94 research outputs found
Ballistic transport and electrostatics in metallic carbon nanotubes
We calculate the current and electrostatic potential drop in metallic carbon
nanotube wires self-consistently, by solving the Green's function and
electrostatics equations in the ballistic case. About one tenth of the applied
voltage drops across the bulk of a nanowire, independent of the lengths
considered here. The remaining nine tenths of the bias drops near the contacts,
thereby creating a non linear potential drop. The scaling of the electric field
at the center of the nanotube with length (L) is faster than 1/L (roughly
). At room temperature, the low bias conductance of large
diameter nanotubes is larger than due to occupation of non crossing
subbands. The physics of conductance evolution with bias due to the
transmission Zener tunneling in non crossing subbands is discussed
Simulation of direct source-to-drain tunnelling using the density gradient formalism: Non-Equilibrium Greens Function calibration
Quantum mechanical confinement effects, gate, hand-to-hand and source-to-drain tunnelling will dramatically affect the characteristics of future generation nanometre scaled devices. It has been demonstrated already that first-order quantum corrections, which satisfactorily describe quantum confinement effects, can be introduced into efficient TCAD orientated drift-diffusion simulators using the density gradient approach. In this paper we refer to Non-Equilibrium Green's Function simulations in order to calibrate the density gradient formalism in respect of both confinement and source-to-drain tunnelling using different effective masses in directions normal and parallel to the conducting channel. We demonstrate that the density gradient formalism can describe accurately the current characteristics in sub 20 nm double gate MOSFETs
Analysis of band-gap formation in squashed arm-chair CNT
The electronic properties of squashed arm-chair carbon nanotubes are modeled
using constraint free density functional tight binding molecular dynamics
simulations. Independent from CNT diameter, squashing path can be divided into
{\it three} regimes. In the first regime, the nanotube deforms with negligible
force. In the second one, there is significantly more resistance to squashing
with the force being nN/per CNT unit cell. In the last regime,
the CNT looses its hexagonal structure resulting in force drop-off followed by
substantial force enhancement upon squashing. We compute the change in band-gap
as a function of squashing and our main results are: (i) A band-gap initially
opens due to interaction between atoms at the top and bottom sides of CNT. The
orbital approximation is successful in modeling the band-gap opening at
this stage. (ii) In the second regime of squashing, large
interaction at the edges becomes important, which can lead to band-gap
oscillation. (iii) Contrary to a common perception, nanotubes with broken
mirror symmetry can have {\it zero} band-gap. (iv) All armchair nanotubes
become metallic in the third regime of squashing. Finally, we discuss both
differences and similarities obtained from the tight binding and density
functional approaches.Comment: 16 pages and 6 figures, To appear in PR
Two-Dimensional Quantum Model of a Nanotransistor
A mathematical model, and software to implement the model, have been devised to enable numerical simulation of the transport of electric charge in, and the resulting electrical performance characteristics of, a nanotransistor [in particular, a metal oxide/semiconductor field-effect transistor (MOSFET) having a channel length of the order of tens of nanometers] in which the overall device geometry, including the doping profiles and the injection of charge from the source, gate, and drain contacts, are approximated as being two-dimensional. The model and software constitute a computational framework for quantitatively exploring such device-physics issues as those of source-drain and gate leakage currents, drain-induced barrier lowering, and threshold voltage shift due to quantization. The model and software can also be used as means of studying the accuracy of quantum corrections to other semiclassical models
Two Dimensional Quantum Mechanical Modeling of Nanotransistors
Quantization in the inversion layer and phase coherent transport are
anticipated to have significant impact on device performance in 'ballistic'
nanoscale transistors. While the role of some quantum effects have been
analyzed qualitatively using simple one dimensional ballistic models, two
dimensional (2D) quantum mechanical simulation is important for quantitative
results. In this paper, we present a framework for 2D quantum mechanical
simulation of a nanotransistor / Metal Oxide Field Effect Transistor (MOSFET).
This framework consists of the non equilibrium Green's function equations
solved self-consistently with Poisson's equation. Solution of this set of
equations is computationally intensive. An efficient algorithm to calculate the
quantum mechanical 2D electron density has been developed. The method presented
is comprehensive in that treatment includes the three open boundary conditions,
where the narrow channel region opens into physically broad source, drain and
gate regions. Results are presented for (i) drain current versus drain and gate
voltages, (ii) comparison to results from Medici, and (iii) gate tunneling
current, using 2D potential profiles. Methods to reduce the gate leakage
current are also discussed based on simulation results.Comment: 12 figures. Journal of Applied Physics (to appear
The inequality of charge and spin diffusion coefficients
Since spin and charge are both carried by electrons (or holes) in a solid, it is natural to assume that charge and spin diffusion coefficients will be the same. Drift-diffusion models of spin transport typically assume so. Here, we show analytically that the two diffusion coefficients can be vastly different in quantum wires. Although we do not consider quantum wells or bulk systems, it is likely that the two coefficients will be different in those systems as well. Thus, it is important to distinguish between them in transportmodels, particularly those applied to quantum wire based devices
Electronic transport through carbon nanotubes -- effects of structural deformation and tube chirality
Atomistic simulations using a combination of classical forcefield and
Density-Functional-Theory (DFT) show that carbon atoms remain essentially sp2
coordinated in either bent tubes or tubes pushed by an atomically sharp AFM
tip. Subsequent Green's-function-based transport calculations reveal that for
armchair tubes there is no significant drop in conductance, while for zigzag
tubes the conductance can drop by several orders of magnitude in AFM-pushed
tubes. The effect can be attributed to simple stretching of the tube under tip
deformation, which opens up an energy gap at the Fermi surface.Comment: To appear in Physical Review Letter
Theory and simulation of quantum photovoltaic devices based on the non-equilibrium Green's function formalism
This article reviews the application of the non-equilibrium Green's function
formalism to the simulation of novel photovoltaic devices utilizing quantum
confinement effects in low dimensional absorber structures. It covers
well-known aspects of the fundamental NEGF theory for a system of interacting
electrons, photons and phonons with relevance for the simulation of
optoelectronic devices and introduces at the same time new approaches to the
theoretical description of the elementary processes of photovoltaic device
operation, such as photogeneration via coherent excitonic absorption,
phonon-mediated indirect optical transitions or non-radiative recombination via
defect states. While the description of the theoretical framework is kept as
general as possible, two specific prototypical quantum photovoltaic devices, a
single quantum well photodiode and a silicon-oxide based superlattice absorber,
are used to illustrated the kind of unique insight that numerical simulations
based on the theory are able to provide.Comment: 20 pages, 10 figures; invited review pape
Heat conductance is strongly anisotropic for pristine silicon nanowires
We compute atomistically the heat conductance for ultra-thin pristine silicon
nanowires (SiNWs) with diameters ranging from 1 to 5 nm. The room temperature
thermal conductance is found to be highly anisotropic: wires oriented along the
direction have 50-75% larger conductance than wires oriented along the
and directions. We show that the anisotropies can be qualitatively
understood and reproduced from the bulk phonon band structure. Ab initio
density functional theory (DFT) is used to study the thinnest wires, but
becomes computationally prohibitive for larger diameters, where we instead use
the Tersoff empirical potential model (TEP). For the smallest wires, the
thermal conductances obtained from DFT- and TEP calculations agree within 10%.
The presented results could be relevant for future phonon-engineering of
nanowire devices.Comment: 7 pages, 5 figure
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