• black phosphorus
• Josephson junction
• superconductivity

The discovery of graphene in 2004 launched the research field of two-di\-men\-sion\-al ($2$D) materials. These layered materials are stacked together only by van der Waals interactions, which makes their exfoliation possible. To date, a variety of $2$D materials have been realized, ranging from insulating hexagonal Boron Nitride (h-BN) and wide band gap ($1.5-2.5$~eV) Transition Metal Dichalcogenides (TMDs) to zero band gap semimetal graphene.
With the increasing interest in these materials, semiconducting black-Phosphorus (bP), the most stable allotrope of phosphorus (P), attracted renewed attention because of its layered crystal structure. In 2014, black-Phosphorus joined the $2$D materials family, thanks to the demonstration that it can be mechanically exfoliated down to a monolayer.

Few-layer black-Phosphorus has unique properties among the members of this family. Its band gap value depends on the flake thickness and is very susceptible to in-plane strain. The band gap can be tuned in a wide range, from $1-2$~eV for monolayer bP to the bulk value of $0.3$~eV, filling the gap between graphene and TMDs. Moreover, bP is characterized by a strong structural in-plane anisotropy, with the consequence that its electronic, optical, and thermal transport properties are highly anisotropic too. Thus, bP is a promising $2$D material for novel applications in opto- and nano-electronic.

Two-dimensional materials have attracted increasing interest also in superconductive electronics, as normal materials for Josephson junctions (JJs). The most remarkable property of these devices is the Josephson effect, which consists in dissipationless flowing of a phase-coherent current (supercurrent) between two superconducting electrodes, coupled by a weak-link. The normal layer material, which can be an insulator, a semiconductor, or a metal, plays a crucial role in the junction behavior.
The choice of a semiconductor is aimed at combining superconducting devices with state-of-the-art of semiconductor electronics. The main feature of these devices is that the supercurrent varies not only with temperature and magnetic field, but also with an applied electric field, as a result of the semiconductor characteristics.

Despite its peculiar properties, only few theoretical and experimental studies have been performed in this research field with bP. The realization of bP-based Josephson junctions may lead to a deeper understanding of superconductivity in this material. In particular, novel studies on superconducting properties, like supercurrent and Andreev reflections, in anisotropic materials could be carried out.
Indeed, the goal of this research is to go towards the realization of such devices, and on longer timescales, to demonstrate an anisotropic characteristic in the Josephson supercurrent flow.

One of the most important parameters for JJs is the transparency of the interface between the superconductor and the normal material. However, it is extremely challenging to obtain good quality contacts to $2$D van der Waals materials because of their pristine surface with no dangling bonds, which makes it difficult to form strong bonds with a metal. With black-Phosphorus, a further challenge is its thickness-dependent high reactivity to oxygen and moisture, which quickly degrades material quality and device performance, since the presence of uncontrolled oxidation can change the contact behavior. Indeed, the only experimental study, available up to now, on JJs with bP as barrier reports the processing of vertical niobium/black-Phosphorus/niobium junctions in an entirely in-situ fabrication system in ultra-high-vacuum ($<10^{-8}$~Torr) conditions.

To address these challenges, we devised an ex-situ and accessible fabrication protocol in order to obtain planar junctions based on few-layer black-Phosphorus as weak-link between two niobium (Nb) electrodes. However, both few-layer bP and Nb are extremely prone to deteriorate in air. Thus, we had to devise an adequate strategy to overcome the problems related to the formation of an insulating layer at the superconductor/semiconductor interface and to the flake deterioration. This can be done minimizing bP flake exposure to air and moisture, performing cleaning procedures with oxygen and argon plasma before the metal deposition, and using a polymer capping layer to prevent degradation of the flakes during measurements.

Therefore, we performed a series of experiments on planar niobium/ black-Phosphorus/niobium junctions fabricated with different ex-situ protocols. The experiments were carried out in a wet $^3$He cryostat with a base temperature of $300$~mK and a superconducting magnet that can generate fields up to $9$~T, and standard two- and four-probe techniques were used for the transport measurements. In order to investigate the protocols' robustness against the variability between different flakes and different samples, various devices were fabricated, following each of these protocols. We studied their influence on the current-voltage characteristics of the junctions, on the properties of the superconducting metal, and on the black-Phosphorus field-effect characteristics. Furthermore, exploiting other device geometries with multiple contacts on the bP flakes, such as Hall bar and transfer length method, we also investigated, both at room temperature and at low temperature, the contact resistance dependence on the fabrication process. Using a titanium/niobium bilayer as superconducting leads and increasing the time of the argon cleaning procedure results in the lowest contact resistance obtained. Moreover, we observe that our best devices approach the quantum limit for contact resistance. Although we could not observe supercurrent across any junction, our ex-situ fabrication protocol represents a promising alternative to obtain high-quality ohmic contacts on bP flakes and, generally, on other air-unstable materials.