Self-consistent Transport Research Articles

Intrinsic region length scaling of heavily doped carbon nanotube p–i–n junctions

We investigated the dependence of the transport properties of heavily doped intratube single-walled carbon nanotube (SWCNT) p-i-n junctions on the length of the intrinsic region by using empirical self-consistent quantum transport simulations. When the length of the intrinsic region is scaled from a few angstroms to over 10 nanometers, the SWCNT p-i-n junction evolves from a tunneling diode with a large negative rectification and large negative differential resistance to one with a large positive rectification (like a conventional positive rectifying diode). The critical length of the intrinsic length is about 8.0 nm. Therefore, one can obtain nanoscale diodes of different performance types by changing the intrinsic region length.

Self-Consistent Dynamics of Impurities in Magnetically Confined Plasmas: Turbulence Intermittency and Nondiffusive Transport

Self-consistent turbulent transport of high-concentration impurities in magnetically confined fusion plasmas is studied using a three-dimensional nonlinear fluid global turbulence model which includes ion-temperature gradient and trapped electron mode instabilities. It is shown that the impurity concentration can have a dramatic feedback in the turbulence and, as a result, it can significantly change the transport properties of the plasma. High concentration impurities can trigger strong intermittency that manifests in non-Gaussian heavy tails of the probability density functions of the E × B fluctuations and of the ion-temperature flux fluctuations. At the heart of this self-consistent coupling is the existence of inward propagating ion-temperature fronts with a sharp gradient at the leading edge that give rise to instabilities and avalanchelike bursty transport. Numerical evidence of time nonlocality (i.e., history dependence) in the delayed response of the flux to the gradient is presented.

Interlayer transport in disordered semiconductor electron bilayers

We study the effects of disorder on the interlayer transport properties of disordered semiconductor bilayers by performing self-consistent quantum transport calculations. We find that the addition of material disorder to the system affects the interlayer interactions leading to significant deviations in the interlayer transfer characteristics. In particular, we find that disorder decreases and broadens the tunneling peak, effectively reducing the interacting system to a non-interacting system. Our results suggest that the experimental observation of exchange-enhanced interlayer transport in semiconductor bilayers requires materials with mean free paths larger than the spatial extent of the system.

Modeling of a vertical tunneling graphene heterojunction field-effect transistor

Vertical tunneling field-effect-transistor (FET) based on graphene heterojunctions with layers of hBN is simulated by self-consistent quantum transport simulations. It is found that the asymmetric p-type and n-type conduction is due to work function difference between the graphene contact and the tunneling channel material. Modulation of the bottom-graphene-contact plays an important role in determining the switching characteristic of the device. Due to the electrostatic short-channel-effects stemming from the vertical-FET structure, the output I-V characteristics do not saturate. The scaling behaviors the vertical-FET as a function of the gate insulator thickness and the thickness of the tunneling channel material are examined.

Zener tunneling: Congruence between semi-classical and quantum ballistic formalisms

We compare the results of self-consistent ballistic quantum transport simulation of Zener tunneling in InAs, a direct bandgap semiconductor, with corresponding semi-classical solutions using the well-known Kane’s model and the Wentzel-Kramers-Brillouin (WKB) approximation. We find a qualitative difference between solutions obtained from Kane’s formulation and rigorous quantum-mechanical formalism. However, the WKB solution, with evaluation of action integral along the tunneling paths determined from the nearest neighbor sp3s* Hamiltonian, is shown to provide qualitative agreement. We discuss the issues involved in fitting semi-classical solutions with their quantum counterpart and also present a brief comparison of our results with experimental data.

Understanding Surfactant/Graphene Interactions Using a Graphene Field Effect Transistor: Relating Molecular Structure to Hysteresis and Carrier Mobility

Manipulation of transport hysteresis on graphene transistors and understanding electron transfer between graphene and polar/ionic adsorbates are important for the development of graphene-based sensor devices and nonvolatile memory electronics. We have investigated the effects of commonly used surfactants for graphene dispersion in aqueous solution on transport characteristics of graphene transistors. The adsorbates are found to transfer electrons to graphene, scatter carrier transport, and induce additional electron-hole puddles when the graphene is on an SiO(2) substrate. We relate the change in transport characteristics to specific chemical properties of a series of anionic, cationic, and neutral surfactants using a modification of a self-consistent transport theory developed for graphene. To understand the effects of surfactant adsorbates trapped on either side of the graphene, suspended devices were fabricated. Strong hysteresis is observed only when both sides of the graphene were exposed to the surfactants, attributable to their function as charge traps. This work is the first to demonstrate the control of hysteresis, allowing us to eliminate it for sensor and device applications or to enhance it to potentially enable nonvolatile memory applications.

Pseudospin transfer torques in semiconductor electron bilayers

We use self-consistent quantum transport theory to investigate the influence of electron-electron interactions on interlayer transport in semiconductor electron bilayers in the absence of an external magnetic field. We conclude that, even though spontaneous pseudospin order does not occur at zero field, interaction-enhanced quasiparticle tunneling amplitudes and pseudospin transfer torques do alter tunneling I-V characteristics, and can lead to time-dependent response to a dc bias voltage.

Dynamics and transport in mean-field coupled, many degrees-of-freedom, area-preserving nontwist maps

Area-preserving nontwist maps, i.e., maps that violate the twist condition, arise in the study of degenerate Hamiltonian systems for which the standard version of the Kolmogorov-Arnold-Moser (KAM) theorem fails to apply. These maps have found applications in several areas including plasma physics, fluid mechanics, and condensed matter physics. Previous work has limited attention to maps in 2-dimensional phase space. Going beyond these studies, in this paper, we study nontwist maps with many-degrees-of-freedom. We propose a model in which the different degrees of freedom are coupled through a mean-field that evolves self-consistently. Based on the linear stability of period-one and period-two orbits of the coupled maps, we construct coherent states in which the degrees of freedom are synchronized and the mean-field stays nearly fixed. Nontwist systems exhibit global bifurcations in phase space known as separatrix reconnection. Here, we show that the mean-field coupling leads to dynamic, self-consistent reconnection in which transport across invariant curves can take place in the absence of chaos due to changes in the topology of the separatrices. In the context of self-consistent chaotic transport, we study two novel problems: suppression of diffusion and breakup of the shearless curve. For both problems, we construct a macroscopic effective diffusion model with time-dependent diffusivity. Self-consistent transport near criticality is also studied, and it is shown that the threshold for global transport as function of time is a fat-fractal Cantor-type set.

Plasma size scaling of avalanche-like heat transport in tokamaks

The influence of plasma size on global ion temperature gradient turbulence is studied with the full-f Eulerian code GT5D (Idomura et al 2009 Nucl. Fusion 49 065029). The gyrokinetic model includes a consistent neoclassical electric field as well as a fixed-power source operator, enabling long-time simulations with self-consistent turbulent transport and equilibrium profiles. The effects of plasma size (from ρ* = 1/100 to ρ* = 1/225) are studied by scaling the minor radius a and the input power. For the first time, worse-than-Bohm scaling is observed under experimentally realistic conditions. For all plasma sizes, avalanches propagate over significant radii but their propagation depends on the radial electric shear. It is found that this quantity does not scale with ρ* due to the building up of intrinsic momentum. Such a dependence can be inferred from a force balance relation, which remains approximately valid in nonlinear simulations. An adaptive parallel momentum source has been implemented in GT5D to damp the parallel momentum profile. The new scan then reveals that the radial electric shear scales with ρ* while the transport is globally higher. These simulations therefore suggest that intrinsic momentum reduces heat transport. This work also addresses another important issue in gyrokinetics: it is shown that for fixed initial physical parameters the turbulent quasi-steady-state is statistically independent of the initial conditions.

Stability analysis of multiple nonequilibrium fixed points in self-consistent electron transport calculations

We present a method to perform stability analysis of nonequilibrium fixed points appearing in self-consistent electron transport calculations. The nonequilibrium fixed points are given by the self-consistent solution of stationary, nonlinear kinetic equation for single-particle density matrix. We obtain the stability matrix by linearizing the kinetic equation around the fixed points and analyze the real part of its spectrum to assess the asymptotic time behavior of the fixed points. We derive expressions for the stability matrices within Hartree-Fock and linear response adiabatic time-dependent density functional theory. The stability analysis of multiple fixed points is performed within the nonequilibrium Hartree-Fock approximation for the electron transport through a molecule with a spin-degenerate single level with local Coulomb interaction.

Annealing dependent performance of organic bulk-heterojunction solar cells: A theoretical perspective

Organic photovoltaic (OPV) technology promises a relatively inexpensive option for the solar energy conversion, provided its efficiency increases beyond the current level (∼7–8%) along with significant improvement in operational lifetime. To achieve this high efficiency/reliability, a systematic theoretical approach is required to optimize the underlying device fabrication process. In this article, we use an anneal-time dependent process-device co-simulation framework (the phase-field model for phase separation coupled with the self-consistent drift-diffusion transport for free carriers) to explore the effects of the process conditions (e.g., annealing temperature, anneal duration) on the performance of organic solar cells. Our results explain experimentally observed annealing effects on the solar cell performance, namely, (i) peak of the short circuit current, (ii) insensitivity of the open circuit voltage, (iii) low fill factor, etc., that would otherwise be deemed anomalous from the perspective of conventional solar cells. As such, this work offers a detailed analysis of the effects of annealing on OPV morphology and its electrical performance. This work also provides a theoretical framework for the optimization of process conditions, which might eventually lead to higher efficiency/reliability of the organic photovoltaic technology.

Secondary electron emission and self-consistent charge transport in semi-insulating samples

Electron beam induced self-consistent charge transport and secondary electron emission (SEE) in insulators are described by means of an electron-hole flight-drift model (FDM) now extended by a certain intrinsic conductivity (c) and are implemented by an iterative computer simulation. Ballistic secondary electrons (SE) and holes, their attenuation to drifting charge carriers, and their recombination, trapping, and field- and temperature-dependent detrapping are included. As a main result the time dependent “true” secondary electron emission rate δ(t) released from the target material and based on ballistic electrons and the spatial distributions of currents j(x,t), charges ρ(x,t), field F(x,t), and potential V(x,t) are obtained where V0 = V(0,t) presents the surface potential. The intrinsic electronic conductivity limits the charging process and leads to a conduction sample current to the support. In that case the steady-state total SE yield will be fixed below the unit: i.e., σ = η + δ < 1.

Computational study of the Seebeck coefficient of one-dimensional composite nano-structures

The Seebeck coefficient (S) of composite nano-structures is theoretically explored within a self-consistent electro-thermal transport simulation framework using the non-equilibrium Green’s function method and a heat diffusion equation. Seebeck coefficients are determined using numerical techniques that mimic experimental measurements. Simulation results show that, without energy relaxing scattering, the overall S of a composite structure is determined by the highest barrier within the device. For a diffusive, composite structure with energy relaxation due to electron-phonon scattering, however, the measured S is an average of the position-dependent values with the weighting factor being the lattice temperature gradient. The results stress the importance of self-consistent solutions of phonon heat transport and the resulting lattice temperature distribution in understanding the thermoelectric properties of a composite structure. It is also clarified that the measured S of a composite structure reflects its power generation performance rather than its cooling performance. The results suggest that the lattice thermal conductivity within the composite structure might be engineered to improve the power factor over the bulk by avoiding the conventional trade-off between S and the electrical conductivity.

Theoretical study of the role of metallic contacts in probing transport features of pure and defected graphene nanoribbons

Understanding the roles of disorder and metal/graphene interface on the electronic and transport properties of graphene-based systems is crucial for a consistent analysis of the data deriving from experimental measurements. The present work is devoted to the detailed study of graphene nanoribbon systems by means of self-consistent quantum transport calculations. The computational formalism is based on a coupled Schrödinger/Poisson approach that respects both chemistry and electrostatics, applied to pure/defected graphene nanoribbons (ideally or end-contacted by various fcc metals). We theoretically characterize the formation of metal-graphene junctions as well as the effects of backscattering due to the presence of vacancies and impurities. Our results evidence that disorder can infer significant alterations on the conduction process, giving rise to mobility gaps in the conductance distribution. Moreover, we show the importance of metal-graphene coupling that gives rise to doping-related phenomena and a degradation of conductance quantization characteristics.

Heterojunction Vertical Band-to-Band Tunneling Transistors for Steep Subthreshold Swing and High on Current

We propose a heterojunction vertical tunneling field-effect transistor and show using self-consistent ballistic quantum transport simulations that it can provide very steep subthreshold swings and high on current, thereby improving the scalability of TFETs for high performance. The turn-on in the pocket region of the device is dictated by the modulation of heterojunction barrier height. The steepness of turn-on is increased because of simultaneous onset of tunneling in the pocket and the region underneath and also due to the contribution of vertical tunneling in the pocket to the current. These factors can be engineered by tuning heterojunction band offsets.

Radiation hydrodynamical instabilities in cosmological and galactic ionization fronts

Ionization fronts, the sharp radiation fronts behind which H/He ionizing photons from massive stars and galaxies propagate through space, were ubiquitous in the universe from its earliest times. The cosmic dark ages ended with the formation of the first primeval stars and galaxies a few hundred Myr after the Big Bang. Numerical simulations suggest that stars in this era were very massive, 25 - 500 solar masses, with H II regions of up to 30,000 light-years in diameter. We present three-dimensional radiation hydrodynamical calculations that reveal that the I-fronts of the first stars and galaxies were prone to violent instabilities, enhancing the escape of UV photons into the early intergalactic medium (IGM) and forming clumpy media in which supernovae later exploded. The enrichment of such clumps with metals by the first supernovae may have led to the prompt formation of a second generation of low-mass stars, profoundly transforming the nature of the first protogalaxies. Cosmological radiation hydrodynamics is unique because ionizing photons coupled strongly to both gas flows and primordial chemistry at early epochs, introducing a hierarchy of disparate characteristic timescales whose relative magnitudes can vary greatly throughout a given calculation. We describe the adaptive multistep integration scheme we have developed for the self-consistent transport of both cosmological and galactic ionization fronts.

Investigation on the dynamic conductance of mesoscopic system based on the self-consistent transport theory

According to the self-consistent electronic dynamic transport theory of mesoscopic system, we present the dynamic conductance of mesoscopic structure. As an application of this theory, we employ a coherent mesoscopic parallel-plate capacitor model in the present study. The results show that the dynamic conductance of system depends on the frequency of external field and Fermi energy of system, and is a complex with a finite imaginary part. For a smaller frequency, the conductance shows a similar feature to dc case, but with the increase of the frequency of external fields, substantial deviations between dc case and ac case are observed, the dynamic conductance of system presents a peak structure with Fermi energy varying. For a given Fermi energy, the dynamic conductance is oscillatory with frequency varying, moreover some negative imaginary parts of conductance are observed. The negative imaginary part implies the capacitive behavior, and positive imaginary part refers to the inductive behavior.

Finite-temperature pseudospin torque effect in graphene bilayers

We use self-consistent quantum transport theory to investigate the influence of layer Fermi energy and temperature on the interlayer transport properties of bilayer graphene in the regime of excitonic superfluidity. We conclude that at low temperature the critical tunneling currents and quasiparticle penetration depths are well explained by the existing zero-temperature pseudospin torque model. However, when the thermal broadening associated with finite-temperature transport is included, we find that modes injected above the gap lead to increased critical interlayer currents and quasiparticle penetration in to the gapped regime beyond what current analytical models predict. We find that the increases in the critical tunneling current are due to thermal smearing smoothing out the quantum interference effects present at low temperature leading to a slower pseudospin precession about the pseudospin Bloch sphere.

Local strain in tunneling transistors based on graphene nanoribbons

A band-to-band tunneling field-effect transistor (FET) can achieve a subthreshold slope steeper than 60 mV/dec at room temperature, but the on-current is low due to existence of the tunneling barrier. Graphene has a monolayer-thin body which is amenable to strain. By using self-consistent quantum transport simulations, we show that with local strain applied at the tunneling junction between the source and the channel in a graphene nanoribbon tunneling FET, the on-current can be significantly improved by over a factor of 10 with the same off-current, no matter at the ballistic limit or in the presence of inelastic phonon scattering.

Analysis of InAs vertical and lateral band-to-band tunneling transistors: Leveraging vertical tunneling for improved performance

Using self-consistent quantum transport simulation on realistic devices, we show that InAs band-to-band Tunneling Field Effect Transistors (TFET) with a heavily doped pocket in the gate-source overlap region can offer larger ON current and steeper subthreshold swing as compared to conventional tunneling transistors. This is due to an additional tunneling contribution to current stemming from band overlap along the body thickness. However, a critical thickness is necessary to obtain this advantage derived from “vertical” tunneling. In addition, in ultra small InAs TFET devices, the subthreshold swing could be severely affected by direct source-to-drain tunneling through the body.