| Monolayer ferroelectric films are greatly restricted in the application of miniaturized integrated circuits and electronic devices by their performance limitations.By using the stress/strain and interlayer coupling effect in the ferroelectric multilayer films,not only dielectric,ferroelectric and piezoelectric properties of single layer ferroelectric films can be improved,but also some new properties can be obtained.In order to exploit advantages and overcome disadvantages of Ba Ti O3 and Bi Fe O3 films,Ba Ti O3 and Bi Fe O3 multilayer films have been constructed in this thisis.At the same time,some new physical phenomena are studied,such as the self-polarization of ferroelectric films,negative capacitance in the bilayer films,and so on.Additionally,the formation mechanisms of these new physical phenomena are also exploited and proposed.The upward self-polarizations accompanied by stable depolarization fields(Ed)have been found in the low-temperature crystallized LNO/Ba Ti O3 and LNO/Bi Fe O3 films through the domain structure and I-V curve characterization.It has been found that the formation process of upward self-polarization of ferroelectric film is related to its Curie temperature(Tc)through study on the effects of films texture,the oxygen vacancies aggregation at the interface induced by the compressive stress and low-temperature crystallization on the selfpolarization of LNO/ Ba Ti O3 and LNO/Bi Fe O3 films.Thus,the formation mechanism of self-polarization of polycrystalline ferroelectric films is proposed on the basis of the above study.The formation mechanism reveals that the fundamental cause of self-polarization of ferroelectric films is the electric field(Es)formed by the oxygen vacancies aggregation at the interface drived by the compressive stress.Moreover,Ba Ti O3 and Bi Fe O3 films preferential nucleation on the LNO buffer layer is the reason for such low temperature crystallization and high oriention of LNO/Ba Ti O3 and LNO/Bi Fe O3 films,which is also crucial for the self-polarization of ferroelectric films.The strain-driven R-M phase transitions has been found in the Bi Fe O3 films based on the phase structures of Li+-Nb5+ co-doped Bi Fe O3 films and Ba Ti O3/Bi Fe O3 bilayer films.These low symmetry M phases are thought to serve as structure bridges between R and T phases and considered to reduce the energy barriers of R-T phase transition because of the symmetry-allowed polarization rotation.Furthermore,the self-polarization behaviours are also found in the Li+-Nb5+ co-doped Bi Fe O3 films and Ba Ti O3/Bi Fe O3 bilayer films,the fundamental cause of self-polarization is also the Es formed by the aggregation of oxygen vacancies at the interface.The coupling effects of low symmetry and self-polarization is helpful to improve the piezoelectric properties of Bi Fe O3 films(>110pm/V),which is comparable with that of lead-based piezoelectric materials.Besides,the greatly reduced leakage current of Bi Fe O 3 films also contributes to the enhanced piezoelectric properties.The dc bias assisstant model is proposed to realize stable negative capacitances in the bilayer films.In principle,the dc bias assisstant model requires two films with greatly different dielectric constant,and the film with small dielectric constant must be ferroelectric.Based on the model,Bi Fe O3/Ba Ti O3 bilayer films are chosen as study objects because of the large difference between the dielectric constant of Ba Ti O3 and Bi Fe O3.Stable negative capacitances are observed directly in the capacitance-frequency curves when dc bias electric fileds are larger than the critical electric field.It has been found that the critical electric field can be greatly reduced by regulating the thickness fraction of Ba Ti O 3 film and the accompanied electric field of self-polarization,which makes the small/zero dc bias assisstant negative capacitance possible.Sandwich composite films composed of two ferroelectric films with similar dielectric constant are helpful to distribute electric fields uniformly in the composite films,which is different from the dc bias assisstant model above.Therefore,amorphous Ba Ti O3 and crystalline Bi Fe O3 film with similar dielectric constant are used to construct the LNO/Bi Fe O3/Ba Ti O3/Bi Fe O3 sandwich composite film with high breakdown strength and energy storage density.It can be obtained that the energy storage density of LNO/Bi Fe O3/Ba Ti O3/Bi Fe O3 sandwich film reaches 18.4J/cm3 at the electric field of 1666 k V/cm.Through comparision the breakdown electric fied and energy storage density of sandwich c omposite films with different amorphous and crystalline phase ratioes,It has been found that the large breakdown electric field and Pm-Pr can be obtained simultaneously in the the sandwich film composed of amorphous and crystalline only when the ratio of amorphous phase is suitable,and then the large energy storage density is obtained. |
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