Preparation And Investigation On The Properties Of Multiferroic Bifeo₃ -based Thin Films

As a typical single phase multiferroic materials, BiFeO₃ shows the coexistence of ferroelectric ordering with Curie temperature of 850°C and magnetic ordering with Néel temperature of 370°C at room temperature, which makes it have great potentials in a wide range of applications, including data storage, spintronics, sensors, microelectromechanical systems, etc. However, for the case of BiFeO₃ films fabricated using metal organic deposition method, the fatal drawbacks are the higher leakage current, larger coercive field, weaker saturated magnetization, and the saturated P-E hysteresis loops can only be obtained in the films thicker than 400 nm. Therefore, in order to meet the requirement of future microelectronic devices, it is imperative to reduce the leakage current, thickness, coercive field, as well as enhance the ferroelectric and ferromagnetic properties for the BiFeO₃-based ferroelectric thin films.Aiming at these obstacles, this paper puts forward two methods to reduce the leakage current, improve the ferroelectric, piezoelectric and ferromagnetic properties of BiFeO₃-based ferroelectric thin films, one is forming the double-layered structure, the other is doping lanthanide element. In this paper, the BiFeO₃-based films are fabricated on different substrates using a metal organic deposition method combining layer-by-layer technique. The effects of Bi_3.5Nd_0.5Ti₃O₁₂, PbZr_0.2Ti_0.79Nb_0.01O₃ buffer layers as well as lanthanide doping on the structure, leakage current, ferroelectric and magnetic properties of BiFeO₃-based films are studied systematically. The main contents and conclusions are summarized as follows:Bi_3.5Nd_0.5Ti₃O₁₂ buffer layers with different thicknesses (0, 40, 80, 160 nm) are fabricated on ITO/Si substrates, the effects of the thicknesses of Bi_3.5Nd_0.5Ti₃O₁₂ buffer layers on the structure, leakage current and multiferroic properties of BiFeO₃ thin films are studies. The experimental results demonstrate that that the leakage currents in the BiFeO₃ films deposited on the Bi_3.5Nd_0.5Ti₃O₁₂ buffer layers are about two or three orders of magnitude lower than that of the film deposited directly on the ITO/Si substrate. The BiFeO₃ films deposited on the 40-nm-thick Bi_3.5Nd_0.5Ti₃O₁₂ buffer layer exhibits the highest volume fraction of (110)-oriented grains, the largest remanent polarization, the best charge-retaining property, and the film can be easily magnetized. Further increase of the Bi_3.5Nd_0.5Ti₃O₁₂ buffer layer thicknesses result in the degradation of the volume fraction of (110)-oriented grains, rectangularity of P-E hysteresis loops, reduction of the Pr value, as well as deterioration of the charge-retaining ability. Therefore, it can be conclude that the optimum thickness of Bi_3.5Nd_0.5Ti₃O₁₂ buffer layer is 40 nm, which is enough for reducing the leakage current of BiFeO₃ film and improve its multiferroic property.A series of Bi_1-xSm_xFeO₃ and Bi_1-xTb_xFeO₃ films are fabricated on ITO/glass substrate, the influences of Sm dopant (0～20%) and Tb (0～16%) on the structure and electrical properties of BiFeO₃ films are investigated, respectively. The results demonstrate that all Bi_1-xSm_xFeO₃ and Bi_1-xTb_xFeO₃ films show similar polycrystalline structures with a strongest diffraction peak of (110). For the case of Bi_1-xSm_xFeO₃ system, the structure transition may occur near x=0.14, the largest remanent piezoelectric coefficient was observed when x=0.16. The leakage currents of Bi_1-xSm_xFeO₃ films can be reduced by doping of Sm, thus, well saturated and rectangular P-E hysteresis loops are observed. The remanent polarization of Bi_1-xSm_xFeO₃ films decrease monotonically with the increase of the Sm doping content. No double P-E hysteresis loops are observed in the whole Sm doping range. The phenomena occurred in Bi_1-xSm_xFeO₃ system can observed in Bi_1-xTb_xFeO₃ system as well, however, the doping content where the structure transition taken place and the remanent piezoelectric coefficient reaches its maximum is different from that in Bi_1-xSm_xFeO₃ system. In Bi_1-xTb_xFeO₃ system, the structure transition occurs near x=0.11, at which doping content, the films exhibited the largest remanent piezoelectric coefficient and saturated magnetization. The different phenonmena observed in the Bi_1-xSm_xFeO₃ and Bi_1-xTb_xFeO₃ systems may be due to the fact that the ionic radiuses of Sm3+ and Tb3+ are different from each other, resulting in different structural distortions of BiFeO₃ films. In order to further reduce the coercive field and thickness, enhance the breakdown characteristics of BiFeO₃-based thin films, the 40-nm-thick PbZr_0.2Ti_0.79Nb_0.01O₃ thin film deposited on Pt(111)/Ti/SiO2/Si substrate is employed as buffer layer, Bi_0.89Tb_0.11FeO₃ films with different thicknesses are then fabricated on the PbZr_0.2Ti_0.79Nb_0.01O₃ buffer layer. As a result of the small coercive field and strong anti-aging property of PbZr_0.2Ti_0.79Nb_0.01O₃ buffer layer, the leakage currents, coercive fields, as well as the asymmetry coercivities of the as-deposited Bi_0.89Tb_0.11FeO₃ films are remarkably decreased. The leakage currents of Bi_0.89Tb_0.11FeO₃ films decrease with the increase of the thickness of Bi_0.89Tb_0.11FeO₃ films. The remanent polarization and coercive field for all Bi_0.89Tb_0.11FeO₃ films are 45⁵0μC/cm2 and 200 kV/cm, respectively, and show weak dependent on the film thickness. The 200-nm-thick Bi_0.89Tb_0.11FeO₃ film shows almost no obvious fatigue and charge loss.In summary, the leakage current and multiferroic properties of BiFeO₃-based films are significantly improved by forming the double-layered structure and doping the lanthanide element, which can pave the way for the development of future microelectronic devices.