1,374 research outputs found
Electric field induced charge injection or exhaustion in organic thin film transistor
The conductivity of organic semiconductors is measured {\it in-situ} and
continuously with a bottom contact configuration, as a function of film
thickness at various gate voltages. The depletion layer thickness can be
directly determined as a shift of the threshold thickness at which electric
current began to flow. The {\it in-situ} and continuous measurement can also
determine qualitatively the accumulation layer thickness together with the
distribution function of injected carriers. The accumulation layer thickness is
a few mono layers, and it does not depend on gate voltages, rather depends on
the chemical species.Comment: 4 figures, to be published in Phys. Rev.
Magnetotransport in a two-dimensional electron system in dc electric fields
We report on nonequilibrium transport measurements in a high-mobility
two-dimensional electron system subject to weak magnetic field and dc
excitation. Detailed study of dc-induced magneto-oscillations, first observed
by Yang {\em et al}., reveals a resonant condition that is qualitatively
different from that reported earlier. In addition, we observe dramatic
reduction of resistance induced by a weak dc field in the regime of separated
Landau levels. These results demonstrate similarity of transport phenomena in
dc-driven and microwave-driven systems and have important implications for
ongoing experimental search for predicted quenching of microwave-induced
zero-resistance states by a dc current.Comment: Revised version, to appear in Phys. Rev.
Diffusion of Mn interstitials in (Ga,Mn)As epitaxial layers
Magnetic properties of thin (Ga,Mn)As layers improve during annealing by
out-diffusion of interstitial Mn ions to a free surface. Out-diffused Mn atoms
participate in the growth of a Mn-rich surface layer and a saturation of this
layer causes an inhibition of the out-diffusion. We combine high-resolution
x-ray diffraction with x-ray absorption spectroscopy and a numerical solution
of the diffusion problem for the study of the out-diffusion of Mn interstitials
during a sequence of annealing steps. Our data demonstrate that the
out-diffusion of the interstitials is substantially affected by the internal
electric field caused by an inhomogeneous distribution of charges in the
(Ga,Mn)As layer.Comment: 11 pages, 5 figure
Metal nanofilm in strong ultrafast optical fields
We predict that a metal nanofilm subjected to an ultrashort (single
oscillation) optical pulse of a high field amplitude at
normal incidence undergoes an ultrafast (at subcycle times ) transition to a state resembling semimetal. Its reflectivity is
greatly reduced, while the transmissivity and the optical field inside the
metal are greatly increased. The temporal profiles of the optical fields are
predicted to exhibit pronounced subcycle oscillations, which are attributed to
the Bloch oscillations and formation of the Wannier-Stark ladder of electronic
states. The reflected, transmitted, and inside-the-metal pulses have non-zero
areas approaching half-cycle pulses. The effects predicted are promising for
applications to nanoplasmonic modulators and field-effect transistors with
petahertz bandwidth
Space-charge mechanism of aging in ferroelectrics: an exactly solvable two-dimensional model
A mechanism of point defect migration triggered by local depolarization
fields is shown to explain some still inexplicable features of aging in
acceptor doped ferroelectrics. A drift-diffusion model of the coupled charged
defect transport and electrostatic field relaxation within a two-dimensional
domain configuration is treated numerically and analytically. Numerical results
are given for the emerging internal bias field of about 1 kV/mm which levels
off at dopant concentrations well below 1 mol%; the fact, long ago known
experimentally but still not explained. For higher defect concentrations a
closed solution of the model equations in the drift approximation as well as an
explicit formula for the internal bias field is derived revealing the plausible
time, temperature and concentration dependencies of aging. The results are
compared to those due to the mechanism of orientational reordering of defect
dipoles.Comment: 8 pages, 4 figures. accepted to Physical Review
Efficient nonlinear room-temperature spin injection from ferromagnets into semiconductors through a modified Schottky barrier
We suggest a consistent microscopic theory of spin injection from a
ferromagnet (FM) into a semiconductor (S). It describes tunneling and emission
of electrons through modified FM-S Schottky barrier with an ultrathin heavily
doped interfacial S layer . We calculate nonlinear spin-selective properties of
such a reverse-biased FM-S junction, its nonlinear I-V characteristic, current
saturation, and spin accumulation in S. We show that the spin polarization of
current, spin density, and penetration length increase with the total current
until saturation. We find conditions for most efficient spin injection, which
are opposite to the results of previous works, since the present theory
suggests using a lightly doped resistive semiconductor. It is shown that the
maximal spin polarizations of current and electrons (spin accumulation) can
approach 100% at room temperatures and low current density in a nondegenerate
high-resistance semiconductor.Comment: 7 pages, 2 figures; provides detailed comparison with earlier works
on spin injectio
Surface potential at a ferroelectric grain due to asymmetric screening of depolarization fields
Nonlinear screening of electric depolarization fields, generated by a stripe
domain structure in a ferroelectric grain of a polycrystalline material, is
studied within a semiconductor model of ferroelectrics. It is shown that the
maximum strength of local depolarization fields is rather determined by the
electronic band gap than by the spontaneous polarization magnitude.
Furthermore, field screening due to electronic band bending and due to presence
of intrinsic defects leads to asymmetric space charge regions near the grain
boundary, which produce an effective dipole layer at the surface of the grain.
This results in the formation of a potential difference between the grain
surface and its interior of the order of 1 V, which can be of either sign
depending on defect transition levels and concentrations. Exemplary acceptor
doping of BaTiO3 is shown to allow tuning of the said surface potential in the
region between 0.1 and 1.3 V.Comment: 14 pages, 11 figures, submitted to J. Appl. Phy
Resonant tunnelling features in the transport spectroscopy of quantum dots
We present a review of features due to resonant tunnelling in transport
spectroscopy experiments on quantum dots and single donors. The review covers
features attributable to intrinsic properties of the dot as well as extrinsic
effects, with a focus on the most common operating conditions. We describe
several phenomena that can lead to apparently identical signatures in a bias
spectroscopy measurement, with the aim of providing experimental methods to
distinguish between their different physical origins. The correct
classification of the resonant tunnelling features is an essential requirement
to understand the details of the confining potential or predict the performance
of the dot for quantum information processing.Comment: 18 pages, 7 figures. Short review article submitted to
Nanotechnology, special issue on 'Quantum Science and Technology at the
Nanoscale
Photoreflectance and surface photovoltage spectroscopy of beryllium-doped GaAs/AlAs multiple quantum wells
We present an optical study of beryllium delta-doped GaAs/AlAs multiple quantum well (QW) structures designed for sensing terahertz (THz) radiation. Photoreflectance (PR), surface photovoltage (SPV), and wavelength-modulated differential surface photovoltage (DSPV) spectra were measured in the structures with QW widths ranging from 3 to 20 nm and doping densities from 2×10(10) to 5×10(12) cm(–2) at room temperature. The PR spectra displayed Franz-Keldysh oscillations which enabled an estimation of the electric-field strength of ~20 kV/cm at the sample surface. By analyzing the SPV spectra we have determined that a buried interface rather than the sample surface mainly governs the SPV effect. The DSPV spectra revealed sharp features associated with excitonic interband transitions which energies were found to be in a good agreement with those calculated including the nonparabolicity of the energy bands. The dependence of the exciton linewidth broadening on the well width and the quantum index has shown that an average half monolayer well width fluctuations is mostly predominant broadening mechanism for QWs thinner than 10 nm. The line broadening in lightly doped QWs, thicker than 10 nm, was found to arise from thermal broadening with the contribution from Stark broadening due to random electric fields of the ionized impurities in the structures. We finally consider the possible influence of strong internal electric fields, QW imperfections, and doping level on the operation of THz sensors fabricated using the studied structures. © 2005 American Institute of Physic
Single-particle states in spherical Si/SiO quantum dots
We calculate ground and excited electron and hole levels in spherical Si
quantum dots inside SiO in a multiband effective mass approximation.
Luttinger Hamiltonian is used for holes and the strong anisotropy of the
conduction electron effective mass in Si is taken into account. As boundary
conditions for electron and hole wave functions we use continuity of the wave
functions and the velocity density at the boundary of the quantum dots.Comment: 8 pages, 5 figure
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