Film thickness dependence of ferroelectric properties of c-axis-oriented epitaxial Bi₄Ti₃O₁₂ thin films prepared by metalorganic chemical vapor deposition

The film thickness dependence of the ferroelectricity of c-axis-oriented epitaxially grown Bi₄Ti₃O₁₂ thin films was investigated. The c-axis-oriented Bi₄Ti₃O₁₂ films with thicknesses from 35 to 400 nm were epitaxially grown on (100)CaRuO₃//(100)SrTiO₃ substrates by metalorganic chemical vapor deposition. The dielectric constant and the coercive field of the films were almost independent of the film thickness. These values were about 151 and 18.8 kV/cm, respectively. The spontaneous polarization P_s was almost constant at 4.0 μC/cm² and was independent of the film thickness above 140 nm. This value is almost the same as the reported value for the single crystal along the c axis. However, the P_s value decreased with decreasing film thickness below 100 nm. Moreover, the remanent polarization continuously decreased as the film thickness decreased below 400 nm. As a result, the ferroelectric property of the epitaxially grown Bi₄Ti₃O₁₂ films also depended on the film thickness and was similar to that of simple perovskite-structured ferroelectric thin films, such as Pb(Zr, Ti)O₃ and (Pb, La)(Zr, Ti)O₃ thin films.