Abstract
Following significant progress in the visualization and characterization of Majorana end modes in hybrid systems of semiconducting nanowires and superconducting islands, much attention is devoted to the investigation of the electronic structure at the buried interface between the semiconductor and the superconductor. The properties of that interface and the structure of the electronic wave functions that occupy it determine the functionality and the topological nature of the superconducting state induced therein. Here we study this buried interface by performing spectroscopic mappings of superconducting aluminum islands epitaxially grown in situ on indium arsenide nanowires. We find unexpected robustness of the hybrid system as the direct contact with the aluminum islands does not lead to any change in the chemical potential of the nanowires, nor does it induce a significant band bending in their vicinity. We attribute this to the presence of surface states bound to the facets of the nanowire. Such surface states, which are present also in bare nanowires prior to aluminum deposition, pin the Fermi level, thus rendering the nanowires resilient to surface perturbations. The aluminum islands further display Coulomb blockade gaps and peaks that signify the formation of a resistive tunneling barrier at the InAs-Al interface. The extracted interface resistivity, , will allow us to proximity induce superconductivity with negligible Coulomb blockade effects by islands with interface area as small as . At low energies we identify a potential energy barrier that further suppresses the transmittance through the interface. A corresponding barrier exists in bare semiconductors between surface states and the accumulation layer, induced to maintain charge neutrality. Our observations elucidate the delicate interplay between the resistive nature of the InAs-Al interface and the ability to proximitize superconductivity and tune the chemical potential in semiconductor-superconductor hybrid nanowires.
3 More- Received 3 June 2019
- Revised 6 September 2019
DOI:https://doi.org/10.1103/PhysRevX.10.011002
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Over the past several decades, interfaces between semiconductors and metals have been investigated intensively because they are crucial for semiconductor-based technologies. Recently, researchers have turned their attention to the interface between semiconducting nanowires and superconducting electrodes. These hybrid systems are promising platforms for inducing topological superconductivity, which is a potential resource for a robust form of quantum technology. We use scanning tunneling microscopy and spectroscopic mappings to explore how indium arsenide nanowires respond to superconducting aluminum islands deposited on the nanowires.
We find evidence for the presence of dense surface-localized electronic states in the nanowires that pin the electron density. This renders the electronic spectrum resilient to the introduction of metallic islands, which means that the nanowires are more robust to external perturbations than previously thought. To study the interface between the nanowire and the metal electrodes, we look at a quantization in the electron occupancy due to charging (a Coulomb blockade) that occurs whenever the metallic droplet is small enough. The detection of this charging effect indicates that the metallic island is fairly decoupled from the semiconducting nanowire underneath it, which allows us to characterize the substantial resistive barrier between the two.
Our findings of pinning of the electron occupancy in semiconducting nanowires and the characterization of the resistive barrier that forms at their interface with metallic electrodes will allow researchers to optimize the design of such hybrid systems and potentially improve their performance for quantum applications.