ALD-grown two-dimensional TiS metal contacts for MoS₂ field-effect transistors

Metal contacts to MoS₂ field-effect transistors (FETs) play a determinant role in the device electrical characteristics and need to be chosen carefully. Because of the Schottky barrier (SB) and the Fermi level pinning (FLP) effects that occur at the contact/MoS₂ interface, MoS₂ FETs often suffer from high contact resistance (R_c). One way to overcome this issue is to replace the conventional 3D bulk metal contacts with 2D counterparts. Herein, we investigate 2D metallic TiS (x ∼ 1.8) as top contacts for MoS₂ FETs. We employ atomic layer deposition (ALD) for the synthesis of both the MoS₂ channels as well as the TiS contacts and assess the electrical performance of the fabricated devices. Various thicknesses of TiS are grown on MoS₂, and the resultant devices are electrically compared to the ones with the conventional Ti metal contacts. Our findings show that the replacement of 5 nm Ti bulk contacts with only ∼1.2 nm of 2D TiS is beneficial in improving the overall device metrics. With such ultrathin TiS contacts, the ON-state current (I_ON) triples and increases to ∼35 μA μm^-1. R_c also reduces by a factor of four and reaches ∼5 MΩ μm. Such performance enhancements were observed despite the SB formed at the TiS/MoS₂ interface is believed to be higher than the SB formed at the Ti/MoS₂ interface. These device metric improvements could therefore be mainly associated with an increased level of electrostatic doping in MoS₂, as a result of using 2D TiS for contacting the 2D MoS₂. Our findings are also well supported by TCAD device simulations.