Study Of Second Harmonic Generation From Domains And Domain Walls In Perovskite Ferroelectric Thin Films

For the development and applications of low-energy and high-efficiency memory devices,ferroelectric and multiferroic thin films have attracted broad attention due to their unique read/write modes and controlled domain distribution.The discovery of new material,manipulating the domain states and detecting the domain distribution are the focuses of this field.As an optical characterization technique to study the ferroic ordering,second harmonic generation（SHG）is a contactless,non-invasive and flexible method for probing materials with broken inversion-symmetry.However,the complex crytal structures and domain variants in the microscopic scale complicate the analysis,and the application of SHG in this field needs to be further developed.In this thesis,the room-temperature ferroelectric properties of Sr Ti O₃ and domain configurations of Bi Fe O₃are systematically studied by SHG.The main content is listed as following:1.Investigation of out-of-plane ferroelectric properties of strontium titanate films by SHG technique.In this section,structural phase transition and out-of-plane ferroelectricity induced by oxygen vacancy and epitaxial strain in Sr Ti O₃ films are investigated by SHG that using different incident configurations.The density of oxygen vacancy defects was controlled by varying the oxygen pressure in the film growth process,and the epitaxial strain states were controlled by lattice constant of the substrates.Structures and c/a ratio of three tetragonal Sr Ti O₃ films were confirmed by reciprocal space mapping,and the out-of-plane ferroelectric polarization of the Sr Ti O₃ films was confirmed by varying the incidence angles in SHG measurements.It is found that the tetragonal Sr Ti O₃ film with both oxygen vacancy defects and epitaxial strain has the largest c/a ratio（～1.064）and the strongest out-of-plane ferroelectric polarization.This study provides a new approach to improve the ferroelectricity of SrTiO₃ films for the applications of room-temperature electronic devices.Meanwhile,this part also provides a reference for the discussion of SHG properties of Bi Fe O₃ films in the following chapters.2.Investigation of SHG properties of stripe domains and domain walls in Bi Fe O3 thin films.In this section,a non-zero SHG contribution from the domain walls between 71°striped domains in Bi Fe O₃ is demonstrated by using SHG techniques including non-analyzed SHG measurement,continuous polarization measurement of SHG（CPSHG）and SHG spatial scanning.Furthermore,Néel-type evolution of the polar vectors inside the domain walls is clarified as well.First,high-quality Bi Fe O₃ films with quasi-rhombohedral structures were grown on Sr Ti O₃ and Dy Sc O₃ substrates by pulsed laser deposition,which contain single-domain and striped domains respectively.Both the intensity and polarization of SHG signals from ferroelectric domains and domain walls were systematically characterized through the CPSHG technique.It is found that the SHG polarization mainly lies along the direction of the net in-plane ferroelectric vector,regardless of the polarization direction of fundamental frequency light.In addition,non-negligible SHG contribution from the domain walls is confirmed by comparing the scanning images of a well-aligned domain area that under different polarization set-ups.The Néel-type evolution of the polar vectors inside the 71°domain walls is determined by analyzing corresponding SHG signals.This work enriches and promotes the application of SHG techniques in the study of ferroelectricity,controlled domain structure and domain walls,and also provides an important reference for the discussion in the next chapter.3.Investigation of SHG characteristics of the mixed-phase structures in Bi Fe O3thin films.Following the works above,SHG properties of purely quasi-tetragonal phase and mixed-phase structures in Bi Fe O₃ thin films are investigated in this section by CPSHG.Based on their SHG polarization characteristics,optical images of complex phase distribution are also obtained.First,the mixed-phase Bi Fe O₃ films were grown on Ca_0.96Ce_0.04Mn O₃-buffered La Al O₃ substrates by using pulsed laser deposition.Through the CPSHG experiments,it is found that the SHG signal of the mixed-phase region is formed by linear superposition of the respective SHG contribution of the quasi-tetragonal and quasi-rhombohedral phases.This explicit relationship can be applied to the other mixed-phase areas in overall Bi Fe O₃ films.This study not only demonstrates that the proposed SHG technique is a powerful tool for investigating complex phase structures,but also provides new insight for the development of optical readout of novel ferroelectric memory devices.