arXiv subjects

Mark Lundstrom

Publications and source records attributed to Mark Lundstrom.

At least 37 records · Page 2Linked to original sources

arXiv2011

Scaling of the energy gap in pattern-hydrogenated graphene

Recent experiments show that a substantial energy gap in graphene can be induced via patterned hydrogenation on an iridium substrate. Here, we show that the energy gap is roughly proportional to $\sqrt{N_{H}}/N_{C}$ when disorder is accounted for, where $N_H$ and $N_C$ denote concentration of hydrogen and carbon atoms, respectively. The dispersion relation, obtained through calculation of the momentum-energy resolved density of states, is shown to agree with previous angle-resolved photoemission spectroscopy results. Simulations of electronic transport in finite size samples also reveal a similar transport gap, up to 1eV within experimentally achievable $\sqrt{N_{H}}/N_{C}$ value.

cond-mat.mes-hall↗

arXiv2011

An efficient algorithm to calculate intrinsic thermoelectric parameters based on Landauer approach

The Landauer approach provides a conceptually simple way to calculate the intrinsic thermoelectric (TE) parameters of materials from the ballistic to the diffusive transport regime. This method relies on the calculation of the number of propagating modes and the scattering rate for each mode. The modes are calculated from the energy dispersion (E(k)) of the materials which require heavy computation and often supply energy relation on sparse momentum (k) grids. Here an efficient method to calculate the distribution of modes (DOM) from a given E(k) relationship is presented. The main features of this algorithm are, (i) its ability to work on sparse dispersion data, and (ii) creation of an energy grid for the DOM that is almost independent of the dispersion data therefore allowing for efficient and fast calculation of TE parameters. The inclusion of scattering effects is also straight forward. The effect of k-grid sparsity on the compute time for DOM and on the sensitivity of the calculated TE results are provided. The algorithm calculates the TE parameters within 5% accuracy when the K-grid sparsity is increased up to 60% for all the dimensions (3D, 2D and 1D). The time taken for the DOM calculation is strongly influenced by the transverse K density (K perpendicular to transport direction) but is almost independent of the transport K density (along the transport direction). The DOM and TE results from the algorithm are bench-marked with, (i) analytical calculations for parabolic bands, and (ii) realistic electronic and phonon results for $Bi_{2}Te_{3}$.

cond-mat.mes-hall↗

arXiv2011

Relating electronic structure to thermoelectric device performance

Realistic thermoelectric modeling and simulation tools are needed to explain the experiments and for device design. In this paper, we present a simple computational technique to make use of rigorous band structure calculations in thermoelectric device design. Our, methodology, based on Landauer theory, provides a way to benchmark effective mass level models against rigorous band structures. It can also be used for determining the temperature-dependent Fermi-level from a given doping density and numerically generated density-of-states, which can then be used with the numerically generated distribution of channels to evaluate the temperature dependent electrical conductivity, thermopower, and electronic thermal conductivity. We illustrate the technique for silicon, for which well-calibrated band structure and transport data is available, but the technique is more generally suited to complex thermoelectric materials and provides an easy way to make use of first principles band structure calculations in device design.

cond-mat.mes-hall↗

arXiv2011

Substrate Gating of Contact Resistance in Graphene Transistors

Metal contacts have been identified to be a key technological bottleneck for the realization of viable graphene electronics. Recently, it was observed that for structures that possess both a top and a bottom gate, the electron-hole conductance asymmetry can be modulated by the bottom gate. In this letter, we explain this observation by postulating the presence of an effective thin interfacial dielectric layer between the metal contact and the underlying graphene. Electrical results from quantum transport calculations accounting for this modified electrostatics corroborate well with the experimentally measured contact resistances. Our study indicates that the engineering of metal- graphene interface is a crucial step towards reducing the contact resistance for high performance graphene transistors.

cond-mat.mes-hall↗

arXiv2011

On the Best Bandstructure for Thermoelectric Performance

The conventional understanding that a bandstructure that produces a Dirac delta function transport distribution (or transmission in the Landauer framework) maximizes the thermoelectric figure of merit, ZT, is revisited. Thermoelectric (TE) performance is evaluated using a simple tight binding (TB) model for electron dispersion and three different scattering models: 1) a constant scattering time, 2) a constant mean-free-path, and 3) a scattering rate proportional to the density-of-states. We found that a Dirac delta-function transmission never produces the maximum ZT. The best bandstructure for maximizing ZT depends on the scattering physics. These results demonstrate that the selection of bandstructure to maximize TE performance is more complex than previously thought and that a high density-of-states near the band edge does not necessarily improve TE performance.

cond-mat.mtrl-sci↗

arXiv2011

Signatures of disorder in the minimum conductivity of graphene

Graphene has been proposed as a promising material for future nanoelectronics because of its unique electronic properties. Understanding the scaling behavior of this new nanomaterial under common experimental conditions is of critical importance for developing graphene-based nanoscale devices. We present a comprehensive experimental and theoretical study on the influence of edge disorder and bulk disorder on the minimum conductivity of graphene ribbons. For the first time, we discovered a strong non-monotonic size scaling behavior featuring a peak and saturation minimum conductivity. Through extensive numerical simulations and analysis, we are able to attribute these features to the amount of edge and bulk disorder in graphene devices. This study elucidates the quantum transport mechanisms in realistic experimental graphene systems, which can be used as a guideline for designing graphene-based nanoscale devices with improved performance.

cond-mat.mes-hall↗

arXiv2010

Landauer vs. Boltzmann and Full Dispersion vs. Debye Model Evaluation of Lattice Thermal Conductivity

Using a full dispersion description of phonons, the thermal conductivities of bulk Si and Bi2Te3 are evaluated using a Landauer approach and related to the conventional approach based on the Boltzmann transport equation. A procedure to extract a well-defined average phonon mean-free-path from the measured thermal conductivity and given phonon-dispersion is presented. The extracted mean-free-path has strong physical significance and differs greatly from simple estimates. The use of simplified dispersion models for phonons is discussed, and it is shown that two different Debye temperatures must be used to treat the specific heat and thermal conductivity (analogous to the two different effective masses needed to describe the electron density and conductivity). A simple technique to extract these two Debye temperatures is presented and the limitations of the method are discussed.

cond-mat.mtrl-sci↗

arXiv2009

On Landauer vs. Boltzmann and Full Band vs. Effective Mass Evaluation of Thermoelectric Transport Coefficients

The Landauer approach to diffusive transport is mathematically related to the solution of the Boltzmann transport equation, and expressions for the thermoelectric parameters in both formalisms are presented. Quantum mechanical and semiclassical techniques to obtain from a full description of the bandstructure, E(k), the number of conducting channels in the Landauer approach or the transport distribution in the Boltzmann solution are developed and compared. Thermoelectric transport coefficients are evaluated from an atomistic level, full band description of a crystal. Several example calculations for representative bulk materials are presented, and the full band results are related to the more common effective mass formalism. Finally, given a full E(k) for a crystal, a procedure to extract an accurate, effective mass level description is presented.

cond-mat.mes-hall↗

arXiv2009

Conductance asymmetry of graphene pn junction

We use the non-equilibrium Green function (NEGF) method in the ballistic limit to provide a quantitative description of the conductance of graphene pn junctions - an important building block for graphene electronics devices. In this paper, recent experiments on graphene junctions are explained by a ballistic transport model, but only if the finite junction transition width, Dw, is accounted for. In particular, the experimentally observed anamolous increase in the resistance asymmetry between nn and np junctions under low source/drain charge density conditions is also quantitatively captured by our model. In light of the requirement for sharp junctions in applications such as electron focusing, we also examine the pn junction conductance in the regime where Dw is small and find that wavefunction mismatch (so-called pseudo-spin) plays a major role in sharp pn junctions.

cond-mat.mes-hall↗

arXiv2008

Bandstructure Effects in Silicon Nanowire Electron Transport

Bandstructure effects in the electronic transport of strongly quantized silicon nanowire field-effect-transistors (FET) in various transport orientations are examined. A 10-band sp3d5s* semi-empirical atomistic tight-binding model coupled to a self consistent Poisson solver is used for the dispersion calculation. A semi-classical, ballistic FET model is used to evaluate the current-voltage characteristics. It is found that the total gate capacitance is degraded from the oxide capacitance value by 30% for wires in all the considered transport orientations ([100], [110], [111]). Different wire directions primarily influence the carrier velocities, which mainly determine the relative performance differences, while the total charge difference is weakly affected. The velocities depend on the effective mass and degeneracy of the dispersions. The [110] and secondly the [100] oriented 3nm thick nanowires examined, indicate the best ON-current performance compared to [111] wires. The dispersion features are strong functions of quantization. Effects such as valley splitting can lift the degeneracies especially for wires with cross section sides below 3nm. The effective masses also change significantly with quantization, and change differently for different transport orientations. For the cases of [100] and [111] wires the masses increase with quantization, however, in the [110] case, the mass decreases. The mass variations can be explained from the non-parabolicities and anisotropies that reside in the first Brillouin zone of silicon.

cond-mat.mtrl-sci↗

arXiv2006

Performance Analysis of a Ge/Si Core/Shell Nanowire Field Effect Transistor

We analyze the performance of a recently reported Ge/Si core/shell nanowire transistor using a semiclassical, ballistic transport model and an sp3s*d5 tight-binding treatment of the electronic structure. Comparison of the measured performance of the device with the effects of series resistance removed to the simulated result assuming ballistic transport shows that the experimental device operates between 60 to 85% of the ballistic limit. For this ~15 nm diameter Ge nanowire, we also find that 14-18 modes are occupied at room temperature under ON-current conditions with ION/IOFF=100. To observe true one dimensional transport in a <110> Ge nanowire transistor, the nanowire diameter would have to be much less than about 5 nm. The methodology described here should prove useful for analyzing and comparing on common basis nanowire transistors of various materials and structures.

cond-mat.mes-hall↗

arXiv2006

The role of self-heating and hot-phonons in metallic single walled carbon nanotubes

The role of self-heating and hot-phonons on electronic transport in metallic single walled carbon nanotube is examined by solving a coupled electron-optical phonon (OP) Boltzmann transport equation and the classical heat equation. We find that self-heating reduces the high bias conductance, thus explaining the short OP mean free paths observed experimentally, while the non-equilibrium OP population reduces the overall current level. Comparison with experimental data shows that for tubes on a substrate, no significant population of hot phonons builds up under the normal range of applied bias. These results shed light on OP phonon decay paths in carbon nanotubes.

cond-mat.mes-hall↗

arXiv2005

A Theoretical Investigation of Surface Roughness Scattering in Silicon Nanowire Transistors

In this letter, we report a three-dimensional (3D) quantum mechanical simulation to investigate the effects of surface roughness scattering (SRS) on the device characteristics of Si nanowire transistors (SNWTs). We treat the microscopic structure of the Si/SiO2 interface roughness directly by using a 3D finite element technique. The results show that 1) SRS reduces the electron density of states in the channel, which increases the SNWT threshold voltage, and 2) the SRS in SNWTs becomes more effective when more propagating modes are occupied, which implies that SRS is more important in planar metal-oxide-semiconductor field-effect-transistors with many transverse modes occupied than in small-diameter SNWTs with few modes conducting.

cond-mat.mes-hall↗

arXiv2004

On the Validity of the Parabolic Effective-Mass Approximation for the Current-Voltage Calculation of Silicon Nanowire Transistors

This paper examines the validity of the widely-used parabolic effective-mass approximation for computing the current-voltage (I-V) characteristics of silicon nanowire transistors (SNWTs). The energy dispersion relations for unrelaxed Si nanowires are first computed by using an sp3d5s* tight-binding model. A semi-numerical ballistic FET model is then adopted to evaluate the I-V characteristics of the (n-type) SNWTs based on both a tight-binding dispersion relation and parabolic energy bands. In comparison with the tight-binding approach, the parabolic effective-mass model with bulk effective-masses significantly overestimates SNWT threshold voltages when the wire width is <3nm, and ON-currents when the wire width is <5nm. By introducing two analytical equations with two tuning parameters, however, the effective-mass approximation can well reproduce the tight-binding I-V results even at a \~1.36nm wire with.

cond-mat.mes-hall↗

arXiv2004

A Quantum Mechanical Approach for the Simulation of Si/SiO2 Interface Roughness Scattering in Silicon Nanowire Transistors

In this work, we present a quantum mechanical approach for the simulation of Si/SiO2 interface roughness scattering in silicon nanowire transistors (SNWTs). The simulation domain is discretized with a three-dimensional (3D) finite element mesh, and the microscopic structure of the Si/SiO2 interface roughness is directly implemented. The 3D Schrodinger equation with open boundary conditions is solved by the non-equilibrium Green's function method together with the coupled mode space approach. The 3D electrostatics in the device is rigorously treated by solving a 3D Poisson equation with the finite element method. Although we mainly focus on computational techniques in this paper, the physics of SRS in SNWTs and its impact on the device characteristics are also briefly discussed.

cond-mat.mes-hall↗

arXiv2004

Performance Evaluation of Ballistic Silicon Nanowire Transistors with Atomic-basis Dispersion Relations

In this letter, we explore the bandstructure effects on the performance of ballistic silicon nanowire transistors (SNWTs). The energy dispersion relations for silicon nanowires are evaluated with an sp3d5s* tight binding model. Based on the calculated dispersion relations, the ballistic currents for both n-type and p-type SNWTs are evaluated by using a semi-numerical ballistic model. For large diameter nanowires, we find that the ballistic p-SNWT delivers half the ON-current of a ballistic n-SNWT. For small diameters, however, the ON-current of the p-type SNWT approaches that of its n-type counterpart. Finally, the carrier injection velocity for SNWTs is compared with those for planar metal-oxide-semiconductor field-effect transistors, clearly demonstrating the impact of quantum confinement on the performance limits of SNWTs.

cond-mat.mes-hall↗

arXiv2004

Self-Aligned Ballistic Molecular Transistors and Electrically Parallel Nanotube Arrays

Carbon nanotube field-effect transistors with structures and properties near the scaling limit with short (down to 50 nm) channels, self aligned geometries, palladium electrodes with low contact resistance and high-k dielectric gate insulators are realized. Electrical transport in these miniature transistors is near ballistic up to high biases at both room and low temperatures. Atomic layer deposited (ALD) high-k films interact with nanotube sidewalls via van der Waals interactions without causing weak localization at 4 K. New fundamental understanding of ballistic transport, optical phonon scattering and potential interfacial scattering mechanisms in nanotubes are obtained.

cond-mat.other↗

arXiv2004

A Three-Dimensional Quantum Simulation of Silicon Nanowire Transistors with the Effective-Mass Approximation

The silicon nanowire transistor (SNWT) is a promising device structure for future integrated circuits, and simulations will be important for understanding its device physics and assessing its ultimate performance limits. In this work, we present a three-dimensional quantum mechanical simulation approach to treat various SNWTs within the effective-mass approximation. We begin by assuming ballistic transport, which gives the upper performance limit of the devices. The use of a mode space approach (either coupled or uncoupled) produces high computational efficiency that makes our 3D quantum simulator practical for extensive device simulation and design. Scattering in SNWTs is then treated by a simple model that uses so-called Buttiker probes, which was previously used in metal-oxide-semiconductor field effect transistor (MOSFET) simulations. Using this simple approach, the effects of scattering on both internal device characteristics and terminal currents can be examined, which enables our simulator to be used for the exploration of realistic performance limits of SNWTs.

cond-mat.other↗