Subgap spectroscopy in hybrid nanowires by nm-thick tunnel barriers

Tunneling spectroscopy is a well known technique to measure subgap states in hybrid semiconductor-superconductor nanowires when searching for signatures of Majorana zero modes (MZMs). Currently, semiconductor at the ends of hybrids combined with local gates is generally used as a tunnel barrier in spectroscopy measurememts. Besides the limitation of detecting only the states close to the hybrid ends, such gate-defined tunnel probes have been shown to cause features in subgap spectra that can mimic the signatures of MZMs and additionally complicate measurement interpretations. In this work, we develop an alternative type of tunnel barriers in order to overcome these limitations. After the growth of superconducting Al on a semiconducting InSb nanowire, a precisely controlled in-situ oxidation of Al is performed to yield a nm-thick AlOx layer. In such thin isolating layer, tunnel junctions can be arbitrarily defined at any point along the hybrid region by shadow-wall angle-deposition of normal metal leads. We utilize this and make multiple tunnel probes along single nanowire hybrids. This allows us to successfully detect the subgap spectra at various distances from the hybrid ends and to identify Andreev bound states (ABSs) of various spatial extension residing inside the hybrids.