Ferroelectricity in Ultrathin BaTiO3 Films: Probing the Size Effect by Ultraviolet Raman Spectroscopy

We demonstrate a dramatic effect of film thickness on the ferroelectric phase transition temperature, Tc, in strained BaTiO3 films grown on SrTiO3 substrates. Using variable temperature ultraviolet Raman spectroscopy enables measuring Tc in ¯lms as thin as 1.6 nm, and film thickness variation from 1.6 to 10 nm leads to Tc tuning from 70 to about 925 K. Raman data are consistent with synchrotron x-ray scattering results, which indicate the presence of of 180± domains below Tc, and thermodynamic phase-field model calculations of Tc as a function of thickness.