We report the interface properties of atomic-layer-deposited Al2O3 thin films on ultraviolet/ozone (UV/O3)-treated multilayer MoS2 crystals. The formation of S-O bonds on MoS2 after low-power UV/O3 treatment increased the surface energy, allowing the subsequent deposition of uniform Al2O3 thin films. The capacitance-voltage measurement of Au-Al2O3-MoS2 metal oxide semiconductor capacitors indicated n-type MoS2 with an electron density of ∼10(17) cm(-3) and a minimum interface trap density of ∼10(11) cm(-2) eV(-1). These results demonstrate the possibility of forming a high-quality Al2O3-MoS2 interface by proper UV/O3 treatment, providing important implications for their integration into field-effect transistors.

High contact resistance of transition-metal dichalcogenide (TMD)-based devices is one of the bottlenecks that limit the application of TMDs in various domains. Contact resistance of TMD-based devices is strongly related to the interface chemistry and band alignment at the contact metal/TMD interfaces. To understand the metal/MoS 2 interface chemistry and band alignment, Ni and Ag metal contacts are deposited on MoS 2 bulk and chemical vapor deposition bilayer MoS 2 (2L-MoS 2 ) film samples under ultrahigh vacuum (∼3 × 10 −11 mbar) and high vacuum (∼3 × 10 −6 mbar) conditions. X-ray photoelectron spectroscopy is used to characterize the interface chemistry and band alignment of the metal/MoS 2 stacks. Ni forms covalent contact on MoS 2 bulk and 2L-MoS 2 film by reducing MoS 2 to form interfacial metal sulfides. In contrast, van der Waals gaps form at the Ag/MoS 2 bulk and Ag/ 2L-MoS 2 film interfaces, proved by the absence of an additional metal sulfide chemical state and the detection of Ag islands on the surface. Different from other metal/MoS 2 systems studied in this work, Ag shows potential to form an Ohmic contact on MoS 2 bulk regardless of the deposition ambient. Fermi levels (E F 's) are pinned near the intrinsic E F of the 2L-MoS 2 film with high defect density regardless of the work function of the metal, which highlights the impact of substrate defect density on the E F pinning effect and contact resistance.

We report the effect of Al2O3 encapsulation on the carrier mobility and contact resistance of multilayer MoS2 thin-film transistors by statistically investigating 70 devices with SiO2 bottom-gate dielectric. After Al2O3 encapsulation by atomic layer deposition, calculation based on Y-function method indicates that the enhancement of carrier mobility from 24.3 cm2 V−1 s−1 to 41.2 cm2 V−1 s−1 occurs independently from the reduction of contact resistance from 276 kΩ·μm to 118 kΩ·μm. Furthermore, contrary to the previous literature, we observe a negligible effect of thermal annealing on contact resistance and carrier mobility during the atomic layer deposition of Al2O3. These results demonstrate that Al2O3 encapsulation is a useful method of improving the carrier mobility of multilayer MoS2 transistors, providing important implications on the application of MoS2 and other two-dimensional materials into high-performance transistors.

We report the photoluminescence quenching in monolayer MoS2, MoSe2, WS2, and WSe2 by atomic layer deposited Al2O3 encapsulation. The negative shift and broadening of photoluminescence emission suggested electron doping after encapsulation. The further reduction, softening, and broadening of the A1g mode in Raman spectra also suggested electron doping after Al2O3 encapsulation. To investigate the origin of photoluminescence quenching, we fabricated bottom-gate MoS2 transistors on SiO2/Si substrates. Under a 405-nm-laser, Al2O3-encapsulated MoS2 transistors exhibited enhanced electron photocurrent, suggesting that photoluminescence quenching was dominated by hole transfer to encapsulation-induced trap states. These results demonstrated the importance of defect control for the dielectric deposition in achieving high-performance optoelectronic devices based on monolayer transition metal dichalcogenides.

We report the variability of electrical properties of Ti contacts in back-gated multilayer MoS 2 thin-film transistors based on mechanically exfoliated flakes. By measuring current-voltage characteristics from room temperature to 240°C, we demonstrate the formation of both ohmic and Schottky contacts at the Ti-MoS 2 junctions of MoS 2 transistors fabricated using identical electrode materials under the same conditions. While MoS 2 transistors with ohmic contacts exhibit a typical signature of band transport, those with Schottky contacts indicate thermally activated transport behavior for the given temperature range. These results provide the experimental evidence of the variability of Ti metal contacts on MoS 2 , highlighting the importance of understanding the variability of electronic properties of naturally occurring MoS 2 for further investigation.