Nanocrystalline monoclinic HfO2 films were sputter deposited on fused silica substrates, air annealed at 573 to 1273 K to affect crystallite growth, and analyzed by x-ray diffraction and spectrophotometry. Lattice expansion occurs with diminishing crystallite size. O 2p→Hf 5d interband absorption dominates the optical edge at energy E≥6.24 eV, with an optical band gap, Eo=5.48±0.023, which is independent of crystallite size. However, the strength of a localized resonant band, with onset at 5.65 eV and maximum at 5.94 eV, is affected by crystallite size. Its polaronic origin in a perfect HfO2 lattice is discussed.

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Orthorhombic (o) HfTiO4 is crystallized when sputter deposited hafnia-titania nanolaminates with ultrathin layers and bilayer (HfO2)0.5(TiO2)0.5 composition are annealed between 573 and 1173 K. However, o-HfTiO4 demixes after annealing at 1273 K, a result not predicted from bulk thermodynamics. X-ray diffraction and Raman microscopy are used here to study structural changes as o-HfTiO4 demixes upon long-term annealing at 1273 K into Ti-doped monoclinic HfO2 and Hf-doped rutile TiO2. We conclude that o-HfTiO4 crystallized at low temperature is intrinsically metastable. A space group symmetry analysis shows that demixing can be accomplished by a continuous phase transition chain.

Nanolaminate films with a nominal 5 nm HfO2–4 nm TiO2 bilayer architecture are sputter deposited on unheated fused silica and Au-coated glass substrates. Films on fused silica are postdeposition annealed from 573 to 1273 K and characterized by x-ray diffraction, scanning electron microscopy, Raman microscopy, and UV-visible-near IR spectrophotometry. The films show weak but progressive crystallization into orthorhombic (o) HfTiO4 when annealed up to 973 K. o-HfTiO4 is expected to form under bulk thermodynamic equilibrium conditions in the case of complete mixing of the bilayer components. Annealing above 973 K produces a crystallization sequence that is not predicted by bulk thermodynamics, ultimately involving o-HfTiO4 demixing to form monoclinic HfO2 doped with Ti and rutile TiO2 doped with Hf. These phases have a higher atomic density than o-HfTiO4 and segregate into discrete mesoscopic features. The authors propose that o-HfTiO4 demixing into higher density phases is a mechanism for thermal stress relief at high temperature. Demixing results in a major loss of optical transparency in the visible and ultraviolet spectral regions.

Nanolaminate HfO2–TiO2 films are grown by reactive sputter deposition on unheated fused SiO2, sequentially annealed at 573to973K, and studied by x-ray diffraction. A nanocrystalline structure of orthorhombic (o) HfTiO4 adjacent to an interface followed by monoclinic (m) Hf1−xTixO2 is identified. m-Hf1−xTixO2, a metastable phase, is isomorphous with m-HfO2 and a high pressure phase, m-HfTiO4. A Vegard’s law analysis shows that the Ti atomic fraction in m-Hf1−xTixO2 is much greater than Ti equilibrium solubility in m-HfO2. A space group-subgroup argument proposes that m-Hf1−xTixO2 arises from an o∕m-HfTiO4 second order phase transition to accommodate the larger Hf atom.