Influence of tantalum on mechanical, ferroelectric and dielectric properties of Bi-excess Bi3.25La0.75Ti3O12 thin film

The Bi-excess Bi_3.25La_0.75Ti₃O₁₂ (BLT) and Bi_3.25La_0.75Ti_2.91Ta_0.09O₁₂ (BLTT) thin films were deposited on Pt(1 1 1)/Ti/SiO₂/Si substrates using rf-magnetron sputtering method. The effect of tantalum on mechanical, ferroelectric and dielectric properties of BLT thin films was investigated. XRD patterns reveal that BLT and BLTT thin films have a same second crystalline phase Bi₁₂TiO₂₀, but the former has a randomly oriented major Bi₄Ti₃O₁₂ phase while the major phase in the later is preferentially oriented along 1 1 7 direction. XPS analysis shows that the substitution of Ta⁵⁺ has a significant influence on Ti³⁺ defects. Mechanical response of thin films is discussed in the light of nano indentation analysis which shows that both the hardness and modulus are reduced with the addition of Ta⁵⁺. Ferroelectric properties were improved significantly by Ta⁵⁺ doping due to the reduction of Ti³⁺ defects, and an enhanced remnant polarization (2P_r) of 32.5 μC/cm² and a decreased coercive field (2E_c) of 260 kV/cm were obtained. In addition, the Ta substitution elevated the dielectric constant and slightly raised the Curie temperature.