Thesis presented December 16, 2022

Abstract:
Recently, new quantum phenomena have been the subject of intense research, notably for the development of quantum computers. Many quantum electronic devices use Josephson junctions. In these junctions, two superconductors are separated by a thin layer of a normal metal, allowing the superconducting current to pass. By replacing the normal metal with a semiconductor, new electronic transport regimes become accessible. Due to a low critical temperature, traditional aluminum-based hybrid superconductor-semiconductor devices cannot be used under high magnetic fields. Therefore, research is moving towards the use of superconducting materials with higher critical temperatures. Yet, changing the materials constituting a device requires the development of new manufacturing processes. This takes place through a thorough understanding of the synthesis of materials and their interfaces.
In this regard, the objective of my thesis was to develop and characterize hybrid interfaces based on InAs and InSb semiconductors to realize Josephson junctions. I first studied the growth of tilted InAs nanowires by molecular beam epitaxy. I used the gold-catalyzed "vapor-liquid-solid" mechanism for the growth of InAs wires. I explored the different growth parameters available. I then deposited superconductors by different techniques on these nanowires and analyzed their interfaces. I showed that the annealing temperature before growth is a key parameter to produce homogeneous nanowires with low size dispersion and high density. By optimizing the annealing temperature and the V/III ratio, I was able to reduce the size dispersion and better control the growth rate. I then developed a process to deposit an amorphous MoGe superconductor around the nanowires. I obtained ultra-thin, smooth and homogeneous shells.
I then analyzed the crystalline structure of Sn/InSb hybrid interfaces. I studied Sn thin films prepared on InSb by Prof. Palmstrøm's group at UCSB by X-ray diffraction (XRD). Then, I participated in a fully in-situ experiment at the European Synchrotron Radiation Facility. I deposited Sn shells at cryogenic temperatures on InSb nanowire samples provided by Prof. Bakkers' group at TU Eindhoven. I used grazing incidence XRD to study the crystal structure of the wires before and after deposition at 80K. I observed that tin films deposited on InSb substrates at cryogenic temperatures are in the cubic α crystalline phase. The tetragonal ß phase of tin appears in the thin films after the deposition of a protective AlO_x layer. This suggests that the deposition of AlO_x by electron beam evaporation provides sufficient heat to initiate the transformation of α- into β-Sn. In contrast to the thin-film geometry, the study conducted at ESRF showed that the shells formed during 80K around InSb nanowires are in the ß-Sn crystalline structure.
In conclusion, the research presented here is crucial for three reasons. (1) I have developed tilted nanowire arrays, those are necessary for the creation of etch-free Josephson junctions; (2) I have developed a superconductor deposition process that preserves the InAs crystal structure. (3) I have determined the crystalline phases of tin deposited under different experimental conditions at 80 K on InSb substrates and nanowires. These new insights are a step forward the understanding of hybrid materials and interfaces and their integration into quantum devices.

Keywords:
Superconductor, epitaxy, semiconductor