Effect Of Mn On The Structure And Properties Of BiFeO₃-based Thin Films

BiFeO₃, as a multiferroic lead-free material at room temperature, shows high ferroelectric Curie temperature (～850℃) and antiferromagnetic Néel temperature (～370℃). Recently, it is demonstrated that morphotropic phase boundary can be driven by the doping of lanthanides and strain in the BiFeO₃ thin films. These films, exhibiting a large piezoelectric response comparable to that of PZT, are a promise candidate to substitute PZT family materials in the future application of information storage, piezoelectric sensors, microelectromechanical (MEMS) systems and multifunctional devices. However, BiFeO₃ thin films, especially for those prepared by chemical solution method, suffer a large leakage current. Furthermore, the large coercive field, insulating properties and the long-term reliability should be improved for the future application. Therefore, it is crucial to resolve the problem and enhance the remanent polarization and piezoelectric coefficient of BiFeO₃ thin films.As reported in this thesis, the leakage current of BiFeO₃-based thin films is restrained and the piezoelectric properties are preserved by controlling accurately the doping content of Mn. The process parameters are adjusted to obtain high crystallinity in order to achieve better ferroelectric properties. BiFeO₃-based thin films are prepared on the ITO/glass substrates by metal organic deposition method combining layer by layer annealing process. The effect of Mn substitution on the structure, leakage current, ferro- and piezoelectric properties of Bi_0.86Sm_0.14FeO₃ thin films is investigated. On the other hand, the effect of each layer thickness and annealing temperature on the growth mode and electrical properties of BiFe_0.95Mn_0.05O₃ is studied. The main conclusions are as follows:In all the Bi_0.86Sm_0.14Fe_1-xMn_xO₃ (x=0.00, 0.01, 0.03, 0.05) films doped different concentrations Mn, Bi_0.86Sm_0.14Fe0.99Mn0.01O₃ film shows lower leakage current and coercive field (272 kV/cm) as well as larger remanent polarization (Pr=53.6μc/cm2) and out-of-plane piezoelectric coefficient (d33=146 pm/V) than those of Bi_0.86Sm_0.14FeO₃ film. This phenomenon indicates that it is necessary to dope 1 at.% Mn in Bi_0.86Sm_0.14FeO₃ film in order to obtain the intrinsic ferro- and piezoelectric properties. However, excessive Mn can lead to the sharply decrease of the piezoresponse and the deterioration of charge retaining capability of the films to some extent. The negative influence brought by high contents of Mn doping can be explained by the structure deviation from the MPB of BSFO film and less contribution from irreversible movement of non-180°domain walls in aged BSFMO films doped with 3 and 5 at. % Mn.BiFe_0.95Mn_0.05O₃ films with different each layer thickness (42 nm,31 nm,25 nm) are deposited on the ITO/glass substrates by metal organic deposition method combining layer by layer annealing process. The film with a each layer thickness of 25 nm achieves homoepitaxial growth and form the column through grains from substrate to film surface. This growth mode avoids multilayer structure and reduces the amount of grain boundaries as well as the crystalline imperfection to a great extent. While the multilayer growth mode, bringing many grain boundaries and defects, is found in the films with each layer thickness of 42 nm and 31 nm. With the decrease of each layer thickness of films, the leakage current density exhibits slightly increase due to the shorter leakage path resulting from the larger grain and less boundaries in the film with a thinner each layer. The each layer thickness of 25 nm film is almost no pulse polarization loss while for the each layer thickness of 42 nm film, pulse polarization loss is up to 27 %. The reason is that the latter has more defects, resulting in serious aging which can destroy the retention performance of thin film. The film with each layer thickness of 25 nm possesses a hysteresis loop showing much larger rectangular degree than the other two samples, the largest (82μC/cm2) remanent polarization, and the smallest non-symmetry of coercive field. This result is related with the slightly aging degree of the film. Therefore, the best single layer thickness is 25 nm for preparing BiFe_0.95Mn_0.05O₃ thin films. The each layer thickness of 25 nm is sufficient to achieve the columnar through growth, and acquire excellent electrical properties such as smaller leakage current density, large remanent polarization and improved retention performance.For the BiFe_0.95Mn_0.05O₃ films annealed at different temperatures, the film with the each layer thickness of 25 nm shows high (110)-oriented preferred degree annealed at higher temperatures. However, a large leakage current density is observed in this film which is result from the shorter leakage paths in the film. For the films annealed at lower temperatures, the remanent polarization of those is increase with the raise of annealing temperature while it decrease when the annealing temperature up to 600℃. Furthermore, the remanent polarization increases obviously with the reduction of each layer thickness. These results may originate from two factors: leakage current and aging effect. As the raise of the annealing temperature, the crystallinity of films increases and amount of defects lowers. As a result, smaller aging effect leads to the increase of remanent polarization. On the other hand, high crystallinity can induce larger leakage current which can reduce the effective electric field for the switching of domains and decrease the remanent polarization. The retention performance of BiFe_0.95Mn_0.05O₃ films is improved by the raise of annealing temperature while it decrease when the annealing temperature up to 600℃, which is consistent with the result of remanent polarization. Therefore, the best annealing temperature is 575℃. BiFe_0.95Mn_0.05O₃ film annealed at 575℃shows lower leakage current density, large remanent polarization(86μC/cm2) and superior retention properties.