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A Compton camera is a gamma-ray imaging device, especially in the sub-mega-electron volt to higher than mega-electron volt range. Compton cameras have recently attracted attention as an environmental survey tool. However, owing to their limited sensitivity, Compton camera images often suffer from various artifacts, especially when the event statistics are low. To address this challenge, several deep learning models have been proposed to enhance the quality of reconstructed images with limited statistics. However, during the event selection phase of a typical Compton camera image reconstruction, a significant number of events that potentially reflect the source distribution are generally discarded. Effective utilization of these discarded events has the potential to estimate an accurate source distribution from limited statistical data. Thus, we initially developed ComptonNet-v1, a framework designed to directly estimate source distribution by integrating all measured events into a single model. To explicitly implement the difference in contribution between events that interact solely with scatterers, solely with absorbers, or with both, we developed ComptonNet-v2, which integrates these events individually. Consequently, our proposed models exhibited superior performance in both quantitative and qualitative assessments compared with existing models, even under low event statistics. In the future, we plan to implement a more memory-efficient model to estimate the distribution of complex source shapes. [ABSTRACT FROM AUTHOR]

Effective cleaning of tin contamination on the collecting mirrors in extreme ultraviolet source is one of the key techniques to improve throughput and cost performance of extreme ultraviolet lithography. Hydrogen radicals produced in hydrogen plasma that is induced by wideband extreme ultraviolet radiation are expected to be utilized for in situ tin contamination cleaning in extreme ultraviolet sources. In this Letter, we clarified absolute density and cleaning ability of the hydrogen radicals produced by intense extreme ultraviolet pulse through ground state population density measurement by laser-induced fluorescence technique. The experimentally obtained radical parameters coincided well with simulation results and collisional radiative model. It was found that the extreme ultraviolet induced plasma was in quasi-steady state with abundant amount of hydrogen radicals in ground state. Further, it was found that the in situ tin contamination cleaning in extreme ultraviolet lithography source would become more practical with increase in operational parameters, such as extreme ultraviolet emission intensity, gas pressure, and radical production cross section. [ABSTRACT FROM AUTHOR]

Epitaxial thin films of ferromagnetic rare-earth nitride, GdN, were synthesized using pulsed laser deposition (PLD) on (001) MgO, pseudo-cubic (001) YAlO3, and (001) TiN buffered (001) MgO substrates. X-ray high-resolution reciprocal space mappings confirmed the epitaxial relationship between GdN and the substrates. The use of a TiN buffer layer changed the growth direction of the GdN films from (001) to (111). The ferromagnetic behavior of the films was characterized, and it was found that the magnetic easy axis could be tuned according to the crystal growth direction. These results suggest that PLD is a viable method for synthesizing epitaxial GdN films with tunable magnetic properties. The ability to control the crystal growth direction and magnetic easy axis of GdN films could be useful for developing spintronic devices. [ABSTRACT FROM AUTHOR]

One of the critical issues in lithography using extreme ultraviolet (EUV) light is tin contamination of the EUV collector mirrors in the tin-based LPP-EUV light source. The contamination can be removed by the reaction of tin atoms with hydrogen radicals producing stannane (SnH4), which is gaseous at the normal temperature. Hydrogen radicals can be produced from hydrogen molecules through photo-dissociation and photo-ionization induced by broadband radiation emitted from the EUV light source. In this work, an efficient production of hydrogen radical using vacuum ultraviolet (VUV) light emitted from laser-produced high-Z plasma is experimentally demonstrated. A two-dimensional radiation hydrodynamic simulation, coupled with photoionization and photo-dissociation cross sections, also shows the efficient hydrogen radical production by increased VUV light emission, as observed in the experiment. [ABSTRACT FROM AUTHOR]

We investigate second-harmonic generation (SHG) light from a Pt surface in atmosphere under terahertz (THz) pulses. THz pulse-modulated SHG intensity, Δ I 2 ω , shows a clear time profile of the THz field, which is similar to that of the conventional electro-optic sampling. The result can be explained by interference between THz field-induced second-harmonic light from air molecules in an optical path and a local oscillator from a Pt surface, whereby heterodyne detection of the THz waveform can be achieved. Using numerical calculations of a wave equation, we discuss the contribution of the Gouy phases of all the pulses, including near-infrared, SHG, and THz pulses, and identification of effective nonlinear susceptibility of the Pt surface. Our method, simply using a polished metal surface and air molecules, does not suffer from phonons or phase matching in solid-state optics and does not require any power supply, bias voltage, or fabrication process, but it offers a simple and gapless sampling method for broadband THz pulses. Here, we demonstrate the gapless detection of a broadband THz pulse in the region of 0.2–20 THz using this method. [ABSTRACT FROM AUTHOR]

We have demonstrated the execution of several programs stored in an instruction memory by a rapid single-flux-quantum bit-serial 8-bit microprocessor named CORE e2h. We employed a minimal instruction set architecture composed of 13 instructions based on the reduced instruction set computer. We integrated a 128-bit instruction memory and a 128-bit data memory with an arithmetic logic unit, two registers, a program counter, an instruction register, and a controller unit on a single chip. The bit-serial operation was performed by an on-chip clock generator, while the system clocks are provided from room-temperature electronics. The CORE e2h was made up of 11 000 Nb/AlOx/Nb Josephson junctions and fabricated with the Advanced Industrial Science and Technology 10-kA/cm2, 9-Nb-layer process. We obtained the correct results for all the executed programs, including the integer division, the two kinds of summation algorithms, the Euclidean algorithm, and finding the greatest divisor. These test programs contained a single or nested double loop, and the maximum number of executed instructions was 205. We confirmed the stable operation with the DC bias margins of around 10% at 50-GHz for different test programs. The measured electric power consumption was 2.5 mW at 4.2 K, and the estimated computing power was 500 × 106 instructions/s. [ABSTRACT FROM AUTHOR]

An ultrafast atomic motion of a photo-induced coherent phonon of bismuth at low temperatures was directly observed with time-resolved x-ray diffraction. A cryostat with a window that is transparent to both optical lasers and x rays enabled versatile diffraction measurements in a wide temperature range including below 10 K. It is found that an atomic displacement in a fully symmetric A1g phonon mode is suppressed at low temperatures. This result indicates that the displacive excitation process is suppressed in the phonon generation with the decrease in temperature. [ABSTRACT FROM AUTHOR]

We demonstrated the generation of perpendicularly spin-polarized electronic currents using a tunnel spin-filtering effect through insulative Fe-rich cobalt ferrite CoxFe3−xO4+δ (I-CFO) barriers with perpendicular magnetic anisotropy (PMA). The I-CFO films grown on conductive Fe-rich cobalt ferrite CoyFe3−yO4 (C-CFO) films, which were deposited on additional I-CFO buffer layers on MgO(001) substrates, exhibited PMA induced by an epitaxial strain. Magnetic tunnel junctions (MTJs), which comprise C-CFO electrode layers, I-CFO barrier layers, and perpendicularly magnetized Co/{Tb/Co}15/Co spin detection layers, showed a tunnel magnetoresistance (TMR) effect. This indicated that spin-polarized tunnel currents were injected into the spin detection layers. A spin injection efficiency of −28% was observed for the MTJs with an I-CFO barrier of 3.0 nm in thickness at 100 K. The voltage dependence of the TMR effect indicates that the spin-injection efficiency is affected by voltage-dependent changes in the effective spin-dependent barrier width. The combination of spinel-type C-CFO and I-CFO films with well-controlled compositions and lattice strains is, therefore, applicable as a spin-injection source for spintronics devices when perpendicularly spin-polarized electronic currents are required. [ABSTRACT FROM AUTHOR]

Epitaxial growth of lead-free (K,Na)NbO3 (KNN) thin films on (001)SrRuO3/Pt/ZrO2/Si substrates was achieved by using RF magnetron sputtering. X-ray diffraction measurements revealed that lattice constants (a1 = 0.3987 nm, a2 = 0.3959 nm, a3 = 0.4011 nm) of epitaxial KNN thin films with a pseudo cubic system were close to KNN single crystals. Vertical piezoelectric force microscopy observation indicated that the spontaneous polarization Ps with a downward direction was dominant, and the epitaxial KNN thin films were naturally polarized. The epitaxial KNN thin films exhibited low relative dielectric constant (εr ∼ 267). In addition, piezoelectric coefficients |e31,f| showed a constant value of about 6.5 C m−2 with the increase in applied positive voltages. Relatively high converse |e31,f| values were obtained at low applied voltages due to an intrinsic piezoelectric effect. Therefore, the epitaxial KNN thin films might enable piezoelectric microelectromechanical systems driven at low applied voltages. [ABSTRACT FROM AUTHOR]

In this study, n-type SiC Schottky barrier diodes (SBDs) with various doping concentrations ( N d = 4 × 10 15 – 1 × 10 19 cm − 3 ) were fabricated, and their forward and reverse current–voltage (I–V) characteristics were analyzed focusing on tunneling current. Numerical calculation with the fundamental formula of tunneling current gives good agreement with experimental forward and reverse I–V curves in the heavily doped SiC SBDs ( N d > 2 × 10 17 cm − 3 ). The analysis of the energy where electron tunneling most frequently occurs revealed that field emission (FE) tunneling dominates conduction instead of thermionic field emission (TFE) under a higher electric field in reverse-biased heavily doped SiC SBDs, while forward I–V characteristics are described only by TFE. In addition, the critical electric field for the TFE–FE transition is quantitatively clarified by carefully considering the sharply changing electric field distribution in SiC with a high donor concentration. [ABSTRACT FROM AUTHOR]

The unprecedented stability and accuracy of optical atomic clocks extend their role not only in frequency metrology but also in fundamental physics and geodesy. In particular, excellent stability of optical lattice clocks accessing a fractional uncertainty of 10 − 18 in less than an hour opens a new avenue for chronometric leveling, which resolves a height difference of one cm in a short averaging time. However, for field use of such clocks, there remains a challenge in developing a transportable system that can operate outside the laboratory. In this Perspective, we describe transportable optical lattice clocks and discuss their future applications to chronometric leveling. [ABSTRACT FROM AUTHOR]

To accelerate the development of GaN power devices, reproducible fabrication of p-type GaN (p-GaN) segments by ion-implantation (I/I) that enables selective-area doping is preferred. In this Letter, the results of time-resolved photoluminescence (PL) and spatially resolved cathodoluminescence (CL) measurements on p-GaN fabricated by I/I are presented. The p-GaN samples of a 220-nm-deep box-shaped Mg concentration profile ([Mg] = 3 × 10 18 cm − 3 ) were fabricated by sequential I/I of Mg and N followed by atmospheric-pressure post-implantation annealing (PIA). During PIA, Mg redistribution was guided by N-I/I-induced vacancies. The intensities of PL peaks originating from Mg acceptors were increased by increasing the PIA temperature (Ta), indicating progressive activation of Mg and simultaneous decrease in the concentration of nonradiative recombination centers (NRCs). As a result, a record-long room-temperature PL lifetime for the near-band edge emission of Ga-polar Mg-implanted p-GaN (1.4 ps) was obtained when Ta = 1300 °C. Because the species of major vacancy-type defects in the present sample was determined as vacancy clusters comprised of few Ga-vacancies (VGa) and N-vacancies (VN) such as (VGaVN)2–3, the NRC concentration is roughly estimated at around 10 17 cm − 3 provided that (VGaVN)2–3 are major NRCs with the electron-capture-coefficient of 7 × 10 − 6 cm 3 s − 1 . Cross-sectional CL linescan images revealed certain activation of Mg from the surface to the leading edge of the box-shaped [Mg] profile, while NRCs remained at the depths right below the box-shaped profile. The results support advantages of using the vacancy-guided Mg redistribution technique for controlling the activated [Mg] profile and realizing p-type Ga-polar GaN:Mg segments by I/I. [ABSTRACT FROM AUTHOR]

We evaluated topological protection for edge and higher-order corner states in topological diffusion systems based on the breathing kagome lattice. In the kagome lattice, the corner states appear at the corner boundary where all three Wannier centers in nontrivial unit cells are located. The three Wannier centers in a unit cell can be placed on the obtuse- and acute-corner boundaries utilizing the armchair boundary, generating topological acute- and obtuse-corner states. For another representative zigzag boundary, only the acute-corner unit cell has three Wannier centers located at the boundary; hence, only the acute-corner state appears. Our band analysis and numerical studies show that the topologically protected decay behavior for armchair boundaries is as robust as that for zigzag boundaries, unlike wave phenomena with space and time periodicities. Our findings can guide the flexible design of topological diffusion applications such as heat localization and recovery systems. [ABSTRACT FROM AUTHOR]

To reduce the threshold current density (Jth) of ultraviolet (UV)-B AlGaN-based laser diodes, we investigated the critical parameters aiming to increase the injection efficiency ηi and the optical confinement factor Γ. Optimization of the thickness of the waveguide layer, the average Al content of the p-type AlGaN cladding layer, and the film thickness of the cladding layer demonstrated that the device characteristics can be improved. This optimization achieved a reduction in Jth to 13.3 kA cm−2 at a lasing wavelength of 300 nm, thus offering the lowest Jth value yet achieved for a UV-B laser diode. [ABSTRACT FROM AUTHOR]

Control of the molecular orientation in a glassy film is a crucial issue, not only for an understanding of the fundamental processes of organic amorphous film formation but also for enhancement of the performance of organic light-emitting diodes (OLEDs) by increasing their light-outcoupling efficiency. In this study, the molecular orientation in codeposited films composed of a host molecule and a disk-shaped emitter that exhibits thermally activated delayed fluorescence is investigated systematically. It is found that the orientation of the transition dipole moment (TDM) of the disk-shaped emitters is strongly dependent on the glass transition temperature and the polarization of the host molecules, and almost perfectly horizontal orientation of the TDM of the disk-shaped emitters can be realized. Our findings clarify the role of the host–guest dipole–dipole interaction in the molecular orientation, and it will enable the expansion of both the molecular design and the material combination rules for high-performance OLEDs. [ABSTRACT FROM AUTHOR]

Electroreflectance (ER) spectra of β-Ga2O3 were measured using Schottky barrier diodes. The ER spectra were well fitted to a third derivative line shape function by assuming three excitonic transitions. The transition energies exhibited moderate blueshifts with an external reverse bias by reflecting the contribution of transitions at M0 type critical points. Optical anisotropy was selectively observed in the polarized ER spectra. The fittings for the third derivative line shape function gave relatively large broadening parameters (0.11–0.89 eV) due mainly to the large exciton-longitudinal-optical-phonon interaction in β-Ga2O3. The ER spectra exhibited a third derivative line shape with a Franz-Keldysh oscillation superimposed on it. The results demonstrate that ER measurements are useful for investigating optical anisotropy in β-Ga2O3 and related alloys. [ABSTRACT FROM AUTHOR]

We report the fabrication of both antidot lattices and unidirectional stripe patterns upon molecular beam epitaxy grown MgZnO/ZnO heterostructures. The magnetoresistance of these high mobility devices exhibits commensurability oscillations associated with ballistic transport of carriers executing orbital motion within the geometry of the imposed modulation. [ABSTRACT FROM AUTHOR]

A high-electron-mobility transistor (HEMT) structure consisting of a 15-nm-thick quaternary AlGaInN top barrier layer, a thin 200-nm-thick unintentionally doped (UID) GaN channel layer, and an AlN back barrier was grown on a single-crystal AlN substrate by metal–organic chemical vapor deposition. The HEMTs fabricated on the grown heterostructure exhibited DC pinch-off characteristics with no large negative resistance owing to the high thermal conductivity of the AlN substrate. The 2-μm-gate-length devices showed the maximum current density of 500 mA/mm and the peak transconductance of 120 mS/mm with the threshold voltage of approximately −4 V. The off-state leakage was less than 30 μA/mm thanks to the thin UID-GaN channel with good crystal quality. The pulsed I–V characteristics for the fabricated HEMTs showed much smaller drain current decreasing compared to the same structured HEMTs with a C-doped GaN back barrier on a SiC substrate. This indicates that the UID-GaN channel grown on the AlN substrate had no harmful impurity levels that cause the current collapse phenomenon. The above-mentioned results demonstrate that the combination of the thin UID-GaN channel and the AlN back barrier works effectively in GaN HEMTs, and it will contribute to the device design and fabrication for future high-power/high-frequency applications. [ABSTRACT FROM AUTHOR]

Magnetic skyrmions, which exhibit Brownian motion in solids, are considered good candidates as information carriers in devices, such as Brownian computers. Voltage control of skyrmions is essential for the ultralow power consumption of such devices. However, the gate operation must be realized with hysteresis-free voltage effects that are independent of ion migration for high-speed devices. In this study, we manipulated the skyrmion diffusion in a Ta|Co-Fe-B|Ta|MgO stacking structure by fabricating a device with a gate introducing an out-of-plane electrical field. Using feedback control, we rectified skyrmion diffusion in one direction, with the number of skyrmions passing through the gate wire from left to right N→ = 28 and from right to left N← = 43. Devices comprising Ta|Co-Fe-B|Pt|MgO junctions were fabricated, and a change in the density of skyrmions was observed upon the application of an out-of-plane electrical field. The creation or annihilation of skyrmions was dependent on the sign of the applied voltage. Furthermore, the skyrmions exhibited no hysteresis during the voltage sweep. Subsequently, the voltage dependence of the hysteresis loops in magneto-optical Kerr signals corresponding to the M–H curve was measured. However, no change was observed, nor was there any change in the saturated magnetization or perpendicular magnetic anisotropy. This result implied the voltage control of the Dzyaloshinskii–Moriya interaction. [ABSTRACT FROM AUTHOR]

Photoluminescence spectroscopy measurements are performed on suspended carbon nanotubes in a field-effect configuration, and the gate voltage dependence of photoluminescence spectra are compared for the pristine and the molecularly adsorbed states of the nanotubes. We quantify the molecular screening effect on the trion binding energies by determining the energy separation between the bright exciton and the trion emission energies for the two states. The voltage dependence shows narrower voltage regions of constant photoluminescence intensity for the adsorbed states, consistent with a reduction in the electronic bandgap due to screening effects. The charge neutrality points are found to shift after molecular adsorption, which suggests changes in the nanotube chemical potential or the contact metal work function. [ABSTRACT FROM AUTHOR]

Scanning transmission electron microscopy-energy dispersive X-ray spectroscopy measurements performed on Cu(In,Ga)Se2 (CIGS) thin-film devices grown with RbF postdeposition treatment (RbF-PDT) revealed the formation of a RbInSe2 compound at the p-CIGS/n-CdS heterointerface. However, this type of Rb-compound formation was not observed with elemental In-free ternary CuGaSe2 (CGS) devices. The film surface of CIGS grown with RbF-PDT was found to be Ga-depleted; thus, the practical interface structure turned out to be CIGS/(CuInSe2, RbInSe2)/CdS. In contrast to the significant improvements observed in In-containing CIGS photovoltaic device performance with RbF-PDT, no significant improvements have been observed in In-free CGS devices thus far. These results suggest that the presence of elemental In plays a key role in obtaining beneficial alkali Rb effects for enhancing device performance as well as surface modification with RbF-PDT. [ABSTRACT FROM AUTHOR]

We present nanoscale friction measurements performed on sputter-deposited high entropy alloy (HEA) sulfide thin films [(VNbTaMoW)S2] via atomic force microscopy. The results reveal (i) the influence of deposition time on the film morphology and (ii) the presence of isolated areas of low friction on film surfaces. We compare the friction results on HEA sulfide thin films with those on a prototypical solid lubricant, sputter-deposited molybdenum disulfide (MoS2), and find that they are superior in terms of lubricative performance. Variable temperature x-ray diffraction, performed up to 973 K, reveals that HEA sulfide thin films exhibit improved oxidation resistance when compared with MoS2 films. Combined, our results show that HEA sulfide thin films have considerable potential as oxidation-resistant solid lubricant coatings. [ABSTRACT FROM AUTHOR]

We investigated the spin transport in nano-scale silicon (Si)-based spin-valve devices with Fe electrodes, MgO/Ge tunnel barriers, and a 20 nm-long Si channel. We observed a clear spin-valve effect when a magnetic field was applied in the film plane along and perpendicular to the Si channel transport direction. Systematic investigations of the bias voltage dependence, temperature dependence, and magnetic-field direction dependence of the magnetoresistance indicate that the observed spin-valve effect is governed by the spin transport through the nano-scale Si channel. The spinvalve effect remains observable up to 200 K. For the device with MgO/Ge tunnel barriers, with a bias voltage of 1.7V at 50K, the spin-dependent output voltage is 13 mV, which is among the highest values reported so far. [ABSTRACT FROM AUTHOR]

The magnetic structure and the effect of a magnetic field on its domain structure were investigated in a magnetoelectric Y-type hexaferrite, Ba1.3Sr0.7CoZnFe11AlO22, by means of mapping with a micro-focused and circularly polarized X-ray beam in the resonant X-ray diffraction. It was revealed that this hexaferrite exhibits a magnetic order characterized by two distinct antiferromagnetic components: incommensurate helical and commensurate collinear ones, which can be explained as the development of the so-called alternating longitudinal conical structure. A multidomain state due to the handedness of the helical component, i.e., spin-chirality, is transformed into nearly a mono-domain one by using only a magnetic field. Furthermore, the sign of the spinchirality in the mono-domain state is reversed by reversing the sign of a magnetic field. These results demonstrate that the spin-chirality in this hexaferrite can be manipulated by a magnetic field alone at room temperature. [ABSTRACT FROM AUTHOR]

Graphene was used as the topmost electrode for a metal-oxide-semiconductor planar-type electron emission device. With several various layers, graphene as a gate electrode on the thin oxide layer was directly deposited by gallium vapor-assisted chemical vapor deposition. The maximum efficiency of the electron emission, defined as the ratio of anode current to cathode current, showed no dependency on electrode thickness in the range from 1.8 nm to 7.0 nm, indicating that electron scattering on the inside of the graphene electrode is practically suppressed. In addition, a high emission current density of 1–100 mA/cm2 was obtained while maintaining a relatively high electron emission efficiency of 0.1%–1.0%. The graphene-oxide-semiconductor planar-type electron emission device has great potential to achieve both high electron emission efficiency and high electron emission current density in practical applications. [ABSTRACT FROM AUTHOR]

We present an interesting property of broadband coherent radiation emitted by a single microbunch, or a single-cycle density modulation in an electron beam passing through an undulator, and describe its application to a frequency upconversion of the single-cycle modulation. This is based on the fact that a monocycle pulse is generated by focusing the coherent radiation with a unit-magnification optical system, which propagates without diffraction like the Bessel beam, at a reduced group velocity equal to the average electron velocity in the undulator. Calculations show that a single microbunch with a length of 46-nm formed in a 2-GeV and 2-kA electron beam can be upconverted to a 4-nm long microbunch through interaction with focused coherent radiation. [ABSTRACT FROM AUTHOR]

Nanoscale crystallographic textures have received very little attention in research on surface plasmons using metallic nanostructures. A single-crystalline metallic nanostructure with a controlled crystallographic texture is expected to reduce optical losses. We elucidated the grain growth mechanism in silver thin films deposited on a highly transparent SiO2 substrate by electron backscatter diffraction methods with nanoscale resolution. At higher substrate temperatures, the grain growth was facilitated but the preferred orientation was not achieved. Moreover, we fabricated a single-crystalline silver nanopillar in a (111)-oriented large growing grain, which was controlled by varying the substrate temperature during film deposition by focused ion-beam milling. Furthermore, the light intensity of the scattering spectrum was measured for a single-crystalline silver nanopillar (undersurface diameter: 200 nm) for which surface plasmon resonance was observed. The single-crystalline silver nanopillar exhibits a stronger and sharper spectrum than the polycrystalline silver nanopillar. These results can be applied to the direct fabrication of a single-crystalline silver nanopillar using only physical processing. [ABSTRACT FROM AUTHOR]

Electronic state of charge carriers, in particular, in highly doped regions, in thin-film transistors of a semicrystalline conducting polymer poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene), has been studied by using field-induced electron spin resonance (ESR) spectroscopy. By adopting an ionic-liquid gate insulator, a gate-controlled reversible electrochemical hole-doping of the polymer backbone is achieved, as confirmed from the change of the optical absorption spectra. The edge-on molecular orientation in the pristine film is maintained even after the electrochemical doping, which is clarified from the angular dependence of the g value. As the doping level increases, spin 1/2 polarons transform into spinless bipolarons, which is demonstrated from the spin-charge relation showing a spin concentration peak around 1%, contrasting to the monotonic increase in the charge concentration. At high doping levels, a drastic change in the linewidth anisotropy due to the generation of conduction electrons is observed, indicating the onset of metallic state, which is also supported by the temperature dependence of the spin susceptibility and the ESR linewidth. Our results suggest that semicrystalline conducting polymers become metallic with retaining their molecular orientational order, when appropriate doping methods are chosen. [ABSTRACT FROM AUTHOR]

Current collapse is suppressed up to 800V of drain voltage in our proposed device, Hybrid-Drainembedded Gate Injection Transistor (HD-GIT), where an additional p-GaN layer is grown on the AlGaN barrier layer and is connected to the drain electrode. We present, based on a device simulation and electroluminescence study, that the hole injection from the additional drain-side p-GaN at the OFF state compensates the hole emission in the epilayer. As a result, the gate-drain access region is not negatively charged at the OFF state, resulting in the drastic suppression of current collapse in HD-GIT. [ABSTRACT FROM AUTHOR]

Vertical-cavity organic lasers are fabricated with melt-grown crystals of a cyano-substituted thiophene-phenylene co-oligomer. Due to lying molecular orientation, surface-emitting lasing is achieved even in the half-cavity crystal grown on a distributed Bragg reflector (DBR) under optical pumping at room temperature. Anticrossing splits in angle-resolved photoluminescence spectra suggest the formation of exciton-polaritons between the cavity photons and the confined Frenkel excitons. By constructing the full-cavity structure sandwiched between the top and bottom DBRs, the lasing threshold is reduced to one order, which is as low as that of the half cavity. Around the threshold, the time profile of the full-cavity emission is collapsed to a pulsed shape accompanied by a finite turn-on delay. We discuss these observed characteristics in terms of a polariton contribution to the conventional photon lasing. [ABSTRACT FROM AUTHOR]

To improve the efficiencies of electronic devices, materials with variable work functions are required to decrease the energy level differences at the interfaces between working layers. Here, we report a method to obtain silver nanoparticles with tunable work functions, which have the same silver core of 5 nm in diameter and are capped by myristates and 1-octanethoilates self-assembled monolayers, respectively. The silver nanoparticles capped by organic molecules can form a uniform two-dimensional sheet at air-water interface, and the sheet can be transferred on various hydrophobic substrates. The surface potential of the two-dimensional nanoparticle sheet was measured in terms of Kelvin probe force microscopy, and the work function of the sheet was then calculated from the surface potential value by comparing with a reference material. The exchange of the capping molecules results in a work function change of approximately 150-250 meV without affecting their hydrophobicity. We systematically discussed the origin of the work function difference and found it should come mainly from the anchor groups of the ligand molecules. The organic molecule capped nanoparticles with tunable work functions have a potential for the applications in organic electronic devices. [ABSTRACT FROM AUTHOR]

We observed clear memristive switching of the tunnel resistance of magnetic tunnel junctions consisting of MnAs/GaAs/AlAs/GaAs:MnAs nanoparticles when a loop of pulse currents was applied on the junctions. Here, the GaAs:MnAs layer contains NiAs-type hexagonal MnAs ferromagnetic nanoparticles embedded in a GaAs matrix. The memristive switching was observed at current densities as low as 103A/cm2 and was insensitive to external magnetic fields. A model of memristive switching was proposed assuming the trap and release of space charges in the GaAs matrix that affect the electrostatic potential of the MnAs nanoparticles under the Coulomb blockade regime. Our model is consistent with the observed temperature dependence of the memristance ratio. [ABSTRACT FROM AUTHOR]

We present a comparative study on the hydrodynamic behaviour of plasmas generated by material ablation by the irradiation of nanosecond extreme ultraviolet (EUV or XUV) or infrared laser pulses on solid samples. It was clarified that the difference in the photon energy deposition and following material heating mechanism between these two lights result in the difference in the plasma parameters and plasma expansion characteristics. Silicon plate was ablated by either focused intense EUV pulse (λ=9-25 nm, 10 ns) or laser pulse (λ=1064 nm, 10 ns), both with an intensity of ~109W/cm². Both the angular distributions and energy spectra of the expanding ions revealed that the photoionized plasma generated by the EUV light differs significantly from that produced by the laser. The laser-generated plasma undergoes spherical expansion, whereas the EUV-generated plasma undergoes planar expansion in a comparatively narrow angular range. It is presumed that the EUV radiation is transmitted through the expanding plasma and directly photoionizes the samples in the solid phase, consequently forming a high-density and high-pressure plasma. Due to a steep pressure gradient along the direction of the target normal, the EUV plasma expands straightforward resulting in the narrower angular distribution observed. [ABSTRACT FROM AUTHOR]

A single layer of graphene with dimensions of 20 mm x 20 mm was grown directly on an insulating substrate by chemical vapor deposition using Ga vapor catalysts. The graphene layer showed highly homogeneous crystal quality over a large area on the insulating substrate. The crystal quality of the graphene was measured by Raman spectroscopy and was found to improve with increasing Ga vapor density on the reaction area. High-resolution transmission electron microscopy observations showed that the synthesized graphene had a perfect atomic-scale crystal structure within its grains, which ranged in size from 50 nm to 200 nm. [ABSTRACT FROM AUTHOR]

High speed imaging of high power impulse magnetron sputtering discharges has revealed that ionization is localized in moving ionization zones but localization disappears at high currents for high yield targets. This offers an opportunity to study the effect ionization zones have on ion energies. We measure that ions have generally higher energies when ionization zones are present, supporting the concept that these zones are associated with moving potential humps. We propose that the disappearance of ionization zones is caused by an increased supply of atoms from the target which cools electrons and reduces depletion of atoms to be ionized. [ABSTRACT FROM AUTHOR]

Single crystals of La(O,F)BiSSe were grown by using a CsCl flux method. Single crystal X-ray structural analysis reveals that the crystal structure is isostructural to the BiS2- or BiSe2-based compounds crystallizing with space group P4/nmm (lattice parameters a=4.1110(2)Å, c=13.6010(7)Å). However, the S atoms are selectively occupied at the apical site of the Bi-SSe pyramids in the superconducting layer. The single crystals show a superconducting transition at around 4.2K in the magnetic susceptibility and resistivity measurements. The superconducting anisotropic parameter is determined to be 34-35 from its upper critical magnetic field. The anisotropy is in the same range with that of other members of the La(O,F)BiCh2 (Ch=S, Se) family under ambient pressure. [ABSTRACT FROM AUTHOR]

A single layer of graphene with dimensions of 20 mm × 20 mm was grown directly on an insulating substrate by chemical vapor deposition using Ga vapor catalysts. The graphene layer showed highly homogeneous crystal quality over a large area on the insulating substrate. The crystal quality of the graphene was measured by Raman spectroscopy and was found to improve with increasing Ga vapor density on the reaction area. High-resolution transmission electron microscopy observations showed that the synthesized graphene had a perfect atomic-scale crystal structure within its grains, which ranged in size from 50 nm to 200 nm. [ABSTRACT FROM AUTHOR]

We adopt superconductor/ferromagnet/insulator/superconductor (SFIS) Josephson junctions (JJs) as both switching JJs and intrinsic π phase shifters in superconductor digital circuits. The critical current density (Jc) and characteristic voltage (Vc) of the SFIS junctions are about 22 A/cm2 and 22 μV, respectively. The intrinsic π phase shift is confirmed by measuring the suppressed nominal critical current Icn and half-period-shifted modulation pattern of a π–π–π superconducting quantum interference device (SQUID) that contains three π-JJs in a superconducting loop. A single-flux-quantum (SFQ) circuit composed of a DC/SFQ, Josephson transmission line (JTL), and SFQ/DC converter based on SFIS JJs alone is demonstrated at 4.2 K. The energy dissipation of the SFQ/DC converter decreases by 80% because some JJs are self-biased by the π phase shifter. The intrinsic circulating currents induced by the π phase shifters lead to a narrow bias margin (±5%) and even error function, which can be solved by parameters optimization or circuit initialization in the future. The half-modulation period (Φ0/2) of a half-flux-quantum (HFQ) SQUID (a partial HFQ JTL) exhibits propagation of HFQ between π–π–π SQUIDs, indicating that more complex HFQ circuits can be developed with π-JJs alone in the future. [ABSTRACT FROM AUTHOR]

Artificial superlattice thin films of lead zirconate titanate (PZT) were epitaxially grown on silicon substrates, and the influence of superlattice strain on their piezoelectric properties was investigated. The c-axis oriented PZT superlattice thin film consists of two different PZT layers, Pb(Zr0.65Ti0.35)O3 (PZT-65) and Pb(ZrxTi1−x)O3 (PZT-X: x = X/100 = 0.3–0.9), with a 4 nm period. Satellite peaks were clearly observed in x-ray diffraction patterns, and cross-sectional composition measurements confirmed the superlattice structure with good interfaces, showing an alternate change in Zr and Ti compositions. Ferroelectric properties varied significantly depending on the PZT-X composition, and in particular, the PZT-65/PZT-30 superlattice thin film showed nearly the same ferroelectricity as the tetragonal phase under a large compressive strain of PZT-65 from the PZT-30 layer. For the PZT-65/PZT-X (X = 30–58) superlattice thin films, the PZT-65 layers received a compressive strain, and a relatively large piezoelectric coefficient, which did not depend on the PZT-X composition, was obtained. However, a decrease in piezoelectricity was observed in PZT-65/PZT-X (X = 70 and 90), where the PZT-65 layers experienced tensile strain. This result indicates that the piezoelectric properties of PZT superlattice thin films can be controlled by the interlayer strain. [ABSTRACT FROM AUTHOR]

We performed the shot noise measurement in epitaxial Fe/MgO/Fe-based magnetic tunneling junctions (MTJs) with various MgO thicknesses between 1.1 nm and 1.625nm. While the Fano factor to characterize the shot noise is very close to 1 in MTJs with MgO barrier thicker than 1.2nm, the magnetic configuration-dependent reduction of the Fano factor for MTJs with thin MgO barrier was observed, which is mainly due to the existence of leakage current. By using a simple parallel circuit model, we demonstrated that the contribution of the leak current can be sensitively derived from the shot noise. [ABSTRACT FROM AUTHOR]

Charge carriers in high-mobility organic thin-film transistors of alkylated dinaphthothienothiophene (C10-DNTT) have been directly observed by field-induced electron spin resonance (FI-ESR) down to 4K. FI-ESR spectra of π-electron hole carriers of C10-DNTT exhibited clear anisotropy, indicating a highly organized end-on molecular orientation at the device interface. The intra-grain and inter-grain carrier motion were probed by the motional narrowing effect of the ESR spectra. The intra-grain motion was clearly observed even at 4K, showing intrinsically high mobility of C10-DNTT crystallites. On the other hand, significantly low activation energy of ~10 meV for inter-grain carrier hopping, compared with pristine DNTT, was observed, which shows that the alkyl substitution drastically enhances the carrier mobility of DNTT system. [ABSTRACT FROM AUTHOR]

We have investigated the film morphology and the field effect mobility of 2,7- dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) thin films which were formed by spin coating on the SiO2 substrate with solution-processed graphene electrodes. The domain size and the density of aggregates in the C8-BTBT film showed the same dependence on the spinning speed. These competitive two factors (domain size and density of aggregates) give an optimum spinning speed, at which the field effect mobility of C8-BTBT transistor showed a maximum (2.6 cm2/V s). This result indicates the importance of spinning speed in the fabrication of solution processed organic thin film transistors by spin coating. [ABSTRACT FROM AUTHOR]

Photoelectric emission from organic and metal thin films is generally observed with irradiation of photon energy larger than 4 eV. In this paper, however, we report photoelectric emission from Ag on a zinc-phthalocyanine (ZnPc) layer at a photon energy of 3.4 eV. The threshold energy for this photoelectric emission is much smaller than the work function of Ag estimated by conventional photoelectron spectroscopy. The photoelectric emission by low-energy photons is significant for Ag thicknesses of less than 1 nm. Photoelectron spectroscopy and morphological study of the Ag/ZnPc suggest that the anomalous photoelectric emission from the Ag surface is caused by a vacuum level shift at the Ag/ZnPc interface and by surface plasmons of the Ag nanoparticles. [ABSTRACT FROM AUTHOR]

Random surface texturing of an optically thick film to increase the path length of scattered light rays, first proposed nearly thirty years ago, has thus far remained the most effective approach for photon absorption over the widest set of conditions. Here, using recent advances in computational electrodynamics, we describe a general strategy for the design of a silicon thin film applicable to photovoltaic cells based on a quasi-resonant approach to light trapping where two partially disordered photonic-crystal slabs, stacked vertically on top of each other, have large absorption that surpasses the Lambertian limit over a broad bandwidth and angular range. [ABSTRACT FROM AUTHOR]

Optical absorptance data, and a strong correlation between solar cell open circuit voltages and the ionization potentials of a wide range of differently substituted fullerene acceptors, are presented as empirical evidence for identical, or at least very similar, band gaps in all substituted C60 and C70 based fullerenes. Both the number and kind of substituents in this study are sufficiently varied to imply generality. While the band gaps of the fullerenes remain the same for all the different substitutions, their ionization potentials vary greatly in a span of more than 0.4 eV. The merits and drawbacks of using these results together with photoelectron based techniques to determine relative fullerene energy levels for, e.g., organic solar cell applications compared to more direct electrochemical methods are also discussed. [ABSTRACT FROM AUTHOR]

A major concern in organic solar cells today is to continue to improve their stability. Aluminum-titanium bilayer cathodes have been reported to prolong the shelf-lives of organic solar cells to be more than 3 months in air without encapsulation. However, the stabilization mechanism was unsolved, and further investigation into the mechanism was required. We probed the surface morphology of different cathodes with atomic force microscope and explored elemental depth profiles in different devices with time of flight-secondary ion mass spectrometry to analyze the mechanism, in addition to optimization of the cell structure. Based on the experimental observations, we attribute the improved shelf-lives to three functions of the bilayer cathodes, e.g., the elimination of large cracks, the suppression of interdiffusion, and the sustainable oxygen scavenging. These findings provide information useful for rationally designing efficient structures that stabilize organic solar cells. [ABSTRACT FROM AUTHOR]

Plasmon hybridization in a stacked pair of graphene ribbons is investigated at terahertz frequencies. We fabricated and characterized an array of graphene ribbons on a heavily-doped Si substrate with a SiO2 gap layer in between. The interaction between graphene plasmons and their mirror images in the substrate forms a coupled system, mimicking a graphene ribbon pair, thereby dramatically modifies the mode profiles and resultant spectral responses. Magnetic plasmon resonances arising from the plasmon hybridization are demonstrated at terahertz frequencies. The corresponding numerical simulations reveal the important resonant behavior of structured graphene and their hybridized systems. [ABSTRACT FROM AUTHOR]

Thermal strain effect on the piezoelectricity was investigated for (Na,Bi)TiO3-BaTiO3 (NBT-BT) thin films on various substrates with (100) and (110) orientations. The compressive-thermal strain caused an increase in the remnant polarization unaccompanied by a decrease in dielectric permittivity, which resulted in enhancement of the piezoelectric coefficient, e*31. For films on (110) substrates, the crystal lattices were distorted to orthorhombic symmetry under both the tensile- and compressive-strains and the in-plane anisotropy of piezoelectricity emerged with e*31 as large as -19 to -20.6 C/m2. We conclude that the large piezoelectricity in NBT-BT films on (110) substrates is related to the appearance of orthorhombic phase. [ABSTRACT FROM AUTHOR]

Intermediate band (IB) solar cell structures based on ZnTeO highly mismatched alloy were examined to demonstrate a photocurrent induced by a two-photon excitation (TPE) process. Two types of the devices, with and without a blocking layer for the IB, are prepared. The device with a blocked IB exhibits small external quantum efficiency (EQE) in photon energy range in which electron transitions from valence band (VB) to IB take place, implying the electron accumulation in IB. The enhancement of EQE is observed in TPE experiments as a result of electron transition from VB to conduction band via IB. [ABSTRACT FROM AUTHOR]

Tensile-strained Si pillars are desired to achieve three-dimensional (3D) transistors with high speed. Effects of ultraviolet (UV) light irradiation on tensile strains in Si pillars covered with SiN:H stress-liners are investigated as a function of the wavelength (172–436 nm). Tensile-stress enhancement is found at specific wavelengths (200–330 nm) under low-temperature heating (400 °C), where dehydrogenation in SiN:H is clearly detected by infrared absorption measurements. On the other hand, equilibrium dehydrogenation by high-temperature heating (>700 °C) without UV-irradiation does not cause tensile-stress enhancement. This nonequilibrium dehydrogenation at low temperatures opens up possibility of 3D transistors with high carrier mobility. [ABSTRACT FROM AUTHOR]