EPITAXIAL ZIRCONIA THIN-FILMS FROM AQUEOUS PRECURSORS

Heteroepitaxial single crystal thin films of ZrO2 (3-40 mol % Y2O3) have been deposited on single crystal, (100) oriented ZrO2 (9.5 MOI % Y2O3) 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: ZrO2 (40 mol % Y2O3)]. 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-degrees-C (approximately 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.