Pulsed Laser Deposition And Ferroelectric Properties Of Bi_3.6Ho_0.4Ti₃O₁₂ Thin Films

As a ferroelectric functional material, rare earth elements doped bismuth titanate (Bi4Ti3O12) with large remanent polarization and low crystallization temperature is a possible substitute for lead zirconate titanate and bismuth-strontium tantalate in ferroelectric random access memory. The purpose of the present dissertation is to prepare a holmium doped bismuth titanate (Bi3.6Ho0.4Ti3O12) thin film and study its ferroelectric property, expecting to provide a environmentally-friendly ferroelectric material with good performance.As a ferroelectric functional material, rare earth elements doped bismuth titanate (Bi4Ti3O12) with large remanent polarization and low crystallization temperature is a possible substitute for lead zirconate titanate and bismuth-strontium tantalate in ferroelectric random access memory.Firstly, Ho3+doped Bi4Ti3O12 (Bi4-xHoxTi3O12) powders were fabricated by solid state reaction using Bi2O3, TiO2 and HO2O3 micro-powders as the raw materials. Then, Bi4-xHoxTi3O12 ceramic targets were obtained by densifying the synthesized powders using hot-pressed sintering. The influences of process parameters (synthesis temperature, sitering temperature, Bi content and Ho doping content) on the structure and property of the Bi4-xHoxTi3O12 powders and ceramic targets were studied, and the appropriate preparation conditions were achieved. That is, Bi4-xHoxTi3O12 powders were synthesized at 900℃for 8h with 3wt% excessive content of Bi2O3, and the powders were further sintered at 850℃-30MPa for 2h to prepare the Bi4-xHoxTi3O12 ceramics. The appropriate Ho content is 0.4, and Bi3.6Ho0.4Ti3O12 ceramic obtained under the appropriate preparation conditions showed pure phase and dense structure (about 99.4%), whose dielectric constant, dielectric loss and remanent polarization were 207.3,0.0066 and 6.96μC/cm2, respectively.Subsequently, we used the dense and pure phase Bi3.6Hoo.4Ti3O12 ceramic as the target to prepare Bi6Ho0.4Ti3O12 thin films on Pt(111)/Ti/SiO2/Si substrates by pulsed laser deposition. The effects of deposited parameters, such as substrate temperature, laser energy density and oxygen pressure, on the phase, orientation, surface morphology and ferroelectric property were mainly investigated. The results showed that the crystallinity of the as-deposited films increased with substrate temperature, and pure phase and well-crystallized Bi3.6Ho0.4Ti3O12 thin films were obtained at 600℃and 700℃with c-axis and random orientations, respectively. As the laser energy density increased, the phases of the films changed from Bi2Ti2O7 to Bi4Ti3O12, and pure Bi3.6Ho0.4Ti3O12 thin films were obtained at laser energy density higher than 24.53J/cm2. The pure Bi3.6Ho0.4Ti3O12 thin films could also be obtained by raising oxygen pressure. The grain sizes increased with oxygen pressure at first and then reduced, while the orientation changed little. The Bi3.6Ho0.4Ti3O12 thin film deposited under the appropriate conditions (29.3J/cm2-700℃-10Pa) exhibited a pure phase, smooth and dense surface, and the maximum remanent polarization was 22.74μC/cm2.