Modification Of Bi₄Ti₃O₁₂ Series Ferroelectric Thin Films

Bismuth layered structure ferroelectrics(BLSFs) thin films have been widely investigated recently.Among them,as a typical kind of layer-structured ferroelectrics, Bi₄Ti₃O₁₂(BTO) ferroelectric thin film has attracted much attention due to their potential applications in nonvolatile ferroelectric random access memory(NvFRAM) devices.In this thesis,we summarized the basic properties of ferroelectrics and application of ferroelectric film in memory.BTO has been considered one of the promising ferroelectirc materials.However,the low 2Pr and poor fatigue property of BTO thin film limit its industrial application.Aiming at those problems,the dopping effects of A-site and B-site substitution and A/B-sites cosubstitution of BTO thin films were discussed.Considering the easy volatilization of Bi₂O₃ in process of heat treatment,we studied the effect of Bi₂O₃ excess content on the microstructure,ferroelectric and dielectric properties of the Bi_3.25La_0.75Ti₃O₁₂(BLT) films.In addition,the influences of buffer layers on structure and electrical properties of BLT thin film were investigated.The research result basicly attained what we expected.The main results are as follows:The deposited BLT were annealed at 600℃—800℃for 20 minutes respectively. Study result reveals that the polarization behavior is affected by both the annealing temperature and the amount of Bi₂O₃ excess content.Excess Bi₂O₃ added to the BLT system can enhance the polarization and dielectric properties due to compensation of Bi loss occurred during calcinations and weaken the polarization and dielectric properties due to formation of a secondary phase such as bismuth oxide.For example,The BLT film sample derived from 7.5 mol%exhibits a remnant polarization(2Pr) 25.26μC/cm²,and coercive voltage(Vc) of 3.62 V,together with a dielectric constant of 252.6 and a dielectric loss of 0.066.Also,the BLT thin film sample derived from 7.5 mol%Bi₂O₃ excess shows the best fatigue resistance characteristics and about 9.2%polarization degradation after 1×10⁹ read/write switching cycles.A-site Praseodymium doped BTO thin films with composition Bi_2.9Pr_0.9Ti₃O₁₂ (BPT-0.9) were prepared after post annealed at different temperature.Based on it,for BPT-0.9 film annealed at 750℃,the effect of different annealing time on ferroelectric properties was discussed.It was found that when annealed for a short time(10 min), relatively broaden peaks were observed for the BPT-0.9 thin film,implying low crystallinity.With the increase of the annealing time length,the diffraction peaks become stronger,indicating the growth in crystallinity in the BLT films.However,when the annealing time is above 25 min,a pyrochlore phase peak appears in X-ray diffraction patterns.It indicates that too long annealing time is unfavorable to crystallinity of BPT-0.9 films.Bi_3.63Pr_0.3Ti₃O₁₂(BPT-0.3) films were deposited by RF-magnetron sputtering with substrate temperature varied from 500 to 750℃and the effect of substrate temperature on growth orientation was investigation.Films deposited below 650℃showed no preferred c-axis orientation and(117)peak is the strongest peak.As the substrate temperature was increased,however,the intensity of the film(00l)-plane peaks increased leading to c-axis preferred orientation in the film at 750℃.B-site Zr-doping Bi_3.25La_0.75Ti_3-xZr)xO₁₂(BLTZ-x,x=0,0.2,0.5,0.8,1.2 and 1.6) were prepared successful.With the increasing of dopant concentration,the remanent polarization(2Pr) first increases then decrease while there is almost no obvious difference in the Vc values.In comparison with no Zr-doped BLT,the fatigue test indicates that appropriated Zr-doped BLTZ-0.2 exhibite better fatigue resistance characteristics.B-site Mn-doped Bi_3.25La_0.75Ti_3-xMn_xO₁₂(BLTM-x,x=0.02,0.04,0.06å’Œ0.08) were prepared by a solid-state reaction technique.It was found that The Curie temperature of Mn-doped BLT was steadily shifted to lower temperature with increasing Mn-doping content.It was the increase in crystal structure symmetry that led to a decrease in the Curie temperature of the samples.In addition,the remnant polarization of BLTM samples increased with the Mn-doping content,which might be due to modification in the orientation of grains and enhance of grains size.Previous research found there are some disadvantages when used Pt as substrate, such as larger stress due to lattice dismatch and interdifusion between Pt and ferroelectric film when heat treatment.Through introduce buffer layer,such Bi₂O₃ or TiO₂,could change crystallization orientation and limit interdifusion.Because of those would improve the ferroelectric properties of BLT films.