Exploration Of Physical Property Regulation Of Perovskite-Oxide Thin Films By Defects

The purpose of this paper is to explore the effects of oxygen and cation defects on the physical properties of perovskite oxide films and the regulation mechanism in detail.According to the types of defects,the work of this paper can be divided into two parts:oxygen vacancy and strontium vacancy.The LaAlO3/SrTiO3 two-dimensional electronic liquid system and SrRuO3/Nb:SrTiO3 Schottky junction memristors were selected to study the effect of oxygen vacancy concentration on the lattice structure and electrical transport properties.SrTiO3/BaTiO3 superlattices were selected to study the regulation of ferroelectric properties of superlattices by Sr vacancies.The specific contents of the paper are as follows:The first chapter is the introduction of the paper.First,the importance of defect research in perovskite oxides is introduced.Then,the research background and mechanism of LaAlO3/SrTiO3 two-dimensional electronic liquid system,the discovery and mechanism of memristors,the research status of ferroelectric SrTiO3 and the regulation methods of ferroelectricity are introduced.In the second chapter,we introduce all the experimental techniques used in this paper,including the preparation of thin films,the characterization of surface morphology,lattice structure and electronic structure,as well as the measurement methods of electrical transport,ferroelectricity and dielectric properties.In the third chapter,we study the relationship between the oxygen vacancy concentration in LaAlO3 lattice,the lattice structure and interface conductivity of LaAlO3/SrTiO3 two-dimensional electronic liquid system.A series of LaAlO3 thin films with a thickness of 20 layers were grown on TiO2-terminated SrTiO3 substrates by pulsed laser deposition equipped with a reflective high-energy electron diffractometer.The in-plane lattice constant measurements showed that all the samples are in-plane strained.The out-of-plane lattice constant measurements showed that the oxygen vacancy concentration in LaAlO3 layer didn't increase with the decrease of oxygen growth pressure,while the interfacial conductivity increases gradually.We thus changed the laser fluence and grew several other groups of samples to find out the reason og this anomaly.Through systematic comparison,it was found that the lattice constant of LaAlO3 is determined by the growth rate of the film.The slower the growth rate is,the lower the oxidation degree of the plume is,and the lower the oxygen content is.And the more oxygen vacancies will expand the lattice.Finally,we measured the chemical composition of the samples.The results of X-ray photoelectron spectroscopy and X-ray fluorescence spectroscopy were consistent,which showed that the La/Al atomic ratio change was not large enough to cause the change of the lattice constants we observed,so we proved that the change of LaAlO3 lattice was caused by the change of oxygen vacancy concentration.In Chapter four,we study the effect of oxygen vacancy concentration on the performance of SrRuO3/Nb:SrTiO3 Schottky junction memristors.We change the oxygen vacancy concentration in SrRuO3 thin films by changing the growth oxygen pressure and laser fluence.The lattice constant measurements and X-ray absorption spectra showed that the oxygen vacancy concentration increases with the decrease of growth oxygen pressure and laser fluence.The temperature dependent I-V characteristic test showed that the resistance switching ratio of the sample increases gradually with the decrease of temperature,and the ratio is the largest when the oxygen vacancy concentration is the median value,and is 5 times larger than that of the other samples.Then we calculated the temperature dependence of the barrier height of the Schottky junction using the thermoelectron emission model.It was found that the Schottky barrier height decreases with the decrease of temperature,and the Schottky barrier height of the sample with the largest resistance switching ratio is the smallest.Therefore,our results demonstrated that the change of oxygen vacancy concentration and Schottky barrier plays a decisive role in the memristor performance.In Chapter five,we study the effect of Sr 'vacancies on the ferroelectricity of SrTiO3/BaTiO3 superlattices.By optimizing the growth conditions of SrTiO3,room-temperature ferroelectric SrTiO3 with Sr defects was obtained.The existence of Sr defects was determined by Rutherford backscattering,and the ferroelectric properties of SrTiOwere characterized by piezoresponse force microscopy.Subsequently,a series of SrTiO3/BaTiO3 superlattices were grown using ferroelectric SrTiO3.The number of layers of BaTiO3 was 8,and the number of layers of SrTiO3 were 4,6,8 and 10.The ferroelectricity measurements of the superlattices showed that when the SrTiO3 layer thickness was 6,the ferroelectric properties of the superlattices were the strongest,and the remanent polarization value was enhanced by 70%compared to that of the single-phase BaTiO3 films grown under the same conditions.The temperature dependent dielectric measurements also showed that the ferroelectric Curie temperature of this sample is 40 K higher than that of BaTiO3.The modulation of the ferroelectric remnant field 2Pr coincides well with the out-of-plane and in-plane octahedral rotation ratio y/a,suggesting a strong correlation between the octahedral rotation and the ferroelectric distortion.Our study demonstrates the importance of constructing superlattices to enhance the ferroelectric properties of the defect engineered materials to be better than the clean materials.In the sixth chapter,we summarize the entire thesis and propose some future research plans.