Investigation On The Orientation Control And Electrical Properties Of Lanthanides Doped BiFeO₃ Thin Films

Lead-based ferroelectric materials such as PZT and PMN-PT are extensively used in nonvolatile memories and piezoelectric devices because of their excellent ferroelectric and piezoelectric properties. However, the lead pollution of these materials is a serious threat to the environment, and the search for “green” substitutes becomes the focus of considerable attention. In the past decades, numerous alternative materials, such as(Ba,Sr)TiO3,(Bi,Na)TiO3,(K,Na)NbO3, and BiFeO3(BFO), have been investigated. Among them, BFO is considered to be a promising candidate because of its superior multiferroic properties and lower processing temperature of thin film.BiFeO3 films,which have anisotropic properties, have different performances in different orientations. So, learn to control the orientation is of great help to improve the ferroelectric and piezoelectric properties of BiFeO3 films. For example, films with(100) preferred orientation have well piezoelectric properties. However, films with(110) preferred orientation have excellent ferroelectric properties. So, we can use the films with(100) preferred orientation to produce piezoelectric sensors, and use the films with(110) preferred orientation to manufacture ferroelectric memory. In this paper, different solvents, different chelating agent, different thickness, different pretreatment temperature on films crystallization degree and crystal orientation were researched. The study contents are listed below.1.The influence of different solvents, methanol, acetic acid, ethylene glycol and acetic acid(1:1) on the degree of crystallization and orientation of Bi0.95La0.05FeO3(BLFO) films were studied. Thin films prepared using a single solvent have well crystallization compared with that of mixed solvent. When Methanol as solvent, the degree of(110) orientation is higher than(012) orientation; When Acetic acid as solvent, the result is contrary; Both the degree of(110) and(012) orientation are low when using ethylene glycol and acetic acid as solvent. In addition, BLFO thin films were prepared on(100)-oriented LaNiO3(LNO)substrates, the degree of crystallization and the degree of(110) orientation can be changed through regulating the solvent ratio.2.The influence of different chelating agents such as citric acid, tartaric acid and urea, on the degree of crystallization and orientation of BLFO films were studied. Different chelating agents have no obvious effects on the films crystallization and the degree of orientation.3.BLFO films with different layer thicknesses(28, 34, 42, 50nm/l) were prepared on ITO/glass substrates. Moderate film thickness can promote the growth of(110)-oriented grains.Too thick or thin thickness can not promote the growth of(110)-oriented grains. Thefilm with 50 nm/l layer thickness has strong(012) diffraction peak.4.BLFO films with different pretreatment temperatures(350 ℃, 375 ℃, 400 ℃,425 ℃, 450 ℃) were prepared on ITO/glass substrates. The moderate pretreatment temperature can promote the growth of(110)-oriented grains. Both high and low pretreatment temperature are detrimental to the growth of(110)-oriented grains. The results of(012)-oriented grains are opposite to the(110)-oriented grains.Thick films with a thickness >1 μm are required in dielectric, piezoelectric, and pyroelectric devices, for example, piezoelectric actuators, pyroelectric infrared detectors, and pressure sensors, for high sensing and driving performance. However, thick films always require a high temperature to crystallize compared with thin films. Considering that crystal growth velocity is related to the diffusion coefficient of ions. The Bi ions, which have a relatively high diffusion coefficient, may contribute to improving the degree of crystallinity of BiFeO3-based ferroelectric films. Therefore, improved electrical properties can be expected in BiFeO3-based thick films with appropriate excess Bi content. In this study, lead-free BLFO ferroelectric thick films with different excess Bi contents(5% to 20%) were deposited on LNO/Si substrates by using the sol–gel method. The film thickness is about 1.4 μm. The effect of excess Bi content and annealing temperature on the electrical properties of the BLFO thick films has been investigated. The study contents are listed below.1.BLFO thick films with different excess Bi contents(5%, 10%, 10%, 20%) were prepared on LNO/Si substrates. The degree of crystallization and the growth of(100)-oriented grains of BLFO films can be effectively improved by increasing the excess Bi content. Too much excess Bi contents will form Bi-rich phases at the grain boundaries, hindering the movement of grain boundaries and limiting the growth of thick films.The film with 15%excess Bi has lower leakage currents and higher degree of crystallization. But adding too much Bi will greatly increases leakage currents. The film with 20% excess Bi has highest leakage currents. With the increase of excess Bi content, the remanent polarization of the films increases and the coercive field decreases. The film with 15 % excess Bi has highest remanent polarization and lowest coercive field. However, the excess Bi contents continued to increase, but the decrease of the remanent polarization, increase of the coercive field. The dielectric constant of films changes with the changes of excess Bi content, and the change rule is identical to the leakage currents.2.BLFO thick films with different pretreatment temperatures(550 ℃, 575 ℃) and different excess Bi contents were prepared on LNO/Si substrates. The crystallization degreeof films can be improved by increase the annealing temperature, but at the same time, it’s more likely to form secondary phases. With the increase of excess Bi content, the effects of increasing annealing temperature on the leakage currents gradually reduced. While the excess Bi content beyond 15%, the effects of increasing annealing temperature on the leakage currents gradually increased. For the films with 5 % excess Bi and 10 % excess Bi, increasing the annealing temperature can improve the ferroelectric performance. For the film with 20 %excess Bi, rising the annealing temperature can deteriorate the ferroelectric performance. For the films with 5 % excess Bi, the dielectric constant shows no obvious change when we increase annealing temperature. For other films, dielectric constant increases with the increases of annealing temperature.