Impact and Origin of Interface States in MOS Capacitor with Monolayer MoS and HfO High-k Dielectric.

Two-dimensional layered semiconductors such as molybdenum disulfide (MoS) at the quantum limit are promising material for nanoelectronics and optoelectronics applications. Understanding the interface properties between the atomically thin MoS channel and gate dielectric is fundamentally important for enhancing the carrier transport properties. Here, we investigate the frequency dispersion mechanism in a metal-oxide-semiconductor capacitor (MOSCAP) with a monolayer MoS and an ultra-thin HfO high-k gate dielectric. We show that the existence of sulfur vacancies at the MoS-HfO interface is responsible for the generation of interface states with a density (D) reaching ~7.03 × 10 cm eV. This is evidenced by a deficit S:Mo ratio of ~1.96 using X-ray photoelectron spectroscopy (XPS) analysis, which deviates from its ideal stoichiometric value. First-principles calculations within the density-functional theory framework further confirms the presence of trap states due to sulfur deficiency, which exist within the MoS bandgap. This corroborates to a voltage-dependent frequency dispersion of ~11.5% at weak accumulation which decreases monotonically to ~9.0% at strong accumulation as the Fermi level moves away from the mid-gap trap states. Further reduction in D could be achieved by thermally diffusing S atoms to the MoS-HfO interface to annihilate the vacancies. This work provides an insight into the interface properties for enabling the development of MoS devices with carrier transport enhancement.