325 research outputs found
Quantum confinement effects in Si/Ge heterostructures with spatially ordered arrays of self-assembled quantum dots
Magnetotunneling spectroscopy was employed to probe the confinement in
vertical Si/Ge double-barrier resonant tunneling diodes with regularly
distributed Ge quantum dots. Their current-voltage characteristics reveal a
step-like behavior in the vicinity of zero bias, indicating resonant tunneling
of heavy-holes via three-dimensionally confined unoccupied hole states in Ge
quantum dots. Assuming parabolic confinement we extract the strength of the
confinement potential of quantum dots.Comment: 4 pages, 3 figure
SiGeSn/GeSn hetero- and multiple quantum well structures for optoelectronics on Si
Advanced information technology has to be able to cope with the enormous amounts and rates of data requirements. New architectures of computing systems, such as neuromorphic computing, will enable deep learning and massive parallel data handling. However, it will need also large amounts of data for training as well as fast transfer rates of data between logic and storage devices. Here, advanced chip and board designs, including silicon optical interposer may allow much higher density of signal traces between co-packaged chips. In particular co-packaged silicon photonic chips allow optical interconnections between systems-in-package. Thus silicon interposer can directly contain photonic devices based on group alloys. In a long term vision this technology might be enabled by GeSn lasers permitting to connect optically individual chips within the system-in-package.In the past years significant progress has been made to develop optically active devices based on Si. A direct band gap for GeSn alloys containing more than 8.5% of Sn was demonstrated and the optically pumped GeSn laser were reported [1,2]. In order to improve the device performance and achieve electrical operation at sufficiently low power still severe challenges have to be met. The GeSn active region has to be embedded in a heterostructure providing optical waveguiding and efficient carrier injection. The active region may contain quantum well structures to warrant low threshold currents and room temperature operation.
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MBE Growth of Al/InAs and Nb/InAs Superconducting Hybrid Nanowire Structures
We report on \textit{in situ} growth of crystalline Al and Nb shells on InAs
nanowires. The nanowires are grown on Si(111) substrates by molecular beam
epitaxy (MBE) without foreign catalysts in the vapor-solid mode. The metal
shells are deposited by electron-beam evaporation in a metal MBE. High quality
supercondonductor/semiconductor hybrid structures such as Al/InAs and Nb/InAs
are of interest for ongoing research in the fields of gateable Josephson
junctions and quantum information related research. Systematic investigations
of the deposition parameters suitable for metal shell growth are conducted. In
case of Al, the substrate temperature, the growth rate and the shell thickness
are considered. The substrate temperature as well as the angle of the impinging
deposition flux are explored for Nb shells. The core-shell hybrid structures
are characterized by electron microscopy and x-ray spectroscopy. Our results
show that the substrate temperature is a crucial parameter in order to enable
the deposition of smooth Al layers. Contrary, Nb films are less dependent on
substrate temperature but strongly affected by the deposition angle. At a
temperature of 200{\deg}C Nb reacts with InAs, dissolving the nanowire crystal.
Our investigations result in smooth metal shells exhibiting an impurity and
defect free, crystalline superconductor/InAs interface. Additionally, we find
that the superconductor crystal structure is not affected by stacking faults
present in the InAs nanowires.Comment: 8 pages, 10 figures, 1 tabl
Thickness dependence of electron-electron interactions in topological p-n junctions
Electron-electron interactions in topological p-n junctions consisting of
vertically stacked topological insulators are investigated. n-type Bi2Te3 and
p-type Sb2Te3 of varying relative thicknesses are deposited using molecular
beam epitaxy and their electronic properties measured using low-temperature
transport. The screening factor is observed to decrease with increasing sample
thickness, a finding which is corroborated by semi-classical Boltzmann theory.
The number of two-dimensional states determined from electron-electron
interactions is larger compared to the number obtained from
weak-antilocalization, in line with earlier experiments using single layers.Comment: 38 pages, 5 figures, 1 tabl
Induced Superconductivity in Hybrid Au/YBa2Cu3O7-x Electrodes on Vicinal Substrates
Superconducting electrodes are an integral part of hybrid Josephson junctions
used in many applications including quantum technologies. We report on the
fabrication and characterization of superconducting hybrid Au/YBa2Cu3O7-x
(YBCO) electrodes on vicinal substrates. In these structures, superconducting
CuO2-planes face the gold film, resulting in a higher value and smaller
variation of the induced energy gap compared to the conventional Au/YBCO
electrodes based on films with the c-axis normal to the substrate surface.
Using scanning tunneling microscopy, we observe an energy gap of about 10-17
meV at the surface of the 15- nm-thick gold layer deposited in situ atop the
YBCO film. To study the origin of this gap, we fabricate nanoconstrictions from
the Au/YBCO heterostructure and measure their electrical transport
characteristics. The conductance of the nanoconstrictions shows a series of
dips due to multiple Andreev reflections in YBCO and gold providing clear
evidence of the superconducting nature of the gap in gold. We consider the
Au/YBCO electrodes to be a versatile platform for hybrid Josephson devices with
a high operating temperature
Current-induced magnetization switching in a magnetic topological insulator heterostructure
We present the current-induced switching of the internal magnetization
direction in a magnetic topological insulator/topological insulator
heterostructure in the quantum anomalous Hall regime. The switching process is
based on the bias current dependence of the coercive field, which is attributed
to the effect of the spin-orbit torque provided by the unpolarized bias
current. Increasing the bias current leads to a decrease in the magnetic order
in the sample. When the applied current is subsequently reduced, the magnetic
moments align with an externally applied magnetic field, resulting in
repolarization in the opposite direction. This includes a reversal of the spin
polarisation and hence a reversal of the chiral edge mode. Possible
applications in spintronic devices are discussed.Comment: 6 pages, 3 figures (5 pages and 5 figures in supplementary
information
Electrical resistance of individual defects at a topological insulator surface
Three-dimensional topological insulators host surface states with linear
dispersion, which manifest as a Dirac cone. Nanoscale transport measurements
provide direct access to the transport properties of the Dirac cone in real
space and allow the detailed investigation of charge carrier scattering. Here,
we use scanning tunnelling potentiometry to analyse the resistance of different
kinds of defects at the surface of a (Bi0.53Sb0.47)2Te3 topological insulator
thin film. The largest localized voltage drop we find to be located at domain
boundaries in the topological insulator film, with a resistivity about four
times higher than that of a step edge. Furthermore, we resolve resistivity
dipoles located around nanoscale voids in the sample surface. The influence of
such defects on the resistance of the topological surface state is analysed by
means of a resistor network model. The effect resulting from the voids is found
to be small compared to the other defects
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