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

Abstract

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.

Conductive Atomic Force Microscopy of Semiconducting Transition Metal Dichalcogenides and Heterostructures

“…Recently, soft O 2 plasma functionalization revealed an effective method to finely tailor the SBH of multilayer MoS 2 , and this approach was exploited to demonstrate MoS 2 field effect transistors with ambipolar (i.e., both n-and p-type) behavior [29]. In this context, nanoscale electrical analyses by C-AFM showed how the SBH of MoS 2 can be tuned by increasing the plasma exposure time [29]. Figure 6b reports the SBH map after the 300 s soft plasma treatment, which results in a broader SBH distribution, with Φ B ranging from 0.21 to 0.58 eV, as shown by the histogram of the Φ B values (see Figure 6e).…”

“…Hence, starting from a narrow distribution of low SBHs for electrons in the case of pristine MoS 2 , the SBH map was modified after a 600 s O 2 plasma treatment into a broad distribution formed by nanometric patches with low SBH for holes in a background with low SBH for electrons. These SBH inhomogeneities in the O 2 plasma treated samples were associated to lateral variations of the incorporated oxygen concentration in the MoS 2 surface region [29]. Back-gated FETs were fabricated with Ni source and drain contacts deposited on pristine MoS 2 (see schematic in Figure 7a) or on areas selectively exposed to O 2 plasma functionalization for 600 s (see schematic in Figure 7c).…”

Vertical Transistors Based on 2D Materials: Status and Prospects

“…To date, p-type MoS2 transistors have been fabricated by using high work function MoOx contacts [64]. Recently, multilayer MoS2 transistors with ambipolar behavior have been demonstrated by selective-area p-type doping in the source/drain regions with O2 plasma [65]. Besides MoS2, other TMDs have been also considered for FETs' fabrication.…”

Advanced Scanning Probe Microscopy of Graphene and Other 2D Materials

“…The nanoscale Schottky barrier distribution at the surface of multilayer MoS 2 could be tailored by varying the incorporated oxygen concentration by O 2 plasma functionalization. Whereas a narrow SBH distribution (0.2-0.3 eV) was measured for pristine MoS 2 , a broader distribution (from 0.2 to 0.8 eV) in the modified one allowed both electrons and holes injection (Figure 4a-c) [90]. An attractive application of electrical mode SPMs is the use of conductive probes to induce electrochemical reactions and to pattern materials in electric field-induced nanolithography processes [91].…”