First-principles Study For Mechanism Of The Leakage Current In Ferroelectric Film

Ferroelectric(FE) materials have been extensively applied in the multifunctional electronic devices due to their excellent physical properties, particularly, in the FE memories. FE memory is a kind of nonvolatile memory device, and it could overcome the shortcomings of the traditional memory. But the development of the FE memory has been very slow due to the FE failure problem. However, with the continual decrease of the thickness of FE thin film, when it reaches microns or nanometers order of magnitude, the leakage current is the main cause to the FE failure of FE thin film. The leakage current of FE thin film is directly related to whether FE memory can be really applied, and it has always been the hot spot of scientific researches.There are still a lot of influencing factors of FE memory leakage current except for the thickness of the film, such as interface, processing temperature, defect and domain wall, etc. In these factors, the most common and the highest proportion of the two factors are defect and domain wall. In this thesis, defect and domain wall on the influence of the FE thin film leakage current is systematically studied by the first principles calculation method.First of all, we take the PbTiO3 ferroelectric thin film materials as the research model and studied the oxygen defects effect on the leakage current. The study found that oxygen defects can easily spread to the interface, and as accompanied by oxygen defects moving to the interface, the leakage current of film is gradually increased. By the metal cation doping, we found that Cu and V doping for oxygen defects has pin effect and makes the leakage current decrease, however, the Fe and Al doped makes the leakage current increase. But the ionic radius of V is more close to the ionic radius of Ti than that of the Cu, so V can more easily dope FE thin film than Cu. Therefore,V is more suitable for doping to control the size of the FE thin film leakage current.This conclusion found a new way for controlling the leakage current caused by oxygen defects.Secondly, we studied the influence of the 180° domain on the FE thin film leakage current systematically. The study indicated that 180° domain wall,perpendicular to the electrode, greatly increased the leakage current of FE thin films,However, the parallel to the electrode direction of 180° domain wall reduces theleakage current of FE thin films. Through the calculation of the transmission coefficient and band structure, we found that the main reason for the 180° domain wall leakage current of perpendicular to the electrode increase, which is 180° domain wall introduced extra electronic transport channel and reduced the band gap of FE thin film. Our research shows that we can suppress the size of the films leakage current by reducing the 180° domain wall perpendicular to the electrode direction, or by adjusting the direction of the domain wall. These theoretical studies have important guiding significance for improving the performance of FE memory.