Fabrication Of La-modified PbTiO₃ And Ba_0.8Sr_0.2TiO₃ Thin Films By Pulsed Laser Deposition

Ferroelectric thin films have attracted much attention in recent years for nonvolative random-access memory (NvRAM) devices, but each of the promising candidates has its own drawbacks for practical applications. Recently, La-modified lead titanate (Pb1-xLax)Ti1-x/4O3 (PLT) solid solution has received considerable attention for its interesting dielectric, ferroelectric, pyroelectric, piezoelectric, and nonlinear electro-optic properties. Researchers recently proposed that it might serve as an alternative material to PZT films in the NvRAM applications. Additionally, relaxor behavior is always a key subject in the field of ferroelectric material. However, the relaxor characteristics of PLT materials have only been reported for bulk PLT and there are few published data on the relaxor behavior of PLT thin films.In this paper, we developed an optimized method of deposition to prepare a set of PLT(x) thin films by pulsed laser deposition. PT/PLT(x)/Pt capacitor structures with good ferroelectric and electronic properties were successfully obtained. The dielectric, ferroelectric and leakage characteristic were compared in view of different La mol% content. Based on recent published research, explanation about the experimental results was also given.Remanent polarization and coercive field decreased with the increasing of La content. The crystal structure transformed from tetragonal to cubic when La concentration increased to 28mol%. As far as Pt/PLT(14)/Pt was concerned, it showed a remanent polarization value of 20 C/cm2(2Pr) with a coercive field of 70kV/cm(2Ec). The most significant conclusion that can be drawn was its fatigue-free characteristic. Using switching voltage of 5v and measuring voltage of 5v under a bipolar pulse of 1MHz in frequency, thePLT(14) film showed no noticeable changes after being subjected to 3 109 read /write repetitive cycles. From the test of leakage current, PLT(x) thin films showed good insulating property as well.The La-modified thin films with 220nm thickness exhibited diffuse phase transition through the testing of temperature dependence of dielectric permittivity. The abnormal dielectric phenomenon was explained according to the observed relaxor behavior.The influence of post-annealing on the properties of the ferroelectric thin-film capacitors is another component of this thesis. The leakage characteristics and dielectric properties of Pt/Ba0.8Sr0.2TiO3/Pt at different annealing steps were discussed in 5th chapter. High leakage currents and low-frequency dielectric relaxation were found in as-deposited capacitors after they had been post-annealed in nitrogen at 550 and subsequently annealed in oxygen at 350 . The mechanism of this phenomenon was discussed.