164 research outputs found
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
Large-signal model of 2DFETs: compact modeling of terminal charges and intrinsic capacitances
We present a physics-based circuit-compatible model for double-gated
two-dimensional semiconductor based field effect transistors, which provides
explicit expressions for the drain current, terminal charges and intrinsic
capacitances. The drain current model is based on the drift-diffusion mechanism
for the carrier transport and considers Fermi-Dirac statistics coupled with an
appropriate field-effect approach. The terminal charge and intrinsic
capacitance models are calculated adopting a Ward-Dutton linear charge
partition scheme that guarantees charge-conservation. It has been implemented
in Verilog-A to make it compatible with standard circuit simulators. In order
to benchmark the proposed modeling framework we also present experimental DC
and high-frequency measurements of a purposely fabricated monolayer MoS2 FET
showing excellent agreement between the model and the experiment and thus
demonstrating the capabilities of the combined approach to predict the
performance of 2DFETs.Comment: 7 pages, 6 figure
CVD Graphene Contacts for Lateral Heterostructure MoS Field Effect Transistors
Intensive research is carried out on two-dimensional materials, in particular
molybdenum disulfide, towards high-performance transistors for integrated
circuits. Fabricating transistors with ohmic contacts is challenging due to the
high Schottky barrier that severely limits the transistors' performance.
Graphene-based heterostructures can be used in addition or as a substitute for
unsuitable metals. We present lateral heterostructure transistors made of
scalable chemical vapor-deposited molybdenum disulfide and chemical
vapor-deposited graphene with low contact resistances of about 9
k{\Omega}{\mu}m and high on/off current ratios of 10${^8}. We also present a
theoretical model calibrated on our experiments showing further potential for
scaling transistors and contact areas into the few nanometers range and the
possibility of a strong performance enhancement by means of layer optimizations
that would make transistors promising for use in future logic circuits.Comment: 23 page
Practical and accurate calculations of Askaryan radiation
An in-depth characterization of coherent radio Cherenkov pulses from particle
showers in dense dielectric media, referred to as the Askaryan effect, is
presented. The time-domain calculation developed in this article is based on a
form factor to account for the lateral dimensions of the shower. It is
computationally efficient and able to reproduce the results of detailed
particle shower simulations with high fidelity in most regions of practical
interest including Fresnel effects due to the longitudinal development of the
shower. In addition, an intuitive interpretation of the characteristics of the
Askaryan pulse is provided. We expect our approach to benefit the analysis of
radio pulses in experiments exploiting the radio technique.Comment: Replaced with version published Phys. Rev.
Measurement of the cosmic ray spectrum above eV using inclined events detected with the Pierre Auger Observatory
A measurement of the cosmic-ray spectrum for energies exceeding
eV is presented, which is based on the analysis of showers
with zenith angles greater than detected with the Pierre Auger
Observatory between 1 January 2004 and 31 December 2013. The measured spectrum
confirms a flux suppression at the highest energies. Above
eV, the "ankle", the flux can be described by a power law with
index followed by
a smooth suppression region. For the energy () at which the
spectral flux has fallen to one-half of its extrapolated value in the absence
of suppression, we find
eV.Comment: Replaced with published version. Added journal reference and DO
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