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

One of the main challenges to exploit molybdenum disulfide (MoS) 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 MoS 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 MoS surface can be tailored at nanoscale using soft O plasma treatments. The morphological, chemical, and electrical modifications of MoS 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 MoS to a broader distribution (from 0.2 to 0.8 eV) after 600 s O 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 MoS multilayer surface, as shown by STEM/EELS analyses and confirmed by ab initio density functional theory (DFT) calculations. Back-gated multilayer MoS FETs, fabricated by self-aligned deposition of source/drain contacts in the O plasma functionalized areas, exhibit ambipolar current transport with on/off current ratio I/I ≈ 10 and field-effect mobilities of 11.5 and 7.2 cm V s 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 O plasma functionalized MoS surface.