Showing papers by "David K. Ferry published in 2002"
TL;DR: Dynamical tunneling gives rise to a set of resonances characterized by two quantum numbers, which leads to conductance oscillations and concentration of wave functions near stable and unstable periodic orbits.
Abstract: We argue that many major features in electronic transport in realistic quantum dots are not explainable by the usual semiclassical approach, due to the contributions of the quantum-mechanical tunneling of the electrons through the Kolmogorov-Arnol'd-Moser islands. We show that dynamical tunneling gives rise to a set of resonances characterized by two quantum numbers, which leads to conductance oscillations and concentration of wave functions near stable and unstable periodic orbits. Experimental results agree very well with our theoretical predictions, indicating that tunneling has to be taken into account to understand the physics of transport in generic nanostructures.
63 citations
TL;DR: In this paper, it was shown that transport in open quantum dots can be mediated by single eigenstates, even when the dot leads support several propagating modes, and that the broadening of these few robust states, whose wave functions are generally localized within the interior of the dot, is independent of the lead width.
Abstract: We show that transport in open quantum dots can be mediated by single eigenstates, even when the dot leads support several propagating modes. The broadening of these few robust states, whose wave functions are generally localized within the interior of the dot, is found to be virtually independent of the lead width. Our results therefore indicate that a proper discussion of the specific nature of the individual eigenstates of the closed system is critical to determining their influence on transport through open dots.
52 citations
TL;DR: This paper compares the two most popular quantum potential techniques: density gradient and the effective potential approaches, in terms of their justification, accuracy and computational efficiency, and discusses the capability of the density gradient formalism to handle direct source-to-drain tunnelling in double-gate MOSFETs.
Abstract: As MOSFETs are scaled to sub 100 nm dimensions, quantum mechanical confinement in the direction normal to the silicon dioxide interface and tunnelling (through the gate oxide, band-to-band and from source-to-drain) start to strongly affect their characteristics. Recently it has been demonstrated that first order quantum corrections can be successfully introduced in self-consistent drift diffusion-type models using Quantum Potentials. In this paper we describe the introduction of such quantum corrections within a full 3D drift diffusion simulation framework. We compare the two most popular quantum potential techniques: density gradient and the effective potential approaches, in terms of their justification, accuracy and computational efficiency. The usefulness of their 3D implementation is demonstrated with examples of statistical simulations of intrinsic fluctuation effects in decanano MOSFETs introduced by discrete random dopants. We also discuss the capability of the density gradient formalism to handle direct source-to-drain tunnelling in sub 10 nm double-gate MOSFETS, illustrated in comparison with Non-Equilibrium Green's Functions simulations.
44 citations
TL;DR: In this paper, a pulsed voltage input and a four-point measurement were used to determine the room temperature velocity characteristic of bulk gallium nitride test structures with an etched constriction.
Abstract: A pulsed voltage input and a four-point measurement were used to determine the room temperature velocity–field characteristic of bulk gallium nitride test structures with an etched constriction. A peak electron velocity of approximately 2.5×107 cm/s was attained at a field of 180 kV/cm, which corresponds closely to theoretical predictions.
42 citations
TL;DR: In this article, the effective potential is employed to account for the quantum mechanical effects of charge setback and elevated ground-state energy in the inversion layer of FD SOI MOSFETs.
Abstract: In this work, the effective potential is employed to account for the quantum mechanical effects of charge setback and elevated ground-state energy in the inversion layer of fully depleted (FD) SOI MOSFETs. We use the effective potential along with a three-dimensional Poisson solver and a Monte Carlo transport kernel to illustrate these quantum mechanical effects on the output characteristics of the transistor. It is demonstrated that the inclusion of such effects has a significant influence on the threshold voltage, carrier energy, and drive current of the device.
36 citations
TL;DR: In this paper, the use of the effective potential to incorporate quantum effects in device models is discussed, and it is shown that while threshold shifts and charge set-back are handled well, tunneling is not well handled by this approach, or by any other local potential approach.
Abstract: We discuss the use of the effective potential to incorporate quantum effects in device models. While threshold shifts and charge set-back are handled well, tunneling is not well handled by this approach, or by any other local potential approach.
33 citations
TL;DR: In this article, a Wigner function quantum Monte Carlo simulation of resonant tunneling diodes is presented, showing tunneling and correlation build-up during resonance, as evidence of a working model.
Abstract: Results obtained from a Wigner function quantum Monte Carlo simulation of resonant tunneling diodes are presented. Methods of introducing partial weights of electrons through an affinity as well as methods for incorporating negative values of the Wigner function are discussed. Results showing tunneling and correlation build-up during resonance are presented as evidence of a working model.
32 citations
TL;DR: In this article, the fluctuations in the threshold voltage and electron drift velocity in ultrasmall devices due to different numbers and distribution of impurity atoms in the device active region were investigated.
Abstract: We present results for the fluctuations in the threshold voltage and electron drift velocity in ultrasmall devices due to different numbers and distribution of impurity atoms in the device active region. We find that fluctuations in the threshold voltage VT are due to both the actual number and position of the dopant atoms. For the devices being considered, the correlation of the threshold voltage (average drift velocity) to the number of dopant atoms in a 10 nm range at various depths shows that the atoms in the top 15–20 nm (top 8 nm) beneath the channel have the most impact.
31 citations
TL;DR: In this article, the results of Gaussian wave-packet tunneling through a single barrier structure and RTD operation achieved from a particle-based Ensemble Monte Carlo (EMC) simulation that is based on the Wigner distribution function (WDF) are presented.
Abstract: We present results of both Gaussian wave-packet tunneling though a single barrier structure and RTD operation achieved from a particle-based Ensemble Monte Carlo (EMC) simulation that is based on the Wigner distribution function (WDF). Methods of including the Wigner potential into the EMC, to incorporate naturally quantum phenomena, via a particle property we call the affinity are discussed. Results showing tunneling and correlation build-up in both cases are presented.
31 citations
TL;DR: In this paper, an electrically tunable semiconductor quantum waveguide implementation of an inverter gate in a GaAs/AlGaAs heterostructure was presented, in which the output of the waveguide structure may be selected via the application of an appropriate magnetic field or electrical bias.
Abstract: In recent years, quantum computing and information theory has received a great deal of attention as a means of drastically improving the computational speed and resources traditionally associated with current binary implementations. We present an electrically tunable semiconductor quantum waveguide implementation of an inverter gate in a GaAs/AlGaAs heterostructure in which the output of the waveguide structure may be selected via the application of an appropriate magnetic field or electrical bias. The resulting behavior observed by our implementation shows a great deal of promise for an eventual semiconductor realization of this basic qubit structure.
30 citations
TL;DR: In this article, the form of the electron-phonon interaction in two-dimensional and quasi-one-dimensional, InGaAs quantum wires was compared, and evidence for a strongly enhanced interaction was found in the quasi-1D wire, and was suggested to result from the presence of the singularities in its electronic density of states.
Abstract: Electron-heating measurements are used to compare the form of the electron–phonon interaction in two-dimensional, and quasi-one-dimensional, InGaAs quantum wires. Evidence for a strongly enhanced interaction is found in the quasi-one-dimensional wire, and is suggested to result from the presence of the singularities in its electronic density of states. The Bloch–Gruneisen criterion is easily violated in this wire, and its energy-loss function is found to show a weak temperature dependence, which is argued to result from a saturation of scattering processes in the uppermost one-dimensional subband.
TL;DR: In this article, the experimental and theoretical studies of high field transport in bulk GaN have been conducted using EPM calculations for the bandstructure, together with an empirical valence shell model for the lattice dynamics in cubic GaN.
Abstract: We report on experimental and theoretical studies of high field transport in bulk GaN. Theoretically, we have investigated the electron-phonon interaction in this material based on empirical pseudopotential method (EPM) calculations for the bandstructure, together with an empirical valence shell model for the lattice dynamics in cubic GaN. The rigid-ion model is used to calculate the electron-phonon scattering rate for all modes, and to extract a deformation potential for use in high field transport calculations. A full band Monte-Carlo (FBMC) method is used to simulate high field transport in GaN. In the experimental studies, we have fabricated etched constrictions for performing pulse I-V measurements in bulk GaN and GaN/AlGaN heterostructures. The experimentally extracted velocity-field characteristics are in good agreement with transport calculations up to fields as high as 1.5 x 10 5 V/cm, where velocities up to 2.5 x 10 7 cm/s are measured.
TL;DR: In this paper, the authors studied the quantum-interference characteristics of a 25 nm, trench-type, InGaAs quantum-wire field-effect transistor realized by selective epitaxy, and found very different behavior from that typically exhibited by disordered wires.
Abstract: We study the quantum-interference characteristics of a 25 nm, trench-type, InGaAs quantum-wire field-effect transistor realized by selective epitaxy, and find very different behavior from that typically exhibited by disordered wires. The amplitude of the magnetoresistance fluctuations is exponentially suppressed at high fields, where evidence of an Aharonov–Bohm effect is observed. The exponential suppression appears to be consistent with theoretical predictions for the influence of magnetic field on the scattering rate in clean wires, while the Aharonov–Bohm effect points to an interference process in which the one-dimensional subbands of the wire themselves constitute well-resolved paths for electron interference.
TL;DR: In this article, the impact of surface roughness and impurity scattering on the transport transient response and steady-state regime in a 50 nm gate-length nMOSFET was studied.
Abstract: A 50 nm nMOSFET has been studied by Ensemble Monte Carlo (EMC) simulation including a novel physical model for the treatment of surface roughness and impurity scattering in the Si inversion layer. In this model, we use a bulk-like phonon and impurity scattering model and surface-roughness scattering in the silicon inversion layer, coupled with the effective/smoothed potential approach to account for space quantization effects. This approach does not require a self-consistent solution of the Schrodinger equation. A thorough account of how these scattering mechanisms affect the transport transient response and steady-state regime in a 50 nm gate-length nMOSFET is given in this paper. A set of I/sub ds/-V/sub ds/ curves for the transistor is shown. We find that the smoothing of the potential to account for quantum effects has a strong impact on the electron transport properties, both in transient and steady-state regimes. We also show results for the impact that impurity and surface-roughness scattering mechanisms have on the average velocity of the carriers in the channel and the current flowing through the device. It was found that time-scales as short as 0.1-0.2 ps are enough to reach a steady-state channel electron average velocity.
TL;DR: In this article, a two-dimensional (2-D) coupled Monte Carlo-Poisson solver was used to simulate a 250 nm n-channel MOSFET with a 70-nm p/sup +/-implant placed at the source end of the channel (achievable by focused-ion-beam (FIB) implantation).
Abstract: A highly asymmetric 250 nm n-channel MOSFET, with a 70-nm p/sup +/-implant placed at the source end of the channel (achievable by focused-ion-beam (FIB) implantation, so the device is named FIBMOS), has been simulated using a two-dimensional (2-D) coupled Monte Carlo-Poisson solver, in which quantum confinement effects have been taken into account by incorporating an effective potential scheme into the particle simulator. Although the device is a long-channel one, its performance is dictated by the highly doped p/sup +/-implant at the source end of the channel, and it is crucial to properly account for the quantum-confinement effects in transport, especially at the implant/oxide interface. We show that parameters such as threshold voltage and device transconductance are extremely sensitive to the proper treatment of quantization effects. On the other hand, the built-in electric field, due to the pronounced asymmetry caused by the presence of the p/sup +/-implant, drastically influences the carrier transport, and consequently, the device output characteristics, in particular the magnitude of the velocity overshoot effect and the low-field electron mobility.
TL;DR: This paper derives an equation of motion for the reduced system density matrix, which is time convolutionless and free of the partial trace with respect to the environment states, and investigates the application of the approach in the description of far-from-equilibrium systems.
Abstract: Evolution of a system, coupled to its environment and influenced by external driving fields, is an old problem that remains of interest. In this paper, we derive an equation of motion for the reduced system density matrix, which is time convolutionless and free of the partial trace with respect to the environment states. This new approach uses an extension of the projection-operator technique, which incorporates an isomorphism between the system's Liouville space and the unit eigenspace of the projection operator induced by the uniform environment density matrix. Numerical application of the present approach is particularly useful in large externally driven systems, as the partial-trace-free equation is given in terms of submatrices significantly smaller than the matrices in the conventional time-convolutionless approaches, which alleviates the computational burden. We also show that all time-convolutionless approaches, conventional or partial-trace-free, are based upon a hidden underlying assumption of time reversibility of the system's evolution. This feature puts significant constraints on applicability of time-convolutionless approaches when employing approximations that yield time irreversibility. Also, we investigate the application of the approach in the description of far-from-equilibrium systems.
TL;DR: In this paper, the authors used a pulsed voltage input with a 200 ns pulse width and a four-point measurement in a 50 Ω environment to determine the room temperature velocity-field characteristic of the structures.
TL;DR: In this article, the role of quantum-mechanical space quantization effects on the operation of a 50 nm MOSFET device, an asymmetric 250 nm FIBMOS device and a narrow-width SOI device was investigated.
Abstract: We investigate the role of the quantum-mechanical space-quantization effects on the operation of a 50 nm MOSFET device, an asymmetric 250 nm FIBMOS device and a narrow-width SOI device structure. We find that space-quantization effects give rise to larger average displacement of the carriers from the interface proper and lower sheet electron density in both the regular and the asymmetric MOSFET device structures. The effect is even more pronounced in the narrow-width SOI device due to the presence of a two-dimensional confinement (both vertical and along the width direction). The reduction in the sheet electron density, in turn, gives rise to shift in the devices threshold voltage, on the order of 100–200 mV, depending upon the device structure being investigated. This leads to 20–40% decrease of the device on-state current which depends upon the gate bias. Hence, to properly describe the operation of future ultra-small devices it is mandatory to incorporate quantum-mechanical space quantization effects into existing classical device simulators (drift-diffusion, hydrodynamics or Monte Carlo particle-based simulators) since first-principle quantum-mechanical calculations (direct solution of the many-body Schrodinger equation, Green's functions method, etc.) are still limited to one-dimensional structures and rely on a number of approximations.
TL;DR: In this article, the influence of temperature and drive current on the Shubnikov-de Haas oscillations was studied as a means to investigate the nature of the electron-phonon interaction in an etched InGaAs quantum wire.
Abstract: The influence of temperature and drive current on the Shubnikov-de Haas oscillations is studied as a means to investigate the nature of the electron–phonon interaction in an etched InGaAs quantum wire. The temperature dependence of the energy-relaxation time, and the current dependence of the electron temperature, appear consistent with an energy relaxation process that is dominated by three-dimensional electron–phonon scattering. A deviation from this behavior is found in measurements performed at 1.6 K, however, and simple estimates suggest that this may be associated with the increased importance at low temperatures of phonon confinement in the etched wire.
TL;DR: In this paper, the authors apply the effective potential approach when modeling a conventional 50-nm MOSFET device and an SOI device structure to calculate the current and estimate the device threshold voltage increase due to the lateral quantization.
TL;DR: In this article, the Anderson model is used to model the electron interaction strength within a quantum dot to be strongly peaked at specific energies, which is similar to the metal-insulator transition observed in open quantum dots and dot arrays.
TL;DR: In this article, a many-body model of transport in open quantum dots is proposed based on the idea of an enhanced electron-electron interaction in the vicinity of a quasibounded state.
Abstract: We formulate a many-body model of transport in open quantum dots, which is based upon the idea of an enhanced electron–electron interaction in the vicinity of a quasibound state. Our studies suggest that the effect of including this peaked interaction is to increase the amplitude of the conductance fluctuations, beyond the value expected from a single-particle treatment. While the role of interactions in transport through open dots has attracted little theoretical attention, our results demonstrate the presence of interaction-induced corrections to the transport in these structures.
TL;DR: In this paper, it was shown that transport in open quantum dots can be mediated by single eigenstates, even when the leads allow several propagating modes, and that the broadening of these states can be virtually independent of lead width.
TL;DR: In this article, the role of discrete dopant distributions on the threshold voltage of the device within the framework of the effective potential was explored by examining the currentvoltage behavior as well as the electron distributions within the device.
Abstract: We use the effective potential to include quantum mechanical effects in thin SOI MOSFETs simulated with 3D Monte Carlo. We explore the role of discrete dopant distributions on the threshold voltage of the device within the framework of the effective potential by examining the current-voltage behavior as well as the electron distributions within the device. We find that simulations with the effective potential produce a similar shift in current as classical simulations when the dopants are considered to have a random discrete distribution instead of a uniform distribution.
TL;DR: In this article, simultaneous measurement of electron and hole transient transport in an Al0.3Ga0.7As-based p-i-n semiconductor nanostructure by using picosecond/subpicosecond Raman spectroscopy is presented.
Abstract: We report experimental results on simultaneous measurement of electron as well as hole transient transport in an Al0.3Ga0.7As-based p–i–n semiconductor nanostructure by using picosecond/subpicosecond Raman spectroscopy. Electron and hole velocity overshoots are directly observed. It is demonstrated that at T=300 K, E=15 kV/cm, and electron-hole pair density n≅5×1017 cm−1, electron overshoots its steady-state value by a factor of about 7; whereas hole about 2.5. These experimental results are discussed and explained.
TL;DR: In this article, several quantized levels in the low temperature magneto-conductance of the quantum dot array system fabricated by a split-metal gate technique on GaAs/AlGaAs 2DEG layer were studied.
Abstract: We have studied several quantized levels in the low temperature magneto-conductance of the quantum dot array system fabricated by a split-metal-gate technique on GaAs/AlGaAs 2DEG layer The gate-voltage dependence of the effective g -value at high fields has been discussed in the dot array It is noted that the depopulation effect at the quantum point contacts leads to a large decrease of the effective g -value
01 Dec 2002
TL;DR: In this paper, the authors describe a method of simulating electron transport in semiconductor devices that operate in the quantum regime, where the electrons are confined to two dimensions (2D) and transport is ballistic.
Abstract: In this paper, we describe a method of simulating electron transport in semiconductor devices that operate in the quantum regime. Specifically, devices formed in which the electrons are confined to two dimensions (2D) and transport is ballistic. Modeling such structures using a finite difference approach, we describe how the conductance can be calculated using a numerically stabilized variant of the transfer matrix approach derived from the 2D Schrodinger equation. Examining the example of a quantum point contact, we also describe how this method can be efficiently coupled to a Poisson solver to allow self-consistency to be achieved.
07 Nov 2002
TL;DR: In this paper, the authors used the effective potential model in 3D Monte Carlo calculations of ultra-thin SOI MOSFETs and found that the model recovers the expected trend in threshold voltage shift with shrinking silicon thickness, down to a thickness of approximately 3 nm.
Abstract: The success of the effective potential method of including quantum confinement effects in simulations of MOSFETs is based on the ability to calculate ahead of time the extent of the Gaussian wave-packet used to describe the electron. In the calculation of the Gaussian, the inversion layer is assumed to form in a triangular potential well, from which a suitable standard deviation can be obtained. The situation in an ultra-thin SOI MOSFET is slightly different, in that the potential well has a triangular bottom, but there is a significant contribution to the confinement from the rectangular barriers formed by the gate oxide and the buried oxide (BOX). For this more complex potential well, it is of interest to determine the range of applicability of the constant standard deviation effective potential model. In this work we include this effective potential model in 3D Monte Carlo calculations of the threshold voltage of ultra-thin SOI MOSFETs. We find that the effective potential recovers the expected trend in threshold voltage shift with shrinking silicon thickness, down to a thickness of approximately 3 nm.
TL;DR: Using super-symmetric quantum mechanics, the authors showed that natural transitions occur between degenerate eigenstates and that the fundamental structure of the quantum qubit should be that of a quantum plane wave.
TL;DR: In this paper, the authors investigate the low-field transport in a highly asymmetric MOSFET structure, characterized by a p+-implant at the source end, by using a Monte Carlo-Poisson simulation with the quantum effects incorporated through an effective potential.
Abstract: Though asymmetric MOSFET structures are being designed in response to small-geometry effects, the performance estimates of such devices often rely on the conventional device description, and neglect to properly account for the interplay between quantum effects and the effects of asymmetry. In this paper, we investigate the low-field transport in a highly asymmetric MOSFET structure, characterized by a p+-implant at the source end, by using a Monte Carlo—Poisson simulation with the quantum effects incorporated through an effective potential. We observe that highly-pronounced asymmetry leads to ballistic transport features, which become suppressed by the inclusion of quantum effects. We prove that mobility degradation is an essentially non-equilibrium signature of quantum mechanics, independent of the well-established equilibrium signatures (charge set-back and gap widening). Consequently, in order to properly estimate the device performance, it becomes important to account for the channel mobility degradation due to quantum effects.