Investigation On The Mechanism Of Aging And Self-polarization Of BiFeO₃-based Thin Films

As a lead free ferroelectric material, BiFeO3(BFO) has been investigated extensively for the potential applications in information storage and microelectromechanical systems due to its superior ferroelectric and piezoelectric properties in thin films. It has been demonstrated that the BFO films with desired properties can be obtained in N2or an atmosphere with low oxygen pressure at the annealing temperatures ranging from500to670℃, which are much lower than its Curie temperature (-850℃). Therefore, the film will be inevitably subjected to aging during the deposition or annealing process. The aging has been demonstrated to be due to that the ferroelectric domains are gradually stabilized by the defect complexes (AFe3+2+)’-(VO2-)" formed between oxygen vacancies and low-valence ions. It is well known that the aging effect has strong negative influence on the long-term reliability of the practical devices. However, the defect complexes formed during aging can limit the movement of the oxygen vacancies, which will reduce the leakage current of the film. It can be deduced that the properties of the films can be improved by a certain extent of aging effect. On the other hand, it is well known that if the ferroelectric thin films are used in the field of pyroelectric, piezoelectric and photoelectric application, they usually should be polarized by large electric field at high temperatures. Thus, the poling process can be omitted in the as-deposited films with self-polarized, which can lessen the technological process and reduce the costs.To solve the aging problem and confirm the relationship between the aging and the properties of BFO films based on the aging mechanism as well as investigate the reason that induce the self-polarized effect in the BFO-based film. The BFO-based films were deposited on different electrodes using a metal organic decomposition process combined with layer-by-layer annealing technique. The effect of LaNiO3(LNO) thickness, annealing temperature and aging time on the structure and properties of BFO-based films were investigated systematically. Meanwhile, we studied the influence of annealing temperature and ion substitution on the self-polarization of BFO-based films. The main contents and conclusions are listed below: 1. The BiFe0.99W0.01O3films were fabricated on the Pt/SiO2and LNO(100)/Pt/SiO2substrates with different LNO thickness (40,80,120nm). The influence of electrodes and the thickness of LNO on the structure, crystallinity and piezoelectric properties of BiFe0.99W0.01O3films were investigated. It was found that, compared to that of Pt electrode, the LNO electrode can favor the improvement the crystallinity and aging resistance of the BiFe0.99W0.01O3films. The highest remanent piezoelectric coefficient d33is118.1pm/V, which can be observed in the film deposited on the LNO with the thickness of80nm.2. The BiFe0.995W0.00503films were fabricated on the LNO(100)/SiO2substrates at the annealing temperatures ranging from425to500℃. The influence of annealing temperature on the structure, surface morphology, leakage current, ferroelectric and piezoelectric properties of BiFe0.995W0.005O3films were investigated. It was found that the film can be crystallized at450℃, which is the lowest crystallization temperature for BFO-based films reported up to now. The leakage currents for all the crystallized films increase with the increase of annealing temperature. Well saturated polarization-electric field hysteresis loops can be obtained in all the crystallized BiFe0.995W0.005O3films. No domain backswitching can be observed in all the crystallized films, suggesting that0.5mol%doping content of W6-is enough for eliminating the aging in the films. The remanent piezoelectric coefficient d33of the film annealed at450℃is as high as128pm/V, which is comparable to that obtained in the BiFe0.99W0.01O3film annealed at500℃. This indicates that the decrease in doping content can reduce the annealing temperature.3. The BFO films were fabricated on LNO(100)/SiO2substrates at annealing temperatures ranging from400to475℃. The effect of annealing temperature on the structure, leakage current and piezoelectric properties of BFO films were investigated. Moreover, the influence of aging time on the piezoelectric properties of the BFO film annealed at various temperatures was discussed. It was found that the leakage currents for all the BFO films increase with the increase of annealing temperature. The piezoresponse of BFO films increase with the aging time, and thereafter decrease. This phenomenon indicates that the piezoelectric properties of BFO films can be improved by aging to some extent. But the aging still affects the long-term reliability of the devices. 4. The BFO, BiFe0.97Ti0.03O3, BiFe0.97Ti0.015Zn0.015O3and BiFe0.97Zn0.03O3films were fabricated on LNO(100)/SiO2substrates. The influence of annealing temperature on the self-polarization of BFO film as well as the effect of ions substitution on the self-polarization of BFO-based films were discussed. It was found that the self-polarization in the pure BFO film is mainly induced by the epitaxial stress between LNO electrode and BFO film. The opposite direction of the self-polarization can be observed in the Ti4+and Zn2+ions doped BFO-based films.In a word, the aging problem of BFO-based film is solved by modifying the thickness of LNO electrode and annealing temperature. We also discussed the influence of aging time on the properties of BFO film, which provide a foundation for the using of aging behavior. Moreover, the mechanism of the self-polarization in the BFO-based film was investigated in this work. This can pave the way for the utilization of the film in the future micro-devices.