6,543 research outputs found
Detection of a single-charge defect in a metal-oxide-semiconductor structure using vertically coupled Al and Si single-electron transistors
An Al-AlO_x-Al single-electron transistor (SET) acting as the gate of a
narrow (~ 100 nm) metal-oxide-semiconductor field-effect transistor (MOSFET)
can induce a vertically aligned Si SET at the Si/SiO_2 interface near the
MOSFET channel conductance threshold. By using such a vertically coupled Al and
Si SET system, we have detected a single-charge defect which is tunnel-coupled
to the Si SET. By solving a simple electrostatic model, the fractions of each
coupling capacitance associated with the defect are extracted. The results
reveal that the defect is not a large puddle or metal island, but its size is
rather small, corresponding to a sphere with a radius less than 1 nm. The small
size of the defect suggests it is most likely a single-charge trap at the
Si/SiO_2 interface. Based on the ratios of the coupling capacitances, the
interface trap is estimated to be about 20 nm away from the Si SET.Comment: 5 pages and 5 figure
A method of recognition of hand drawn line patterns
Method of recognition of hand drawn line pattern
Heterostructure unipolar spin transistors
We extend the analogy between charge-based bipolar semiconductor electronics
and spin-based unipolar electronics by considering unipolar spin transistors
with different equilibrium spin splittings in the emitter, base, and collector.
The current of base majority spin electrons to the collector limits the
performance of ``homojunction'' unipolar spin transistors, in which the
emitter, base, and collector all are made from the same magnetic material. This
current is very similar in origin to the current of base majority carriers to
the emitter in homojunction bipolar junction transistors. The current in
bipolar junction transistors can be reduced or nearly eliminated through the
use of a wide band gap emitter. We find that the choice of a collector material
with a larger equilibrium spin splitting than the base will similarly improve
the device performance of a unipolar spin transistor. We also find that a
graded variation in the base spin splitting introduces an effective drift field
that accelerates minority carriers through the base towards the collector.Comment: 9 pages, 2 figure
Electronic Interface Reconstruction at Polar-Nonpolar Mott Insulator Heterojunctions
We report on a theoretical study of the electronic interface reconstruction
(EIR) induced by polarity discontinuity at a heterojunction between a polar and
a nonpolar Mott insulators, and of the two-dimensional strongly-correlated
electron systems (2DSCESs) which accompany the reconstruction. We derive an
expression for the minimum number of polar layers required to drive the EIR,
and discuss key parameters of the heterojunction system which control 2DSCES
properties. The role of strong correlations in enhancing confinement at the
interface is emphasized.Comment: 7 pages, 6 figures, some typos correcte
Energy and momentum relaxation dynamics of hot holes in modulation doped GaInNAs/GaAs quantum wells
We present the studies of energy and momentum relaxation dynamics of nonequilibrium holes in GaxIn1−xNyAs1−y/GaAs quantum well modulation doped with Be. Experimental results show that the real-space transfer (RST) of hot holes occurs via thermionic emission from the high-mobility GaInNAs quantum wells into the low-mobility GaAs barriers at a threshold electric field of F ∼ 6 kV/cm at T = 13 K. At this field the hole drift velocity saturates at vd ∼ 1×107 cm/s. A slight increase in the field above the threshold leads to the impact ionization of acceptors in the barriers by the nonequilibrium holes. We observe and model theoretically a negative differential mobility effect induced by RST that occurs at an electric field of F ∼ 7 kV/cm. The observed current surge at electric fields above 7 kV/cm is attributed to the hole multiplication induced by shallow impurity breakdown in the GaAs barrier and impact ionization in the high-field domain regime associated with the packet of RST of holes in the well
Nanostructured electrodes for thermionic and thermo-tunneling devices
Recently, new quantum features have been studied in the area of ridged
quantum wells (RQW). Periodic ridges on the surface of the quantum well layer
impose additional boundary conditions on the electron wave function and reduce
the quantum state density. Electrons, rejected from forbidden quantum states,
have to occupy the states with higher energy. As a result, Fermi energy in RQW
increases and work function (WF) decreases. We investigate low WF electrode,
com-posed from a metal RQW layer and a base substrate. The substrate material
was selected so that electrons were confined to the RQW. The WF value depends
on ridge geometry and electron confinement. We calculate WF in the metal RQW
films grown both on a semiconductor and metal substrates. In the case of
semiconductor substrate, wide band gap materials are preferable as they allow
more reduction in RQW work function. In the case of metal substrate, low Fermi
energy materials are preferable. For most material pairs, the WF was reduced
dramatically. Such structures, can serve as electrodes for room temperature
thermionic and thermotunnel energy converters and coolers.Comment: 8 pages, 5 figures, 2 table
Charge state of the O molecule during silicon oxidation through hybrid functional calculations
We study the charge state of the diffusing O molecule during silicon
oxidation through hybrid functional calculations. We calculate charge
transition levels of O in bulk SiO and use theoretical band offsets to
align these levels with respect to the Si band edges. To overcome the band-gap
problem of semilocal density fuctionals, we employ hybrid functionals with both
predefined and empirically adjusted mixing coefficients. We find that the
charge transition level in bulk SiO occurs at 1.1 eV
above the silicon conduction band edge, implying that the O molecule
diffuses through the oxide in the neutral charge state. While interfacial
effects concur to lower the charge transition level, our estimates suggest that
the neutral charge state persists until silicon oxidation.Comment: 4 pages, 3 figure
Local conductivity and the role of vacancies around twin walls of (001)-BiFeO3 thin films
BiFeO3 thin films epitaxially grown on SrRuO3-buffered (001)-oriented SrTiO3
substrates show orthogonal bundles of twin domains, each of which contains
parallel and periodic 71o domain walls. A smaller amount of 109o domain walls
are also present at the boundaries between two adjacent bundles. All as-grown
twin walls display enhanced conductivity with respect to the domains during
local probe measurements, due to the selective lowering of the Schottky barrier
between the film and the AFM tip (see S. Farokhipoor and B. Noheda, Phys. Rev.
Lett. 107, 127601 (2011)). In this paper we further discuss these results and
show why other conduction mechanisms are discarded. In addition we show the
crucial role that oxygen vacancies play in determining the amount of conduction
at the walls. This prompts us to propose that the oxygen vacancies migrating to
the walls locally lower the Schottky barrier. This mechanism would then be less
efficient in non-ferroelastic domain walls where one expects no strain
gradients around the walls and thus (assuming that walls are not charged) no
driving force for accumulation of defects
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