Table of contents

The source of carrier compensation in metalorganic vapor phase epitaxy (MOVPE)-grown n-type GaN was quantitatively investigated by Hall-effect measurement, deep-level transient spectroscopy, and secondary ion mass spectrometry. These analysis techniques revealed that there were at least three different compensation sources. The carrier compensation for samples with donor concentrations below 5 × 10¹⁶ cm⁻³ can be explained by residual carbon and electron trap E3 (E_C − 0.6 eV). For samples with higher donor concentrations, we found a proportional relationship between donor concentration and compensating acceptor concentration, which resulted from a third source of compensation. This is possibly due to the self-compensation effect.

Al₂O₃/n-GaN MOS-capacitors grown by metalorganic chemical vapor deposition with in-situ- and ex-situ-formed Al₂O₃/GaN interfaces were characterized. Capacitors grown entirely in situ exhibited ∼4 × 10¹² cm⁻² fewer positive fixed charges and up to ∼1 × 10¹³ cm⁻² eV⁻¹ lower interface-state density near the band-edge than did capacitors with ex situ oxides. When in situ Al₂O₃/GaN interfaces were reformed via the insertion of a 10-nm-thick GaN layer, devices exhibited behavior between the in situ and ex situ limits. These results illustrate the extent to which an in-situ-formed dielectric/GaN gate stack improves the interface quality and breakdown performance.

Longitudinal piezoelectric constant (e₃₃) values of wurtzite materials, which are listed in a structure database, are calculated and analyzed by using first-principles and statistical learning methods. It is theoretically shown that wurtzite materials with high e₃₃ generally have small lattice constant ratios (c/a) almost independent of constituent elements, and approximately expressed as e₃₃ ∝ c/a − (c/a)₀ with ideal lattice constant ratio (c/a)₀. This relation also holds for highly-piezoelectric ternary materials such as Sc_xAl_1−xN. We conducted a search for high-piezoelectric wurtzite materials by identifying materials with smaller c/a values. It is proposed that the piezoelectricity of ZnO can be significantly enhanced by substitutions of Zn with Ca.

We demonstrate that a broad emission band is observable at room temperature in the vicinity of the carbon-related C-line detected at cryogenic temperatures in electron-irradiated Si. Its spectral shape is different from similar shapes of the bands due to dislocations, oxygen precipitates, and thermal donors. The band is annealed out at 450 °C and its intensity ratio to the band-edge emission has a positive correlation with carbon concentration in the same manner as the C-line. We deduce that the band has a very similar origin to the C-line and discuss the possibility of carbon quantification by using the ratio as an index.

Mo-doped Sb_1.8Te materials and electrical devices were investigated for high-thermal-stability and high-speed phase-change memory applications. The crystallization temperature (t_c = 185 °C) and 10-year data retention (t_10-year = 112 °C) were greatly enhanced compared with those of Ge₂Sb₂Te₅ (t_c = 150 °C, t_10-year = 85 °C) and pure Sb_1.8Te (t_c = 166 °C, t_10-year = 74 °C). X-ray diffraction and transmission electron microscopy results show that the Mo dopant suppresses crystallization, reducing the crystalline grain size. Mo_2.0(Sb_1.8Te)_98.0-based devices were fabricated to evaluate the reversible phase transition properties. SET/RESET with a large operation window can be realized using a 10 ns pulse, which is considerably better than that required for Ge₂Sb₂Te₅ (∼50 ns). Furthermore, ∼1 × 10⁶ switching cycles were achieved.

A normal-incidence quantum cascade detector coupled by a nanopore array structure (NPS) is demonstrated. The NPS is fabricated on top of an In_0.53Ga_0.47As contact layer by inductively coupled plasma etching using anodic aluminum oxide as a mask. Because of the nonuniform volume fraction at different areas of the device mesa, the NPS acts as subwavelength random gratings. Normal-incidence light can be scattered into random oblique directions for inter-sub-band transition absorption. With normal incidence, the responsivities of the device reach 24 mA/W at 77 K and 15.7 mA/W at 300 K, which are enhanced 2.23 and 1.96 times, respectively, compared with that of the 45°-edge device.

In this work, we realize tunneling propagation through spoof surface plasmon polariton transmission lines loaded with magnetoinductive metamaterial channels above a high cutoff frequency. Magnetoinductive metamaterial channels consist of split-ring resonators, and two different structures are proposed. Samples are fabricated, and both measurements and simulations indicate a near-perfect tunneling propagation around 17 GHz. The proposed methodology could be exploited as a powerful platform for investigating tunneling surface plasmons from radio frequencies to optical frequencies.

We propose a grating-coupled graphene metamaterial structure at a terahertz band. An obvious tunable plasmon-induced transparency can be achieved by changing the graphene Fermi level, and there is a very high absorption peak in the spectrum. Thus, this structure can be used to realize a terahertz modulator or absorption device with two outstanding functions. Using phase changes, we have also studied the slow light effect, and we find that there is a broad ultrahigh-group-index band, and the maximum group index can reach 216. This research may open up new avenues for slow light devices.

Tunable multiband polarization conversion and manipulation are achieved by introducing vanadium dioxide (VO₂) into a planar spiral asymmetric chiral metamaterial. Numerical simulations demonstrate that when VO₂ is in the insulating state, circularly polarized electromagnetic waves are emitted at two distinct resonant frequencies. When VO₂ is in the metallic state, the number of resonant frequencies changes from two to four. In addition, the initial left-handed and right-handed circularly polarized transmitted waves correspondingly transform into right and left ones. Moreover, the surface current distributions are studied in order to investigate the transformation behaviors of both the insulating and metallic states.

A high-performance transverse-magnetic (TM) mode-pass polarizer for mid-infrared wavelengths based on a silicon nitride–silicon subwavelength grating (SWG) waveguide is proposed. The designed device supports the Bloch mode for the TM mode only, whereas the transverse-electric mode is reflected by the SWG. Simulations show that with a device length of 7 µm, this polarizer is capable of achieving an ultrahigh polarization extinction ratio (PER) of ∼50 dB and an insertion loss of ∼0.3 dB at a wavelength of 2 µm. The device bandwidth is increased to 60 nm for PER > 30 dB. The polarizer also shows good tolerance to fabrication uncertainties.

We report the fabrication of low-droop high-efficiency green c-plane light-emitting diodes (LEDs) utilizing GaN tunnel junction (TJ) contacts. The LED epitaxial layers with a top p-GaN layer were grown by metal organic chemical vapor deposition and an n⁺⁺-GaN layer was deposited by molecular beam epitaxy to form a TJ. The TJ LEDs were then compared with equivalent LEDs having a tin-doped indium oxide (ITO) contact. The TJ LEDs exhibited a higher performance and a lower efficiency droop than did the ITO LEDs. At 35 A/cm², the external quantum efficiencies for the TJ and ITO LEDs were 31.2 and 27%, respectively.

We demonstrate a novel torsion sensor based on a twisted photonic crystal fiber with an embedded liquid rod waveguide. Only one resonant dip appears in the transmission spectrum in the range of 1,350–1,650 nm, which is associated with the directional coupling between the core and rod modes. The relative position of the fiber core with respect to the liquid rod waveguide is altered owing to the torsion stress, which changes the phase-matching condition and leads to a resonant-wavelength shift. The helical structure of the liquid rod waveguide can improve the torsion sensitivity of the device and provide an ability to distinguish the rotation direction. The measured torsion sensitivities were as high as ∼203 and ∼208 nm·mm·rad⁻¹ in the clockwise and counterclockwise rotations, respectively.

We demonstrate an ultrabright narrow-band two-photon source at the 1.5 µm telecom wavelength for long-distance quantum communication. By utilizing a bow-tie cavity, we obtain a cavity enhancement factor of 4.06 × 10⁴. Our measurement of the second-order correlation function G⁽²⁾(τ) reveals that the linewidth of 2.4 MHz has been hitherto unachieved in the 1.5 µm telecom band. This two-photon source is useful for obtaining a high absorption probability close to unity by quantum memories set inside quantum repeater nodes. Furthermore, to the best of our knowledge, the observed spectral brightness of 3.94 × 10⁵ pairs/(s·MHz·mW) is also the highest reported over all wavelengths.

We investigate free-layer size D dependence of effective anisotropy field in nanoscale CoFeB/MgO magnetic tunnel junctions by homodyne-detected ferromagnetic resonance. The effective anisotropy field $H_{K}^{\text{eff}}$ monotonically increases with decreasing D for a device with the reference-layer size much larger than the free-layer size. In contrast, $H_{K}^{\text{eff}}$ does not increase in a monotonic manner for a device with the reference-layer size comparable to the free-layer size. We reveal that the difference can be explained by the variation of the anisotropy field in the vicinity of the device edge.

We report on electron spin relaxation measurements of Er³⁺ dopants in a Y₂SiO₅ crystal using an electron paramagnetic resonance spectrometer based on a Josephson bifurcation amplifier. We observed the change in the induced flux as a function of time for two spin transitions (at different crystallographic sites) after an excitation microwave pulse or a change in the static magnetic field. Low-microwave-power measurements showed relaxation times of approximately 10 h at 20 mK, and 1/T₁ followed a T² dependence between 30 and 200 mK. We observed no difference in behavior between the two transitions. The microwave power and temperature dependences suggest that a phonon-bottleneck-like process limits relaxation.

Here, we present an analytic formula for the domain-wall depinning current from artificial triangular notches driven by the spin–orbit torque combined with the Dzyaloshinskii–Moriya interaction. Interestingly, in contrast to the magnetic-field-driven depinning, the depinning current is governed solely by the notch slope angle, irrespective of the notch depth and wire width. An analytic formula is proposed to explain the present observation on the basis of the variational principle for minimum energy states. The validity of the formula is verified via micromagnetic simulation, confirming the detailed effects of the spin–orbit torque and Dzyaloshinskii–Moriya interaction strengths.

The large-signal modulation characteristics of a GaN-based micro-LED have been studied for Gbps visible-light communication. With an increasing signal modulation depth the modulation bandwidth decreases, which matches up with the increase in the sum of the signal rise time and fall time. By simulating the band diagram and the carrier recombination rate of the micro-LED under large-signal modulation, carrier recombination and the carrier sweep-out effect are analyzed and found to be the dominant mechanisms behind the variation of modulation bandwidth. These results give further insight into improving the modulation bandwidth for high-speed visible-light communication.

We investigated the effect of post-annealing on the doping of graphene with MoO₃ in this study. The as-deposited molybdenum oxide thin film prepared using our method was not completely oxidized; in addition, it was in an amorphous state, due to which its doping effect was not significant. As the post-deposition annealing temperature was increased, the oxidation and crystallization of the molybdenum oxide progressed and the doping effect increased accordingly. After annealing at 350 °C, the holes were the most doped and the sheet resistance was the lowest. The doped graphene film obtained in this study shows higher doping effect and stability compared to other dopants.

We demonstrate the creation of single-atom-sharpened tungsten tips via nanosecond pulse application under tensile stresses. The formation process and resultant tip structures were observed in situ at the atomic resolution by high-resolution transmission electron microscopy. It was found that the single-atom-sharpened tips have high stability sufficient for the dramatic turnaround in various fine tip techniques.

Owing to their phonon scattering and interfacial thermal resistance (ITR) characteristics, inorganic multilayers (MLs) have attracted considerable attention for thermal barrier applications. In this study, a-Si/a-Ge MLs with layer thicknesses ranging from 0.3 to 5 nm and different interfacial elemental mixture states were fabricated using a combinatorial sputter-coating system, and their thermal conductivities were measured via a frequency-domain thermo-reflectance method. An ultra-low thermal conductivity of κ = 0.29 ± 0.01 W K⁻¹ m⁻¹ was achieved for a layer thickness of 0.8 nm. The ITR was found to decrease from 8.5 × 10⁻⁹ to 3.6 × 10⁻⁹ m² K W⁻¹ when the interfacial density increases from 0.15 to 0.77 nm⁻¹.

Nitrogen-vacancy (NV) centers in diamonds are expected for high-performance quantum sensing devices. The NV centers in heteroepitaxial diamond films on Si substrates have more potential to enable low-cost and large-area sensors than typical single-crystal diamond substrates and to support the emergence of diamond/Si hybrid devices. In this paper, NV centers were formed in (111) heteroepitaxial diamond films on Si substrates with preferential atomic alignment in the [111] direction. In addition, incorporation of silicon-vacancy centers, which decrease the sensitivity of sensors, from the 3C-SiC/Si substrate was effectively suppressed using oxygen gas in the growth environment.

In vertical GaN PN diodes (PNDs) grown entirely by metal–organic chemical vapor deposition (MOCVD), large current nonuniformity was observed. This nonuniformity was induced by macrosteps on the GaN surface through modulation of carbon incorporation into the n-GaN crystal. It was eliminated in a hybrid PND consisting of a carbon-free n-GaN layer grown by hydride vapor phase epitaxy (HVPE) and an MOCVD-regrown p-GaN layer. The hybrid PND showed a fairly low on-resistance (2 mΩ cm²) and high breakdown voltage (2 kV) even without a field plate electrode. These results clearly indicated the strong advantages of the HVPE-grown drift layer for improving power device performance, uniformity, and yield.

The configuration of the interfacial misfit array at In_xGa_1−xSb/GaAs interfaces with different indium compositions and thicknesses grown by metalorganic chemical vapor deposition was systematically analyzed using X-ray diffraction (XRD) reciprocal space maps (RSMs). These analyses confirmed that the epilayer relaxation was mainly contributed to by the high degree of spatial correlation of the 90° misfit array (correlation factors <0.01). The anisotropic peak-broadening aspect ratio was found to have a non-linear composition dependence as well as be thickness-dependent, related to the strain relaxation of the epilayer. However, the peak-broadening behavior in each RSM scan direction had different composition and thickness dependences.

A gel state exists in the solution–solid conversion process. We found that solidification can be promoted by irradiating the gel with ultraviolet (UV) light. In this study, a patterning method without using a vacuum system or employing photoresist materials has been proposed wherein solidification was applied to a gel by UV irradiation. Indium oxide gel, indium gallium oxide gel, lanthanum zirconium oxide gel, and lanthanum ruthenium oxide gels were successfully patterned by using our technique. Moreover, an oxide thin-film transistor was fabricated by our novel patterning method and was successfully operated.

We propose a new asynchronous measurement system to visualize the amplitude and phase distribution of a frequency-modulated electromagnetic wave. The system consists of three parts: a nonpolarimetric electro-optic frequency down-conversion part, a phase-noise-canceling part, and a frequency-tracking part. The photonic local oscillator signal generated by electro-optic phase modulation is controlled to track the frequency of the radio frequency (RF) signal to significantly enhance the measurable RF bandwidth. We demonstrate amplitude and phase measurement of a quasi-millimeter-wave frequency-modulated continuous-wave signal (24 GHz ± 80 MHz with a 2.5 ms period) as a proof-of-concept experiment.

The physical mechanism of metal ablation induced by femtosecond laser irradiation was investigated in this study. Calculations based on finite-temperature density functional theory indicate that condensed copper becomes unstable at high electron temperatures due to an electronic entropy effect. Based on these results, an electronic entropy-driven mechanism is proposed to explain the metal ablation. Furthermore, a mathematical model is developed to simulate the ablation depth, where the effect of the electronic entropy is included. This mathematical model can quantitatively describe the experimental data in the low-laser-fluence region, where the electronic entropy effect is determined to be especially important.

Resistive pulse sensing (RPS) is an interesting analytical system in which micro- to nanosized pores are used to evaluate particles or small analytes. Recently, molecular immobilization techniques to improve the performance of RPS have been reported. The problem in functionalization for RPS is that molecular immobilization by chemical reaction is restricted by the pore material type. Herein, a simple functionalization is performed using mussel-inspired polydopamine as an intermediate layer to connect the pore material with functional molecules.