194 research outputs found
Identification of the major cause of endemically poor mobilities in SiC/SiO2 structures
Materials with good carrier mobilities are desired for device applications,
but in real devices the mobilities are usually limited by the presence of
interfaces and contacts. Mobility degradation at semiconductor-dielectric
interfaces is generally attributed to defects at the interface or inside the
dielectric, as is the case in Si/SiO2 structures, where processing does not
introduce detrimental defects in the semiconductor. In the case of SiC/SiO2
structures, a decade of research focused on reducing or passivating interface
and oxide defects, but the low mobilities have persisted. By invoking
theoretical results and available experimental evidence, we show that thermal
oxidation generates carbon di-interstitial defects inside the semiconductor
substrate and that they are a major cause of the poor mobility in SiC/SiO2
structures
Defect-related hysteresis in nanotube-based nano-electromechanical systems
The electronic properties of multi-walled carbon nanotubes (MWCNTs) depend on the positions of their walls with respect to neighboring shells. This fact can enable several applications of MWCNTs as nano-electromechanical systems (NEMS). In this article, we report the findings of a first-principles study on the stability and dynamics of point defects in double-walled carbon nanotubes (DWCNTs) and their role in the response of the host systems under inter-tube displacement. Key defect-related effects, namely, sudden energy changes and hysteresis, are identified, and their relevance to a host of MWCNT-based NEMS is highlighted. The results also demonstrate the dependence of these effects on defect clustering and chirality of DWCNT shells
Atomic-Scale Dynamics of the Formation and Dissolution of Carbon Clusters in SiO2
Oxidation of SiC produces SiO2 while CO is released. A `reoxidation' step at
lower temperatures is, however, necessary to produce high-quality SiO2. This
step is believed to cleanse the oxide of residual C without further oxidation
of the SiC substrate. We report first-principles calculations that describe the
nucleation and growth of O-deficient C clusters in SiO2 under oxidation
conditions, fed by the production of CO at the advancing interface, and their
gradual dissolution by the supply of O under reoxidation conditions. We predict
that both CO and CO2 are released during both steps.Comment: RevTex, 4 pages, 2 ps figures, to appear in Phys. Rev. Lett. (June
25, 2001
Atomic origin of high temperature electron trapping in MOS devices
MOSFETs based on wide band-gap semiconductors are suitable for operations at
high temperature, at which additional atomic-scale processes that are benign at
lower temperatures can get activated which results in device degradation.
Recently significant enhancement of electron trapping was observed under
positive bias in SiC MOSFETs at temperatures higher than 150{\deg}C. Here we
report first-principle calculations showing that the enhanced electron trapping
is associated with thermally activated capturing of a second electron by an
oxygen vacancy in SiO2, by which the vacancy transforms into a new structure
that comprises one Si dangling bond and a bond between a five-fold and a
four-fold Si atoms. The results suggest a key role of oxygen vacancies and
their structural reconfigurations in the reliability of high-temperature MOS
devices
Spin-dependent resonant tunneling through quantum-well states in magnetic metallic thin films
Quantum-well (QW) states in {\it nonmagnetic} metal layers contained in
magnetic multilayers are known to be important in spin-dependent transport, but
the role of QW states in {\it magnetic} layers remains elusive. Here we
identify the conditions and mechanisms for resonant tunneling through QW states
in magnetic layers and determine candidate structures. We report
first-principles calculations of spin-dependent transport in epitaxial
Fe/MgO/FeO/Fe/Cr and Co/MgO/Fe/Cr tunnel junctions. We demonstrate the
formation of sharp QW states in the Fe layer and show discrete conductance
jumps as the QW states enter the transport window with increasing bias. At
resonance, the current increases by one to two orders of magnitude. The
tunneling magnetoresistance ratio is several times larger than in simple spin
tunnel junctions and is positive (negative) for majority- (minority-) spin
resonances, with a large asymmetry between positive and negative biases. The
results can serve as the basis for novel spintronic devices.Comment: 4 figures in 5 eps file
Unified Band Theoretic Description of Electronic and Magnetic Properties of Vanadium Dioxide Phases
The debate about whether the insulating phases of vanadium dioxide (VO2) can
be described by band theory or must be described by a theory of strong electron
correlations remains unresolved even after decades of research. Energy-band
calculations using hybrid exchange functionals or including self-energy
corrections account for the insulating or metallic nature of different phases,
but have not yet successfully accounted for the observed magnetic orderings.
Strongly-correlated theories have had limited quantitative success. Here we
report that, by using hard pseudopotentials and an optimized hybrid exchange
functional, the energy gaps and magnetic orderings of both monoclinic VO2
phases and the metallic nature of the high-temperature rutile phase are
consistent with available experimental data, obviating an explicit role for
strong correlations. We also report a potential candidate for the newly-found
metallic monoclinic phase and present a detailed magnetic structure of the M2
monoclinic phase
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