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

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