Infrared study of BaTiO3 thin films

Thin films of BaTiO3 are prepared on silicon substrates by spin-coating processing. The crystalline grain sizes are determined by AFM observation. The average diameter of the grains is about 50 nm in films thicker than 60 nm. In the films thinner than 30 nm the grain size decreases. Far-infrared absorption spectra are measured by FTIR spectrometer as a function of thickness. The frequencies of phonons observed in films thicker than 30 nm shows good correspondence with those of bulk crystals. However, in thinner films of 10 nm the profiles of the absorption lines are rather different. The intensities of the low-frequency modes at 180, 250 and 510 cm(-1) decrease as the thickness of film decreases, whereas the 375 cm(-1) mode component retains its intensity. This suggests the change of crystal structure below the grain size of 30 nm.