Directed evolution of α-grains in thin metastable-Al2O3 films deposited on Si(100) after post-deposition annealing

A 250-nm-thick Al2O3 film was deposited on a Si(100) by a radio-frequency magnetron sputtering and annealed at 1100 °C for various periods of time in air. In the matrix composed of fine metastable-Al2O3 grains of 50-100 nm in diameter, large α-Al2O3 grains of about 2-10 μm in diameter appeared, interestingly aligning themselves along various directions. The compressive stress developed in the alumina films because the thermal expansion coefficient of the film was higher than that of the silicon substrate. The stress distribution in the film is expected to be inhomogeneous due to some discontinuities or defects, such as arrays of dislocation pits and steps on the surface of the Si substrate, which could be generated by intersections of the substrate surface and the slip and twin planes in the Si substrate. The enhanced phase transformation into α-Al2O3 along various directions is suggested to arise from such discontinuities or defects.