Ferroelectricity in Ultrathin ${\mathrm{BaTiO}}_3$ 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 ${\mathrm{BaTiO}}_3$ films grown on ${\mathrm{SrTiO}}_3$ 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° domains below $T_c$, and thermodynamic phase-field model calculations of $T_c$ as a function of thickness.

Figure 1

Room temperature Raman spectra of four ${\mathrm{BaTiO}}_3$ ultrathin films on ${\mathrm{SrTiO}}_3$ substrates compared to the spectrum of a bare substrate. Arrows mark the phonon peaks of the ${\mathrm{BaTiO}}_3$ films. The inset shows Raman spectra of 1.6 nm-thick ${\mathrm{BaTiO}}_3$ film and ${\mathrm{SrTiO}}_3$ substrate measured at 10 K, and the difference between the two spectra.

Figure 2

Temperature evolution of Raman spectra of a 2.4 nm-thick ${\mathrm{BaTiO}}_3$ film on a ${\mathrm{SrTiO}}_3$ substrate. Arrows mark the phonon peaks of the film used for $T_c$ determination. The inset shows the temperature dependencies of normalized Raman intensities of the peaks at $\sim 180$ and $540{\mathrm{cm}}^{-1}$ for 2.4 and 10 nm ${\mathrm{BaTiO}}_3$ films capped with 10 nm ${\mathrm{SrTiO}}_3$. The dash-dotted lines are linear fits.

Figure 3

$T_c$ as a function of the ${\mathrm{BaTiO}}_3$ layer thickness, as determined from Raman data for all films studied (symbols). The dash-dotted line is a result of the phase-field model calculation with open-circuit boundary conditions.

Figure 4

In-plane reciprocal space map around the ${\mathrm{BaTiO}}_3$ 303 peak at 500 K for a 10-nm-thick ${\mathrm{BaTiO}}_3$ film capped with 10 nm of ${\mathrm{SrTiO}}_3$. A redder hue indicates higher intensity (log scale). The reciprocal lattice units (rlu) are in terms of the ${\mathrm{SrTiO}}_3$ substrate.