guest ::
| Home > Publications database > Growth and characterization of InAs nanowire-based Josephson junctions |
| Home > Publications database > Growth and characterization of InAs nanowire-based Josephson junctions |
| Dissertation / PhD Thesis | FZJ-2022-04488 |
2022
RWTH Aachen University
This record in other databases:
Please use a persistent id in citations: doi:10.18154/RWTH-2022-08606
Abstract: This work delves into the growth mechanism as well as structural and electrical characterization of InAs nanowires (NWs) for Josephson junctions. The superconductors used in this case are aluminum and niobium. Josephson junctions are an essential component of a superconducting qubit. This work describes the evolution of the Josephson junctions within the state-of-the-art and achieving higher transparency of the semiconductor/superconductor interfaces. The first part of the work deals with the optimization of the selective area growth. This method offers greater control of the growth of nanowires and provides higher uniformity. Parameters such as temperature, indium growth rate, and arsenic beam equivalent pressure (BEP) have been optimized to achieve a growth yield of 95%, using a 20 nm thick silicon dioxide mask on a Si(111) substrate. Eventually, the InAs nanowires are grown and optimized for diameters of 70-80 nm and lengths of 4-5 μm. Additional experiments have been performed to dope the InAs nanowires with tellurium. In the case of Josephson junctions, they offer a huge asset, with a doping range of 1× 10^18 cm^−3 to 1× 10^19 cm^−3. An increase in the conductance of these nanowires is observed with increased doping and thereby an enhanced critical current of the Josephson junctions. Moreover, Te doping has shown an impact on the diameter and the length of the nanowires, since it is a surfactant. Atom probe tomography investigations performed on these nanowires show additional (211) lateral facets, that shift the hexagonal structure of the InAs nanowire to a partly dodecagon structure at Te doping concentrations greater than 1×10^19 cm^−3. Furthermore, the transparency of the InAs/superconductor interface has been tuned. A defect-free interface and a smooth film of a superconductor is apre-requisite for a high-quality Josephson junction, since this ensures a good coupling between the materials. A complete in-situ method has been adopted, to grow Al and Nb, onto the nanowires, thereby eliminating, any possible exposure of the semiconductor surface to the ambient. To achieve defect-free semiconductor/superconductor interfaces, a brief degassing step is introduced to the nanowires before the growth of the superconducting metals such as aluminum or niobium. This process, ensured enhanced transparency between the materials, thereby strengthening the coupling, by that improving the proximity effect. To be brief, the proximity effect induces Cooper pairs into the semiconductor, i.e. it turns the NW partly into a superconductor. Furthermore, the growth parameters of the metals evaporated are optimized to produce a smooth and defect-free interface and are investigated systematically. Lastly, the in-situ approach is expanded to encompass the fabrication of Josephson junctions at ultra-high vacuum conditions and to include other superconducting and capping materials in the process. The substrates made for this purpose have been prepared in such a way that two nanowires grow in a square trench at 90° to the planes of the trench. The growth windows for the NW growth are meticulously and selectively placed in such a way that one NW shadows the other during the metal evaporation, thus, causing a junction on the latter wire. The superconductors used in this process are optimized to create smooth and defect-free layers. In the case of aluminum, the growth of the metals is found to depend more on the temperature than on the angle of deposition. In contrast, for Nb, the angle of evaporation has a huge effect on the smoothness of the film. The investigations presented in these sections include transmission electron microscopy and corresponding low-temperature electrical measurements. This shadow approach, increased the metal evaporation angles onto the nanowires, from 30° to 87°, thus causing smooth and defect-free layers. This has also been shown to increase the interface transparency, between the NW and the superconductors. Lastly, this platform has also been used to demonstrate the growth of complex NW networks and multiple Josephson junctions.
Keyword(s): Information and Communication (1st) ; Condensed Matter Physics (2nd) ; nanowires
|
The record appears in these collections: |