Materials for epitaxial superconducting heterostructures: Nb, NbN and NbO/Nb

Abstract

Josephson Junctions are junctions that operate under the Josephson effect, the tunnelling of electrons through a non-superconducting layer between two superconducting layers. They are popular candidates for the realisation of quantum bits due to the low dissipation inherent to superconductors making long coherence times possible. In practice, two level system defects present in the barrier layer at the nano and atomic-scale can interact with qubits leading to decoherence. The ultimate goal of this project is to successfully implement new Josephson Junction architectures and designs. In service of this goal, the aims of this report are twofold. The first is to investigate the formation of a crystalline NbO barrier layer on an epitaxial Nb film that could replace the traditional AlO 𝑥 barrier that currently limits junction performance. The second is to investigate the fabrication of a superconducting 𝛿-NbN film for use in a spin-triplet superconductor. In the first half of the first section of this study, Nb films were grown on c-plane Al 2 O 3 and the relationship between the film’s thickness and its superconducting properties was investigated. It was found that as thickness decreased, transition temperatures decreased as well as was expected. The formation of an oxide layer on the surface of the films that were exposed to atmosphere was noted through XRR measurements. This oxide layer was utilized in the second half of this section where the formation of a NbO(111)-terminated Nb(110) thin film was attempted. The thin film was subjected to anneals of ∼ 700◦𝐶. X-ray photoelectron spectroscopy measurements confirmed the reduction of surface oxides such as Nb 2 O 5 and NbO 2 to NbO but also the formation of NbC. Removal of NbC via Ar sputtering was unsuccessful. A higher temperature anneal (∼ 2000◦𝐶) may remove the carbide and lead to the formation of a NbO-terminated Nb thin film. In the second half of this study, the growth of superconducting 𝛿-NbN thin films on c-plane Al 2 O 3 via DC magnetron sputtering and their potential application in a spin triplet superconducting heterostructure were investigated. X-ray diffraction patterns revealed that under nitrogenation during growth led to the formation of 𝛾-Nb 4 N 3 and 𝛽-Nb 2 N mixed phase films which were not superconducting. An increase in N 2 during growth saw a shift in phase from 𝛾-Nb 4 N 3 to 𝛿-NbN, however 𝛽-Nb 2 N was still present and the films were still not superconducting. The mixture of phases present in the films may be affecting their superconducting properties. A subsequent post anneal of the first film did not return a superconducting film. The phase of this film post anneal could not be successfully determined due to issues with the XRD apparatus. Revisions of the growth process such as the modulation of Ar gas flow or growth on a different substrate may lead to the realisation of superconducting 𝛿-NbN.