110 research outputs found
Conductance properties of rough quantum wires with colored surface disorder
Effects of correlated disorder on wave localization have attracted
considerable interest. Motivated by the importance of studies of quantum
transport in rough nanowires, here we examine how colored surface roughness
impacts the conductance of two-dimensional quantum waveguides, using direct
scattering calculations based on the reaction matrix approach. The
computational results are analyzed in connection with a theoretical relation
between the localization length and the structure factor of correlated
disorder. We also examine and discuss several cases that have not been treated
theoretically or are beyond the validity regime of available theories. Results
indicate that conductance properties of quantum wires are controllable via
colored surface disorder.Comment: 19 pages, 7 figure
Diffusive Transport in Quasi-2D and Quasi-1D Electron Systems
Quantum-confined semiconductor structures are the cornerstone of modern-day
electronics. Spatial confinement in these structures leads to formation of
discrete low-dimensional subbands. At room temperature, carriers transfer among
different states due to efficient scattering with phonons, charged impurities,
surface roughness and other electrons, so transport is scattering-limited
(diffusive) and well described by the Boltzmann transport equation. In this
review, we present the theoretical framework used for the description and
simulation of diffusive electron transport in quasi-two-dimensional and
quasi-one-dimensional semiconductor structures. Transport in silicon MOSFETs
and nanowires is presented in detail.Comment: Review article, to appear in Journal of Computational and Theoretical
Nanoscienc
Electron mobility in silicon nanowires
The low-field electron mobility in rectangular silicon nanowire (SiNW)
transistors was computed using a self-consistent Poisson-Schr\"{o}dinger-Monte
Carlo solver. The behavior of the phonon-limited and surface-roughness-limited
components of the mobility was investigated by decreasing the wire width from
30 nm to 8 nm, the width range capturing a crossover between two-dimensional
(2D) and one-dimensional (1D) electron transport. The phonon-limited mobility,
which characterizes transport at low and moderate transverse fields, is found
to decrease with decreasing wire width due to an increase in the
electron-phonon wavefunction overlap. In contrast, the mobility at very high
transverse fields, which is limited by surface roughness scattering, increases
with decreasing wire width due to volume inversion. The importance of acoustic
phonon confinement is also discussed briefly
On Landauer vs. Boltzmann and Full Band vs. Effective Mass Evaluation of Thermoelectric Transport Coefficients
The Landauer approach to diffusive transport is mathematically related to the
solution of the Boltzmann transport equation, and expressions for the
thermoelectric parameters in both formalisms are presented. Quantum mechanical
and semiclassical techniques to obtain from a full description of the
bandstructure, E(k), the number of conducting channels in the Landauer approach
or the transport distribution in the Boltzmann solution are developed and
compared. Thermoelectric transport coefficients are evaluated from an atomistic
level, full band description of a crystal. Several example calculations for
representative bulk materials are presented, and the full band results are
related to the more common effective mass formalism. Finally, given a full E(k)
for a crystal, a procedure to extract an accurate, effective mass level
description is presented.Comment: 33 pages, 8 figure
Identification of new transitions and mass assignments of levels in Pr
The previously reported levels assigned to 151,152,153Pr have recently been
called into question regarding their mass assignment. The above questioned
level assignments are clarified by measuring g-transitions tagged with A and Z
in an in-beam experiment in addition to the measurements from 252Cf spontaneous
fission (SF) and establish new spectroscopic information from to
in the Pr isotopic chain. The isotopic chain 143-153Pr has been studied from
the spontaneous fission of 252Cf by using Gammasphere and also from the
measurement of the prompt g-rays in coincidence with isotopically-identified
fission fragments using VAMOS++ and EXOGAM at GANIL. The latter were produced
using 238U beams on a 9Be target at energies around the Coulomb barrier. The
g-g-g-g data from 252Cf (SF) and those from the GANIL in-beam A- and Z-gated
spectra were combined to unambiguously assign the various transitions and
levels in 151,152,153Pr and other isotopes. New transitions and bands in
145,147,148,149,150Pr were identified by using g-g-g and g-g-g-g coincidences
and A and Z gated g-g spectra. The transitions and levels previously assigned
to 151,153Pr have been confirmed by the (A,Z) gated spectra. The transitions
previously assigned to 152Pr are now assigned to 151Pr on the basis of the
(A,Z) gated spectra. Two new bands with 20 new transitions in 152Pr and one new
band with 7 new transitions in 153Pr are identified from the g-g-g-g
coincidence spectra and the (A,Z) gated spectrum. In addition, new g-rays are
also reported in 143-146Pr. New levels of 145,147-153Pr have been established,
reliable mass assignments of the levels in 151,152,153Pr have been reported and
new transitions have been identified in 143-146Pr showing the new avenues that
are opened by combining the two experimental approaches.Comment: Accepted in Phys. Rev.
Determining the electronic performance limitations in top-down fabricated Si nanowires with mean widths down to 4 nm
Silicon nanowires have been patterned with mean widths down to 4 nm using top-down lithography and dry etching. Performance-limiting scattering processes have been measured directly which provide new insight into the electronic conduction mechanisms within the nanowires. Results demonstrate a transition from 3-dimensional (3D) to 2D and then 1D as the nanowire mean widths are reduced from 12 to 4 nm. The importance of high quality surface passivation is demonstrated by a lack of significant donor deactivation, resulting in neutral impurity scattering ultimately limiting the electronic performance. The results indicate the important parameters requiring optimization when fabricating nanowires with atomic dimensions
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