In this study, a 20 cycle atomic layer-deposited (ALD) TiO 2 dielectric interfacial layer was inserted between WS 2 and other metals (Ti and Pd) to lower the Schottky barrier height through a mechanism such as Fermi-level depinning as in a silicon device. In addition to dramatically reducing contact resistance, unipolar transfer characteristics could be achieved in WS 2 FETs by using a TiO 2 dielectric interfacial layer with both low and high work function metal-WS 2 contact systems (Ti-WS 2 and Pd-WS 2 , respectively).Figure 1 a shows the schematic of a back-gate WS 2 FET with a 20 cycle ALD TiO 2 interfacial layer. The channel length and width are ≈3 and ≈20 µm, respectively. A lattice constant of 0.65 nm for the WS 2 was confi rmed as shown in a representative high resolution transmission electron microscopy (HRTEM) image of the metal-TiO 2 -WS 2 stack (Figure 1 b). TiO 2 thickness ≈2.7 nm is expected based on the deposition rate of 0.135 nm per cycle. The fi gure inset is an electron diffraction pattern obtained from a Ti-TiO 2 -WS 2 stack structure. Raman spectra for bi-and multilayer WS 2 are shown in Figure 1 c, and corresponding optical images are shown in Figure 1 d. The Raman spectra for WS 2 include two modes: the E 1 2g mode, which corresponds to the in-plane vibrational motion of atoms, and the A 1g mode, which is attributed to the out-of-plane vibrational motion of atoms. The thickness of the WS 2 layer can be extracted from the shift in the wavenumber. As the thickness increases, the wavenumber between E 1 2g and A 1g and the absolute intensity increase, while the intensity ratio of E 1 2g to A 1g decreases. [ 43 ] Figure 2 a shows the transfer characteristics ( I DS -V BG ) of WS 2 FETs with a Ti electrode (work function ≈4.3 eV). The drain bias was increased from 0.1 to 0.5 V in 0.1 V steps. The transfer characteristics showed bipolar behavior with a hole current for the negative gate bias region and electron current for the positive gate bias region. This is typical behavior for Schottky barrier FETs with a midgap-like contact metal. The effective position of the Fermi level appears to have shifted to the near midgap of WS 2 , forming a high Schottky barrier for both electrons and holes as shown in Figure 2 e. With a metal-semiconductor contact, when the effective workfunction of a metal is shifted by extrinsic mechanisms, the barrier height can be defi ned considering the Fermi-level pinning factor as shown in the equation below 1 B,n 0 gs M s gs 0where B,n 0 Φ is the barrier height, M φ is the work function of the metal electrode, s χ is the electron affi nity of the semiconductor, gs γ is parameter for the gap states, and 0 φ is a charge neutrality position. [ 44 ] The shift in the location of Fermi level can be caused by many factors such as surface states, metal-induced gap states (MIGS), defect states, or disorder-induced gap states, etc. [36][37][38]45,46 ] Recently, 2D transition metal dichalcogenides (TMDs) have attracted intense research interests due to their unique electrical, optica...