In atomic layer deposition, expose the surface to a molybdenum precursor, purge unreacted precursor and volatile products, expose it to a sulfur reactant, and purge again. Each half-reaction should saturate the available reactive sites rather than continue indefinitely with dose. Repetition builds a molybdenum disulfide film, with nucleation, incubation and layer closure checked independently.
Self-limited surface chemistry is not the same as depositing exactly one complete crystal monolayer per cycle. A cycle may add only a fraction of a layer, and initially isolated islands can thicken before all bare substrate is covered. Reactions on hydroxylated silicon dioxide can also differ from those on the comparatively inert MoS basal surface. A precursor pair must actually show saturation under the chosen conditions; simply alternating gases is insufficient evidence of ALD.
In metalorganic chemical vapor deposition, metalorganic and sulfur-containing precursors are typically supplied simultaneously or with overlapping residence times, and film growth depends on decomposition, transport and surface reaction rates. Both methods involve chemistry, but ALD separates saturating half-reactions and purge steps. An ALD process need not use a metalorganic precursor, and pulsed CVD is not automatically ALD.
Potential advantages are reproducible thickness increments, large-area uniformity, conformal access to features and compatibility with moderate-temperature integration. Saturated exposure can make coverage less sensitive to small precursor-flux variations than an unsaturated deposition rate. The cycle count provides a useful process variable for matching an optical or electrical thickness target.
For a monolayer device, control of nucleation and coalescence is especially valuable, because excessive second-layer growth and uncovered patches both change the optical response. The advantages are conditional: a reliable ALD window and suitable precursor chemistry are required, and conformality does not imply a single-crystal film. Long exposure and purge steps reduce throughput, residual ligands or impurities can remain, and post-growth crystallization may require temperatures that the integrated device cannot tolerate.
Many independently oriented nuclei form on an amorphous support and then coalesce before they can become one large crystal. The SiO substrate provides no unique epitaxial orientation. At relatively low deposition temperature, short surface diffusion lengths and slow bond rearrangement restrict the distance over which deposited species can find a well-ordered configuration. Island collisions therefore leave grain boundaries; precursor residues and incomplete sulfurization can add disorder.
Raising temperature or annealing can improve crystallinity, but may conflict with the ALD reaction window, substrate compatibility or sulfur retention. Small grains are consequently a frequent process outcome, not a fundamental theorem that ALD can never form a well-ordered layer. A direct ALD experiment on SiO-covered wafers found nanocrystalline MoS by diffraction and electron microscopy, while demonstrating self-limited precursor exposure behaviour.[ref]
Wafer-scale, conformal and direct growth of MoS2 thin films by atomic layer deposition by Yujin Jang and others. 2016, SiO2 substrates, self-limited growth and nanocrystalline microstructure
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