Ohmic Contact Engineering for Two-Dimensional Materials

One of the major areas of semiconductor device research is the development of transparent or ohmic contacts between semiconductors and metal electrodes for the efficient injection of charge carriers into the conduction channel. Fast-emerging two-dimensional (2D) materials with atomically flat surfaces, free of dangling bonds, are intuitively promising to form ohmic contacts with metals. However, the contacts of 2D devices usually possess a large Schottky barrier and rarely follow the Schottky-Mott rule, because of interfacial effects such as Fermi-level pinning. Herein, we summarize recent progress and developments in contact engineering of 2D materials for the realization of ohmic contacts in 2D electronic devices. The basic physics of contacts for both Si and 2D materials is briefly introduced. A variety of engineering strategies are subsequently introduced, including band matching, doping, phase engineering, insertion of buffer layers, 2D/metal van der Waals contacts, and edge contacts. Finally, opportunities and challenges for optimizing contacts for future 2D electronics are discussed.