Contact Engineering for Dual-Gate MoS2 Transistors Using O2 Plasma Exposure

The benefits of O-2 plasma exposure at the contact regions of dual-gate MoS2 transistors prior to metal deposition for high performance electron contacts are studied and evaluated. Comparisons between devices with and without the exposure demonstrate significant improvements due to the formation of a high-quality contact interface with low electron Schottky barrier (similar to 0.1 eV). Topographical and interfacial characterizations are used to study the contact formation on MoS2 from the initial exfoliated surface through the photolithography process and Ti deposition. Fermi level pinning near the conduction band is shown to take place after photoresist development leaves residue on the MoS2 surface. After O-2 plasma exposure and subsequent Ti deposition, Ti scavenges oxygen from MoOx and forms TiOx. Electrical characterization results indicate that photoresist residue and other contaminants present after development can significantly impact electrical performance. Without O-2 plasma exposure at the contacts, output characteristics of MoS2 FETs demonstrate nonlinear, Schottky-like contact behavior compared to the linearity observed for contacts with exposure. O-2 plasma allows for the removal of the residue present at the surface of MoS2 without the use of a high-temperature anneal. A low conduction band offset and superior carrier injection are engineered by employing the reactive metal Ti as the contact to deliberately form TiO2. Dual-gate MoS2 transistors with O-2 plasma exposure at the contacts demonstrate linear output characteristics, lower contact resistance (similar to 20X reduction), and higher field effect mobility (similar to 15X increase) compared to those without the treatment. In addition, these results indicate that device fabrication process induced effects cannot be ignored during the formation of contacts on MoS2 and other 2D materials.