Contact engineering for two-dimensional van der Waals semiconductors

Two-dimensional (2D) semiconductors represent the most promising post-silicon channel materials for ultimate electronics. However, the unique atomic thickness renders them incompatible with traditional atomic doping technique through ion implantation and thermal activation, which poses a key challenge for constructing ohmic contacts with 2D semiconductors. In the last decade, constant efforts have been devoted to address this critical challenge. In this article, by casting light on the origins of contact resistance between electrodes and 2D semiconductors, we review various strategies of contact engineering for 2D van der Waals semiconductors and the steady progress achieved in this specific issue, in order to provide guidance for device design and integration of 2D semiconductors for next-generation electronics.