Electrical contacts to 2D materials

Two dimensional (2D) semiconductors like molybdenum disulphide (MoS2) and molybdenum diselenide (MoSe2) belonging to the family of transition metal dichalcogenides (TMDC) show electronic and optical properties relevant for applications in prospective quantum technologies including nanoelectronics, nano-sensoric, photonics, spinorbitronics and neuromorphic computation. Currently, however, the performance of 2D based devices is limited by the large contact resistance induced by Schottky barriers formed at the metal-TMDC interface alongside the low mobility of the charge carriers. In this thesis, a comprehensive protocol for the fabrication on-demand of ohmic and Schottky contacts on relevant 2D semiconductors is developed. Single and few monolayer 2D MoS2 and 2D MoSe2 flakes with different thicknesses are produced by mechanical exfoliation, transferred onto SiO2/Si substrates via deterministic dry transfer and are thoroughly characterised with optical microscopy, scanning electron microscopy, atomic force microscopy and Raman spectroscopy. The flakes are contacted with Ti and Au in the flake-on-contact and in the contact-on-flake configurations. The contacts exhibit either ohmic or Schottky behaviour, depending on the contact material and on the thickness of the 2D material. By applying a gate voltage, the carrier mobility is increased and, as a result, the contact resistance is significantly reduced. Schottky 2D field-effect-transistor behaviour is obtained both in: (i) back-gating, where the gate voltage is applied perpendicular to the sample plane through the SiO2/Si substrate, and (ii) top-gating, with an ionic gel deposited onto the TMDC layers. These experimental results pave the way for improving the performance of atomically thin TMDC devices and for exploring their transport phenomena.