Phase selection and transition in Hf-rich hafnia-titania nanolaminates

摘要

Hf-rich hafnia-titania nanolaminate films with five HfO_2-TiO_2 bilayer architectures (0.64 to 0.94 Hf atom fraction) were sputter deposited on unheated fused silica substrates, annealed post-deposition from 573 to 1273 K, and analyzed by x-ray diffraction to study phase selection and transition. Isochronal annealing for 1 h intervals from 573 to 1173 K produces weak crystallization into monoclinic (m) HfO_2 doped with Ti, i.e., m-Hf_(1-x)Ti_xO_2. The amount of Ti incorporated into m-HfO_2 depends upon both architecture and overall stoichiometry, but in all but the coarsest architecture, exceeds the bulk solubility limit of x = 0.05. Initial annealing at 1273 K produces significant crystallization into a biphasic structure, m-Hf_91-x)Ti_xO_2 and orthorhombic (o) HfTiO_4. From bulk phase equilibrium considerations, o-HfTiO_4 is expected to crystallize under conditions of interfacial bilayer mixing. However, upon further annealing at 1273 K, o-HfTiO_4 proves to be unstable. o-HfTiO_4 demixing inevitably occurs independent of architecture and stoichiometry, resulting in final crystallization products after 96 h at 1273 K that are m-Hf_(1-x)Ti_xO_2 with x ≈ 0.05 and TiO_2 doped with Hf. We suggest that o-HfTiO_4 instability arises from a driving force to form domains similar to those found in the low temperature in/commensurate structures of ZrTiO_4. A detailed crystallographic group-subgroup analysis of the o (Pbcn) → m (P2_1/c) transition shows that these domains can be represented by an orientation twin in the latter structure and their creation can be achieved by a single step second-order phase transition.