Thickness dependence of the crystallization of β-Fe2O3/α-Al2O3(0001) thin films grown by sputtering

The crystallization of alpha-Fe2O3/alpha-Al2O3(0001) thin films has been studied using real-time synchrotron x-ray scattering and atomic force microscope. In the sputter-grown amorphous films of various thicknesses at room temperature, we find the coexistence of alpha-Fe2O3 and Fe3O4 interfacial crystallites (similar to 50-angstrom-thick), well aligned to the alpha-Al2O3[0001] direction. The amorphous precursor is crystallized to the epitaxial alpha-Fe2O3 grains in three steps with annealing temperature; i) the growth of the well aligned alpha-Fe2O3 interfacial crystallites to approximately 200-angstrom-thick, together with the transformation of the Fe3O4 crystallites to the alpha-Fe2O3 crystallites (< 400 degrees C), ii) the growth of the less aligned alpha-Fe2O3 grains on top of the well aligned grains (> 400 degrees C), and iii) the nucleation of the different less aligned alpha-Fe2O3 grains directly on the alpha-Al2O3 substrate (> 600 degrees C). The surface evolution of the amorphous precursor films after annealing is consistent with the microstructure evolution during the crystallization.