First-principles Study Of Interface And Strain Effects On Leakage Current In Ferroelectric Film

Ferroelectric memories are the most promising nonvolatile memory device, meanwhile, they are also the drive to promote the development of ferroelectric materials. But for the ferroelectric memory, reliability due to the ferroelectric failure is obstacle to its development. The ferroelectric failure includes fatigue, imprint and retention loss. When the ferroelectric film is in small size or fall into nanoscale, leakage current is one of the factors which result in the ferroelectric failure. Therefore, the tuning of leakage current has been always a hot research for ferroelectric workers. However, the diversity of influence factors and the complexity of conductive mechanism bring some difficulties to study leakage current. With the development of film preparation technology and the miniaturization trend of device, the interface and strain effects among the many factors that influence the leakage current in ferroelectric film become important. At the same time, the interface and strain effects are essential in the film preparation. In this thesis, we studied the interface and strain effects on the leakage current by first principles calculation and obtained the tuning regulation for current, which provides theoretical basis for interface and strain engineering of ferroelectric films. The primary coverage of this thesis is as follows:(1) With typical perovskite ferroelectric PbTiO3(PTO), we investigated the influence of the different electrodes on the film leakage current by first principles calculation and non-equilibrium Green’s function method. It is found that the leakage current depends not only on the electrode types but also on the interface termination of ferroelectric film. On the whole, current with oxide electrode is greater than that with metal electrode and the PTO film with TiO2-terminated interface shows a larger leakage current than that with PbO-terminated interfaces. The interface barriers for the four ferroelectric capacitors were also investigated to analyze the intrinsic mechanism of interface effects. It is found that the leakage variation of different capacitors is in good accordance with the results of potential barrier, which provides theory guidance for the films preparation.(2) With typical perovskite ferroelectric PbTiO3(PTO) and SrTiO3(STO) as buffer layer, the effects of adding buffer layers on the ferroelectric polarization symmetry and leakage current were studied. It is found that the polarization symmetry of PTO thin films can be destroyed by the introduction of STO buffer layer in different degree and polarization state which points upward is stabilized relative to the other one. Breaking inversion symmetry is one of the reasons for imprint failure in ferroelectric memories, but for some ferroelectric tunnel junctions or piezoelectric sensors, we can improve electrical properties by using the polarization asymmetry of films. Meanwhile, in the study of leakage current, it is found that the addition of STO system makes leakage current increasing, and because leakage current of STO is much larger than PTO in the same conditions.(3) With typical perovskite ferroelectric BaTiO3(BTO), the strain effects on the polarization and leakage current of ferroelectric films were studied. It is found that compressive strain can increase polarization in c-axis and reduce leakage current; tensile strain can reduce polarization in c-axis and increase leakage current. Our research shows that the biaxial plane strain can effectively tune the polarization and leakage current of ferroelectric films, so it is the essential way to improvement of properties by epitaxial strain tuning.