Selective area growth and stencil lithography for in situ fabricated quantum devices

Abstract

ith their experimental verification in 2007, topological insulators (TIs) render a new and fascinating class of materials 1 . A band inversion in the bulk of three-dimensional (3D) TIs creates a 2D metallic subspace at the physical surface of these 3D crystals. The charge carriers of the 2D metal (Dirac electrons) have their spin locked to the momentum, which leads to a topological protection of the subspace 2-4 . This intrinsic quantumspin texture enables the realization of novel technologies, which range from spintronics to quantum computing. Particularly in combination with superconductors (S), TIs promise new quantum devices. Networks of TI nanostructures in proximity to superconductive islands have been predicted to host non-Abelian Majorana modes at the ends and at the crossing points of the networks [5][6][7][8] . Braiding of these elusive modes, that is, exchanging the position of Majorana modes in a 2D plane (Supplementary Fig. 2), resembles topologically protected quantum operations in the Majorana platform. Topological quantum bits (qubits), which use Majorana modes 9,10 to store and process quantum information, are expected to compute fault tolerantly with minimal need for error correction [11][12][13][14] .Topological qubits require high-quality (multi-terminal) Josephson junctions (JJs) 12,15,16 . The simplest type of such a JJ is a two-terminal S-TI-S device (Fig. 1). The Josephson effect 17 allows for an electrical current to conduct dissipationlessly across a lateral junction of two close-by superconductive electrodes separated by a weak link of non-superconductive material. In conventional lateral JJs, the supercurrent is mediated by Andreev bound states (ABS), which effectively transport Cooper pairs across the weak link 18 . In S-TI-S junctions the Dirac system forms a weak link. The quantum spin texture of the Dirac system causes an additional transport channel, known as Majorana bound states (MBS), which adds to conventional ABS 19 . In contrast to ABS, MBS facilitate single-electron transport across the weak link 20 . The contribution of MBS to a supercurrent can be detected via Shapiro response measurements 19,[21][22][23][24] . MBS manifest themselves by a suppression of odd Shapiro steps in low-temperature transport experiments under radio frequency (RF) radiation, due to their 4π-periodic energy-phase dependency 25 .To create and preserve MBS in S-TI-S junctions, the Dirac system in between the superconductive electrodes needs to be conserved (Fig. 1b). Surface oxidation 26,27 and reactions with water molecules at ambient conditions 28 can lead to additional non-topological states at the surface of (Bi,Sb)-based TIs. These superimpose locally with the Dirac system, and thus allow for additional scattering events that could destroy the MBS. To avoid surface degradation in (Bi,Sb)-based TIs, an in situ deposited protective AlO x capping layer on top of the topological surface is often employed 29,30 . Although such capping layers protect the topological surface states for ex situ fabricat...

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

“…Deposition of different materials from the various directions further increases the possible functionality. Shadow epitaxy also applies to planar structures such as selective area grown nanostructures [ 13,14,32 ] or vapor–liquid–solid NWs grown parallel to the substrate [ 29 ] (Section 9, Supporting Information). Finally, we discuss the potential for large‐scale fabrication using vertical device structures in Section 10, Supporting Information.…”

“…Finally, we show that the platform is compatible with both half‐shell and full‐shell hybrid geometries and allows for functional devices to be encapsulated in situ with passivating dielectrics, thus protecting sensitive elements. The flexibility of the shadow structure engineering, the realization of key geometries not possible with previous shadow techniques, [ 29,30,32 ] the demonstrated ability to explore new materials and the significant increase in device performance suggests that shadow epitaxy will become vital to future device optimization. While the focus here is on SE/SU hybrids, shadow epitaxy is equally applicable for any metal/SE/insulator combination, extending the scope to other applications where pristine surfaces and interfaces are key.…”

Multiterminal Josephson Effect

“…On the experimental side, although nanowire network systems proximitized by superconductors have been investigated as a promising platform for topological quantum computation, the experiments are still limited to measuring two-terminal properties [38][39][40][41][42][43]. Multiterminal supercurrents were reported in graphene-based junctions [44], where a heating effect of coexisting disspative currents on supercurrent transport was mainly discussed.…”

Proximity‐Effect‐Induced Superconductivity in Nb/Sb2Te3‐Nanoribbon/Nb Junctions

“…First evidence of the existence of Majorana zero modes in topological material based Josephson junctions have been reported already. [16,[25][26][27] However, there still exist many challenges to realize topological Josephson junctions, therefore, clarifying the physical mechanism of proximity effect induced superconductivity in 3D TI materials is beneficial for advancing the field of topological quantum computing.…”