Search

We use the 6×6 Luttinger–Kohn Hamiltonian to study the effects of the spin-orbit split-off band on the properties of holes tunneling across quantum wells. The transmission coefficients of heavy and light holes decrease sharply (antiresonances) at energies coinciding with the formation of boundstates in two effective potential wells for the light and SO bands. The origin of these effective potential wells is due to the difference between the threshold energies for free propagation in the quantum well and cladding layers for holes in the light and SO bands. The dwell times of holes increase sharply at the antiresonance energies and are found to reach a maximum at some values of the transverse wave vector for holes incident with a fixed energy. The probability of heavy- to light-hole conversion (and vice versa) is found to increase at the antiresonance energies. These results are compared with the predictions of a one-band effective-mass description of hole transport to stress the importance of the coupling between the hole subbands on the tunneling properties of holes across quantum wells.

This paper examines the implications of differences in hole and electron transport properties on the microwave performance potential of InP-based Pnp HBTs. In particular, the carrier mobility, diffusion length, saturation velocity and ballistic transport properties for holes vs electrons are compared and their implications for Pnp device performance, such as the current gain and high frequency performance. The effects of the doping level on the mobility and minority carrier lifetime are included using empirical models fit to the available experimental data. The results show that, while the hole's transport properties are inferior to those for the electron, the minority carrier lifetime is larger for holes than electrons so that the hole diffusion length in the base is smaller but comparable to the electron's. As a result, Pnp HBTs with adequate current gain can be achieved with only a modest reduction in the base width compared with the Npn. For the device's microwave performance, the hole's characteristics determine the base transit time and the collector delay time, which limit the Pnp HBT's high frequency performance.

We analyze the importance of space-charge effects in the cathode to anode gap region of a recently proposed metal/CdS(cadmium sulfide)/LaS(lanthanum sulfide) cold cathode. Our approach is based on an ensemble Monte Carlo description of electron transport assuming ballistic injection across the CdS and LaS layers. Under this approximation, the energy spectrum of the injected beam entering the air gap can be determined exactly as a function of the applied bias across the CdS layer. The effects of shot noise in the injected current are taken into account. For some of the biasing conditions considered here, space-charge effects are quite drastic and lead to dynamical effects which are responsible for the onset of current self-quenching similar to the Child–Langmuir regime of operation of thermionic cathodes. The limiting anode current density is found to be much larger than the Child–Langmuir limit. In the presence of strong space-charge effects, large oscillations in the minimum of the electrostatic potentia...

An analytical model which matches thermionic-emission-diffusion of holes across the emitter-base heterojunction with drift-diffusion transport across a graded base has been developed and used to examine the performance capabilities of InP-based Pnp heterojunction bipolar transistors (HBTs). Hole drift-diffusion across the emitter-base space charge region is shown to be of comparable importance to thermionic emission in controlling hole injection into the base. The effects of compositional base grading on the recombination currents is also taken into account. Compositional grading of the base is shown to enhance the device’s current gain by as much as a factor of 10 by reducing recombination in the quasi-neutral base. More importantly, compositional base grading significantly reduces the base transit time which improves the device’s peak cutoff frequency by as much as a factor of 1.5. A cutoff frequency as high as 35 GHz is found to be possible. The analysis indicates that composition grading of the base c...

The electronic structure of the LaS surface and CdS/LaS interface is calculated by means of a first-principles electronic-structure method for slab and bulk geometries. The calculations were based on the local-density approximation to density-functional theory and made use of linear muffin-tin orbitals as basis functions for solving the Kohn-Sham equations. The observed low work function of LaS is reproduced by our theory and the calculated surface relaxation of LaS is found to be very small. It is further found that NaCl structured layers of LaS should grow in a well-behaved, epitaxial way on a CdS substrate with a ZnS structure. The interlayer spacing between the first atomic layer of LaS and the topmost layer of the CdS substrate is found to be close to the interlayer distances in both LaS and CdS. The LaS/CdS geometry should thus be possible to grow, and may be a good candidate for cold electron emitter devices.

Self-heating effects in graded AlGaAs/GaAs heterojunction bipolar transistors are analyzed using an ensemble Monte Carlo description of electron transport from emitter to subcollector including space-charge effects through a self-consistent solution of Poisson’s equation. For a given collector current density, the temperature rise in the intrinsic portion of the device is calculated using a simple analytical treatment. The temperature dependence of the scattering rates and various device parameters is included in the Monte Carlo simulations. The extent of velocity overshoot in the collector and its dependence on the device temperature is analyzed. The collector delay time is most affected by self-heating effects leading to a degradation of the unity current gain cut-off frequency of graded heterojunction bipolar transistors. For the biasing conditions considered here, the unity current gain cut-off frequency is found to be as much as 18% below its value calculated while neglecting self-heating effects.

The performance capabilities of InP-based pnp heterojunction bipolar transistors (HBT's) have been investigated using a drift-diffusion transport model based on a commercial numerical simulator. The low hole mobility in the base is found to limit the current gain and the base transit time, which limits the device's cutoff frequency. The high electron majority carrier mobility in the n/sup +/ InGaAs base allows a reduction in the base doping and width while maintaining an adequately low base resistance. As a result, high current gain (>300) and power gain (>40 dB) are found to be possible at microwave frequencies. A cutoff frequency as high as 23 GHz and a maximum frequency of oscillation as high as 34 GHz are found to be possible without base grading. Comparison is made with the available, reported experimental results and good agreement is found. The analysis indicates that high-performance pnp InP-based HBT's are feasible, but that optimization of the transistor's multilayer structure is different than for the npn device.

One potential application of cold cathodes is flat panel displays. At present, the market for emissive displays is based on cathode ray tubes (CRTs) and liquid crystal displays (LCDs). However, these technologies do not offer the combination of bright, sharp images in a low power, lightweight and flat package. Cold cathodes should offer these advantages in the not-too-distant future. Since the international market for displays by the year 2000 is predicted to reach over $20 billion (US), an increasing number of innovative cold cathode concepts should be expected.

In this article we study the effect of dynamically modifying the weight matrix on the performance of a neural associative memory. The dynamic modification is implemented by adding, at each step, the outer product of the current state, scaled by a suitable constant η, to the correlation weight matrix. For single-shot synchronous dynamics, we analytically obtain the optimal value of η. Although knowledge of the noise percentage is required for calculating the optimal value of η, a fairly good choice of η can be made even when the amount of noise is not known. Experimental results are provided in support of the analysis. The efficacy of the proposed modification is also experimentally verified for the case of asynchronous updating with transient length > 1.

Starting with the $4\ifmmode\times\else\texttimes\fi{}4$ Luttinger-Kohn Hamiltonian, we develop a scattering-matrix approach to study coherent hole transport through the valence-band energy profile across the emitter-base junction of typical abrupt and graded $\mathrm{Pnp}$ heterojunction bipolar transistors. The tunneling and reflection coefficients of heavy and light holes are calculated for the upper and lower $2\ifmmode\times\else\texttimes\fi{}2$ Hamiltonians obtained through a unitary transform of the $4\ifmmode\times\else\texttimes\fi{}4$ Luttinger-Kohn Hamiltonian. The probability for a transition from heavy (light) to light (heavy) hole while tunneling across the emitter-base junction is calculated as a function of the value of the wave vector parallel to the emitter-base heterointerface for both abrupt and graded heterojunctions. For holes injected from the emitter into the base, the probability of heavy- to light-hole conversion is shown to be quite different when calculated with the upper and lower Hamiltonians. On the other hand, the probability of light- to heavy-hole conversion is nearly the same for the upper and lower Hamiltonians. The energy dependence of the heavy- and light-hole tunneling coefficients is shown to be quite different from those calculated using a parabolic-band model, in which the effects of mixing and anisotropy in the valence band are neglected.

We identify a physical mechanism responsible for the onset of chaos in an asymmetric superconducting Wheatstone bridge of overdamped Josephson junctions while focusing on the dynamics of the transverse junction. The dynamics of the transverse junction are shown to be affected by an effective-noise-current term whose presence eventually leads to the onset of chaos in the bridge. This effective-noise-current term results from the competition of circulating currents in the upper and lower loops of the bridge. For some bridges, the effective-noise-current term has a profound influence on the value of the dc biasing current at which a nonzero average voltage appears across the transverse junction of the bridge.

We analyze the importance of current crowding in a new cold cathode emitter that consists of a thin wide band-gap semiconductor material sandwiched between a metallic contact and a low work function semimetallic thin film. Potential material candidates are suggested to achieve low-voltage (

The average base transit time is computed using a current impulse response technique for three typical abrupt Npn heterojunction bipolar transistors as a function of the emitter‐base bias, VBE. This technique is based on a hybrid model of carrier transport incorporating a quantum‐mechanical analysis of carrier injection at the emitter‐base junction and a Monte Carlo analysis of base transport. For typical AlGaAs/GaAs and InP/InGaAs structures, the base transit time first increases with VBE, reaches a maximum, and then decreases towards a value close to the one predicted using a semi‐Maxwellian injection of carriers into the base at an energy equal to the emitter‐base conduction band spike. For a typical InAlAs/InGaAs structure, the average base transit time is found to decrease with an increase in VBE. For all structures, we show that there is a correlation between the bias dependence of the average base transit time and the bias dependence of the average number of collisions per carrier (calculated for c...

Ensemble Monte Carlo simulations of electron transport through a new aluminum gallium nitride/gallium nitride (AlGaN/GaN) cold cathode emitter are reported. We analyze the energy spectrum of carriers prior to being injected into a low work function slab of Lanthanum Hexaboride (LaB 6 ) as a function of the ramp energy of the carriers at the AlGaN/GaN heterojunction. Plasmon scattering is shown to be the major scattering mechanism in the structure leading to substantial shift towards the low kinetic energy end of the energy spectrum of the carriers injected into the low work function Lanthanum Hexaboride thin film. Intervalley scattering is found to dominate in the depletion layer at the GaN/LaB 6 interface. Design improvements to increase the efficiency of the cold cathode are suggested.

We analyze a new cold cathode emitter which consists of a thin wide band gap semiconductor material sandwiched between a metallic material, and a low work function semimetallic thin film. We show that under forward bias operation the electrons captured in the low work function material are responsible for an effective reduction of the semimetallic film work function, together with a substantial increase of the cathode emitted current. The dynamic work function shift is shown to increase with the amount of injected current. Potential material candidates are suggested to achieve low‐voltage (

Current interest in spintronics is largely motivated by a belief that spin based devices (e.g. spin field effect transistors) will be faster and consume less power than their electronic counterparts. Here we show that this is generally untrue. Unless materials with extremely strong spin orbit interaction can be developed, the spintronic devices will not measure up to their electronic cousins. We also show that some recently proposed modifications of the original spin field effect transistor concept of Datta and Das [Appl. Phys. Lett., Vol. 56, 665 (1990)] actually lead to worse performance than the original construct.

We study the onset of intrinsic bistability and accompanying hysteresis in a single quantum point contact (QPC) with in-plane side gates in the presence of lateral spin-orbit coupling. The hysteresis in the conductance versus common gate voltage applied to the two side gates exists only if the narrow portion of the QPC is long enough. The hysteresis is absent if the effects of electron-electron interaction are neglected but increases with the strength of the electron-electron interaction. The hysteresis appears in the region of conductance anomalies, i.e., less than 2e2/h, and is due to multistable spin textures in these regions., Comment: Applied Physics Letters (2013)

We propose a new device to create a tunable all-electric spin polarizer: a quantum point contact (QPC) with four gates -- two in-plane side gates in series. The first pair of gates, near the source, is asymmetrically biased to create spin polarization in the QPC channel. The second set of gates, near the drain, is symmetrically biased and that bias is varied to maximize the amount of spin polarization in the channel. The range of common mode bias on the first set of gates over which maximum spin polarization can be achieved is much broader for the four gate structure compared with the case of a single pair of in-plane side gates., Comment: 4 figures, to appear in Applied Physics Letters

We used residual gas analysis (RGA) to identify the species desorbed during field emission (FE) from a carbon nanotube (CNT) fiber. The RGA data show a sharp threshold for H2 desorption at an external field strength that coincides with a breakpoint in the FE data. A comprehensive model for the gradual transition of FE from adsorbate-enhanced CNTs at low bias to FE from CNTs with reduced H2 adsorbate coverage at high bias is developed which accounts for the gradual desorption of the H2 adsorbates, alignment of the CNTs at the fiber tip, and importance of self-heating effects with applied bias., Comment: 18 pages 3 figures

A model to simulate electron transport through the base region of abrupt heterojunction bipolar transistors has been developed taking into account the finite probability for electrons in the base to tunnel through the emitter‐base spike back into the emitter. The average base transit time is calculated as a function of the emitter‐base voltage using the impulse response technique. For all biases, the average base transit time is found to be smaller than its value computed while neglecting the finite probability for electrons with energy below the emitter‐base spike to tunnel back into the emitter. For the case of an AlGaAs/GaAs structure, the average base transit time is found to increase with the forward emitter‐base voltage.

We compute the current‐voltage characteristics of unbalanced superconducting Wheatstone bridges biased with constant and radio frequency current sources. An analytical expression for the critical supercurrent at which a nonzero voltage appears across the transverse junction of the bridge is derived. Analytical expressions of the first three Shapiro steps are also given and found to be in good agreement with numerical simulations.

The forward delay time (τF) of heterojunction bipolar transistors (HBTs) is calculated using a hybrid model of carrier transport. A rigorous quantum-mechanical treatment of electron tunneling and thermionic emission across the spike at the emitter-base junction is used to determine the energy of the electron flux injected into the base region. This flux is used as an initial distribution in a regional Monte Carlo simulator to model electron transport from base to sub-collector. In this paper, we estimate the base transit time using the impulse response technique and the collector delay time using the expression of Laux and Lai (IEEE Electron Device Letters, 11, 174, 1990). Improvements to the hybrid model proposed here to reduce some of the discrepancies between measured and calculated values of ƒτ for some InAlAs/InGaAs and InP/InGaAs structures reported in the literature are discussed.

We describe an analytical approach to modeling NPN InP-based heterojunction bipolar transistors using a Gummel Poon model. Starting from an initial, physical description of the device's epitaxial structure and geometry, this physics-based model is utilized to simulate the device's operation using a thermionic-field-diffusion model for carrier injection at the emitter and diffusive transport across the base. This approach incorporates recent advances in understanding of device operation and improved models of parasitics. The cutoff frequency and maximum frequency of oscillation are calculated as a function of collector current density and compared with experimental measurements. For the cutoff frequency, the results of this model underestimate the high frequency performance of the device by as much as 50% at the higher collector current levels. To improve the model and to allow simulation of narrower base widths, a more rigorous analysis of tunneling through the emitter spike and ballistic transport across the base and base-collector depletion regions has been included. Quantum mechanical tunneling of electrons through the emitter-base spike has been taken into account for accurate description of the energy distribution of electron injection into the base. The resulting electron flux distribution is used as an initial distribution in a regional Monte Carlo simulator to model electron transport from the emitter edge of the base to the subcollector. The results are then used to calculate an average base transit time and base-collector space charge region delay time which can be incorporated in the analytical model.

A hybrid model is developed to simulate electron transport through the emitter‐base heterojunction and the base region of abrupt heterojunction bipolar transistors. The energy distribution of the injected electron flux through the emitter‐base junction is calculated using a rigorous quantum‐mechanical treatment of electron tunneling and thermionic emission across the spike at the emitter‐base junction. The results are compared with those predicted by the conventional thermionic‐field‐emission model. For both models, the electron fluxes injected across the emitter‐base junction are used as initial energy distributions in a regional Monte Carlo calculation to model electron transport through the base. The average base transit times are calculated using the impulse response technique as a function of the emitter‐base voltage. The differences between the thermionic‐field‐emission model and the rigorous quantum‐mechanical approaches to model electron transport through abrupt heterojunction bipolar transistors are pointed out.

In their rocksalt structure, rare-earth monosulfides offer a more stable alternative to alkali metals to attain low or negative electron affinity when deposited on various III-V and II-VI semiconductor surfaces. In this article, we first describe the successful deposition of Lanthanum Monosulfide via pulsed laser deposition on Si and MgO substrates and alumina templates. These thin films have been characterized by X-ray diffraction, atomic force microscopy, high resolution transmission electron microscopy, ellipsometry, Raman spectroscopy, ultraviolet photoelectron spectroscopy and Kelvin probe measurements. For both LaS/Si and LaS/MgO thin films, the effective work function of the submicron thick thin films was determined to be about 1 eV from field emission measurements using the Scanning Anode Field Emission Microscopy technique. The physical reasons for these highly desirable low work function properties were explained using a patchwork field emission model of the emitting surface. In this model, nanocrystals of low work function materials having a orientation perpendicular to the surface and outcropping it are surrounded by a matrix of amorphous materials with higher work function. To date, LaS thin films have been used successfully as cold cathode emitters with measured emitted current densities as high as 50 A/cm2. Finally, we describe the successful growth of LaS thin films on InP substrates and, more recently, the production of LaS nanoballs and nanoclusters using Pulsed Laser Ablation.

We study of the appearance and evolution of several anomalous (i.e., G < G(0) D 2e(2)/h) conductance plateaus in an In(0.52)Al(0.48)As/InAs quantum point contact (QPC). This work was performed at T = 4:2 K as a function of the offset bias ΔV(G) between the two in-plane gates of the QPC. The number and location of the anomalous conductance plateaus strongly depend on the polarity of the offset bias. The anomalous plateaus appear only over an intermediate range of offset bias of several volts. They are quite robust, being observed over a maximum range of nearly 1 V for the common sweep voltage applied to the two gates. These results are interpreted as evidence for the sensitivity of the QPC spin polarization to defects (surface roughness and impurity (dangling bond) scattering) generated during the etching process that forms the QPC side walls. This assertion is supported by non-equilibrium Green function simulations of the conductance of a single QPC in the presence of dangling bonds on its walls. Our simulations show that a spin conductance polarization as high as 98% can be achieved despite the presence of dangling bonds. The maximum in is not necessarily reached where the conductance of the channel is equal to 0:5G(0).

We have recently shown that asymmetric lateral spin orbit coupling (LSOC) resulting from the lateral in-plane electric field of the confining potential of a side-gated quantum point contact (QPC) can be used to create a strongly spin-polarized current by purely electrical means1 in the absence of applied magnetic field. Using the non-equilibrium Green function formalism (NEGF) analysis of a small model QPC2, three ingredients were found to be essential to generate the strong spin polarization: an asymmetric lateral confinement, a LSOC induced by the lateral confining potential of the QPC, and a strong electron-electron (e-e) interaction. In this paper, NEGF is used to study how the spin polarization is affected by the presence of impurities in the central portion of the QPC. It is found that the number, location, and shape of the conductance anomalies, occurring below the first quantized conductance plateau (G 0 =2e2/h), are strongly dependent on the nature (attractive or repulsive) and the locations of the impurities. We show that the maximum of the conductance spin polarization is affected by the presence of impurities. For QPCs with impurities off-center, a conductance anomaly appears below the first integer step even for the case of symmetric bias on the two side gates. These results are of practical importance if QPCs in series are to be used to fabricate all-electrical spin valves with large ON/OFF conductance ratio.

The use of a thin mixed layer consisting of an inert diluent material and a light emitting material between the hole-transport layer and electron-transport layer of organic light-emitting diodes leads to an increase in the external quantum efficiency. The efficiency improvement is highly dependent on the thickness of the diluted light-emitting layer and driving current. Significant improvement seen at low current densities is explained in terms of effective hole confinement by the mixed layer while a modest decreases in efficiency at higher current densities may be attributed to luminescence quenching at the hole-transport layer/inert diluents material interface. The phenomena are demonstrated with three different inert diluents materials. A maximum external quantum efficiency improvement of about 40% is found for a diluted light-emitting layer thickness between 40 {\AA} and 60 {\AA}., Comment: 12 pages, 3 figures, to appear in Journal of Nanoelectronics and Optoelectronics

We have measured the longitudinal (T1) and transverse (T2) spin relaxation times in the organic semiconductor tris(8-hydroxyquinolinolato aluminum) - also known as Alq3 - at different temperatures and under different electric fields driving current. These measurements shed some light on the spin relaxation mechanisms in the organic. The two most likely mechanisms affecting T1 are hyperfine interactions between carrier and nuclear spins, and the Elliott-Yafet mode. On the other hand, the dominant mechanism affecting T2 of delocalized electrons in Alq3 remains uncertain, but for localized electrons (bound to defect or impurity sites), the dominant mechanism is most likely spin-phonon coupling.

Abstract With 2.0 Giga watt (GW) already installed and more than 50.0 GW planned in Europe by 2020 as reported by the European Wind Energy Association, the installation of bottom mounted offshore wind turbine in very shallow water (less than 40 m / 120 ft) is now well established. The foundation and the offshore installation costs represent approximately 50% of the total development cost of an offshore wind farm and this proportion increases with the water depth. Hence, floating wind turbine is the solution for deep offshore, which does not mean far from the coast. In summer 2009, the world's first full scale offshore floating wind turbine, has been installed in Norway in 220 m water depth. The offshore floating wind turbine is now becoming a reality considering the following advantages:Offshore floating wind farms offer a more constant and predictable wind resource.Location of wind farms in areas or countries with no shallow water is made possible.When necessary, the location of wind farms far from the coast greatly reduces the visual impact.Wind farms can be developed in areas subject to earthquake.The floating substructure design can be used for projects in various site locations.Integration and installation procedures are simplified.Anchoring installation does not require sophisticated vessels. This paper will focus on the ongoing development of a breakthrough solution with the goal to reduce the cost for offshore floating wind applications. The overall concept is based on the use of a vertical axis wind turbine that offers a much lower centre of gravity than the more standard horizontal axis wind turbine concepts. Combined to an effective mooring system, this new floating wind turbine can economically compete with bottom mounted solutions in shallow water, bearing in mind that integration of the turbine in the floater and installation of the floating wind turbine must remain as simple and as safe as possible. Further more, in order to reduce as much as possible the cost of the electricity generated, a wind turbine dedicated to the offshore environment is actually designed with a reduced number of parts and auxiliaries assuring low maintenance (no pitch, no yaw, and no gearbox). The test results of a vertical axis wind turbine onshore prototype which has been running for several months will be presented as well as the development plan for a 2 Mega watt (MW) onshore prototype and a full scale 2 MW floating prototype to be installed in France in 2012. Introduction Oil & Gas industry has a vast experience in floating production units. Starting more than 35 years ago, there is to date around 540 floating units on all seven seas. These floating units can be classified into four major families: tension leg platform (TLP), semi submersible, spar and ship shape. The distribution of such units is as follows:25 TLP the first one was Conoco Hutton, installed in 1984.84 Semi the first one was BHP Argyll, installed in 1975.17 Spar the first one was Anadarco Neptune, installed in 1996.414 ship shapes: 149 Floating Production Storage and Offloading (FPSO), the first one was Cities Service Indonesia Inc's Poleng PB FPSO barge installed in 1975 and 265 Floating Storage Units (FSU), Sevans and other mobile (early) production units.

A two-dimensional ?-potential Kronig?Penney model for quasi-one-dimensional (Q1D) disordered systems is used to study analytically the influence of a constant electric field on the inverse localization length (LL). Based on the Green's function formalism we have calculated LL as a function of the incoming energy E, electric field F, length L of the Q1D sample, number of modes M in the transverse direction and the amount of disorder w. We show that, for large L in Q1D systems, states are weakly localized, i.e.?we deal with power-law localization. With increasing electric field in Q1D mesoscopic systems a transition from exponential to a power-law behavior takes place, as in 1D systems. We note that the graphs showing the inverse LL change significantly with increasing F (for fixed M) rather than with increasing M (for fixed F). We also show that the graphs representing the ratio of the corresponding localization length without and with electric field collapse for all modes M into a universal curve in the Q1D strip model.

Various experimental groups have reported observation of both integer and fractional giant Shapiro steps in the I‐V curves of N×M arrays of superconducting‐normal metal‐superconducting (SNS) Josephson junctions. The size of these steps have been investigated both experimentally and theoretically as a function of temperature, external magnetic field, and rf current. In this report, we investigate the possibility of controlling the size of these giant Shapiro steps by an external gate voltage by considering arrays of superconductor‐semiconductor‐superconductor junctions. We show that giant Shapiro steps of different orders can be quenched over different gate voltage ranges in these arrays.

With the advent of sophisticated growth techniques such as Molecular Beam Epitaxy and Metal Organic Chemical Vapor Deposition, the calculation of the energy boundstates and electron wave‐functions of the one‐electron Schrödinger equation has received a lot of attention over the last decade. With the more recent fabrication of quantum wires and dots, it seems now imperative to extend the boundstates calculation to systems containing only a few electrons. Hereafter, we investigate the effect of electron exchange and Coulomb interactions on the boundstates of a two‐electron system in a square quantum well. The technique is based on a general Alternating Direction Implicit algorithm ( T. Singh and M. Cahay, SPIE Vol. 1675, Quantum Wells and Superlattice Physics IV (1992), p.11) combined with a Fourier spectrum analysis of the two‐particle wavefunction correlation , , where χ1, χ2 are the coordinates of the two electrons. The precise location of the energy eigenvalues requires the appropriate use of window functions before calculating the Fourier transform of the correlation function. We also compare our results for the boundstate energies with those obtained using a first order time‐independent perturbation theory.

The use of asymmetrically biased quantum point contacts in semiconductor heterostructures paves the way for the realization of an all-electric spin field-effect transistor.

In the past, the quantum mechanical tunneling time through simple rectangular barrier has been obtained by various theoretical approaches including the dwell time, the phase delay time, the Larmor clock time and also using the numerical analysis of wave packets. The agreement among these approaches over a range of incident electron energy is far from satisfactory. In this manuscript, analytical expressions for the tunneling time are derived based on the group velocity approach (referred hereafter as the average particle time, tau /sub APT/) for single and double rectangular potential barriers under zero bias. The results of the single barrier case, including the limiting value of the tunneling time for various energy limits, are compared with these previous tunneling time calculations. The tau /sub APT/ results provide physically meaningful tunneling times for zero and infinite incident energy limits of the electron. The tau /sub APT/ for the double barrier structure is computed from the analytical solution as a function of the incident energy of the electron for two experimentally studied resonant tunneling structures. For both the single and double barrier cases, the effect of the structure parameters such as barrier width, height, and well width on the tau /sub APT/ are obtained and reported. >

We review our recent study of the conductance of side gated quantum point contacts (QPCs) in the presence of lateral spin-orbit scattering (LSOC). As shown in Phys. Rev. B 80, 155440 (2009), a small difference of bias between the two side gates of a QPC leads to an inversion asymmetry in the LSOC on opposite edges of the channel. This triggers a small spin imbalance in the QPC. The latter increases substantially when the effects of electron-electron interaction are included in the QPC. This is accompanied by a wide variety of anomalous conductance plateaus as the dimensions of the QPC are varied. In this paper, the spin polarization of the conductance a is studied as a function of the device geometry. A simple model of two QPCs in series predict an ON/OFF ratio equal to 1/(1 − α2) which is around 86 at T = 4.2K for the device parameters investigated. A tandem of QPCs could therefore act as an efficient all-electric spin valve. Design issues for operation at higher temperature are discussed.

A numerical calculation of the patch-field distribution across nano-patchwork surfaces has been developed. Results show that the low work function nanosize zones are intrinsically protected by an electrostatic screen, which is induced by the surrounding area having higher work function. In presence of an applied field, during field emission, a preferential opening of the surface barrier just over the nano-patches induces a field emission array of parallel e-beams whose geometrical distribution is defined by the positions of the nano-patches.

The transmission line technique to solve the time independent Schrodinger equation is modified to include the effect of elastic scattering by introducing a scattering impedance Z s = 4 jV 0 /ℏ. The method is applied to calculate the current-voltage and traversal time characteristics of double barrier resonant tunneling structures. The peak current density for a random distribution of scatterers in the quantum well decreases while it increases for scattering at a fixed location compared to its value in the absence of any scatterers. Traversal time decreases with applied bias and shows distinct dips at resonance. The traversal time through an asymmetric resonant tunneling structure is also investigated.

We have studied phase coherent electron transport in a disordered semiconductor quantum wire subjected to a weak magnetic field. Our model treats disorder (elastic scattering) with scattering matrices and uses Weber functions to calculate the elements of the scattering matrices. The Weber functions account for the magnetic field exactly. We find that the universal conductance fluctuations and the two-terminal Landauer resistance of a disordered structure are surprisingly sensitive to whether the scatters are attractive or repulsive and the difference between the effects of attractive and repulsive scatterers is accentuated by the presence of evanescent states.

We study the influence of impurity scattering on the probability of electron wave switching between coupled quantum wells while varying the location of the impurities in an electron-wave directional coupler. We use a transfer-matrix formalism to solve the coupled-mode equations describing the electron propagation assuming that only one symmetric and one antisymmetric states are available for conduction in the directional coupler. If the Fermi energy is not large compared to the strength of the potential energy profile, the maximum probability of electron transfer between the wells can be substantially reduced from the predicted value of 100% for perfectly symmetric impurity-free directional couplers. The probability of transfer is strongly dependent on the location of the impurities. Attractive and repulsive impurities affect the probability of transfer differently. The potential of the impurities is modeled as a delta-potential with strength equal to a few feV- cm 2 . The relevance of our simulations for the study of directional couplers proposed recently is discussed.

We have calculated the two-dimensional spatial distributions of several transport variables such as the current density, carrier concentration, chemical potential, space-charge electric field, residual resitivity dipole potential and electromigration forces in disordered mesoscopic structures subjected to arbitrary magnetic fields. These fully quantum mechanical calculations shed new light on such magnetotransport phenomena as the integer quantum Hall effect, the formation of magnetic bound states, magnetic response of curent vortices caused by quantum interference between scatterers, the nature of residual resistivity dipoles in phase coherent transport, the difference between majority- and minority-carrier mobilities in the quantum mechanical regime, the electron wind force and direct force of electromigration in a mesoscopic system, and a variety of other phenomena.

Recently several proposals of quantum interference devices have appeared based on the close proximity of two coupled electron waveguides [1,2]. The steady-state analysis of these new electron-wave directional couplers indicates that these devices are feasible using current fabrication technology [2], and could be used as basic switches in high-performance digital circuit at low enough temperature. In order to investigate the potential high-frequency applications of these newly proposed electron directional couplers, we have studied the transfer of wavepackets between parallel quantum wells using a block alternating direction implicit technique to solve the two dimensional time-dependent Schrodinger equation following the scalar ADI scheme of Douglas and Gunn [3]. While injecting a wavepacket in one well, we show that only a partial transfer between wells (close to 85%) is reached even for perfectly double-quantum wells (in the direction of growth). We also study the amount of transfer in asymmetric configurations and while varying the total energy of the wavepacket. The length and time scales for maximum transfer are found to be around a few thousand angstroms and picoseconds, respectively.

Pnp InP/InGaAs heterojunction bipolar transistors have been modeled using a modified Gummel–Poon model, and the results are compared with experimental measurements and results from a commercial simulator. The model provides a good description of the transistor’s high frequency performance and describes the falloff in device performance at high current densities. The model overestimates the current gain at low current densities by neglecting hole recombination in the base side of the emitter–base space charge region.

This work focuses on improving the crystalline property of the LaS thin films by improving the deposition conditions. Many factors can affect the quality of laser deposited films, including vacuum level, laser energy, laser repetition rate, substrate temperature, target to substrate distance, background gas pressure, and substrate bias., 18th International Vacuum Nanoelectronics Conference, 2005

In electronics, information has been traditionally stored, processed and communicated using an electron's charge. This paradigm is increasingly turning out to be energy-inefficient, because movement of charge within an information-processing device invariably causes current flow and an associated dissipation. Replacing charge with the "spin" of an electron to encode information may eliminate much of this dissipation and lead to more energy-efficient "green electronics". This realization has spurred significant research in spintronic devices and circuits where spin either directly acts as the physical variable for hosting information or augments the role of charge. In this review article, we discuss and elucidate some of these ideas, and highlight their strengths and weaknesses. Many of them can potentially reduce energy dissipation significantly, but unfortunately are error-prone and unreliable. Moreover, there are serious obstacles to their technological implementation that may be difficult to overcome in the near term. This review addresses three constructs: (1) single devices or binary switches that can be constituents of Boolean logic gates for digital information processing, (2) complete gates that are capable of performing specific Boolean logic operations, and (3) combinational circuits or architectures (equivalent to many gates working in unison) that are capable of performing universal computation., Comment: Topical Review

A nonequilibrium Green's function formalism is used to study the conductance of a side-gated quantum point contact (QPC) in the presence of the lateral spin-orbit coupling (LSOC). A small difference of bias voltage between the two side gates (SGs) leads to an inversion asymmetry in the LSOC between the opposite edges of the channel. In the single-electron modeling of transport, this triggers a spontaneous but insignificant spin polarization in the QPC. However, the spin polarization of the QPC is enhanced substantially when the effect of electron-electron interaction is included. The spin polarization is strong enough to result in the occurrence of a conductance plateau at $0.5{G}_{0}$ $({G}_{0}=2{e}^{2}/h)$ in the absence of any external magnetic field. In our simulations of a model QPC device, the 0.5 plateau is found to be quite robust and survives up to a temperature of 40 K. The spontaneous spin polarization and the resulting magnetization of the QPC can be reversed by flipping the polarity of the source to drain bias or the potential difference between the two SGs. These numerical simulations are in good agreement with recent experimental results for side-gated QPCs made from the low band-gap semiconductor InAs.