Contact morphologies for ultrafast optoelectronics in 2D materials

Two-dimensional van der Waals materials are two-dimensional crystals with strong covalent in-plane bonds and weak van der Waals interaction between the layers with a variety of different electronic, optical and mechanical properties. A very prominent class of two-dimensional materials are transition metal dichalcogenides and amongst them particularly the semiconducting subclass. Their properties include bandgaps in the near-infrared to the visible range and a decent charge carrier mobility. These characteristics make the materials highly attractive for both fundamental research as well as innovative device applications. For most 2D semiconductor devices, one of the largest challenges is to prepare low resistance contacts. That holds particularly for optoelectronic devices to avoid depletion fields at Schottky barriers. In general, a combination of a top contact and edge contact is used to describe the morphology of contacts to 2D materials [1]. The top contact is either formed by a van der Waals gap between a metal and the 2D semiconductor or by the creation of covalent bonds to the 2D semiconductor, leading to a metallization of the contact region. Best contacts so far have been achieved by inducing a metallic phase transition within the 2D semiconductors, e.g. via a phase transition from 2H to 1T in MoS
₂
FETs [2] or by using graphene as contact material [3,4].