Table of contents

Large sizes (9×12 mm²) semipolar GaN substrates with the plane orientation of (2021) were obtained by using the ammonothermal method. Their structural quality was investigated by using the X-ray diffraction whereas the optical quality was probed by absorptionlike (contactless electroreflectance) and emissionlike (photoluminescence) techniques. It is shown that these substrates are characterized by very narrow X-ray diffractions peaks [17 and 21 arcsec for the (2021) and (1010) peak, respectively] and the GaN energy gap-related transition is clearly observed in contactless electroreflectance and photoluminescence spectra. A defect-related emission is also observed in photoluminescence spectra but its intensity is much weaker that the energy-gap related emission.

Low ohmic contact resistance m-plane AlGaN/GaN heterojunction field-effect transistors were demonstrated using a regrown n⁺-GaN contact layer. An ohmic contact resistance of 0.25 Ω·mm was obtained with an 80-nm-thick Si-doped regrown GaN contact layer deposited by metal organic chemical vapor deposition. Enhancement-mode m-plane AlGaN/GaN heterojunction field-effect transistors were demonstrated with a threshold voltage of +1.4 V and 2.0 mΩ·cm² on-state resistances at +5 V of gate–source voltage.

The current–voltage characteristics of Schottky barrier diodes formed on GaN(0001) free-standing substrates with net donor concentrations of 7.6×10¹⁵–1.4×10¹⁷ cm^-3 are discussed. The substrates were grown by hydride vapor phase epitaxy. Ni Schottky contacts were directly formed on chemical–mechanical-polished Ga-polar faces of the substrates. Nearly ideal characteristics for both directions were obtained. The ideality factors for forward characteristics are 1.02–1.05, very close to unity. The reverse characteristics agree well with calculations based on thermionic-field emission theory without any fitting parameter.

Based on the first-principles calculations within the self-interaction-corrected local density approximation, we propose a materials design for high efficiency photovoltaic solar cells. By using the Korringa–Kohn–Rostoker coherent potential approximation method, the electronic structure and mixing energy of CuIn_1-xGa_xSe₂ (CIGS) are calculated. It is found that (1) CIGS has positive and convex mixing energy; thus, it has a tendency towards spinodal nano-decomposition, and (2) the band alignment between CuInSe₂ and CuGaSe₂ is type II. Based on these findings, it is pointed out that efficient electron–hole separation and multiexciton effect upon photoexcitation can be expected in the spinodal decomposed CIGS for realizing high-efficiency energy conversion.

Cu₂ZnSnS₄ (CZTS) thin films were investigated as photoelectrodes for H₂ evolution from water under solar light. Surface modifications with Pt, CdS, and TiO₂ enhanced the incident photon-to-current conversion efficiency of CZTS thin film electrode for H₂ production from water by three orders of magnitude to 40% (λ= 600 nm). The solar energy conversion efficiency in H₂ production using modified CZTS reached 1.2%. Our results demonstrate that the photoelectrochemical properties of CZTS were significantly improved by surface modification, which suggests the feasibility of CZTS as a photocathode for a photoelectrochemical water-splitting system.

BiFeO₃ and BiFe_1-xW_xO₃ (x = 0.005, 0.01, 0.02) films were epitaxially deposited on LaNiO₃(100)/Si substrates using a metal organic decomposition process. Well-saturated polarization–electric field hysteresis loops can be observed in all BiFe_1-xW_xO₃ films owing to their leakage currents being lower than that of the BiFeO₃ film. Additionally, the domains in BiFe_0.995W_0.005O₃ and BiFe_0.99W_0.01O₃ films can be uniformly switched using a piezoelectric-mode atomic force microscope (AFM) due to the efficient deaging effect of W⁶⁺. More importantly, the BiFe_0.99W_0.01O₃ film exhibits the highest piezoelectric coefficient (∼132 pm/V) among all the films, making it promising for applications in high-temperature piezoelectric devices and AFM-tip-based data storage.

We report on high-performance solution-processed n-channel organic thin-film transistors based on a long-chain alkyl-substituted fullerene derivative, C₆₀-fused N-methylpyrrolidine-meta-dodecyl phenyl (C60MC12), by surface modification of an insulator. C60MC12 films were fabricated on self-assembled monolayer (SAM)-treated gate insulators by spin-coating, which was facilitated by hydrophilic patterning of the edge of the substrate. X-ray diffraction revealed that the crystallinity of C60MC12 films was improved by SAM treatment. The octadecyltrichlorosilane-treated device demonstrated a high mobility of 0.4–0.5 cm² V^-1 s^-1, which is comparable with those of amorphous silicon thin-film transistors. An improvement in bias stress stability by the SAM treatment was also observed.

Organic thin-film solar cells based on a bulk heterojunction utilizing the phthalocyanine derivative 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH₂) have been studied. C6PcH₂ is soluble in common organic solvents such as chloroform, and the blend uniform thin film with the fullerene derivative 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM) could be fabricated by a spin-coating method. Solar cells with an indium–tin-oxide/polymer hole transport layer/C6PcH₂:PCBM/Al structure, the active layer of which was prepared by a wet process using a low-weighted molecular system, have demonstrated a high external quantum efficiency of more than 70% in the Q-band absorption region of C6PcH₂ and a high energy conversion efficiency of 3.1%.

The first 30-mW-class semipolar blue light-emitting diode (LED) on a free-standing (10) GaN substrate has been demonstrated by using microscale periodic backside structures. The light extraction efficiency and corresponding output power were greatly enhanced, by up to 2.8-fold (bare chip) compare with conventional devices. At a driving current of 20 mA, the LED showed an output power of 31.1 mW and an external quantum efficiency of 54.7%. Semipolar GaN LED technology is now comparable to commercial c-plane blue LED technology, not only in terms of internal material properties but also in terms of chip processing techniques.

We have developed dual-probe scanning near-field optical microscopy (SNOM) to visualize detailed carrier diffusion/recombination processes and applied it to the assessment of the local carrier dynamics in an InGaN single quantum well. It is clearly demonstrated that the carrier motion is strongly affected by the potential distribution within InGaN; potential ridges prevent carriers from diffusing outside them, whereas potential peaks cause carriers to travel a roundabout route around them. As a consequence, carriers anisotropically diffuse for several hundred nanometers along a specific direction toward a strong-photoluminescence domain. Thus, the dual-probe SNOM technique is a powerful nanoscopic tool, and may be versatile for characterizing photonic materials.

Linearly polarized light emission is analyzed in nonpolar light-emitting diodes (LEDs) covering the blue to green spectral range. In photoluminescence, m-plane GaInN/GaN structures reach a polarization ratio from 0.70 at 460 nm to 0.89 at 515 nm peak wavelength. For a-plane structures, the polarization ratio is 0.53 at 400 nm and 0.60 at 480–510 nm. In electroluminescence the polarization ratio is 0.77 at 505 nm in 350×350 µm²m-plane devices at 20 mA. Such a device should allow 44% power saving compared with nonpolarized c-plane LEDs combined with a polarizing filter, as commonly used in LED-backlit liquid crystal displays.

Silicon nanoparticles synthesized using low-energy (35 keV) silver ion beam implantation in crystalline Si exhibit enhanced radiative recombination efficiency due to resonant coupling of localized surface plasmon polariton to the bound excitons confined at the nanoscale Si interface. A photoluminescence (PL) enhancement of more than 20 times is observed due to the local field effect induced by the metal nanoparticles at 250 K. At 15 K, fourfold enhancement in the radiative recombination rate is observed as the presence of the PL due to Ag ion implantation is 400 ps compared with 2.1 ns without the metal ions.

The polarity and mechanism of terahertz radiation from native n-type InN excited by femtosecond optical pulses are investigated. The optical properties, electron concentrations, and crystalline quality are characterized by photoluminescence and Raman scattering spectra. The electron concentrations are estimated to be between 0.35×10¹⁹ and 3.87×10¹⁹ cm^-3. The polarity of terahertz radiation field from the samples with higher electron concentrations is opposite to that from p-InAs, indicating that the dominant radiation mechanism is the drift current. However, the samples with lower electron concentrations show the same polarity as p-InAs. Under this condition, the radiation mechanism is dominated by the photo-Dember effect.

We demonstrate the monolithic integration of a silica-based arrayed waveguide grating (AWG) filter and silicon (Si) variable optical attenuators (VOAs) by connecting them with spot-size converters (SSCs). The AWG is based on a silica waveguide with an index contrast (Δ) of ∼3%, and the Si VOA is based on a rib-type Si wire waveguide with a p–i–n lateral diode structure. The integrated device performs wavelength demultiplexing in 16 channels and has fast optical power attenuation with a -3 dB bandwidth of 50 MHz in each channel.

We have demonstrated volumetric optical recording using an all-semiconductor picosecond laser, which generated optical pulses with a duration of 3 ps and a maximum peak power of 100 W at a wavelength of 404 nm and a repetition frequency of 1 GHz. This pulsed laser system efficiently induced multiphoton absorption in the recording media due to its high peak power and high repetition rate. The recording marks were formed as submicrometer voids inside a single thick recording layer by multiphoton absorption. A clear readout signal was obtained from the recorded marks.

We present the performances of a superconducting nanowire single-photon detector (SNSPD) in an ultralow-temperature region from 16 mK to 4 K. The system detection efficiency of the SNSPD showed saturation in the bias-current and temperature dependences, and reached the considerably high value of 15% for 100 Hz dark count rate below 1.4 K at a wavelength of 1550 nm even without an optical cavity structure. We found that the dark count exists even at 16 mK and black body radiation becomes its dominant origin in the low temperatures for fiber-coupled devices.

We propose a concept of mono-material multilayer interference optical filter that can be fabricated using simple micro-electro-mechanical systems (MEMS) technologies. The key point of this concept is to control the refractive index of each layer by sub-wavelength structure (SWS), not by selecting different materials. A homogeneous composition of material results in robustness in cryogenic thermal cycle, which is essential for highly sensitive infrared observations. We have fabricated antireflection (AR) SWS layers on silicon wafers as a demonstration of this concept. The evaluation of optical characteristics shows that the SWS layers can be used as layers of interference filters in infrared wavelength.

A high-refractive-index glass with the composition of 0.30La₂O₃–0.70Nb₂O₅ was prepared in bulk form by containerless processing without using network former oxides. The glass transition temperature was 725 °C and the first crystallization peak temperature was 805 °C. The glass was colorless and transparent from 345 nm to 7.3 µm. The refractive index was higher than 2.3 in the visible range, and the Abbe number estimated from the Drude–Voigt relationship was 20.4. These results indicate that the niobate glass system would be useful for optical applications in the visible to infrared region.

Hybrid moth-eye structures were proposed to enhance broadband antireflection properties. An ultralow average reflectance down to 0.11% over the solar spectral range has been achieved, which demonstrates a ∼50% enhancement in broadband antireflection capability as compared with corresponding uniform moth-eye structures. This is attributed to a smaller refractive index gradient with respect to structure height. These hybrid structures can be applied to solar cells for higher light-to-electricity conversion efficiency.

Emerging free-electron lasers (FELs) at short wavelengths open a new possibility for ultrafast microscopy of transient phenomena at high spatial resolution due to the ultrashort pulse duration. As a first step, we performed sub-picosecond snapshot imaging with extreme ultraviolet FEL by adopting a recently proposed holographic technique, holography with extended reference by autocorrelation linear differential operation (HERALDO). We point out that HERALDO enables efficient single-shot phase-contrast imaging by carefully designing the aperture size of the reference without sacrificing spatial resolution. The successful reconstruction of a test pattern from a single-shot hologram demonstrates an effective method of taking phase-contrast sub-picosecond snapshots.

In this paper, we report the improved lasing threshold in cholesteric liquid crystal (ChLC) lasers using an in-plane helix alignment, where the helix axis lies in the in-plane direction of the cell. The in-plane helix alignment of ChLCs was obtained by applying an in-plane electric field in a cell with homeotropic surface treatment while cooling the sample from the isotropic phase. The energy threshold of this device was 0.3 mJ/(cm²·pulse), which was found to be less than 1/35 that of conventional planarly aligned ChLC lasers.

Spinel ferrite Zn_1-xCo_xFe₂O₄ (0≤x≤1) (111) composition-spread thin film was epitaxially grown on an MgAl₂O₄(111) substrate by combinatorial pulsed laser deposition. Sequential deposition of the two end-compounds with monolayer-period led to the formation of solid solution with addressable composition. Point-by-point ultraviolet–visible magnetic circular dichroism and Faraday rotation angle spectra showed a significant variation with x, and the latter spectra combined with absorption spectra led to the rapid evaluation of magnetooptical performance factor as functions of x and photon energy. A significant enhancement of ultraviolet magnetooptical effect was observed by substituting a small amount of Zn (≤30 at. %) for Co in CoFe₂O₄.

Current-induced magnetization switching (CIMS) in CoFeB (pinned layer)/MgO/Fe₄N (free layer) magnetic tunnel junctions (MTJs) was investigated at room temperature and an inverse CIMS phenomenon was clearly identified. From the current-field magnetic phase diagram, determined from the dependence of the switching field on the bias current, the direction of spin-transfer torque was found to be opposite to that observed in CoFeB/MgO/CoFeB-MTJs. The inverse CIMS observed in this study cannot be explained only by the conventional theory, which states that the direction of the spin-transfer torque is determined by the sign of the spin polarization of the pinned layer.

The quantum nature of a sub-millimeter sized superconducting inductor–capacitor (LC) resonator is been demonstrated. By employing a series of pulse sequences that induce half a cycle of vacuum Rabi oscillations, we put a single microwave photons into an LC resonator from a superconducting flux qubit. Sub-Poissonian photon number distributions obtained by the Fourier transformation of observed quantum oscillations between the |n,e> and |n+1,g> states (n: photon number in the resonator, g,e: qubit state) indicates the successful generation of non-classical photon states in an LC resonator.

This letter describes the fabrication of amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) by direct transfer printing. An a-IGZO layer and a silicon dioxide (SiO₂) layer were sequentially sputtered on a poly(dimethylsiloxane) (PDMS) stamp; the stamp was then pressed onto a glass substrate on which a gate metal had been previously deposited. Then, a-IGZO/SiO₂ layers were successfully transferred by simply releasing the stamp from the substrate; a bottom-gate TFT was finally constructed. The measured current–voltage characteristics exhibited good field-effect mobility exceeding 10 cm² V^-1 s^-1. The on/off current ratio and subthreshold slope were 4×10⁵ and 0.86 V/decade, respectively.

We have evaluated the influence of practical factors on the conversion efficiency of hot carrier solar cells, from which photogenerated carriers are extracted before being completely thermalized. Equilibration and thermalization of the carriers, and energy dissipation associated with hot carrier extraction were involved in a thermodynamic modeling. Among them, thermalization has been found to have the greatest impact. Even though a 1 ns thermalization time could be realized, the conversion efficiency is close to the Shockley–Queisser limit (34%) under 1 sun irradiation, and lower than the limiting values of triple-junction cells (around 60%) at 1000 sun.

We report vertically aligned ZnO nanostructures grown using periodically polarity-inverted (PPI) ZnO heterostructures on (0001) Al₂O₃ without the aid of a catalyst. In order to fabricate the PPI ZnO templates, the polarity control of ZnO film via in situ crystal polarity control methods using CrN and Cr₂O₃ buffer layers was used. Vertically aligned ZnO nanowires or nanowalls were formed only on the Zn-polar regions. The self-catalyzed process on the Zn-polar surface was most likely responsible for this selective growth. Finally, we determined the mechanism of nanowall formation on the basis of the coalescence of several nanowires in the defined area.

We report on ternary InGaAs nanowires (NWs) with a <110> orientation laterally grown on a GaAs (311)B substrate in the Au-particle-catalyzed vapor–liquid–solid mode. We determined the structure and composition of the lateral NWs using transmission electron microscopy and X-ray energy-dispersive spectrometry measurements. With an In/Ga source mole ratio of 20:80, the composition of the lateral NW is In_0.04Ga_0.96As. We found that the addition of 4% indium changed the cross section of the lateral NW from the triangular shape of GaAs NWs to a trapezoidal shape.

Sub-2-nm-size basic ligand Au nanoparticles were chemically synthesized and chemisorbed on an acidic self-assembled monolayer/Au(111) substrate by acid–base interaction. Coulomb blockade behaviors with clear Coulomb gaps were observed in current–voltage (I–V) and log I–V curves of the chemisorbed Au nanoparticles by scanning tunneling spectroscopy at room temperature. By fitting the measured I(V) and log I(V) to a Coulomb blockade model, we estimated the charging energy of one electron on the Au nanoparticles to be 10 times greater than the thermal energy kT; the tunneling resistance of the Au core–Au(111) surface was evaluated to be 3.5 GΩ ±15%.

A novel cross-sectional X-ray diffraction method was developed to evaluate crystal structures of multiwalled carbon nanotube (CNT) forests. Aligned degree of the graphene volume in the CNT forest could be quantified by this analysis. The domain sizes of the CNTs in the forests were estimated using a modified Scherrer's equation for bending multilayered graphene sheets. The statistical information of the crystal structure of the CNT forests including the degree of alignment, domain size, layer numbers, and interlayer spacing d₀₀₂ will contribute to developing applications of CNT forests for devices and functional materials.

We have realized high-performance p- and n-type top-gate carbon nanotube field-effect transistors (CNFETs) by controlling the charges introduced at the interface between the gate insulator and the substrate. We also fabricated a complementary metal–oxide–semiconductor (CMOS) inverter on an individual single-walled carbon nanotube (SWNT). This CMOS inverter exhibited a high voltage gain of 26 and a large noise margin of ∼70% of V_dd/2. The proposed technique provides air-stable and high-performance carbon nanotube CMOS devices that are compatible with the Si CMOS process. The effects of interface charges on the mechanisms of CNFET operation have also been investigated on the basis of potential calculations.

We demonstrate the fabrication of high-quality fully relaxed SiGe layers on a silicon-on-insulator (SOI) substrate by rapid melt growth. A compositional gradient and crystallographic defects are confined to a region between the relaxed SiGe and residual SOI layers. The degradation of surface roughness during rapid thermal annealing is suppressed by the capping SiO₂ layer. The growth mechanism is discussed based on the phase diagram of the Si–Ge system and the depth profile of the Ge concentration.

An infinite-layer structure artificial superlattice thin film, [CaFeO₂]/[SrFeO₂], was prepared by low-temperature reduction of a brownmillerite-structure artificial superlattice thin film [CaFeO_2.5]/[SrFeO_2.5] grown on a SrTiO₃(001) substrate. Both the CaFeO_2.5 and SrFeO_2.5 brownmillerite structures changed to the infinite-layer structures and unusual square-planar-coordination FeO₂ plane was stabilized in the infinite-layer superlattice. The low-temperature reduction technique is useful for making superlattices that cannot be made by simply depositing a film.

The atomic structure of sub-monolayer depositions fabricated by an alternating deposition of 0.5 monolayer InAs and 16 monolayer GaAs is revealed by cross-sectional scanning tunneling microscopy. The resulting InAs-rich structures are about 5 nm wide and laterally separated from each other by about 2 nm, yielding a very high density above 10¹² cm^-2. The InAs-rich material is not only found within the deposition layer, but remarkably segregated with a segregation coefficient R∼0.7 over several monolayers along the growth direction. A similar segregation coefficient is found in the case of only 4 monolayer GaAs spacer thickness, revealing a more general growth mechanism for sub-monolayer depositions.

Single-crystalline thin films of indium iron oxides with two-dimensional triangular lattice structures were successfully grown on ZnO(001) substrates using pulsed laser deposition. Thin films of ferrimagnetic InFe₂O₄ with uniaxial magnetic anisotropy were found to grow in vacuum, while weak-ferromagnetic InFeO₃ thin films were formed under O₂ atmosphere. The key point for the growth of high-quality films is the control of volatilization of In during film deposition, which can be achieved by changing the growth temperature, O₂ pressures, and target compositions.

Relatively high-pressure (several hundred Pa) plasma was used for very high rate deposition of silicon thin films. We studied the loss kinetics of hydrogen atoms, which influence microcrystalline silicon film property in capacitively coupled plasma at relatively high-pressure using vacuum ultraviolet laser absorption spectroscopy. Increasing the pressures from 66.5 to 665 Pa, we found that the decay time constants of H atom were significantly lengthened from measurements of afterglow plasmas. In addition, surface-loss probabilities of the H atom decreased as the pressure increased.

We have devised a new resist-coating method using spin coating for creating three-dimensional (3D) nanostructures in semiconductors. Using this method, we were able to successfully coat poly(methyl methacrylate) (PMMA) films on the vertical side faces of micrometer-order Si blocks. The use of a solvent with low viscosity was found to be effective for obtaining good uniformity in resist thickness. Moreover, Monte Carlo simulations of electron scattering revealed that fine patterning on both the opposite side faces of a Si block should be possible by simultaneous development followed by electron beam (EB) writing from opposite directions, despite electron scattering.

Ge content and thickness variations of epitaxial Si_1-xGe_x layers on Si(100) were nondestructively measured using a long-focal-length, polychromator-based, multiwavelength micro-Raman spectroscopy system, which is designed for noncontact, in-line process and material property monitoring. The Ge content and thickness variations measured using the Raman system showed good agreement with results from both high-resolution X-ray diffraction (HRXRD) and secondary ion mass spectroscopy (SIMS) analysis. The nondestructive depth profiling capability of the multiwavelength Raman spectroscopy was verified on Si_1-xGe_x layers on Si(100) intentionally profiled with a Ge content gradient.

High-resolution neutron dark-field imaging has been applied to study magnetic domains in sheets of commercial grain-oriented electrical steel enveloped in an insulating layer. The resulting images have been compared to those obtained using conventional magnetic powder patterns and a clear correlation has been found, with the neutron technique providing complementary information about the internal structures of the material. An understanding of the magnetic structure of these materials is important for improving the performance characteristics – particularly minimizing core loss – in the electrical devices in which they are used.

We investigated the optimum architecture for confining fluorescent probes in microwells on a Si substrate by covering it with a lipid bilayer. We modified the structure of the wells to prevent the lipid membrane from falling into them, and the overhang shape at the aperture improved the probability of confinement. The fluorescence intensity from the calcein confined in the wells remained unchanged for one hour or more, indicating that the probes remain stably in the wells without flowing out. An artificial cell sealed with the suspended membrane is a promising tool for the functional analysis of membrane proteins.

An impulsive force generated by focusing a femtosecond laser into water through an objective lens was directly measured utilizing an atomic force microscope (AFM). We analyzed the transient bending movement of the AFM cantilever initiated by the impulsive force and suggested a procedure to quantify it. The order was 10^-12 N s, corresponding to the average force of µN in µs timescale. The quantification creates a new way of estimating intercellular adhesion strength, because it is possible to dissociate cell–cell adhesion by the impulsive force.

A large hydroxyapatite (HA) sheet of 50 mm diameter was constructed using a new fabrication process. The HA sheet was prepared by pulsed laser deposition (PLD) and then separated from the substrate. An intermediate layer of spin-coated photoresist between the PLD film and the substrate was dissolved in acetone to enable the separation. Sufficient crystallinity was obtained after postannealing. The area of the HA sheet prepared by the proposed process was approximately 20 times larger than that of the HA sheet prepared by the previous process. The proposed process can also be used for larger sheets. The HA sheet has promise in medical and dental applications.