Abstract
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 (Dit) 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 Dit 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.
Highlights
Two-dimensional (2D) MoS2 has attracted a lot of interests for electronics applications due to its excellent electrical properties, such as high mobility[1], near-ideal subthreshold swing[2] and high on/off current ratio[3,4,5,6]
Sulfur vacancies are responsible for the defects in mechanical exfoliation (ME) and chemical vapor deposition (CVD) while antisite defects with molybdenum replacing sulfur dominate in thermal evaporation
We find that the presence of sulfur vacancies is responsible for the generation of interface states that causes the frequency dispersion in the accumulation regime of the MoS2 metal-oxide-semiconductor capacitor (MOSCAP), which exhibits a dependence on the applied gate voltage
Summary
Two-dimensional (2D) MoS2 has attracted a lot of interests for electronics applications due to its excellent electrical properties, such as high mobility[1], near-ideal subthreshold swing[2] and high on/off current ratio[3,4,5,6]. The high mobility achieved through the adoption of high-k dielectric in MoS2 field-effect transistor is comparable to that of thin-film silicon[9], opening up a wealth of opportunities for its application in future electronics. A monolayer MoS2 nano-sheet is deposited onto the HfO2/Si substrate by magnetron sputtering approach. A magnetron sputtering method was reported to synthesize wafer-scale, high-uniformity and high-purity MoS2 thin film with good control of thickness using a one-step process[17]. To-date, the experimentally attainable hole mobility and on/off ratio of SiO2-based MoS2 transistors fabricated using the magnetron sputtering method have been reported to be ~12.2 cm[2] V−1 s−1 and ~103, respectively[17]. Carrier transport study of the magnetron-sputtered MoS2 with an integrated high-k gate dielectric have not been reported so far. Sulfur vacancies are responsible for the defects in ME and CVD while antisite defects with molybdenum replacing sulfur dominate in thermal evaporation
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