Crystallization and segregation in vitreous rutile films annealed at high temperature

Vitreous titania films with rutile short-range order were sputter deposited on unheated fused silica substrates, sequentially annealed at 973 and 1273 K, and examined by Raman microscopy, scanning electron microscopy, and x-ray diffraction. A segregated microstructure developed after the 1273 K anne...

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Bibliographic Details
Published in:Journal of vacuum science & technology. A, Vacuum, surfaces, and films Vacuum, surfaces, and films, 2005-11, Vol.23 (6), p.1568-1574
Main Authors: Omari, M. A., Sorbello, R. S., Aita, C. R.
Format: Article
Language:English
Subjects:
ANNEALING
CRYSTALLIZATION
DEPOSITION
EPITAXY
MATERIALS SCIENCE
MICROSTRUCTURE
MONOCLINIC LATTICES
RAMAN SPECTRA
RUTILE
SCANNING ELECTRON MICROSCOPY
SEGREGATION
SILICA
SPACE GROUPS
TEMPERATURE RANGE 0400-1000 K
TEMPERATURE RANGE 1000-4000 K
TETRAGONAL LATTICES
THERMAL EXPANSION
THERMAL STRESSES
THIN FILMS
TITANIUM OXIDES
X-RAY DIFFRACTION
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Summary:Vitreous titania films with rutile short-range order were sputter deposited on unheated fused silica substrates, sequentially annealed at 973 and 1273 K, and examined by Raman microscopy, scanning electron microscopy, and x-ray diffraction. A segregated microstructure developed after the 1273 K anneal. This microstructure consists of supermicron-size craters dispersed in a matrix of submicron rutile crystals. Ti–O short-range order in the craters is characteristic of a mixture of two high pressure phases, m - Ti O 2 (monoclinic P 2 1 ∕ c space group) and α - Ti O 2 (tetragonal Pbcn space group). We calculated that a high average compressive stress parallel to the substrate must be accommodated in the films at 1273 K, caused by the difference in the thermal expansion coefficients of titania and fused silica. The formation of the segregated microstructure is modeled by considering two processes at work at 1273 K to lower a film’s internal energy: crystallization and nonuniform stress relief. The Gibbs–Thomson relation shows that small m - Ti O 2 crystallites are able to form directly from vitreous Ti O 2 at 1273 K. However, the preferred mechanism for forming α - Ti O 2 is likely to be by epitaxial growth at crystalline rutile twin boundaries (secondary crystallization). Both phases are denser than crystalline rutile and reduce the average thermal stress in the films.
ISSN:0734-2101
1520-8559
DOI:10.1116/1.2091094