Abstract
Hybrid superconductor-semiconductor devices are currently one of the most promising platforms for realizing Majorana zero modes. Their topological properties are controlled by the band alignment of the two materials, as well as the electrostatic environment, which are currently not well understood. Here, we seek to fill in this gap and address the role of band bending and superconductor-semiconductor hybridization in such devices by analyzing a gated single Al-InAs interface using a self-consistent Schrödinger-Poisson approach. Our numerical analysis shows that the band bending leads to an interface quantum well, which localizes the charge in the system near the superconductor-semiconductor interface. We investigate the hybrid band structure and analyze its response to varying the gate voltage and thickness of the Al layer. This is done by studying the hybridization degrees of the individual subbands, which determine the induced pairing and effective factors. The numerical results are backed by approximate analytical expressions which further clarify key aspects of the band structure. We find that one can obtain states with strong superconductor-semiconductor hybridization at the Fermi energy, but this requires a fine balance of parameters, with the most important constraint being on the width of the Al layer. In fact, in the regime of interest, we find an almost periodic dependence of the hybridization degree on the Al width, with a period roughly equal to the thickness of an Al monolayer. This implies that disorder and shape irregularities, present in realistic devices, may play an important role for averaging out this sensitivity and, thus, may be necessary for stabilizing the topological phase.
4 More- Received 21 January 2018
DOI:https://doi.org/10.1103/PhysRevX.8.031040
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
Interfaces between metals and semiconductors are ubiquitous in semiconducting electronics. Such interfaces have recently attracted renewed attention in the context of engineered topological superconducting nanowires, which harbor charge-neutral, zero-energy edge excitations, the so-called Majorana zero modes. Remarkably, these zero modes can, in principle, be harnessed for noise-immune quantum computing. Here, we analyze a gated single Al-InAs interface using numerical and analytical methods. The Al-InAs system is currently the most promising candidate for the creation of topological superconductors because of its extremely clean interfaces.
There is strong experimental evidence of Majorana zero modes in metal-semiconductor interfaces when spin-orbit coupling and a magnetic field are present. But these modes are accessible only below a critical temperature, at which the metal becomes superconducting. The systems are typically operated at temperatures well below 1 K. Many microscopic details of the interfaces and the degree of metal-semiconductor hybridization that control topological properties—including the nature of the chemical bounds at the interface, the degree of interaction between the electron’s charge and spin, and the electrostatic landscape—have remained unclear, however. Here, we resolve the energy spectrum and the electrostatic landscape of the Al-InAs interface. We show that strong hybridization is achievable but that it requires a fine balance of parameters and a controlled Al width. We also demonstrate that the degree of hybridization depends almost periodically on the Al width, with a period roughly equal to the thickness of the Al monolayer. This finding implies that disorder and shape irregularities, present in realistic devices, may average out this sensitivity and stabilize the topological phase.
We expect that our findings will help improve designs of the next generation of Majorana devices and shed light on engineering semiconductor-superconductor hybrids.