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In this work, the formation of micrometer-size crystalline monolayer (1L) MoS2 flakes with triangular shape and a central multilayer core is obtained by the sulfurization at 800 °C of pre-deposited ultrathin MoOx films (1.2–1.8 nm) on c-sapphire substrates. The thickness uniformity, crystalline quality, doping and strain distribution in the MoS2 flakes as a function of the initial MoOx thickness was evaluated by micro-Raman (μR) mapping. The excellent crystalline quality of the triangular 1L-MoS2 flakes was confirmed by micro-photoluminescence (μPL) maps, showing a very intense peak at ∼1.85 eV, that decreases in the central part, as expected for multilayers MoS2. A biaxial strain of ∼0.38–0.4% was deduced from the μPL data, in perfect agreement with μR results. Our results show how the sulfurization of pre-deposited MoOx films on c-sapphire allows, under proper conditions, to obtain 1L-MoS2 flakes with quality comparable to the one typically reported by the conventional chemical vapour deposition, with important implications for device applications.

A systematic study was performed on the effects of surface treatments before Atomic Layer Deposition (ALD) growth of Al 2 O 3 thin films on (0001)AlGaN/GaN substrate. The AlGaN/GaN surface was treated either with: H 2 O 2 :H 2 SO 4 (A treatment) and H 2 O 2 :H 2 SO 4 + H 2 O:HF (B treatment). After surface wet-treatments, Al 2 O 3 was immediately deposited at 250 °C by Plasma Enhanced ALD from trimethylaluminum precursor. The film thickness was measured to be about 27 nm using transmission electron microscopy and different structural evolution was observed under electron beam analysis, re-arranging from amorphous as-deposited films to polycrystalline or epitaxial films depending on the pre-deposition treatment. Surface morphology obtained by atomic force microscopy in tapping mode showed smooth Al 2 O 3 layers with lower roughness in the case of films deposited after B treatment. Dielectric properties have been evaluated for films deposited after both A and B treatments and better dielectric properties have been observed for film fabricated after B treatment. Moreover, dielectric properties improved after post-deposition annealing at high temperature.