Highly Transparent Gatable Superconducting Shadow Junctions

Abstract

Gate-tunable junctions are key elements in quantum devices based on hybrid semiconductor–superconductor materials. They serve multiple purposes ranging from tunnel spectroscopy probes to voltage-controlled qubit operations in gatemon and topological qubits. Common to all is that junction transparency plays a critical role. In this study, we grow single-crystalline InAs, InSb, and InAs1–x Sb x semiconductor nanowires with epitaxial Al, Sn, and Pb superconductors and in situ shadowed junctions in a single-step molecular beam epitaxy process. We investigate correlations between fabrication parameters, junction morphologies, and electronic transport properties of the junctions and show that the examined in situ shadowed junctions are of significantly higher quality than the etched junctions. By varying the edge sharpness of the shadow junctions, we show that the sharpest edges yield the highest junction transparency for all three examined semiconductors. Further, critical supercurrent measurements reveal an extraordinarily high I C R N, close to the KO-2 limit. This study demonstrates a promising engineering path toward reliable gate-tunable superconducting qubits.

Reappearance of first Shapiro step in narrow topological Josephson junctions

“…On the verge in between basic material research and technological implementation, novel fabrication techniques are used to pursue a high device quality with high yield ( 17 , 19 ). One main aspect for superconducting topological hybrid devices is to ensure a highly transparent interface between the topological matter and the superconducting metal deposited.…”

“…One main aspect for superconducting topological hybrid devices is to ensure a highly transparent interface between the topological matter and the superconducting metal deposited. Complex in situ deposition techniques have been developed in these regards, which have been reported to lead to contaminant-free interfaces, high transparencies, and a strong proximity coupling ( 17 , 19 ). We recently reported on molecular beam epitaxy (MBE)–grown, in situ defined TI Josephson junctions on ternary compound (Bi 0.06 Sb 0.94 ) 2 Te 3 (BST) thin films ( 17 ).…”

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

“…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.…”