arXiv subjects

Kimberly A. Dick

Publications and source records attributed to Kimberly A. Dick.

33 records · Page 2Linked to original sources

arXiv2021

Symmetry-controlled singlet-triplet transition in a double-barrier quantum ring

We engineer a system of two strongly confined quantum dots to gain reproducible electrostatic control of the spin at zero magnetic field. Coupling the dots in a tight ring-shaped potential with two tunnel barriers, we demonstrate that an electric field can switch the electron ground state between a singlet and a triplet configuration. Comparing our experimental co-tunneling spectroscopy data to a full many-body treatment of interacting electrons in a double-barrier quantum ring, we find excellent agreement in the evolution of many-body states with electric and magnetic fields. The calculations show that the singlet-triplet energy crossover, not found in conventionally coupled quantum dots, is made possible by the ring-shaped geometry of the confining potential.

cond-mat.mes-hall↗

arXiv2021

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.

cond-mat.mes-hall↗

arXiv2021

Electronic band structure of narrow-bandgap p-n nanojunctions in heterostructured nanowires measured by electron energy loss spectroscopy

The electronic band structure of complex nanostructured semiconductors has a considerable effect on the final electronic and optical properties of the material and, ultimately, on the functionality of the devices incorporating them. Valence electron energy-loss spectroscopy (VEELS) in the transmission electron microscope (TEM) provides the possibility of measuring this property of semiconductors with high spatial resolution. However, it still represents a challenge for narrow-bandgap semiconductors, since an electron beam with low energy spread is required. Here we demonstrate that by means of monochromated VEELS we can study the electronic band structure of narrow-gap materials GaSb and InAs in the form of heterostructured nanowires, with bandgap values down to 0.5 eV, especially important for newly developed structures with unknown bandgaps. Using complex heterostructured InAs-GaSb nanowires, we determine a bandgap value of 0.54 eV for wurtzite InAs. Moreover, we directly compare the bandgaps of wurtzite and zinc-blende polytypes of GaSb in a single nanostructure, measured here as 0.84 and 0.75 eV, respectively. This allows us to solve an existing controversy in the band alignment between these structures arising from theoretical predictions. The findings demonstrate the potential of monochromated VEELS to provide a better understanding of the band alignment at the heterointerfaces of narrow-bandgap complex nanostructured materials with high spatial resolution. This is especially important for semiconductor device applications where even the slightest variations of the electronic band structure at the nanoscale can play a crucial role in their functionality.

cond-mat.mtrl-sci↗

arXiv2020

Selective tuning of spin-orbital Kondo contributions in parallel-coupled quantum dots

We use co-tunneling spectroscopy to investigate spin-, orbital-, and spin-orbital Kondo transport in a strongly confined system of InAs double quantum dots (QDs) parallel-coupled to source and drain. In the one-electron transport regime, the higher symmetry spin-orbital Kondo effect manifests at orbital degeneracy and no external magnetic field. We then proceed to show that the individual Kondo contributions can be isolated and studied separately; either by orbital detuning in the case of spin-Kondo transport, or by spin splitting in the case of orbital Kondo transport. By varying the inter-dot tunnel coupling, we show that lifting of the spin degeneracy is key to confirming the presence of an orbital degeneracy, and to detecting a small orbital hybridization gap. Finally, in the two-electron regime, we show that the presence of a spin-triplet ground state results in spin-Kondo transport at zero magnetic field.

cond-mat.mes-hall↗

arXiv2019

Independent control of nucleation and layer growth in nanowires

Control of the crystallization process is central to developing novel materials with atomic precision to meet the demands of electronic and quantum technology applications. Semiconductor nanowires grown by the vapor-liquid-solid process are a promising material system in which the ability to form components with structure and composition not achievable in bulk is well-established. Here we use in situ TEM imaging of GaAs nanowire growth to understand the processes by which the growth dynamics are connected to the experimental parameters. We find that two sequential steps in the crystallization process - nucleation and layer growth - can occur on similar time scales and can be controlled independently using different growth parameters. Importantly, the layer growth process contributes significantly to the growth time for all conditions, and will play a major role in determining material properties. The results are understood through theoretical simulations correlating the growth dynamics, liquid droplet and experimental parameters.

physics.app-ph↗

arXiv2019

Limits of III-V nanowire growth based on particle dynamics

Crystal growth of semiconductor nanowires from a liquid droplet depends on the stability of this droplet at the liquid-solid interface. By combining in-situ transmission electron microscopy with theoretical analysis of the surface energies involved, we show that truncation of the interface can increase the stability of the droplet, which in turn increases the range of parameters for which successful nanowire growth is possible. In addition to determining the limits of nanowire growth, this approach allows us to experimentally estimate relevant surface energies, such as the GaAs $\{11\bar{2}0\}$ facet.

physics.app-ph↗

arXiv2019

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.

cond-mat.mes-hall↗

arXiv2019

In situ analysis of catalyst composition during gold catalyzed GaAs nanowire growth

Semiconductor nanowires offer the opportunity to incorporate novel structures and functionality into electronic and optoelectronic devices. A clear understanding of the nanowire growth mechanism is essential for well-controlled growth of structures with desired properties, but the understanding is currently limited by a lack of empirical measurements of important parameters during growth, such as catalyst particle composition. However, this is difficult to accurately determine by investigating post-growth. We report direct measurement of the catalyst composition of individual gold seeded GaAs nanowires inside an electron microscope as they grow. The Ga content in the catalyst during growth increased with both temperature and Ga precursor flux. A direct comparison of the calculated phase diagrams of the Au-Ga-As ternary system to the measured catalyst composition not only lets us estimate the As content in the catalyst but also indicates the relevance of phase diagrams to understanding nanowire growth.

physics.app-ph↗

arXiv2019

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 ($t$) tunable by one order of magnitude. Large single-particle energy separations (up to 10 meV) and $|g^*|$ factors ($\sim$10) enable detailed studies of the $B$-field-induced transition from a singlet-to-triplet ground state as a function of $t$. In particular, we investigate how the magnitude of the spin-orbit-induced singlet-triplet anticrossing depends on $t$. For cases of strong coupling, we find values of 230 $μ$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.

cond-mat.mes-hall↗

arXiv2019

Individually addressable double quantum dots formed with nanowire polytypes and identified by epitaxial markers

Double quantum dots (DQDs) hold great promise as building blocks for quantum technology as they allow for two electronic states to coherently couple. Defining QDs with materials rather than using electrostatic gating allows for QDs with a hard-wall confinement potential and more robust charge and spin states. An unresolved problem is how to individually address these quantum dots, which is necessary for controlling quantum states. We here report the fabrication of double quantum dot devices defined by the conduction band edge offset at the interface of the wurtzite and zinc blende crystal phases of InAs in nanowires. By using sacrifical epitaxial GaSb markers selectively forming on one crystal phase, we are able to precisely align gate electrodes allowing us to probe and control each QD independently. We hence observe textbook-like charge stability diagrams, a discrete energy spectrum and electron numbers consistent with theoretical estimates and investigate the tunability of the devices, finding that changing the electron number can be used to tune the tunnel barrier as expected by simple band diagram arguments.

cond-mat.mes-hall↗

arXiv2019

Spatial Control of Multiphoton Electron Excitations in InAs Nanowires by Varying Crystal Phase and Light Polarization

We demonstrate the control of multiphoton electron excitations in InAs nanowires (NWs) by altering the crystal structure and the light polarization. Using few-cycle, near-infrared laser pulses from an optical parametric chirped-pulse amplification system, we induce multiphoton electron excitations in InAs nanowires with controlled wurtzite (WZ) and zincblende (ZB) segments. With a photoemission electron microscope, we show that we can selectively induce multiphoton electron emission from WZ or ZB segments of the same wire by varying the light polarization. Developing \textit{ab-initio GW} calculations of 1st to 3rd order multiphoton excitations and using finite-difference time-domain simulations, we explain the experimental findings: While the electric-field enhancement due to the semiconductor/vacuum interface has a similar effect for all NW segments, the 2nd and 3rd order multiphoton transitions in the band structure of WZ InAs are highly anisotropic, in contrast to ZB InAs. As the crystal phase of NWs can be precisely and reliably tailored, our findings opens up for new semiconductor optoelectronics with controllable nanoscale emission of electrons through vacuum or dielectric barriers.

physics.optics↗

arXiv2018

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.

cond-mat.mes-hall↗

arXiv2017

Annealing of Au, Ag and Au-Ag alloy nanoparticle arrays on GaAs (100) and (111)B

Part of developing new strategies for fabrications of nanowire structures involves in many cases the aid of metal nanoparticles (NPs). It is highly beneficial if one can define both diameter and position of the initial NPs and make well-defined nanowire arrays. This sets additional requirement on the NPs with respect to being able to withstand a pre-growth annealing process (i.e. de- oxidation of the III-V semiconductor surface) in an epitaxy system. Recently, it has been demonstrated that Ag may be an alternative to using Au NPs as seeds for particle-seeded nanowire fabrication. This work brings light onto the effect of annealing of Au, Ag and Au-Ag alloy NP arrays in two commonly used epitaxial systems, the Molecular Beam Epitaxy (MBE) and the Metalorganic Vapor Phase Epitaxy (MOVPE). The NP arrays are fabricated with the aid of Electron Beam Lithography on GaAs 100 and 111B wafers and the evolution of the NPs with respect to shape, size and position on the surfaces are studied after annealing using Scanning Electron Microscopy (SEM). We find that while the Au NP arrays are found to be stable when annealed up to 600 $^{\circ}$C in a MOVPE system, a diameter and pitch dependent splitting of the particles are seen for annealing in a MBE system. The Ag NP arrays are less stable, with smaller diameters ($\leq$ 50 nm) dissolving during annealing in both epitaxial systems. In general, the mobility of the NPs is observed to differ between the two the GaAs 100 and 111B surfaces. While the initial pattern is found be intact on the GaAs 111B surface for a particular annealing process and particle type, the increased mobility of the NP on the 100 may influence the initial pre-defined positions at higher annealing temperatures. The effect of annealing on Au-Ag alloy NP arrays suggests that these NP can withstand necessary annealing conditions for a complete de-oxidation of GaAs surfaces.

cond-mat.mtrl-sci↗

arXiv2016

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.

cond-mat.mes-hall↗

arXiv2016

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.

cond-mat.mes-hall↗