Oriented Growth Of Ferroelectric Bi_3.15Nd_0.85Ti₃O₁₂ Thin Films Of Layered-perovskite Structure

Rare-earth substituted Bi₄Ti₃O₁₂ thin films of layered-perovskite structure are of large spontaneous polarization and fatigue-free. In this type of ferroelectrics, the major component of spontaneous polarization lies along the a-axis. The a-axis oriented growth (or near a-axis oriented) of these films is important for their device applications, particularly on metal electrodes.Bi_3.15Nd_0.85Ti₃O₁₂(BNdT) thin films of either high a/b-axes orientation or low alb-axes orientation were grown on standard-type (111)Pt/Ti/SiO₂/Si(100) substrates through sol-gel technique, using Bi(NO₃)₃·5H₂O, Nd₂O₃, Ti(OC₄H₉)₄ as raw materials. The remnant polarization (2P_r) value of the highly alb-axes-oriented BNdT films is about 58μC/cm² , larger than that of the lowly alb-axes-oriented BNdT films (40μC/cm² ). Both of the films have excellent fatigue-free and retention properties. Transmission electron microscopy (TEM) investigation shows that highly a/b-axes oriented BNdT films consist of columnar grains of 150-300 nm in lateral size. It was found that heating rapidly for crystallization is crucial for the nucleation of a/b-axes oriented grains.The Bi_3.15Nd_0.85Ti₃O₁₂ thin films of a/b-axes orientation were also grown on (001)YSZ substrates through a sol-gel process. The volume fraction of a/b-axes oriented grains in the films was estimated to be about 48% according to the X-ray pole figure. TEM investigation confirms that the film is a/b-axes oriented, and the lateral sizes of the columnar grains is 100-200 nm.