Nanovoid formation by change in amorphous structure through the annealing of amorphous Al2O3 thin films

作者：Tane M^*; Nakano S; Nakamura R; Ogi H; Ishimaru M; Kimizuka H; Nakajima H

来源：Acta Materialia, 2011, 59(11): 4631-4640.

DOI：10.1016/j.actamat.2011.04.008

• Nanovoid
• Amorphous oxides
• Elastic properties
• Molecular dynamics simulations
• Nanoporous

The formation mechanism of a high density of nanovoids by annealing amorphous Al2O3 thin films prepared by an electron beam deposition method was investigated. Transmission electron microscopy observations revealed that nanovoids similar to 1-2 nm in size were formed by annealing amorphous Al2O3 thin films at 973 K for 1-12 h, where the amorphous state was retained. The elastic stiffness, measured by a picosecond laser ultrasound method, and the density, measured by X-ray reflectivity, increased drastically after the annealing process, despite nanovoid formation. These increases indicate a change in the amorphous structure during the annealing process. Molecular dynamics simulations indicated that an increase in stable AlO6 basic units and the change in the ring distribution lead to a drastic increase in both the elastic stiffness and the density. It is probable that a pre-annealed Al2O3 amorphous film consists of unstable low-density regions containing a low fraction of stable AlO6 units and stable high-density regions containing a high fraction of stable AlO6 units. Thus, local density growth in the unstable low-density regions during annealing leads to nanovoid formation (i.e., local volume shrinkage).

• 出版日期2011-6