An overview of the results can be divided into 3 material platform categories exploring:
1. single hybrid nanowires grown using VLS growth approach
2. hybrid nanowires grown using novel scalable SAG approach
3. basic interface properties in planar samples (layer by layer)
1) Vapor Liquid Solid (VLS) epitaxy is a useful platform for exploring new material combinations without compromising on quality. Typically, an ‘ex-situ’ lithography step followed by etching is required to pattern the superconductor. With conventional ex-situ/top-down processing it is difficult to avoid etching residues and damaged surfaces, which deteriorates the device’s performance. In our research we contrived a new VLS methodology to create a gate tunable Josephson junction ‘in-situ’ via shadowing (Fig. 1b) which can be sued with a variety of semiconductors e.g. InAs, InSb and InAsxSb1-x and superconductor combinations (Al, Pb, Sn). The research has resulted in more than 25 research papers in high impact journals, and several collaborations with leading measurement groups globally.
2) Selective Area Growth. Despite the popularity of VLS nanowires among researchers looking for Majorana Fermions, this growth platform is not suitable for complex design needed for qubit operations and large-scale manufacturability. Therefore, it became our objective to prioritize a scalable approach, and we turned to a selective area growth (SAG) method wherein nanowires are grown selectively inside lithographically defined trenches. We have progressed considerably and achieved huge success in forming scalable planar networks grown by Molecular Beam Epitaxy, a growth process that traditionally has not been considered as suitable for selective area growth.
The hallmark of the low-disorder transport in the semiconductor nanowires with induced superconductivity is attainment of the ballistic transport regime. Thus, for quantifying the disorder in afore-mentioned nanowires, carrier mobilities (peak transconductance) are measured at low temperatures using field effect devices (Fig. 2 c,d) and Hall-bar devices (Fig. 2 e,f).
As we progressed, we realized that the quality of platform does not meet the requirements of forming clear evidence of such quantum states. So, focus changed to improve the growth process and resulted in 6 publications and many more promising experiments are on the way. As an example, we have successfully grown SAG nanowires (Figure 2b), achieved dislocation free interfaces and significantly improved purity.
3) Single hybrid interfaces. The electronic properties of hetero-interfaces like band-offset, energy level positions of the interface states are of paramount importance for engineering the hybrid nanowire quantum device, where proximity coupling of a semiconductor to a s-wave superconductor induces topological superconductivity in the former. Therefore, we focused our research efforts for determining the band alignment of the semiconductor/superconductor epitaxial interface. The first material system investigated by us was planar InAs/Al. We successfully used angle-resolved photoelectron spectroscopy (ARPES), photon energy dependent core-level spectroscopy, and self-consistent electronic structure calculations to determine the band alignment of the epitaxial interface. Many material combinations (including ferromagnetic components) were grown and analyzed and have given new insight to the electronic structure of some of the most promising structures for topological quantum computing.
