Nanocrystalline Tin Oxide Thin Films via Liquid Flow Deposition

Nanocrystalline films of SnO₂ were deposited by liquid flow deposition (LFD), i.e., by flowing aqueous solutions of SnCl₄·5H₂O and HCl over single-crystalline silicon substrates at 80°C. The substrates were either oxidized and fully hydrolyzed (bare silicon) or oxidized, hydrolyzed, and then coated with siloxy-anchored organic self-assembled monolayers (SAMs). Continuous, adherent films formed on sulfonate- and thioacetate-functionalized SAMs; adherent but sometimes discontinuous films formed on bare silicon and methyl-functionalized SAMs. The films contained equiaxed cassiterite crystals, ～4–10 nm in size. The film thickness increased linearly with deposition time. The maximum growth rate observed was 85 nm·h^?1 on sulfonate SAM, and the maximum film thickness obtained was 1 μm. A new dimensionless parameter, the normalized residence time, τ, was introduced for the purpose of interpreting the influence of solution conditions (i.e., degree of supersaturation, as controlled via pH, and tin concentration) and flow characteristics (flow rate and the configuration of the deposition chamber) on the growth rate in LFD processes. The results were consistent with a particle attachment mechanism for film growth and inconsistent with heterogeneous nucleation on the substrate.