Statistical Reproducibility of Selective Area Grown InAs Nanowire Devices.

New approaches such as selective area growth (SAG), where crystal growth is lithographically controlled, allow the integration of bottom-up grown semiconductor nanomaterials in large-scale classical and quantum nanoelectronics. This calls for assessment and optimization of the reproducibility between individual components. We quantify the structural and electronic statistical reproducibility within large arrays of nominally identical selective area growth InAs nanowires. The distribution of structural parameters is acquired through comprehensive atomic force microscopy studies and transmission electron microscopy. These are compared to the statistical distributions of the cryogenic electrical properties of 256 individual SAG nanowire field effect transistors addressed using cryogenic multiplexer circuits. Correlating measurements between successive thermal cycles allows distinguishing between the contributions of surface impurity scattering and fixed structural properties to device reproducibility. The results confirm the potential of SAG nanomaterials, and the methodologies for quantifying statistical metrics are essential for further optimization of reproducibility.

Cryogenic multiplexing using selective area grown nanowires.

Bottom-up grown nanomaterials play an integral role in the development of quantum technologies but are often challenging to characterise on large scales. Here, we harness selective area growth of semiconductor nanowires to demonstrate large-scale integrated circuits and characterisation of large numbers of quantum devices. The circuit consisted of 512 quantum devices embedded within multiplexer/demultiplexer pairs, incorporating thousands of interconnected selective area growth nanowires operating under deep cryogenic conditions. Multiplexers enable a range of new strategies in quantum device research and scaling by increasing the device count while limiting the number of connections between room-temperature control electronics and the cryogenic samples. As an example of this potential we perform a statistical characterization of large arrays of identical quantum dots thus establishing the feasibility of applying cross-bar gating strategies for efficient scaling of future selective area growth quantum circuits. More broadly, the ability to systematically characterise large numbers of devices provides new levels of statistical certainty to materials/device development.

InAs/MoRe Hybrid Semiconductor/Superconductor Nanowire Devices.

Implementing superconductors capable of proximity-inducing a large energy gap in semiconductors in the presence of strong magnetic fields is a major goal toward applications of semiconductor/superconductor hybrid materials in future quantum information technologies. Here, we study the performance of devices consisting of InAs nanowires in electrical contact with molybdenum-rhenium (MoRe) superconducting alloys. The MoRe thin films exhibit transition temperatures of ∼10 K and critical fields exceeding 6 T. Normal/superconductor devices enabled tunnel spectroscopy of the corresponding induced superconductivity, which was maintained up to ∼10 K, and MoRe-based Josephson devices exhibited supercurrents and multiple Andreev reflections. We determine an induced superconducting gap lower than expected from the transition temperature and observe gap softening at finite magnetic field. These may be common features for hybrids based on large-gap, type II superconductors. The results encourage further development of MoRe-based hybrids.

Andreev Interference in the Surface Accumulation Layer of Half-Shell InAsSb/Al Hybrid Nanowires.

Understanding the spatial distribution of charge carriers in III-V nanowires proximity coupled to superconductors is important for the design and interpretation of experiments based on hybrid quantum devices. In this letter, the gate-dependent surface accumulation layer of half-shell InAsSb/Al nanowires is studied by means of Andreev interference in a parallel magnetic field. Both uniform hybrid nanowires and devices featuring a short Josephson junction fabricated by shadow lithography, exhibit periodic modulation of the switching current. The period corresponds to a flux quantum through the nanowire diameter and is consistent with Andreev bound states occupying a cylindrical surface accumulation layer. The spatial distribution is tunable by a gate potential as expected from electrostatic models.

Superconductivity and Parity Preservation in As-Grown In Islands on InAs Nanowires.

We report in situ synthesis of crystalline indium islands on InAs nanowires grown by molecular beam epitaxy. Structural analysis by transmission electron microscopy showed that In crystals grew in a tetragonal body-centered crystal structure within two families of orientations relative to wurtzite InAs. The crystalline islands had lengths < 500 nm and low-energy surfaces, suggesting that growth was driven mainly by surface energy minimization. Electrical transport through In/InAs devices exhibited Cooper pair charging, evidencing charge parity preservation and a pristine In/InAs interface, with an induced superconducting gap ∼ 0.45 meV. Cooper pair charging persisted to temperatures > 1.2 K and magnetic fields ∼ 0.7 T, demonstrating that In/InAs hybrids belong to an expanding class of semiconductor/superconductor hybrids operating over a wider parameter space than state-of-the-art Al-based hybrids. Engineering crystal morphology while isolating single islands using shadow epitaxy provides an interesting alternative to previous semiconductor/superconductor hybrid morphologies and device geometries.

Epitaxial Pb on InAs nanowires for quantum devices.

Semiconductor-superconductor hybrids are widely used to realize complex quantum phenomena, such as topological superconductivity and spins coupled to Cooper pairs. Accessing new, exotic regimes at high magnetic fields and increasing operating temperatures beyond the state-of-the-art requires new, epitaxially matched semiconductor-superconductor materials. One challenge is the generation of favourable conditions for heterostructural formation between materials with the desired properties. Here we harness an increased knowledge of metal-on-semiconductor growth to develop InAs nanowires with epitaxially matched, single-crystal, atomically flat Pb films with no axial grain boundaries. These highly ordered heterostructures have a critical temperature of 7 K and a superconducting gap of 1.25 meV, which remains hard at 8.5 T, and therefore they offer a parameter space more than twice as large as those of alternative semiconductor-superconductor hybrids. Additionally, InAs/Pb island devices exhibit magnetic field-driven transitions from a Cooper pair to single-electron charging, a prerequisite for use in topological quantum computation. Semiconductor-Pb hybrids potentially enable access to entirely new regimes for a number of different quantum systems.

Shadow Epitaxy for In Situ Growth of Generic Semiconductor/Superconductor Hybrids.

Uniform, defect-free crystal interfaces and surfaces are crucial ingredients for realizing high-performance nanoscale devices. A pertinent example is that advances in gate-tunable and topological superconductivity using semiconductor/superconductor electronic devices are currently built on the hard proximity-induced superconducting gap obtained from epitaxial indium arsenide/aluminum heterostructures. Fabrication of devices requires selective etch processes; these exist only for InAs/Al hybrids, precluding the use of other, potentially superior material combinations. This work introduces a crystal growth platform-based on 3D structuring of growth substrates-which enables synthesis of semiconductor nanowire hybrids with in situ patterned superconductor shells. The platform eliminates the need for etching, thereby enabling full freedom in the choice of hybrid constituents. All of the most frequently used superconducting hybrid device architectures are realized and characterized. These devices exhibit increased yield and electrostatic stability compared to etched devices, and evidence of ballistic superconductivity is observed. In addition to aluminum, hybrid structures based on tantalum, niobium, and vanadium are presented.

Quantum-Confinement-Enhanced Thermoelectric Properties in Modulation-Doped GaAs-AlGaAs Core-Shell Nanowires.

Nanowires (NWs) hold great potential in advanced thermoelectrics due to their reduced dimensions and low-dimensional electronic character. However, unfavorable links between electrical and thermal conductivity in state-of-the-art unpassivated NWs have, so far, prevented the full exploitation of their distinct advantages. A promising model system for a surface-passivated one-dimensional (1D)-quantum confined NW thermoelectric is developed that enables simultaneously the observation of enhanced thermopower via quantum oscillations in the thermoelectric transport and a strong reduction in thermal conductivity induced by the core-shell heterostructure. High-mobility modulation-doped GaAs/AlGaAs core-shell NWs with thin (sub-40 nm) GaAs NW core channel are employed, where the electrical and thermoelectric transport is characterized on the same exact 1D-channel. 1D-sub-band transport at low temperature is verified by a discrete stepwise increase in the conductance, which coincided with strong oscillations in the corresponding Seebeck voltage that decay with increasing sub-band number. Peak Seebeck coefficients as high as ≈65-85 µV K-1 are observed for the lowest sub-bands, resulting in equivalent thermopower of S2 σ ≈ 60 µW m-1 K-2 and S2 G ≈ 0.06 pW K-2 within a single sub-band. Remarkably, these core-shell NW heterostructures also exhibit thermal conductivities as low as ≈3 W m-1 K-1 , about one order of magnitude lower than state-of-the-art unpassivated GaAs NWs.

Superconducting vanadium/indium-arsenide hybrid nanowires.

We report MBE synthesis of InAs/vanadium hybrid nanowires. The vanadium was deposited without breaking ultra-high vacuum after InAs nanowire growth, minimizing any effect of oxidation and contamination at the interface between the two materials. We investigated four different substrate temperatures during vanadium deposition, ranging from -150 °C to 250 °C. The structural relation between vanadium and InAs depended on the deposition temperature. The three lower temperature depositions gave vanadium shells with a polycrystalline, granular morphology and the highest temperature resulted in vanadium reacting with the InAs nanowire. We fabricated electronic devices from the hybrid nanowires and obtained a high out-of-plane critical magnetic field, exceeding the bulk value for vanadium. However, size effects arising from the nanoscale grains resulted in the absence of a well-defined critical temperature, as well as device-to-device variation in the resistivity versus temperature dependence during the transition to the superconducting state.

Quantum Transport and Sub-Band Structure of Modulation-Doped GaAs/AlAs Core-Superlattice Nanowires.

Modulation-doped III-V semiconductor nanowire (NW) heterostructures have recently emerged as promising candidates to host high-mobility electron channels for future high-frequency, low-energy transistor technologies. The one-dimensional geometry of NWs also makes them attractive for studying quantum confinement effects. Here, we report correlated investigations into the discrete electronic sub-band structure of confined electrons in the channel of Si δ-doped GaAs-GaAs/AlAs core-superlattice NW heterostructures and the associated signatures in low-temperature transport. On the basis of accurate structural and dopant analysis using scanning transmission electron microscopy and atom probe tomography, we calculated the sub-band structure of electrons confined in the NW core and employ a labeling system inspired by atomic orbital notation. Electron transport measurements on top-gated NW transistors at cryogenic temperatures revealed signatures consistent with the depopulation of the quasi-one-dimensional sub-bands, as well as confinement in zero-dimensional-like states due to an impurity-defined background disorder potential. These findings are instructive toward reaching the ballistic transport regime in GaAs-AlGaAs based NW systems.

A conducting polymer with enhanced electronic stability applied in cardiac models.

Electrically active constructs can have a beneficial effect on electroresponsive tissues, such as the brain, heart, and nervous system. Conducting polymers (CPs) are being considered as components of these constructs because of their intrinsic electroactive and flexible nature. However, their clinical application has been largely hampered by their short operational time due to a decrease in their electronic properties. We show that, by immobilizing the dopant in the conductive scaffold, we can prevent its electric deterioration. We grew polyaniline (PANI) doped with phytic acid on the surface of a chitosan film. The strong chelation between phytic acid and chitosan led to a conductive patch with retained electroactivity, low surface resistivity (35.85 ± 9.40 kilohms per square), and oxidized form after 2 weeks of incubation in physiological medium. Ex vivo experiments revealed that the conductive nature of the patch has an immediate effect on the electrophysiology of the heart. Preliminary in vivo experiments showed that the conductive patch does not induce proarrhythmogenic activities in the heart. Our findings set the foundation for the design of electronically stable CP-based scaffolds. This provides a robust conductive system that could be used at the interface with electroresponsive tissue to better understand the interaction and effect of these materials on the electrophysiology of these tissues.

Electron-beam patterning of polymer electrolyte films to make multiple nanoscale gates for nanowire transistors.

We report an electron-beam based method for the nanoscale patterning of the poly(ethylene oxide)/LiClO4 polymer electrolyte. We use the patterned polymer electrolyte as a high capacitance gate dielectric in single nanowire transistors and obtain subthreshold swings comparable to conventional metal/oxide wrap-gated nanowire transistors. Patterning eliminates gate/contact overlap, which reduces parasitic effects and enables multiple, independently controllable gates. The method's simplicity broadens the scope for using polymer electrolyte gating in studies of nanowires and other nanoscale devices.