QDev Seminar: Magdhi Kirti, Università degli Studi di Trieste

Optimization of semiconductor - superconductor nanosystems for the development of Andreev qubits

Two-dimensional electron systems confined near the surface of narrowband semiconductors have piqued interest due to their ease of integration with superconductors, allowing for new hybrid device systems. Such hybrid systems lay the foundations of a novel solid-state platform for scalable quantum computing based on Andreev quantum bits (qubits). High-quality superconducting thin films with transparent interfaces to a low-D semiconductor are expected to improve coherence time as well as to offer strong qubit-qubit coupling; therefore, these Semiconductor-superconductor hybrid systems resulting in Andreev qubits are among the most promising candidates1. InAs 2D electron gases (2DEGs) are the ideal semiconductor systems due to their vanishing Schottky barrier; however, their exploitation is limited by the nonavailability of commercial lattice-matched substrates2,3. For this work, a great effort has been made in the investigation of the structural and transport properties of InAs quantum wells grown by molecular beam epitaxy on GaAs (001) substrates over the years, to realize 2D electron gases with high electron mobility at low temperature. Due to the large lattice mismatch (7%) between the active InAs layer and the GaAs substrate, a step-graded buffer layer structure was employed to adapt the two different lattice parameters4.

These high mobility semiconductor heterostructures, were then used for integration into hybrid platforms. On the shallow 2DEGs, in-situ growth of Al films is demonstrated by Molecular Beam Epitaxy. Despite of the observed multidomain structure on Al films we obtained the state-of-the-art electrical properties and the superconducting proximity effect was observed in a Josephson junction. The growth protocol developed could thus set a new standard for the fabrication of Andreev qubits on GaAs technology.

Acknowledgements: And-QC FETOPEN H2020 project
(1) J.S.Lee et al.,Nano Lett. 2019, 19, 30833090.
(2) D. Ercolani et al., Phys. Rev. B,2008, 77, 235307.
(3) F. Capotondi et al., Journal of crystal growth, 2005, 278, 538543.
(4) K.S. Wickramasinghe et al., Appl. Phys. Lett., 2018, 113, 262104.