Critical thickness of ultrathin ferroelectric BaTiO3 films

To investigate the critical thickness of ferroelectric $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ (BTO) films, we fabricated fully strained $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3∕\mathrm{BTO}∕\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ heterostructures on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrates by pulsed laser deposition with in situ reflection high-energy electron diffraction. We varied the BTO layer thickness from $3\text{to}30\mathrm{nm}$ . By fabricating $10\times 10\mu {\mathrm{m}}^2$ capacitors, we could observe polarization versus electric-field hysteresis loops, which demonstrate the existence of ferroelectricity in BTO layers thicker than $5\mathrm{nm}$ . This observation provides an experimental upper bound of $5\mathrm{nm}$ for the critical thickness. The BTO thickness-dependent scaling of the remanent polarization agrees with the predictions of recent first-principle simulations [J. Junquera and P. Ghosez, Nature 422, 506 (2003)].