Far-infrared study of PbTiO3 thin films deposited by sol-gel processing

Thin films of the perovskite ferroelectric PbTiO3 on silicon single crystal substrates were prepared by sol-gel processing. The correlation between the thickness and the Brain size of films was determined by SEM and AFM. In the films with thickness above 0.1 mu m the average grain size is about 0.1 mu m independent of the thickness, whereas in films thinner than 0.1 mu m the grain size decreases as the thickness decreases. Absorption profiles of the far-infrared spectra of films in which the grain sizes are greater than 0.1 mu m are quite similar to those observed in bulk crystals, whereas in the thinner films those of the absorption lines are rather different. Moreover, the lowest-frequency mode at 80 cm(-1), which corresponds to the ferroelectric soft mode in the tetragonal phase, decreases its frequency and intensity as the grain size decreases. The possibility of the occurence of structural changes around the grain size of about 60 nm has been pointed out.