Accumulation-Type Ohmic Contacts Between Degenerately Doped and Nearly Intrinsic 2D Semiconductors

Abstract

Abstract Body: Two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs) have captivated the attention of the scientific community for about a decade now because they have demonstrated a multitude of desirable properties for flexible electronics and optoelectronics. However, a major materials challenge of 2D semiconductors is that they tend to form a substantial Schottky barrier (SB) with most metals used for making electrical contacts, severely limiting both the fundamental research and electronics/optoelectronics applications of 2D semiconductors. Efforts to eliminate/minimize the SB by selecting contact metals with high (low) work function for holes (electrons) have been hampered by the nearly ubiquitous presence of Fermi-level pinning. Van der Waals (vdW) contacts have recently emerged as a promising contact-engineering strategy to minimize the Fermi-level pinning effect at the metal/2D semiconductor interface. Electrical contacts formed by transferring prefabricated metal electrodes onto 2D semiconductors demonstrated SB heights (SBHs) approaching the Schottky-Mott limit, which was attributed to the minimization of structural and chemical defects at the interface by avoiding direct metal deposition on the 2D semiconductors. However, these contacts are still depletion-type with a notable positive SBH even when the work function of metal is significantly larger than the ionization energy of 2D semiconductors, indicating that Fermi-level pinning is still present. To date, accumulation-type ohmic contacts completely free of a SB have not been experimentally demonstrated in 2D semiconductor FET devices to the best of our knowledge. We report the realization of accumulation-type vdW Ohmic contacts between nearly intrinsic WSe2 and degenerately p-doped MoS2 (p+-MoS2). Dual-gated WSe2 field-effect transistors (FETs) with p+-MoS2 bottom-contacts were fabricated as a test structure to demonstrate true Ohmic contacts free of a Schottky barrier without the need of electrostatically gating the contact regions. Despite that the back-gate electric field is screened by the p+-MoS2 bottom-contacts and the nearly intrinsic nature of WSe2, these devices exhibit ohmic behavior and similarly high performance in the back-gate as well as in the top-gate configuration (with the other gate grounded), including linear output characteristics, a high on/off ratio of 108, and two-terminal extrinsic field-effect mobility exceeding 130 cm2V-1s-1 at room temperature, which approaches the intrinsic mobility of few-layer WSe2 (~ 200 cm2V-1s-1) with further reduction of the series resistance of excess p+-MoS2. The observed ohmic behavior persists to low temperatures. Theoretical modeling shows that a p+-MoS2/WSe2 junction effectively acts as an accumulation-type metal/semiconductor ohmic contact, as signified by an extremely thin depletion region of ~ 1nm in the p+-MoS2 side of the junction and that a substantial accumulation layer of free holes in the WSe2 side. The accumulation-type ohmic contacts between p+-MoS2 and WSe2 is further verified by the results of dual-gated measurements. We attribute the accumulation-type ohmic contacts to nearly ideal metal-semiconductor interface of a vdW junction consisting of 2D materials with similar crystalline and electronic structures, where the SBH is dictated by the Schottky-Mott rule. Because the work function of p+-MoS2 (acting as a 2D metal) is slightly larger than the ionization energy of WSe2 as verified by X-ray photoelectron spectroscopy (XPS), a strong hole accumulation region develops in WSe2 at the vdW interface as predicted by the Schottky-Mott rule. The 2D vdW contacts with negligible Fermi-level pinning can also be used as the baseline to study the mechanism of Fermi-level pinning at the metal/2D-semiconductor interface. *This work was partially supported by NSF (DMR-2004445; ECCS-1849578) and the Kaskas Scholarship Funds.