Crystallization, metastable phases, and demixing in a hafnia-titania nanolaminate annealed at high temperature

Abstract

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.

Addendum to “Phase selection and transition in Hf-rich hafnia-titania nanolaminates” (on SiO2) [J. Appl. Phys. 109, 123523 (2011)]: Hafnon formation

“…1,3 The aforementioned double peak vicinal to the rutile standard, labeled A and B, as well as a lower angle peak, labeled C, were also observed after 24 h at 1273 K. Peak A is attributed to the (110) planes of TiO 2 :Hf as before, 1-3 but peak B is now attributed to hafnon (200) planes. 8 Peak C is attributed to hafnon (101) planes.…”

“…[1][2][3] The as-grown film consists of nanocrystalline orthorhombic (o) HfTiO 4 . (o-HfTiO 4 is intrinsically unstable and demixes upon annealing above 973 K. 1,3 ). Post-deposition air annealing was carried out first at 1173 K for up to 192 h (denoted stage I).…”

Influence of Material Composition on Structural and Optical Properties of HfO2-TiO2 Mixed Oxide Coatings

“…In the literature, the HfO 2 -TiO 2 pseudobinary systems have been investigated previously by others [17,[23][24][25], especially in view of their electrical properties [26][27][28]. As was shown by Li et al [26], mixed oxides of HfO 2 -TiO 2 had, for example, higher permittivity compared to undoped HfO 2 .…”

Ti-catalyzed HfSiO4 formation in HfTiO4 films on SiO2 studied by Z-contrast scanning electron microscopy

“…We previously reported that orthorhombic (o) HfTiO 4 in sputter deposited films on fused SiO 2 substrates demixes upon annealing to form monoclinic (m) HfO 2 and rutile (r) TiO 2 . 6,7 When demixing occurs at high temperature (1273 K), crystalline hafnon (HfSiO 4 ) with a zircon archetype lattice is formed, 8 indicating a reaction between the film and the substrate. This last result is surprising because HfSiO 4 does not form in a HfO 2 -on-SiO 2 film subjected to a similar annealing regimen.…”

“…2(a)) shows characteristic round features associated with demixing. [6][7][8] The Raman shifts acquired from a feature and the surrounding matrix are shown in Fig. 2(b).…”