Schottky Barrier Height Engineering for Electrical Contacts of Multilayered MoS2 Transistors with Reduction of Metal-Induced Gap States

The difficulty in Schottky barrier height (SBH) control arising from Fermi-level pinning (FLP) at electrical contacts is a bottleneck in designing high-performance nanoscale electronics and optoelectronics based on molybdenum disulfide (MoS2). For electrical contacts of multilayered MoS2, the Fermi level on the metal side is strongly pinned near the conduction-band edge of MoS2, which makes most MoS2 channel field-effect transistors (MoS2 FETs) exhibit n-type transfer characteristics regardless of their source/drain (S/D) contact metals. In this work, SBH engineering is conducted to control the SBH of electrical top contacts of multilayered MoS2 by introducing a metal-interlayer-semiconductor (MIS) structure which induces the Fermi-level unpinning by a reduction of metal induced gap states (MIGS). An ultrathin titanium dioxide (TiO2) interlayer is inserted between the metal contact and the multilayered MoS2 to alleviate FLP and tune the SBH at the S/D contacts of multilayered MoS2 FETs. A significant alleviation of FLP is demonstrated as MIS structures with 1 nm thick TiO2 interlayers are introduced into the S/D contacts. Consequently, the pinning factor (S) increases from 0.02 for metal-semiconductor (MS) contacts to 0.24 for MIS contacts, and the controllable SBH range is widened from 37 meV (50-87 meV) to 344 meV (107-451 meV). Furthermore, the Fermi-level unpinning effect is reinforced as the interlayer becomes thicker. This work widens the scope for modifying electrical characteristics of contacts by providing a platform to control the SBH through a simple process as well as understanding of the FLP at the electrical top contacts of multilayered MoS2.