| Due to the low cost, good stretchability and flexibility of base-metallic foils and the excellent dielectric, ferroelectric and piezoelectric functionality of the ferroelectric oxide thin films, ferroelectric thin film devices directly integrated on base-metallic substrates such as Ni or Cu, have shown great potentials in applications such as flexible and stretchable high density embedded capacitor, structure health monitoring system, and microelectromechanical system. However, due to the difficulties in controlling the interface and defects in the films during fabrication, there are still great technical challenges in developing devices based on this kind of integrated structures. Especially, the high leakage current in the ferroelectric thin film is one of the key limitations in the device applications. Investigation on the conduction mechanism of leakage current in the integrated structure would be helpful to understand the deep underlying physics and propose methods to control the leakage current in devices.In the research presented in this thesis, we used a typical ferroelectric material BaTiO3(BTO) as a model system. By using a chemical solution deposition technique named polymer-assisted deposition(PAD), we have successfully grown BTO thin films on base-metallic Ni substrates. The leakage conductive mechanisms have been systematically studied. The correlation between the growth condition and leakage mechanisms as well as the underlying physics have been explored. Some methods to reduce the leakage current of BaTiO3/Ni integrated structure have been proposed.By using a pre-oxidized buffer layer and thermal treatment in reducing ambient, we have successfully fabricated BTO thin films on Ni substrates using the PAD technique. Based on the analysis of the leakage current density(J) and electric field(E), we firstly systematically investigate the leakage conduction mechanisms in the special Au/Ni/BTO/Ni structure, which is a typical metal/ferroelectric/base-meal integrated structure, fabricated by using the PAD technique. We found that the leakage current is ohmic conduction mechanism at low electric field intensity(less than 10 kV/cm) whatever positive and negative bias. When the electric field intensity is high, under negative bias, leakage current is Schottky emission mechanism. Under positive bias, when the test voltage from the low to the high, leakage current is P-F mechanism and SCLC mechanism with traps. On the other hand, leakage current is SCLC mechanism without traps. The interface between the ferroelectric thin film and the top electrode is Schottky contact whereas the one between the ferroelectric thin film and the bottom substrate is Ohmic contact. The results are different from similar structures prepared using physical vapor deposition or the noble-matel/ferroelectric/noble-metal structures. This may result from the special processing of PAD which would induce relatively high density of oxygen vacancies at the surface of the BTO film as well as the interdiffusion at the BTO/Ni interface.Based on the analysis of the leakage mechanisms and the physical model of their correlation with the interface and defects in the films, we proposed three methods to reduce the leakage current of the integrated structure.1. To optimize the leakage performance by adjusting the thickness of nickel oxide buffer layer between the substrate layer and thin film layer. The leakage current decreased significantly with the increase of immersing time of Ni substrate in hydrogen peroxide solution which determines the thickness of the nickel oxide layer. By comparing the dielectric and ferroelectric performance of the integrated structure, the optimized immersing time is 120 minutes. Based on the analysis of the leakage mechanisms, we believed that the nickel oxide buffer layer with a proper thickness can not only compensate the oxygen vacancies in the BaTiO3 thin film during the annealing process, but also improve the wettability of the precursor solution on the substrate and inhibit the interdiffusion between the thin film layer and the substrate layer, as well as minimize the interfacial capacitance. Thus, the performance of thin film can be improved.2. To reduce the leakage current by adding a post annealing process in the rapid thermal annealing(RTA) furnace. The results indicated that when the second annealing time was 60 minutes, the dielectric loss was reduced by 46%, and the leakage current was reduced by two orders of magnitude than the reference sample. Ferroelectric polarization strength reached a maximum. However, further increase the RTA time would increase the leakage current again. Based on the analysis of the leakage mechanisms, we suggested that the second annealing affects not only the interface barrier but also the bulk defect concentration, which could be attributed to the competition of the compensation of oxygen vacancies and the interdiffusion between BTO films and Ni substrates. It is demonstrated that a proper treatment of a second annealing in oxygen could reduce the leakage current density and the dielectric loss effectively.3. To reduce the leakage current by changing stoichiometric ratio of precursor solution and making the BTO thin film layer with excess titanium. We found that part of the excess titanium elements existed in the form of Ti O2 in the BTO thin films. The leakage current and dielectric loss have been reduced obviously in the BTO films with excess Ti. Based on the analysis of the leakage mechanism and the defect interaction, we suggest that the reduction of the leakage current is related with the phase separation of TiO2 which would decrease the conductivity at the grain boundaries, as well as the reaction of Ti ions with oxygen vacancies which would decrease the mobile carriers in the film.The research in this thesis is not only a systematic fundamental research on the leakage mechanisms of the special metal/ferroelectric oxide/base-metal integrated structures, but also an exploration on the methods to reduce the leakage current and improve the performance of this kind of structures, which would be pave the way for their applications. |
PDF Full Text Request