Shadow-wall lithography of ballistic superconductor–semiconductor quantum devices

Abstract

The realization of hybrid superconductor–semiconductor quantum devices, in particular a topological qubit, calls for advanced techniques to readily and reproducibly engineer induced superconductivity in semiconductor nanowires. Here, we introduce an on-chip fabrication paradigm based on shadow walls that offers substantial advances in device quality and reproducibility. It allows for the implementation of hybrid quantum devices and ultimately topological qubits while eliminating fabrication steps such as lithography and etching. This is critical to preserve the integrity and homogeneity of the fragile hybrid interfaces. The approach simplifies the reproducible fabrication of devices with a hard induced superconducting gap and ballistic normal-/superconductor junctions. Large gate-tunable supercurrents and high-order multiple Andreev reflections manifest the exceptional coherence of the resulting nanowire Josephson junctions. Our approach enables the realization of 3-terminal devices, where zero-bias conductance peaks emerge in a magnetic field concurrently at both boundaries of the one-dimensional hybrids.

Spin-mixing enhanced proximity effect in aluminum-based superconductor-semiconductor hybrids

“…In Figure 3, we show the results of both tunneling spectroscopy and Coulomb blockade measurements on InSb/Al/Pt nanowires. The fabrication follows our shadow-wall lithography method (5,6), of which details can be found in the supplementary materials (24). In Fig.…”

“…On the superconductor side, aluminum has become the material of choice. Thin shells made of this metal combined with an oxide-free interface result in clean electronic transport (5)(6)(7). This includes suppressed sub-gap tunneling conductance (hard induced gap) and parity-conserving transport (8), enabling the search for topological superconductivity.…”

Single-shot fabrication of semiconducting-superconducting nanowire devices

“…Here, we establish a fabrication method based on our previously introduced shadow-wall lithography technique 20 that overcomes these problems and enables the synthesis of high-quality hybrid devices in a single shot. The complete elimination of processing after the formation of the delicate semiconductor-superconductor interface is the critical aspect of our approach.…”

“…It can have vast applications, from sparking rapid exploration of different combinations of semiconductors and superconductors to paving the route to more complex devices. Specifically, we exploit the properties of InSb nanowires and Al films, a combination of materials that is promising for the study of Majorana physics 13,17,20 . Nanowires are typically ∼ 10 µm long 4 , and the Al thin films can be grown homogeneously with thicknesses as low as 5 nm.…”

“…We emphasize that the ad-hoc design of shadow walls and the double-angle evaporation to- gether with the isolation of the bond pads as discussed in ref. 20 allow for fabricating device without creating electrical shorts. Critically, the chip is mounted in a dilution refrigerator within a few hours after the evaporation, preventing the formation of chemical intermixing at the fragile InSb/Al interface.…”