Electron Microscopic Studies of Structure and Crystallization of Amorphous Metal Oxide Films

High-resolution transmission electron microscopy using axial beam illumination was carried out on vacuum-deposited WO 3 , TiO 2 , and MoO 3 films. In as-deposited WO 3 films thinner than 100 Å and films heated at 350°C for 2∼5 h which gave rise to amorphous haloes in electron diffraction, crossed fringe-like structure images of the order of 10∼20 Å was observed. This means that the amorphous films consisted of micro-crystallites in which the W–O 6 octahedra were arrayed like crystalline WO 3 . When the films were heated at 350°C for 10 h, they grew to crystallites of WO 3 as large as 100 Å. On the other hand, in WO 3 films thicker than about 200 Å, crystals of a few microns grew after shorter heat treatment. Similar results were obtained for the TiO 2 and MoO 3 films. The differnce in crystallization between the very thin films and the thicker films is interpreted by considering the heat generated during crystallization and its dispersion.