Nucleation mechanism during WS2 plasma enhanced atomic layer deposition on amorphous Al2O3 and sapphire substrates.

The structure, crystallinity and properties of as-deposited two-dimensional (2D) transition metal dichalcogenides are determined by nucleation mechanisms in the deposition process. 2D materials grown by atomic layer deposition (ALD) in the absence of a template are polycrystalline or amorphous. Little is known about their nucleation mechanisms. Therefore, the nucleation behavior of WS₂ during plasma enhanced ALD from WF₆, H₂ plasma and H₂S at 300 °C is investigated on amorphous ALD Al₂O₃ starting surface and on monocrystalline, bulk sapphire. Preferential interaction of the precursors with the Al₂O₃ starting surface promotes fast closure of the WS₂ layer. The WS₂ layers are fully continuous at WS₂ content corresponding to only 1.2 WS₂ monolayers. On amorphous Al₂O₃, (0002) textured and polycrystalline WS₂ layers form with grain size of 5 to 20 nm due to high nucleation density (∼10¹⁴ nuclei/cm²). The WS₂ growth mode changes from 2D (layer-by-layer) growth on the initial Al₂O₃ surface to three-dimensional (Volmer-Weber) growth after WS₂ layer closure. Further growth proceeds from both WS₂ basal planes in register with the underlying WS₂ grain and from or over grain boundaries of the underlying WS₂ layer with different in-plane orientation. In contrast, on monocrystalline sapphire, WS₂ crystal grains can locally align along a preferred in-plane orientation. Epitaxial seeding occurs locally albeit a large portion of crystals remain randomly oriented, presumably due to the low deposition temperature. The WS₂ sheet resistance is 168 MΩμm, suggesting that charge transport in the WS₂ layers is limited by grain boundaries. [ABSTRACT FROM AUTHOR]