Paper 2D-ThP8
Surface Functionalization of Few-layer MoS₂ for Atomic Layer Deposition using Gold Chloride Salts

Thursday, November 2, 2017, 6:30 pm, Room Central Hall

Transition metal dichalcogenides such as MoS₂ have attracted much interest in the field of nanoelectronics in recent years. MoS₂ is a layered material with a hexagonal structure similar to graphene. Unlike graphene, however, MoS₂ is a semiconducting material with an indirect band gap of ~1.2 eV in bulk form and a direct band gap of ~1.8 eV in monolayer form. As such, MoS₂ has attracted interest as a possible channel material in field-effect transistors. An important feature of the modern field-effect transistor is the gate dielectric. In the past decade, the electronics industry has transitioned from using native silicon oxide as the gate dielectric to using high-k metal oxides deposited via atomic layer deposition. Field-effect devices fabricated from MoS₂ will thus require the growth of metal oxides on the MoS₂ surface. Unfortunately, the sulfur-terminated MoS₂ surface is hydrophobic and not conducive to metal oxide film growth using atomic layer deposition. As such, the surface must be functionalized prior to deposition. Here, we report a novel wet chemistry method for functionalization of MoS₂ surfaces using gold chloride salts.

Mechanically exfoliated MoS₂ surfaces are treated by immersion in a solution of HAuCl₄ or AuCl₃ for 5-60 seconds and are subsequently subjected to atomic layer deposition of Al₂O₃ using trimethylaluminum and water as precursors. We measure the effectiveness of the surface treatment by investigating the post-deposition surface topography using atomic force microscopy. Our hypothesis is that immersion of the MoS₂ surfaces in the gold chloride solution will leave behind adsorbed gold chloride molecules which render the surface hydrophilic and amenable to the growth of metal oxide films. While untreated surfaces show island growth, gold chloride-treated surfaces show more coalesced films. Treatment for at least 30 seconds results in a reduction of the film RMS roughness by a factor of 4 compared to untreated surfaces after deposition of 3 nm (nominal thickness) Al₂O₃ at 200°C. Film surface coverage improves from 50-60% on untreated surfaces to 80-99% on surfaces treated for 10 seconds and finally to complete film coverage on surfaces treated for 30 and 60 seconds. Surface roughness and coverage is investigated as a function of film thickness and deposition temperature, and provides information on initial growth mechanisms and process conditions for the growth of conformal, high-quality films.