[Paper Review] InSbAs Two-Dimensional Electron Gases as a Platform for Topological Superconductivity

Christian M. Moehle, Chung Ting Ke+8 more|arXiv (Cornell University)|Jan 1, 2021

Topological Materials and Phenomena31 references57 citations

TL;DR

This paper demonstrates that InSbAs two-dimensional electron gases (2DEGs) with tunable spin-orbit coupling and large g-factors, when interfaced with in-situ grown aluminum, form a robust platform for topological superconductivity. The hybrid system exhibits a hard superconducting gap and enables stable operation of phase-controllable Josephson junctions, superconducting islands, and quasi-1D structures—key architectures for probing Majorana zero modes.

ABSTRACT

<p>Topological superconductivity can be engineered in semiconductors with strong spin-orbit interaction coupled to a superconductor. Experimental advances in this field have often been triggered by the development of new hybrid material systems. Among these, two-dimensional electron gases (2DEGs) are of particular interest due to their inherent design flexibility and scalability. Here, we discuss results on a 2D platform based on a ternary 2DEG (InSbAs) coupled to in situ grown aluminum. The spin-orbit coupling in these 2DEGs can be tuned with the As concentration, reaching values up to 400 meV Å, thus exceeding typical values measured in its binary constituents. In addition to a large Landé g-factor of ∼55 (comparable to that of InSb), we show that the clean superconductor-semiconductor interface leads to a hard induced superconducting gap. Using this new platform, we demonstrate the basic operation of phase-controllable Josephson junctions, superconducting islands, and quasi-1D systems, prototypical device geometries used to study Majorana zero modes. </p>

Motivation & Objective

• To develop a hybrid 2DEG-superconductor platform with enhanced spin-orbit coupling and large g-factor for topological superconductivity.
• To overcome the challenge of poor superconducting proximity effect in InSb by using a ternary InSbAs 2DEG with a clean, in-situ Al interface.
• To demonstrate device architectures—Josephson junctions, superconducting islands, and quasi-1D systems—suitable for probing Majorana zero modes.
• To achieve a hard induced superconducting gap via a pristine semiconductor-superconductor interface in a ternary 2DEG system.

Proposed method

• Growth of InSb1−xAsx 2DEGs on GaAs substrates via molecular beam epitaxy with precise control of As concentration (x = 0 to 0.240).
• In-situ capping with 7 nm of aluminum after semiconductor growth to ensure a clean, high-quality interface.
• Use of a 2 ML InAs screening layer to prevent intermixing and maintain interface quality.
• Fabrication of Hall bars and top-gated devices to tune electron density and measure transport properties.
• Application of magneto-transport measurements at 300 mK to extract spin-orbit coupling via weak anti-localization (WAL) fitting with the Iordanskii-Lyanda-Geller-Pikus (ILP) model.
• Employment of tunneling spectroscopy and differential conductance mapping in phase-controllable Josephson junctions, superconducting islands, and quasi-1D superconducting strips to probe Andreev bound states and potential Majorana zero modes.

Experimental results

Research questions

• RQ1 Can InSbAs 2DEGs with tunable As concentration achieve stronger spin-orbit coupling than InAs or InSb?
• RQ2 Does the in-situ Al capping on InSbAs 2DEGs produce a hard superconducting gap suitable for topological superconductivity?
• RQ3 Can prototypical topological device architectures—phase-controllable Josephson junctions, superconducting islands, and quasi-1D structures—be stably operated in this hybrid system?
• RQ4 What is the role of the g-factor and spin-orbit coupling in enabling the topological regime at experimentally accessible magnetic fields?

Key findings

• The linear Rashba spin-orbit parameter in InSbAs 2DEGs reaches up to 400 meV·Å, significantly exceeding values in InSb (~100 meV·Å) and InAs.
• A large g-factor of approximately 55 is measured, comparable to InSb and favorable for entering the topological regime at low magnetic fields.
• The pristine Al/InSbAs interface enables a hard induced superconducting gap, confirmed by spectroscopy measurements.
• Phase-controllable Josephson junctions show clear flux-dependent gap modulation, consistent with Andreev bound states.
• Superconducting islands exhibit 2e-periodic Coulomb oscillations at zero bias, indicating coherent Cooper pair transport.
• Quasi-1D superconducting strips show a zero-energy state emerging at ~0.7 T under parallel magnetic field, a potential signature of Majorana zero modes.