Yttrium-induced phase-transition technology for forming perfect ohmic contact in two-dimensional MoS2 transistors

Abstract The van der Waals (vdW) strategy is promising for overcoming the Fermi pinning challenge in two-dimensional (2D) transistors. However, the lack of advanced-node lithography-compatible methods hinders wafer-scale integrated manufacturing of vdW contacts. We report a yttrium-doping-induced phase-transition technology for converting semiconducting trigonal prismatic (2H) into heavily doped metallic octahedral (1T) MoS2 to achieve suitable vdW band alignment and ideal ohmic contacts. This Y-doped 1T-MoS2 is a vdW metallic buffer, and it improves carrier transfer efficiency from the metal electrode to 2H MoS2. We developed a solid-state-source three-step doping method (plasma-deposition-annealing) and achieved Å-thickness surface doping with yttrium atoms replacing sulfur atoms in the top surface of MoS2, thus forming self-aligned Y-doped 1T-MoS2 vdW ohmic contacts lithographically patterned in the source and drain regions. We fabricated 10-nm MoS2 FETs with Y-doped 1T-MoS2 vdW contacts in 2-inch wafers exhibiting nearly ideal ohmic contact with an ignorable Schottky barrier, achieved the smallest contact-resistance Rc in all 2D TMD FETs (less than 117-ohm micrometers) and a total-resistance (235-ohm micrometers) more than three times smaller than those of the best-reported 2D n-type FETs (greater than 800-ohm micrometers), showing high performance with an excellent on-current density of 1.22 milliamperes per micrometer at a small drain voltage of 0.7 V, a high ballistic ratio of 80%, and a record transconductance of 3.2 millisiemens per micrometer.