Preparation And Physical Properties Of Na_0.5Y_0.5TiO₃ Epitaxial Thin Films

Na_0.5Y_0.5TiO₃（NYTO）is a novel lead-free ferroelectric material.At present,the related studies of NYTO are mainly concentrated on polycrystalline samples.The preparation and studies of high-quality epitaxial NYTO thin films are urgently needed in order to understand the physical properties of NYTO.In this thesis,NYTO epitaxial thin films were prepared by RF magnetron sputtering,and the structure and physical properties of NYTO films were studied in detail.The main research contents are as follows:The epitaxial NYTO thin films were prepared by RF magnetron sputtering using LSCO or SRO as bottom electrode.The structures of Pt/NYTO/SRO and Pt/NYTO/LSCO sandwich capacitors were fabriacted,and the effects of bottom electrode materials on ferroelectric and dielectric properties of NYTO films were studied.The NYTO thin films grown on SRO and LSCO bottom electrodes show obvious ferroelectric properties,the remnant polarizations were 1.12μC/cm² and 0.60μC/cm²,the dielectric constant were 221（100 kHz）and 137（100kHz）,respectively.We attributed this difference between NYTO films and bottom electrode to the lattice matching.Then,the structure of Pt/NYTO/Nb:STO sandwich capacitor was constructed,and its ferroelectric,dielectric,leakage and resistive properties were studied.It shows a saturation ferroelectric hysteresis loop in this structure,the corresponding remnant polarization and dielectric are 0.12μC/cm² and 221（100 kHz）,respectively.The NYTO/Nb:STO interface is Schottky contact,which makes the capacitor have rectifying characteristics,further impacting the ferroelectric properties of the NYTO films.Resistive switching in the structure of Pt/NYTO/Nb:STO only exists when the Schottky junction is at positive bias due to the influence of the Schottky barrier at the interface.The NYTO films annealed in air and oxygen atmosphere respectively,and the effects of annealing atmosphere on the ferroelectric,dielectric and resistive properties of NYTO films were investigated.It is found that annealing can reduce the oxygen vacancies and improve the ferroelectric and dielectric properties of NYTO thin films in the oxygen atmosphere,the remnant polarization was 0.43μC/cm² and the dielectric constant was 260（100 kHz）.The ferroelectric and dielectric properties of NYTO films are very weak annealed in air,.In addition,the annealing of oxygen atmosphere can also reduce leakage current and enhance resistive switching properties（increase switching window）of NYTO films,while annealing leads to the opposite result in air atmosphere.This is explained based on the oxygen vacancy in the films.