Photovoltaic Effects In Transition-metal Oxide Based Hetrostructures

Compared to traditional heterojunctions,the transition metal-oxide heterojunctions involve varieties of physical properties.Especially for the strongly-related materials which usually have some unique nature(such as,superconducting,ferroelectrics),the behaviors of the heterojunction would be altered significantly and even emerging some very useful functions.Furthermore,its photovoltaic effect also usually shows some novel behaviors.These studies are of application prospects for thin-film photovoltaic devices.In this dissertation,we have systematically investigated the photovoltaic effect and electronic transportation characteristics of the superconducting heterojunction and the ferroelectric heterojunctions.The whole thesis is divided into six chapters.In chapter 1,the basic characteristics of the heterojunctions and photovoltaic properties are briefly summarized,and then the superconducting heterojunctions,ferroelectric heterojunctions,and metal/Nb-doped SrTiO3 junctions with their interfaces states,photovoltaic mechanisms,physical properties and recently progresses are systematically introduced.In chapter 2,we reported the design of a multi-functional sample holder to measure the physical properties of the metal or semiconductors,especially under the magnetic field,the light irradiation and electrical field at very low temperature.This multi-functional sample holder can also be used to measure the dielectric,ferroelectric and photovoltaic properties of the solid sample.In chapter 3,the photovoltaic properties of YBa2Cu3O7-?/Nb-doped SrTiO3 heterostructures were investigated systemically under laser irradiation with different wavelengths from 365 nm to 640 nm.The Voc appears under the laser illumination with photon energy 2.4 eV,far below the band gap of the Nb-doped SrTiO3 3.2 eV.The temperature dependencies of the Voc and short-current show kinks around the structural phase transition temperature of SNTO.In chapter 4,the photovoltaic effect and photo-assisted diode behavior of Pt/BiFeO3/Nb-doped SrTiO3 hetero structure with different BiFeO3 thicknesses(60 nm and 200nm)were investigated systematically under two wavelengths(365 nm and 445 nm).We found that the short-current density Jsc shows 2 orders of magnitude difference between 365 nm and 445 nm owing to their different photo-excitation mechanisms.The photo-assisted diode-rectifying characteristic revealed that the rectifying ratio increases with increasing irradiation intensities but threshold voltage is almost unchanged.In chapter 5,the J-V curves in Au(Pt)/SNTO heterostructures were investigated under different laser wavelengths and the photovoltaic effect is strongly dependent on the photon energy,indicating the Schottky model is appropriate.We also investigated the I-V rectifying and C-V characteristics of Au/0.7wt%SNTO,and the compared the fitting Schottky barrier value among the three methods.And temperature dependent photovoltage is manipulated by the thermal activation.In chapter 6,the vortex phase transition in BaFe1.8Co0.2As2 single crystals was studied systematically via measuring the current-voltage(I-V)characteristics by rotating the crystals in magnetic fields from H//c to H//ab.The angle-resolved I-V curves clearly show a three-dimensional vortex glass(VG)scaling collapse.The angular dependent VG transition field Hg and critical current density Jc can be approximately described by the anisotropic Ginzburg-Landau model.Based on the above results,the angle-resolved vortex phase diagram is constructed,indicating the anisotropic superconducting properties of the system.