Development of semiconductor-superconductor nanosystems for application in quantum computation.

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 radically new solid-state platform for scalable quantum computing based on Andreev quantum bits (qubits). These Semiconductor-superconductor hybrid systems resulting in Andreev qubits are among the most promising candidates, as high-quality superconducting thin films with transparent interfaces to a low-D semiconductor will improve coherence time as well as offer strong qubit-qubit coupling. InAs 2D electron gases (2DEGs) are the ideal semiconductor systems due to their vanishing Schottky barrier; however, their exploitation is limited by the non-availability of commercial lattice-matched substrates. 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 parameters. We have optimized the buffer layer to reduce the residual strain in the quantum well region. Corresponding to this strain reduction, we see an increase of the electron mobility up to almost 106cm2/Vs, in line with state of art samples on InP substrates. To understand the limiting factors of mobility in detail, we studied low temperature scattering mechanisms on these 2DEGs. The insights gained from this research enabled us to enhance the buffer layer to achieve higher electron mobilities. These high mobility semiconductor heterostructures, were then used for integration into hybrid platforms. Topological superconducting regime was achieved with the optimization of mobility and distance of the QW from the surface to have proximity effect. This recipe allowed to reach a hybrid system with mobility around 8.6 X 104 cm2/Vs for 2DEGs at 10 nm from the surface, with a charge around 4.95 X 1011/cm2 with silicon δ doping. On these shallow 2DEGs in-situ growth of aluminum films on near-surface InAs 2DEGs by Molecular Beam Epitaxy is demonstrated. Despite of the observed multidomain structure we obtained state of art electrical properties and 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.