Showing papers on "Biasing published in 2010"

Voltage-induced perpendicular magnetic anisotropy change in magnetic tunnel junctions

Abstract: A voltage-induced perpendicular magnetic anisotropy change in an ultrathin FeCo layer was observed in an epitaxial magnetic tunnel junction (MTJ) structure. A spin-transfer induced ferromagnetic resonance measurement technique was used under various bias voltage applications to evaluate the anisotropy change. From the peak frequency shifts, we could estimate that a surface magnetic anisotropy change of 15 μJ/m2 was induced by an electric field application of 400 mV/nm in the MTJ with a 0.5 nm thick FeCo layer. The realization of voltage-induced anisotropy changes in an MTJ structure should have a large impact on the development of electric-field driven spintronic devices.

Fabrication and Characterization of the Charge-Plasma Diode

Abstract: We present a new lateral Schottky-based rectifier called the charge-plasma diode realized on ultrathin silicon-on-insulator. The device utilizes the workfunction difference between two metal contacts, palladium and erbium, and the silicon body. We demonstrate that the proposed device provides a low and constant reverse leakage-current density of about 1 fA/μm with ON/OFF current ratios of around 107 at 1-V forward bias and room temperature. In the forward mode, a current swing of 88 mV/dec is obtained, which is reduced to 68 mV/dec by back-gate biasing.

Bias-voltage dependence of perpendicular spin transfer torque in asymmetric MgO-based magnetic tunnel junctions

Abstract: Spin-transfer torque allows the magnetization of nanopillar devices to be switched electrically. Incorporating asymmetries into the design of such a device generates a linear out-of-plane torque component that could help prevent the unwanted spontaneous reversal of the nanopillar’s magnetization. Spin-transfer torque1,2 (STT) allows the electrical control of magnetic states in nanostructures3,4,5. The STT in magnetic tunnel junctions (MTJs) is of particular importance owing to its potential for device applications6,7. It has been demonstrated8,9,10,11 that the MTJ has a sizable perpendicular STT ( , field-like torque), which substantially affects STT-driven magnetization dynamics. In contrast to symmetric MTJs where the bias dependence of is quadratic8,9,10,12,13, it is theoretically predicted that the symmetry breaking of the system causes an extra linear bias dependence11. Here, we report experimental results that are consistent with the predicted linear bias dependence in asymmetric MTJs. The linear contribution is quite significant and its sign changes from positive to negative as the asymmetry is modified. This result opens a way to design the bias dependence of the field-like term, which is useful for device applications by allowing, in particular, the suppression of the abnormal switching-back phenomena.

Space charge limited conduction with exponential trap distribution in reduced graphene oxide sheets

Abstract: We elucidate on the low mobility and charge traps of the chemically reduced graphene oxide (RGO) sheets by measuring and analyzing temperature dependent current-voltage characteristics. The RGO sheets were assembled between source and drain electrodes via dielectrophoresis. At low bias voltage the conduction is Ohmic while at high bias voltage and low temperatures the conduction becomes space charge limited with an exponential distribution of traps. We estimate an average trap density of 1.75×1016 cm−3. Quantitative information about charge traps will help develop optimization strategies of passivating defects in order to fabricate high quality solution processed graphene devices.

Effects of nitrogen pressure and pulse bias voltage on the properties of Cr-N coatings deposited by arc ion plating

Abstract: Cr-N coatings were deposited on 1Cr18Ni9Ti stainless steel in the pure N(2) atmosphere by arc ion plating (AIP). The relationships between deposition parameters and coating properties were investigated. X-ray diffraction showed a phase transformation from CrN+Cr(2)N+Cr-->CrN+Cr-->CrN and the CrN preferred orientation changed from (200) to (220) as N(2) pressure increased. Increasing bias voltage led to CrN preferred orientation changed from (200) to (220) and the formation of Cr(2)N. XPS results indicated that chemical composition of the coatings changed as N(2) pressure increased but it changed little with bias voltage. The lower melting point of chromium nitride formed on target surface induced the increase of macroparticles and deposition rate with increasing N(2) pressure; and bias voltage had an obvious effect on reducing macroparticles of the Cr-N coatings. Residual stresses were measured by substrate curvature technique, and the changing tendency coincided with the microhardness of the coatings. (C) 2009 Elsevier B.V. All rights reserved.

Current-assisted thermally activated flux liberation in ultrathin nanopatterned NbN superconducting meander structures

Abstract: We present results from an extensive study of fluctuation phenomena in superconducting nanowires made from sputtered NbN. Nanoscale wires were fabricated in form of a meander and operated at a constant temperature T0.4Tc
0. The superconducting state is driven close to the electronic phase transition by a high bias current near the critical one. Fluctuations of sufficient strength temporarily drive a section of the meander structure into the normal-conducting state, which can be registered as a voltage pulse of nanosecond duration. We considered three different models
vortex-antivortex pairs, vortex edge barriers, and phase-slip centers to explain the experimental data. Only thermally excited vortices, either via unbinding of vortex-antivortex pairs or vortices overcoming the edge barrier, lead to a satisfactory and consistent description for all measurements.

Nonvolatile resistive switching memory based on amorphous carbon

Abstract: Resistive memory effect has been found in carbon nanostructure-based devices by Standley et al. [Nano Lett. 8, 3345 (2008)]. Compared to nanostructures, hydrogenated amorphous carbon (a-C:H) has much more controllable preparation processes. Study on a-C:H-based memory is of great significance to applications of carbon-based electronic devices. We observed nonvolatile resistance memory behaviors in metal/a-C:H/Pt structures with device yield 90%, ON/OFF ratio >100, and retention time >105 s. Detailed analysis indicates that the resistive switching originates from the formation/rupture of metal filaments due to the diffusion of the top electrodes under a bias voltage.

Tunable band gap in hydrogenated bilayer graphene.

Abstract: We have studied the electronic structural characteristics of hydrogenated bilayer graphene under a perpendicular electric bias using first-principles density functional calculations. The bias voltage applied between the two hydrogenated graphene layers allows continuous tuning of the band gap and leads to transition from semiconducting to metallic state. Desorption of hydrogen from one layer in the chair conformation yields a ferromagnetic semiconductor with a tunable band gap. The implications of tailoring the band structure of biased system for future graphene-based device applications are discussed.

Modeling Scale-Dependent Bias on the Baryonic Acoustic Scale with the Statistics of Peaks of Gaussian Random Fields

Abstract: Models of galaxy and halo clustering commonly assume that the tracers can be treated as a continuous field locally biased with respect to the underlying mass distribution. In the peak model pioneered by Bardeen et al.[Astrophys. J. 304, 15 (1986)], one considers instead density maxima of the initial, Gaussian mass density field as an approximation to the formation site of virialized objects. In this paper, the peak model is extended in two ways to improve its predictive accuracy. First, we derive the two-point correlation function of initial density peaks up to second order and demonstrate that a peak-background split approach can be applied to obtain the k-independent and k-dependent peak bias factors at all orders. Second, we explore the gravitational evolution of the peak correlation function within the Zel'dovich approximation. We show that the local (Lagrangian) bias approach emerges as a special case of the peak model, in which all bias parameters are scale independent and there is no statistical velocity bias. We apply our formulas to study how the Lagrangian peak biasing, the diffusion due to large scale flows, and the mode coupling due to nonlocal interactions affect the scale dependence of bias from small separations up to themore » baryon acoustic oscillation (BAO) scale. For 2{sigma} density peaks collapsing at z=0.3, our model predicts a {approx}5% residual scale-dependent bias around the acoustic scale that arises mostly from first order Lagrangian peak biasing (as opposed to second order gravity mode coupling). We also search for a scale dependence of bias in the large scale autocorrelation of massive halos extracted from a very large N-body simulation provided by the MICE Collaboration. For halos with mass M > or approx. 10{sup 14}M{sub {center_dot}/}h, our measurements demonstrate a scale-dependent bias across the BAO feature which is very well reproduced by a prediction based on the peak model.« less

Characterization of Multicrystalline Silicon Modules with System Bias Voltage Applied in Damp Heat

Abstract: Because it is considered economically favorable to build arrays with high system voltage by serially connecting photovoltaic (PV) modules, it is necessary to explore the potential long-term degradation mechanisms that the modules may incur under such electrical potential. We performed accelerated lifetime testing of multicrystalline silicon PV modules in 85°C/85% relative humidity (RH) and 45°C/30% RH while placing the active layer in either positive or negative 600 V bias with respect to the grounded module frame. A negative bias applied to the active layer leads to more rapid and catastrophic module power degradation compared to a positive bias. This negative bias degradation is associated with significant shunting of individual cells as indicated by electroluminescence, thermal imaging, and I-V curves. Mass spectroscopy results support ion migration as one of the causes. Electrolytic corrosion is seen occurring with the silicon nitride antireflective coating and silver gridlines, and there is ionic transport of metallization at the encapsulant interface observed with damp heat and applied bias. Leakage current and module degradation are found to be highly dependent on the module construction, with factors such as encapsulant and front glass resistivity affecting performance. Measured leakage currents range from about the same as those seen in published reports of modules deployed in Florida (USA) to about 100 times higher when undergoing environmental chamber testing.

Electrically Controlled Giant Piezoresistance in Silicon Nanowires

Abstract: Herein we demonstrate giant piezoresistance in silicon nanowires (NWs) by the modulation of an electric field-induced with an external electrical bias. Positive bias for a p-type device (negative for an n-type) partially depleted the NWs forming a pinch-off region, which resembled a funnel through which the electrical current squeezed. This region determined the total current flowing through the NWs. In this report, we combined the electrical biasing with the application of mechanical stress, which impacts the charge carriers’ concentration, to achieve an electrically controlled giant piezoresistance in nanowires. This phenomenon was used to create a stress-gated field-effect transistor, exhibiting a maximum gauge factor of 5000, 2 orders of magnitude increase over bulk value. Giant piezoresistance can be tailored to create highly sensitive mechanical sensors operating in a discrete mode such as nanoelectromechanical switches.

Instability in threshold voltage and subthreshold behavior in Hf–In–Zn–O thin film transistors induced by bias-and light-stress

Abstract: Electrical bias and light stressing followed by natural recovery of amorphous hafnium-indium-zinc-oxide (HIZO) thin film transistors with a silicon oxide/nitride dielectric stack reveals defect density changes, charge trapping and persistent photoconductivity (PPC) In the absence of light, the polarity of bias stress controls the magnitude and direction of the threshold voltage shift (ΔVT), while under light stress, VT consistently shifts negatively In all cases, there was no significant change in field-effect mobility Light stress gives rise to a PPC with wavelength-dependent recovery on time scale of days We observe that the PPC becomes more pronounced at shorter wavelengths

Tunnel magnetoresistance with improved bias voltage dependence in lattice-matched Fe/spinel MgAl2O4/Fe(001) junctions

Abstract: We fabricated fully epitaxial Fe/MgAl2O4/Fe(001) magnetic tunnel junctions using plasma oxidation of an Mg/Al bilayer. The MgAl2O4 showed a (001)-oriented spinel-type structure, and there were few misfit dislocations at the interfaces between the MgAl2O4 and the two Fe layers due to a small lattice mismatch (∼1%). Tunnel magnetoresistance (TMR) ratios up to 117% (165%) were obtained at room temperature (15 K). The bias voltage for one-half of the zero-bias TMR ratio (Vhalf) was relatively large, ranging from 1.0 to 1.3 V at room temperature, which is attributed to the small misfit dislocation density.

Rate-Limiting Processes Determining the Switching Time in a Ag2S Atomic Switch

Abstract: The switching time of a Ag2S atomic switch, in which formation and annihilation of a Ag atomic bridge is controlled by a solid-electrochemical reaction in a nanogap between two electrodes, is investigated as a function of bias voltage and temperature. Increasing the bias voltage decreases the switching time exponentially, with a greater exponent for the lower range of bias than that for the higher range. Furthermore, the switching time shortens exponentially with raising temperature, following the Arrhenius relation with activation energy values of 0.58 and 1.32 eV for lower and higher bias ranges, respectively. These results indicate that there are two main processes which govern the rate of switching, first, the electrochemical reduction Ag+ + e−→Ag and, second, the diffusion of Ag+ ions. This investigation advances the fundamental understanding of the switching mechanism of the atomic switch, which is essential for its successful device application.

Measuring the external quantum efficiency of two-terminal polymer tandem solar cells

Abstract: Tandem configurations, in which two cells are stacked and connected in series, offer a viable approach to further increase the power conversion efficiency (PCE) of organic solar cells. To enable the future rational design of new materials it is important to accurately assess the contributions of individual subcells. Such accurate measurement of the external quantum efficiency (EQE) of the subcells of two-terminal organic or polymer tandem solar cells poses specific challenges, caused by two characteristics of these cells, i.e. a sub-linear light intensity dependence of the current and a field-assisted charge collection. These properties necessitate that EQE experiments are carried out under representative illumination conditions and electrical bias to maintain short-circuit conditions for the addressed subcell. We describe a method to determine the magnitudes of the bias illumination and bias voltage during EQE measurements, based on the behavior of single junction cells and optical modeling. The short-circuit current densities of the subcells obtained by convolution of the EQE with the AM1.5G solar spectrum are consistent with those obtained from optical modeling and correctly predict the current density–voltage characteristics of the tandem cell under AM1.5G conditions.

Graphene Magnetic Field Sensors

Abstract: Graphene extraordinary magnetoresistance (EMR) devices have been fabricated and characterized in varying magnetic fields at room temperature. The atomic thickness, high carrier mobility and high current carrying capabilities of graphene are ideally suited for the detection of nanoscale sized magnetic domains. The device sensitivity can reach 10 mV/Oe, larger than state of the art InAs 2DEG devices of comparable size and can be tuned by the electric field effect via a back gate or by imposing a biasing magnetic field.

High spin-filter efficiency in a Co ferrite fabricated by a thermal oxidation

Abstract: Co ferrites fabricated by a thermal oxidation have been tested as a spin filter. Spin-filter efficiencies of 44% and 4.3% were confirmed at 10 K and RT, respectively, using a magnetic tunnel junction of Pt/CoFe2O4/MgO/Co. By increasing the bias voltage, the tunneling magnetoresistance (TMR) at 10 K increases then decreases. This is interpreted as a tunneling via spin-down conduction band in the Co ferrite. Since the electron at RT is attributed to a hopping conduction on the defect states in the Co ferrite, the reduction of the defects will be the key to achieving a higher spin-filter effect.

A new single-photon avalanche diode in 90nm standard CMOS technology

Abstract: We report on the first implementation of a single-photon avalanche diode (SPAD) in 90nm complementary metal oxide semiconductor (CMOS) technology. The detector features an octagonal multiplication region and a guard ring to prevent premature edge breakdown using a standard mask set exclusively. The proposed structure emerged from a systematic study aimed at miniaturization, while optimizing overall performance. The guard ring design is the result of an extensive modeling effort aimed at constraining the multiplication region within a well-defined area where the electric field exceeds the critical value for impact ionization. The device exhibits a dark count rate of 8.1 kHz, a maximum photon detection probability of 9% and the jitter of 398ps at a wavelength of 637nm, all of them measured at room temperature and 0.13V of excess bias voltage. An afterpulsing probability of 32% is achieved at the nominal dead time. Applications include time-of-flight 3D vision, fluorescence lifetime imaging microscopy, fluorescence correlation spectroscopy, and time-resolved gamma/X-ray imaging. Standard characterization of the SPAD was performed in different bias voltages and temperatures.

Temperature dependent negative capacitance behavior in (Ni/Au)/AlGaN/AlN/GaN heterostructures

Abstract: The temperature dependent capacitance–voltage (C–V) and conductance–voltage (G/x–V) characteristics of (Ni/Au)/Al0.22Ga0.78N/AlN/GaN heterostructures were investigated by considering the series resistance (Rs) effect in the temperature range of 80–390 K. The experimental results show that the values of C and G/x are strongly functioning of temperature and bias voltage. The values of C cross at a certain forward bias voltage point (�2.8 V) and then change to negative values for each temperature, which is known as negative capacitance (NC) behavior. In order to explain the NC behavior, we drawn the C vs I and G/x vs I plots for various temperatures at the same bias voltage. The negativity of the C decreases with increasing temperature at the forward bias voltage, and this decrement in the NC corresponds to the increment of the conductance. When the temperature was increased, the value of C decreased and the intersection point shifted towards the zero bias direction. This behavior of the C and G/x values can be attributed to an increase in the polarization and the introduction of more carriers in the structure. Rs values increase with increasing temperature. Such temperature dependence is in obvious disagreement with the negative temperature coefficient of R or G reported in the literature. The intersection behavior of C–V curves and the increase in Rs with temperature can be explained by the lack of free charge carriers, especially at low temperatures.

Impact of plasma effects on the performance of silicon sensors at an X-ray FEL

Abstract: The impact of electron hole plasmas on silicon sensor performance was studied with a multi-channel Transient Current Technique (mTCT) setup. Electron hole densities of up to 10 16 cm � 3 (equivalent to 10 5 focused 12 keV photons) were created with sub-ns lasers (660 and 1015 nm) and the time resolved current pulses of segmented sensors (4 channels simultaneously) were read out by a 2.5 GHz oscilloscope. Measurements for strip sensors of 280mm thickness and 80mm pitch as well as 450mm thickness and 50mm pitch were carried out showing an increase of the charge collection time and an increase of the charge spread (charge cloud explosion) with increased charge carrier density. These effects were studied as a function of the applied bias voltage and electron hole density. It was shown that for the current AGIPD sensor design plasma effects in p-in-n sensors of 450mm thickness are negligible if at least 500 V bias is applied.

Design and Analysis of a Tunable Miniaturized-Element Frequency-Selective Surface Without Bias Network

Abstract: A novel, tunable miniaturized-element frequency-selective surface that does not require additional bias networks is presented. This spatial filter is composed of two wire-grids printed on opposite sides of a substrate and connected to each other with an array of varactors using plated via holes. Varactor diodes are positioned between the grids. Via sections and metallic pads are fabricated and create a dc path for biasing the varactors with the grids themselves. This configuration eliminates the need for any additional network, and therefore resolves the design difficulties associated with the spurious response of the bias network. An equivalent circuit model is developed to facilitate the design procedure. Full-wave numerical simulations are used to validate the results based on the circuit model. Simulations show that by altering the capacitance of the varactors from 0.1 to 1 pF, a frequency tunability from 8 to 10 GHz with an almost constant bandwidth can be achieved.

Write and erase scheme for resistive memory device

Abstract: A method for programming a two terminal resistive memory device, the method includes applying a bias voltage to a first electrode of a resistive memory cell of the device; measuring a current flowing through the cell; and stopping the applying of the bias voltage if the measured current is equal to or greater than a predetermined value.

An accurate high-speed single-electron quantum dot pump.

Abstract: Using standard microfabrication techniques, it is now possible to constructdevicesthatappear toreliablymanipulateelectronsone atatime.These devices have potential use as building blocks in quantum computing devices, or as a standard of electrical current derived only from a frequency and the fundamental charge. To date, the error rate in semiconductor 'tuneable-barrier' pump devices, those which show most promise for high-frequency operation, have not been tested in detail. We present high-accuracy measurements of the current from an etched GaAs quantum dot pump, operated at zero source-drain bias voltage with a single ac-modulated gate at 340MHz driving the pump cycle. By comparison with a reference current derived from primary standards, we show that the electron transfer accuracy is better than 15 parts per million. High- resolution studies of the dependence of the pump current on the quantum dot tuning parameters also reveal possible deviations from a model used to describe the pumping cycle.

Voltage tunable short wire-pair type of metamaterial infiltrated by nematic liquid crystal

Abstract: In this letter, we report an experimental demonstration on voltage tunable short wire-pair metamaterial with negative permeability at microwave frequencies After infiltrated by nematic liquid crystal, magnetic resonance of short wire-pair can be effectively tuned via relatively low static electric field The results show, by increasing bias voltage from 0 to 100 V, magnetic resonance is continuously and reversibly shifted from 991 down to 955 GHz Moreover, the effective permeability of metamaterial, for operation frequency around magnetic resonance, varies from negative to positive values Numerical analysis has a good agreement with experimental results

Small Signal Equivalent Circuit Modeling for AlGaN/GaN HFET: Hybrid Extraction Method for Determining Circuit Elements of AlGaN/GaN HFET

Abstract: The developments in AlGaN/GaN heterojunction field effect transistors (HFETs) are beginning to allow harnessing of the great potential of this technology in high-power radio-frequency (RF) applications. However, the integration of HFET into a circuit environment requires accurate small and large signal modeling of the device operating under various biasing conditions. The conventional small signal equivalent circuit modeling methods consist of “cold” measurements for extracting parasitic elements, and on-bias measurements in determining the intrinsic device circuit elements. Also, the optimization routines are often explored directly using the “hot” measurement data to minimize errors. In this paper, an 18-element small signal equivalent circuit model for AlGaN/GaN HFET is proposed and implemented, and contrasted to various de-embedding methods. Among the methods treated, the hot-FET optimization extraction based on the parasitic capacitances obtained from cold measurements leads to the smallest error between the simulated S-parameters and the measured ones at all bias points employed, with an average error of about 5%. This hybrid extraction algorithm is strengthened by imposing constraints to avoid any nonphysical convergence. We note that the extrinsic parasitics determined by this method differ considerably from the values obtained by cold-FET measurements, which implies that the assumption on the bias independency for extrinsic parameters in the latter method might be questionable for AlGaN/GaN HFET.

Modeling and Characterization of Current Gain Versus Temperature in 4H-SiC Power BJTs

Abstract: Accurate physical modeling has been developed to describe the current gain of silicon carbide (SiC) power bipolar junction transistors (BJTs), and the results have been compared with measurements. Interface traps between SiC and SiO2 have been used to model the surface recombination by changing the trap profile, capture cross section, and concentration. The best agreement with measurement is obtained using one single energy level at 1 eV above the valence band, a capture cross section of 1 × 10-5 cm2, and a trap concentration of 2 × 1012 cm-2. Simulations have been performed at different temperatures to validate the model and characterize the temperature behavior of SiC BJTs. An analysis of the carrier concentration at different collector currents has been performed in order to describe the mechanisms of the current gain fall-off at a high collector current both at room temperature and high temperatures. At room temperature, high injection in the base (which has a doping concentration of 3 × 1017 cm-3) and forward biasing of the base-collector junction occur simultaneously, causing an abrupt drop of the current gain. At higher temperatures, high injection in the base is alleviated by the higher ionization degree of the aluminum dopants, and then forward biasing of the base-collector junction is the acting mechanism for the current gain fall-off. Forward biasing of the base-collector junction can also explain the reduction of the knee current with increasing temperature by means of the negative temperature dependence of the mobility.