Epitaxial zirconia thin films from aqueous precursors

Heteroepitaxial single crystal thin films of ZrO_2 (3–40 mol % Y_2O_3) have been deposited on single crystal, (100) oriented ZrO_2 (9.5 mol % Y_2O_3) substrates, using aqueous precursor solutions of zirconium acetate and yttrium nitrate. Film compositions crystallizing with the cubic structure had a lattice mismatch up to 1.59% [film composition: ZrO_2 (40 mol% Y_2O_3)]. Precursor films were deposited by spin coating, pyrolyzed to form the oxide, and heated at high temperatures to promote epitaxial growth. Cross-sectional TEM observations of thin films annealed at 600 °C (∼0.3 T _m) show the film to be composed of two distinct regions: an epitaxial layer, 0–6 nm thick, immediately adjacent to the substrate surface, and a porous nanocrystalline region (5–10 nm grain size) comprising the bulk of the film. At higher temperatures, the epitaxial layer grows by consuming the nanocrystalline material. Porosity accumulates at the growing interface, producing a dense epitaxial layer. Lattice mismatch is accommodated by a combination of misfit dislocations and epitaxial film strain. Calculations indicate that the energy required to accommodate the lattice mismatch can be a significant fraction of the total driving energy of grain boundary elimination, suggesting that growth may be completely arrested if the misfit between film and substrate exceeds a critical value.