Abstract
The discovery of graphene by Andre Geim and Novoselov sparked growing interest in two-dimensional (2D) systems, both for applied and fundamental physics research. Among these, the transition metal dichalcogenides (TMDs) form a family of 2D materials with unique properties. TMDs are crystalline materials of the form MX2 where M is a transition metal and X2 are two chalcogen atoms, most forming layered structures. They can exist as monolayers with a honeycomb structure comparable to graphene. They exhibit a wide range of band gaps (from 0 to 2 eV), combined with strong spin-orbit couplings. Of all the TMDs, MoS2 remains the most studied to date. MoS2 monolayers have a large direct band gap (1.9 eV) located at the K points of the reciprocal lattice. This ‘gap’ qualifies them for many photonic applications, and their excellent mechanical flexibility makes them a promising semiconductor material for flexible electronics.In 2012, it was observed for the first time that an electrostatic control of doping of a few MoS2 layers induced a transition from the insulating phase to a metallic phase, until reaching a superconducting phase at high electronic density, with a temperature critical can reach 10.8 K. Following this discovery, two experimental groups have independently demonstrated that the critical fields in the plane of the monolayer material were several times higher than those of several layers, and six times higher than the Pauli limits. Due to spin-orbit coupling (SOC) and the absence of a center of inversion, electron spins are oriented in the direction perpendicular to the 2D plane. This type of coupling is usually called 'Ising SOC' to distinguish it from Rashba SOC, which keeps electron spins in in-plane directions. Due to the strong out-of-plane pinning of spins of the electrons forming the Cooper pairs, the external in-plane magnetic fields are much less effective in separating them and suppressing superconductivity. As a result, critical in-plane magnetic fields in MoS2 can reach several Teslas. This type of unconventional superconductor is called Ising superconductor, and appears to be a candidate for the formation of topological states such as Majorana fermions. More generally, this coupling makes MoS2-based devices promising for spintronics and valleytronics.The original objective of this project, although still far from being achieved, is to develop a Josephson junction controllable by electrostatic grids. These would allow the formation of S-I-S to S-M-S and even S-S'-S junctions, based on single crystals of TMD. S, M, and I represent the superconducting, metallic, and insulating regions, respectively, with S' being a slightly different superconducting state. Monocrystals (multilayer or monolayer) can be used as the sole material forming the junction, where the electronic phase will be tuned by local gates made of carbon nanotubes (CNTs). Consequently, the proposed junction will be made of a single material and will be fully checked in situ. The superconducting state of MoS2 will be achieved with the use of gates based on ionic liquid.To date, different transport regimes have been reported in the insulating regime in the literature. Also, a precise description of the transition to a metallic regime and the diffusion mechanisms in this regime are still needed. Moreover, even for apparently more basic problems, such as at the interface of contact with a metal, there is no complete study of the mechanisms involved depending on doping and temperature. This thesis presents our efforts in the development of a controlled fabrication process for MoS2 CVD-based components. Many approaches attempting to make the contacts more ohmic, and to dope as efficiently as possible, have been studied. After observing that in-situ annealing was the most efficient, we studied in detail the transport in the insulating and metallic regimes and the transition between these two regimes, as well as the transport at the gold/MoS2 interface.