Showing papers on "Biasing published in 2006"

Electric-field control of exchange bias in multiferroic epitaxial heterostructures.

Abstract: The magnetic exchange between epitaxial thin films of the multiferroic (antiferromagnetic and ferroelectric) hexagonal YMnO3 oxide and a soft ferromagnetic (FM) layer is used to couple the magnetic response of the FM layer to the magnetic state of the antiferromagnetic one. We will show that biasing the ferroelectric YMnO3 layer by an electric field allows control of the magnetic exchange bias and subsequently the magnetotransport properties of the FM layer. This finding may contribute to paving the way towards a new generation of electric-field controlled spintronic devices.

Nonlinear spin current and magnetoresistance of molecular tunnel junctions.

Abstract: We report on a theoretical study of spin-polarized quantum transport through a Ni-bezenedithiol(BDT)-Ni molecular magnetic tunnel junction (MTJ). Our study is based on carrying out density functional theory within the Keldysh nonequilibrium Green's function formalism, so that microscopic details of the molecular MTJ are taken into account from first principles. A magnetoresistance ratio of $\ensuremath{\sim}27%$ is found for the Ni-BDT-Ni MTJ which declines toward zero as bias voltage is increased. The spin currents are nonlinear functions of bias voltage, even changing sign at certain voltages due to specific features of the coupling between molecular states and magnetic leads.

Measurement of Current-Induced Local Heating in a Single Molecule Junction

Abstract: We have studied the current-induced local heating effects in single molecules covalently bound to two electrodes by measuring the force required to break the molecule-electrode bonds under various conditions. The breakdown process is thermally activated, which is used to extract the effective temperature of the molecular junction as a function of applied bias voltage. We have also performed first-principles calculations of both local heating and current-induced force effects, and the results are in good agreement with the experimental findings.

High efficiency wavelength conversion of 10 Gb/s data in silicon waveguides

Abstract: Efficient wavelength conversion via four-wave-mixing in silicon-on-isolator p-i-n waveguides has been realized. By reverse biasing the p-i-n diode structure formed along the silicon rib waveguide, the nonlinear absorption due to two photon absorption induced free carrier absorption is significantly reduced, and a wavelength conversion efficiency of -8.5 dB has been achieved in an 8 cm long waveguide at a pump intensity of 40 MW/cm2. A high-speed pseudo-random bit sequence data at 10 Gb/s rate is converted to a new wavelength channel in the C-band with clear open eye diagram and no waveform distortion. Conversion efficiency as functions of pump power, wavelength detuning, and bias voltages, have been investigated. For shorter waveguides of 1.6 cm long, a conversion bandwidth of > 30 nm was achieved.

Electrical detection of spin accumulation at a ferromagnet-semiconductor interface.

Abstract: We show that the accumulation of spin-polarized electrons at a forward-biased Schottky tunnel barrier between Fe and -GaAs can be detected electrically. The spin accumulation leads to an additional voltage drop across the barrier that is suppressed by a small transverse magnetic field, which depolarizes the spins in the semiconductor. The dependence of the electrical accumulation signal on magnetic field, bias current, and temperature is in good agreement with the predictions of a drift-diffusion model for spin-polarized transport.

Germanium-on-SOI Infrared Detectors for Integrated Photonic Applications

Abstract: An overview of recent results on high-speed germanium-on-silicon-on-insulator (Ge-on-SOI) photodetectors and their prospects for integrated optical interconnect applications are presented. The optical properties of Ge and SiGe alloys are described and a review of previous research on SOI and SiGe detectors is provided as a motivation for the Ge-on-SOI detector approach. The photodetector design is described, which consists of lateral alternating p- and n-type surface contacts on an epitaxial Ge absorbing layer grown on an ultrathin-SOI substrate. When operated at a bias voltage of -0.5 V, 10mumtimes10 mum devices have dark current Idark, of only ~10 nA, a value that is nearly independent of finger spacing S, between S=0.3mum and 1.3mum. Detectors with S=1.3mum have external quantum efficiencies eta, of 52% (38%) at lambda=895 nm (850 nm) with corresponding responsivities of 0.38 A/W (0.26 A/W). The wavelength-dependence of eta agrees fairly well with expectations, except at longer wavelengths, where Si up-diffusion into the Ge absorbing layer reduces the efficiency. Detectors with 10 mumtimes10 mum area and S=0.6mum have -3-dB bandwidths as high as 29 GHz, and can simultaneously achieve a bandwidth of 27 GHz with Idark=24 nA, at a bias of only -1 V, while maintaining high efficiency of eta=46%(33%), at lambda=895 nm (850 nm). Analysis of the finger spacing and area-dependence of the device speed indicates that the performance at large finger spacing is transit-time-limited, while at small finger spacing, RC delays limit the bandwidth. Methods to improve the device performance are presented, and it is shown that significant improvement in the speed and efficiency both at lambda=850 and 1300 nm can be expected by optimizing the layer structure design

Ambipolar charge transport in organic field-effect transistors

Abstract: A model describing charge transport in disordered ambipolar organic field-effect transistors is presented. The basis of this model is the variable-range hopping in an exponential density of states developed for disordered unipolar organic transistors. We show that the model can be used to calculate all regimes in unipolar as well as ambipolar organic transistors, by applying it to experimental data obtained from ambipolar organic transistors based on a narrow-gap organic molecule. The threshold voltage was determined independently from metal insulator semiconductor diode measurements. An excellent agreement between theory and experiment is observed over a wide range of biasing regimes and temperatures.

Electronic memory effects in diodes from a zinc oxide nanoparticle-polystyrene hybrid material

Abstract: Current-voltage characteristics of diode structures with an active layer of a zinc oxide nanoparticle-polystyrene hybrid material (1:2 by weight) deposited by spin coating from solution were investigated. Aluminum and poly(3,4-ethylenedioxythiophene):polystyrene-sulfonate were used as electrodes. After a forming step, the conduction under reversed bias voltage can be raised or lowered in a gradual and reversible manner by applying forward and reverse bias voltages, respectively. Electrically induced switching between states with high and lower conductivities is possible on a time scale of 100ms and the conduction levels remain stable for over 1h.

Ambipolar charge transport in organic field-effect transistors

Abstract: A model describing charge transport in disordered ambipolar organic field-effect transistors is presented. The basis of this model is the variable-range hopping in an exponential density of states developed for disordered unipolar organic transistors. We show that the model can be used to calculate all regimes in unipolar as well as ambipolar organic transistors, by applying it to experimental data obtained from ambipolar organic transistors based on a narrow-gap organic molecule. The threshold voltage was determined independently from metal insulator semiconductor diode measurements. An excellent agreement between theory and experiment is observed over a wide range of biasing regimes and temperatures.

Potential-biased, asymmetric waveforms for charge-injection with activated iridium oxide (AIROF) neural stimulation electrodes

Abstract: The use of potential biasing and biphasic, asymmetric current pulse waveforms to maximize the charge-injection capacity of activated iridium oxide (AIROF) microelectrodes used for neural stimulation is described. The waveforms retain overall zero net charge for the biphasic pulse, but employ an asymmetry in the current and pulse widths of each phase, with the second phase delivered at a lower current density for a longer period of time than the leading phase. This strategy minimizes polarization of the AIROF by the charge-balancing second phase and permits the use of a more positive anodic bias for cathodal-first pulsing or a more negative cathodic bias for anodal-first pulsing to maximize charge injection. Using 0.4-ms cathodal-first pulses, a maximum charge-injection capacity of 3.3 mC/cm/sup 2/ was obtained with an 0.6-V bias (versus Ag|AgCl) and a pulse asymmetry of 1:8 in the cathodal and anodal pulse widths. For anodal-first pulsing, a maximum charge capacity of 9.6 mC/cm/sup 2/ was obtained with an asymmetry of 1:3 at an 0.1-V bias. These measurements were made in vitro in carbonate-buffered saline using microelectrodes with a 2000 /spl mu/m/sup 2/ surface area.

Method and apparatus for controlling driving current of illumination source in a display system

Abstract: The present application describes a programmable current controller for regulating an operating driving current flowing through an illumination source. The driving current is regulated according to a digital reference corresponding to a predetermined operating current for the illumination source. The digital reference can be converted into a reference electrical parameter (current or voltage). The reference electrical parameter is compared with an operating electrical parameter (current or voltage) corresponding to the operating driving current of the illumination source. Based on the comparison, a driving bias current is generated, which is used to regulate the operating driving current of the illumination source.

Spin-filter effect in magnetite nanowire.

Abstract: Spin-dependent electron transport in individual magnetite (Fe3O4) nanowires contacted with normal metallic electrodes was investigated. Such a configured device demonstrated a spin-filter effect, that is, only the minority spin carriers can transport through the magnetite nanowire due to its negative spin polarization. An anomalous positive magnetoresistance similar to 7.5% is observed at room temperature. Moreover, the magnetoresistance can be controlled via bias voltage.

Temperature Dependence of Three-Terminal Molecular Junctions with Sulfur End-Functionalized Tercyclohexylidenes

Abstract: We have studied the gate and temperature dependence of molecular junctions containing sulfur end-functionalized tercyclohexylidenes. At low temperatures we find temperature-independent transport; at temperatures above 150 K the current increases exponentially with increasing temperature. Over the entire temperature range (10 -300 K), and for different gate voltages, a simple toy model of transport through a single level describes the experimental results. In the model, the temperature dependence arises from the Fermi distribution in the leads and in a three-parameter fit we extract the level position and the tunnel rates at the left and right contact. We find that these parameters increase as the bias voltage increases.

Piezoelectric strain in AlGaN∕GaN heterostructure field-effect transistors under bias

Abstract: Micro-Raman spectroscopy was used to study piezoelectric strain in AlGaN∕GaN heterostructure field-effect transistors under bias. The measurements were made through the transparent SiC substrate. Strain in the GaN layer varied over the device area and was dependent on bias voltage, and affected, in particular, the gate-drain gap and area underneath the drain contact. The observed strain in GaN was shown to be related to the electric field component normal to the surface. Finite element simulations of electric field distribution show good qualitative agreement with the experimental data. Effects of strain on Raman temperature measurements in transistors are also discussed.

Electro-optic modulation

Abstract: A silicon electro-optic waveguide modulator is formed using a metal-oxide-semiconductor (MOS) configuration. Various embodiments are described using different modes of operation of the MOS diode and gate oxide thicknesses. In one example, a high-speed submicron waveguide active device is formed using silicon-on-insulator. A micro-ring resonator intensity-modulator exhibits switching times on the order of tens of pS with modulation depth of 73% with a bias voltage of 5 volts.

Gate-length biasing for runtime-leakage control

Abstract: Leakage power has become one of the most critical design concerns for the system level chip designer. While lowered supplies (and consequently, lowered threshold voltage) and aggressive clock gating can achieve dynamic power reduction, these techniques increase the leakage power and, therefore, causes its share of total power to increase. Manufacturers face the additional challenge of leakage variability: Recent data indicate that the leakage of microprocessor chips from a single 180-nm wafer can vary by as much as 20/spl times/. Previously proposed techniques for leakage-power reduction include the use of multiple supply and gate threshold voltages, and the assignment of input values to inactive gates, such that leakage is minimized. The additional design space afforded by the biasing of device gate lengths to reduce chip leakage power and its variability is studied. It is well known that leakage power decreases exponentially and delay increases linearly with increasing gate length. Thus, it is possible to increase gate length only marginally to take advantage of the exponential leakage reduction, while impairing performance only linearly. From a design-flow standpoint, the use of only slight increases in gate length preserves both pin and layout compatibility; therefore, the authors' technique can be applied as a postlayout enhancement step. The authors apply gate-length biasing only to those devices that do not appear in critical paths, thus assuring zero or negligible degradation in chip performance. To highlight the value of the technique, the multithreshold voltage technique, which is widely used for leakage reduction, is first applied and then gate-length biasing is used to show further reduction in leakage. Experimental results show that gate-length biasing reduces leakage by 24%-38% for the most commonly used cells, while incurring delay penalties of under 10%. Selective gate-length biasing at the circuit level reduces circuit leakage by up to 30% with no delay penalty. Leakage variability is reduced significantly by up to 41%, which may lead to substantial improvements in the manufacturing yield and the product cost. The use of gate-length biasing for leakage optimization of cell instances is also assessed, in which: 1) not all timing arcs are timing critical and/or 2) the rise and fall transitions are not both timing critical at the same time.

Experimental characterization of collapse-mode CMUT operation

Abstract: This paper reports on the experimental characterization of collapse-mode operation of capacitive micromachined ultrasonic transducers (CMUTs). CMUTs are conventionally operated by applying a direct current (DC) bias voltage less than the collapse voltage of the membrane, so that the membrane is deflected toward the bottom electrode. In the conventional regime, there is no contact between the membrane and the substrate; the maximum alternating current (AC) displacement occurs at the center of the membrane. In collapse-mode operation, the DC bias voltage is first increased beyond the collapse voltage, then reduced without releasing the collapsed membrane. In collapse-mode operation, the center of the membrane is always in contact with the substrate. In the case of a circular membrane, the maximum AC displacement occurs along the ring formed between the center and the edge of the membrane. The experimental characterization presented in this paper includes impedance measurements in air, pulse-echo experiments in immersion, and one-way optical displacement measurements in immersion for both conventional and collapse-mode operations. A 205-mum times 205-mum 2-D CMUT array element composed of circular silicon nitride membranes is used in the experiments. In pulse-echo experiments, a custom integrated circuit (IC) comprising a pulse driver, a transmit/receive switch, a wideband low-noise preamplifier, and a line driver is used. By reducing the parasitic capacitance, the use of a custom IC enables pulse-echo measurements at high frequencies with a very small transducer. By comparing frequency response and efficiency of the transducer in conventional and collapse regimes, experimental results show that a collapsed membrane can be used to generate and detect ultrasound more efficiently than a membrane operated in the conventional mode. Furthermore, the center frequency of the collapsed membrane can be changed by varying the applied DC voltage. In this study, the center frequency of a collapsed transducer in immersion is shown to vary from 20 MHz to 28 MHz with applied DC bias; the same transducer operates at 10 MHz in the conventional mode. In conventional mode, the maximum peak-to-peak pressure is 370 kPa on the transducer surface for a 40-ns, 25-V unipolar pulse excitation. In collapse mode, a 25-ns, 25-V unipolar pulse generates 590 kPa pressure at the surface of the transducer

Direct observation of half-metallic energy gap in Co2MnSi by tunneling conductance spectroscopy

Abstract: Magnetic tunnel junctions with a Co2MnSi∕Al–O∕CoFe structure are prepared by magnetron sputtering and investigated with respect to the energy gap near the Fermi energy level. The plasma oxidation time for the Al–O barrier is found to affect the condition of the Co2MnSi∕Al–O interface. The optimized sample (50s oxidation time) exhibits a magnetoresistance ratio of 159% and tunneling spin polarization of 0.89 at 2K. The bias voltage dependence of tunneling conductance (dI∕dV−V) reveals a clear half-metallic energy gap at 350–400meV for Co2MnSi, with an energy separation of just 10meV between the Fermi energy and the bottom edge of conduction band.

Electrowetting on superhydrophobic SU-8 patterned surfaces

Abstract: Electrowetting on micro-patterned layers of SU-8 photoresist with an amorphous Teflon (R) coating has been observed. The cosine of the contact angle is shown to be proportional to the square of the applied voltage for increasing bias. However, this does not apply below 40 V and we suggest that this may be explained in terms of penetration of fluid into the pattern of the surface. Assuming that the initial application of a bias voltage converts the drop from Cassie-Baxter to Wenzel regime, we have used this as a technique to estimate the roughness factor of the surface.

Current-voltage characteristics through a single light-sensitive molecule

Abstract: A light-sensitive molecular switch based on single azobenzene molecule has been proposed recently C. Zhang, M. H. Du, H. P. Cheng, X. G. Zhang, A. E. Roitberg, and J. L. Krause, Physical Review Letters 92, 158301 2004 . Here we investigate the stability of the molecular switch under finite bias. Using a firstprinciples method that combines the nonequilibrium Green’s function technique and density functional theory, we compute the current-voltage curves for both trans and cis configurations of the azobenzene molecule connected to two gold leads between bias voltages of 0 and 1 V. We find that the current through the trans configuration is significantly higher than that through the cis configuration for most biases, suggesting that the molecular switch proposed previously is stable under the finite bias. A negative differential conductance NDR is found for the cis configuration at 0.8 V. Analysis of the band structure of the leads and the molecular states reveals that the transmission through the highest occupied molecular orbital state of the molecule is suppressed significantly at this bias voltage, which causes the NDR.

Fast-response no-bias-bend liquid crystal displays using nanostructured surfaces

Abstract: We present results of a fast-response no-bias-bend (NBB) liquid crystal display, made possible by using a nanostructured alignment layer. Such alignment layers allow high pretilt angles of over 45° to be fabricated reliably. Thus, a stable bend configuration pi-cell can be achieved without applying any bias voltage to the cell. This NBB cell has a total on-off response time of less than 1.8ms and is faster than the corresponding optically compensated bend cell with a low pretilt angle.

Magnetic device having multilayered free ferromagnetic layer

Abstract: Magnetic multilayer structures, such as magnetic or magnetoresistive tunnel junctions (MTJs) and spin valves, having a magnetic biasing layer formed next to and magnetically coupled to the free ferromagnetic layer to achieve a desired stability against fluctuations caused by, e.g., thermal fluctuations and astray fields. Stable MTJ cells with low aspect ratios can be fabricated using CMOS processing for, e.g., high-density MRAM memory devices and other devices, using the magnetic biasing layer. Such multilayer structures can be programmed using spin transfer induced switching by driving a write current perpendicular to the layers. Each free ferromagnetic layer can include two or more layers and may be a multilayered free ferromagnetic stack that includes first and second ferromagnetic layers and a non-magnetic spacer between the first and second ferromagnetic layers.

In situ scanning tunnelling spectroscopy of inorganic transition metal complexes

Abstract: Redox molecules with equilibrium potentials suitable for electrochemical control offer perspectives in nanoscale and single-molecule electronics. This applies to molecular wiring, but also towards higher sophistication such as transistor or diode function. Most recent nanoscale or single-molecule functional systems are, however, fraught with operational limitations such as cryogenic temperatures and ultra-high vacuum, or lack of electrochemical potential control. We report here cyclic voltammetry (CV) using single-crystal Au(111)- and Pt(111)-electrodes and electrochemical in situ scanning tunnelling microscopy (STM) of a class of Os(II)/(III)- and Co(II)/(III)-complexes, the former novel in molecular electronics. The complexes are robust, with ligand groups suitable for linking the complexes to the Au(111)- and Pt(111)-surfaces via N- and S-donor atoms. The CV data reflect monolayer behaviour. Interfacial ET of the Os-complexes is fast, k0ET ≥ 106 s−1, while the Co-complex reacts much more slowly, k0ET ≈ (1−3) × 103 s−1. In situ STM of the Os-complexes shows a maximum in the tunnelling current/overpotential relation at constant bias voltage with up to 50-fold current rise. The peak position follows systematically the bias voltage and equilibrium potential, in keeping with theoretical frames for two-step electron transfer (ET) of in situ STM of redox molecules. The molecular conductivity behaves broadly similarly. The Co-complex also shows a tunnelling spectroscopic feature but much weaker than the Os-complexes. This can be ascribed to the much smaller interfacial ET rate constant, again caused by large intramolecular nuclear reorganization and weak electronic coupling to the substrate electrode. Overall the study has mapped the properties of target molecules needed for stable electronic switching, of possible importance in molecular electronics towards the single-molecule level, in room temperature condensed matter environment.

Electronic Circuit, Method of Driving Electronic Circuit, Electro-Optical Device, Method of Driving Electro-Optical Device, and Electronic Apparatus

Abstract: To provide an electronic circuit, a method of driving the electronic circuit, an electro-optical device, a method of driving the electro-optical device and an electronic apparatus, capable of reducing deviations in threshold voltages of transistors. A pixel circuit 20 is constructed with three transistors of a driving transistor Trd, an adjusting transistor Trc and a switching transistor Trs, and two capacitors of a first capacitor C 1 and a second capacitor C 2 . Further, a source of the adjusting transistor Trc is connected to a voltage supply line VL for supplying a driving voltage Vdd through a control transistor Q in common with the sources of the adjusting transistors Trc of other pixel circuits, the voltage supply line VL being provided at the right end side of an active matrix part.

Short-wavelength (λ≈3.05μm) InP-based strain-compensated quantum-cascade laser

Abstract: The design and implementation of a short-wavelength quantum-cascade laser based on the strain-compensated In0.73Ga0.27As–In0.55Al0.45As–AlAs heterosystem on InP is described. Lasers with a reduced level of doping in the active region require a larger bias voltage and emit at shorter wavelength; the emission wavelength is 3.05μm at T≈80K. The lasers operate up to T≈150K and electroluminescence persists up to room temperature, where the peak position is close to 3.3μm. The short-wavelength limit of such lasers is evaluated based on the dependence of their maximum operation temperatures and on the probable energies of the indirect valleys in the active region.

Current transport studies of ZnO/p-Si heterostructures grown by plasma immersion ion implantation and deposition

Abstract: Rectifying undoped and nitrogen-doped ZnO∕p-Si heterojunctions were fabricated by plasma immersion ion implantation and deposition. The undoped and nitrogen-doped ZnO films were n type (n∼1019cm−3) and highly resistive (resistivity ∼105Ωcm), respectively. While forward biasing the undoped-ZnO∕p-Si, the current follows Ohmic behavior if the applied bias Vforward is larger than ∼0.4V. However, for the nitrogen-doped-ZnO∕p-Si sample, the current is Ohmic for Vforward 2.5V. The transport properties of the undoped-ZnO∕p-Si and the N-doped-ZnO∕p-Si diodes were explained in terms of the Anderson model and the space charge limited current model, respectively.

The mechanical properties evaluation of the CrN coatings deposited by the pulsed DC reactive magnetron sputtering

Abstract: The chromium nitride coatings have been deposited by the bipolar symmetric pulsed DC reactive magnetron sputtering process at different temperature and pulse substrate bias. The pulse frequencies of target power and the substrate bias were kept at 2 kHz and 50 kHz, respectively. The nanoindenter, scratch and Daimler-Benz Rockwell-C adhesion tests were adopted to evaluate the mechanical properties of the CrN coatings. The preferred orientation of the CrN coatings changed from (111) to (200) with increasing substrate temperature and negative bias voltage applied. The hardness and adhesion properties of the coatings also increased with substrate temperature and a − 290 V bias voltage. A CrN film with 21 GPa in hardness and good adhesion property was deposited at 300 °C and − 290 V bias. It is observed that through the bipolar symmetry pulsed DC reactive magnetron sputtering process, the CrN films with sufficient adhesion performance was achieved in this work.

Investigation of Self-Heating Effects in Submicrometer GaN/AlGaN HEMTs Using an Electrothermal Monte Carlo Method

Abstract: An electrothermal Monte Carlo (MC) method is applied in this paper to investigate electron transport in submicrometer wurtzite GaN/AlGaN high-electron mobility transistors (HEMTs) grown on various substrate materials including SiC, Si, GaN, and sapphire. The simulation method is an iterative technique that alternately runs an MC electronic simulation and solves the heat diffusion equation using an analytical thermal resistance matrix method. Results demonstrate how the extent of the thermal droop in the Id-Vds characteristics and the device peak temperature depend upon both the biasing conditions and the substrate material type. Polarization effects are considered in the simulations, as they greatly influence electron transport in GaN/AlGaN HEMTs by creating a highly concentrated two-dimensional electron gas (2DEG) at the GaN/AlGaN interface. It is shown that a higher 2DEG density provides the devices with a better current handling capability but also increases the importance of the thermal effects