Low-Energy He⁺ Ions Induced Functionalization of the MoS₂ Surface for ALD HfO₂ Growth Enhancement

Abstract

Atomically thin molybdenum disulfide (MoS2) is a promising semiconducting material for next-generation electronics and photonics. These applications often require its pairing with ultrathin Atomic Layer Deposited (ALD) high-k dielectrics, which may not only improve the performance of fabricated field-effect transistors (FET) but also protect against harmful interactions with the ambient environment. However, because ALD on the pristine MoS2 surface suffers from prolonged nucleation, resulting in ultrathin continuous film growth difficulty, its preliminary functionalization becomes a necessary aspect. To date, the most reliable way of MoS2 functionalization is ion beam exposure, which enables to produce point defects, the presence of which should improve the precursor chemisorption during the following ALD growth. In this work, we demonstrated the possibility of the sulfur vacancy (V S) concentration tuning both at the sub-surface level of a bulk crystal and in few-layer films by controlling the He+ ion energy and the irradiation dose. Based on X-ray photoelectron spectroscopy (XPS) analysis, the bulk crystal was prone to significant sulfur loss even under 500 eV He+ irradiation, which is governed by physical sputtering selectivity. In contrast to the bulk crystal, for the case of few-layer MoS2 films, just the Frenkel V S formation was observed, as indicated by Raman spectroscopy, since no stoichiometry changes were detected. Generated sulfur vacancies are readily passivated through Mo-OH species during subsequent water vapor exposure in the ALD reactor, which resulted in a significantly enhanced ALD HfO2 growth. As a result, the possibility of the ultrathin (<3 nm) and continuous film growth on the irradiated bulk MoS2 surface was demonstrated, and for the monolayer MoS2 film, an increase in the surface density of ALD active sites by a factor of ≈2 was observed.