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| Home > Publications database > MBE growth and characterization of Te-doped InAs nanowires and InAs/superconductor hybrid structures |
| Master Thesis | FZJ-2016-06202 |
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2016
Abstract: This work investigates the growth mechanism of tellurium doped InAs/superconductor hybrid structures as well as their electrical properties. Semiconductor/superconductor structures are of interest for both, fundamental research and applied science. The junctions might become an important building block in future quantum computing networks. However, already in present time gate-controllable Josephson junctions constitute an attractive system for research and applications.One part of this thesis is devoted to the doping mechanism of InAs nanowires using the group VI material tellurium. Nanowire doping is of great interest for a broad range of applications such as field-effect transistors, p-n junctions and optoelectronic devices. In case of gateable Josephson junctions, they constitute an important component, as successfully doped wires lead to a higher critical current Ic, i.e. an enhanced superconducting gap. Applying simple two-point but also field effect measurements, we find that Te-doping is indeed effective. The impact of the tellurium incorporation onto the morphology as well as the crystal structure is investigated using SEM, TEM and XRD measurements. The TEM and XRD measurements are enabled by co-operations with other groups. The results show that the occurrence of zinc blende segments within the wurtzite dominated crystal structure is enhanced due to Te-doping. Moreover, the addition of Te adatoms during growth leads to an increase in wire diameter and decrease in length.The second part of this work will focus on the investigation of the molecular-beam-epitaxial growth of the superconducting metals aluminum and niobium onto InAs nanowires. Here, an impurity- and defect-free semiconductor/superconductor interface is crucial in order to assure a good superconducting coupling between both materials. The coupling mechanism is based on the proximity effect and leads to an induction of Cooper pairs into the semiconductor, though only if the contacts bear a high transparency. The optimal parameters to produce a smooth surface as well as an impurity-free interface are investigated systematically. In case of Al, the growth regimes are found to depend strongly on the growth temperature. In case of Nb, the growth angle and growth rate have the main influence, respectively. Our investigations result in the finding of growth parameters suitable for Al and Nb shell deposition on InAs nanowires, enabling highly transparent contacting.Finally, the quality of the Te-doped Al/InAs/Al Josephson junction produced is classified via low temperature measurements, provided by the group of T. Schäpers. The characteristic voltage Vc extracted, suggests that the interface produced, in combination with the carrier incorporation, results in a comparably high figure of merit Vc, i.e. a high grade junction.
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