Engineering hybrid epitaxial InAsSb/Al nanowires for stronger topological protection

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Engineering hybrid epitaxial InAsSb/Al nanowires for stronger topological protection

Joachim E. Sestoft, Thomas Kanne, Aske Nørskov Gejl, Merlin von Soosten, Jeremy S. Yodh, Daniel Sherman, Brian Tarasinski, Michael Wimmer, Erik Johnson, Mingtang Deng, Jesper Nygård, Thomas Sand Jespersen, Charles M. Marcus, and Peter Krogstrup

Phys. Rev. Materials 2, 044202 – Published 12 April 2018

Abstract

The combination of strong spin-orbit coupling, large $g$ factors, and the coupling to a superconductor can be used to create a topologically protected state in a semiconductor nanowire. Here we report on growth and characterization of hybrid epitaxial InAsSb/Al nanowires, with varying composition and crystal structure. We find the strongest spin-orbit interaction at intermediate compositions in zinc-blende ${InAs}_{1 - x} {Sb}_{x}$ nanowires, exceeding that of both InAs and InSb materials, confirming recent theoretical studies. We show that the epitaxial InAsSb/Al interface allows for a hard induced superconducting gap and $2 e$ transport in Coulomb charging experiments, similarly to experiments on InAs/Al and InSb/Al materials, and find measurements consistent with topological phase transitions at low magnetic fields due to large effective $g$ factors. Finally we present a method to grow pure wurtzite InAsSb nanowires which are predicted to exhibit even stronger spin-orbit coupling than the zinc-blende structure.

Received 29 December 2017

DOI:https://doi.org/10.1103/PhysRevMaterials.2.044202

Physics Subject Headings (PhySH)

Chemical bonding Composition Proximity effect Semiconductors Superconductivity Topological materials Topological quantum computing

Condensed Matter, Materials & Applied PhysicsQuantum Information, Science & Technology

Authors & Affiliations

Joachim E. Sestoft¹, Thomas Kanne¹, Aske Nørskov Gejl¹, Merlin von Soosten², Jeremy S. Yodh¹, Daniel Sherman¹, Brian Tarasinski³, Michael Wimmer³, Erik Johnson^2,4, Mingtang Deng¹, Jesper Nygård², Thomas Sand Jespersen², Charles M. Marcus¹, and Peter Krogstrup^1,*

¹Center for Quantum Devices and Station Q Copenhagen, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark
²Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark
³QuTech and Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands
⁴Department of Wind Energy, Technical University of Denmark, Risø Campus, 4000 Roskilde, Denmark

^*[email protected]

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Issue

Vol. 2, Iss. 4 — April 2018

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Images

Figure 1
(a) Sb concentration, $x$ , as a function of NW length, $L$ , with a steplike profile of the As/Sb molar fraction along the NW. The inset shows the NWs as grown on the substrate. (b) False-colored scanning electron micrograph of a typical device. Yellow, Ti/Au contacts and gates; gray, ${InAs}_{1 - x} {Sb}_{x}$ NW; $V_{SD}$ , the applied voltage bias; $I$ , measured current; $V_{G}$ , gate voltage controlling the chemical potential on individual segments. (c) Trace/retrace (black/gray) of differential conductance for $x = 0.18$ as a function of $V_{G}$ showing aperiodic reproducible fluctuation with amplitude $\sim e^{2} / h$ . (d) Averaged differential conductance change, $〈 Δ g 〉$ , as a function of magnetic field showing suppression of the weak antilocalization effect around $B = 0$ for all concentrations. Red overlay is the fit from Eq. (1), from which $l_{SO}$ is extracted. (e) Spin-orbit length (left $y$ axis) as a function of Sb concentration. A noticeable minimum in $l_{SO}$ is observed around $x \sim 0.5$ , for the three different gate averaging amplitudes, $V_{avg} = 4$ V, 6 V, and 8 V. The Rashba coefficient (right $y$ axis) is a conversion of the average $l_{SO}$ displaying a qualitative maximum around $x \sim 0.5$ .
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Figure 2
(a) $7^{\circ}$ rotation between the $[1 \bar{1} 0]$ and the $[11 \bar{2}]$ directions in the semiconductor and the superconductor at $x = 0.34$ , respectively. Scale bar is the same for (a)–(c). (b) Rotation between the two $[11 \bar{2}]$ directions for $x = 0.64$ is $6^{\circ}$ . (c) No observed rotation between the $[1 \bar{1} 0]$ and [111] for $x = 0.84$ . (d) Bicrystal Burger circuit on a long segment of an interface for $x = 0.34$ showing two edge dislocations with respect to 3:2 domain match. Color bar shows relative bending of the interplane distances with respect to a reference region away from the interface. Blue is upwards bending; red is downwards bending. (e), (f) The two dislocations from (d).
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Figure 3
(a) False-colored scanning electron micrograph of a NIS device. (b) Differential conductance as a function of $V_{SD}$ plotted on a linear (blue) and logarithmic scale (red) showing a highly suppressed differential conductance within $Δ = \pm 230 μ eV$ . (c) Conductance as a function of $V_{SD}$ and $V_{G}$ showing two symmetric ABS within $Δ \sim 230 μ eV$ at two different gate configurations. Unbroken and dashed lines indicate the gate configuration for (d) and (e), respectively. (d), (e) Conductance as a function of $V_{SD}$ and $B_{∥}$ showing the magnetic field evolution of two ABS that merge and form zero-bias peaks at $\sim 350$ and 400 mT, respectively. (f) Conductance as a function of $V_{SD}$ corresponding to line cuts (blue and red squares) seen in (d); shows the zero-bias peak with strongly suppressed differential conductance symmetrically around the peak, signature of a hard topological gap.
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Figure 4
(a) False-colored SEM image of the measured NISIN device. (b) Cartoon of the cross sections indicated in (a) by three colored dashed lines illustrates the top-gate material sequencing. (c) Differential conductance, $g$ , as a function of $V_{G}$ and $V_{SD}$ showing $2 e$ -spaced Coulomb diamonds within $Δ \sim 0.15$ meV. For $V_{SD} > 0.15$ mV, $1 e$ periodic Coulomb resonances are evident, verifying Cooper pair tunneling within $V_{SD} \sim \pm 0.15$ mV. (d) Differential conductance at zero bias as a function of $V_{G}$ and $B_{∥}$ . Splitting from $2 e$ to $1 e$ initiates at $B^{*} = B_{∥} = 150$ mT and is evenly $1 e$ spaced at $B^{* *} = B_{∥} = 250$ mT, suggesting a smooth transition from $2 e$ to $1 e$ .
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Figure 5
(a) Top-view SEM micrograph of the NWs grown in the six directions parallel to the substrate. Inset show a $45^{\circ}$ tilted SEM micrograph of three kinked structures. (b) HR-TEM micrograph of the InAsSb/Al interface shows a coherent epitaxial crystal match. The black and white arrows indicate the orientations of the hybrid system, where a 3:2 domain match with no rotation is observed. (c) False-colored SEM micrograph of a NIS device composed of normal-metal leads connected to an ${InAs}_{0.7} {Sb}_{0.3}$ WZ NW grown along the $[1 \bar{1} 00]$ direction. Inset shows normalized EDX intensities for In, As, and Sb from a region of the NW shown by the white box. (d) Differential conductance as a function of $V_{SD}$ plotted on a linear (red trace) and logarithmic scale (blue trace) showing a superconducting hard gap with highly suppressed differential conductance within $Δ = \pm 225 μ eV$ .
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