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

Abstract

The quality of interfaces and surfaces is crucial for the performance of nanoscale devices. A pertinent example is the close tie between current progress in gate-tunable and topological superconductivity using semiconductor/superconductor electronic devices and the hard proximity-induced superconducting gap obtained from epitaxial indium arsenide/aluminium heterostructures. Fabrication of devices requires selective etch processes; these only exist for InAs/Al hybrids, which precludes the use of other, potentially better material combinations in functional devices. We present a crystal growth platform based on three-dimensional structuring of growth substrates for synthesising semiconductor nanowires with in-situ patterned superconductor shells, which enables independent choice of material by eliminating etching. 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.One dimensional semiconductor (SE) nanowires (NWs) proximity coupled to superconductors (SU) have attracted considerable attention from the condensed matter community since the prediction 1,2 and observation of Majorana zero-modes 3-5 , which have been proposed as a basis for topologically protected quantum information processors 6,7 . To ensure topological protection, methods for growing disorder-free 'hard-gap' SE/SU epitaxial hybrids were developed 8-10 . These materials utilise bottom-up crystal growth of InAs nanowires with uniform epitaxial aluminium coatings, an approach which has been extended to high mobility two-dimensional systems 11,12 and selective area grown networks 13,14 . The success of epitaxial InAs/Al hybrids lies in the ability to realise important device classes such as normal metal spectroscopic devices, 5,9,11,12 Josephson Junctions 15-18 for gate-controlled transmon qubits 19,20 , and superconducting Majorana islands 21-23 , using top-down processing to selectively remove the Al. A limitation of this method is that relying on post-process etching inherently limits materials choice. For instance, despite strong incentives to utilise technologically important superconductors such as Nb 24 and NbTiN 25 -which exhibit higher transition temperatures, critical magnetic fields and superconducting energy gaps -selectively removing Nb from InAs remains an unsolved problem. Similarly, InSb is an attractive semiconductor due to its high mobility, g-factor and strong spin-orbit coupling 25-28 . Yet, selectively removing even aluminum from InSb without damage is impossible with known methods. Thus, most potential improvements in epitaxial SE/SU technology are predicated on developing a materials-independent method for device fabrication. An attractive approach to eliminate etching is to employ an in-situ 'shadow approach' to mask specific segments along the NW from supe...

Multiterminal Quantized Conductance in InSb Nanocrosses

“…Finally, we demonstrate a new concept for merging the NC geometry with pre-defined substrate structures to enable in situ patterning of epitaxial superconductors, which have been shown to substantially enhance the performance of hybrid devices. [22][23][24]…”

“…[8] However the fragility of InSb is a challenge for devices based on InSb/Al epitaxial hybrids as all known etchings of Al also severely damages the InSb semiconductor and degrade device performance. Recently, in situ "shadow" approaches [18,[21][22][23][24] have been developed allowing patterning of epitaxial superconductor growth, yielding reproducible transport characteristics and observations of ballistic transport at B = 0 T for single NWs. These in situ shadow concepts, developed for single NWs, are however, incompatible with the NC geometry, where controlled shadows of the central junction region are required for most applications.…”

Fully in situ Nb/InAs-nanowire Josephson junctions by selective-area growth and shadow evaporation

“…Thus, for example, aluminium with a small superconducting gap and a small critical magnetic eld but large superconducting coherence length is a common choice. 1,13,16 For higher temperatures or higher magnetic elds operation, superconductors such as Nb and its alloys, 3,15,17,18 Pb, 19,20 or V, 21,22 are used. However, detailed analysis of these semiconductor/superconductor systems are just starting to emerge.…”

“…A successful method to circumvent these problems is the in situ deposition of the superconducting material on the semiconductor nanowires. 18,[22][23][24] Another important issue is the residual material le on the structure from wet-chemical etching technique, that is normally used for the fabrication of Josephson junctions with a small gap. However, recently it was demonstrated that closely separated superconducting electrodes can also be achieved by shadow evaporation technique, i.e.…”

“…either by using a nanowire which crosses another [25][26][27] or using a patterned, suspended SiO 2 layer as a shadow or stencil mask. 18,22 All of these methods, have a common trait to potentially reach the so-called short channel limit, in which the current through the device is only carried by a single Andreev bound state. Such mesoscopic junctions are one of the main requirements for topological systems based on Majorana fermions.…”