201 research outputs found
Electron-hole interactions in coupled InAs-GaSb quantum dots based on nanowire crystal phase templates
We report growth and characterization of a coupled quantum dot structure that
utilizes nanowire templates for selective epitaxy of radial heterostructures.
The starting point is a zinc blende InAs nanowire with thin segments of
wurtzite structure. These segments have dual roles: they act as tunnel barriers
for electron transport in the InAs core, and they also locally suppress growth
of a GaSb shell, resulting in coaxial InAs-GaSb quantum dots with integrated
electrical probes. The parallel quantum dot structure hosts spatially separated
electrons and holes that interact due to the type-II broken gap of InAs-GaSb
heterojunctions. The Coulomb blockade in the electron and hole transport is
studied, and periodic interactions of electrons and holes are observed and can
be reproduced by modeling. Distorted Coulomb diamonds indicate voltage-induced
ground-state transitions, possibly a result of changes in the spatial
distribution of holes in the thin GaSb shell.Comment: 8 pages, 7 figure
Single-electron transport in InAs nanowire quantum dots formed by crystal phase engineering
We report electrical characterization of quantum dots formed by introducing
pairs of thin wurtzite (WZ) segments in zinc blende (ZB) InAs nanowires.
Regular Coulomb oscillations are observed over a wide gate voltage span,
indicating that WZ segments create significant barriers for electron transport.
We find a direct correlation of transport properties with quantum dot length
and corresponding growth time of the enclosed ZB segment. The correlation is
made possible by using a method to extract lengths of nanowire crystal phase
segments directly from scanning electron microscopy images, and with support
from transmission electron microscope images of typical nanowires. From
experiments on controlled filling of nearly empty dots with electrons, up to
the point where Coulomb oscillations can no longer be resolved, we estimate a
lower bound for the ZB-WZ conduction-band offset of 95 meV.Comment: 9 pages 9 figure
Schottky barrier and contact resistance of InSb nanowire field effect transistors
Understanding of the electrical contact properties of semiconductor nanowire
(NW) field effect transistors (FETs) plays a crucial role in employing
semiconducting NWs as building blocks for future nanoelectronic devices and in
the study of fundamental physics problems. Here, we report on a study of the
contact properties of Ti/Au, a widely used contact metal combination, to
individual InSb NWs via both two-probe and four-probe transport measurements.
We show that a Schottky barrier of height is
present at the metal-InSb NW interfaces and its effective height is gate
tunable. The contact resistance () in the InSb NWFETs is also
analyzed by magnetotransport measurements at low temperatures. It is found that
at on-state exhibits a pronounced magnetic field dependent
feature, namely it is increased strongly with increasing magnetic field after
an onset field . A qualitative picture that takes into account
magnetic depopulation of subbands in the NWs is provided to explain the
observation. Our results provide a solid experimental evidence for the presence
of a Schottky barrier at Ti/Au-InSb NW interfaces and can be used as a basis
for design and fabrication of novel InSb NW based nanoelectronic devices and
quantum devices.Comment: 12 pages, 4 figure
Spectroscopy and level detuning of few-electron spin states in parallel InAs quantum dots
We use tunneling spectroscopy to study the evolution of few-electron spin
states in parallel InAs nanowire double quantum dots (QDs) as a function of
level detuning and applied magnetic field. Compared to the much more studied
serial configuration, parallel coupling of the QDs to source and drain greatly
expands the probing range of excited state transport. Owing to a strong
confinement, we can here isolate transport involving only the very first
interacting single QD orbital pair. For the (2,0)-(1,1) charge transition, with
relevance for spin-based qubits, we investigate the excited (1,1) triplet, and
hybridization of the (2,0) and (1,1) singlets. An applied magnetic field splits
the (1,1) triplet, and due to spin-orbit induced mixing with the (2,0) singlet,
we clearly resolve transport through all triplet states near the avoided
singlet-triplet crossings. Transport calculations, based on a simple model with
one orbital on each QD, fully replicate the experimental data. Finally, we
observe an expected mirrored symmetry between the 1-2 and 2-3 electron
transitions resulting from the two-fold spin degeneracy of the orbitals.Comment: 17 pages, 8 figure
Electrical properties of InAs1−xSbx and InSb nanowires grown by molecular beam epitaxy
Results of electrical characterization of Au nucleated InAs₁ˍₓSbₓnanowiresgrown by molecular beam epitaxy are reported. An almost doubling of the extracted field effect mobility compared to reference InAsnanowires is observed for a Sb content of x = 0.13. Pure InSbnanowires on the other hand show considerably lower, and strongly diameter dependent, mobility values. Finally, InAs of wurtzite crystal phase overgrown with an InAs₁ˍₓSbₓ shell is found to have a substantial positive shift in threshold voltage compared to reference nanowires.This work received financial support from the Nanometer
Structure Consortium at Lund University (nmC@LU), the
Swedish Research Council (VR), the Swedish Foundation for
Strategic Research (SSF), and the Knut and Alice Wallenberg
Foundation (KAW). It also received financial support from
the French National Research Agency (ANR), TERADOT
project, under Contract No.ANR-11-JS04-002-01
Electrical control of spins and giant g-factors in ring-like coupled quantum dots
Emerging theoretical concepts for quantum technologies have driven a
continuous search for structures where a quantum state, such as spin, can be
manipulated efficiently. Central to many concepts is the ability to control a
system by electric and magnetic fields, relying on strong spin-orbit
interaction and a large g-factor. Here, we present a new mechanism for spin and
orbital manipulation using small electric and magnetic fields. By hybridizing
specific quantum dot states at two points inside InAs nanowires, nearly perfect
quantum rings form. Large and highly anisotropic effective g-factors are
observed, explained by a strong orbital contribution. Importantly, we find that
the orbital and spin-orbital contributions can be efficiently quenched by
simply detuning the individual quantum dot levels with an electric field. In
this way, we demonstrate not only control of the effective g-factor from 80 to
almost 0 for the same charge state, but also electrostatic change of the ground
state spin
Vapor-solid-solid growth dynamics in GaAs nanowires
Semiconductor nanowires are promising material systems for coming of age
nanotechnology. The usage of the vapor solid solid (VSS) route, where the
catalyst used for promoting axial growth of nanowire is a solid, offers certain
advantages compared to the common vapor liquid solid (VLS) route (using liquid
catalyst). The VSS growth of group-IV elemental nanowires have been
investigated by other groups in situ during growth in a transmission electron
microscope (TEM). Though it is known that compound nanowire growth has
different dynamics compared to monoatomic semiconductors, the dynamics of VSS
growth of compound nanowires has not been understood. Here we investigate VSS
growth of compound nanowires by in situ microscopy, using Au-seeded GaAs as a
model system. The growth kinetics and dynamics at the wire-catalyst interface
by ledge-flow is studied and compared for liquid and solid catalysts at similar
growth conditions. Here the temperature and thermal history of the system is
manipulated to control the catalyst phase. In the first experiment discussed
here we reduce the growth temperature in steps to solidify the initially liquid
catalyst, and compare the dynamics between VLS and VSS growth observed at
slightly different temperatures. In the second experiment we exploit thermal
hysteresis of the system to obtain both VLS and VSS at the same temperature.
The VSS growth rate is comparable or slightly slower than VLS growth. Unlike in
the VLS case, during VSS growth we see several occasions where a new layer
starts before the previous layer is completely grown, i.e. multilayer growth.
Understanding the VSS growth mode enables better control of nanowire properties
by widening the range of usable nanowire growth parameters
Tuning the two-electron hybridization and spin states in parallel-coupled InAs quantum dots
We study spin transport in the one- and two-electron regimes of
parallel-coupled double quantum dots (DQDs). The DQDs are formed in InAs
nanowires by a combination of crystal-phase engineering and electrostatic
gating, with an interdot tunnel coupling () tunable by one order of
magnitude. Large single-particle energy separations (up to 10 meV) and
factors (10) enable detailed studies of the -field-induced transition
from a singlet-to-triplet ground state as a function of . In particular, we
investigate how the magnitude of the spin-orbit-induced singlet-triplet
anticrossing depends on . For cases of strong coupling, we find values of
230 eV for the anticrossing using excited-state spectroscopy. Experimental
results are reproduced by calculations based on rate equations and a DQD model
including a single orbital in each dot.Comment: 5 pages, 4 figure
Parameter space mapping of InAs nanowire crystal structure
Crystal structure and defects have been shown to have a strong impact on III-Vnanowire properties. Recently, it was demonstrated that the issue of random stacking and polytypism in semiconductornanowires can often be controlled using accessible growth parameters (such as temperature, diameter, and V/III ratio). In addition, it has been shown that crystal phase can be tuned selectively between cubic zinc blende and hexagonal wurtzite within individual nanowires of III-V materials such as InAs. In order for such results to be generally applied to different growth setups, it is necessary to fully explore and understand the trends governing crystal phase dependencies on all accessible growth parameters, including how they relate to each other. In this study, the authors have systematically investigated the influence of temperature, diameter, V/III ratio, and total mass flow on the crystal structure of InAsnanowiresgrown by metal-organic vapor phase epitaxy over a broad parameter range. The authors observed that each of these accessible parameters can affect the resulting crystal structure, and that the trends for each parameter are affected by the magnitude of the others. The authors also noted that most of the parameter dependencies are nonlinear and, in fact, exhibit threshold values at which structure changes discontinuously. By optimizing each of the growth parameters, it is shown that pure ZB or pure WZ phase can be achieved for several different sets of growth conditions. The roles of nucleation kinetics, thermodynamics, and precursor chemistry are also discussed to compare the results to current nanowiregrowth models. The results in this work should facilitate comparison of data and transfer of knowledge between different growth systems and techniques, which, in turn, should lead to greater understanding of polytypism in nanowires and greater control and freedom in nanowire crystal phase engineering.This work was supported by the Nanometer Structure
Consortium at Lund University nmC@LU, the Swedish
Foundation for Strategic Research SSF, the Swedish Research
Council VR, and the Knut and Alice Wallenberg
Foundation
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