Table of contents

The path to the realization of GaN p–n junction blue light-emitting devices [light-emitting diodes (LEDs) and laser diodes (LDs)] is reviewed. In the early 1970s, I was strongly convinced of the prospects of GaN towards the realization of blue LEDs and short-wavelength LDs, and in 1979, I decided to adopt the metalorganic vapor phase epitaxy (MOVPE) method for the growth of GaN, which was never employed at that time, for this purpose. My group developed low-temperature (LT-) buffer layer technology based on MOVPE to drastically improve the crystal quality of GaN, which led to the realization of low-resistivity p-type GaN, its p–n junction, and the conductivity control of n-type GaN, and finally to the realization of blue light-emitting devices, such as LEDs and LDs.

We investigate the effect of molecular ordering in the semiconductor of inverted staggered-type organic thin-film transistors (OTFTs) on device parameters. Molecular ordering is controlled by modifying the gate insulator with self-assembled monolayers and clarified by X-ray diffraction analysis. The reduction in the density of free carriers in a semiconductor with the alkyl-modified surface reduces off-current (I_OFF) and increases contact resistance (R_C). In contrast, I_OFF increases and R_C decreases for a phenyl-modified surface. Ultraviolet photoemission studies revealed that differences in the molecular ordering and the surface dipole moment caused by the insulator surfaces are crucial for device parameters for OTFTs.

Ultrananocrystalline diamond (UNCD)/nonhydrogenated amorphous carbon (a-C) composite films were deposited on unheated WC containing Co by coaxial arc plasma deposition. The hardness of the film is 51.3 GPa, which is comparable with the highest values of hard a-C films deposited on nonbiased substrates. The deposited film is approximately 3 µm thick, which is one order larger than that of hard a-C films. The internal compressive stress is 4.5 GPa, which is evidently smaller than that of comparably hard a-C films. The existence of a large number of grain boundaries in the UNCD/a-C film might play a role in the release of the internal stress.

On the surface of as-grown β-Ga₂O₃ single crystals that are cut and polished, we found nanometer-sized grooves elongated in the [001] direction. We confirmed that these grooves terminate within the crystals in the [010] direction. This proves that the grooves are different from micropipes penetrating crystals. Their typical length and width are 50–1200 nm in the [001] direction and ∼40 nm in the [100] direction, respectively. The grooves tend to form an array in the [001] direction. The type of nanometer-sized grooves should be essentially different from etch pits.

The charge carrier mobilities along the vertical and lateral directions of perylene platelet single crystals were measured by the time-of-flight (TOF) method. In the lateral directional measurement, the entire region between electrodes was irradiated to obtain measurable signals. The transient photocurrent was different from the conventional TOF measurements; hence, we developed an analytic method for lateral directional measurement. The electron mobilities along the thickness and lateral directions were 0.33 and 2.0 cm²·V⁻¹·s⁻¹ and the hole mobilities were 0.12 and 0.6 cm²·V⁻¹·s⁻¹, respectively.

Edge-defined fed-grown $(\bar{2}01)$ β-Ga₂O₃ single crystals with high electron concentration of 3.9 × 10¹⁸ cm⁻³ at 300 K were characterized by Hall effect measurement, and Schottky barrier diodes have been demonstrated. Electron mobility was as high as 74 cm²/(V·s) at 300 K regardless of the high doping concentration. The electron concentration did not change substantially in the low temperature below 160 K. This properties can be explained by the two-band model due to the inter-band conduction. On the Schottky barrier diodes, the rectification characteristics were clearly observed, and the current density of 96.8 A/cm² at the forward voltage of 1.6 V was obtained.

Using a SiN_x insertion layer to reduce dislocations, enhanced photovoltaic properties could be obtained in p–i–n InGaN/GaN heterojunction solar cell. To investigate the influence of the dislocations on the photovoltaic behaviors, a sample grown without SiN_x insertion layer was identically prepared for comparison. From optical properties measurements, the reduction in the number of non-radiative centers and a stronger In localization effect was shown in the sample with SiN_x insertion layer. However, the quantum confined stark effect was almost negligible in both the samples. Electrical properties measurement showed reduced saturation current and increased shunt resistance in the sample with SiN_x insertion layer due to the reduced dislocation density. By comparing these results and using a numerical model, the influence of the dislocation density on the different photovoltaic properties such as open-circuit voltage and fill factor has been confirmed.

The electrical properties of In/Ti/Al/Au contacts to N-polar n-GaN (n_d = 5 × 10¹⁸ cm⁻³) for high-power vertical light-emitting diodes were investigated at various thicknesses of the In layer, and compared with those of Ti/Al/Au contacts. Before annealing, both the Ti/Al/Au and In/Ti/Al/Au contacts were ohmic. After annealing at 300 °C for 1 min, all of the samples exhibited some degradation of their electrical properties, although the In/Ti/Al/Au samples were more thermally stable. After annealing at 300 °C for 60 min, the Ti/Al/Au contacts became non-ohmic, while the In (5 nm)/Ti/Al/Au contacts remained ohmic with a contact resistivity of 2.6 × 10⁻⁴ Ω cm². X-ray photoemission spectroscopy (XPS) results showed that, for all of the samples, annealing caused an increase in the content of interfacial oxygen. Based on the XPS and electrical results, the annealing dependence of the electrical characteristics of In/Ti/Al/Au contacts are described and discussed.

We present a systemic study of the terahertz (THz) optical conductivity of a strongly correlated La_0.33Pr_0.34Ca_0.33MnO₃ (LPCMO) thin film on a LaAlO₃ substrate. The measurements are carried out by THz time-domain spectroscopy in the temperature regime from 15 to 105 K. The frequency-dependent optical conductivity in the metallic phase region of the samples exhibits a non-Drude-like response. We find that below 105 K, both the real and imaginary parts of the complex conductivity can be reproduced by the Drude–Smith model. The important sample and material parameters of the LPCMO thin film (such as the persistence of velocity, the ratio of carrier density to effective mass, and electronic scattering time) can be determined by fitting experimental data. The results obtained agree with those obtained from four-probe electrical transport measurements.

InP nanowires and InP/GaInAs/InP core–multishell nanowires were successfully grown on an InP(111)B substrate by low-pressure metal organic vapor phase epitaxy (MOVPE) using an indium catalyst. The self-catalytic vapor–liquid–solid (VLS) mode was used to obtain high-quality nanowires in which a deposited indium droplet acts as the catalyst instead of a metal particle, as in the case of the conventional VLS mode. InP core nanowire structures dependent on growth temperature and preheating temperature were obtained. InP/GaInAs/InP core–multishell nanowire structures, densities, and optical properties were investigated at various flow rates of trimethylindium (TMI) during the growth of InP core nanowires and the growth time of the GaInAs shell layer was also studied. The growth volume and density of nanowires were mainly dependent on growth temperature and preheating temperature, respectively. The height of nanowires was dependent on the TMI flow rate in the InP core nanowire growth, and the thickness of GaInAs shell layer was controlled by adjusting the growth time of the GaInAs shell layer. The photoluminescence (PL) intensity increased with increasing nanowire height and the peak wavelength was controlled by adjusting the thickness of the GaInAs shell layer.

Vertically conducting deep-ultraviolet (DUV) light-emitting diodes (LEDs) with a polarization-induced backward-tunneling junction (PIBTJ) were grown by metal–organic chemical vapor deposition (MOCVD) on 6H-SiC substrates. A self-consistent solution of Poisson–Schrödinger equations combined with polarization-induced theory was applied to simulate the PIBTJ structure, energy band diagrams, and free-carrier concentration distribution. AlN and graded Al_xGa_1−xN interlayers were introduced between the PIBTJ and multiple quantum well layers to avoid cracking of the n-Al_0.5Ga_0.5N top layer. At a driving current of 20 mA, an intense DUV emission at ∼288 nm and a weak shoulder at ∼386 nm were observed from the AlGaN top layer side. This demonstrates that the PIBTJ can be used to fabricate vertically conducting DUV LED on SiC substrates.

We simulated screw dislocations with the Burgers vector parallel to the [0001] direction in 4H-SiC by a classical molecular dynamics method. A stable structure of an extended dislocation generated by the dissociation of a screw dislocation was identified by calculating the strain energy caused by dislocation cores and stacking faults. As a result, we conclude that the most expected structure of the extended dislocation is made of partial dislocations with the Burgers vector b = 1/2c + 1/2c (c is equal to the thickness of one period in the c-axis direction of 4H-SiC) and the stacking fault that is parallel to the a-plane, and that the distance between the dislocation cores is less than about 44 Å.

We demonstrate that compressively strained Si/Si_1−xC_x heterostructures are epitaxially grown on Ar ion implanted Si substrates. The ion-implantation-induced defects are found to promote strain relaxation in the Si_1−xC_x layers, which accompanies an increase in the substitutional C concentrations. The top Si layers are strained on the Si_1−xC_x layers for all samples, and thus the increase in the substitutional C concentration based on strain relaxation in the Si_1−xC_x layers is very important for the control of the compressive strain in the top Si layer.

Metal-induced crystallization was applied to an electrodeposited Ge film on an insulator. It was confirmed that crystallization occurred at 150 °C for 1 h in ambient N₂ and that Cu, which was used as an electrode for plating, started diffusing into the Ge film even at 100 °C. The diffused Cu was distributed uniformly in the film, and the ratio of Cu to Ge was ∼2.5. A fine particulate pattern, attributed to the effect of the Cu diffusion, was observed on the surface by scanning electron microscopy. We considered that the crystallization of the electrodeposited Ge occurred because of the diffusion of Cu from the electroplate electrode. Consequently, (220)-oriented Ge was obtained. The maximum grain size of the crystallized 120-nm-thick Ge film was 240 nm.

The successful formation of abrupt phosphorus (P) δ-doping profiles in germanium (Ge) is reported. When the P δ-doping layers were grown by molecular beam epitaxy (MBE) directly on Ge wafers whose surfaces had residual carbon impurities, more than a half the phosphorus atoms were confined successfully within a few nm of the initial doping position even after the growth of Ge capping layers on the top. On the other hand, the same P layers grown on Ge buffer layers that had much less carbon showed significantly broadened P concentration profiles. Current–voltage characteristics of Au/Ti/Ge capping/P δ-doping/n-Ge structures having the abrupt P δ-doping layers with carbon assistance showed excellent ohmic behaviors when P doses were higher than 1 × 10¹⁴ cm⁻² and the capping layer thickness was as thin as 5 nm. Therefore, the insertion of carbon around the P doping layer is a useful way of realizing ultrashallow junctions in Ge.

Indium is becoming one of the most important dopant species for silicon crystals used in photovoltaics. In this work we have investigated the behavior of indium in silicon crystals grown by the Czochralski pulling process. The experiments were performed by growing 200 mm crystals, which is a standard diameter for large volume production, thus the data reported here are of technological interest for the large scale production of indium doped p-type silicon. The indium segregation coefficient and the evaporation rate from the silicon melt have been calculated to be 5 × 10⁻⁴ ± 3% and 1.6 × 10⁻⁴ cm·s⁻¹, respectively. In contrast to previous works the indium was introduced in liquid phase and the efficiency was compared with that deduced by other authors, using different methods. In addition, the percentage of electrically active indium at different dopant concentrations is calculated and compared with the carrier concentration at room temperature, measured by four-point bulk method.

Chemical mist deposition (CMD) of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was investigated in terms of cavitation frequency f, solvent, flow rate of nitrogen, substrate temperature T_s, and substrate dc bias V_s as variables for efficient PEDOT:PSS/crystalline silicon (c-Si) heterojunction solar cells. The high-speed-camera and differential mobility analysis characterizations revealed that the average size and flux of PEDOT:PSS mist depend on f, type of solvent, and V_s. Film deposition occurred when positive V_s was applied to the c-Si substrate at T_s of 30–40 °C, whereas no deposition of films occurred with negative V_s, implying that the film is deposited mainly from negatively charged mist. The uniform deposition of PEDOT:PSS films occurred on textured c-Si(100) substrates by adjusting T_s and V_s. The adhesion of CMD PEDOT:PSS film to c-Si was greatly enhanced by applying substrate dc bias V_s compared with that of spin-coated film. The CMD PEDOT:PSS/c-Si heterojunction solar cell devices on textured c-Si(100) in 2 × 2 cm² exhibited a power conversion efficiency η of 11.0% with better uniformity of the solar cell parameters. Furthermore, η was increased to 12.5% by adding an AR coating layer of molybdenum oxide MoO_x formed by CMD. These findings suggest that CMD with negatively charged mist has great potential for the uniform deposition of organic and inorganic materials on textured c-Si substrates by suitably adjusting T_s and V_s.

InGaN/GaN self-organized quantum dots can provide useful advantages over quantum wells for the realization of long-wavelength visible light sources because the dots are formed by strain relaxation. A III–nitride based laser emitting in the red (λ ∼ 630 nm), which has not been demonstrated with quantum wells, would be useful for a host of applications. We have investigated the epitaxy and characteristics of self-organized InGaN/GaN multiple layer quantum dots grown by plasma-assisted molecular beam epitaxy and have optimized their properties by tuning the growth parameters. Red-emitting (λ ∼ 630 nm) quantum dots have radiative lifetime ∼2.5 ns and internal quantum efficiency greater than 50%. Edge-emitting red-lasers with multi-dot layers in the active region exhibit an extremely low threshold current density of 1.6 kA/cm², a high temperature coefficient T₀ = 240 K, and a large differential gain dg/dn = 9 × 10⁻¹⁷ cm².

We have investigated the effect of a Ta diffusion barrier layer on the electrical characteristics of AuBe/Au contacts on a p-GaP window layer for AlGaInP-based light-emitting diodes (LEDs). It was shown that after annealing at 500 °C, the AuBe/Ta/Au contacts exhibited nearly 2 orders of magnitude lower specific contact resistance (2.8 × 10⁻⁶ Ω·cm²) than the AuBe/Au contacts (1.0 × 10⁻⁴ Ω·cm²). The LEDs with and without the Ta diffusion barrier layer showed an external quantum efficiency of 14.03 and 13.5% at 50 mA, respectively. After annealing at 500 °C, the AuBe/Ta/Au contacts showed a higher reflectance (92.8% at 617 nm) than that of the AuBe/Au contacts (87.7%). X-ray photoemission spectroscopy (XPS) results showed that the Ga 2p core level for the annealed AuBe/Au samples shifted to higher binding energies, while this level shifted towards lower binding energies for the AuBe/Ta/Au samples. Depth profiles using Auger electron spectroscopy (AES) showed that annealing of the AuBe/Au samples caused the outdiffusion of both Be and P atoms into the metal contact, while for the AuBe/Ta/Au samples, the outdiffusion of Be atoms was blocked by the Ta barrier layer and more Be atoms were indiffused into GaP. The annealing-induced electrical degradation and ohmic contact formation are described and discussed based on the XPS and electrical results.

We report a feasibility study of a terahertz imaging system with resonant tunneling diodes (RTDs) that oscillate at 0.30 THz. A pair of RTDs acted as an emitter and a detector in the system. Terahertz reflection images of opaque samples were acquired with our RTD imaging system. A spatial resolution of 1 mm, which is equal to the wavelength of the RTD emitter, was achieved. The signal-to-noise ratio (SNR) of the reflection image was improved by 6 dB by using polarization optics that reduced interference effects. Additionally, the coherence of the RTD enabled a depth resolution of less than 3 µm to be achieved by an interferometric technique. Thus, RTDs are an attractive candidate for use in small THz imaging systems.

A bulk heterojunction (BHJ) organic photovoltaic cell employing tetra-tert-butyl zinc phthalocyanine as electron donor and [6,6]-phenyl C₆₁-buturic acid methyl ester as electron acceptor has been fabricated. The effect of TiO_x cathode interlayer, the weight ratio of donor:acceptor in the photoactive layer, and the thermal annealing of photoactive layer on the performance of the cells were investigated. The results show that the insertion of TiO_x layer leads to an increase in the photocurrent density of the cells by 11 times compared to those without cathode interlayer. Atomic force microscopy images reveal the formation of well-connected percolated pathways for each phase separated component (donor and acceptor) upon annealing of the film at 150 °C. An important aspect of the present BHJ photovoltaic cell is that it has been obtained by simple wet processes, and most of the fabrication steps have been carried out in ambient air without the use of a glove box.

Conjugated polymers have been widely studied as active materials for organic solar cells, which are a promising replacement for silicon solar cells. A novel electron donor polymer of poly{bi(dodecyl)thiophene-thieno[3,4-c]pyrrole-4,6-dione} (PBTTPD) blended with an electron acceptor, [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM), gave a device power conversion efficiency as high as 7.3%. In the present work, we performed time-resolved absorption change spectroscopy at various mixture ratios of PC₇₁BM in the PBTTPD/PC₇₁BM blend. Among the obtained time constants, the shortest (60 fs) and the longest (500 fs and 15 ps) were assigned to the production and relaxation of charge-transfer (CT) states, respectively. The PC₇₁BM blended with PBTTPD was found to suppress interchain carrier transport and increase intrachain carrier transport. The prolonged lifetime of the CT state in the equally blended PBTTPD/PC₇₁BM film increases the probability of charge separation and thus increases the power conversion efficiency of the device.

The effect of passivation films on a Si quantum dot superlattice (QDSL) was investigated to generate high photocurrent in solar-cell applications. Three types of passivation films, sputter-grown amorphous silicon carbide (a-SiC), hydrogenated a-SiC (a-SiC:H), and atomic-layer-deposited aluminum oxide (ALD-Al₂O₃), were used to passivate the Si QDSLs containing a stack of four 4 nm Si nanodisks (NDs) and 2 nm silicon carbide (SiC) films fabricated by neutral beam etching (NBE). Because of the high surface-to-volume ratio typically present in quantum Si-NDs formed in the top-down NBE process, there is a tendency to form larger surface dangling bonds on untreated Si-ND surfaces as well as to have short distance (<10 nm) between high-aspect-ratio nanopillars of stacked 4 nm Si-NDs/2 nm SiC films, which conventionally sputter SiC films cannot uniformly cover. Therefore, we optimized the passivation techniques with an ALD-Al₂O₃ film. Scanning electron microscopy (SEM) analysis helped to explain the surface morphology before and after the passivation of the QDSLs. After the completion of the passivation process, the quality of the top surface films of the QDSLs was analyzed from the surface roughness by atomic force microscopy (AFM) analysis, which revealed that ALD-Al₂O₃ passivated films had the smallest roughness (RMS) of 1.09 nm with respect to sputter-grown a-SiC (RMS: 1.75 nm) and a-SiC:H (RMS: 1.54 nm) films. Conductive atomic force microscopy (CAFM) revealed that ALD-Al₂O₃ passivation decreased the surface-leakage current as a result of proper passivation of side-wall surface defects in the QDSLs. The carrier transport characteristics were extracted from the QDSLs using the photovoltaic (PV) properties of p⁺⁺/i/n⁺ solar cells, where the QDSLs consisted of different passivation layers acting as intermediate layers (i-layers) between the high-doping-density p⁺⁺ Si (1 × 10²⁰ cm⁻³) and n⁺ Si (1 × 10¹⁹ cm⁻³) substrates. High-doping-density p⁺⁺ Si acted as a hole conductor instead of a photocarrier generator, hence, we could observe the PV properties of the i-layers. The highest short-circuit current density of 4.75 mA cm⁻² was generated from the QDSL with the ALD-Al₂O₃-passivated surface, which is suitable for high-efficiency QD solar cells compared with a-SiC-passivated (0.04 mA cm⁻²) and a-SiC:H-passivated (0.37 mA cm⁻²) QDSL surfaces.

A trace material detection system was developed on the basis of cavity-enhanced absorption spectroscopy (CEAS) using a fiber-coupled passively locked external cavity diode laser (PLEC-DL) in the near-infrared (NIR) wavelength region. The oscillation range of an antireflection-coated diode laser (AR-DL) coupled into an external cavity could be simply selected with a narrowband bandpass filter (1 nm), resulting in a stable wavelength oscillation in the wideband tunability between 1640 and 1680 nm. The external cavity acts as a trace material sensor that exhibits excellent flexibility because it is free from the DL source and is carefully designed with mirrors having reflectivities of ca. 99.995 and 99.99%. Trace-level detection was successfully demonstrated with the developed sensor having a minimum detectable absorption coefficient of 2.4 × 10⁻⁸ cm⁻¹, which corresponds to 0.15 ppm for CH₄ concentration without interference from H₂O absorption lines under atmospheric pressure.

In this paper, we theoretically and experimentally study the performance of an entanglement-based quantum key distribution (QKD) system using single-photon detectors (SPDs) with poor afterpulse characteristics. We reveal that the afterpulse fraction (P_a) in an SPD does not impose a bound on the lowest limit of the error rate in sifted keys of an entanglement-based QKD system. Secure secret key sharing is possible even when P_a is large, for example, exceeding 100%. The system performance in terms of the final key rate is found to be dominated by the parameter η/(1 + P_a) of the SPD, where η is the detection efficiency. The operation conditions of the SPD should be optimized so as to have the maximal η/(1 + P_a), while retaining sufficiently low dark counts. The experimental results were in good agreement with the theoretical predictions. A visibility of 90%, which is sufficiently high for secure secret key sharing in a QKD protocol, was obtained in twofold interference experiments even by using an SPD with P_a exceeding 100%.

Cr₂Ge₂Te₆ (CGT), a layered ferromagnetic insulator, has attracted a great deal of interest recently owing to its potential for integration with Dirac materials to realize the quantum anomalous Hall effect (QAHE) and to develop novel spintronics devices. Here, we study the uniaxial magnetic anisotropy energy of single-crystalline CGT and determine that the magnetic easy axis is directed along the c-axis in its ferromagnetic phase. In addition, CGT is an insulator below the Curie temperature. These properties make CGT a potentially promising candidate substrate for integration with topological insulators for the realization of the high-temperature QAHE.

In this study, the nanoimprint lithography (NIL) technique used to fabricate III–V compound nanowires was investigated. A silicon mold and thermoplastic polymer mr-I 7010R were used for hot embossing nanoimprint lithography. The mold was patterned by e-beam lithography with two masks exposed with different dosages to reduce the proximity effect. The filling capability and residual layer thickness of the thermoplastic polymer were optimized at the embossing temperature of 125 °C. A 73 nm GaAs nanowire was obtained by the mold coated with an antisticking layer.

We study the electronic properties of graphene nanoribbons with zigzag and armchair edges under a parallel electric field generated by two planar electrodes with a potential barrier simulating an insulating layer of electrodes in a FET structure using density functional theory combined with an effective screening medium method. Our calculations show that the nearly free electron (NFE) states strongly depend on the mutual arrangements of graphene nanoribbons with respect to the electric field. In contrast, the electronic energy bands associated with the π electrons are insensitive to the relative direction of the ribbon with respect to the external electric field. We also observe that the electric field concentration around the edges leads to the orientation dependence of the NFE states on the field.

The cyclotron transition line-width for a system of electrons interacting with the flexural wave of phonons confined in a quantum well structure of silicon was calculated using the optical conductivity formula derived by the projection-reduction method. Only a few confined phonons with low energy make a significant contribution to the line-width, which increases with increasing temperature. The well width and magnetic field dependence of the line-width are complicated and the flexural mode contributes to the line-width more strongly than the dilatational mode at low magnetic fields and for small well widths.

The effect of substrate temperature (T_S) and pulse duration (PD) on Mg incorporation, surface quality, and photoresponse properties of MgZnO films grown via PMOCVD were studied. Films grown at T_S ranging from 500 to 700 °C but at identical PDs had band gaps varying from 3.38 to 3.87 eV, corresponding to Mg content between x = 0.06 and 0.27. The film with Mg content of 0.27 was the smoothest and achieved at 630 °C-optimal T_S. Additionally, pulse time effect was studied by growing films at the same T_S but different PDs. A film grown at PD of 12 s has incorporated ∼40% higher Mg than one grown in a continuous mode (PD → ∞), indicting the cruciallity of PMOCVD to realize high Mg film. The peak response spectra of photodetectors were also varied with T_S and PD, in accordance with Mg content in the films.

We studied the growth of Si at the surface of epitaxial graphene on 6H-SiC(0001). Characteristic flower-like islands with a thickness of 2 to 3 nm nucleated during the growth from 290 to 420 K. The islands became featureless in growth at higher temperatures. The growth was predominantly governed by diffusion-limited aggregation. The diffusion energy was evaluated to be 0.21 eV from the temperature-dependent decrease in the density of the islands.

In this investigation, silver (Ag) films of varying thickness (25–100 nm) were grown on cupric oxide (CuO) on silicon and quartz. The CuO preparation was carried out by the thermal oxidation annealing of copper (Cu) thin films deposited by DC magnetron sputtering. The physical properties of the prepared films were studied by different techniques. Rutherford backscattering spectroscopy (RBS) analysis indicated that the Ag film thickness was about 25–100 nm. X-ray diffraction (XRD) results showed that by increasing Ag thickness, the film crystallinity was improved. Also, atomic force microscopy (AFM) and scanning electron microscopy (SEM) results demonstrated that the surface morphology and the grain size were affected by the Ag film thickness. Furthermore, the electrical resistivity of films determined by four-point probe measurements versus the Ag film thickness was discussed. A reduction in the optical band gap energy of CuO is observed from 1.51 to 1.42 eV with an increase in Ag film thickness to 40 nm in Ag/CuO films.

Amorphous indium–gallium–zinc oxide (a-IGZO) films were deposited by DC magnetron sputtering and post-annealed in air at 300–1000 °C for 1 h to investigate the crystallization behavior in detail. X-ray diffraction, electron beam diffraction, and high-resolution electron microscopy revealed that the IGZO films showed an amorphous structure after post-annealing at 300 °C. At 600 °C, the films started to crystallize from the surface with c-axis preferred orientation. At 700–1000 °C, the films totally crystallized into polycrystalline structures, wherein the grains showed c-axis preferred orientation close to the surface and random orientation inside the films. The current–gate voltage (I_d–V_g) characteristics of the IGZO thin-film transistor (TFT) showed that the threshold voltage (V_th) and subthreshold swing decreased markedly after the post-annealing at 300 °C. The TFT using the totally crystallized films also showed the decrease in V_th, whereas the field-effect mobility decreased considerably.

We report on the results of the crystal growth of hen-egg lysozyme by magnetically levitating crystals in a small amount of buffer solution. The concentrations of lysozyme and the precipitating agent (gadolinium chloride) were 6.53 wt % and 0.362 mol/kg, respectively. Gadolinium chloride, which induces the magneto-Archimedes effect, was utilized to levitate the crystals with B_z · (dB_z/dz) = 22.46 T²/m, where B_z is the vertical (z) component of the magnetic flux density vector. Although the collected crystals were small, we succeeded in maintaining the levitation of the crystals into a specific place in the liquid phase from the beginning of nucleation. In situ observation revealed that a state of pseudo-weightlessness was generated in the vicinity of the magnet bore edge, and small crystals were concentrated inside the domain moving along an hourglass-shaped surface. We found by numerical computations that the formation of the hourglass-shaped domain is attributable to the radial component of the magnetic force.

We have developed a heavy-ion computed tomography (IonCT) system using a scintillation screen and an electron-multiplying charged coupled device (EMCCD) camera that can measure a large object such as a human head. In this study, objective with the development of the system was to investigate the possibility of applying this system to heavy-ion treatment planning from the point of view of spatial resolution in a reconstructed image. Experiments were carried out on a rotation phantom using ¹²C accelerated up to 430 MeV/u by the Heavy-Ion Medical Accelerator in Chiba (HIMAC) at the National Institute of Radiological Sciences (NIRS). We demonstrated that the reconstructed image of an object with a water equivalent thickness (WET) of approximately 18 cm was successfully achieved with the spatial resolution of 1 mm, which would make this IonCT system worth applying to the heavy-ion treatment planning for head and neck cancers.

Owing to the atomically small thickness and moderate bandgap of MoS₂, the compound is expected to be a channel material for future short-channel and low-leakage transistors. However, the high contact resistance between a metal and MoS₂ is a serious issue. Although many studies have been conducted to reduce contact resistance, the variability of contact resistance has not been investigated. In this study, we fabricated MoS₂ transistors and evaluated their electrical properties. A large discrepancy in electrical characteristics, which originates from contact resistance variability was observed. We found that the contact resistance variability is due to the peeling of a metal from MoS₂, which originates from the weak cohesion of the metal to MoS₂ and the thermal contraction of the metal. To reduce thermal contraction, a thin contact metal is utilized. As a result, better adhesion of the metal and suppression of contact resistance variability are observed, which indicates that the reduction in the thermal contraction of metals is important to reduce contact resistance and its variability.

Screen printing is a method commonly used for making electrodes for crystalline silicon solar cells. Although the screen-printing method is fast and easy, screen-printed electrodes have a porous structure, high contact resistance, and low aspect ratio. On the other hand, plated electrodes have low contact resistance and narrow electrode width. Therefore, the plating method could be substituted for the screen-printing method in crystalline silicon solar cells. During the plating process, ghost plating can appear at the surface when the quality of the passivation layer is poor, causing an increase in the recombination rate. In this paper, light-induced plating was applied to the fabrication of electrodes, and various passivation layers were investigated to remove ghost plating in crystalline silicon solar cells. These included, (1) SiN_x deposited by plasma-enhanced chemical vapor deposition (PECVD), (2) a double SiN_x layer formed by PECVD, (3) a double layer with thermal silicon oxide and SiN_x deposited by PECVD, and (4) a double layer comprising SiN_x and SiO_x formed by PECVD. For the plated solar cells, a laser was used to remove various antireflection coating (ARC) layers and phosphoric acid was spin-coated onto the doped silicon wafer prior to laser ablation. Also, a screen-printed solar cell was fabricated to compare plated solar cells with screen-printed solar cells. As a result, we found that a thermal SiO₂/PECVD SiN_x layer showed the lowest pinhole density and its wet vapor transmission rate was characterized. The solar cell with the thermal SiO₂/PECVD SiN_x layer showed the lowest J₀₂ value, as well as improved V_oc and J_sc.

We examined the atomic concentrations and the weight densities of SiC surfaces irradiated with remote nitrogen plasmas. The unique approach of this work is that we compared the SiC surface irradiated with atomic nitrogen with that irradiated with a mixture of atomic nitrogen and molecular nitrogen in the metastable $\text{A}^{3}\Sigma _{\text{u}}^{ + }$ state. As a result, it was found that molecular nitrogen in the $\text{A}^{3}\Sigma _{\text{u}}^{ + }$ state has a higher efficiency than atomic nitrogen in the nitriding of SiC surfaces. The weight density measurements have revealed the removal of Si and C from the SiC surface by the irradiation of remote nitrogen plasma. These results suggest that the formation of volatile molecules is less significant when the SiC surface is irradiated with molecular nitrogen in the metastable $\text{A}^{3}\Sigma _{\text{u}}^{ + }$ state.

The effect of implanted carbon (C) on silicon (Si) self-diffusion has been investigated using pre-amorphized ²⁸Si/^natSi multilayers. The isotope multilayers were pre-amorphized by Ge implantation followed by C implantation, and annealed at 950 °C. Because of the presence of C, the Si self-diffusion was slower in 30 min annealing than the self-diffusion without C. This was attributed to the trapping of Si self-interstitials by C. On the other hand, the Si self-diffusion with C was faster in 2 h annealing than the self-diffusion without C, except in the end-of-range (EOR) defect region. The cause of this enhanced diffusion was understood as the retardation of Ostwald ripening of EOR defects by C trapped at the defects. In the EOR defect region, however, Si self-diffusion was slower than the self-diffusion without C in both 30 min and 2 h annealing owing to the presence of C. Relaxation of the tensile strain associated with the EOR defects by the trapped C was proposed to be the main cause of the retarded diffusion in the EOR region.

This study describes our newly fabricated resonant thermal sensors based on vanadium oxide and investigates the temperature dependences of their resonant frequencies and Q factor. The suspended vanadium oxide resonators are microfabricated using Au or SiO₂ as the sacrificial layer. The resonant frequency of the fabricated vanadium oxide resonators linearly varies with temperature, and the value of temperature coefficient of the resonant frequency is −1308 ppm/K in the range of 20–100 °C. The averaged Q factor in this range was 540. The temperature and thermal resolution of the vanadium oxide resonator are estimated as 1.7 mK/$\sqrt{\text{Hz}}$ and 4.3 nW/$\sqrt{\text{Hz}}$, respectively, which are higher than those of a Si resonator having similar dimensions and under similar conditions. Therefore, the feasibility that vanadium oxide is a promising material for resonant thermal sensors is indicated.

In this paper, an underwater acoustic (UWA) communication scheme for mobile platforms is proposed. The proposed scheme is based on the orthogonal signal division multiplexing (OSDM) scheme, which offers highly reliable UWA communication. However, OSDM is not suitable for mobile platforms as it is — it requires a receiver array and a large calculation cost for equalization. To establish a reliable link with small communication platforms, we design OSDM that can perform reliable communication without the need for an array and can reduce receiver complexity using the time-diversity technique (TD), and evaluate its performance in experiments. The experimental results suggest that OSDM-TD can simultaneously achieve power-efficient communications and receiver complexity reduction, and can realize small-scale communication platforms. In detail, OSDM-TD achieved almost the same communication quality as conventional OSDM, in exchange for an effective data rate. Moreover, the power efficiency of OSDM-TD was almost the same as that of conventional OSDM with two receiver array elements, although the calculation cost of OSDM-TD was far below that of conventional OSDM. As a result, it was found that OSDM-TD is suitable for UWA communication for mobile nodes whose capacity and computational resources are severely limited.

We have fabricated planar parabolic refractive X-ray lenses made of quartz glass for high-energy X-ray focusing by optical lithography and dry etching techniques. We succeeded in 100-µm-deep etching, realizing highly efficient microfocusing for high-energy X-rays. We measured the size and photon flux of the X-rays focused by two lenses with a crossed geometry at BL13XU of SPring-8. High-flux (more than 2 × 10⁹ photons/s) microbeams with 25 and 30 keV X-rays were successfully obtained.

We have developed a compact compression test stage for synchrotron radiation (SR) micro-Laue diffraction (MLD) measurements to investigate the deformation behavior of Mg–Zn–Y alloys with long-period stacking-ordered (LPSO) structures. The stage can compress a small sample with a size of 0.3 × 0.3 × 1.0 mm³. The loading can be changed from 0 to 100 N. Using this compression test stage, MLD experiments were performed on a directionally solidified Mg₈₅Zn₆Y₉ alloy polycrystal with a single 18R-type LPSO phase. We succeeded in revealing the change in the grain boundaries with increasing compression loading.

We obtained local optical absorption spectra of MoS₂ with a spatial resolution of approximately 200 nm using scanning near-field optical microscopy with a supercontinuum laser light source, and we found that the absorption spectra exhibited a significant site dependence on the MoS₂ monolayer crystal. We found clear relationships between local optical absorption spectra and photoluminescence intensities. At a site that exhibited a weak photoluminescence, the intensity of the A exciton optical absorption was also weak, and its line shape was significantly deformed, which suggests the influence of n-doping. The results indicate a significant inhomogeneity in the n-doping levels on a single sheet.

A probe method to measure the diffusion coefficient and the ion temperature in the edge plasma is proposed. The probe is constructed as an asymmetric double probe, where one electrode is sensitive to the direction of magnetic field while the other is insensitive to it. The ratio of ion saturation currents to two electrodes is used to determine the diffusion coefficient from which the ion temperature is estimated. The saturation currents are calculated by taking the anisotropic diffusion to the probe into account. The present probe method is applicable in the edge plasma even with such a complicated magnetic field as that in helical devices.

The Langmuir–Blodgett (LB) film based on the ditetradecyldimethylammonium–Au(dmit)₂ [2C₁₄N⁺Me₂–Au(dmit)₂] salt shows a high room-temperature conductivity of 40 S/cm with a metallic temperature dependence. However, the solvent for spreading the material at the air/water interface is a $1:1$ mixture of benzene and acetonitrile, which should be substituted by a less hazardous solvent considering the health effects. Here, we report on the substitution of the solvent by a less hazardous one — a $1:1$ mixture of toluene and acetone; the 2C₁₄N⁺Me₂–Au(dmit)₂ LB film fabricated using the mixture also exhibits a high room-temperature conductivity together with a metallic temperature dependence.

Iron-sulfide-oxide thin films, which are promising candidates for solar cell materials, were deposited by electrochemical deposition. As-deposited and annealed films were characterized by Mössbauer spectroscopy, X-ray diffraction (XRD), and Raman scattering at room temperature. The as-deposited film is amorphous, and the oxygen content is about 1/4 of the sulfur content (S/Fe ≈ 1.5, O/Fe ≈ 0.4). The Mössbauer spectrum for the as-deposited film is a doublet with a broad line profile having hyperfine parameters similar to those of FeS₂ pyrite or marcasite. This indicates that Fe atoms are in the Fe²⁺ low-spin state, as in FeS₂.