Study On Perovskite Epitaxial Oxide Thin Films And Superlattices

Oxides with perovskite structure, because of its physical and chemical properties such as dielectric, ferroelectric, ferromagnetic, conducting, superconducting and optical nonlinearity properties, have became the hot topics at the forefront of today’s research, including ferroelectric effect of PbTiO3,(Pb,La)(Zr,Ti)O3, etc; giant magnetoresistance of (La,Ca)MnO3. Presently, people are taking advantage of a variety of theoretical and experimental means to study the physical laws of electric, ferroelectric, superconducting, ferromagnetic, magnetoresistive properties of perovskite structure materials, and to explore the relationship between structure and properties of perovskite materials. Based on this background, we chose to study the perovskite oxides as a research object.This thesis is arranged as follows:at the beginning, we briefly summarize the basic structure and physical properties of the perovskite structure materials, then pay much attention on the two typical perovskite structure oxides-rare earth manganese oxide Yttrium Manganese Oxide YMnO3and La-doped ferroelectric oxide (Pb,La) TiO3. Secondarily, we synthesized these samples of the two materials bypulsed laser deposition technology. And then we characterized the physical properties (including ferroelectric, ferromagnetic, transport properties, dielectric properties and the photovoltaic effect) of these films and superlattices.. The specific content is as follows: (1) Orthorhombic YMnO3thin films were epitaxially grown on bare and LaNiO3buffered (001)-STO substrates by pulsed laser deposition under various oxygen pr essures from5to30Pa. The crystal structure and micros tructure of these films have been characterized by both X-ray diffractions and transmission electron microscopy. The leakage current, modeled as the space charge limited current (SCLC) mechanism, decreased significantly with the increase of oxygen content. It is further found that the magnetic property of films is greatly enhanced in YMnO3films grown under high oxygen pressure, which can be explained decreased oxygen vacancies. In addition, bipolar switching behavior was obtained only in the films grown under30Pa oxygen pressure, which is attributed to the decrease of voltage-driven oxygen vacancy migration.(2) We deposited the PLT25/GSO single layer film and ferroelectric superlattices with different period numbers and thicknesses on STO-(100) substrates buffered with SrRuO3by pulsed laser deposition. X-ray diffraction and transmission electron microscopy indicate that superlattices with each period have obvious satellite peak and clearly layered structure. These results substantiate the highly crystalline quality of these films. The current-voltage curve shows that the leakage current has exponential relationship with voltage. Leakage current is dependent on the components of superlattices, the thickness of the ferroelectric material and the substrate type. Ferroelectric hysteresis loops show that, compared to the ferroelectric monolayer, superlattices have decreased polarization and lower leakage current. More importantly, the photovoltaic (PV) effect on ferroelectric superlattice can be observed. And the PV effect is much higher than that in the monolayer films. And then further study shows the photocurrent of ferroelectric superlattices is strongly dependent on the periodic number. To understand these phenomena, we attribute the PV effect to the internal built-in electrical field of ferroelectric layers.