Growth And Characterization Of Bi_3.25La_0.75Ti₃O₁₂ Thin Film For Ferroelectric Memory

Ferroelectric nonvolatile memories have attracted much attention since last decades due to their potential advantages of high speed,low power consumption,nonvolatility and anti-radialization.Bi_3.25La_0.75Ti₃O₁₂(BLT) thin film is one of the most promising materials for application in ferroelectric memories.In this thesis,properties and growth of BLT thin films were studied.The BLT thin films were deposited on Pt/TiO₂/SiO₂/p-Si(100)substrates by Sol-Gel method and the effect of annealing paraments on their properties were investigated.The microstructure and ferroelectric properties are influenced greatly by annealing temperature. The co-effect of annealing temperature and ambient results in optimal ferroelectric properties of BLT thin films in 20%O₂ in mixed gas of oxygen and nitrogen at 750℃.The structure,crystallization degree and morphology are also influenced by annealing pressure. The thin films annealed under oxygen pressure of 0.1 atm are a-axis preferred oriented and acquire excellent ferroelectric properties.Bi₄Ti₃O₁₂(BIT),BLT,Bi_3.99Ti_2.97V_0.03O₁₂(BTV) and Bi_3.24La_0.75Ti_2.97V_0.03O₁₂(BLTV) were prepared for the study of the effect of the B-site V substitution on the BLT thin films. The BLTV thin film shows randomly oriented with fine rod-like grains.Raman spectra show the A-site La substitution exerts influence on Ti ions in B sites of the BIT thin film, TiO₆(or VO₆) symmetry decreases and Ti-O(or V-O) hybridization increases for V substitution.On the other hand,the oxygen vacancies are suppressed by the higher-valentcation substitution due to the charge neutrality restriction.Compared with the BLT thin films,the ferroelectric,fatigue and leakage current properties of the BLTV thin films with V substitutition are improved greatly.To acquire the optimal concentration of V substitutition,Bi_3.25-x/3La_0.75Ti_3-xV_xO₁₂ were prepared with different concentration of V substitutition(x:0～8%).The remanent polarization and fatigue properties first increase then decrease with increasing of dopant concentration.When x=3%,the thin film exhibits excellent ferroelectric property(P_r:25.4μc/cm²) and the strongest fatigue endurance up to 10¹⁰ cycles.When x increases to 8%, the properties decrease due to slightly large distortion of crystal lattice and charge defects caused by excessive V substitution.To lower crystallization temperature,the BLTV thin films were deposited by UV-assisted Sol-Gel method.The UV irradiation during pyrolysis promotes the decomposition of C,H,O organic species,which results in good crystallization for the films in the following annealing process.The UV-irradiated BLTV thin films show good ferroelectric property(2P_r:38.7μc/cm²,2E_c:202.4 kV/cm) after annealing at 650℃.Based on the theory of nucleation,both grain size and the barrier height increase with crystallization temperature increasing.The BLT thin films annealed at 800℃display c-axis-oriented columnar grains where the interface nucleation dominates microstructure. The effect of the number of particles on the growth of thin films was simulated by Diffusion Limited Cluster Aggregation(DLCA) and Reaction Limited Aggregation(RLA) models.Thin films exhibit oriented distribution when the number of particles is small, while uniform distribution when the number increasing.And the simulation is consistent with the experimental results.When the number of particles is small,the grains distribute orientedly and their size is large,which reflects the concentrated growth of particles. When the number of particles is increasing,the grains distribute uniformly and their size decreases.