Table of contents

We present polychromatic ultraviolet (UV) spectra from electrically driven light-emitting diodes (LEDs) based on three-dimensional (3D) AlGaN quantum wells (QWs). The LED structure is fabricated on AlN composed of (0001) facets, $\{ 1\bar{1}01\}$ facets, and vicinal (0001) facets with bunched steps. Although subsequent n-AlGaN growth tends to eliminate step bunching, appropriate design of the growth procedures preserves the 3D structure of AlN even in the LED structure. Because the QWs on the (0001) facets and bunched steps emit different colors, the fabricated LED exhibits polychromatic deep UV emission.

In this paper, we report on a vertical GaN trench MOS barrier Schottky (TMBS) rectifier for attaining low leakage current at high temperature and high reverse voltage. At 200 °C, a high blocking voltage of 750 V was achieved at a leakage current of 1 mA/cm². To the best of our knowledge, this blocking voltage is the highest ever reported for GaN Schottky rectifiers operating at such a high temperature. Furthermore, the fabricated TMBS rectifier operated at large forward currents up to 10 A. These results verify that the developed vertical GaN TMBS rectifiers have great potential as high-power and high-temperature devices.

We report a combination of highly doped layers and polarization engineering that achieves highly efficient blue-transparent GaN/InGaN/GaN tunnel junctions (In content = 12%). NPN diode structures with a low voltage drop of 4.04 V at 5 kA/cm² and a differential resistance of 6.51 × 10⁻⁵ Ω·cm² at 3 kA/cm² were obtained. The tunnel junction design with n⁺⁺-GaN (Si: 5 × 10²⁰ cm⁻³)/3 nm p⁺⁺-In_0.12Ga_0.88N (Mg: 1.5 × 10²⁰ cm⁻³)/p⁺⁺-GaN (Mg: 5 × 10²⁰ cm⁻³) showed the best device performance. Device simulations agree well with the experimentally determined optimal design. The combination of low In composition and high doping can facilitate lower tunneling resistance for blue-transparent light-emitting diodes.

Lateral GaN MOSFETs on homoepitaxial p-GaN layers with different Mg doping concentrations ([Mg]) have been evaluated to investigate the impact of [Mg] on MOS channel properties. It is demonstrated that the threshold voltage (V_th) can be controlled by [Mg] along with the theoretical curve. The field effect mobility also shows [Mg] dependence and a maximum field effect mobility of 123 cm² V⁻¹ s⁻¹ is achieved on [Mg] = 6.5 × 10¹⁶ cm⁻³ layer with V_th = 3.0 V. The obtained results indicate that GaN MOSFETs can be designed on the basis of the doping concentration of the p-GaN layer with promising characteristics for the realization of power MOSFETs.

Al-rich III–nitride-based deep-ultraviolet (UV) (275–320 nm) light-emitting diodes are plagued with a low emission efficiency and high turn-on voltages. We report Al-rich (Al,Ga)N metal–insulator–semiconductor UV light-emitting Schottky diodes with low turn-on voltages of <3 V, which are about half those of typical (Al,Ga)N p–i–n diodes. Our devices use a thin AlN film as the insulator and an n-type Al_0.58Ga_0.42N film as the semiconductor. To improve the efficiency, we inserted a GaN quantum-well structure between the AlN insulator and the n-type Al_xGa_1−xN semiconductor. The benefits of the quantum-well structure include the potential to tune the emission wavelength and the capability to confine carriers for more efficient radiative recombination.

Improved turn-on voltages and reduced series resistances were realized by depositing highly Si-doped n-type GaN using molecular beam epitaxy on polarization-enhanced p-type InGaN contact layers grown using metal–organic chemical vapor deposition. We compared the effects of different Si doping concentrations and the addition of p-type InGaN on the forward voltages of p–n diodes and light-emitting diodes, and found that increasing the Si concentrations from 1.9 × 10²⁰ to 4.6 × 10²⁰ cm⁻³ and including a highly doped p-type InGaN at the junction both contributed to reductions in the depletion width, the series resistance of 4.2 × 10⁻³–3.4 × 10⁻³ Ω·cm², and the turn-on voltages of the diodes.

In this Letter, we report on the design optimization of metamorphic InSb/InAs/In(Ga,Al)As/GaAs heterostructures with type-II-in-type-I quantum well (QW) active regions, aimed at the enhancement of their room-temperature photoluminescence (PL). The strong influence of the design of the convex-graded metamorphic buffer layer (MBL) and the value of the MBL inverse step in the range from 2 to 14 mol % In on stresses in such heterostructures, as well as their PL intensity, are discussed. The optimized metamorphic In(Sb,As)/In_0.63Ga_0.37As/In_0.75Al_0.25As/MBL/GaAs structure with the inverse step of 10 mol % demonstrates 3.2–3.5 µm mid-IR PL intensity quenching from liquid-nitrogen to room temperature by a factor of 12.

Carrier recombination and transport processes play key roles in determining optoelectronic performance characteristics such as the efficiency droop and forward voltage in InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs). In this work, we investigate the dominant carrier transport and recombination processes inside and outside the MQW region as functions of injection current from the viewpoint of carrier-energy-loss mechanisms. It is experimentally shown that carrier accumulation and subsequent spill-over at MQW active layers due to the insufficient carrier recombination rate, mainly the radiative recombination rate, explain the dependences of both the efficiency droop and the forward voltage on the injection current.

A GaN/AlGaN-based ultraviolet light-emitting diode (LED) structure with an embedded porous-AlGaN reflector was fabricated by a doping-selective electrochemical (EC) wet-etching process. The n⁺-AlGaN/undoped-AlGaN (u-AlGaN) stack structures with different Al contents were transformed into porous-AlGaN/u-AlGaN stack structures that acted as the embedded distributed Bragg reflectors (DBRs). The porosity of the EC-treated AlGaN layer was increased by decreasing the Al content in the n⁺-AlGaN layer. The reflectivity of the porous-AlGaN DBR structure was measured to be 90% at 379.3 nm with a 37.2 nm stopband width. The photoluminescence emission intensity of the DBR-LED was enhanced by forming the embedded porous-AlGaN DBR structure.

In this work, a switchable plasmonic structure is proposed for reflection-type spatial light modulation in the visible range with subwavelength resolution. This structure is based on a metallic grating in which each resonant cavity couples the incident light into a gap surface plasmon mode and then reflects the light modulated in the cavity. By incorporating an ultrathin layer of the phase-change material Ge₂Sb₂Te₅ at the entrance of the cavity, the optical modulation characteristic of the structure can be switched between two modes. Numerical investigations are conducted to verify the proposed structure, with the focused analysis of two common types of binary modulations: amplitude-only and phase-only modulations.

A waveguide-coupled inverted nested ring resonator structure is proposed and theoretically investigated. The effects of the parameters of such a structure on its transmission properties are demonstrated. Results show that both Fano resonance and coupled-resonator-induced transparency (CRIT) spectra can be produced within this structure, depending on the length ratio of the feedback waveguide and ring. In addition, the slope of the Fano resonance can be tuned and reversed by varying coupling coefficients and effective refractive index. Besides, CRIT-based tunable group delay can be obtained with high transmittance. Therefore, this structure has potential applications in optical sensors and optical communication.

A grating coupler with a trapezoidal hole array was designed and fabricated for perfectly vertical light coupling between a single-mode optical fiber and a silicon waveguide on a silicon-on-insulator (SOI) substrate. The grating coupler with an efficiency of 53% was computationally designed at a 1.1-µm-thick buried oxide (BOX) layer. The grating coupler and silicon waveguide were fabricated on the SOI substrate with a 3.0-µm-thick BOX layer by a single full-etch process. The measured coupling efficiency was 24% for TE-polarized light at 1528 nm wavelength, which was 0.69 times of the calculated coupling efficiency for the 3.0-µm-thick BOX layer.

We report the generation of a carrier-envelope-phase-stable femtosecond pulse at 10 µm with 800 nJ by the down-conversion of a 25-fs-long 800 nm pulse with 1.2 mJ obtained from a standard Ti:sapphire regenerative amplifier. We discuss a simple optical setup for the spectral broadening of the 800 nm pulse by filamentation in air and the intrapulse difference frequency generation in a wide-bandgap nonlinear optical crystal, LiGaS₂. From the measurement of the electric-field waveform of the generated mid-infrared pulse by electrooptic sampling, the maximum electric-field amplitude is estimated to be approximately 11 MV/cm at a beam diameter of 70 µm.

We demonstrate an Er-doped fiber laser (EDFL) mode-locked by a MoS₂ saturable absorber (SA), delivering a 256 fs, 2 nJ soliton pulse at 1563.4 nm. The nonlinear property of the SA prepared by magnetron sputtering deposition (MSD) is measured with a modulation depth (MD) of ∼19.48% and a saturable intensity of 4.14 MW/cm². To the best of our knowledge, the generated soliton pulse has the highest pulse energy of 2 nJ among the reported mode-locked EDFLs based on transition metal dichalcogenides (TMDs). Our results indicate that MSD-grown SAs could offer an exciting platform for high pulse energy and ultrashort pulse generation.

Using synchrotron X-ray powder diffraction, we investigate the charge-density distributions of the layered oxypnictides (LaO)MnAs, (LaO)FeAs, (LaO)NiAs, and (LaO)ZnAs, which are an antiferromagnetic semiconductor, a parent material of an iron-based superconductor, a low-temperature superconductor, and a non-magnetic semiconductor, respectively. For the metallic samples, clear charge densities are observed in both the transition-metal pnictide layers and the rare-earth-oxide layers. However, in the semiconducting samples, there is no finite charge density between the transition-metal element and As. These differences in charge density reflect differences in physical properties. First-principles calculations using density functional theory reproduce the experimental results reasonably well.

New half-metallic magnetic materials and ferromagnetic spin gapless semiconductors have been designed by substituting the transition metal atoms for Pb atoms in methylammonium lead iodide, MAPbI₃ (MA = CH₃NH₃⁺). MAScI₃, MATiI₃, MACrI₃, MAFeI₃, and MANiI₃ are predicted to be half-metallic ferromagnets, MAVI₃ and MAMnI₃ are ferromagnetic semiconductors, and MACoI₃ is a spin gapless semiconductor. The physical properties of methylammonium transition metal triiodides may vary with strain. The high-spin polarization makes the methylammonium transition metal triiodides promising candidates for spintronics applications.

We investigate the effect of tensile strain on spin splitting at the n-type interface of LaAlO₃/SrTiO₃ in terms of spin–orbit coupling coefficient α and spin texture in the momentum space using first-principles density-functional calculations. Our results show that α could be controlled by the tensile strain, and can be enhanced by up to 5 times for a tensile strain of 7%, and the effect of the tensile strain leads to a persistent spin helix, which has a long spin lifetime. The strain effect on the LaAlO₃/SrTiO₃ interface is important for various applications such as spin field-effect transistors and spin-to-charge conversion.

We demonstrate a reliable method for controlling the operation timing and data flow direction in a nanomagnet logic (NML) circuit with spatially uniform magnetic fields. We introduce a data and buffer nanomagnet, which have the same shape, but their magnetic easy axes are at an angle of 45° to each other. We also introduce a ferromagnetically and an antiferromagnetically coupled arrangement of the nanomagnets. Using this method, we perform bit shift operations on 3-bit NML shift registers. This method provides a means of fabricating highly integrated NML circuits with a simplified structure.

We demonstrate that the Faraday effect can be electrically and reversibly switched on and off at room temperature without changing the external magnetic field. A Co ultrathin film deposited on a Pt underlayer was used for the demonstration. An electric field was applied to the surface of the Co film by forming an electric double layer using an ionic liquid. The switching of the Faraday effect was caused by the electric-field-induced ferromagnetic phase transition in the system. The magnitude of the electrical change in the Faraday rotation angle and the ellipticity are also discussed.

We developed depletion-mode vertical Ga₂O₃ trench metal–oxide–semiconductor field-effect transistors by using n⁺ contact and n⁻ drift layers. These epilayers were grown on an n⁺ (001) Ga₂O₃ single-crystal substrate by halide vapor phase epitaxy. Cu and HfO₂ were used for the gate metal and dielectric film, respectively. The mesa width and gate length were approximately 2 and 1 µm, respectively. The devices showed good DC characteristics, with a specific on-resistance of 3.7 mΩ cm² and clear current modulation. An on–off ratio of approximately 10³ was obtained.

Recently, virtual domain walls (VDWs) contained within a narrow gap among discrete magnetic nanoelements without real internal spin structures and thus with a significantly suppressed stochastic nature of domain wall propagation have attracted considerable attention. In this work, the ratchet effect of VDW motion in asymmetrically shaped nanodot chains under alternating magnetic fields was numerically investigated via micromagnetic simulations. The results show that the asymmetric stray field distribution in the gap and the magnetic shape anisotropy of the nanodots are the primary causes of the VDW ratchet effect. The ratchet behavior is found to be controllable via modulation of the frequency and strength of the alternating magnetic field, as well as by modifying the geometric shape of the asymmetric nanodots. A unilateral bandpass filter based on the VDW ratchet effect is proposed.

Vibrational energy transfer from photoexcited single-wall carbon nanotubes (SWCNTs) to coupled proteins is a key to engineering thermally induced biological reactions, for example, in photothermal therapy. Here, we explored vibrational energy transfer from photoexcited SWCNTs to different adsorbed biological materials by means of a femtosecond pump–probe technique. We show that the vibrational relaxation time of the radial breathing modes in SWCNTs depends significantly on the structure of the coupled materials, that is, proteins or biopolymers, indicating that the vibrational energy transfer is governed by overlapping of the phonon densities of states of the SWCNTs and coupled materials.

Remote N₂ plasma treatment is explored as a surface functionalization technique to deposit ultrathin high-k dielectric on single-layer MoS₂. The ultrathin dielectric is used as a tunneling contact layer, which also serves as an interfacial layer below the gate region for fabricating top-gate MoS₂ metal–oxide–semiconductor field-effect transistors (MOSFETs). The fabricated devices exhibited small hysteresis and mobility as high as 14 cm²·V⁻¹·s⁻¹. The contact resistance was significantly reduced, which resulted in the increase of drain current from 20 to 56 µA/µm. The contact resistance reduction can be attributed to the alleviated metal–MoS₂ interface reaction and the preserved conductivity of MoS₂ below the source/drain metal contact.

We use density functional theory to calculate the electronic structure of monolayer and bilayer InSe nanosheets. The interlayer interaction is found to have a large effect on the s orbital distribution of In and Se atoms. The electronic properties of InSe change substantially under in-plane bi-axial strain, including the semiconductor-to-metal transition. Both van der Waals forces and the electron wave function overlap affect the electronic structure tunability in a delicate way. Aside from the band-nature change, the electron-transport ability is expected to be altered, which is important for InSe-based electronic devices.

Metasurfaces consisting of arrays of high-index Mie resonators concentrating/redirecting light are important for integrated optics, photodetectors, and solar cells. Herein, we report the optical properties of low-Ge-content SiGe lens-like Mie resonator island arrays fabricated via dewetting during Ge deposition on a Si(100) surface at approximately 900 °C. We observe enhancement of the Si interaction with light owing to the efficient island-induced light concentration in the submicron-depth Si layer, which is mediated by both near-field Mie resonance leaking into the substrate and far-field light focusing. Such metasurfaces can improve the Si photodetector and solar-cell performance.

AlGaN/GaN high-electron-mobility transistor (HEMT) device layers were grown by metal organic chemical vapor deposition (MOCVD) on commercial engineered QST™ substrates to demonstrate a path to scalable, cost-effective foundry processing while supporting the thick epitaxial layers required for power HEMT structures. HEMT structures on 150 mm Si substrates were also evaluated. The HEMTs on engineered substrates exhibited material quality, DC performance, and forward blocking performance superior to those of the HEMT on Si. GaN device layers up to 15 µm were demonstrated with a wafer bow of 1 µm, representing the thickest films grown on 150-mm-diameter substrates with low bow to date.