Table of contents

We report on the observation of ambipolar transport characteristics of InN:Mg nanowires at room temperature, providing unambiguous evidence that the Fermi level is fundamentally unpinned on the grown surfaces of InN. Such behavior, however, was not observed in Si-doped InN nanowires, which is explained by Si-dopant surface segregation and the resulting accumulation of electrons. Furthermore, this suggests that defects and incorporation of impurities on the grown surfaces of InN are the primary causes of the commonly measured accumulation of electrons and Fermi-level pinning.

In_xGa_1−xN quantum wells (QWs) grown by metalorganic chemical vapor deposition on bulk m-plane GaN substrates with surface miscut −1° toward [0001] ("−1° c-miscut") currently suffer from low indium uptake and broad luminescence linewidth in the blue spectrum. New surface "double miscuts" with miscut components in the combined a- and c-directions exhibit more uniform step flow growth in homoepitaxy than coloaded −1° c-miscut substrates. QWs grown on double miscut surfaces exhibit reproducibly higher indium uptake, narrower and more symmetric electroluminescence lineshapes in the blue spectrum, and higher output power at the peak wavelength than those grown on −1° c-miscut surfaces.

Thick Si-doped AlN layers were homoepitaxially grown by hydride vapor phase epitaxy on AlN(0001) seed substrates. Following the removal of the seed substrate, an n-type AlN substrate with a carrier concentration of 2.4 × 10¹⁴ cm⁻³ was obtained. Vertical Schottky barrier diodes were fabricated by depositing Ni/Au Schottky contacts on the N-polar surface of the substrate. High rectification with a turn-on voltage of approximately 2.2 V was observed. The ideality factor of the diode at room temperature was estimated to be ∼8. The reverse breakdown voltage, defined as the leakage current level of 10⁻³ A/cm², ranged from 550 to 770 V.

Selective layer disordering in an intersubband Al_0.028Ga_0.972N/AlN superlattice using a silicon nitride (SiN_x) capping layer is demonstrated. The SiN_x capped superlattice exhibits suppressed layer disordering under high-temperature annealing. Additionally, the rate of layer disordering is reduced with increased SiN_x thickness. The layer disordering is caused by Si diffusion, and the SiN_x layer inhibits vacancy formation at the crystal surface and ultimately, the movement of Al and Ga atoms across the heterointerfaces. Patterning of the SiN_x layer results in selective layer disordering, an attractive method to integrate active and passive III–nitride-based intersubband devices.

N-polar $(000\bar{1})$ (−c-plane) InGaN light-emitting diodes with emission wavelengths ranging from blue to green to red were fabricated on a c-plane sapphire substrate by metalorganic vapor phase epitaxy. The optimization of growth conditions for −c-plane InGaN/GaN multiple quantum wells was performed. As a result, the extension of the emission wavelength from 444 to 633 nm under a constant current of 20 mA was achieved by changing the growth temperature of quantum wells from 880 to 790 °C.

Hydrothermal-grown bulk ZnO single crystals are investigated before and after gamma-ray irradiation. The irradiation does not alter the optical transparency in the visible region. The gamma rays only induce modified near-band-edge UV emission with blue-shifted peaks and shortened response times. From the initial values before irradiation, the peaks shift by 5 to 6 nm, and the response times shorten by 140 to 440 ps. We attribute these observations to the radiation-induced defects on the bulk crystals. Our results nevertheless lead to the realization of short-wavelength ZnO scintillators that can be utilized in high-energy-radiation environments.

CdSe/CdS core–shell quantum dots (QDs) were synthesized using a facile method in aqueous phase. X-ray diffraction pattern, high-resolution transmission electron microscopy images, and energy dispersive spectroscopy profiles showed that stoichiometric CdSe/CdS QDs were formed. Temperature-dependent photoluminescence spectra showed that the activation energy of CdSe/CdS core–shell QDs was 15 meV. The potential profiles and interband transition energies of the strained type-II CdSe/CdS core–shell QDs were calculated. The calculated interband transition energies slightly decreased from 2.061 to 2.007 eV when the shell thickness increased from 10 to 17 Å. The theoretical interband transition energy of 2.007 eV was in reasonable agreement with the photoluminescence excitonic transition energy of 1.98 eV.

Gallium phosphide bismide (GaP_1−xBi_x) epilayers with x up to 1.0% were grown via molecular beam epitaxy and their photoluminescence spectra were investigated at low temperatures. Surprisingly, the emission spectrum of the GaP_1−xBi_x epilayers was fully described by isolated bismuth-bound exciton recombination at the A and B lines (2.232 and 2.229 eV, respectively) together with their phonon replicas, without a need for any description of recombination from bismuth pair or cluster states. These observations contrast with the typical behavior of energy transfer to lower-lying nitrogen pair states in GaP_1−yN_y at similar impurity concentrations and offer insights into the electronic structure evolution of GaP_1−xBi_x.

We studied the impact of buffered HF (BHF) cleaning on the interface properties of Al₂O₃/InAs/GaSb metal–oxide–semiconductor (MOS) structures fabricated by the ex-situ surface cleaning process. The Al₂O₃/InAs/GaSb MOS structures fabricated with BHF cleaning exhibited lower D_it values than those fabricated with sulfur passivation. In addition, the Al₂O₃/InAs/GaSb MOS structures fabricated with BHF cleaning were robust with respect to the MOS field-effect transistor fabrication process by using W gate metal with PMA in the 250–300 °C range.

Polycrystalline GeSn thin films on Si substrates with a Sn composition up to 4.5% have been fabricated and characterized. The crystalline structure, surface morphology, and infrared (IR) absorption coefficient of the annealed GeSn thin films were carefully investigated. It was found that the GeSn thin films with a Sn composition of 4.5% annealed at 450 °C possessed a desirable polycrystalline structure according to X-ray diffraction (XRD) analyses and Raman spectroscopy analyses. In addition, the absorption coefficient of the polycrystalline GeSn thin films in the IR region was significantly better than that of the single crystalline bulk Ge.

This article reports for the first time the electrical properties of fabricated n-3C-SiC/p-Si heterojunction diodes under external mechanical stress in the [110] direction. An anisotype heterojunction diode of n-3C-SiC/p-Si was fabricated by depositing 3C-SiC onto the Si substrate by low-pressure chemical vapor deposition. The mechanical stress significantly affected the scaling current density of the heterojunction. The scaling current density increases with stress and is explained in terms of a band offset reduction at the SiC/Si interface under applied stress. A reduction in the barrier height across the junction owing to applied stress is also explained quantitatively.

We report on the temperature dependence of the charge transport and activation energy of amorphous silicon carbide (a-SiC) thin films grown on quartz by low-pressure chemical vapor deposition. The electrical conductivity as characterized by the Arrhenius rule was found to vary distinctly under two activation energy thresholds of 150 and 205 meV, corresponding to temperature ranges of 300 to 450 K and 450 to 580 K, respectively. The a-SiC/quartz system displayed a high temperature coefficient of resistance ranging from −4,000 to −16,000 ppm/K, demonstrating a strong feasibility of using this material for highly sensitive thermal sensing applications.

In this study, what occurs at the SiO₂/substrate interface and how Si atoms in SiO₂ behave in interfacial SiO₂ scavenging in a HfO₂ gate stack have been investigated. Since the same scavenging occurs on both Si and SiC substrates, the SiC substrate is used to study the role of the substrate. The characterizations of the SiO₂/SiC interface show no Si growth on the substrate and no consumption of the substrate in the SiO₂ scavenging. Isotope tracing experiments for a HfO₂/SiO₂/Si stack further demonstrate that atomic Si is generated in scavenging and diffuses out through the HfO₂ layer. On the basis of these findings, the reaction at the SiO₂/substrate interface is thermodynamically discussed.

We propose a series of new compositions of AlN-based piezoelectric material based on the results of first-principles calculation. The composition is expressed by $(\alpha _{y},\beta _{1 - y})_{x}$Al_1−xN, where the elements α and β are selected to maintain the charge neutrality of the host AlN. We found that the selection of Mg²⁺ for α and Hf⁴⁺ and Zr⁴⁺ for β with y = 0.5 show good chemical stability with much better piezoelectric properties than pure AlN. Our results indicate broad compositional freedom for improving the piezoelectric properties of AlN by using the co-doping technique.

We adjusted the conductivity of a poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT/PSS) electrode and an injection barrier between the PEDOT/PSS source/drain (S/D) electrode and a pentacene semiconductor by adding HAuCl₄ to a PEDOT/PSS solution. Gold in the PEDOT/PSS S/D electrode was synthesized by a redox reaction between PEDOT/PSS and Au ions. This reaction enhances the conductivity of the PEDOT/PSS S/D electrodes and reduces the injection barrier between the PEDOT/PSS S/D electrodes and the pentacene semiconductor, and causes the field-effect mobility to increase by about 230%. As such, it is considered a very useful method of making high-performance organic thin-film transistors (OTFTs).

We report a novel high-speed response component observed in the transient state in cholesteric liquid crystals (ChLCs) with a uniform lying helix (ULH). When an electric field is applied normal to the helix axis, two well-known physical phenomena are induced: (i) the flexo-electric effect and (ii) the elongation of the helical structure. In this study, we show that the latter effect can be additionally separated into two different components with fast and slow response times, which reorient the LC director along the electric field without and with the helical pitch elongation, respectively.

We proposed a plasmonic structure consisting of uniform metallic nanoslits with a movable dielectric substrate for subwavelength beam manipulation. In this structure, the phase of incident light can be modulated beforehand by the substrate before bumping the metal slits; therefore, the shape of transmitted light from nanoslits can be actively controlled by changing the substrate profile. According to this idea, plasmonic lenses with different functions, such as focusing light to two points, focus length variation, and beam deflection, are designed and investigated numerically. The proposed structure has a unique advantage of multifunction and reduces the difficulty in fabrication.

The fabrication of Au nanowire arrays by mechanical deformation using anodic porous alumina was studied. Porous alumina was pressed onto a Au surface so that Au entered the nanoholes of the porous alumina. The Au nanowire arrays were obtained by dissolving the porous alumina. For the efficient fabrication of nanowires, a nanoimprinting process was applied, which enabled the nanowires to be reproducibly obtained. The porous alumina used as the nanoimprinting mold could be reused. The nanowire arrays could be used as substrates for surface-enhanced Raman scattering measurements owing to the enhancement of the light intensity based on localized surface plasmon resonance.

The complete dispersion relations of transverse magnetic surface plasmon polaritons (TM SPPs) at a metal/birefringent dielectric interface are studied using a rigorous electromagnetic treatment. The exact expression of the wave vector normal to the boundary is derived. The propagation properties and components of the Poynting vectors of four types of TM SPPs are demonstrated with real materials in a wide range of photon energies. On the basis of this analysis, new types of TM SPPs are predicted. Furthermore, a perfect surface wave is realized with real materials in this simple structure. Our results may be useful for plasmonic-based devices associated with anisotropic media.

A plasmonic lithography method is proposed to generate a large-area, period-reduced, subwavelength one-dimensional grating pattern. It is demonstrated with a surface plasmon polaritons interference effect and a hyperbolic metamaterial multilayer structure by UV radiation at 405 nm wavelength. Photoresist patterns of 1.5 × 1.5 cm² area with 350 nm period and 100 nm linewidth were fabricated on polyethylene terephthalate films by using a 700-nm-period Al grating mask and exposed to a transverse magnetic polarized laser. The proposed lithography approach provides a cost-effective and less laborious pathway for the fabrication of large-area periodic nano-patterns.

High-quality InGaN-based visible light-emitting diodes (LEDs) are demonstrated on ScAlMgO₄ (SCAM) (0001) substrates. GaN grown on SCAM by metal–organic vapor phase epitaxy is nearly strain-free with an in-plane compressive strain of −1.26 × 10⁻³, which is much smaller than that in conventional GaN/sapphire owing to the smaller thermal expansion mismatch between GaN and SCAM. We fabricate InGaN/GaN quantum well LEDs on GaN/SCAM templates, and observe bright blue electroluminescence at ∼470 nm wavelength. The device performances of LEDs on SCAM are comparable to those of LEDs on sapphire. Our achievements indicate that highly efficient InGaN-based light emitters are possible on SCAM substrates.

We have grown 50 cm² mono Si ingots by the seed cast technique. The carbon and oxygen concentrations of the ingots have been significantly reduced by improving the gas flow condition and coating. The dislocation density was also reduced by eliminating the extra dislocation generation sources. Owing to these developments, the lifetime of wafers has reached 465 µs. Finally, the efficiency of 18.7% has been achieved, which is comparable to 18.9% of the reference Czochralski (CZ) Si wafer.

GaAs tunnel diodes (TDs) embedded with an InAs quantum dot (QD) layer were grown and their performance was compared with that of TDs without a QD layer. The TDs embedded with a QD layer showed enhanced peak tunnel current density and lower differential resistivity at zero bias compared with the TDs without a QD layer. The samples were then annealed to mimic the overlayer growth process. It was found that the performance degradation after annealing was smaller for the QD-layer-embedded TDs. The improved characteristics of the QD-layer-embedded GaAs TDs make them advantageous for interconnecting unit cells in tandem solar cells.

We investigated the effects of sodium doping on the photocarrier dynamics in Cu₂ZnSnS₄ (CZTS) single crystals using optical pump-THz probe transient reflectivity (THz-TR) and time-resolved photoluminescence (PL) spectroscopy. The THz-TR and PL decay dynamics are influenced by sodium doping, and their sodium-induced changes are consistent with each other. These time-resolved measurements revealed that the lifetime of photocarriers increases with sodium doping. This result indicates that a part of defects is suppressed by doping sodium into CZTS and implies that sodium doping improves the charge transport properties of CZTS, leading to an improvement in the performance of CZTS-based solar cells.

We evaluate whether the measurement stability of Brillouin optical correlation-domain reflectometry (BOCDR) using polymer optical fibers (POFs) can be enhanced by polarization scrambling. In this study, two major factors that affect the signal-to-noise ratio in BOCDR, specifically, the spatial resolution and incident power, are varied, and their influences on distributed measurements with polarization scrambling are experimentally investigated. We thus confirm that in POF-based BOCDR, unlike in BOCDR using standard silica glass fibers, polarization scrambling is an effective means of enhancing the measurement stability only when the spatial resolution is sufficiently low or when the incident power is sufficiently high.

Heavily MgO doped LiTaO₃ crystals have not only good electro-optic properties but also a higher photorefractive damage resistance and wider transmission band than LiNbO₃ and conventional LiTaO₃. We demonstrated an electro-optic polarization conversion type modulator using periodically poled 8 mol % MgO doped congruent LiTaO₃ for the first time. High-quality domain-inverted structures of ∼200 µm period and 10 mm length were formed. A maximum polarization conversion efficiency of 93% was obtained for a wavelength of 980 nm. The temperature acceptance bandwidth of 2.0 °C was in close agreement with the theoretical value.

Scintillation and luminescence properties of orthorhombic Gd₂Si₂O₇:Ce (GPS:Ce) single-crystal scintillators were investigated for temperatures ranging from room temperature (RT) to 573 K. Orthorhombic GPS crystals were grown by using a top-seeded solution growth (TSSG) method. The scintillation light yield of the orthorhombic GPS at RT was ∼2.9 times higher than that of Gd₂SiO₅:Ce (GSO). The light yield values of the orthorhombic GPS (Ce = 2.5%) were almost unchanged for temperatures ranging from RT to 523 K, and at 523 K, were higher than twice the light yield of GSO at RT. These GPS scintillators are expected to contribute to oil exploration at greater depths.

Compact, wavelength-tunable light sources are desired for the enhancement of information communication technology and bio-imaging applications. We propose a compact, wavelength-tunable laser diode with a wide wavelength-tunable range around 1230 nm consisting of a quantum-dot optical amplifier and a silicon photonic tunable filter. High-quality InAs quantum dots grown with the sandwiched sub-nano separator technique were used as the optical gain medium. The wavelength-tunable filter was constructed with ring resonators fabricated using silicon photonics. The single-mode laser oscillation was demonstrated with a 28.5-nm wavelength-tunable range.

We have successfully demonstrated the room-temperature continuous-wave operation of GaN-based vertical-cavity surface-emitting lasers (VCSELs) with all-dielectric reflectors, which were fabricated using epitaxial lateral overgrowth. The VCSELs exhibited a threshold current of 8 mA and a threshold voltage of 4.5 V at a lasing wavelength of 446 nm. The maximum output power was 0.9 mW for an 8-µm-diameter current aperture, which was made possible because of the high thermal conductivity of the GaN substrate.

We report an experimental observation on universality of stochasticity in magnetic domain-wall motion along ferromagnetic nanowires. The domain-wall arrival time exhibits stochasticity depending on the nanowire width and strength of external magnetic field. Strikingly, all of the measured stochasticity data collapse onto a single universal curve that is given by a function of the number of de-pinning events. Such stochasticity has been found to be saturated for nanowires thinner than about 200 nm, which is possibly attributed to the dimensionality transition from two to one dimension. These results provide essential information for operating and/or design of domain-wall mediated devices.

The magnetic and transport properties of Co/Pt multilayer-based bottom-pinned perpendicular magnetic tunnel junctions (pMTJs) on Ru, Hf, and Ru/Hf seed layers (SLs) were investigated after annealing at different temperatures. The perpendicular synthetic antiferromagnetic (pSAF) layer on the Ru SL was found to be thermally robust (after annealing at 400 °C for 30 min). A high tunnel magnetoresistance (TMR) ratio of 100% was achieved at a low resistance-area product (5.5 Ω·µm²) and was stable up to 350 °C. For the stack on Ru SL, TMR degradation after annealing was caused by the degradation of the pMTJ (CoFeB/MgO/CoFeB), while in the Hf and Ru/Hf SL, both the pMTJ and pSAF were affected.

We applied micromagnetic numerical calculations to a study of vortex-state reversal dynamics in half-spheres. We found an additional, heretofore unknown mechanism of vortex-core reversals that occur via the nucleation of a reversed vortex core at the edge of the half-sphere after expulsion of the original core either with or without the reversal of the original chirality, but without formation of the magnetization dip or Bloch point. The vortex-state reversals are affected by the curved geometrical confinement of the half-spheres. Detailed descriptions of the reversal dynamics offer the fundamentals of both vortex polarization and chirality reversals in curved restricted geometries.

The Dzyaloshinskii–Moriya interaction (DMI) is an antisymmetric exchange interaction that plays a decisive role in the formation of chiral magnetic structures and in the determination of magnetoelectric properties. This study investigated the impact of an external voltage on the magneto-static surface waves in a Au/Fe/MgO multilayer. Spin waves were excited and detected using two coplanar waveguides and controlled by an external DC voltage. The DC bias voltage dependence of the resonant frequency in the spin waves revealed that the voltage effect has both directionally symmetric and asymmetric components, signifying voltage control of both interfacial magnetic anisotropy and interface-DMI.

We report on collective dynamics of three-chained magnetic vortices interacting with each other through magnetic dipolar coupling. Three different eigenstates characterized by the phase difference of the rotating cores were clearly observed by means of the spin torque diode effect. We also selectively excited such eigenstates with the same system by tuning the phase difference. These experimental results are well reproduced by analytical calculations based on the linearized Thiele's equation. Moreover, we demonstrate that the spectra obtained for the selective excitations correspond to a part of the dispersion relation for one-dimensional chained vortices.

The effect of electric field (E-field) on the magnetism of FePt thin films in FePt/0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃ (PMN–PT) heterostructures was investigated by anomalous Hall effect measurement. For FePt films of different thicknesses, the coercivity vs E-field curves show a typical butterfly-like loop behavior. Further results indicate that the coercivity variation is composed of the volatile symmetrical butterfly-like loop and nonvolatile hysteresis loop-like parts, which originate from the volatile and nonvolatile strains induced by the E-field in the PMN–PT(001) substrate, respectively. No significant difference has been observed after inserting a 2 nm W interlayer, suggesting that the charge-mediated coercivity variation is negligible in FePt/PMN–PT heterostructures.

Spin-transfer-torque switching in a spin-valve nanopillar with a conically magnetized free layer (c-FL) was theoretically studied using the macrospin model. The c-FL is shown to have advantages such as a low switching current density and short switching time compared with a perpendicularly magnetized free layer (p-FL). A c-FL with a thermal stability of 60k_BT exhibited a switching current density 22% smaller and a switching time 56% shorter than those of a p-FL with the same thermal stability.

The current perpendicular to film plane type giant magnetoresistance (CPP-GMR) effect in Co₂Fe_0.4Mn_0.6Si/Ag₈₃Mg₁₇/Co₂Fe_0.4Mn_0.6Si junctions was investigated. An epitaxially grown 5-nm-thick Ag₈₃Mg₁₇ film having partially ordered L1₂ structure was fabricated on the Co₂Fe_0.4Mn_0.6Si layer by magnetron sputtering. CPP-GMR effects were observed in the submicrometer-sized junctions, and the maximum value of the observed (intrinsic) MR ratio was 40% (48%) at room temperature. The average change in the resistance–area product was 23 mΩ·µm² for the Ag–Mg junctions, which was higher than those of conventional CPP-GMR junctions using a Ag spacer layer.

We report here an experimental technique to generate spinwaves using femtosecond laser pulses. The femtosecond laser pulses induce ultrafast demagnetization, which causes the propagation of the spinwaves from the laser spot. The observed spinwaves exhibit an anisotropic behavior by showing both the forward and backward modes depending on the propagation direction with respect to the in-plane magnetization direction, as confirmed by micromagnetic simulations. The forward mode is found to propagate over a few micrometers with small dissipation, providing a possible spinwave source.

A SiN_x/AlN dielectric stack, which has been shown to provide a practical, robust, and effective passivation for GaN-based lateral heterojunction power switching devices, was characterized in this work to provide insights on the mechanisms of its current collapse suppression ability. The interface between the SiN_x/AlN passivation stack and the AlGaN/GaN structure was characterized by investigating the interface state distribution and its chemical composition. Such interface was found to have much less trap states and significantly less oxidation than that in the heterostructure passivated by an Al₂O₃/AlN stack, validating that SiN_x/AlN passivation is superior to Al₂O₃/AlN passivation.

The electrical characteristics of a series of AlInN high-electron-mobility transistors (HEMTs) with a GaN cap layer ranging from 0 to 26 nm are investigated for power switching applications. The breakdown voltage (V_B), mobility of two-dimensional electron gas, on-state resistance (R_on), and dynamic R_on of the HEMTs are improved by increasing the cap layer thickness. The improved electrical characteristics are attributed to the GaN cap layer, which not only reduces the surface E-field but also raises the conduction band of the barrier layer and effectively prevents electrons from being trapped in the AlInN barrier and above.

Multilevel memories have attracted significant interest because of their larger memory density per unit cell. Here, we investigated multilevel operation with ambipolar carbon nanotube thin-film transistors. Three distinct conduction states and a direct change between any of them were demonstrated by selecting appropriate values for the magnitude and duration of each program/erase voltage pulse. A low operation voltage of 5 V and a short duration of 1 s were obtained by utilizing a bilayer Al₂O₃-epoxy dielectric to enhance the gate modulation efficiency. A tradeoff exists between low-voltage operation and fast switching for a given device.

We have developed a system for the simultaneous measurement of electrical conductance and thermopower of the single benzenedithiol (BDT) molecular junction, which was characterized by inelastic electron tunneling spectroscopy, at low temperature. The simultaneous measurements revealed a negative correlation between the electrical conductance and the thermopower. Strong metal–molecule coupling at the single BDT molecular junction leads to high conductance and low thermopower because of the broadening of the conduction orbital, which explains the negative correlation. The observed fluctuation in conductance and thermopower reflects the change in the metal–molecule contact configuration and molecular orientation.

We performed a first-principles study on two-dimensional (2D) arsenene doped with non-magnetic elements. It was found that dopants (groups III, V, and VII) with odd numbers of valence electrons maintained the semiconducting character of the pristine system, while those (groups IV and VI) with even numbers of valence electrons caused the metallic character to change. Remarkably, the C- and O-doped systems were spin-polarized and could be modulated into half-metals by the external electric field. Our findings reveal a potential method of engineering buckled arsenene for applications in nanoelectronics.

The subthreshold transport properties in exfoliated MoS₂ FETs are reported. The temperature dependence of subthreshold characteristics in multilayered MoS₂ FETs behaves in the same way as that of conventional semiconductors, while conductance fluctuations and random telegraphic signals in the subthreshold region of I_ds–V_gs characteristics were observed much more frequently in monolayered FETs than in multilayered ones. This fact is understandable from the viewpoint of a three-dimensional to two-dimensional percolation transport process in MoS₂ with defects, which should be located in the MoS₂ layer or at the MoS₂/SiO₂ interface. Thus, it is suggested that few-layered MoS₂ FETs are more viable for practical applications from the viewpoint of suppressing defect-induced current fluctuations.

A new method for selective crystallization of the metastable phase (form II) of acetaminophen is described. To obtain form II, we prepared a highly supersaturated solution (σ_I = 3.7) and then applied ultrasonic irradiation at different frequencies. Without ultrasonic irradiation, spontaneous crystallization did not occur within one month in the highly supersaturated condition (σ_I = 3.7). When ultrasonic irradiation at 28 kHz was applied, form II preferentially crystallized. Therefore, we conclude that ultrasonic irradiation can be an effective technique for selectively crystallizing the metastable phase.

In this study, 4H-SiC stripe-shaped trenches preformed on an n⁺ substrate were filled by adding HCl to the chemical vapor deposition process at relatively high pressures. HCl was found capable of counterbalancing the deposition on the mesa top by strong etching, and it thus enabled quasi-selective epitaxial growth across the whole extents of the trenches, where the epilayer preferentially grows from the trench bottom. Using the established technique, the 1-µm-wide 4H-SiC trenches, with an aspect ratio of 5, which is the highest aspect ratio to date, were completely filled at a growth rate above 0.5 µm/h and acquired a flat end surface.

We report electrodeposition of n-type cuprous oxide (Cu₂O) films on p-type CuO films electrodeposited on Ti substrates for forming p-CuO/n-Cu₂O heterostructures. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis revealed that the films had good structural quality, with substrates being well-covered by the films. The p-CuO/n-Cu₂O heterojunctions exhibited good photovoltaic properties and diode characteristics. The surfaces of Cu₂O films subject to ammonium sulfide treatment exhibited enhanced photocurrents. Under AM 1.5 illumniation, the obtained sulfur-treated and annealed Ti/p-CuO/n-Cu₂O/Au solar cell structure yielded energy conversion efficiency of 0.64%, with V_oc = 220 mV and J_sc = 6.8 mA cm⁻².

Interfacial phenomena at the liquid/solid interface under high temperatures were observed in real time to understand the growth process of AlN during solid-source solution growth. In this study, we used an AlN/α-Al₂O₃ template as the substrate; these wide-bandgap materials made the substrate transparent to visible light. Therefore, we observed the morphology of the liquid/solid interface through the template from the bottom. In this investigation, a polycrystal formed because of melt-back etching during the initial stage of growth; nevertheless, we succeeded in obtaining real-time images of interfacial phenomena.

Silicon nanowires (SiNWs) have been grown on Si(100) substrates with and without a thermal oxide layer by rf magnetron sputtering of Si in Ar/H₂. In the experiments, thin Au layers were employed as catalysts, resulting in a significant and substantial growth of randomly oriented, polycrystalline SiNWs, typically 20 µm long and 350 nm in diameter after 60 min of growth on both Si and SiO₂ substrates at 700 °C. These indicate the possibility of providing an alternative method of SiNW growth that does not require toxic feed gases and high-temperature tube furnaces, and hence is suitable for growth on large-diameter substrates in industry.

An AlGaN/GaN-based Ω-shaped nanowire fin-shaped FET (FinFET) with a fin width of 50 nm was fabricated using tetramethylammonium hydroxide (TMAH)-based lateral wet etching. An atomic layer deposited (ALD) HfO₂ side-wall layer served as the etching mask. ALD Al₂O₃ and TiN layers were used as the gate dielectric and gate metal, respectively. The Ω-shaped gate structure fully depletes the active fin body and almost completely separates the depleted fin from the underlying thick GaN buffer layer, resulting in superior device performance. The top-down processing proposed in this work provides a viable pathway towards gate-all-around devices for III–nitride semiconductors.

We demonstrate the selective and controllable undercut etching of the InGaN/GaN multiple quantum well active regions of nonpolar and semipolar laser diode (LD) structures by photoelectrochemical (PEC) etching without external bias. The lateral etch rate ranged from ∼20 nm/min to ∼1.2 µm/min. Metal masks were used to define the undercut and to improve the PEC etch resolution by reducing the scattered light in the system, which contributes to degradation of the lateral etch resolution, as suggested by ray tracing simulations. This resulted in a light-exposed-area: masked-area etch selectivity of ${\sim} 13:1$.

A comparative study on the electrical characteristics of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) with HfLaO and HfLaON gate dielectrics is conducted. With the appropriate incorporation of nitrogen into the HfLaO gate dielectric, the saturation mobility of the TFTs can reach 31.3 cm² V⁻¹ s⁻¹, which is more than twice that of the control sample with the HfLaO gate dielectric, as a result of the passivation of traps at/near the dielectric/a-IGZO interface by the nitrogen incorporation. However, the electrical characteristics (such as saturation mobility, on-current, and on-current/off-current ratio) of the devices tend to deteriorate with excessive nitrogen incorporation owing to nitrogen-related defects generated in the dielectric.