38 research outputs found
Flexural phonon scattering induced by electrostatic gating in graphene
Graphene has an extremely high carrier mobility partly due to its planar
mirror symmetry inhibiting scattering by the highly occupied acoustic flexural
phonons. Electrostatic gating of a graphene device can break the planar mirror
symmetry yielding a coupling mechanism to the flexural phonons. We examine the
effect of the gate-induced one-phonon scattering on the mobility for several
gate geometries and dielectric environments using first-principles calculations
based on density functional theory (DFT) and the Boltzmann equation. We
demonstrate that this scattering mechanism can be a mobility-limiting factor,
and show how the carrier density and temperature scaling of the mobility
depends on the electrostatic environment. Our findings may explain the high
deformation potential for in-plane acoustic phonons extracted from experiments
and furthermore suggest a direct relation between device symmetry and resulting
mobility.Comment: Accepted at Physical Review Letter
First-principles method for electron-phonon coupling and electron mobility: Applications to 2D materials
We present density functional theory calculations of the phonon-limited
mobility in n-type monolayer graphene, silicene and MoS. The material
properties, including the electron-phonon interaction, are calculated from
first-principles. We provide a detailed description of the normalized full-band
relaxation time approximation for the linearized Boltzmann transport equation
(BTE) that includes inelastic scattering processes. The bulk electron-phonon
coupling is evaluated by a supercell method. The method employed is fully
numerical and does therefore not require a semi-analytic treatment of part of
the problem and, importantly, it keeps the anisotropy information stored in the
coupling as well as the band structure. In addition, we perform calculations of
the low-field mobility and its dependence on carrier density and temperature to
obtain a better understanding of transport in graphene, silicene and monolayer
MoS. Unlike graphene, the carriers in silicene show strong interaction with
the out-of-plane modes. We find that graphene has more than an order of
magnitude higher mobility compared to silicene. For MoS, we obtain several
orders of magnitude lower mobilities in agreement with other recent theoretical
results. The simulations illustrate the predictive capabilities of the newly
implemented BTE solver applied in simulation tools based on first-principles
and localized basis sets
Inelastic vibrational signals in electron transport across graphene nanoconstrictions
We present calculations of the inelastic vibrational signals in the
electrical current through a graphene nanoconstriction. We find that the
inelastic signals are only present when the Fermi-level position is tuned to
electron transmission resonances, thus, providing a fingerprint which can link
an electron transmission resonance to originate from the nanoconstriction. The
calculations are based on a novel first-principles method which includes the
phonon broadening due to coupling with phonons in the electrodes. We find that
the signals are modified due to the strong coupling to the electrodes, however,
still remain as robust fingerprints of the vibrations in the nanoconstriction.
We investigate the effect of including the full self-consistent potential drop
due to finite bias and gate doping on the calculations and find this to be of
minor importance
Manipulating the voltage drop in graphene nanojunctions using a gate potential
Graphene is an attractive electrode material to contact nanostructures down
to the molecular scale since it can be gated electrostatically. Gating can be
used to control the doping and the energy level alignment in the nanojunction,
thereby influencing its conductance. Here we investigate the impact of
electrostatic gating in nanojunctions between graphene electrodes operating at
finite bias. Using first principles quantum transport simulations, we show that
the voltage drop across \emph{symmetric} junctions changes dramatically and
controllably in gated systems compared to non-gated junctions. In particular,
for \emph{p}-type(\emph{n}-type) carriers the voltage drop is located close to
the electrode with positive(negative) polarity, i.e. the potential of the
junction is pinned to the negative(positive) electrode. We trace this behaviour
back to the vanishing density of states of graphene in the proximity of the
Dirac point. Due to the electrostatic gating, each electrode exposes different
density of states in the bias window between the two different electrode Fermi
energies, thereby leading to a non-symmetry in the voltage drop across the
device. This selective pinning is found to be independent of device length when
carriers are induced either by the gate or dopant atoms, indicating a general
effect for electronic circuitry based on graphene electrodes. We envision this
could be used to control the spatial distribution of Joule heating in graphene
nanostructures, and possibly the chemical reaction rate around high potential
gradients.Comment: 6 pages, 7 figure
Efficient first-principles calculation of phonon assisted photocurrent in large-scale solar cell devices
We present a straightforward and computationally cheap method to obtain the
phonon-assisted photocurrent in large-scale devices from first-principles
transport calculations. The photocurrent is calculated using nonequilibrium
Green's function with light-matter interaction from the first-order Born
approximation while electron-phonon coupling (EPC) is included through special
thermal displacements (STD). We apply the method to a silicon solar cell device
and demonstrate the impact of including EPC in order to properly describe the
current due to the indirect band-to-band transitions. The first-principles
results are successfully compared to experimental measurements of the
temperature and light intensity dependence of the open-circuit voltage of a
silicon photovoltaic module. Our calculations illustrate the pivotal role
played by EPC in photocurrent modelling to avoid underestimation of the
open-circuit voltage, short-circuit current and maximum power. This work
represents a recipe for computational characterization of future photovoltaic
devices including the combined effects of light-matter interaction,
phonon-assisted tunneling and the device potential at finite bias from the
level of first-principles simulations
Electron-phonon scattering from Green's function transport combined with Molecular Dynamics: Applications to mobility predictions
We present a conceptually simple method for treating electron-phonon
scattering and phonon limited mobilities. By combining Green's function based
transport calculations and molecular dynamics (MD), we obtain a temperature
dependent transmission from which we evaluate the mobility. We validate our
approach by comparing to mobilities and conductivies obtained by the Boltzmann
transport equation (BTE) for different bulk and one-dimensional systems. For
bulk silicon and gold we successfully compare against experimental values. We
discuss limitations and advantages of each of the computational approaches.Comment: 8 pages, 8 figure
Phonon excitation and instabilities in biased graphene nanoconstrictions
We calculate the phonons in a graphene nanoconstriction(GNC) in the presence
of a high current density. The Joule-heating, current-induced forces, and
coupling to electrode phonons is evaluated using first principles
nonequilibrium DFT-NEGF calculations. Close to a resonance in the electronic
structure we observe a strongly nonlinear heating with bias and breakdown of
the harmonic approximation. This behavior results from negatively damped
phonons driven by the current. The effect may limit the stability and capacity
of graphene nanoconstrictions to carry high currents