223 research outputs found
Heteroepitaxy of and on GaAs (111)A by Atomic Layer Deposition: Achieving Low Interface Trap Density
GaAs metalβoxideβsemiconductor devices historically suffer from Fermi-level pinning, which is mainly due to the high trap density of states at the oxide/GaAs interface. In this work, we present a new way of passivating the interface trap states by growing an epitaxial layer of high-k dielectric oxide, , on GaAs(111)A. High-quality epitaxial thin films are achieved by an ex situ atomic layer deposition (ALD) process, and GaAs MOS capacitors made from this epitaxial structure show very good interface quality with small frequency dispersion and low interface trap densities . In particular, the /GaAs interface, which has a lattice mismatch of only 0.04%, shows very low in the GaAs bandgap, below near the conduction band edge. The /GaAs capacitors also show the lowest frequency dispersion of any dielectric on GaAs. This is the first achievement of such low trap densities for oxides on GaAs.Chemistry and Chemical Biolog
Channel Length Scaling of MoS2 MOSFETs
In this article, we investigate electrical transport properties in ultrathin
body (UTB) MoS2 two-dimensional (2D) crystals with channel lengths ranging from
2 {\mu}m down to 50 nm. We compare the short channel behavior of sets of
MOSFETs with various channel thickness, and reveal the superior immunity to
short channel effects of MoS2 transistors. We observe no obvious short channel
effects on the device with 100 nm channel length (Lch) fabricated on a 5 nm
thick MoS2 2D crystal even when using 300 nm thick SiO2 as gate dielectric, and
has a current on/off ratio up to ~109. We also observe the on-current
saturation at short channel devices with continuous scaling due to the carrier
velocity saturation. Also, we reveal the performance limit of short channel
MoS2 transistors is dominated by the large contact resistance from the Schottky
barrier between Ni and MoS2 interface, where a fully transparent contact is
needed to achieve a high-performance short channel device.Comment: 22 pages, 6 figures; ACS Nano, ASAP, 201
Electrons surfing on a sound wave as a platform for quantum optics with flying electrons
Electrons in a metal are indistinguishable particles that strongly interact
with other electrons and their environment. Isolating and detecting a single
flying electron after propagation to perform quantum optics like experiments at
the single electron level is therefore a challenging task. Up to date, only few
experiments have been performed in a high mobility two-dimensional electron gas
where the electron propagates almost ballistically. Flying electrons were
detected via the current generated by an ensemble of electrons and electron
correlations were encrypted in the current noise. Here we demonstrate the
experimental realisation of high efficiency single electron source and single
electron detector for a quantum medium where a single electron is propagating
isolated from the other electrons through a one-dimensional channel. The moving
potential is excited by a surface acoustic wave, which carries the single
electron along the 1D-channel at a speed of 3\mum/ns. When such a quantum
channel is placed between two quantum dots, a single electron can be
transported from one quantum dot to the other, which is several micrometres
apart, with a quantum efficiency of emission and detection of 96% and 92%,
respectively. Furthermore, the transfer of the electron can be triggered on a
timescale shorter than the coherence time T2* of GaAs spin qubits6. Our work
opens new avenues to study the teleportation of a single electron spin and the
distant interaction between spatially separated qubits in a condensed matter
system.Comment: Total 25 pages. 12 pages main text, 4 figures, 5 pages supplementary
materia
An Oscillatory Contractile Pole-Force Component Dominates the Traction Forces Exerted by Migrating Amoeboid Cells
We used principal component analysis to dissect the mechanics of chemotaxis of amoeboid cells into a reduced set of dominant components of cellular traction forces and shape changes. The dominant traction force component in wild-type cells accounted for ~40% of the mechanical work performed by these cells, and consisted of the cell attaching at front and back contracting the substrate towards its centroid (pole-force). The time evolution of this pole-force component was responsible for the periodic variations of cell length and strain energy that the cells underwent during migration. We identified four additional canonical components, reproducible from cell to cell, overall accounting for an additional ~20% of mechanical work, and associated with events such as lateral protrusion of pseudopodia. We analyzed mutant strains with contractility defects to quantify the role that non-muscle Myosin II (MyoII) plays in amoeboid motility. In MyoII essential light chain null cells the polar-force component remained dominant. On the other hand, MyoII heavy chain null cells exhibited a different dominant traction force component, with a marked increase in lateral contractile forces, suggesting that cortical contractility and/or enhanced lateral adhesions are important for motility in this cell line. By compressing the mechanics of chemotaxing cells into a reduced set of temporally-resolved degrees of freedom, the present study may contribute to refined models of cell migration that incorporate cell-substrate interactions
Performance of the CMS Cathode Strip Chambers with Cosmic Rays
The Cathode Strip Chambers (CSCs) constitute the primary muon tracking device
in the CMS endcaps. Their performance has been evaluated using data taken
during a cosmic ray run in fall 2008. Measured noise levels are low, with the
number of noisy channels well below 1%. Coordinate resolution was measured for
all types of chambers, and fall in the range 47 microns to 243 microns. The
efficiencies for local charged track triggers, for hit and for segments
reconstruction were measured, and are above 99%. The timing resolution per
layer is approximately 5 ns
Performance and Operation of the CMS Electromagnetic Calorimeter
The operation and general performance of the CMS electromagnetic calorimeter
using cosmic-ray muons are described. These muons were recorded after the
closure of the CMS detector in late 2008. The calorimeter is made of lead
tungstate crystals and the overall status of the 75848 channels corresponding
to the barrel and endcap detectors is reported. The stability of crucial
operational parameters, such as high voltage, temperature and electronic noise,
is summarised and the performance of the light monitoring system is presented
An Adhesion-Dependent Switch between Mechanisms That Determine Motile Cell Shape
Keratocytes are fast-moving cells in which adhesion dynamics are tightly coupled to the actin polymerization motor that drives migration, resulting in highly coordinated cell movement. We have found that modifying the adhesive properties of the underlying substrate has a dramatic effect on keratocyte morphology. Cells crawling at intermediate adhesion strengths resembled stereotypical keratocytes, characterized by a broad, fan-shaped lamellipodium, clearly defined leading and trailing edges, and persistent rates of protrusion and retraction. Cells at low adhesion strength were small and round with highly variable protrusion and retraction rates, and cells at high adhesion strength were large and asymmetrical and, strikingly, exhibited traveling waves of protrusion. To elucidate the mechanisms by which adhesion strength determines cell behavior, we examined the organization of adhesions, myosin II, and the actin network in keratocytes migrating on substrates with different adhesion strengths. On the whole, our results are consistent with a quantitative physical model in which keratocyte shape and migratory behavior emerge from the self-organization of actin, adhesions, and myosin, and quantitative changes in either adhesion strength or myosin contraction can switch keratocytes among qualitatively distinct migration regimes
Line-Scanning Particle Image Velocimetry: An Optical Approach for Quantifying a Wide Range of Blood Flow Speeds in Live Animals
The ability to measure blood velocities is critical for studying vascular development, physiology, and pathology. A key challenge is to quantify a wide range of blood velocities in vessels deep within living specimens with concurrent diffraction-limited resolution imaging of vascular cells. Two-photon laser scanning microscopy (TPLSM) has shown tremendous promise in analyzing blood velocities hundreds of micrometers deep in animals with cellular resolution. However, current analysis of TPLSM-based data is limited to the lower range of blood velocities and is not adequate to study faster velocities in many normal or disease conditions.We developed line-scanning particle image velocimetry (LS-PIV), which used TPLSM data to quantify peak blood velocities up to 84 mm/s in live mice harboring brain arteriovenous malformation, a disease characterized by high flow. With this method, we were able to accurately detect the elevated blood velocities and exaggerated pulsatility along the abnormal vascular network in these animals. LS-PIV robustly analyzed noisy data from vessels as deep as 850 Β΅m below the brain surface. In addition to analyzing in vivo data, we validated the accuracy of LS-PIV up to 800 mm/s using simulations with known velocity and noise parameters.To our knowledge, these blood velocity measurements are the fastest recorded with TPLSM. Partnered with transgenic mice carrying cell-specific fluorescent reporters, LS-PIV will also enable the direct in vivo correlation of cellular, biochemical, and hemodynamic parameters in high flow vascular development and diseases such as atherogenesis, arteriogenesis, and vascular anomalies
Submicron and Nanometer Structures Technology and Research
Contains reports on twenty research projects and a list of publications.Defense Advanced Research Projects Agency Contract N00019-92-K-0021Joint Services Electronics Program Contract DAAL03-92-C-0001National Science Foundation Grant ECS 90-16437U.S. Army Research Office Grant DAAL03-92-G-0291IBM CorporationU.S. Air Force - Office of Scientific Research Grant F49620-92-J-0064National Science Foundation Grant DMR 87-19217National Science Foundation Grant DMR 90-22933Defense Advanced Research Projects Agency Consortium for Superconducting ElectronicsNational Aeronautics and Space Administration Contract NAS8-36748National Aeronautics and Space Administration Grant NAGW-200
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