Effects of Lanthanides-Substitution on the Ferroelectric Properties of Bismuth Titanate Thin Films Prepared by MOCVD Process

Abstract

Ferroelectric lanthanides-substituted Bi₄Ti₃O₁₂ (Bi 4-X Ln X Ti₃O₁₂, BLnT) thin films approximately 200㎚ in thickness were deposited by metal organic chemical vapor deposition onto Pt(111)/Ti/SiO₂/Si(100) substrates. Many researchers reported that the lanthanides substitution for Bi in the pseudo-perovskite layer caused the distortion of TiO6 octahedron in the a-b plane accompanied with a shift of the octahedron along the a-axis. In this study, the effect of lanthanides (Ln=Pr, Eu, Gd, Dy)-substitution and crystallization temperature on their ferroelectric properties of bismuth titanate (Bi₄Ti₃O₁₂, BIT) thin films were investigated. As BLnT thin films were substituted to lanthanide elements (Pr, Eu, Gd, Dy) with a smaller ionic radius, the remnant polarization (2P r ) values had a tendency to increase and made an exception of the Eu-substituted case because Bi 4-x Eu x Ti₃O₁₂ (BET) thin films had the smaller grain sizes than the others. In this study, we confirmed that better ferroelectric properties can be expected for films composed of larger grains in bismuth layered peroskite materials. The crystallinity of the thin films was improved and the average grain size increased as the crystallization temperature increased from 600 to 720℃. Moreover, the BLnT thin film capacitor is characterized by well-saturated polarization-electric field (P-E) curves with an increase in annealing temperature. The BLnT thin films exhibited no significant degradation of switching charge for at least up to 1.0×10¹¹ switching cycles at a frequency of 1㎒. From these results, we can suggest that the BLnT thin films are the suitable dielectric materials for ferroelectric random access memory applications.