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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