Publications by Thomas Sand Jespersen
- 2022
-
Doubling the mobility of InAs/InGaAs selective area grown nanowires -
Abstract
- Selective area growth (SAG) of nanowires and networks promise a route toward scalable electronics, photonics and quantum devices based on III-V semiconductor materials. The potential of high-mobility SAG nanowires however is not yet fully realized, since interfacial roughness, misfit dislocations at the nanowire/substrate interface and non-uniform composition due to material intermixing all scatter electrons. Here, we explore SAG of highly lattice-mismatched InAs nanowires on insulating GaAs(001) substrates and address these key challenges. Atomically smooth nanowire/substrate interfaces are achieved with the use of atomic hydrogen (a-H) as an alternative to conventional thermal annealing for the native oxide removal. The problem of high lattice mismatch is addressed through an In$_x$Ga$_{1-x}$As buffer layer introduced between the InAs transport channel and the GaAs substrate. The Ga-In material intermixing observed in both the buffer layer and the channel is inhibited via careful tuning of the growth temperature. Performing scanning transmission electron microscopy and x-ray diffraction analysis along with low-temperature transport measurements we show that optimized In-rich buffer layers promote high quality InAs transport channels with the field-effect electron mobility over~10000~cm$^2$V$^{-1}$s$^{-1}$. This is twice as high as for non-optimized samples and among the highest reported for InAs selective area grown nanostructures.
-
Doubling the mobility of InAs/InGaAs selective area grown nanowires -
Abstract
- 2021
-
Andreev interference in the surface accumulation layer of half-shell
InAsSb/Al hybrid nanowires -
Abstract
- 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 InAsSb/Al nanowires was 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, exhibited 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 was tunable by a gate potential as expected from electrostatic models.
Lukas Stampfer, Damon J. Carrad, Dags Olsteins, Christian E. N. Petersen, Sabbir A. Khan, Peter Krogstrup, Thomas S. Jespersen [pdf]
-
Andreev interference in the surface accumulation layer of half-shell
InAsSb/Al hybrid nanowires -
Abstract
- 2020
-
Shadow Epitaxy for In Situ Growth of Generic Semiconductor/Superconductor Hybrids -
Abstract
- 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/aluminium heterostructures. Fabrication of devices requires selective etch processes; these exist only for InAs/Al hybrids, precluding the use of other, potentially superior material combinations. We present a crystal growth platform -- based on three-dimensional structuring of growth substrates -- which enables synthesis of semiconductor nanowire hybrids with in-situ patterned superconductor shells. This platform eliminates the need for etching, thereby enabling full freedom in choice of hybrid constituents. We realise and characterise all the most frequently used architectures in superconducting hybrid devices, finding increased yield and electrostatic stability compared to etched devices, along with evidence of ballistic superconductivity. In addition to aluminium, we present hybrid devices based on tantalum, niobium and vanadium. This is the submitted version of the manuscript. The accepted, peer reviewed version is available from Advanced Materials: http://doi.org/10.1002/adma.201908411 Previous title: Shadow lithography for in-situ growth of generic semiconductor/superconductor devices
Damon J. Carrad, Martin Bjergfelt, Thomas Kanne, Martin Aagesen, Filip Krizek, Elisabetta M. Fiordaliso, Erik Johnson, Jesper Nygård, Thomas Sand Jespersen Journal reference: Advanced Materials (2020) 1908411 [pdf] DOI: 10.1002/adma.201908411
-
Shadow Epitaxy for In Situ Growth of Generic Semiconductor/Superconductor Hybrids -
Abstract