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In the present paper, a theoretical and experimental study of a highly absorptive, weakly reflective coating designed and fabricated on the basis of 3D THz resonant elements is reported. Transmission and reflection of electromagnetic waves from the metamaterial-substrate structure involving a highly absorptive, weakly reflective array of artificial bi-anisotropic elements were analyzed. The samples contained paired right-handed and left-handed helices, due to the fact that the chirality was compensated. The parameters of helices were optimized to achieve roughly identical values of dielectric permittivity and magnetic permeability. As a result, the metamaterial exhibited weak reflectivity in the vicinity of resonance frequency. On the other hand, effective resonance properties of the helices were tuned to ensure substantial absorption of THz radiation. Analytical expressions for the coefficients of radiation reflection and transmission in the samples were derived by solving a boundary-value problem for the propagation of electromagnetic waves in the metamaterial-substrate system. Simulated properties of fabricated structures were compared with experimental data.

The scattering of coherent light from a system with underlying flow can be used to yield essential information about dynamics of the process. In the case of pulsatile flow, there is a rapid change in the properties of the speckle images. This can be studied using the standard laser speckle contrast and also the fractality of images. In this paper, we report the results of experiments performed to study pulsatile flow with speckle images, under different experimental configurations to verify the robustness of the techniques for applications. In order to study flow under various levels of complexity, the measurements were done for three in-vitro phantoms and two in-vivo situations. The pumping mechanisms were varied ranging from mechanical pumps to the human heart for the in vivo case. The speckle images were analyzed using the techniques of fractal dimension and speckle contrast analysis. The results of these techniques for the various experimental scenarios were compared. The fractal dimension is a more sensitive measure to capture the complexity of the signal though it was observed that it is also extremely sensitive to the properties of the scattering medium and cannot recover the signal for thicker diffusers in comparison to speckle contrast.

Compared to conventional NAND flash resistive switching metal-oxide cells show a number of advantages, like an increased endurance, lower energy consumption, and superior switching speed. Understanding the role of defects for the resistive switching phenomenon in metal oxides is crucial for their improvement and thereby also for their acceptance as a next generation data storage device. Strontium titanate (STO) is considered a model material due to its thoroughly investigated defect chemistry. This paper presents a comparative study of the switching kinetics for three different compositions [Sr]/([Sr]+[Ti]) of 0.57 (Sr-rich), 0.50 (stoichiometric STO), and 0.46 (Ti-rich STO). The STO films, deposited by atomic layer deposition, were integrated in Pt/STO/TiN nanocrossbars with a feature size of 100 nm. By analysis of the transient currents, the switching kinetics are investigated between 10 ns and 104 s for the SET and 10 ns and 100 s for the RESET. A clear influence of the composition on the degree of nonlinearity of the switching kinetics was observed. Applying an analytical model for the oxygen vacancy migration, we were able to explain the differences in the SET kinetics by composition-dependent changes in the thermal conductivity and by a lower activation energy for the Ti-rich sample. This might be utilized in design rules of future ReRAM devices.I. INTRODUCTION

This paper examines the structural properties of gallium phosphide layers by high resolution x-ray diffraction and atomic force microscopy measurements. GaP layers are grown on misoriented and nominally exactly oriented silicon (001) substrates by metalorganic vapor phase epitaxy. Structural characterization is performed by reciprocal lattice map and transverse scan measurements of (00l)-reflections (l = 2, 4, 6). Transverse scan line profiles of GaP layers on exactly oriented and misoriented substrates are compared thoroughly and antiphase disorder related satellite peaks are observed on exactly oriented substrates. In addition, results imply that antiphase disorder is self-annihilated on misoriented substrates. The dependence of crystallographic tilt on growth temperature indicates structural coherence. Williamson-Hall-like plot of transverse scans reveals the lateral correlation length of crystalline defects of 79 nm which gives the average size of the mosaic crystallites. In addition, the mosaicity of the GaP layer is 0.042°.

This paper describes performance characteristics of a bridge‐connected magnetic circuit with single output winding in dc control. The difference between positive and negative amplitudes of output current changes with the dc control current. An application of the bridge‐connected magnetic circuit to a dc current detector is presented in this paper.

In this paper, we study the efficiency of several types of all-dielectric, non-resonant, antireflection, and light-trapping coatings for the enhancement of photovoltaic absorption in thin-film silicon solar cells. We compare the enhancement of the photovoltaic absorption offered by a square array of nano-pillar shaped voids in the dielectric covering of the cell with that granted by a flat blooming layer, and a densely packed array of dielectric nanospheres. We optimize these coatings and show that the newly proposed nanostructure allows a significant increase of the photovoltaic absorption. The dependence of antireflection and light-trapping properties on the angle of incidence is numerically investigated, and it is shown that the array of voids keeps optimal also after averaging over the incidence angles.

In this paper, we present a systematic study of the effect of growth parameters on the structural and optical properties of InAs quantum dot (QD) grown under Stranski-Krastanov mode by molecular beam epitaxy. The dot density is significantly reduced from 1.9x10(10) to 0.6x10(10) cm(-2) as the growth rate decreases from 0.075 to 0.019 ML/s, while the island size becomes larger. Correspondingly, the emission wavelength shifts to the longer side. By increasing the indium fraction in the InGaAs capping layer, the emission wavelength increases further. At indium fraction of 0.3, a ground state transition wavelength as long as 1.4 mum with the excited state transition wavelength of around 1.3 mum has been achieved in our dots. The optical properties of QDs with a ground state transition wavelength of 1.3 mum but with different growth techniques were compared. The QDs grown with higher rate and embedded by InGaAs have a higher intensity saturation level from excitation dependent photoluminescence measurements and a smaller intensity decrease from temperature dependent measurements. Finally, single mirror light emitting diodes with a QD embedded in InGaAs have been fabricated. The quantum efficiency at room temperature is 1.3%, corresponding to a radiative efficiency of 21.5%. (C) 2002 American Institute of Physics.

In this paper, low energy electron beam (5–20 keV, 0–500 μAs/cm2) induced damage on a GaN/InGaN/GaN near-surface quantum well structure is studied. Exposure to low energy electron beam is shown to significantly reduce the optical quality of the structure. It is also observed that reducing the electron beam energy causes larger PL intensity reduction. This can be explained by considering the beam penetration depth, which is shown to be smaller with lower e-beam energies. The damage is believed to be attributed to enhanced dislocation mobility upon low energy electron beam irradiation. However, further studies are needed to confirm the mechanism. These results should be taken into consideration in low energy electron beam related sample characterization and preparation.

Recently, a renewed interest in Zener tunneling has arisen because of its increasing impact on semiconductor device performance at nanometer dimensions. In this paper we evaluate the tunnel probability under the action of a nonuniform electric field using a two-band model and arrive at significant deviations from the commonly used Kanes model, valid for weak uniform fields only. A threshold on the junction bias where Kanes model for Zener tunneling breaks down is determined. Comparison with Kanes model particularly shows that our calculation yields a higher tunnel probability for intermediate electric fields and a lower tunnel probability for high electric fields. When performing a current calculation comparing to the WKB approximation for the case of an abrupt p-n junction significant differences concerning the shape of the I-V curve are demonstrated.

We have recently proposed a solid state heat pump based on photon mediated heat transfer between two large-area light emitting diodes coupled by the electromagnetic field and enclosed in a semiconductor structure with a nearly homogeneous refractive index. Ideally the thermophotonic heat pump (THP) allows heat transfer at Carnot efficiency but in reality there are several factors that limit the efficiency. The efficient operation of the THP is based on the following construction factors and operational characteristics: (1) broad area semiconductor diodes to enable operation at optimal carrier density and high efficiency, (2) recycling of the energy of the emitted photons, (3) elimination of photon extraction losses by integrating the emitting and the absorbing diodes within a single semiconductor structure, and (4) eliminating the reverse thermal conduction by a nanometer scale vacuum layer between the diodes. In this paper we develop a theoretical model for the THP and study the fundamental physical limitations and potential of the concept. The results show that even when the most important losses of the THPs are accounted for, the THP has potential to outperform the thermoelectric coolers especially for heat transfer across large temperature differences and possibly even to compete with conventional small scale compressor based heat pumps.

Magnetic nanowires (NWs) are promising as material for use in spintronics and as the precursor of permanent magnets because they have unique properties due to their high aspect ratio. The growth of magnetic Fe whiskers was reported in the 1960s, but the diameter was not on a nanoscale level and the growth mechanism was not fully elucidated. In the present paper, we report the almost vertical growth of Fe NWs on a single crystal yttrium-stabilized zirconia (Y0.15Zr0.85O2) by a thermal CVD method. The NWs show a characteristic taper part on the bottom growing from a trigonal pyramidal nucleus. The taper angle and length can be controlled by changing the growth condition in two steps, which will lead to obtaining uniformly distributed thin Fe NWs for applications.

This paper discusses two new kinds of micromagnetoelastic devices made on silicon wafer whose function is to control the permeability of magnetostrictive soft magnetic thin films by voltage-controlled elastic strain. One is piezoelectric type and the other is electrostatic type. Structure, fabrication process, characteristics, and maximum possible output are discussed. The feasibility of these devices has been clarified although the rate of obtained permeability change was less than 1%. These device characteristics could be improved to 11% for piezoelectric type and to 80% for electrostatic type by the optimization of device dimensions and reduction of process damage to the magnetic film.

This paper discusses free carrier generation by pulsed laser fields as a mechanism to switch the optical properties of semiconductor photonic crystals and bulk semiconductors on an ultrafast time scale. Requirements are set for the switching magnitude, the time-scale, the induced absorption as well as the spatial homogeneity, in particular for silicon at lambda= 1550 nm. Using a nonlinear absorption model, we calculate carrier depth profiles and define a homogeneity length l_hom. Homogeneity length contours are visualized in a plane spanned by the linear and two-photon absorption coefficients. Such a generalized homogeneity plot allows us to find optimum switching conditions at pump frequencies near v/c= 5000 cm^{-1} (lambda= 2000 nm). We discuss the effect of scattering in photonic crystals on the homogeneity. We experimentally demonstrate a 10% refractive index switch in bulk silicon within 230 fs with a lateral homogeneity of more than 30 micrometers. Our results are relevant for switching of modulators in absence of photonic crystals.

Graphene nanomeshes (GNMs) are expected to be a high-performance channel material for metal-oxide-semiconductor field-effect-transistors (MOSFETs), since they can open up a band gap in a large sheet of graphene thin film by simply introducing two-dimensional periodical nanoscale holes. In this paper, we theoretically investigate the electronic band structures and the electron transport properties of GNMs based on a tight-binding approach. We demonstrate that GNMs have the capability of band structure engineering by controlling its neck width and furthermore the potential ability providing high current drivability when applied to a field-effect-transistor channel.

InAs quantum dots (QDs) grown on InP substrates can be used as light emitters in the telecommunication bands. In this paper, we present optical characterization of high-density circular quantum dots (QDots) grown on InP(311)B substrates and elongated dots (QDashes) grown on InP(001) substrates. We study the charge carrier transfer and luminescence thermal quenching mechanisms of the QDots and QDashes by investigating the temperature dependence of their time-integrated and time-resolved photoluminescence properties. This results in two different contributions of the thermal activation energies. The larger activation energies are attributed to the carrier escape to the barrier layer and the wetting layer (WL) from QDots and QDashes, respectively. The smaller activation energies are found to be originated from inter-dot/dash carrier transfer via coupled excited states. The variation of the average oscillator strength associated with the carrier re-distribution is discussed. The relation of the two activation energies is also quantitatively studied with the measurements of excited-state and ground-state energy separations. Finally, we show an approach to isolate individual quantum dots or dashes in a suitable nanostructure.

It has recently been shown how the injection barriers at the electrode interfaces and the built-in voltage (Vbi) of organic light emitting diodes can be obtained from measurements of the differential capacitance at low frequencies, using the voltage and height of a distinct peak in the capacitance-voltages curves. In this paper, we investigate the effects of Gaussian energetic disorder on the analysis, for single-carrier and double-carrier devices. We show how the disorder affects the peak position and height, and how from combined capacitance-voltage measurements and device modeling the injection barriers and Vbi can be determined. As examples, hole-only and double-carrier devices are investigated based on a blue-emitting polyfluorene-triarylamine copolymer.

Two-photon-induced photopolymerization (2PP) has gained increased interest due to the capability of manufacturing three-dimensional structures with very high feature resolution. To assess the suitability of photopolymer systems for 2PP, methods have to be developed that allow a screening of the efficiency of monomer-initiator combinations in the context of high throughput, large processing window and geometric quality of the final parts. In this paper, a method for evaluating 2PP structures is described. For this purpose, the double-bond conversion of fabricated 2PP structures was measured giving quantifiable results about the efficiency of the photoinitiator. The method is based on local measurement of the double-bond conversion of the photopolymer using a microscope in combination with infrared spectroscopy. The obtained double-bond conversion is a measure for the efficiency of the photopolymer system (initiator in combination with monomer), and thus allows to compare different photopolymers in a quantitative way. Beside this evaluation of 2PP structures, fabrication of complex 3D structures was done to determine the limits of the 2PP technology for miscellaneous components.

Electromagnetic cloaks are devices that can be used to reduce the total scattering cross section of various objects. An ideal cloak removes all scattering from an object and thus makes this object "invisible" to the electromagnetic fields that impinge on the object. However, ideal cloaking appears to be possible only at a single frequency. To study cloaking from an electromagnetic pulse we consider propagation of a pulse inside a waveguide with a cloaked metal object inside. There are several ways to achieve cloaking and in this paper we study three such methods, namely, the coordinate-transformation cloak, the transmission-line cloak, and the metal-plate cloak. In the case of the two last cloaks, pulse propagation is studied using experimental data whereas the coordinate-transformation cloak is studied with numerical simulations. The results show that, at least in the studied cases where the cloaked object's diameter is smaller than the wavelength, the cloaks based on transmission-line meshes and metal plates have wider bandwidth than the coordinate-transformation cloak., Comment: 17 pages, 9 figures

In this paper, we study the influences of a channel source-end potential profile on the ballistic transport of carriers in Si metal oxide semiconductor field effect transistors (MOSFETs) based on a quantum-corrected Monte Carlo device simulation. As a result, we found that higher ballistic efficiency is expected in MOSFETs with a heterojunction bottleneck barrier, such as Schottky source/drain MOSFETs, compared to that with the conventional p-n junction source and drain. Such a superior ballistic behavior is demonstrated due to the narrower bottleneck potential profile formed at the source-channel interface.

Linear motors find their utilization in an increasing number of industrial applications. Permanent magnet linear synchronous motors (PMLSMs) are favorable in many applications due to their servo characteristics, robustness, and high force density. The major disadvantage of moving coil type PMLSMs is the cable slab that energizes the coils from the fixed world to the moving parts of the machine. These cable slabs introduce extra wear and dynamical distortions. Moreover, in precision application the cable slab is supported by additional linear drives. These disadvantages can be eliminated if the coils could be powered wirelessly. In this paper two topologies are proposed that are capable of transferring 1 kW of power at most. The transformer part of the CET is characterized by means of two dimensional finite element analysis, and the influence of using additional capacitors to boost the output power is examined. Furthermore, an analysis of the core losses is conducted. Conclusions are drawn from the results.

We have found that molybdenum dioxide (MoO_2) is an excellent dopant for enhancing electrical conductivities in organic hole-transporting layers. We fabricated hole-only devices with an alpha-sexithiophene (α-6T) layer doped with MoO_2 at various concentrations to investigate how doping MoO_2 into the α-6T layers influences the hole-injection and hole-transport characteristics of these layers. We observed a marked increase in electrical conductivity as a result of the MoO_2 doping. The 30-mol % MoO_2-doped α-6T layer had a high electrical conductivity of 8.9±1.3×10^ S/cm. From the results of our visible/near-infrared absorption spectra study of these doped layers, we confirmed that this increase in electrical conductivity is caused by a charge transfer between MoO_2 and α-6T, which leads to an increase in free hole concentration in the doped layers and the formation of an ohmic contact at an electrode/α-6T interface. In the latter part of this paper, we discuss current flow and electroluminescence (EL) characteristics of organic light-emitting diodes (OLEDs) with a 30-mol % MoO_2-doped α-6T hole-transporting layer and a 30-mol % Cs-doped phenyldipyrenylphosphine oxide (POPy_2) electron-transporting layer. We achieved an extremely low driving voltage of 3.1 V required for a current density of 100 mA/cm^2 in the doped OLEDs owing to the use of the α-6T and POPy_2 layers with high carrier mobilities and the excellent p-type MoO_2 and n-type Cs dopants. We demonstrated the enhancement of power efficiencies by ≈2 times in the doped OLEDs compared with undoped OLEDs and observed bright EL at low driving voltages in the doped OLEDs, for example, 100 cd/m^2 at 2.3 V, 1000 cd/m^2 at 2.7 V, and 10 000 cd/m^2 at 3.3 V.

In this paper, we report the structural and electronic properties of pentacene thin films grown on a polycrystalline Co film using atomic force microscopy and ultraviolet photoemission spectroscopy (UPS), respectively. Investigation of this type of interface is of importance for the engineering of hybrid organometallic spintronic devices for which the use of spin polarized electrodes is a prerequisite. Uniform single crystalline areas of pentacene as large as several micrometers, with molecules arranging almost perpendicular to the substrate, were obtained. For the electronic properties at this interface, we have found an energy barrier for the hole injection of about 1 eV, in spite of the fact that the ionization potential of pentacene reported previously equals the work function of Co. A shift of the vacuum level of the same magnitude has also been observed. A comparison of the UPS spectra of the pentacene films with the gas phase spectrum directly indicates that hybridization effects are present at this interface.

This paper demonstrates that the spatial distribution of electronic states of an arsenic ion implanted Si (111) substrate can be observed by using a wavelength tunable second harmonic (SH) microscope in a wide photon energy range from 2hω=1.96 to 5.19 eV. The contrast in the SH intensity images between the As-doped area and the nondoped area depends greatly on the SH photon energy. For 2hω>3 eV, optical second harmonic generation (SHG) from the nondoped area was stronger than from the doped area, and the contrast was reversed for 2hω

For performing electron‐beam‐induced current (EBIC) measurements on sufficiently large samples, the use of a ‘‘planar‐collector geometry’’ (i.e., with the collector covering part of the irradiated surface itself) is very attractive. However, the pertinent theoretical EBIC curves for finite surface‐recombination velocities s have so far been lacking. This paper presents the complete theoretical expressions for arbitrary values of s and diffusion length L. Simple asymptotic solutions are given for point‐ and finite‐size generation sources. Easy methods are developed to facilitate the application of these solutions in the practical evaluation of L and s from experimental EBIC curves. These methods are applied to experimental data available through the literature.

This paper describes a two c‐core type orthogonal‐core made of Fe‐6.5 wt. % Si sheet and its application to power converters. The orthogonal‐core can be used in the high‐frequency region because a Fe‐6.5 wt. % Si sheet has superior soft magnetic properties including high permeability and low core loss. Using the orthogonal‐core, we constructed a parametric oscillation type dc‐ac converter. The dc‐ac converter had voltage regulation, overload protection, noise rejection, and almost sinusoidal output. Good efficiency was obtained in the frequency range from 400 Hz to 1 kHz. Furthermore, we propose a dc‐dc converter for coupling two independent dc power sources. The dc‐dc coupling converter has bilateral power transfer and ease of controlling the transferred power. The basic operation of the dc‐dc coupling converter is described.

International audience; Rare earth ions hosted in solids are good candidates for quantum technologies due to their chemical stability and optical and spin transitions exhibiting long coherence lifetimes. While bulk oxide crystals are usually the preferred host material, the development of a scalable silicon-compatible thin film platform would be desirable. In this paper, we report on the growth of Y2(1-x)Eu2xO3 thin films on silicon in the full range of Eu3+ concentration by direct liquid injection Chemical Vapour Deposition. Our sub-micron polycrystalline films with a strong-(111) texture were grown for all compositions into the bixbyite cubic phase. The variation of growth rates with temperature and flow indicated that deposition occurred through a mass-transport controlled regime. Optical assessment of the Eu-doped thin films showed inhomogeneous linewidths as narrow as 50 GHz and fluorescence lifetimes of 1ms for the lowest concentrations. Finally, a spectral hole was successfully burned in a 200 nm-thin film with a 2 % Eu doping leading to a homogeneous linewidth of 11 MHz. These values are still below those reported for bulk single crystals indicating that additional decoherence mechanisms exist in such nanometric films, which might be alleviated by further improvement of the crystalline quality. Nevertheless, these results pave the way to the use of CVD-grown Eu:Y2O3 thin films as a platform for integrated quantum devices.