High-angle annular dark-field scanning transmission electron microscopy was used to investigate the crystallization mechanism of amorphous hafnium dioxide (HfO2) layers in gate stacks (polysilicon/HfO2/SiON/Si substrate). A 0.9-nm-thick HfO2 layer remained amorphous with a uniform thickness on annealing at 1050 degrees C. In contrast, crystalline islands with a cubic structure formed when a 1.8-nm-thick HfO2 layer was annealed. These islands had commensurate interfaces with both the silicon substrate and the polysilicon film. These results suggest that crystallization is promoted on a silicon surface. (C) 2011 Elsevier B.V. All rights reserved.