Characterization of InAs-Al semiconductor-superconductor hybrid devices

Detta är en Master-uppsats från Lunds universitet/Fasta tillståndets fysik; Lunds universitet/Fysiska institutionen

Sammanfattning: In this thesis we study charge and spin transport through InAs-Al nanowire superconductor-semiconductor hybrid devices. We focus on assessing the quality of the InAs-Al contact interface when using a weak HCl acid to wet-etch the nanowire surface prior to metal evaporation. The contact quality assessment is based on parameters extracted via electrical characterization performed mK temperatures in a dilution refrigerator. In order to successfully study transport through superconductor-semiconductor hybrids it is highly important to achieve a high-quality contact interface. With an ultimate goal of producing devices suitable for the study of hybridized sub-gap states in mind, we discuss the viability of using our proposed fabrication scheme in this context. In hybrid devices there is superconductivity induced in the semiconductor via the proximity effect. The interplay between s-wave superconductivity from the superconductor and the unique properties of the semiconductor, e.g spin-orbit interaction, g'-factor, and tunable electron configuration via external gates, can give rise to exotic superconducting phenomena. Of particular relevance are Majorana bound states, which are candidates for the realization of stable qubits to be used in topological quantum computing. Experimental signatures of Majorana bound states have been reported in devices which are similar to devices presented in this thesis. This thesis attempts to further the understanding of quantum transport through, and in particular the magnetic field behavior of, proximitized nanowire quantum dots. Furthering this understanding proves beneficial when ultimately interpreting transport data in devices which are designed to hold Majorana bound states. High-quality Al contacts on InAs nanowires have been manufactured and measured at mK temperatures. Supercurrents were measured in shunted Al-InAs-Al Josephson junction devices, and the critical magnetic field of the contacts was observed to depend on field direction. Excess currents were extracted in order to estimate the transparency of the contacts. Spectroscopy devices were manufactured study the density of states in a proximitized InAs segment in Al-InAs-Au nanowire-based devices, where a crystal-phase engineered band-offset has been incorporated in the semiconductor junction. These latter devices showed quantum dot behavior and a wide variation in coupling strength to the superconducting lead. We conclude the relative coupling strength primarily from observed degree of Yu-Shiba-Rusinov screening of dot spins, which determines the nature of our observed sub-gap states.

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