Modelling and comparison of trapped fields in (RE)BCO bulk superconductors for activation using pulsed field magnetization

The ability to generate a permanent, stable magnetic field unsupported by an electromotive force is fundamental to a variety of engineering applications. Bulk high temperature superconducting (HTS) materials can trap magnetic fields of magnitude over ten times higher than the maximum field produced by conventional magnets, which is limited practically to rather less than 2 T. In this paper, two large c-axis oriented, single-grain YBCO and GdBCO bulk superconductors are magnetized by the pulsed field magnetization (PFM) technique at temperatures of 40 and 65 K and the characteristics of the resulting trapped field profile are investigated with a view of magnetizing such samples as trapped field magnets (TFMs) in situ inside a trapped flux-type superconducting electric machine. A comparison is made between the temperatures at which the pulsed magnetic field is applied and the results have strong implications for the optimum operating temperature for TFMs in trapped flux-type superconducting electric machines. The effects of inhomogeneities, which occur during the growth process of single-grain bulk superconductors, on the trapped field and maximum temperature rise in the sample are modelled numerically using a 3D finite-element model based on the H-formulation and implemented in Comsol Multiphysics 4.3a. The results agree qualitatively with the observed experimental results, in that inhomogeneities act to distort the trapped field profile and reduce the magnitude of the trapped field due to localized heating within the sample and preferential movement and pinning of flux lines around the growth section regions (GSRs) and growth sector boundaries (GSBs), respectively. The modelling framework will allow further investigation of various inhomogeneities that arise during the processing of (RE)BCO bulk superconductors, including inhomogeneous Jc distributions and the presence of current-limiting grain boundaries and cracks, and it can be used to assist optimization of processing and PFM techniques for practical bulk superconductor applications. © 2014 IOP Publishing Ltd.

Temperature dependence of hole spin relaxation in ultrathin InAs monolayers

The temperature dependence of hole spin relaxation time in both neutral and n-doped ultrathin InAs monolayers has been investigated. It has been suggested that D'yakonov-Perel (DP) mechanism dominates the spin relaxation process at both low and high temperature regimes. The appearance of a peak in temperature dependent spin relaxation time reveals the important contribution of Coulomb scatterings between carriers to the spin kinetics at low temperature, though electron-phonon scattering becomes dominant at higher temperatures. Increased electron screening effect in the n-doped sample has been suggested to account for the shortened spin relaxation time compared with the undoped one. The results suggest that hole spins are also promising for building solid-state qubits.

A low-cost CMOS programmable temperature

A novel uncalibrated CMOS programmable temperature switch with high temperature accuracy is presented. Its threshold temperature T-th can be programmed by adjusting the ratios of width and length of the transistors. The operating principles of the temperature switch circuit is theoretically explained. A floating gate neural MOS circuit is designed to compensate automatically the threshold temperature T-th variation that results form the process tolerance. The switch circuit is implemented in a standard 0.35 mu m CMOS process. The temperature switch can be programmed to perform the switch operation at 16 different threshold temperature T(th)s from 45-120 degrees C with a 5 degrees C increment. The measurement shows a good consistency in the threshold temperatures. The chip core area is 0.04 mm(2) and power consumption is 3.1 mu A at 3.3V power supply. The advantages of the temperature switch are low power consumption, the programmable threshold temperature and the controllable hysteresis.

Reduction of tensile stress in GaN grown on Si(111) by inserting a low-temperature AlN interlayer

This study describes the growth of a low-temperature AlN interlayer for crack-free GaN growth on Si(111). It is demonstrated that, in addition to the lower growth temperature, growth of the AlN interlayer under Al-rich conditions is a critical factor for crack-free GaN growth on Si(111) substrates. The effect of the AlN interlayer thickness and NH3/TMA1 ratios on the lattice constants of subsequently grown high temperature GaN was investigated by X-ray triple crystal diffraction. The results show that the elimination of micro-cracks is related to the reduction of the tensile stress in the GaN epitaxial layers. This was also coincident with a greater number of pits formed in the AlN interlayer grown under Al rich conditions. It is proposed that these pits act as centers for the generation of misfit dislocations, which in turn leads to the reduction of tensile stress. (C) 2004 Elsevier B.V. All rights reserved.

Low-frequency noise properties of GaN Schottky barriers deposited on intermediate temperature buffer layers

Metal-semiconductor-metal (MSM) structures were fabricated by RF-plasma-assisted MBE using different buffer layer structures. One type of buffer structure consists of an AlN high-temperature buffer layer (HTBL) and a GaN intermediate temperature buffer layer (ITBL), another buffer structure consists of just a single A IN HTBL. Systematic measurements in the flicker noise and deep level transient Fourier spectroscopy (DLTFS) measurements were used to characterize the defect properties in the films. Both the noise and DLTFS measurements indicate improved properties for devices fabricated with the use of ITBL and is attributed to the relaxation of residue strain in the epitaxial layer during growth process. (C) 2003 Elsevier Ltd. All rights reserved.

In-situ resistivity measurement of ZnS in diamond anvil cell under high pressure

An effective method is developed to fabricate metallic microcircuits in diamond anvil cell (DAC) for resistivity measurement under high pressure. The resistivity of nanocrystal ZnS is measured under high pressure up to 36.4 GPa by using designed DAC. The reversibility and hysteresis of the phase transition are observed. The experimental data is confirmed by an electric current field analysis accurately. The method used here can also be used under both ultrahigh pressure and high temperature conditions.

High-power and low-threshold-current-density GaAs/AlGaAs quantum cascade lasers

We report on the realization of GaAs/AlGaAs quantum cascade lasers with an emission wavelength of 9.1 mu m above the liquid nitrogen temperature. With optimal current injection window and ridge width of 24 and 60 mu m respectively, a peak output power more than 500 mW is achieved in pulsed mode operation. A low threshold current density J(th) = 2.6 kA/cm(2) gives the devices good lasing characteristics. In a drive frequency of 1 kHz, the laser operates up to 20% duty cycle.

Amorphous silicon nanoparticles in compound films grown on cold substrates for high-efficiency photoluminescence

Silicon nanoparticles have been fabricated in both oxide and nitride matrices by using plasma-enhanced chemical vapour deposition, for which a low substrate temperature down to 50 degreesC turns out to be most favourable. High-rate deposition onto such a cold substrate results in the formation of nanoscaled silicon particles, which have revealed an amorphous nature under transmission electron microscope (TEM) examination. The particle size can be readily controlled below 3.0 nm, and the number density amounts to over 10(12) cm(-2), as calculated from the TEM micrographs. Strong photoluminescence in the whole visible light range has been observed in the as-deposited Si-in-SiOx and Si-in-SiNx thin films. Without altering the size or structure of the particles, a post-annealing at 300 degreesC for 2 min raised the photoluminescence efficiency to a level comparable to the achievements with nanocrystalline Si-in-SiO2 samples prepared at high temperature. This low-temperature procedure for fabricating light-emitting silicon structures opens up the possibility of manufacturing integrated silicon-based optoelectronics.

Growth temperature effect on the optical and material properties of AlxInyGa1-x-yN epilayers grown by MOCVD

AlxInyGa1-x-yN epilayers have been grown by metalorganic chemical vapor deposition (MOCVD) at different temperatures from 800 to 870degreesC. The incorporation of indium is found to increase with decreasing growth temperature, while the incorporation of Al remains nearly constant. The optical properties of the samples have been investigated by photoluminescence (PL) and time-resolved photoluminescence (TRPL) at different temperatures. The results show that the sample grown at 820 C exhibits the best optical quality for its large PL intensity and the absence of the yellow luminescence. Furthermore the temperature-dependent PL and TRPL of the sample reveals its less exciton localization effect caused by alloy fluctuations. In the scanning electron microscopy measurement, much uniform surface morphology is found for the sample grown at 820degreesC, in good agreement with the PL results, The improvement of AlxInyGa1-x-yN quality is well correlated with the incorporation of indium into AlGaN and the possible mechanism is discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

Raman-forbidden mode and oxygen ordering in Bi2Sr2-xLaxCuO6+gamma single crystals annealed in oxygen

A Raman-forbidden phonon mode at about 840 cm(-1) is observed popularly on the surface of pun and La-doped Bi2Sr2-xLaxCuO6+y (0 less than or equal to x less than or equal to 0.8) single crystals annealed in oxygen. A remarkable excitation dependence of this additional line is found. Based on the properties of the structure of the Bi-O layer with excess oxygen atoms and the similarity in the appearance of the Raman-forbidden modes between RBa2Cu3Ox (R = Y, Nd, Gd, Pr) and Bi2Sr2-xLaxCuO6+y systems, we attribute the manifestation of this additional line to the ordering of the interstitial oxygen in the Bi-O layers. Our results provide Raman evidences for confirming that the ordering of the movable oxygen may exist universally in high-temperature superconductors.

Photoluminescence study of multilayer In0.55Al0.45As/Al0.5Ga0.5As quantum dot at various temperature

The photoluminescence of self-assembled multilayer In0.55Al0.45As/Al0.5Ga0.5As quantum dot (QD) was measured at various temperatures. Strong photoluminescence of wetting layer (WL) and quantum dots were observed at the same time. Furthermore, direct excitons thermal transfer process between the wetting layer and quantum dots was observed. In the study of temperature dependence of PL intensity it was found that the PL peak of wetting layer contains two quenching processes: at low temperature, excitons are thermally activated from localized states to extended two-dimensional states and then trapped by QDs; at high temperature excitons quench through the X valley of barriers. Using rate equation excitons thermal transfer and quenching processes were analyzed quantitatively.

High Quality AlGaN Grown on GaN Template with HT-AlN Interlayer

When AlGaN is grown on GaN template, crack networks invariably generate when the thickness of the AlGaN layers over GaN exceeds the critical value. We used thin high temperature deposited AlN layer (HT-AlN) as the interlayer between GaN template and AlGaN epilayer which was very effective in eliminating the cracks in AlGaN epilayer. AlGaN layers with high Al mole fractions were also grown. Characterization showed that the crystalline quality of AlGaN epilayer was fairly good even when the At mole fraction was high.

Temperature-dependent modulation characteristics for 1.3 mu m InAs/GaAs quantum dot lasers

Temperature-dependent modulation characteristics of 1.3 mu m InAs/GaAs quantum dot (QD) lasers under small signals have been carefully studied at various bias currents. Based on experimental observations, it is found that the modulation bandwidth significantly increases when excited state (ES) lasing emerges at high temperature. This is attributed to additional photons emitted by ES lasing which contribute to the modulation response. A rate equation model including two discrete electron energy levels and the level of wetting layer has been used to investigate the temperature-dependent dynamic behavior of the QD lasers. Numerical investigations confirm that the significant jump for the small signal modulation response is indeed caused by ES photons. Furthermore, we identify how the electron occupation probabilities of the two discrete energy levels can influence the photon density of different states and finally the modulation rate. Both experiments and numerical analysis show that the modulation bandwidth of QD lasers at high temperature can be increased by injecting more carriers into the ES that has larger electron state degeneracy and faster carrier's relaxation time than the ground state.

Ge composition saturation behavior during low-temperature Si1-xGex growth by disilane and solid Ge molecular beam epitaxy

Ge composition dependence on the Ge cell temperature has been studied during the growth of Si1-xGex by disilane and solid Ge molecular beam epitaxy at a substrate temperature of 500 degrees C. It is found that the composition x increases and then saturates when the Ge cell temperature increases, which is different from the composition-dependent behavior in growth at high temperature as well as in growth by molecular beam epitaxy using disilane and germane. The enhanced hydrogen desorption from a Ge site alone cannot account for this abnormal composition-variation behavior. We attribute this behavior to the increase of rate constant of H desorption on a Si site when the Ge cell temperature increases.

Stress and resistivity controls on in situ boron doped LPCVD polysilicon films for high-Q MEMS applications

The simultaneous control of residual stress and resistivity of polysilicon thin films by adjusting the deposition parameters and annealing conditions is studied. In situ boron doped polysilicon thin films deposited at 520 ℃ by low pressure chemical vapor deposition (LPCVD) are amorphous with relatively large compressive residual stress and high resistivity. Annealing the amorphous films in a temperature range of 600-800 ℃ gives polysilicon films nearly zero-stress and relatively low resistivity. The low residual stress and low resistivity make the polysilicon films attractive for potential applications in micro-electro-mechanical-systems (MEMS) devices, especially in high resonance frequency (high-f) and high quality factor (high-Q MEMS resonators. In addition, polysilicon thin films deposited at 570 ℃ and those without the post annealing process have low resistivities of 2-5 mΩ·cm. These reported approaches avoid the high temperature annealing process (> 1000℃), and the promising properties of these films make them suitable for high-Q and high-f MEMS devices.