Impact and Origin of Interface States in MOS Capacitor with Monolayer MoS₂ and HfO₂ High-k Dielectric

Two-dimensional layered semiconductors such as molybdenum disulfide (MoS₂) at the quantum limit are promising material for nanoelectronics and optoelectronics applications. Understanding the interface properties between the atomically thin MoS₂ 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 MoS₂ and an ultra-thin HfO₂ high-k gate dielectric. We show that the existence of sulfur vacancies at the MoS₂-HfO₂ interface is responsible for the generation of interface states with a density (D_it) reaching ~7.03 × 10¹¹ cm^-2 eV^-1. 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 MoS₂ 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_it could be achieved by thermally diffusing S atoms to the MoS₂-HfO₂ interface to annihilate the vacancies. This work provides an insight into the interface properties for enabling the development of MoS₂ devices with carrier transport enhancement.