279 research outputs found
Device modeling of long-channel nanotube electro-optical emitter
We present a simple analytic model of nanotube electro-optical emitters,
along with improved experimental measurements using PMMA-passivated devices
with reduced hysteresis. Both the ambipolar electrical characteristics and the
motion of the infrared-emission spot are well described. The model indicates
that the electric field is strongly enhanced at the emission spot, and that
device performance can be greatly improved by the use of thinner gate oxides
Cooling of photoexcited carriers in graphene by internal and substrate phonons
We investigate the energy relaxation of hot carriers produced by
photoexcitation of graphene through coupling to both intrinsic and remote
(substrate) surface polar phonons using the Boltzmann equation approach. We
find that the energy relaxation of hot photocarriers in graphene on commonly
used polar substrates, under most conditions, is dominated by remote surface
polar phonons. We also calculate key characteristics of the energy relaxation
process, such as the transient cooling time and steady state carrier
temperatures and photocarriers densities, which determine the thermoelectric
and photovoltaic photoresponse, respectively. Substrate engineering can be a
promising route to efficient optoelectronic devices driven by hot carrier
dynamics.Comment: related papers at http://tonylow.info
Graphene quantum dots probed by scanning tunneling spectroscopy and transport spectroscopy after local anodic oxidation
Graphene quantum dots are considered as promising alternatives to quantum
dots in III-V semiconductors, e.g., for the use as spin qubits due to their
consistency made of light atoms including spin-free nuclei which both imply
relatively long spin decoherene times. However, this potential has not been
realized in experiments so far, most likely, due to a missing control of the
edge configurations of the quantum dots. Thus, a more fundamental investigation
of Graphene quantum dots appears to be necessary including a full control of
the wave function properties most favorably during transport spectroscopy
measurements. Here, we review the recent success in mapping wave functions of
graphene quantum dots supported by metals, in particular Ir(111), and show how
the goal of probing such wave functions on insulating supports during transport
spectroscopy might be achieved.Comment: 14 pages, review articl
Tip-gating Effect in Scanning Impedance Microscopy of Nanoelectronic Devices
Electronic transport in semiconducting single-wall carbon nanotubes is
studied by combined scanning gate microscopy and scanning impedance microscopy
(SIM). Depending on the probe potential, SIM can be performed in both invasive
and non-invasive mode. High-resolution imaging of the defects is achieved when
the probe acts as a local gate and simultaneously an electrostatic probe of
local potential. A class of weak defects becomes observable even if they are
located in the vicinity of strong defects. The imaging mechanism of tip-gating
scanning impedance microscopy is discussed.Comment: 11 pages, 3 figures, to be published in Appl. Phys. Let
Carbon nanotubes as a tip calibration standard for electrostatic scanning probe microscopies
Scanning Surface Potential Microscopy (SSPM) is one of the most widely used
techniques for the characterization of electrical properties at small
dimensions. Applicability of SSPM and related electrostatic scanning probe
microscopies for imaging of potential distributions in active micro- and
nanoelectronic devices requires quantitative knowledge of tip surface contrast
transfer. Here we demonstrate the utility of carbon-nanotube-based circuits to
characterize geometric properties of the tip in the electrostatic scanning
probe microscopies (SPM). Based on experimental observations, an analytical
form for the differential tip-surface capacitance is obtained.Comment: 14 pages, 4 figure
Role of Single Defects in Electronic Transport through Carbon Nanotube Field-Effect Transistors
The influence of defects on electron transport in single-wall carbon nanotube
field effect transistors (CNFETs) is probed by combined scanning gate
microscopy (SGM) and scanning impedance microscopy (SIM). SGM reveals a
localized field effect at discrete defects along the CNFET length. The
depletion surface potential of individual defects is quantified from the
SGM-imaged radius of the defect as a function of tip bias voltage. This
provides a measure of the Fermi level at the defect with zero tip voltage,
which is as small as 20 meV for the strongest defects. The effect of defects on
transport is probed by SIM as a function of backgate and tip-gate voltage. When
the backgate voltage is set so the CNFET is "on" (conducting), SIM reveals a
uniform potential drop along its length, consistent with diffusive transport.
In contrast, when the CNFET is "off", potential steps develop at the position
of depleted defects. Finally, high-resolution imaging of a second set of weak
defects is achieved in a new "tip-gated" SIM mode.Comment: to appear in Physical Review Letter
Energy dissipation in graphene field-effect transistors
We measure the temperature distribution in a biased single-layer graphene
transistor using Raman scattering microscopy of the 2D-phonon band. Peak
operating temperatures of 1050 K are reached in the middle of the graphene
sheet at 210 KW cm^(-2) of dissipated electric power. The metallic contacts act
as heat sinks, but not in a dominant fashion. To explain the observed
temperature profile and heating rate, we have to include heat-flow from the
graphene to the gate oxide underneath, especially at elevated temperatures,
where the graphene thermal conductivity is lowered due to umklapp scattering.
Velocity saturation due to phonons with about 50 meV energy is inferred from
the measured charge density via shifts in the Raman G-phonon band, suggesting
that remote scattering (through field coupling) by substrate polar surface
phonons increases the energy transfer to the substrate and at the same time
limits the high-bias electronic conduction of graphene.Comment: The pdf-file contains the main manuscript (19 pages, 3 figures) and
the supplement (5 pages, 4 figures
Layer Number Determination and Thickness-dependent Properties of Graphene Grown on SiC
The electronic properties of few-layer graphene grown on the carbon-face of
silicon carbide (SiC) are found to be strongly dependent on the number of
layers. The carrier mobility is larger in thicker graphene because
substrate-related scattering is reduced in the higher layers. The carrier
density dependence of the mobility is qualitatively different in thin and thick
graphene, with the transition occurring at about 2 layers. The mobility
increases with carrier density in thick graphene, similar to multi-layer
graphene exfoliated from natural graphite, suggesting that the individual
layers are still electrically coupled in spite of reports recording non-Bernal
stacking order in C-face grown graphene. The Hall coefficient peak value is
reduced in thick graphene due to the increased density of states. A reliable
and rapid characterization tool for the layer number is therefore highly
desirable. To date, AFM height determination and Raman scattering are typically
used since the optical contrast of graphene on SiC is weak. However, both
methods suffer from low throughput. We show that the scanning electron
microscopy (SEM) contrast can give similar results with much higher throughput
Carrier scattering, mobilities and electrostatic potential in mono-, bi- and tri-layer graphenes
The carrier density and temperature dependence of the Hall mobility in mono-,
bi- and tri-layer graphene has been systematically studied. We found that as
the carrier density increases, the mobility decreases for mono-layer graphene,
while it increases for bi-layer/tri-layer graphene. This can be explained by
the different density of states in mono-layer and bi-layer/tri-layer graphenes.
In mono-layer, the mobility also decreases with increasing temperature
primarily due to surface polar substrate phonon scattering. In
bi-layer/tri-layer graphene, on the other hand, the mobility increases with
temperature because the field of the substrate surface phonons is effectively
screened by the additional graphene layer(s) and the mobility is dominated by
Coulomb scattering.
We also find that the temperature dependence of the Hall coefficient in
mono-, bi- and tri-layer graphene can be explained by the formation of electron
and hole puddles in graphene. This model also explains the temperature
dependence of the minimum conductance of mono-, bi- and tri-layer graphene. The
electrostatic potential variations across the different graphene samples are
extracted.Comment: 18 pages, 7 figure
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