Ambipolar MoS₂ Transistors by Nanoscale Tailoring of Schottky Barrier Using Oxygen Plasma Functionalization

One of the main challenges to exploit molybdenum disulfide (MoS₂) potentialities for the next-generation complementary metal oxide semiconductor (CMOS) technology is the realization of p-type or ambipolar field-effect transistors (FETs). Hole transport in MoS₂ FETs is typically hampered by the high Schottky barrier height (SBH) for holes at source/drain contacts, due to the Fermi level pinning close to the conduction band. In this work, we show that the SBH of multilayer MoS₂ surface can be tailored at nanoscale using soft O₂ plasma treatments. The morphological, chemical, and electrical modifications of MoS₂ surface under different plasma conditions were investigated by several microscopic and spectroscopic characterization techniques, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), conductive AFM (CAFM), aberration-corrected scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). Nanoscale current-voltage mapping by CAFM showed that the SBH maps can be conveniently tuned starting from a narrow SBH distribution (from 0.2 to 0.3 eV) in the case of pristine MoS₂ to a broader distribution (from 0.2 to 0.8 eV) after 600 s O₂ plasma treatment, which allows both electron and hole injection. This lateral inhomogeneity in the electrical properties was associated with variations of the incorporated oxygen concentration in the MoS₂ multilayer surface, as shown by STEM/EELS analyses and confirmed by ab initio density functional theory (DFT) calculations. Back-gated multilayer MoS₂ FETs, fabricated by self-aligned deposition of source/drain contacts in the O₂ plasma functionalized areas, exhibit ambipolar current transport with on/off current ratio I_on/I_off ≈ 10³ and field-effect mobilities of 11.5 and 7.2 cm² V^-1 s^-1 for electrons and holes, respectively. The electrical behavior of these novel ambipolar devices is discussed in terms of the peculiar current injection mechanisms in the O₂ plasma functionalized MoS₂ surface.