Fabrication And Properties Of Doped Bi₄Ti₃O₁₂ Ferroelectric Thin Films

Recently, rare-earth neodymium (Nd) doped Bi₄Ti₃O₁₂ (BTO) ferroelectric thin films have become the most potential materials replacing the conventional Pb(Zr,Ti)O3 thin films for application in non-volatile ferroelectric random access memories (NvFRAM), due to its larger remnant polarization(2Pr), excellent fatigue endurance, relatively low crystallization temperature. However the detailed studies on the mechanism of the improved properties by doping and the dopant influence on crystallization temperature are still lacking. In the thesis, the structure, ferroelectric and leakage current properties of doping BTO thin films, which belong to the family of layer-structured ferroelectrics, have been studied by Raman scattering spectra. These results will be helpful to understand the mechanism of the property improvement and the effect on crystallization temperature caused by doping.Nd-doped Bi₄Ti₃O₁₂ (Bi4-xNdxTi3O12) ferroelectric thin films have been deposited on Pt/Ti/SiO2/Si(111) substrates at different temperature by Sol-Gel process. The effect of the content of Nd and the crystallization temperature on the microstructure and ferroelectric properties have been investigated. It is concluded that all of Nd3+ ions substituted for the Bi3+ ions at the perovskite A site, and are not incorporated into the (Bi2O2)2+ slabs when Nd content (x) is lower than 0.45 by the XRD pattern, Raman spectra and P-E hysteresis measurements. This leads to the increase of remnant polarization of BTO ferroelectric thin film. The 2Pr reaches a maximum value of 32.7μC / cm2when Nd content (x) is equal to 0.45. Pyrochlore phase was found when annealing temperature is lower than 600 0C and single Aurivillius structure phase formed completely at 700 0C.Then vanadium (V)- and manganese (Mn)-doped Bi3.55Nd0.45Ti3O12 ferroelectric thin film samples have been prepared by Sol-Gel process and the microstructure of these samples was analyzed by Raman spectra and their ferroelectricity were measured.. The Bi3.55Nd0.45Ti2.99V0.01O12 (BNTV) films, which annealed in nitrogen, air, and oxygen environments respectively, show smooth, density surface and a textured (00l) and (117) growth. But the BNTV thin film crystallized in nitrogen exhibits the highest 2Pr. Pyrochlore phase in the thin film sample crystallized in the lower temperature was observed. The single Aurivillius phase forms completely and BNTV film shows better ferroelectricity when the annealing temperature is higher than 680℃, which is lower than the crystallization temperature of BNT films. It is found that all of Mn⁴⁺ ions substituted for the Ti⁴⁺ ions at the perovskite B site, and are not incorporated into the A site Bi3+. When the Mn content (x) is equal to 0.01, the thin film Bi3.55Nd0.45Ti2.99Mn0.01O12 (BNTM) exhibits the best ferroelectricity with 2Pr=27μC / cm2 and a coercive voltage of 5.3V. Different from V-doped BNT films, Mn-doping restrains from forming of pyrochlore phase in the BNTM thin films. At the same time, the temperature of the single Aurivillius structure phase in the films starting to be formed increase to about 720℃with Mn-doping, at which the BNTM film crystallized shows a good ferroelectricity.