Structure of Amorphous Aluminum Oxide

Whereas prototypical ${\mathrm{Al}}_2{\mathrm{O}}_3$ is not a glass former, amorphous ${\mathrm{Al}}_2{\mathrm{O}}_3$ can be formed as thin films through vapor deposition and can serve as a structural model for the ${\mathrm{Al}}_2{\mathrm{O}}_3$ glass. The first two-dimensional solid-state NMR experiments for amorphous ${\mathrm{Al}}_2{\mathrm{O}}_3$ thin film reveal that four- and five-coordinated species are predominant (95%), while six-coordinated species are minor. Such a species distribution is remarkably similar to what has been predicted theoretically for ${\mathrm{Al}}_2{\mathrm{O}}_3$ melts. Upon annealing to $800°\mathrm{C}$ the five-coordinated species becomes negligible, indicating the onset of crystallization of ${\mathrm{Al}}_2{\mathrm{O}}_3$.

Figure 1

Cross section TEM image [(left) low- and (right) high-resolution image with a diffraction pattern (inset)] of as-deposited alumina thin film. TEM specimens were prepared on a focused Ga-ion beam (FIB) workstation. Cross section image of lamellae, $\sim 10\mu \mathrm{m}$ wide and $5\mu \mathrm{m}$ deep, lifted out and attached to a Mo half grid. The cutting of the lamellae was done with 7–20 nA at 30 kV. The specimens were milled at 30 kV and 0.5–3 nA and were polished at 5 kV and 70 pA to reduce focused-Ga-ion-beam-induced damage. The final thickness was 40–50 nm. The image was obtained in a TEM [JEM4010 (JEOL Ltd., Co.)].

Figure 2

${}^{27}\mathrm{Al}$ MAS NMR spectra for ${\mathrm{Al}}_2{\mathrm{O}}_3$ thin film, rotor background, and Si substrate. $*$ refers to spinning sidebands.

Figure 3

${}^{27}\mathrm{Al}$ 3QMAS NMR spectra for ${\mathrm{Al}}_2{\mathrm{O}}_3$ thin film. Projections on the isotropic and MAS dimension are also shown. Contour lines are drawn at 5% intervals from relative intensities of 12% to 97% with added lines at 6% and 9%.

Figure 4

${}^{27}\mathrm{Al}$ MAS NMR spectra for ${\mathrm{Al}}_2{\mathrm{O}}_3$ thin film annealed at $800°\mathrm{C}$ and $1200°\mathrm{C}$. Rotor/Si backgrounds were subtracted from the original spectra.