Since the last decade, research on plasma catalysis has attracted keen attention as an emerging type of low-carbon technology. An advantage of plasma is to facilitate non-equilibrium reaction fields on a large scale, which is inaccessible by conventional thermal approaches. Stable molecules such as CO2 and CH4 are activated by electrical energy, paving the way for low-temperature chemistry that departs from energy-intensive heat-dependent systems. Moreover, the power-to-chemical concept could gain momentum with plasma technologies that are driven by renewable energy. Currently, research is accelerating with application initiatives, but at the same time the importance of scientific understanding of plasma catalytic reactions is being recognized more than ever. This review article offers an overview of various plasma technologies in the "plasma alone" and "plasma–catalyst combination" context. Plasma–catalyst combination technology, known as "plasma catalysis", is discussed further to dry methane reforming (CH4 + CO2 = 2CO + 2H2) and the reverse water gas shift reaction (CO2 + H2 = CO + H2O) for a mechanistic insight.
Invited Reviews
Invited Reviews are comprehensive reports of the background and state-of-the-art research on a specific field in applied physics. They provide an overview of the field to nonspecialists and aim to broaden the scope of applied physics.
Recent achievements in the research of heterojunctions fabricated using surface activated bonding (SAB), one of the practically useful direct wafer bonding technologies, are discussed. The response of bonding interfaces to post-bonding annealing is focused. These junctions reveal high thermal tolerance (1000 °C in the case of junctions made of widegap materials) despite differences in coefficients of thermal expansion between bonded materials. Defect layers with several nm thickness formed by the surface activation process at the as-bonded interfaces get faint and their electrical and mechanical properties are improved by annealing. These results show that as-bonded interfaces are in a metastable state, and novel functional devices are likely to be realized by applying wafer processing steps to SAB-based junctions. Characteristics of III–V//Si multijunction solar cells, GaN-on-diamond high electron mobility transistors, and metal-foil based low-loss interconnects that are fabricated by processing SAB-based junctions are described, and future prospects are presented.
We review photoluminescence processes due to donor and acceptor impurities with concentrations ranging from 1 × 1010 to 1 × 1020 cm−3 in both uncompensated and compensated Si at 4.2 K for application to the impurity characterization. Systematic evolution and extinction were observed in the impurity bound exciton and impurity cluster bound exciton emission, donor-acceptor pair emission with and without discrete sharp lines, donor-band—to—acceptor-band emission, and the emission involving the intrinsic degenerate band. These variations occur as results of the transition from isolated impurity levels to interacting impurity levels due to impurity pairing or clustering in the concentration range around 1 × 1016 cm−3, the formation of the impurity bands in the range mid 1017 cm−3, and their unification with the intrinsic bands at critical Mott concentration of 3 × 1018 cm−3. Multiple radiative processes coexist in the transition regions and their intensities are determined by the impurity concentrations and the radiative probability of respective processes.
Low-temperature plasma plays various roles in industrial material processing as well as provides a number of scientific targets, both from theoretical and experimental points of view. Such rich features in variety are based on its complexities, arising from diverse parameters in constituent gas-phase species, working gas pressure, input energy density, and spatial boundaries. When we consider causalities in these complexities, direct application of machine-learning methods is not always possible since levels of complexities are so high in comparison with other scientific research targets. To overcome this difficulty, progresses in plasma diagnostics and data acquisition systems are inevitable, and the handling of a large number of data elements is one of the key issues for this purpose. In this topical review, we summarize previous and current achievements of visualization, acquisition, and analysis methods for complex plasma datasets which may open a scientific and technological category mixed with rapid machine-learning advancements and their relevant outcomes. Although these research trends are ongoing, many reports published so far have already convinced us of various expanding aspects of low-temperature plasma leading to the potential for scientific progress as well as developments of intellectual design in industrial plasma processes.
This paper reviews the status of characterization of defects in β-Ga2O3 crystals grown by edge-defined film-fed growth and hydride vapor phase epitaxy using chemical etching, scanning electron microscopy, focused ion beam scanning ion microscopy, X-ray topography (XRT), and transmission electron microscopy (TEM). The observed defects are classified into four types: dislocations, stacking faults (SFs), twins, and plate-like nanovoids (PNVs). First, we present the detailed characterization of dislocations in the crystal by chemical etching, XRT, and TEM, and discuss possible slip systems. Next, we describe XRT analyses of two types of SFs: SFs 1 lying on the (
) plane and SFs 2 on the (111) and (
) planes. We describe the results for twins found in crystals via high-resolution TEM and electron diffraction analysis, and PNVs corresponding to etch pits on the (010) plane. Finally, we discuss possible generation mechanisms of the defects and their influence on device characteristics.
CuInSe2 and its related I–III–VI2 chalcopyrite compounds have been studied for about 50 years for application to absorbers in polycrystalline thin-film solar cells. Several research groups developed Cu(Ga,In)Se2, Cu(Ga,In)(S,Se)2, and (Cu,Ag)(Ga,In)Se2 solar cells with conversion efficiencies higher than 20%. The bandgap energies of these absorber materials in high-efficiency solar cells are less than 1.2 eV. Currently, wide bandgap chalcopyrite compounds, such as Ga-rich Cu(Ga,In)Se2, Cu(Ga,In)S2, and Ga-rich (Cu,Ag)(Ga,In)Se2, have gained attention as absorber materials for the top cells in tandem structure solar cells. However, the conversion efficiency of these wide bandgap solar cells cannot reach 20%. I–III–VI2 chalcopyrite compounds are composed of three or more elements and their crystal and electronic structures are more complicated than III–V or II–VI compound semiconductors. In particular, phase diagrams containing the I–III–VI2 chalcopyrite compounds are complex and vary greatly depending on the material systems. This article provides an overview of the current state of our materials science understanding of I–III–VI2 chalcopyrite compounds. In particular, we discuss the differences between CuInSe2 and the other chalcopyrite compounds based on their phase diagrams of I2VI–III2VI3 pseudobinary systems such as Cu2Se–In2Se3.
Plasma processing plays an important role in manufacturing leading-edge electronic devices such as ULSI circuits. Reactive ion etching achieves fine patterns with anisotropic features in metal-oxide-semiconductor field-effect transistors (MOSFETs). In contrast, it has been pointed out over the last four decades that plasma processes not only modify the surface morphology of materials but also degrade the performance and reliability of MOSFETs as a result of defect generation in materials such as crystalline Si substrate and dielectric films. This negative aspect of plasma processing is defined as plasma (process)-induced damage (PID) which is categorized mainly into three mechanisms, i.e. physical, electrical, and photon-irradiation interactions. This article briefly discusses the modeling of PID and provides historical overviews of the characterization techniques of PID, in particular, by the physical interactions, i.e. ion bombardment damage.
By actively using not only charge transport of electrons and holes but also their spins, we can create a variety of new phenomena and functional materials. It is highly expected that these new materials and phenomena are applied to the development of next-generation electronics and information technology. This field, called spintronics, is emerging and rapidly making progress in many subfields. This article focuses on one of the most important materials, ferromagnetic semiconductors (FMSs) which have the properties of both ferromagnets and semiconductors, and their applications. We review the recent progress in the studies of FMSs and their applications, particularly, spin transistors.
The fascinating point of 2D and layered materials is that they can be assembled into van der Waals (vdW) heterostructures, in which atomic layers are integrated by vdW force. There are almost infinite potential combinations in vdW heterostructures owing to the multiple degrees of freedom, i.e., the choice of materials, stacking order, and lateral orientation angle at the interfaces. In this article, we review the fabrication technique of vdW heterostructures, which has played an essential role in the development of the 2D materials research field. First, we describe the primary technique of mechanical exfoliation to fabricate and identify high-quality atomic layers. We then discuss the assembly of atomic layers into vdW heterostructures. Finally, we introduce the recent advancement of fabrication techniques using autonomous robotic assembly. We hope this article would help the readers to acquire basic knowledge of vdW assembly and motivate them to fabricate vdW heterostructures.
The femtosecond laser amplifier has attracted much attention as a promising tool for cell processing and manipulation. The focused pulse with high peak intensity induces not only 3D selective laser ablation, but effective generation of a shockwave and stresswaves. Under a microscope, these waves act on cells near the laser focal point as an impulsive force. This force has been applied as an external trigger to estimate intercellular adhesion strength, to manipulate single cells and to investigate the mechanobiology. In this review, we explain the kinetics of the impulsive force based on the femtosecond laser ablation mechanism, and introduce applications to the evaluation of intercellular adhesion, which is an essential issue in cell research for elucidating tissue formation and signal transduction between cells in the physiology and pathology. Based on these achievements, we predict future advanced applications to biology and engineering.
A non-equilibrium electronegative plasma serves as the reactive source for semiconductor dry processing as an advanced technology. This paper reviews the current knowledge about the fundamental processes, structures, dynamics, and functions of high-frequency electronegative plasmas investigated over the past 30 years, and discusses the hidden characteristics originating from a majority of positive and negative ions and a minority of electrons. A unique structure with a negative ion layer is emphasized in terms of the sustaining mechanism underlying capacitively coupled plasma. In a strong electronegativity, the main sustaining mechanism is caused by a cluster of ionizations placed in front of the instantaneous anode by a minority of electrons accelerated from the bulk plasma into the active double layer. A new insight is obtained for how to hold a bulk plasma. The bulk plasma is maintained by a time-averaged net ionization rate equal to the electron attachment by minority electrons under the assistance of a relatively high reduced field E(t)/Ng in order to compensate for the large loss by ion–ion recombination. The structure is quite different from that of an electropositive plasma having a low reduced field under ambipolar diffusion. It is proposed that it will be possible to estimate the high value of E(t)/Ng in bulk plasma in a strongly electronegative plasma on the basis of the static DC breakdown theory in electronegative gas.
Conventional Langmuir probe cannot be used in most reactive plasmas for materials processing, owing to the insulating layers that are deposited on the probe surface. To address this issue, a novel variety of microwave probes has been recently developed for the purpose of monitoring local electron density, which is a key parameter for plasma control. The probe diagnostic must be stable, compact, easy, exhibiting negligible disturbance to a plasma and materials processing. This article presents a review of the diagnostic principle, characteristics and applications of representative probes such as the plasma oscillation probe, surface wave probe (plasma absorption probe), multi-pole resonance probe, hairpin probe and curling probe. The merits and demerits of each probe are presented from a reactive plasma diagnostic point of view. Innovations in the applications of microwave probes to optical emission spectrometry and the monitoring of wall deposits are also presented.
Metal halide perovskites can be easily fabricated with low-temperature solution processes, and they are a promising class of materials for solar cells, light emitters, and nonlinear optical devices. The perovskites are direct-gap semiconductors with sharp absorption edges and a highly efficient luminescence with no Stokes shift. In addition, the perovskites exhibit superior optoelectronic properties such as efficient photon recycling and long free-carrier diffusion. In this review, we discuss the fundamental optical properties of bulk crystals and the operation principles of heterodiode devices based on these novel functional materials.
Phonon engineering is expected to contribute to further development of various fields and technologies such as electronics, photonics, thermal engineering, and materials science. Although phonons inherently exist in condensed matter, their behavior strongly depends on the scale of the system and the materials, and they play a major role in electrical, optical, thermal, and mechanical properties. Therefore, researchers have been attempting to find effective ways to control phonons to modify the material properties — in this regard, nanostructuring was found to be highly effective. Here, we review recent advances in the simulations and experiments on phonon transport and thermal conduction control in nanostructures. We mainly focus on tailored nanostructures, especially phononic crystals of which the design is based on the nanoscale phonon transport property.
We review the recent progress in thin-film silicon solar cells with photonic crystals, where absorption enhancement is achieved by using large-area resonant effects in photonic crystals. First, a definitive guideline for enhancing light absorption in a wide wavelength range (600–1100 nm) is introduced, showing that the formation of multiple band edges utilizing higher-order modes confined in the thickness direction and the introduction of photonic superlattice structures enable significant absorption enhancement, exceeding that observed for conventional random scatterers. Subsequently, experimental evidence of this enhancement is demonstrated for a variety of thin-film Si solar cells: ∼500-nm-thick ultrathin microcrystalline silicon cells, few-µm-thick microcrystalline silicon cells, and ∼20-µm-thick thin single-crystalline silicon cells. The high short-circuit current densities and/or efficiencies observed for each cell structure confirm the effectiveness of using multiple band-edge resonant modes of photonic crystals for enhancing broadband absorption in actual solar cells.
The application of in situ synchrotron X-ray diffraction (XRD) to the molecular-beam epitaxial (MBE) growth of III–V semiconductors is overviewed along with backgrounds of the diffraction theory and instrumentation. X-rays are sensitive not only to the surface of growing films but also to buried interfacial structures because of their large penetration depth. Moreover, a spatial coherence length up to µm order makes X-rays widely applicable to the characterization of low-dimensional structures, such as quantum dots and wires. In situ XRD studies during growth were performed using an X-ray diffractometer, which was combined with an MBE chamber. X-ray reciprocal space mapping at a speed matching a typical growth rate was achieved using intense X-rays available from a synchrotron light source and an area detector. The importance of measuring the three-dimensional distribution of XRD intensity in a reciprocal space map is demonstrated for the MBE growth of two-, one-, and zero-dimensional structures. A large amount of information about the growth process of two-dimensional InGaAs/GaAs(001) epitaxial films has been provided by three-dimensional X-ray reciprocal mappings, including the anisotropic strain relaxation, the compositional inhomogeneity, and the evolution of surface and interfacial roughness. For one-dimensional GaAs nanowires grown in a Au-catalyzed vapor-liquid–solid mode, the relationship between the diameter of the nanowires and the formation of polytypes has been suggested on the basis of in situ XRD measurements. In situ three-dimensional X-ray reciprocal space mapping is also shown to be useful for determining the lateral and vertical sizes of self-assembled InAs/GaAs(001) quantum dots as well as their internal strain distributions during growth.
In recent years, piezoelectric microelectromechanical systems (MEMS) have attracted attention as next-generation functional microdevices. Typical applications of piezoelectric MEMS are micropumps for inkjet heads or micro-gyrosensors, which are composed of piezoelectric Pb(Zr,Ti)O3 (PZT) thin films and have already been commercialized. In addition, piezoelectric vibration energy harvesters (PVEHs), which are regarded as one of the key devices for Internet of Things (IoT)-related technologies, are promising future applications of piezoelectric MEMS. Significant features of piezoelectric MEMS are their simple structure and high energy conversion efficiency between mechanical and electrical domains even on the microscale. The device performance strongly depends on the function of the piezoelectric thin films, especially on their transverse piezoelectric properties, indicating that the deposition of high-quality piezoelectric thin films is a crucial technology for piezoelectric MEMS. On the other hand, although the difficulty in measuring the precise piezoelectric coefficients of thin films is a serious obstacle in the research and development of piezoelectric thin films, a simple unimorph cantilever measurement method has been proposed to obtain precise values of the direct or converse transverse piezoelectric coefficient of thin films, and recently this method has become to be the standardized testing method. In this article, I will introduce fundamental technologies of piezoelectric thin films and related microdevices, especially focusing on the deposition of PZT thin films and evaluation methods for their transverse piezoelectric properties.
During the past quarter century, the development of nanoscience and nanotechnology has been very much influenced and substantiated by the emergence of real nanometer-scale materials headed by fullerenes, carbon nanotubes (CNTs), and graphene, the so-called nanocarbons. This review article deals with some of the recent progress in the syntheses, characterization, and applications of the hybrid materials composed of nanopeapods (CNTs encapsulating atoms, molecules, nanowires, and nanoribbons). All of these studies are closely related to the characteristic usages of the internal nanospace prepared by the CNTs. Furthermore, the two-dimensional (2D) space prepared by two sheets of graphene has also been used as a 2D template for observing some dynamical phenomena of liquidus materials by transmission electron microscopy even under high-vacuum conditions.
CMOS miniaturization is now approaching the sub-10 nm level, and further downscaling is expected. This size scaling will end sooner or later, however, because the typical size is approaching the atomic distance level in crystalline Si. In addition, it is said that electron transport in FETs is ballistic or nearly ballistic, which means that the injection velocity at the virtual source is a physical parameter relevant for estimating the driving current. Channel-materials with higher carrier mobility than Si are nonetheless needed, and the carrier mobility in the channels is a parameter important with regard to increasing the injection velocity. Although the density of states in the channel has not been discussed often, it too is relevant for estimating the channel current. Both the mobility and the density of states are in principle related to the effective mass of the carrier. From this device physics viewpoint, we expect germanium (Ge) CMOS to be promising for scaling beyond the Si CMOS limit because the bulk mobility values of electrons and holes in Ge are much higher than those of electrons and holes in Si, and the electron effective mass in Ge is not much less than that in III–V compounds. There is a debate that Ge should be used for p-MOSFETs and III–V compounds for n-MOSFETs, but considering that the variability or nonuniformity of the FET performance in today's CMOS LSIs is a big challenge, it seems that much more attention should be paid to the simplicity of the material design and of the processing steps. Nevertheless, Ge faces a number of challenges even in case that only the FET level is concerned. One of the big problems with Ge CMOS technology has been its poor performance in n-MOSFETs. While the hole mobility in p-FETs has been improved, the electron mobility in the inversion layer of Ge FETs remains a serious concern. If this is due to the inherent properties of Ge, only p-MOSFETs might be used for device applications. To make Ge CMOS devices practically viable, we need to understand why electron mobility is severely degraded in the inversion layer in Ge n-channel MOSFETs and to find out how it can be increased. In the Si CMOS technology, the SiO2/Si interface has long been investigated and cannot be ignored even after the introduction of high-k gate stack technology. In that sense, the GeO2/Ge interface should be intensively studied to make the best of Ge's advantages. Therefore we first discuss the GeO2/Ge interface with regard to its physical and electrical characteristics. When we regard Ge as a channel material beyond Si for high performance ULSIs, we also have to seriously consider the gate stack scalability and reliability. The source/drain engineering, as well as the gate stack formation, is another challenge in Ge MOSFET design. Both the higher metal/Ge contact resistance and the larger p/n junction leakage current may be the consequences of Ge's intrinsic properties because they are derived from the strong Fermi-level pinning and the narrow energy band gap, respectively. Even if the carrier transport in the channel may be ideally ballistic, these properties should degrade FET properties. The narrower energy band gap of Ge is often addressed, but the higher dielectric constant of Ge is rarely discussed. This is also the case for most of the other high-mobility materials. The dielectric constant is directly and negatively related to short-channel effects, and we have not been able to provide a substantial solution to overcome this hardship. We have to keep this in mind for the short-channel FET operation. Although a number of problems remain to be solved, in this paper, we view the current status of Ge FET technology positively. A number of (but not all) Ge-related challenges have been overcome in the past 10 years, which seems to be a good time to summarize the status of Ge technology, particularly materials engineering aspects rather than device integration issues. Since we cannot cover all of the results published to date, we mainly discuss fundamental aspects based on our experimental results. Remaining challenges are also addressed but not comprehensively. Integration issues are not discussed in this review. Finally, new types of electron devices utilizing Ge's advantages are briefly introduced on the basis of our experimental results.
In this paper, we review the progress in recent studies on the performance enhancement methods for capacitive silicon resonators. We provide information on various fabrication technologies and design considerations that can be employed to improve the performance of capacitive silicon resonators, including low motional resistance, small insertion loss, and high quality factor (Q). This paper contains an overview of device structures and working principles, fabrication technologies consisting of hermetic packaging, deep reactive-ion etching and neutral beam etching, and design considerations including mechanically coupled, movable electrode structures and piezoresistive heat engines.
Scanning nonlinear dielectric microscopy (SNDM) can easily distinguish the dopant type (PN) and has a wide dynamic range of sensitivity from low to high concentrations of dopants, because it has a high sensitivity to capacitance variation on the order of 10−22 F/
. It is also applicable to the analysis of compound semiconductors with much lower signal levels than Si. We can avoid misjudgments from the two-valued function (contrast reversal) problem of dC/dV signals. Under an ultrahigh-vacuum condition, SNDM has atomic resolution. As the extended versions of SNDM, super-higher-order SNDM, local-deep-level transient spectroscopy, noncontact SNDM, and scanning nonlinear dielectric potentiometory have been developed and introduced. The favorable features of SNDM originate from its significantly high sensitivity.
The photovoltaic industry is in a phase of rapid expansion, growing by more than 30% per annum over the last few decades. Almost all commercial solar cells presently use single-crystalline or multicrystalline silicon wafers similar to those used in microelectronics; meanwhile, thin-film compounds and alloy solar cells are currently under development. The laboratory performance of these cells, at 26% solar energy conversion efficiency, is now approaching thermodynamic limits, with the challenge being to incorporate these improvements into low-cost commercial products. Improvements in the optical design of cells, particularly in their ability to trap weakly absorbed light, have also led to increasing interest in thin-film cells based on polycrystalline silicon; these cells have advantages over other thin-film photovoltaic candidates. This paper provides an overview of silicon-based solar cell research, especially the development of silicon wafers for solar cells, from the viewpoint of growing both single-crystalline and multicrystalline wafers.
In this paper, we give an overview of the recent progress in GaN-based high-electron-mobility transistors (HEMTs) developed for mainstream acceptance in the power electronics field. The comprehensive investigation of AlGaN/GaN HEMTs fabricated on a free-standing semi-insulating GaN substrate reveals that an extracted effective lateral breakdown field of approximately 1 MV/cm is likely limited by the premature device breakdown originating from the insufficient structural and electrical quality of GaN buffer layers and/or the GaN substrate itself. The effective lateral breakdown field is increased to 2 MV/cm by using a highly resistive GaN substrate achieved by heavy Fe doping. Various issues relevant to current collapse are also discussed in the latter half of this paper, where a more pronounced reduction in current collapse is achieved by combining two different schemes (i.e., a prepassivation oxygen plasma treatment and a field plate structure) for intensifying the mitigating effect against current collapse. Finally, a novel approach to suppress current collapse is presented by introducing a three-dimensional field plate (3DFP) in AlGaN/GaN HEMTs, and its possibility of realizing true collapse-free operation is described.
The impacts of the progress in the art and science of crystal growth on human life are reviewed. Even before the invention of the transistor, quartz and corundum crystals were used as crystal oscillators and jewel bearings, respectively. However, a major impact of crystal growth on society was experienced with the invention of the transistor, which required high-purity and perfect germanium crystals. Once the importance of crystal growth was clearly recognized, the science of crystal growth also extensively developed. The growth of single crystalline silicon allows us to produce integrated circuits, which are used in all the electronic devices in everyday use. The technological developments in the growth of compound semiconductors have also had a large impact on society through the inventions of the laser diode for optical communication and the p–n junction nitride light-emitting diode toward the realization of a less energy-intensive society. The latter invention was awarded the 2014 Nobel Prize in Physics. Finally, future aspects of crystal growth are discussed.
Power semiconductor devices are key components in power conversion systems. Silicon carbide (SiC) has received increasing attention as a wide-bandgap semiconductor suitable for high-voltage and low-loss power devices. Through recent progress in the crystal growth and process technology of SiC, the production of medium-voltage (600–1700 V) SiC Schottky barrier diodes (SBDs) and power metal–oxide–semiconductor field-effect transistors (MOSFETs) has started. However, basic understanding of the material properties, defect electronics, and the reliability of SiC devices is still poor. In this review paper, the features and present status of SiC power devices are briefly described. Then, several important aspects of the material science and device physics of SiC, such as impurity doping, extended and point defects, and the impact of such defects on device performance and reliability, are reviewed. Fundamental issues regarding SiC SBDs and power MOSFETs are also discussed.
By probing dielectric polarization originating from dipoles and electrons in materials, we can study dynamical carrier behaviors in materials and also in devices. Maxwell displacement current (MDC) measurement allows us to directly probe orientational dipolar motion in monolayers, while electric-field-induced optical second-harmonic generation (EFISHG) measurement allows dynamical electron and hole transport in solids to be probed directly. By probing nonlinear polarization induced in solids by coupling with incident electromagnetic waves of laser beam and dc electric field that originate from moving carriers, long-range carrier motion is visualized. Experiments using a time-resolved EFISHG technique reveal carrier transfer in organic devices such as organic field-effect transistors, organic light-emitting diodes, organic memory devices, and organic solar cells, and thus enable us to model the carrier transport mechanism. We anticipate that this novel technique using EFISHG can be a powerful tool for studying carrier behaviors in organic devices as well as in organic materials.
Fiber lasers, which consist of ideal waveguides of optical fibers, work as stable, practical, and maintenance-free lasers. A passively mode-locked ultrashort pulse fiber laser using the intensity-dependent absorption element has made great progress using a new type of saturable absorber with a nanocarbon material. The techniques of ultrashort pulse amplification and pulse compression were also developed. Using a combination of specialty fibers, ultrawideband pulse sources, such as wavelength tunable ultrashort pulses and supercontinuum, can be demonstrated. These new light sources are useful for laser applications, especially for optical metrology. In this paper, the progress of highly functional ultrashort pulse fiber laser sources and their applications are reviewed mainly on the basis of the works of this author.
Quantum information science has recently attracted a lot of attention. Its applications include secure communication, quantum computation, quantum simulation, and quantum metrology. In these applications, photons are one of the most important physical quanta for their tolerance to decoherence. In this manuscript, we review the recent progress in single-photon/entangled-photon emitters and their applications: heralded single-photon sources using parametric downconversion and their application to quantum key distribution, highly indistinguishable heralded single-photon sources, fiber-coupled solid-state single-photon sources, and ultrabroadband-frequency entanglement generation.
The technical history of when and how the basic understanding of the emission efficiency of organic light-emitting diodes (OLEDs) was established over the last 50 years is described. At first, our understanding of emission efficiency in single-crystal and thin-film electroluminescence (EL) devices in the early stages before the Eastman-Kodak breakthrough, that is, the introduction of the concept of multilayer structures, is examined. Then our contemplation travels from the Eastman-Kodak breakthrough towards the presently widely accepted concept of emission efficiency. The essential issues concerning the emission efficiency of OLEDs are summarized to help readers to obtain a common understanding of OLED efficiency problems, and detailed discussions on the primary factors that determine emission efficiency are given. Finally, some comments on remaining issues are presented.
The integration limit of flash memories is approaching, and many new types of memory to replace conventional flash memories have been proposed. Unlike flash memories, new nonvolatile memories do not require storage of electric charges. The possibility of phase-change random-access memories (PCRAMs) or resistive-change RAMs (ReRAMs) replacing ultrahigh-density NAND flash memories has been investigated; however, many issues remain to be overcome, making the replacement difficult. Nonetheless, ferroelectric RAMs (FeRAMs) and magnetoresistive RAMs (MRAMs) are gradually penetrating into fields where the shortcomings of flash memories, such as high operating voltage, slow rewriting speed, and limited number of rewrites, make their use inconvenient. For instance, FeRAMs are widely used in ICs that require low power consumption such as smart cards and wireless tags. MRAMs are used in many kinds of controllers in industrial equipment that require high speed and unlimited rewrite operations. For successful application of new non-volatile semiconductor memories, such memories must be practically utilized in new fields in which flash memories are not applicable, and their technologies must be further developed.
Extreme ultraviolet (EUV) radiation, the wavelength of which is 13.5 nm, is the most promising exposure source for next-generation semiconductor lithography. The development of EUV lithography has been pursued on a worldwide scale. Over the past decade, the development of EUV lithography has significantly progressed and approached its realization. In this paper, the resist materials and processes among the key technologies of EUV lithography are reviewed. Owing to its intensive development, the resist technology has already closely approached the requirements for the 22 nm node. The focus of the development has shifted to the 16 nm node and beyond. Despite the trade-off relationships among resolution, line edge roughness/line width roughness, and sensitivity, the capability of resist technology will go beyond the 16 nm node.
There are high expectations for coated conductors in electric power applications such as superconducting magnetic energy storage (SMES) systems, power cables, and transformers owing to their ability to contribute to stabilizing and increasing the capacity of the electric power supply grid as well as to reducing CO2 emission as a result of their high critical-current characteristics. Research and development has been performed on wires/tapes and electric power devices worldwide. The Materials and Power Applications of Coated Conductors (M-PACC) Project is a five-year national project in Japan started in 2008, supported by the Ministry of Economy, Trade and Industry (METI) and the New Energy and Industrial Technology Development Organization (NEDO), to develop both coated conductors that meet market requirements and basic technologies for the above-mentioned power applications using coated conductors. In this article, research and development results are reviewed and compared with the interim/final targets of the project, and future prospects are discussed.
Among a series of high-temperature superconducting materials that have been discovered to date, (Bi,Pb)2Sr2Ca2Cu3O10-x is the best candidate for superconducting wires that are long with commercial productivity, and critical current performance. In particular, the controlled overpressure (CT-OP) sintering technique gave us a 100% density of (Bi,Pb)2Sr2Ca2Cu3O10-x portion, which leads to robustness, increase in critical current, and mechanical tolerance. Many application prototypes are already verified and are being evaluated worldwide. Current leads for large magnets and magnetic billet heaters are already commercial products. Commercial applications for power cables, motors for ship propulsion and electric vehicles, and many kinds of magnets are promising in the near future.
The recent discovery of iron-based superconductors has evoked enthusiasm for extensive research on these materials because they form the second high-temperature superconductor family after the copper oxide superconductors and impart an expectation for materials with a higher transition temperature (Tc). It has also been clarified that they have peculiar physical properties including an unconventional pairing mechanism and superconducting properties preferable for application such as a high upper critical field and small anisotropy. This paper reviews the research on thin films, Josephson junctions, and superconducting wires and tapes made from iron-based superconductors, which has been performed toward the realization of future applications. Though there are many technical hurdles toward the practical application of these materials, some promising features such as a high critical current density in thin films under high magnetic fields and advantageous grain boundary properties over copper oxides have been clarified.
One hundred years after the discovery of superconductivity, we are now facing a new era that demands an increase in the superconducting transition temperature Tc. In addition to copper-based superconductors, iron-based superconductors that have been discovered recently have been considered high-temperature superconductors. The similarity and difference between the two high-Tc systems are discussed on the basis of our recent theoretical and experimental understandings. While the pairing mechanism and non-Fermi liquid behaviors in transport properties may have a common origin between the two systems, the strengths of electron correlation are different: Cuprate is a doped Mott insulator, while iron pnictide is an itinerant system with a weak correlation. Pseudogap phenomena in hole-doped cuprates and their absence in electron-doped cuprate are regarded as a consequence of a strong correlation. Recent topics in cuprates about electron–hole asymmetry and pseudogap phenomenon are reviewed from a theoretical viewpoint. For iron pnictides, anisotropic behaviors in antiferromagnetic phases and new iron-selenide superconductors are discussed.
Invited Reviews Volumes 41-50
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The precision measurement of time and frequency is of great interest for a wide range of applications, including fundamental science and technologies that support broadband communication networks and the navigation with global positioning systems (GPSs). The development of optical frequency measurement based on frequency combs has revolutionized the field of frequency metrology, especially research on optical frequency standards. The proposal and realization of the optical lattice clock have further stimulated studies in the field of optical frequency metrology. Optical carrier transfer using optical fibers has been used to disseminate optical frequencies or compare two optical clocks without degrading their stability and accuracy. In this paper, we review the state-of-the-art development of optical frequency combs, standards, and transfer techniques with emphasis on optical lattice clocks. We address recent results achieved at the University of Tokyo and the National Metrology Institute of Japan in respect of frequency metrology with Sr and Yb optical lattice clocks.
Historically, in the mass production of semiconductor devices, exposure tools have been repeatedly replaced with those with a shorter wavelength to meet the resolution requirements projected in the International Technology Roadmap for Semiconductors issued by the Semiconductor Industry Association. After ArF immersion lithography, extreme ultraviolet (EUV; 92.5 eV) radiation is expected to be used as an exposure tool for the mass production at or below the 22 nm technology node. If realized, 92.5 eV EUV will be the first ionizing radiation used for the mass production of semiconductor devices. In EUV lithography, chemically amplified resists, which have been the standard resists for mass production since the use of KrF lithography, will be used to meet the sensitivity requirement. Above the ionization energy of resist materials, the fundamental science of imaging, however, changes from photochemistry to radiation chemistry. In this paper, we review the radiation chemistry of materials related to chemically amplified resists. The imaging mechanisms from energy deposition to proton migration in resist materials are discussed.
Phase-change random access memory (PRAM) technology is reviewed. PRAM uses the phase change between the amorphous state and the crystalline state caused by Joule heating as its memory mechanism. A change in electrical resistance owing to a phase change is detected by a small electric current. The merits of this approach are that the resistance change is more than one order of magnitude, and its simple structure decreases the number of steps in the manufacturing process. Suppression of reset current for the change from the low-resistance crystalline state to the amorphous state and an improvement in durability against set-reset cycles and high-temperature operation will ultimately be achieved.
Resonant tunneling diodes (RTDs) have the potential for use as compact and coherent terahertz (THz) sources operating at room temperature. In this paper, sub-THz and THz oscillators with RTDs integrated on planar circuits are described. Fundamental oscillation up to 0.65 THz and harmonic oscillation up to 1.02 THz were obtained at room temperature in our recent study. Limiting factors for oscillation frequency and output power are theoretically analyzed including tunneling and transit-time effects and parasitic elements. Oscillation frequency and its dependence on RTD size are in good agreement with the measured results. Based on this result, it is shown that fundamental oscillation up to 2.3 THz and an output power of 60 µW at 1 THz are theoretically expected by improving the structures of the RTD and the antenna. Voltage-controlled oscillation, which is useful for the precise control of frequency, is observed in the RTD oscillators. Coherent power combining in an array configuration to achieve high output power as well as mutual injection locking between the array elements are also described.
An investigation of ultrahigh-density ferroelectric data storage based on scanning nonlinear dielectric microscopy (SNDM) is described. To obtain fundamental knowledge of high-density ferroelectric data storage, several studies of nanodomain formation in a congruent lithium tantalate single crystal were conducted. This paper is a summary report consisting of the most recent experimental data from investigations of ferroelectric high density data storage.
Plasma etching technologies such as reactive ion etching (RIE), isotropic etching, and ashing/plasma cleaning are the currently used booster technologies for manufacturing all silicon devices based on the scaling law. The needs-driven conversion from the wet etching process to the plasma/dry etching process is reviewed. The progress made in plasma etching technologies is described from the viewpoint of requirements for the manufacturing of devices. The critical applications of RIE, isotropic etching, and plasma ashing/cleaning to form precisely controlled profiles of high-aspect-ratio contacts (HARC), gate stacks, and shallow trench isolation (STI) in the front end of line (FEOL), and also to form precise via holes and trenches used in reliable Cu/low-k (low-dielectric-constant material) interconnects in the back end of line (BEOL) are described in detail. Some critical issues inherent to RIE processing, such as the RIE-lag effect, the notch phenomenon, and plasma-induced damage including charge-up damage are described. The basic reaction mechanisms of RIE and isotropic etching are discussed. Also, a procedure for designing the etching process, which is strongly dependent on the plasma reactor configuration, is proposed. For the more precise critical dimension (CD) control of the gate pattern for leading-edge devices, the advanced process control (APC) system is shown to be effective.
We can now utilize the wave properties of electrons to conduct fundamental experiments on quantum mechanics because of the development of brighter electron beams as well as the ability directly to image the quantum world by utilizing the phase information of electrons. In this paper, we describe new possibilities that have been generated by electron-phase microscopy using electron microscopes equipped with coherent yet bright electron beams.
The vertical-cavity surface-emitting laser (VCSEL) is becoming a key device in high-speed optical local-area networks (LANs) and even wide-area networks (WANs). This device is also enabling ultraparallel data transfer in equipment and computer systems. In this paper, we will review its physics and the progress of technology covering the spectral band from the infrared to the ultraviolet, by featuring materials, fabrication technology, and performances such as threshold, output power, polarization, modulation and reliability. Lastly, we will touch on its future prospects.
Entanglement is one of the essential resources of quantum information and communication technology. Photons are the most popular and promising media to manipulate entanglement. In this review article, concepts and progress in the generation, observation, and characterization of entangled photons are presented. Starting from underlying theoretical concepts, a historical review on the generation of entangled photons is given. Particularly, recent results on the generation of polarization-entangled photons from semiconductor sources are reviewed and discussed.
The historical background and recent progress in the development of organic electronics and photonic crystals, particularly tunable photonic crystals realized by combining photonic crystal structure with functional organic molecules, are discussed. The novel characteristics of organic electronic devices with mainly conducting polymers, which are related to the optical effects, and the tunable photonic crystals composed of periodic structures of optical wavelength order combined with functional organic materials are demonstrated.
Theoretical and experimental works on the plasticity of carbon nanotubes are reviewed in this paper. Theoretical calculations have clarified that plastic elongation and plastic-bend formation occur only at an appropriate high temperature above the critical tensile strain and the critical curvature for straight nanotubes and elastically bent nanotubes, respectively. The initiation of these processes is 5-7-7-5 defect nucleation, then two 5-7 pairs separate from the defect. The pairs glide away to leave a thinner nanotube with a different chirality for the elongation, and glide from the "belly" position for zigzag tubes and from the back position for armchair tubes toward a position close to the neutral plane of the bent tube for the plastic bend. Current-induced experiments have demonstrated that the plastic-bend formation of nanotubes and their recovery are consistent with the theoretical findings. The onset current is rather low and has strong tube-diameter dependence for plastic-bend formation but is extremely high (close to the sublimation curret) for its recovery.
In this review article, we deal with the structures and properties of novel hybrid nanocarbon materials, which are created by the incorporation of atoms and molecules in hollow spaces of fullerenes and carbon nanotubes (CNTs); these hybrid materials are called endohedral metallofullerenes (in the case of metal atom incorporated fullerenes) and nano-peapods, respectively. Synthesis procedures, structural characterizations by synchrotron powder x-ray diffraction, electronic structures, and magnetic properties of endohedral metallofullerenes are discussed. The structure and properties of nano-peapods by high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), scanning tunneling microscopy (STM) and field effect transistor (FET) transport measurements together with their synthesis procedures are described. The utilization of the low-dimensional nanosized spaces of CNTs to produce novel low-dimensional nanocluster, nanowire and nano-tube materials is also discussed.
A general review on high-temperature superconductivity was made. After prehistoric view and the process of discovery were stated, the special features of high-temperature superconductors were explained from the materials side and the physical properties side. The present status on applications of high-temperature superconductors were explained on superconducting tapes, electric power cables, magnets for maglev trains, electric motors, superconducting quantum interference device (SQUID) and single flux quantum (SFQ) devices and circuits.
Marked improvements in the crystalline quality of GaN enabled the production of GaN-based p–n junction blue-light-emitting and violet-laser diodes. These robust, energetically efficient devices have opened up a new frontier in optoelectronics. A new arena of wide-bandgap semiconductors has been developed due to marked improvements in the crystalline quality of nitrides. In this article, we review breakthroughs in the crystal growth and conductivity control of nitride semiconductors during the development of p–n junction blue-light-emitting devices. Recent progress mainly based on the present authors' work and future prospects of nitride semiconductors are also discussed.
High-power device technology is a key technological factor for wireless communication, which is one of the information network infrastructures in the 21st century, as well as power electronics innovation, which contributes considerably to solving the energy saving problem in the future energy network. Widegap semiconductors, such as SiC and GaN, are strongly expected as high-power high-frequency devices and high-power switching devices owing to their material properties. In this paper, the present status and future prospect of these widegap semiconductor high-power devices are reviewed, in the context of applications in wireless communication and power electronics.
One of the main requirements for Si-based ultrasmall devices is atomic-order control of process technology. Here we show the concept of atomically controlled processing for group IV semiconductors based on atomic-order surface reaction control. By ultraclean low-pressure chemical vapor deposition using SiH4 and GeH4 gases, high-quality low-temperature epitaxial growth of Si, Ge, and Si1-xGex with atomically flat surfaces and interfaces on Si(100) is achieved, and atomic-order surface reaction processes on group IV semiconductor surface are formulated based on a Langmuir-type surface adsorption and reaction scheme. In in-situ doped Si1-xGex epitaxial growth on the (100) surface in a SiH4–GeH4–dopant (PH3, or B2H6 or SiH3CH3)–H2 gas mixture, the deposition rate, the Ge fraction and the dopant concentration are explained quantitatively assuming that the reactant gas adsorption/reaction depends on the surface site material and that the dopant incorporation in the grown film is determined by Henry's law. Self-limiting formation of 1–3 atomic layers of group IV or related atoms in the thermal adsorption and reaction of hydride gases on Si(100) and Ge(100) is generalized based on the Langmuir-type model. Si or SiGe epitaxial growth over N, P or B layer already-formed on Si(100) or SiGe(100) surface is achieved. Furthermore, the capability of atomically controlled processing for advanced devices is demonstrated. These results open the way to atomically controlled technology for ultralarge-scale integrations.
In this review, we introduce the production methods and applications of carbon nanotubes. Carbon nanotubes are now attracting a broad range of scientists and industries due to their fascinating physical and chemical properties. Focusing on the chemical vapor deposition (CVD) method, we will briefly review the history and recent progress of the synthesis of carbon nanotubes for the large-scale production and double-walled carbon nanotube production. We will also describe effective purification methods that avoid structural damage, and discuss the electrochemical, composite, and medical applications of carbon nanotubes.
For the past 30 years, plasma etching technology has led in the efforts to shrink the pattern size of ultralarge-scale integrated (ULSI) devices. However, inherent problems in the plasma processes, such as charge buildup and UV photon radiation, limit the etching performance for nanoscale devices. To overcome these problems and fabricate sub-10-nm devices in practice, neutral-beam etching has been proposed. In this paper, I introduce the ultimate etching processes using neutral-beam sources and discuss the fusion of top-down and bottom-up processing for future nanoscale devices. Neutral beams can perform atomically damage-free etching and surface modification of inorganic and organic materials. This technique is a promising candidate for the practical fabrication technology for future nano-devices.
Structures and properties of liquid crystalline phases formed by bent-core molecules are reviewed. At least eight phases designated as B1–B8 have been found, being unambiguously distinguished from phases formed by usual calamitic molecules due to a number of remarkable peculiarities. In addition to B1–B8 phases, smectic A-like phases and biaxial nematic phases formed by bent-core molecules are also reviewed. The most attractive aspects of this new class of liquid crystals are in polarity and chirality, despite being formed from achiral molecules. The bent-core mesogens are the first ferroelectric and antiferroelectric liquid crystals realized without introducing chirality. Spontaneous chiral deracemization at microscopic and macroscopic levels occurs and is controllable. Moreover, achiral bent-core molecules enhance system chirality. The interplay between polarity and chirality provides chiral nonlinear optic effects. Further interesting phenomena related to polarity and chirality are also reviewed.
Photocatalysis has recently become a common word and various products using photocatalytic functions have been commercialized. Among many candidates for photocatalysts, TiO2 is almost the only material suitable for industrial use at present and also probably in the future. This is because TiO2 has the most efficient photoactivity, the highest stability and the lowest cost. More significantly, it has been used as a white pigment from ancient times, and thus, its safety to humans and the environment is guaranteed by history. There are two types of photochemical reaction proceeding on a TiO2 surface when irradiated with ultraviolet light. One includes the photo-induced redox reactions of adsorbed substances, and the other is the photo-induced hydrophilic conversion of TiO2 itself. The former type has been known since the early part of the 20th century, but the latter was found only at the end of the century. The combination of these two functions has opened up various novel applications of TiO2, particularly in the field of building materials. Here, we review the progress of the scientific research on TiO2 photocatalysis as well as its industrial applications, and describe future prospects of this field mainly based on the present authors' work.
The development of the high electron mobility transistor (HEMT) provides a good illustration of the way a new device emerges and evolves toward commercialization. This article will focus on these events that the author feels might be of interest to young researchers. Recent progress and future trends in HEMT technology are also described.
This paper reports selected recent topics in high-Tc superconductive electronics. Improved process technology for high-Tc digital electronics, the development of a sampling oscilloscope, magnetic immunoassay using a high-Tc superconducting quantum interference device (SQUID), scanning laser-SQUID for integrated circuits testing, terahertz radiation from high-Tc superconductors, and optical control of vortices are reviewed.
V. Viviani et al. [Biochemistry 38 (1999) 8271] were the first to succeed in cloning the red-emitting enzyme from the South American railroad worm, which is the only bioluminescent organism known to emit a red-colored light. The application of red bioluminescence has been our goal because the transmittance of longer-wavelength light is superior to that of the other colors for visualization of biological functions in living cells. Now, different color luciferases, which emit with wavelength maxima ranging from 400 to 630 nm, are available and are being used. For example, based on different color luciferases, Nakajima et al. developed a tricolor reporter in vitro assay system based on these different color luciferases in which the expression of three genes can be monitored simultaneously. On the other hand, bioluminescence resonance energy transfer (BRET) is a natural phenomenon caused by the intermolecular interaction between a bioluminescent protein and a fluorophore on a second protein, resulting in the light from the bioluminescence reaction having the spectrum of the fluorophore. Otsuji et al. [Anal. Biochem. 329 (2004) 230] showed that the change in the efficiency of energy transfer in intramolecular BRET can quantify cellular functions in living cells. In this review, I introduce the basic mechanisms of color tuning in bioluminescent systems and new applications based on color tuning in the life sciences.
Since the middle of the 1990s, X-ray phase imaging including phase tomography has been attracting increasing attention. The advantage of X-ray phase imaging is that an extremely high sensitivity is achieved for weak-absorbing materials, such as biological soft tissues, which generate a poor contrast by conventional methods. Medical and biological imaging is the main target of X-ray phase imaging, and several trials using synchrotron radiation sources and laboratory sources have been made. Measuring and controlling the X-ray phase are also significant for X-ray microscopy with a high spatial resolution, and innovative techniques are attracting intense interest. The progress of X-ray phase imaging is supported by the developments in X-ray sources such as third-generation synchrotron radiation sources, optical elements, and image detectors. This article describes the advantages of using X-ray phase information and reviews various techniques studied for X-ray phase imaging.
Recent progress in ultrafast photonics is reviewed with special emphasis on the research and development activities in Japanese research institutions in the field of optical communication and related measurement technologies. After summarizing the physical natures of ultrashort optical pulses, selected topics are reviewed on such as (1) ultrahigh-bit-rate optical communication employing the combination of optical time division multiplexing (OTDM) and wavelength division multiplexing (WDM), (2) optical components for ultrafast photonics with emphasis on all optical switches including semiconductor optical amplifiers, cascaded second order frequency converters, semiconductor saturable absorber switches, organic dye saturable absorber switches and bistable semiconductor lasers, (3) microwave photonics, emphasizing millimeter-wave/photonic communication technologies, and (4) high-speed optical measurements featuring both compact femtosecond pulse source development and rf magnetic field imaging. Some comments on the future prospect of ultrafast photonics are also given. It is concluded that in order to bring the powerful and versatile capability of ultrafast photonics into the real world, further collaboration between photonics specialists and production engineers/information specialists is strongly desired.
Femtosecond pulsed lasers have been widely used for materials microprocessing. Due to their ultrashort pulse width and ultrahigh light intensity, the process is generally characterized by the nonthermal diffusion process. We observed various induced microstructures such as refractive-index-changed structures, color center defects, microvoids and microcracks in transparent materials (e.g., glasses after the femtosecond laser irradiation), and discussed the possible applications of the microstructures in the fabrication of various micro optical devices [e.g., optical waveguides, microgratings, microlenses, fiber attenuators, and three-dimensional (3D) optical memory]. In this paper, we review our recent research developments on single femtosecond-laser-induced nanostructures. We introduce the space-selective valence state manipulation of active ions, precipitation and control of metal nanoparticles and light polarization-dependent permanent nanostructures, and discuss the mechanisms and possible applications of the observed phenomena.
The bulk acoustic wave filter composed of piezoelectric thin film resonators has many features superior to those of other small filters such as a surface acoustic wave (SAW) filter and a ceramic filter. As it has no fine structure in its electrode design, it has a high Q factor that leads to low-loss and sharp-cut off characteristics and a high power durability particularly in the high-frequency range. Furthermore, it has the potentiality of integrated devices on a Si substrate. In this paper, we review the recent developments of piezoelectric thin film resonator filters in the world, including our development for mobile communication applications. After describing the feature and history of the piezoelectric thin film resonator filters, our technologies are introduced in focusing on the resonator structures, the piezoelectric thin film and electrode film materials, the cavity structures, the filter structure and its design rules and characteristics, comparing with SAW filters. The competition and coexistence between the piezoelectric thin film resonator filters and the SAW filters are also described. In this paper, we describe the development of a piezoelectric thin film resonator from the standpoint of researchers who have a long experience of SAW filter development.
Growth processes of hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (µc-Si:H) from SiH4 and H2/SiH4-glow discharge plasmas are reviewed. Differences and similarities between µc-Si:H and a-Si:H growth reactions in the plasma and on the film-growing surface are discussed, and the nucleus-formation process followed by the epitaxial-like crystal growth process is explained as being processes unique to µc-Si:H. The governing reaction of dangling-bond-defect density in the resulting a-Si:H and µc-Si:H films is also discussed in order to obtain a clue to improve the optoelectronic properties of these materials to enable device applications, particularly to thin-film silicon-based solar cells. Material issues concerning the realization of low-cost high-efficiency solar cells are described, and finally, recent progress in those issues is presented.
In this article, we review recent studies on nanomechanics of biostructures performed in the Laboratory of Biodynamics at Tokyo Institute of Technology. We employed the force spectroscopy mode of the atomic force microscope, to determine the hidden mechanical properties of protein-based biostructures that have made life on the earth so successful. We investigated the mechanical heterogeneity of the internal structure of globular proteins and cell membranes. Single molecules of globular proteins were stretched from their two ends after being sandwiched between the probe of the atomic force microscope and the substrate through a covalent crosslinking system. The resulting force-extension curve revealed mechanical heterogeneities in the conformation of globular proteins. The covalent crosslinking system withstood a tensile force of up to 1.8±0.33 nN (loading rate = 11.7 nN/s) while most of the noncovalently folded protein sub-structures were completely stretched out with less force. The result of force spectroscopy supported a long-standing conjecture that an enzyme cannot simply be a soft material because it must catalyze chemical reactions involving the formation and breakdown of mechanically rigid covalent molecules. Next, the AFM force spectroscopy was applied to determine the force needed to disrupt noncovalently assembled biostructures such as composite biomembranes composed of lipids and proteins. We were able to show that intrinsic membrane proteins that are securely anchored to a lipid bilayer could be pulled out of the membrane with a significantly less force than that required for covalent bond breakdown, but with a force in the comparable range required for the disruption of the internal structures of globular proteins. From the available results from our group and other groups, a new concept of force-based biostructure assembly is emerging.
Technological breakthroughs in growth control of SiC are reviewed. Step-controlled epitaxy by using off-axis SiC {0001} substrates to grow high-quality epitaxial layer is explained in detail. The introduction of substrate off-angles brings step-flow growth, which easily makes polytype replication of SiC at rather low temperatures. Off-angle dependence, rate-determining processes, and temperature dependence of growth rate are discussed. Prediction, whether step-flow growth or two-dimensional nucleation does occur, is given as a function of off-angle, growth temperature, and growth rate. Optical and electrical properties of undoped epitaxial layers are characterized. Impurity doping during the growth is explained. Recent progresses in peripheral technologies for realization of power electronic devices, such as bulk growth, epitaxial growth, ion implantation, MOS interface, ohmic contacts, are introduced. Finally application to high-power electronic devices is briefly described.
Si wafers have contributed to the rapid growth of the semiconductor industry as a basic material for ultra large scale integration (ULSI) through the research and development of new technologies and mass production in response to the various demands of device manufacturers. In this paper, first, the key issues of wafer quality improvement with respect to wafer fabrication technology, gettering and grown-in defects are reviewed. Various wafers currently in use such as annealed wafers, epitaxial wafers and 300 mm diameter wafer are discussed with respect to technology and cost effectiveness. Advanced Si-based wafers represented by silicon on insulator (SOI) and strained SiGe wafers are also described. After discussing the challenge to develop innovative Si wafer technologies which will lead to the future development of ULSI, the other important issues associated with Si wafers such as the re-examination of over-stringent specifications, cost reduction, economically reasonable pricing and the promotion of mutual understanding and cooperation between device makers and wafer makers for the continued development of both industries are emphasized.
Hydrogenated amorphous silicon and related alloy films have attracted much attention because of the wide application of these films in devices such as thin-film transistors and solar cells. However, the degradation of these films caused by intense illumination is a serious shortcoming. In this review, various experimental results concerning this problem and various models for the photocreation of dangling bonds which is thought to be the main origin of the degradation are introduced and discussed. Degradation in the device performance, some efforts to overcome the degradation and some metastable defects other than photocreated ones are also briefly introduced.
The fundamental band gap of InN has been thought to be about 1.9 eV for a long time. Recent developments of metalorganic vapor phase epitaxy (MOVPE) and RF-molecular beam epitaxy (RF-MBE) growth technologies have made it possible to obtain high-quality InN films. A lot of experimental results have been presented very recently, suggesting that the true band-gap energy of InN should be less than 1.0 eV. In this paper, we review the results of the detailed study of RF-MBE growth conditions for obtaining high-quality InN films. The full widths at half maximum (FWHMs) of ω-mode X-ray diffraction (XRD), ω–2θ mode XRD and E2 (high-frequency)-phonon-mode peaks in the Raman scattering spectrum of the grown layer were 236.7 arcsec, 28.9 arcsec and 3.7 cm-1, respectively. The carrier concentration and room temperature electron mobility were 4.9×1018 cm-3 and 1130 cm2/Vs, respectively. Photoluminescence and optical absorption measurements of these high-quality InN films have clearly demonstrated that the fundamental band gap of InN is about 0.8 eV. Studies on the growth and characterization of InGaN alloys over the entire alloy composition further supported that the fundamental band gap of InN is about 0.8 eV.
Multilayer ceramic capacitor (MLCC) production and sales figures are the highest among fine-ceramic products developed in the past 30 years. The total worldwide production and sales reached 550 billion pieces and 6 billion dollars, respectively in 2000. In the course of progress, the development of base-metal electrode (BME) technology played an important role in expanding the application area. In this review, the recent progress in MLCCs with BME nickel (Ni) electrodes is reviewed from the viewpoint of nonreducible dielectric materials. Using intermediate-ionic-size rare-earth ion (Dy2O3, Ho2O3, Er2O3, Y2O3) doped BaTiO3 (ABO3)-based dielectrics, highly reliable Ni-MLCCs with a very thin layer below 2 µm in thickness have been developed. The effect of site occupancy of rare-earth ions in BaTiO3 on the electrical properties and microstructure of nonreducible dielectrics is studied systematically. It appears that intermediate-ionic-size rare-earth ions occupy both A- and B-sites in the BaTiO3 lattice and effectively control the donor/acceptor dopant ratio and microstructural evolution. The relationship between the electrical properties and the microstructure of Ni-MLCCs is also presented.
Invited Reviews Volumes 31-40
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Current information technologies use semiconductor devices and magnetic/optical discs, however, it is foreseen that they will all face fundamental limitations within a decade. This paper reviews the prospects and problems of single molecule devices, including switching devices, wires, nanotubes, optical devices, storage devices and sensing devices for future information technologies and other advanced applications in the next paradigm. The operation principles of these devices are based on the phenomena occurring within a single molecule, such as single electron transfer, direct electron-hole recombination, magnetic/charge storage and regand-receptor reaction. Four possible milestones for realizing the Peta (1015)-floating operations per second (P-FLOPS) personal molecular supercomputer are described, and the necessary technologies are listed. These include, (1) two terminal conductance measurement on single molecule, (2) demonstration of two terminal molecular device characteristics, (3) verification of three terminal molecular device characteristics and (4) integration of the functions of "molecular super chip". Thus, 1000 times higher performance information technologies would be realized with molecular devices.
A low-temperature, uniform, high-density plasma is produced by applying ultrahigh-frequency (UHF) power through a spokewise antenna. The plasma is uniform within ±5% over a diameter of 30 cm. No magnetic field is needed to maintain the high-density plasma. Consequently, the plasma source is fairly simple and lightweight. This plasma creates a high electron density and a low degree of dissociation of the feed gas at the same time because the electron energy distribution function is not Maxwellian (bi-Maxwellian distributions). The plasma characteristics are highly suitable for the precise etching of Al and gate electrodes. Additionally, by the combination of bi-Maxwellian electron energy distribution in the UHF plasma and new fluorocarbon gas chemistries (C2F4, CF3I), selective radical generations of CF2 and CF3 could be realized for high-aspect contact hole patterning of SiO2. A high ion density and a high-energy tail in the electron energy distribution can also be maintained over a wide range of pressure (from 3 to 20 mTorr), whereas in conventional inductively coupled plasma (ICP: 13.56 MHz), the ion density and number of high-energy electrons are drastically reduced when the gas pressure is increased. This indicates that the ionization in the UHF plasma does not depend significantly on gas pressures between 3 and 20 mTorr because the discharge frequency is higher than the frequency of electron collisions in the plasma. As a result, the UHF plasma provides a process window for high-performance etching that is wider than the one provided by an ICP.
Dynamic observation of crystal imperfections has been made by live topography to image diffraction topographs instantaneously, using an intense X-ray source and a video camera. With the aid of synchrotron radiation, a resolution of 6 µm was achieved using an X-ray "Saticon" camera tube having an amorphous Se–As photoconductive layer. The property of the layer made avalanche amplification possible, enabling live topography using a conventional X-ray tube. Usefulness of live topography is demonstrated by in-situ observation of crystal-melt interfaces: Dislocations exist stably at equilibrium but become unstable with deviation from equilibrium leading to a dislocation-free state. This result is explained as the reverse of spiral growth. A formation mechanism of swirl microdefects is proposed on the basis of experimental results which indicate that only vacancies are intrinsic defects in silicon.
Cryodetectors with superconducting sensors have been actively developed and optimized during the past years. This review discusses the operating principles of the two detector concepts, superconducting tunnel junctions and low-temperature bolometer or calorimeter. Progress in research with cryodetectors manifested itself by many successful device demonstrations and as a result, a shift in emphasis from pure detector optimization towards actual applications has been observed. Cryodetectors have been used for X-ray fluorescence, optical photon detection, mass spectroscopy of heavy molecules, etc. Nevertheless, there is still room for further improvement until the detectors will have actually reached their theoretical limitations.
A comprehensive review, including some recent results, of the structures, properties and fabrication methods of inorganic and organic silicon-based materials with backbone dimensions from 0 to 3 is presented. Quantum effects in low-dimensional silicon structures are discussed using organosilicon materials, such as polyhedral compounds and oligosilanes (quantum dots), polysilanes (quantum wires), heterocopolymers (one-dimensional superlattices), and polysilynes (quantum planes). The luminous properties of silicon-based materials are also summarized.
Chlorine-based dry etching of III/V compound semiconductors for optoelectronic applications has been reviewed. The advantages of the ultrahigh-vacuum (UHV)-based electron cyclotron resonance (ECR)-plasma reactive ion beam etching (RIBE) over conventional RF-plasma reactive ion etching (RIE) were emphasized as the capability to use carbon-free, chlorine (Cl2) gas plasmas, controllability of ion energies and compatibility with other UHV-based chambers such as a molecular beam epitaxy (MBE) chamber. The RIBE technique was shown to exhibit excellent laser diode performances, such as extremely low threshold-current, high polarization-controllability and a lifetime of more than 3000 h for structures with more than 1-µm-wide etched-mesa width. The degree of etching-induced damage was evaluated in terms of the nonradiative surface recombination velocity Sr and the possibilities of practical applications of the dry-etched devices were discussed using the Sr values.
We performed recent studies on the effects of magnetic field and hydrostatic pressure on martensitic transformations in some ferrous and nonferrous alloys. The studies clarified the effects of magnetic field and hydrostatic pressure on martensitic transformation start temperature, the nature of magnetoelastic martensitic transformation and the morphology of martensites and transformation kinetics of athermal and isothermal transformations. Transformation start temperatures of all ferrous alloys examined increase with increasing magnetic field, but those of nonferrous alloys, such as Ti–Ni and Cu–Al–Ni shape memory alloys, are not affected. On the other hand, the transformation start temperature decreases with increasing hydrostatic pressure in some ferrous alloys, but increases in Cu–Al–Ni alloys. The magnetic field and hydrostatic pressure dependences of the martensitic start temperature are in good agreement with those calculated by our proposed equations. While investigations in the work on the ferrous Fe–Ni–Co–Ti shape memory alloy, we found that magnetoelastic martensitic transformation appears and, in addition, several martensite plates grow nearly parallel to the direction of the applied magnetic field in the specimen of Fe–Ni alloy single crystal. We further found that the isothermal process in Fe–Ni–Mn alloy changes to the athermal one under magnetic field and the athermal process changes to the isothermal one under hydrostatic pressure. Based on these facts, a phenomenological theory was constructed, which unifies the two transformation processes.
Recent development of technology and understanding of the growth mechanism in heteroepitaxial growth of nitrides on highly-mismatched substrates have enabled us to grow high-quality GaN, AlGaN, GaInN and their quantum well structures. Conductivity control of both n-type and p-type nitrides has also been achieved. These achievements have led to the commercialization of high-brightness blue, green and white light-emitting diodes and to the realization of short wavelength laser diodes and high-speed transistors based on nitrides. The performance of these devices is still progressing, but still requires advances in many areas of materials science and device fabrication.
The requirements for the fabrication technology of 2-dimensional and/or 3-dimensional nanometer-scale heterostructures with III–V compound semiconductors are described. In addition to a fabrication capability with nanometer accuracy, the processes must avoid both undesirable contaminations and any damage effect. To meet these requirements, we have developed in situ electron-beam (EB) processing in which all of the processes, including EB lithography, pattern etching and epitaxial overgrowth, are performed successively in an ultra-high vacuum-based environment. The present status of this technique, i.e. nanometer-scale patterning, cleanliness of the processed surfaces and damage-free characteristics, is discussed. It is also demonstrated that self-organized epitaxy, which is now being intensively studied, can be combined with in situ EB processing as an elemental process.
The development of a coherent electron beam has opened a way to measure, by electron holography, the phase distribution of an electron beam transmitted through a phase object to a precision within 1/100 of the wavelength and to observe vortex dynamics by Lorentz microscopy (defocused electron microscopy under collimated illumination). Objects and fields on the microscopic level, which have been previously inaccessible, are thus becoming observable. Examples are the measurement of a magnetic field distribution inside a vortex and the dynamic observation of vortices.
Granular matter is a typical example of a new topic in statistical (phenomenology) mechanics. Reconsidering granular matter from the physical point of view, several new aspects have been clarified, although granular matter has been studied by engineers for a long (period of) time. This review examines three topics: (1) pattern dynamics of sand ripples and dunes, (2) mathematical structure of a fluidized bed, and (3) convection and turbulence in a vibrating bed. Investigating these topics, it is found that the dynamics of granular matter exhibits many typical nonlinear phenomena, for example, formations of pattern, localized states, and turbulence.
We have applied the spin dynamics technique to conducting polymers, in particular, with the ESR method, and investigated microscopic transport properties in a quasi-one-dimensional (Q1D) polymer chain. This technique is a unique method for obtaining microscopic information on intrinsic transport properties, because macroscopic resistivity is mainly limited by electrical properties of interfacial regions between microcrystals. In this article, the principles and applications of this method will be reviewed in detail. The difference between ESR applied here and 1H NMR will also be discussed, and each characteristic feature will be stressed.
Recently, the transition to ferromagnetic order has been found in crystals of organic compounds not containing metallic elements. The progress of the research is reviewed starting from the initial stage of study to obtain the design strategies to induce ferromagnetic interaction in organic crystals to the discovery of ferromagnetism in p-nitrophenyl nitronyl nitroxide ( p-NPNN; C13H16N3O4). The recent development in the field of molecular magnetism is also described.
Recent progress in Si heterostructure engineering is reviewed from physical and technological viewpoints. Advanced methods to fabricate atomic layer doping structures, Si on insulator structures, and strain controlled double heterostructures, i.e. Si/SiGe/Si and Si/silicide/Si using molecular beam epitaxy, are developed. Detailed characterization provides a comprehensive understanding of the physical phenomena behind these new crystal growth techniques. Application of these advanced methods to novel device fabrication is also discussed.
Electron-lattice interactions in nonmetallic materials are reexamined in the many-electron scheme. The difference in the stable atomic configuration between two electronic states is the origin of the electron-lattice interaction. We show the relationship among the adiabatic potentials, one electron (hole) energy and the lattice elastic energy, paying attention to the electron-hole symmetry. Correct configuration coordinate diagrams for deep-level defects in semiconductors are presented which can be used even when the number of carriers changes due to creation and recombination. Radiative and nonradiative carrier capture and recombination processes at deep-level defects are described consistently with particular attention to the charge of a defect, the thermal and the optical depths of a bound carrier, the correlation between successive electron and hole captures, and the energy dissipation to the lattice through the interaction mode.
A review is given of recent progress in the experimental study of high-Tc superconductors which aims at understanding of the unique electronic structure and spin/charge excitations in the doped CuO2 plane, and ultimately at unravelling the high-Tc mechanism.
Ionized cluster beam (ICB) deposition has been used to form thin films of metals, insulators, semiconductors and organic materials which have unique characteristics when compared to films formed using other techniques. In addition, the use of gas-phase atoms in the form of accelerated clusters has recently shown promise for surface modification applications. A fundamental understanding of ICB deposition and related techniques requires investigations of (1) the mechanisms which lead to the growth of large vapor phase clusters, (2) techniques for determining the size distribution of large vapor clusters, (3) the initial stages of film nucleation, (4) film growth morphology related to lattice mismatch and ion beam parameters. Clarification of the role of clusters in ICB deposition has been greatly aided by atomic scale imaging by transmission electron microscopy and scanning tunnel microscopy in the early stages of film growth. Emphasis is given to the formation of high-quality, epitaxial metallic films. Several applications of ICB films with respect to microelectronics, optical mirrors, compound materials and organic materials are discussed with emphasis on the special characteristics of ICB films. Applications for gas-cluster processing are reviewed.
Recently, superionic conductors (SIC's) have been considered to be a key material for several application fields as well as energy engineering and ceramic technology. We review the recent stage of the basic models and concepts for the physical understanding of high-speed ionic transport in solids. In the latter part of this paper, we briefly review the applications of SIC's to solid-state fuel cells and sensors. As a new application of SIC's, we introduce a technique developed on a complex system consisting of an oxygen ionic conductor and high-Tc oxide super-conductors. Finally, we introduce the possibility of the application of SIC's to optical devices.
This paper reviews recent studies of regularly coiled whiskers of Si3N4 prepared by the chemical vapor deposition (CVD) method as well as those of carbon by the catalytic pyrolysis method. Coiled Si3N4 whiskers have been obtained from a gas mixture of Si2Cl6 and NH3 at 1200°C on substrates on which metal impurity was painted. The most effective impurity for the growth of the whiskers was Ni for the quartz substrate and Fe for the graphite. A vapor liquid solid (VLS) growth mechanism was suggested from morphology of the whiskers. Coiled carbon whiskers have been grown by the catalytic pyrolysis of acetylene at 300-750°C using Ni powder as a catalyst. A small amount of H2S was indispensable for the growth of the coiled carbon whiskers. A Ni compound seed observed on the tip of the pair-coiled carbon whisker is a single crystal. It is suggested that each crystal plane of the Ni compound seed has a different catalytic ability for the growth of the coiled carbon whiskers. The growth mechanism for the coiled carbon whiskers involves the surface diffusion of carbon atoms on the Ni compound seed. Structure of the coiled whiskers of Si3N4 and carbon was investigated by a scanning electron microscope (SEM) and a transmission electron microscope (TEM). Furthermore, extension characteristics of these whiskers were examined.
Lattice-dynamical aspects are coherently applied to the reversible photostructural change (PSC) effect and associated phenomena in chalcogenide glasses. Far-infrared, X-ray photoelectron and optical absorption measurements reveal that photo-induced distortions and quenching in lattice configurations are characterized by increased randomness, which can be reversed by thermal annealing for full recovery. A statistical analysis reveals clearly that PSCs such as photodarkening and photoexpansion are essentially equivalent to a thermally frozen-in effect. The PSCs can be directly traced to the strong electron-lattice coupling and localized bond strain of chalcogenide glasses. A lattice-dynamic energy diagram highlights the importance of the quadratic-term of atomic distortion (δq)2 in relating PSC to the glass transition phenomenon. The photochemical and photodoping effects are then described, on the same basis, in terms of the lattice fluctuation and high fictive temperature.
The collective effort of the author's group on the study of AlGaAs quantum-confined field-effect light emitters is outlined, starting from the physics underlying the light emitters. The developed three-terminal light emitters with functions of current injection and field control of luminescent characteristics demonstrate high-speed switchings of emission intensity at room temperature. The scheme for the high-speed switching of spontaneous emissions does not rely on changes in carrier population at all, but purely on effects of the electric fields on the oscillator strengths in quantum-well active layers of the devices pumped with a very low injection current density, ∼10 A/cm2. The response time, ∼300 ps of the spontaneous luminescence intensity for a pulsed input voltage, is observed to be completely free of the recombination lifetime limitation. Alteration of spontaneous emissions through continuous tuning of emission wavelength by electric fields applied to GaAs quantum wells, together with modification of vacuum field fluctuations of photon systems inside one-dimensional microcavities, is elaborated experimentally to further improve the device characteristics such as external efficiency and spatial coherency of light output, of the field-effect light emitters. The result of the alteration of spontaneous emission indicates the possibility of highly efficient and extremely high-speed light emitters, even with the bonus of a novel function, beam steering.
Invited Reviews Volumes 21-30
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In Penning ionization electron spectroscopy, the kinetic energy of electrons ejected by collisions between targets T (gas or solid) and metastable rare gas atoms A* is analyzed. This electron spectroscopy is selectively sensitive to the outermost surface layer of solids, since metastable atoms do not penetrate into inner layers. Furthermore, it provides information on the local electron distribution of individual orbitals exposed outside the outermost surface layer. The application of these unique features of Penning spectroscopy to the surface characterization of organic solids, including Langmuir-Blodgett films, is discussed. Both a Penning spectroscopy technique and the process of Penning ionization of gas-phase molecules are briefly described.
A comprehensive review of the Nd–Fe–B permanent magnet material is given. A historical survey, the basic magnetic properties of Nd2Fe14B and isomorphous compounds, a phase diagram for the ternary Nd–Fe–B system, processing techniques and magnetic properties of sintered Nd–Fe–B magnets, and the future directions for improvements of this type of permanent magnet are discussed.
Due to the quantum size effect, semiconductor quantum-well structure exhibits many unique material properties which can not be realized in conventional bulk crystals. These unique properties are very attractive for novel electronic and optoelectronic devices. This paper reviews studies on physical properties and application of quantum well structures for optoelectronics, and gives a future forecasting in the progress of this field.
The switching process of a surface stabilized ferroelectric liquid crystal cell is considered on the basis of disclination dynamics for various cell thicknesses. In this report we first summarize the surface stabilized ferroelectric liquid crystal states and their boundaries, and then examine the switching characteristics using stroboscopic micrographs. New experimental findings are (1) the boundary between two uniform states consists of two surface disclinations and one internal disclination, and (2) the domain nucleation, including the internal disclination loop, is responsible for the slow optical response. Finally, we point out several inconsistencies with our simple model regarding molecular orientations and suggest the existence of a selective pre-tilt at both surfaces, which should be controlled from an application point of view.
Electron scattering by various kinds of impurities in semiconductors was studied using millimeter wave and infrared cyclotron resonance, and it was found possible to use linewidth measurement to derive the inverse relaxation time of electrons due to impurities and hence the scattering coefficient as well as the cross section. The study ranged from well-identified impurity species in elemental semiconductors to more obscure ones in compound semiconductors, with both neutral and ionized impurity scattering being treated. The treatment was also extended briefly to dislocations and thermal defects. The results are tabulated for impurities in Ge and Si only, since they are basically extensible, mutatis mutandis, to impurity scattering in compound semiconductors.
A review is given of some remarkable interfacial phenomena exhibited by materials such as liquid crystals and amphiphilic molecules. Emphasis is placed on recent evidence of peculiar molecular ordering at the interface not found in the bulk. The nature of the ordering is often quite unexpected and sometimes contradicts our intuitive feelings. Because the common feature of the molecular ordering is high density and a high degree of orientational and in some cases positional order, it is proposed that the interfaces be used as a field for molecular manipulation in the preparation of new materials.
Surface atomic structure analysis by low-energy ion scattering spectroscopy (ISS) is reviewed, with particular emphasis on quantitative surface atomic structure analysis by ISS. The important differences between ISS and Rutherford backscattering spectroscopy (RBS), some basic characteristics of ISS, a special type of ISS called impact-collision ion scattering spectroscopy (ICISS), and the general features of the shadow cone in the energy range of ISS are discussed as a basis for the description of particular examples of ISS studies which follow. The examples are mainly concerned with the analysis of the atomic arrangement, defect structure, thermal vibration, and electron spatial distribution of the (001) and (111) surfaces of TiC.
Amorphous ribbons of pure PbTiO3 were obtained by the rapid solidification technique and were confirmed to be in the amorphous state by several different experimental techniques. Structural studies revealed clusters with radii up to about 150 Å, larger than those so far found in other materials such as metals or semiconductors in the amorphous state. Dielectric and Raman spectral studies confirmed that the material remains in the amorphous state indefinitely. Low-temperature studies showed that not only the amorphous state but also the crystalline state of PbTiO3 has a dielectric constant minimum in the mK region, a characteristic of the amorphous state.
Comprehensive descriptions are given of the static and dynamic behaviors of electrooptic bistable devices. The static behaviors include the characteristics of optical output versus input and optical output versus phase retardation, and several modifications of operational characteristics. The transient and unstable responses, including the switching response, periodic self-pulsation, optical monostable pulse generation and chaotic behavior, are also discussed for electrooptic bistable devices with different feedback configurations.
This paper discusses the fundamental aspects of theories describing the electromagnetic surface modes in microcorystals and their optical response. The relationships between the theories are emphasized, and three characteristics of the surface modes are summarized. Observed infrared spectra of MgO microcrystals and Raman spectra of GaP microcrystals are presented to illustrate how the surface phonon modes can be identified experimentally. The effect of surface plasmon resonance in an Ag-island film on the absorption and Raman scattering of a copper phthalocyanine (dye) coating is also discussed. Comments are also made on the extension of surface mode spectroscopy and related topics.
A short review is presented of the theoretical background of a physical model for the quantification of Auger electron spectroscopy (AES) for surface analysis. The recent studies on the data-base for the inelastic mean free paths (IMFP) by Seah and Dench and systematic calculations of the backscattering factors (R) by Shimizu and Ichimura have now enabled standard quantitative corrections comparable to those widely used in electron probe microanalysis, to be accomplished. For quantitative corrections of wider practical use, the present paper proposes the use of functional representations of the backscattering factors for different angles of incidence (ψ) for primary energies ranging from 3 to 10 keV as follows: R=1+(2.34-2.10Z0.14)×U-0.35+(2.58Z0.14-2.98) for ψ=0°, R=1+(0.462-0.777Z0.20)×U-0.32+(1.15Z0.20-1.05) for ψ=30°, R=1+(1.21-1.39Z0.13)U-0.33+(1.94Z0.131.88) for ψ=45°, where U is the ratio of the primary energy to the binding energy, and Z is the atomic number of a sample.
When Si is in contact with metal film, it readily reacts at low temperatures (≤200°C) leading to several interesting effects. For example, thick (∼1000 Å) SiO2 growth for a short time (∼10 min) due to Si–Au reaction and uniform silicide layer formations at Si/Pd, Pt, Ni interfaces. Since Si is a typical covalent semiconductor with high melting point (∼1400°C), without the presence of such effect of metal to weaken the covalent bond of Si adjacent to the metal, the above reactions rarely occur. As a possible mechanism of the bond-weakening, the present author proposes a model postulating electronic screening of Coulomb interaction responsible for the covalent bonding due to mobile free electrons in the metal films. This "Screening model" seems to be evidenced through observations of initial stages of Si-Au and -Pd reaction by both electron and ion scattering spectroscopies. In addition, new usage of the channeling effect of MeV He+ ions is demonstrated to be a powerful tool for interface and surface studies.
The principle of optically detected magnetic resonance (ODMR) is briefly described, bearing in mind its application to amorphous semiconductors. The ODMR measurements including those which are time-resolved are reviewed on amorphous semiconductors, particularly on hydrogenated amorphous silicon (a-Si: H). The nature of the recombination centres in a-Si: H is also discussed.
A consistent quantum-mechanical description is given of some atomistic properties of hydrogen isotopes in bcc metals, including in particular the energy and wave functions of a hydrogen atom, its preference of the type of interstitial sites, distortion of the surrounding lattice, and its effects on crystal structure. A general systematics in the site-preference–a transition from T sites to O sites in more congested circumstances–is noted and explained physically. Results of our recent experiments on hydrogen in V and Fe are also presented.
Electromechanical interactions in the phase transformation of crystals are discussed from the macroscopic and crystallographic points of view. The depolarizing-field effects are described in connection with the electromechanical couplings in piezoelectric media. The electrical conditions determining effective elastic constants of the sounds are elucidated. Morphic effects in the phase transition are argued from the two aspects of "induced effect" and "symmetry superposition". The similar argument is also applied to the case of ferromagnetic materials, and the macroscopic description is given of the magnetomechanical interactions.
Thin-film photodiodes with the graded-composition structures of amorphous Se–As–Te have been developed. The physical mechanism of the buit-in-field effect in these highly resistive photodiodes has been clarified. In order to realize complicated composition distributions, multi-layer evaporation technology has also been developed. The physical properties of multi-layered films and those of the uniform amorphous film have been compared, and it has been shown that a multi-layered film of 1 nm periodicity can be regarded as an almost uniform amorphous material. Built-in-field effect photoreceptors can be utilized not only for TV pickup tubes but also for highly sensitive xerographic plate and other solid-state sensors.
The current understanding of thermally induced microdefects in Czochralski-grown silicon crystals is briefly reviewed and our investigations of the defects are described. The microdefects originate in oxygen precipitation occurring during thermal treatments after crystal growth. Both homogeneous and heterogeneous nucleation models have been proposed for the oxygen precipitation. The homogeneous nucleation model is contradicted because a low density of microdefects are induced in recent high-quality crystals even at high oxygen concentrations. A heterogeneous nucleation model is proposed, based on detailed investigations of the thermal behaviors of microdefects. It is demonstrated that the oxygen precipitation is governed by nucleation sites (carbon atoms) and the thermal history of wafers after crystal growth besides the oxygen concentration of the wafer.
Invited Reviews Volumes 1-20
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As examples of the best use of the so-called AC calorimetry technique, several results are described from various viewpoints. The behavior of heat capacity at ferroelectric, antiferroelectric and structural phase transitions is surveyed from the standpoint of critical behavior, of jump at first order transition, or of a sensitive metod of finding a new phase transition. In two- and three-dimensional antiferromagnets, the critical behavior is discussed with an emphasis on the crossover from the Ising to the Heisenberg system. In the two-dimensional antiferromagnets, the crossover is revealed from the dependence of the critical amplitude on the strength of the Ising-like anisotropy. A hyperscaling relation is proposed at nematic-to-smectic A transition of liquid crystals. Finally, studies of the frequency dependence of heat capacities in the denaturation of proteins and in the order-disorder transition of alloys are reported.
A description is given of the present status of studies on the Peierls transition, which is a metal-insulator transition characteristic of an electronically one-dimensional system. Fundamental properties of a one-dimensional electron system are discussed and the concept of charge-density wave is explained. Physical properties of TTF-TCNQ and its family (TSeF-TCNQ, HMTTF-TCNQ, HMTSF-TCNQ, TMTSF-DMTCNQ and NMP-TCNQ) are described in relation to the Peierls transition, and brief descriptions are given of other one-dimensional conductors, MX3 (M=Ta, Nb; X=S, Se) and K2Pt(CN)4Br0.3·3H2O.
Since the first announcement of low-loss fiber by Corning Glass Works in 1970, remarkable progress has been made in glass fibers for optical communication both in fabrication techniques and in fiber transmission characteristics. Various fabrication methods have been proposed and examined; the outside vapor-phase oxidation (OVPO) method, the modified chemical vapor deposition (MCVD) method, and the vapor-phase axial deposition (VAD) method, to name typical examples. These processes have enabled us to obtain graded-index multi-mode fibers with low loss and broad bandwidth, as well as low-loss single-mode fibers. In particular, the development of low transmission-loss fibers in the long-wavelength band opened up a new low-loss window in the wavelength bands of 1.3 µm and 1.55 µm. This review paper describes recent progress in the fabrication methods and transmission characteristics of optical fibers, together with future trends and items for research in the field of optical communications.
The history of modelocking of the semiconductor laser is reviewed. The theory of modelocking as it relates to the semiconductor laser diode system is developed and discussed. Experiments on semiconductor lasers at MIT and the Bell Laboratories under both active and passive modelocking conditions are described.
A historical survey of progress in the study of solid solution hardening is given, and then the present state of knowledge is explained. The advances in statistical treatment related to the motion of dislocations through random arrays of solute atoms promoted by the recent discovery of inertial effects is most spectacular.
A comprehensive review is given on the studies of the nature and low temperature(≈500°C) annealing behavior of disorders generated in silicon single crystal substrates during impurity ion implantation. Interaction of disorders and impurity atoms during annealing treatment is analysed over each layer along ion trajectories. This is because the concentrations of both disorders and implanted impurity atoms have unique distribution profiles in the substrate. On the assumption that a carrier compensation center is left where the recovery of each implantation generated amorphous cluster occurs, it is found to be essential, for the electrical activation of implanted impurity atoms, that spatial overlapping of the amorphous clusters occurs. Excess vacancies generated during annealing from amorphous layers are thought to thought to contribute to the anomalous behavior of solid solubility of impurity atoms involved in regrown layers. A better understanding of the phenomena that govern implantation and subsequent annealing offered would open a way for new applications of implantation technology.
Since the invention of semiconductor lasers a decade and a half ago the first chance for the laser to enter into practical systems has finally come, that is, as a light source for optical communications. As a reliable communication system component, the laser has to be reproducible, designable and reliable. A review is made on the technological progress in crystal growth, processing and characterization for reproducibility. This, in turn, brought about the agreement between theory and experiment or, in other words, the development of device physics. A review is also made of reliability physics to meet requirements for stable system operation and easy maintenance. A bright future for semiconductor lasers is expected.
This article surveys theoretical and experimental work on piezo- and pyroelectricity of polymers in the 1970's with special emphasis on the origins of these properties. The origins are classified into three types: (A) internal strain (§2), (B) strain- and temperature-dependences of spontaneous polarization (§3), and (C) elastic and/or dielectric heterogeneity of a system with embedded charges (§4). The origin of piezo- and pyroelectricity of poly(vinylidene fluoride) is discussed as a typical example of electret (§5). Piezoelectric relaxations of polymers are discussed in some detail (§6). Methods of measurements of piezo- and pyroelectric constants of polymer films (§7) and applications of polymer films as new transducer materials (§8) are briefly reviewed.
Recent progress in the understanding of the improper ferroelectric phase transitions is reviewed. The phase transition in ammonium Rochelle salt is discussed from the symmetry point of view. The phase transitions in some improper ferroelectrics, such as langbeinite, boracite and dicalcium strontium propionate, are discussed and several problems on these materials, which are still controversial, are pointed out. In connection with the isomorphous phase transition recently discovered in dicalcium strontium propionate, the role of identity representation in the theory of phase transitions is briefly discussed.
Following a brief introductory remark on the holographic grating, the possibility of designing concave gratings for a specific application is discussed, giving basic ideas of the grating design, actual design procedures, and feasible ways of improving the existing design methods. As a proof of the usefulness of the design method developed by the authors, spot diagrams and experimental results are given for aberration-reduced holographic concave gratings that have been designed and produced specifically for a Seya-Namioka monochromator. The efficiency, stray light, and polarization characteristics of holographic and conventional concave gratings are compared for the purpose of giving some idea about the relative merits of the two kinds of gratings. The design method is also applied to the determination of the ruling parameters needed for the production of mechanically ruled stigmatic concave gratings.