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

Atomically thin molybdenum disulfide(MoS2) is a promisingsemiconducting material for next-generation electronics and photonics.These applications often require its pairing with ultrathin AtomicLayer Deposited (ALD) high-k dielectrics, which maynot only improve the performance of fabricated field-effect transistors(FET) but also protect against harmful interactions with the ambientenvironment. However, because ALD on the pristine MoS2 surfacesuffers from prolonged nucleation, resulting in ultrathin continuousfilm growth difficulty, its preliminary functionalization becomesa necessary aspect. To date, the most reliable way of MoS2 functionalization is ion beam exposure, which enables to producepoint defects, the presence of which should improve the precursorchemisorption during the following ALD growth. In this work, we demonstratedthe possibility of the sulfur vacancy (V (S)) concentration tuning both at the sub-surface level of a bulk crystaland in few-layer films by controlling the He+ ion energyand the irradiation dose. Based on X-ray photoelectron spectroscopy(XPS) analysis, the bulk crystal was prone to significant sulfur losseven under 500 eV He+ irradiation, which is governed byphysical 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 Ramanspectroscopy, since no stoichiometry changes were detected. Generatedsulfur vacancies are readily passivated through Mo-OH species duringsubsequent water vapor exposure in the ALD reactor, which resultedin a significantly enhanced ALD HfO2 growth. As a result,the possibility of the ultrathin (<3 nm) and continuous film growthon the irradiated bulk MoS2 surface was demonstrated, andfor the monolayer MoS2 film, an increase in the surfacedensity of ALD active sites by a factor of & AP;2 was observed.