Contact Engineering for Dual-Gate MoS 2 Transistors Using O 2 Plasma Exposure

The benefits of O 2 plasma exposure at the contact regions of dual-gate MoS 2 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 (∼0.1 eV). Topographical and interfacial characterizations are used to study the contact formation on MoS 2 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 MoS 2 surface. After O 2 plasma exposure and subsequent Ti deposition, Ti scavenges oxygen from MoO x and forms TiO x . 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 MoS 2 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 MoS 2 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 TiO 2 . Dual-gate MoS 2 transistors with O 2 plasma exposure at the contacts demonstrate linear output characteristics, lower contact resistance (∼20× reduction), and higher field effect mobility (∼15× 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 MoS 2 and other 2D materials.