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Considering the strong built-in electric field caused by polarization in wurtzite nitride heterostructures, the energy levels and wave functions of the two-dimensional electrons in AlyGa1−yN back-barrier Al2O3/InxAl1−xN/GaN heterostructures are calculated using the finite element difference method. The dispersion relations and electrostatic potentials of optical phonons are obtained by the transfer matrix method. The electron mobility under optical phonon scattering is studied based on the theory of Lei–Ting force balance equation. The influences of AlyGa1−yN back barrier, ternary mixed crystal effect, and size effect are also analyzed by comparing with the Al2O3/InxAl1−xN/GaN heterostructure without back barrier. It is found that the introduction of a back barrier can attenuate the effect of gate-insulating layer, but enhance the effect of barrier and channel layers on electronic states. Then, the interaction between electrons and optical phonons is weakened, so the electron mobility in AlyGa1−yN back-barrier Al2O3/InxAl1−xN/GaN heterostructure is higher than that in Al2O3/InxAl1−xN/GaN structure under the same conditions. It is also found that using a thinner AlyGa1−yN film with y = 0.25 as the back-barrier layer of Al2O3/InxAl1−xN/GaN heterostructure is more conducive to improving 2DEG mobility. These conclusions can provide references for the preparation of InxAl1−xN/GaN heterojunction devices. [ABSTRACT FROM AUTHOR]

Considering the anisotropy of wurtzite semiconductors, the interface optical phonons in double-channel AlGaN/GaN heterostructures are investigated by using a dielectric continuous model and transfer matrix method. Also, the ternary mixed crystal effect and size effect on the dispersion relations and electrostatic potentials of phonons are analyzed in detail. The results show that there are six branches of interface phonon modes in a double-channel heterostructure. For some values of Al composition, however, the phonon mode with the highest frequency may not exist, especially when the thicknesses of materials and the wave vectors of phonons are small. The ternary mixed crystal effect and size effect not only influence the values of phonon frequency and electrostatic potential, but also change the vibration mode of interface phonons. This suggests that the interface phonon vibrations can be controlled to reduce their adverse effects by changing the Al composition of AlGaN and the thickness of each layer in a double-channel heterostructure. [ABSTRACT FROM AUTHOR]

Applying a finite difference method and modified random-element-isodisplacement model, the mobility of electrons in the two dimensional electron gas in Al2O3/AlGaN/GaN double heterojunctions is calculated in consideration of scattering from interface and half-space optical phonons based on the theory of force balance equation. Considering the effect of ternary mixed crystals and built-in electric fields, the electronic wave functions and corresponding eigen-energies are obtained by solving Schr€odinger equations. The results show that electronic mobility decreases with increasing Al from a small component, and then increases with the increasing Al. Other effects such as the size, fixed charges at Al2O3/AlGaN interface, and temperature are also discussed. It is found that the thickness of AlGaN layer increases the mobility, and the fixed charges also increase the mobility but within a certain range, whereas the thickness of Al2O3 layer and temperature reduce the mobility. Some of our results are compared with the experimental data and our conclusion is helpful for designing high electron mobility transistors. Published by AIP Publishing. [ABSTRACT FROM AUTHOR]

The Planck constant, with its mathematical symbol h, is a fundamental constant in quantum mechanics that is associated with the quantization of light and matter. It is also of fundamental importance to metrology, such as the definition of ohm and volt and the latest definition of kilogram. One of the first measurements to determine the Planck constant is based on the photoelectric effect; however, the values thus obtained so far have exhibited a large uncertainty. The accepted value of the Planck constant, 6.626 070 15 × 10−34 J s, is obtained from one of the most precise methods, the Kibble balance, which involves the quantum Hall effect, the Josephson effect, and the use of the international prototype of the kilogram or its copies. Here, we present a precise determination of the Planck constant by modern photoemission spectroscopy technique. Through the direct use of Einstein's photoelectric equation, the Planck constant is determined by accurately measuring the energy position of the gold Fermi level using light sources with various photon wavelengths. The precision of the Planck constant as measured in this work, 6.626 10(13) × 10−34 J s, is improved by four to five orders of magnitude from the previous photoelectric effect measurements. We propose that this direct method of photoemission spectroscopy has potential to further increase its measurement precision of the Planck constant to be comparable to the most accurate methods available at present. [ABSTRACT FROM AUTHOR]

Spin information carried by magnons is attractive for computing technology, and the development of magnon-based computing circuits is of great interest. However, magnon transport in insulators has been challenging, different from the clear physical picture for spin transport in conductors. Here, we investigate the lateral transport properties of thermally excited magnons in yttrium iron garnet (YIG), a model magnetic insulator. Polarity reversals of detected spins in non-local geometry devices have been experimentally observed and are strongly dependent on temperature, YIG film thickness, and injector-detector separation distance. A competing two-channel transport model for thermally excited magnons is proposed, which is qualitatively consistent with the spin signal behavior. In addition to the fundamental significance for thermal magnon transport, our work furthers the development of magnonics by creating an easily accessible magnon source with controllable transport. [ABSTRACT FROM AUTHOR]

We report a practical superconducting terahertz (THz) source, comprising a stack of Bi2Sr2CaCu2O8 intrinsic Josephson junctions (IJJs) and a vanadium dioxide (VO2) tunable attenuator with coplanar interdigital contacts. The electrical triggering phase transitions are observed not only at room temperature, but also at low temperatures, which provides a proof of the electrical triggering. Applying this, the VO2 attenuator is implemented for the independent regulations on the emission powers from the IJJ THz emitter, remaining frequencies and temperatures unchanged. The attenuation can be tuned smoothly and continuously within a couple of volts among which the maximum is, respectively, -5.6 dB at 20K or -4.3 dB at 25K. Such a power-adjustable radiation source, including the VO2 attenuator, can further expand its practicability in cryogenic THz systems, like superconducting THz spectrometers. [ABSTRACT FROM AUTHOR]

We investigate the electrical manipulation of the phase transition in ultrathin FeRh films through a combination of ionic liquid and oxide gating. The 5 nm-thick FeRh films show an antiferromagneticferromagnetic transition at around 275K with in-plane magnetic field of 70 kOe. A negative gate voltage seriously suppresses the transition temperature to ~248K, while a positive gate voltage does the opposite but with a smaller tuning amplitude. The formation of electric double layer associated with a large electric field induces the migration of oxygen ions between the oxide gate and the FeRh layer, producing the variation of Fe moments in antiferromagnetic FeRh accompanied by the modulation of the transition temperature. Such a modulation only occurs within several nanometers thick scale in the vicinity of FeRh surface. The reversible control of FeRh phase transition by electric field might pave the way for non-volatile memories with low power consumption. [ABSTRACT FROM AUTHOR]

Nanocrystalline antiperovskite GaNxMn3 powders were prepared by mechanically milling. The micrograin GaNxMn3 exhibits an abrupt volume contraction at the antiferromagnetic (AFM) to paramagnetic (PM) (AFM-PM) transition. The temperature window of volume contraction (ΔT) is broadened to 50 K as the average grain size (〈D〉) is reduced to ~30 nm. The corresponding coefficient of linear thermal expansion (α) reaches ~ -70 ppm/K, which are comparable to those of giant NTE materials. Further reducing 〈D〉 to ~10 nm, ΔT exceeds 100 K and α remains as large as -30 ppm/K (-21 ppm/K) for x = 1.0 (x = 0.9). Excess atomic displacements together with the reduced structural coherence, revealed by high-energy X-ray pair distribution functions, are suggested to delay the AFM-PM transition. By controlling 〈D〉, giant NTE may also be achievable in other materials with large lattice contraction due to electronic or magnetic phase transitions. [ABSTRACT FROM AUTHOR]

We investigate the electrical manipulation of Co/Ni magnetization through a combination of ionic liquid and oxide gating, where HfO2 with a low O2- ion mobility is employed. A limited oxidation-reduction process at the metal/HfO2 interface can be induced by large electric field, which can greatly affect the saturated magnetization and Curie temperature of Co/Ni bilayer. Besides the oxidation/reduction process, first-principles calculations show that the variation of d electrons is also responsible for the magnetization variation. Our work discloses the role of gate oxides with a relatively low O2- ion mobility in electrical control of magnetism, and might pave the way for the magneto-ionic memory with low power consumption and high endurance performance. [ABSTRACT FROM AUTHOR]

We report on Bi2Sr2CaCu2O8 (BSCCO) intrinsic Josephson junction stacks with improved cooling, allowing for a remarkable increase in emission frequency compared to the previous designs. We started with a BSCCO stack embedded between two gold layers. When mounted in the standard way to a single substrate, the stack emits in the range of 0.43-0.82 THz. We then glued a second, thermally anchored substrate onto the sample surface. The maximum voltage of this better cooled and dimension-unchanged sample was increased and, accordingly, both the emission frequencies and the tunable frequency range were significantly increased up to 1.05 THz and to 0.71 THz, respectively. This double sided cooling may also be useful for other "hot" devices, e.g., quantum cascade lasers. [ABSTRACT FROM AUTHOR]

We describe the development of a glucose sensor through direct incorporation of an enzyme (glucose oxidase) into the gate of an organic thin film transistor (OTFT). We show that glucose diffusion is the key determinant of the device response time and present a mechanism of glucose sensing in these devices that involves protonic doping of the transistor channel via enzymatic oxidation of glucose. The integrated OTFT sensor is sensitive across 4 decades of glucose concentration; a range that encompasses both the blood and salivary glucose concentration levels. As such, this work acts as a proof-of-concept for low-cost printed biosensors for salivary glucose. [ABSTRACT FROM AUTHOR]

An edge toroidal charge exchange recombination spectroscopy (eCXRS) diagnostic, based on a heating neutral beam injection (NBI), has been deployed recently on the Experimental Advanced Superconducting Tokamak (EAST). The eCXRS, which aims to measure the plasma ion temperature and toroidal rotation velocity in the edge region simultaneously, is a complement to the exiting core CXRS (cCXRS). Two rows with 32 fiber channels each cover a radial range from ~2.15 m to ~2.32 m with a high spatial resolution of ~5-7 mm. Charge exchange emission of Carbon VI CVI at 529.059 nm induced by the NBI is routinely observed, but can be tuned to any interested wavelength in the spectral range from 400 to 700 nm. Double-slit fiber bundles increase the number of channels, the fibers viewing the same radial position are binned on the CCD detector to improve the signal-to-noise ratio, enabling shorter exposure time down to 5 ms. One channel is connected to a neon lamp, which provides the real-time wavelength calibration on a shot-to-shot basis. In this paper, an overview of the eCXRS diagnostic on EAST is presented and the first results from the 2015 experimental campaign will be shown. Good agreements in ion temperature and toroidal rotation are obtained between the eCXRS and cCXRS systems. [ABSTRACT FROM AUTHOR]

The design and performance of the first vacuum ultraviolet (VUV) laser-based angle-resolved photoemission (ARPES) system are described. The VUV laser with a photon energy of 6.994 eV and bandwidth of 0.26 meV is achieved from the second harmonic generation using a novel nonlinear optical crystal KBe2BO3F2. The new VUV laser-based ARPES system exhibits superior performance, including superhigh energy resolution better than 1 meV, high momentum resolution, superhigh photon flux, and much enhanced bulk sensitivity, which are demonstrated from measurements on a typical Bi2Sr2CaCu2O8 high temperature superconductor. Issues and further development related to the VUV laser-based photoemission technique are discussed. [ABSTRACT FROM AUTHOR]

We propose that a defect complex comprising a suboxide Hf–Si bond and an interfacial dangling bond is responsible for the stress-induced buildup of interface traps in Si/SiO2/HfO2 capacitors. With the aid of first-principles calculations, we show that these defects possess a symmetric double-well energy minimum with a moderate intervening barrier. The calculated activation energies suggest a relatively easy hopping of H atoms between the two energy minima (a field-aided shuttling mechanism). This mechanism can explain the experimentally measured oscillations of interface-trap densities during switched-bias conditions following x-ray irradiation or constant-voltage stress. [ABSTRACT FROM AUTHOR]

The 1/f noise magnitude of n-channel metal-oxide-semiconductor field effect transistors is found to decrease by up a factor of ∼3 after 18 years of room-temperature aging. This decrease is largest in devices with high-temperature post-gate-oxidation N2 annealing, which increases the densities of O vacancies and strained Si–Si bonds near the Si/SiO2 interface. Approximately 100 mV positive threshold voltage shifts are observed for all device types during aging. These results suggest that the aging related changes in 1/f noise may well be caused by the relaxation of strained Si–Si bonds associated with O vacancies in the near-interfacial SiO2. [ABSTRACT FROM AUTHOR]

The effects of the surface treatments on the performance of Ti/Al/Ti/Au Ohmic contacts to undoped AlGaN/GaN heterostructure are investigated. Contact resistance and specific contact resistivity as low as 0.25 Ω mm and 5.85×10-7 Ω cm2, respectively, have been achieved through pretreatment in CCl2F2 plasma followed by buffered oxide etch solution, with rapid thermal annealing at 900 °C for 30 s in a N2 ambient. The electrical quality and annealing temperature dependence of the metallization scheme are illustrated. Furthermore, excellent edge acuity is also demonstrated for this annealed Ti/Al/Ti/Au Ohmic contact. [ABSTRACT FROM AUTHOR]

We report first-principles calculations that elucidate the mechanisms that underlie key features of negative-bias temperature instability (NBTI). We show that the depassivation of Si–H bonds by protons released in the Si substrate is consistent with the observed increase in interface-trap density. The calculated activation energy of 0.36 eV is in excellent agreement with observations for long stress times. Adequate amounts of hydrogen, needed to initiate depassivation, are likely to exist in the substrate, trapped in complexes with dopants. The role of holes in the H release mechanism is identified. Finally, we explain how the above mechanisms can account for various experimental NBTI observations. [ABSTRACT FROM AUTHOR]

Fluorine is known to have both a beneficial and adverse role on the characteristics of Si–SiO2-based devices. Here we report the results of first-principles calculations in terms of which we elucidate this dual behavior. On one hand, we find that Si–F interfacial bonds are resistant to depassivation by hydrogen species, with the process being shut down in the presence of holes. However, we also show that any excess of fluorine has a negative effect since it either creates stretched bonds, which can act as carrier traps, or disrupts an abrupt interface by cleavage of Si–Si bonds. [ABSTRACT FROM AUTHOR]

Negative bias-temperature instability (NBTI) in metal–oxide–semiconductor capacitors with SiOxNy/HfO2 gate dielectrics is compared to those with thermal SiO2 oxides. Activation energies for interface and oxide-trap charge densities for each device type, estimated from capacitance–voltage measurements versus temperature and electric field, lie in the range 0.2–0.4 eV. This suggests that the release of hydrogen from, e.g., oxide protrusions in Si, followed by the lateral motion of protons along the interface (activation energy ∼0.3 eV), may play a key role in NBTI. Passivation reactions between protons and Si–H can create interface traps, and proton capture by sub-oxide bonds (O vacancies) can lead to positive trapped-oxide charge. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Nanocrystalline antiperovskite GaNxMn3 powders were prepared by mechanically milling. The micrograin GaNxMn3 exhibits an abrupt volume contraction at the antiferromagnetic (AFM) to paramagnetic (PM) (AFM-PM) transition. The temperature window of volume contraction (ΔT) is broadened to 50 K as the average grain size (〈D〉) is reduced to ~30 nm. The corresponding coefficient of linear thermal expansion (α) reaches ~ -70 ppm/K, which are comparable to those of giant NTE materials. Further reducing 〈D〉 to ~10 nm, ΔT exceeds 100 K and α remains as large as -30 ppm/K (-21 ppm/K) for x = 1.0 (x = 0.9). Excess atomic displacements together with the reduced structural coherence, revealed by high-energy X-ray pair distribution functions, are suggested to delay the AFM-PM transition. By controlling 〈D〉, giant NTE may also be achievable in other materials with large lattice contraction due to electronic or magnetic phase transitions. [ABSTRACT FROM AUTHOR]