Unipolar p-type monolayer WSe_(2) field-effect transistors with high current density and low contact resistance enabled by van der Waals contacts

High-performance field-effect transistors (FETs) based on atomically thin two-dimensional (2D) semiconductors have demonstrated great promise in post-Moore integrated circuits. However, unipolar p-type 2D semiconductor transistors yet remain challenging and suffer from low saturation current density (less than 10 µA·µm^(−1)) and high contact resistance (larger than 100 kΩ·µm), mainly limited by the Schottky barrier induced by the mismatch of the work-functions and the Fermi level pinning at the metal contact interfaces. Here, we overcome these two obstacles through van der Waals (vdW) integration of high work-function metal palladium (Pd) as the contacts onto monolayer WSe2 grown by chemical vapor deposition (CVD) method. We demonstrate unipolar p-type monolayer WSe2 FETs with superior device performance: room temperature on-state current density exceeding 100 µA·µm^(−1), contact resistance of 12 kΩ·µm, on/off ratio over 107, and field-effect hole mobility of ~ 103 cm2·V^(−1)·s^(−1). Electrical transport measurements reveal that the Fermi level pinning effect is completely effectively eliminated in monolayer WSe2 with vdW Pd contacts, leading to a Schottky barrier-free Ohmic contact at the metal-semiconductor junctions. Combining the advantages of large-scale vdW contact strategy and CVD growth, our results pave the way for wafer-scale fabrication of complementary-metal-oxide-semiconductor (CMOS) logic circuits based on atomically thin 2D semiconductors.

Synthesis of wafer-scale monolayer MoS_(2) on sapphire: Unlocking the influence of key growth parameters

Large-scale synthesis of high-quality two dimensional(2D)semiconductors,such as molybdenum disulfide(MoS_(2)),is a prerequisite for their lab-to-fab transition.It is crucial to systematically explore and understand the influence of key synthetic conditions on the nucleation,uniformity,and quality of MoS_(2) wafers.Here,we report the epitaxial growth of high-quality and uniform monolayer MoS_(2) films on 2-in c-plane sapphire by chemical vapor deposition(CVD)method under optimized growth conditions(0–1 mg NaCl,adequate S/Mo ratio,and the addition of 0–1 sccm O2).We systematically explore the influence of critical synthetic conditions on the nucleation,and stitching of MoS_(2) domains over the wafer scale,including the dosage of the alkali metal salt NaCl additive,the evaporation temperature of MoO_(3),the distance between MoO_(3) and the substrate,and the flow rate of O_(2).Among them,the dosage of NaCl and the S/Mo ratio have important influences on the quality and film coverage of MoS_(2),while the flow rate of O_(2) plays a key role in controlling the nucleation density and domain size.We further discovered that a-plane sapphire could easily guide the unidirectional growth of MoS_(2) without the need for other specific synthetic conditions compared with c-plane and m-plane sapphire.The field-effect transistors(FETs)fabricated from the full-coverage films show an average and the highest mobilities of 28.5 and around 45 cm^(2)·V−1·s^(-1),respectively.