148 research outputs found
Enhanced shot noise in carbon nanotube field-effect transistors
We predict shot noise enhancement in defect-free carbon nanotube field-effect
transistors through a numerical investigation based on the self-consistent
solution of the Poisson and Schrodinger equations within the non-equilibrium
Green functions formalism, and on a Monte Carlo approach to reproduce injection
statistics. Noise enhancement is due to the correlation between trapping of
holes from the drain into quasi-bound states in the channel and thermionic
injection of electrons from the source, and can lead to an appreciable Fano
factor of 1.22 at room temperature.Comment: 4 pages, 4 figure
Shot noise suppression in quasi one-dimensional Field Effect Transistors
We present a novel method for the evaluation of shot noise in quasi
one-dimensional field-effect transistors, such as those based on carbon
nanotubes and silicon nanowires. The method is derived by using a statistical
approach within the second quantization formalism and allows to include both
the effects of Pauli exclusion and Coulomb repulsion among charge carriers. In
this way it extends Landauer-Buttiker approach by explicitly including the
effect of Coulomb repulsion on noise. We implement the method through the
self-consistent solution of the 3D Poisson and transport equations within the
NEGF framework and a Monte Carlo procedure for populating injected electron
states. We show that the combined effect of Pauli and Coulomb interactions
reduces shot noise in strong inversion down to 23 % of the full shot noise for
a gate overdrive of 0.4 V, and that neglecting the effect of Coulomb repulsion
would lead to an overestimation of noise up to 180 %.Comment: Changed content, 7 pages,5 figure
Drift velocity peak and negative differential mobility in high field transport in graphene nanoribbons explained by numerical simulations
We present numerical simulations of high field transport in both suspended
and deposited armchair graphene nanoribbon (A-GNR) on HfO2 substrate. Drift
velocity in suspended GNR does not saturate at high electric field (F), but
rather decreases, showing a maximum for F=10 kV/cm. Deposition on HfO2 strongly
degrades the drift velocity by up to a factor of 10 with respect to suspended
GNRs in the low-field regime, whereas at high fields drift velocity approaches
the intrinsic value expected in suspended GNRs. Even in the assumption of
perfect edges, the obtained mobility is far behind what expected in
two-dimensional graphene, and is further reduced by surface optical phonons.Comment: 4 pages, 4 figure
Growth-Induced Strain in Chemical Vapor Deposited Monolayer MoS2: Experimental and Theoretical Investigation
Monolayer molybdenum disulphide (MoS) is a promising two-dimensional (2D)
material for nanoelectronic and optoelectronic applications. The large-area
growth of MoS has been demonstrated using chemical vapor deposition (CVD)
in a wide range of deposition temperatures from 600 {\deg}C to 1000 {\deg}C.
However, a direct comparison of growth parameters and resulting material
properties has not been made so far. Here, we present a systematic experimental
and theoretical investigation of optical properties of monolayer MoS grown
at different temperatures. Micro-Raman and photoluminescence (PL) studies
reveal observable inhomogeneities in optical properties of the as-grown single
crystalline grains of MoS. Close examination of the Raman and PL features
clearly indicate that growth-induced strain is the main source of distinct
optical properties. We carry out density functional theory calculations to
describe the interaction of growing MoS layers with the growth substrate as
the origin of strain. Our work explains the variation of band gap energies of
CVD-grown monolayer MoS, extracted using PL spectroscopy, as a function of
deposition temperature. The methodology has general applicability to model and
predict the influence of growth conditions on strain in 2D materials.Comment: 37 pages, 6 figures, 10 figures in supporting informatio
Ultra Low Specific Contact Resistivity in Metal-Graphene Junctions via Atomic Orbital Engineering
A systematic investigation of graphene edge contacts is provided.
Intentionally patterning monolayer graphene at the contact region creates
well-defined edge contacts that lead to a 67% enhancement in current injection
from a gold contact. Specific contact resistivity is reduced from 1372
{\Omega}m for a device with surface contacts to 456 {\Omega}m when contacts are
patterned with holes. Electrostatic doping of the graphene further reduces
contact resistivity from 519 {\Omega}m to 45 {\Omega}m, a substantial decrease
of 91%. The experimental results are supported and understood via a multi-scale
numerical model, based on density-functional-theory calculations and transport
simulations. The data is analyzed with regards to the edge perimeter and
hole-to-graphene ratio, which provides insights into optimized contact
geometries. The current work thus indicates a reliable and reproducible
approach for fabricating low resistance contacts in graphene devices. We
provide a simple guideline for contact design that can be exploited to guide
graphene and 2D material contact engineering.Comment: 26 page
Simulation of contact resistance in patterned graphene
While trying to exploit graphene in Radio Frequency applications, the reduction of the contact resistance (Rc) is probably one of the most challenging technological issues to be solved. Graphene patterning under the metal has been demonstrated to be a promising solution, leading to a reduction of Rc by up to a factor of 20, probably due to an increased conductivity at the borders of the patterns of graphene. This technology is still at the early stage and a complete understanding of the physical mechanisms at play is lacking. To this purpose we propose a multi- scale approach based on first-principle calculations, and the solution of the continuity equation to compute Rc in the considered patterned contacts
The double-edged sword of inflammation in inherited retinal degenerations: Clinical and preclinical evidence for mechanistically and prognostically impactful but treatable complications
We present retrospective data from our clinical research efforts of the past several years alongside a review of past and current clinical and preclinical data independently by several investigators supporting our clinical evidence for the importance of inflammation in inherited retinal degenerations (IRDs). We show how inflammation is a complicating factor in IRDs but, if recognized and managed, also a great opportunity to mitigate disease severity immediately, improve patient prognosis and quality of life, extend the treatment windows for gene-specific and agnostic therapeutic approaches, mitigate the impact of inflammatory complications on the accurate estimate of vision changes in IRD natural history studies, improve the chances of safer outcomes following cataract surgery, and potentially reduce the likelihood of inflammatory adverse events and augment the efficacy of viral vector-based treatment approaches to IRDs. Manuscript contribution to the field. Inflammation has been suspected to be at play in IRDs since the beginning of the 1900s and became a research focus through the early 1990s but was then largely abandoned in favor of genetic-focused research. Thanks to regained cognizance, better research tools, and a more holistic approach to IRDs, the recent reappraisal of the role of inflammation in IRDs has brought back to the surface its importance. A potential confounder in natural history studies and a limiting factor in clinical trials if not accounted for, inflammation can be managed and often offers an opportunity for immediately improved prognosis and outcomes for IRD patients. We present our retrospective clinical evidence for connections with a measurable secondary autoimmune component that can develop in IRDs and contribute to vision loss but is at least in part treatable. We also present ample lines of evidence from the literature corroborating our clinical observations at the preclinical level
Vertical Heterostructures between Transition-Metal Dichalcogenides -- A Theoretical Analysis of the NbS/WSe junction
Low-dimensional metal-semiconductor vertical heterostructures (VH) are
promising candidates in the search of electronic devices at the extreme limits
of miniaturization. Within this line of research, here we present a
theoretical/computational study of the NbS/WSe metal-semiconductor
vertical hetero-junction using density functional theory (DFT) and conductance
simulations. We first construct atomistic models of the NbS/WSe VH
considering all the five possible stacking orientations at the interface, and
we conduct DFT and quantum-mechanical (QM) scattering simulations to obtain
information on band structure and transmission coefficients. We then carry out
an analysis of the QM results in terms of electrostatic potential, fragment
decomposition, and band alignment. The behavior of transmission expected from
this analysis is in excellent agreement with, and thus fully rationalizes, the
DFT results, and the peculiar double-peak profile of transmission. Finally, we
use maximally localized Wannier functions, projected density of states (PDOS),
and a simple analytic formula to predict and explain quantitatively the
differences in transport in the case of epitaxial misorientation. Within the
class of Transition-Metal Dichalcogenide systems, the NbS/WSe vertical
heterostructure exhibits a wide interval of finite transmission and a
double-peak profile, features that could be exploited in applications.Comment: 22 pages main text, 11 pages supplementar
Statistical theory of shot noise in quasi-1D Field Effect Transistors in the presence of electron-electron interaction
We present an expression for the shot noise power spectral density in
quasi-one dimensional conductors electrostatically controlled by a gate
electrode, that includes the effects of Coulomb interaction and of Pauli
exclusion among charge carriers. In this sense, our expression extends the well
known Landauer-Buttiker noise formula to include the effect of Coulomb
interaction through induced fluctuations in the device potential. Our approach
is based on the introduction of statistical properties of the scattering matrix
and on a second-quantization many-body description. From a quantitative point
of view, statistical properties are obtained by means of Monte Carlo
simulations on a ensemble of different configurations of injected states,
requiring the solution of the Poisson-Schrodinger equation on a
three-dimensional grid, with the non-equilibrium Green functions formalism. In
a series of example, we show that failure to consider the effects of Coulomb
interaction on noise leads to a gross overestimation of the noise spectrum of
quasi-one dimensional devices
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