and spectral analysis, we confirmed that the fabricated device possessed Γ-point single-mode oscillation at wavelengths above 505 nm. Our results have the potential to further expand the applications of PCSELs and semiconductor lasers in visible region.

To address the issue of the high cost of GaN substrates, a recycling process for GaN substrates using a laser slicing technique was investigated. The channel properties of lateral MOSFETs and the reverse characteristics of vertical PN diodes, which represent the main components of vertical power devices, exhibited no degradation either before and after laser slicing or due to the overall GaN substrate recycling process. This result indicates that the proposed recycling process is an effective method for reducing the cost of GaN substrates and has the potential to encourage the popularization of GaN vertical power devices.

A unique field termination structure combining a three-step field plate with nitrogen ion implantation to enhance the reverse breakdown performance of Pt/β-Ga₂O₃ Schottky barrier diodes (SBDs) and NiO/β-Ga₂O₃ heterojunction diodes (HJDs) is reported. The fabricated devices showed a low R_on,sp of 6.2 mΩ cm² for SBDs and 6.8 mΩ cm² for HJDs. HJDs showed a 0.8 V turn-on voltage along with an ideality factor of 1.1 leading to a low effective on-resistance of 18 mΩ cm². The devices also showed low reverse leakage current (<1 mA cm⁻²) and a breakdown voltage of ∼1.4 kV. These results offer an alternative, simpler route for fabricating high-performance kilovolt-class β-Ga₂O₃ diodes.

Recent first-principles approaches to semiconductors are reviewed, with an emphasis on theoretical insight into emerging materials and in silico exploration of as-yet-unreported materials. As relevant theory and methodologies have developed, along with computer performance, it is now feasible to predict a variety of material properties ab initio at the practical level of accuracy required for detailed understanding and elaborate design of semiconductors; these material properties include (i) fundamental bulk properties such as band gaps, effective masses, dielectric constants, and optical absorption coefficients; (ii) the properties of point defects, including native defects, residual impurities, and dopants, such as donor, acceptor, and deep-trap levels, and formation energies, which determine the carrier type and density; and (iii) absolute and relative band positions, including ionization potentials and electron affinities at semiconductor surfaces, band offsets at heterointerfaces between dissimilar semiconductors, and Schottky barrier heights at metal–semiconductor interfaces, which are often discussed systematically using band alignment or lineup diagrams. These predictions from first principles have made it possible to elucidate the characteristics of semiconductors used in industry, including group III–V compounds such as GaN, GaP, and GaAs and their alloys with related Al and In compounds; amorphous oxides, represented by In–Ga–Zn–O; transparent conductive oxides (TCOs), represented by In₂O₃, SnO₂, and ZnO; and photovoltaic absorber and buffer layer materials such as CdTe and CdS among group II–VI compounds and chalcopyrite CuInSe₂, CuGaSe₂, and CuIn_1−xGa_xSe₂ (CIGS) alloys, in addition to the prototypical elemental semiconductors Si and Ge. Semiconductors attracting renewed or emerging interest have also been investigated, for instance, divalent tin compounds, including SnO and SnS; wurtzite-derived ternary compounds such as ZnSnN₂ and CuGaO₂; perovskite oxides such as SrTiO₃ and BaSnO₃; and organic–inorganic hybrid perovskites, represented by CH₃NH₃PbI₃. Moreover, the deployment of first-principles calculations allows us to predict the crystal structure, stability, and properties of as-yet-unreported materials. Promising materials have been explored via high-throughput screening within either publicly available computational databases or unexplored composition and structure space. Reported examples include the identification of nitride semiconductors, TCOs, solar cell photoabsorber materials, and photocatalysts, some of which have been experimentally verified. Machine learning in combination with first-principles calculations has emerged recently as a technique to accelerate and enhance in silico screening. A blend of computation and experimentation with data science toward the development of materials is often referred to as materials informatics and is currently attracting growing interest.

To realize high-capacity Ge anodes for next-generation Li⁺-ion batteries, a multilayer anode with a C(top)/Ge(middle)/C(bottom) structure was developed, where nanostructured amorphous Ge (a-Ge) and amorphous-like carbon films with a grain size of 10–20 nm were deposited sequentially by high-pressure Ar sputtering at 500 mTorr. Compared with the a-Ge anode, the C(top)/a-Ge(middle)/C(bottom) multistacking layer anode showed improved capacity degradation for repeated lithiation/delithiation reactions and achieved a high capacity of 910 mAh g⁻¹ with no capacity fading after 90 cycles at a C-rate of 0.1.

This study investigated the crystallographic plane dependence of the reaction of AlN and AlGaN using heated-pressurized water under saturated vapor pressure. The results show that the reaction strongly depends on the crystallographic orientation plane, with no reaction in the +c-plane, the formation of an AlOOH-altered layer in the −c-plane, and etching in the a- and m-planes. These results suggest that the exfoliation mechanism of AlGaN grown on periodically formed AlN nanopillars on sapphire substrates using heated-pressurized water involves etching of a- and m-plane crystals, demonstrating that the proposed method is highly reproducible and versatile for large-diameter wafer exfoliation.

The present study focused on CO₂ capture, storage, and conversion through the innovative integration of plasma–ionic liquid (IL) technology. For the first time, we employed plasma-IL technology to confront climate change challenges. We utilized 1-Butyl-3-methylimidazolium chloride IL to capture and store CO₂ under atmospheric pressure, and subsequently employed plasma to induce the transformation of IL-captured CO₂ into CO. Furthermore, we performed computer simulations to enhance our understanding of the CO₂ and CO capture processes of water and IL solutions. This comprehensive approach provides valuable insights into the potential of plasma–IL technology as a viable solution for climate change.

We demonstrate a master oscillator power amplifier system that emits single-frequency, high-energy optical pulses at 1539 nm using an Er, Yb:glass planar waveguide amplifier with a normalized frequency of the waveguide of 4.4. A maximum pulse energy of 9.7 mJ is observed at a repetition frequency of 500 Hz. The signal to noise ratio is 25 dB and is independent of the repetition frequency from 100 to 500 Hz. The beam quality factor M² of the output is 1.03 thanks to the small normalized frequency of the planar waveguide.

1550 nm wavelength photonic-crystal surface-emitting lasers (PCSELs) are attractive for optical communication and eye-safe sensing applications. In this study, we present InP-based PCSELs featuring a double-lattice photonic-crystal structure designed for high-power single-mode operation at a wavelength of 1550 nm. These PCSELs demonstrate output powers exceeding 300 mW under continuous-wave conditions at 25 °C. Additionally, highly stable single-mode oscillation with a side-mode suppression ratio of over 60 dB is verified at temperatures from 15 °C to 60 °C. Measurement and simulation of photonic band structures reveal the impacts of the threshold gain margin and optical coupling coefficient on the single-mode stability.

A unique field termination structure combining a three-step field plate with nitrogen ion implantation to enhance the reverse breakdown performance of Pt/β-Ga₂O₃ Schottky barrier diodes (SBDs) and NiO/β-Ga₂O₃ heterojunction diodes (HJDs) is reported. The fabricated devices showed a low R_on,sp of 6.2 mΩ cm² for SBDs and 6.8 mΩ cm² for HJDs. HJDs showed a 0.8 V turn-on voltage along with an ideality factor of 1.1 leading to a low effective on-resistance of 18 mΩ cm². The devices also showed low reverse leakage current (<1 mA cm⁻²) and a breakdown voltage of ∼1.4 kV. These results offer an alternative, simpler route for fabricating high-performance kilovolt-class β-Ga₂O₃ diodes.

In this research, a 1064 nm single-longitudinal-mode (SLM) vortex beam was generated from an annular-pumped Nd:YVO₄ microchip laser, in which a circular Dammann grating was used to reshape the 808 nm pump light into an annular intensity profile. As a result, the laser emitted a SLM LG₀₁ beam with a maximum output power of 192 mW. A straightforward technique for producing SLM vortex beams is made available by this work.

In this study, we explore the rejuvenation of a Zener diode degraded by high electrical stress, leading to a leftward shift, and broadening of the Zener breakdown voltage knee, alongside a 57% reduction in forward current. We employed a non-thermal annealing method involving high-density electric pulses with short pulse width and low frequency. The annealing process took <30 s at near-ambient temperature. Raman spectroscopy supports the electrical characterization, showing enhancement in crystallinity to explain the restoration of the breakdown knee followed by improvement in forward current by ∼85%.

An emerging concept of "nanoarchitectonics" has been proposed as a way to apply the progress of nanotechnology to materials science. In the introductory parts, we briefly explain the progress in understanding materials through nanotechnology, the overview of nanoarchitectonics, the effects of nanoarchitectonics on the development of functional materials and devices, and outline of nanoarchitectonics intelligence as a main subject of this review paper. In the following sections, we explain the process of constructing intelligent devices based on atomic switches, in which the behavior of atoms determines the device functions, by integrating them with nanoarchitectonics. The contents are categorized into (i) basic operation of atomic switch, (ii) artificial synapse, (iii) neuromorphic network system, (iv) hetero-signal conversion, (v) decision making device, and (vi) atomic switch in practical uses. The atomic switches were originally relatively simple ON/OFF binary-type electrical devices, but their potential as multi-level resistive memory devices for artificial synapses and neuromorphic applications. Furthermore, network-structured atomic switches, which are complex and have regression pathways in their structure and resemble cranial neural circuits. For example, A decision-making device that reproduces human thinking based on a principle different from brain neural circuits was developed using atomic switches and proton-conductive electrochemical cells. Furthermore, atomic switches have been progressively developed into practical usages including application in harsh environments (e.g. high temperature, low temperature, space). Efforts toward information processing and artificial intelligence applications based on nanoarchitectonics tell remarkable success stories of nanoarchitectonics, linking the control of atomic motion to brain-like information control through nanoarchitecture regulations.

Nanopores are cost-effective digital platforms, which can rapidly detect and identify biomolecules at the single-molecule level with high accuracy via the changes in ionic currents. Furthermore, nanoscale deoxyribonucleic acid and proteins, as well as viruses and bacteria that are as small as several hundred nanometers and several microns, respectively, can be detected and identified by optimizing the diameters of a nanopore according to the sample molecule. Thus, this review presents an overview of the methods for fabricating nanopores, as well as their electrical properties, followed by an overview of the transport properties of ions and analyte molecules and the methods for electrical signal analysis. Thus, this review addresses the challenges of the practical application of nanopores and the countermeasures for mitigating them, thereby accelerating the construction of digital networks to secure the safety, security, and health of people globally.

The past several decades have witnessed a vast array of developments in printable organic semiconductors, where successes both in synthetic chemistry and in printing technology constituted a key step forward to the realization of printed electronics. In this Review, we highlight specifically materials science, charge transport, and device engineering of—two-dimensional single crystals—. Defect-free organic single-crystalline wafers manufactured via a one-shot printing process allow remarkably reliable implementations of organic thin-film transistors with decently high carrier mobility up to 10 cm² V⁻¹ s⁻¹, which has revolutionized the current printing electronics to be able to meet looming internet of things challenges. This Review focuses on the perspective of printing two-dimensional single crystals with reasonable areal coverage, showing their promising applications for practical devices and future human society, particularly based on our recent contributions.

Major features of silicon carbide (SiC) power devices include high blocking voltage, low on-state loss, and fast switching, compared with those of the Si counterparts. Through recent progress in the material and device technologies of SiC, production of 600–3300 V class SiC unipolar devices such as power metal-oxide-semiconductor field-effect transistors (MOSFETs) and Schottky barrier diodes has started, and the adoption of SiC devices has been demonstrated to greatly reduce power loss in real systems. However, the interface defects and bulk defects in SiC power MOSFETs severely limit the device performance and reliability. In this review, the advantages and present status of SiC devices are introduced and then defect engineering in SiC power devices is presented. In particular, two critical issues, namely defects near the oxide/SiC interface and the expansion of single Shockley-type stacking faults, are discussed. The current physical understanding as well as attempts to reduce these defects and to minimize defect-associated problems are reviewed.

Engineered quantum systems enabling novel capabilities for computation and sensing have blossomed in the last decade. Architectures benefiting from combining complementary physical systems have emerged as promising approaches for quantum technologies. A new class of hybrid quantum systems based on collective spin excitations in ferromagnetic materials has led to the diverse set of platforms outlined in this review article. The coherent interaction between microwave cavity modes and spin-wave modes is presented as a key ingredient for the development of more complex hybrid systems. Indeed, quanta of excitation of the spin-wave modes, called magnons, can also interact coherently with optical photons, phonons, and superconducting qubits in the fields of cavity optomagnonics, cavity magnomechanics, and quantum magnonics, respectively. Notably, quantum optics experiments in magnetically-ordered solid-state systems are within reach thanks to quantum magnonics. Applications of hybrid quantum systems based on magnonics for quantum information processing and quantum sensing are briefly outlined.

We have developed the GaN-based distributed feedback laser diode (DFB-LD) with the detuning of +5 nm to obtain smaller temperature dependence of the threshold current. We found that the current-light characteristics almost overlapped up to 300 mW between 25^○C and 80^○C. The estimated characteristic temperature is about 2550 K. These indicate that our DFB-LD is promising for applications that require small temperature dependence in the output power and oscillation wavelength at constant operation current without precise temperature control.

We demonstrate a master oscillator power amplifier system that emits single-frequency, high-energy optical pulses at 1539 nm using an Er, Yb:glass planar waveguide amplifier with a normalized frequency of the waveguide of 4.4. A maximum pulse energy of 9.7 mJ is observed at a repetition frequency of 500 Hz. The signal to noise ratio is 25 dB and is independent of the repetition frequency from 100 to 500 Hz. The beam quality factor M² of the output is 1.03 thanks to the small normalized frequency of the planar waveguide.

1550 nm wavelength photonic-crystal surface-emitting lasers (PCSELs) are attractive for optical communication and eye-safe sensing applications. In this study, we present InP-based PCSELs featuring a double-lattice photonic-crystal structure designed for high-power single-mode operation at a wavelength of 1550 nm. These PCSELs demonstrate output powers exceeding 300 mW under continuous-wave conditions at 25 °C. Additionally, highly stable single-mode oscillation with a side-mode suppression ratio of over 60 dB is verified at temperatures from 15 °C to 60 °C. Measurement and simulation of photonic band structures reveal the impacts of the threshold gain margin and optical coupling coefficient on the single-mode stability.

Improved p-GaN gate reliability is achieved through a simple oxygen compensation technique (OCT), which involves oxygen plasma treatment after gate opening and subsequential wet etching. The OCT compensates for the Mg acceptors near the p-GaN surface, leading to an extended depletion region under the same gate bias and thus reducing the electric field. Furthermore, the Schottky barrier height also increases by OCT. Consequently, suppressed gate leakage current and enlarged gate breakdown voltage are achieved. Notably, the maximum applicable gate bias also increases from 4 V to 8.1 V for a 10-year lifetime at a failure rate of 1%.

A unique field termination structure combining a three-step field plate with nitrogen ion implantation to enhance the reverse breakdown performance of Pt/β-Ga₂O₃ Schottky barrier diodes (SBDs) and NiO/β-Ga₂O₃ heterojunction diodes (HJDs) is reported. The fabricated devices showed a low R_on,sp of 6.2 mΩ cm² for SBDs and 6.8 mΩ cm² for HJDs. HJDs showed a 0.8 V turn-on voltage along with an ideality factor of 1.1 leading to a low effective on-resistance of 18 mΩ cm². The devices also showed low reverse leakage current (<1 mA cm⁻²) and a breakdown voltage of ∼1.4 kV. These results offer an alternative, simpler route for fabricating high-performance kilovolt-class β-Ga₂O₃ diodes.

In this research, a 1064 nm single-longitudinal-mode (SLM) vortex beam was generated from an annular-pumped Nd:YVO₄ microchip laser, in which a circular Dammann grating was used to reshape the 808 nm pump light into an annular intensity profile. As a result, the laser emitted a SLM LG₀₁ beam with a maximum output power of 192 mW. A straightforward technique for producing SLM vortex beams is made available by this work.

In this study, we explore the rejuvenation of a Zener diode degraded by high electrical stress, leading to a leftward shift, and broadening of the Zener breakdown voltage knee, alongside a 57% reduction in forward current. We employed a non-thermal annealing method involving high-density electric pulses with short pulse width and low frequency. The annealing process took <30 s at near-ambient temperature. Raman spectroscopy supports the electrical characterization, showing enhancement in crystallinity to explain the restoration of the breakdown knee followed by improvement in forward current by ∼85%.

Deep levels near the surface of 4H-SiC after dry oxidation were investigated. A large and broad peak appeared in the low-temperature range of deep level transient spectroscopy (DLTS) spectra after oxidation of SiC at 1300^oC, indicating multiple deep levels energetically located near the conduction band edge are generated inside SiC by thermal oxidation. Analyses of the DLTS spectra acquired with changing the bias voltage revealed that the majority of deep levels is located very near the SiC surface, within about 6 nm-deep region from the surface. The area density of the observed deep levels is higher than 3×10¹² cm^-2.

LiNbO3 (LNO) is currently intensively studied as an important ferroelectric material. In this work, polycrystalline LNO films were prepared through a sputtering technique, and their ferroelectricity-related resistive switching property was investigated using a device structure of PtSi/SiO2/LNO/Pt. The device exhibits a volatile resistance switching property at lower positive sweeping voltages and a stable bipolar nonvolatile switching property at higher sweeping voltages. The resistive switching mechanism of the device is discussed based on the domain wall conductivity characteristics of the polycrystalline LNO thin films. The PtSi/SiO2/LNO/Pt memristor device has potential applications in memory and artificial neural synapses.

We report a monolithic heterodyne receiver by using the AlGaN/GaN nonlinear transmission line as a local oscillator and a mixer simultaneously. This heterodyne receiver has a high radio frequency bandwidth from 80 to 360 GHz and a high intermediate frequency bandwidth of 18 GHz. These results indicate that the nonlinear transmission line receiver has promising potential in broadband spectrum analysis.

Physical reservoir computing (PRC) is useful for edge computing, although the challenge is to improve computational performance. In this study, we developed an inverted input method, the inverted input is additionally applied to a physical reservoir together with the original input, to improve the performance of the ion-gating reservoir. The error in the second-order nonlinear equation task was 7.3 × 10⁻⁵, the lowest error in reported PRC to date. Improvement of high dimensionality by the method was confirmed to be the origin of the performance enhancement. This inverted input method is versatile enough to enhance the performance of any other PRC.

We study the terahertz (THz) magnetic field pulse enhanced by a spiral-shaped antenna resonator (SAR). We deposit the SAR on the surface of a terbium-gallium-garnet crystal, which has a large Verdet constant, and measure the Faraday rotation angle for strong THz pulse excitation by magneto-optical sampling (MOS) with NIR light. The determined magnetic field strength and field-enhancement spectrum are consistent with the theoretical predictions. This first report of the detection of a Tesla-class picosecond magnetic field pulse by MOS is expected to be useful in research on the control of magnetization in spintronic devices.

MEMS resonant sensing devices require both HF (f) and low dissipation or high quality factor (Q) to ensure high sensitivity and high speed. In this study, we investigate the resonance properties and energy loss in the first three resonance modes, resulting in a significant increase in f‧Q product at higher orders. The third order resonance exhibits an approximately 15-fold increase in f‧Q product, while the Q factor remains nearly constant. Consequently, we achieved an ultrahigh f‧Q product exceeding 10¹² Hz by higher-order resonances in single-crystal diamond cantilevers.

Ce:Li₆Y(BO₃)₃ (LYBO) is a well-known candidate for thermal neutron detection with a very high Li concentration (3.06 × 10²²/cm³). So far, as-grown crystals exhibit a milky appearance that compromises their performance as scintillators. Current work demonstrates, for the first time, the growth of scattering-free undoped and Ce-doped LYBO by a thermal quenching process. The origin and features of the scattering centers are investigated in detail. Furthermore, the annealing treatment for the scintillation activation is studied, finding that the reduction in oxygen vacancies is mandatory. Under thermal neutron irradiation, the annealed scattering-free Ce:LYBO single crystal achieves a record-high light yield of 6200 ph/n in a single decay with a lifetime as short as 24 ns.

Thermal healing of focused ion beam-implanted defects in GaN is investigated by off-axis electron holography in TEM. The data reveal that healing starts at temperatures as low as about 250 °C. The healing processes result in an irreversible transition from defect-induced Fermi level pinning near the VB toward a midgap pinning induced by the crystalline-amorphous transition interface. Based on the measured pinning levels and the defect charge states, we identify the dominant defect type to be substitutional carbon on nitrogen sites.

The Si-nano dot substrates formed using the ultrathin silicon oxide films were applied to fabricate CaSi₂ films. The CaSi₂ formed by this process was identified as the metastable phase 2H as the main component, and the 1H structure existed partially at the grains of the 2H phase. Although no experimental reports exist for the formation of 2H-CaSi₂ crystal, the Si-nano dot substrates are considered as the high-entropy substrate to form the metastable phases. We experimentally determined the lattice parameter of the 2H phase by the annular dark field–scanning transmission electron microscopy observations using the Si as an internal standard sample.