Crystallization of sol-gel-derived strontium-bismuth-tantalate thin films on silicon substrates

Abstract

Crystal structure of sol-gel derived SrBi2Ta2O9 thin films (thickness < 100 nm) deposited on silicon substrates has been investigated as a function of annealing temperature and time by means of X-ray diffraction. P-type Si(100) substrates were treated in a diluted HF solution after chemical cleaning and subsequently a precursor solution for SrBi2Ta2O9 was deposited on the Si substrates by spin coating technique. After drying at 423 K in the atmosphere the samples were annealed in a furnace. At annealing temperatures of 823-973 K the crystallization from amorphous phase to a fluorite-type structure (space group: Fm3m) was confirmed, but a ferroelectric phase (space group: A21am) was not observed even after annealing for 10 hours. As annealing temperature was higher than 1003 K, at first diffraction only from the fluorite-type structure was observed, and then diffraction from A21am structure appears after a specific time t0; it depends on the annealing temperature. The diffraction intensities from the fluorite-type structure decreased and those from A21am structure increased with annealing time. Finally diffraction only from the A21am structure was observed. These results mean that crystal structure of the SrBi2Ta2O9 thin film was gradually transformed from fluorite into A21am structure with time. By atomic force microscope it was confirmed that grain growth occurred simultaneously with the structural transformation. Furthermore, X-ray reflectivity analysis indicated that an interfacial layer was formed during the crystallization and the thickness of the layer increased with annealing time.