Synthesis And Property Manipulation Of Freestanding SrTiO₃ Films And Corresponding Heterostructure

The ultra-thin two-dimensional materials have attracted great attention due to great mechanical and semiconductor properties.The typical two-dimensional materials,transition metal oxide perovskites,can be manipulated due to the close coupling interaction between charge,spin,orbit and lattice.Many novel physical properties(such as ferroelectricity,magnetism,high-temperature superconducting and so on)can be proved.Similar to the traditional two-dimensional materials,reduced-dimensional metal oxide perovskites will have greater potential as an ideal flexible material.In this thesis,we synthesis the perovskite films and manipulate their physical properties,which mainly focus on the interesting physical phenomena tuned by strain effect.Typically,the strain engineering of perovskite films depends on lattice mismatch between films and substrates.This method has been widely used to control the structure and properties of many materials,which,however,is fundamentally limited and is unable to produce continuously controllable strain due to the lack of available substrates.Also,this method has various possible non-intrinsic effects such as the generated defects during growth,which may cause additional strain relaxation.These constraints in turn limit the range of strain states that can be achieved in practice,which will also hinder the development of rational design.To solve these problems,we choose Sr₃Al₂O₆ as the sacrificial layer to systhesis the freestanding films,the physical properties of which will be manipulated in this thesis.Here,we mainly discuss SrTiO₃system and the corresponding heterostructure.We have successfully systhesised the freestanding SrTiO₃ films and freestanding SrTiO₃/LaAlO₃/SrTiO₃ heterostructures to discuss their novel physical properties under different strain states.The main results can be concluded as the following three parts:(1)High quality freestanding SrTiO₃ thin films are obtained by oxide molecular beam epitaxy(oxide-MBE).It is found that under tensile strain of about 1.1%,the 15u.c.freestanding SrTiO₃will transfer from quantum-paraelectricty to room-temperature-ferroelectricity,which can be demonstrated from capacitance measurements and the first-principle calculation.The purely in-plane ferroelectric domains are characterized by piezoresponse force microscopy,and the ferroelectric domain size dependence of film thickness is discussed.The ferroelectric properties of freestanding SrTiO₃ films with thickness(4?15 unit cell)at room temperature have been realized.It is proved that the in-plane ferroelectric domain density will become larger with decreasing thickness.(2)The transfer method of two-dimensional ultra-thin freestanding films is improved.Epoxy adhesive can be used to transfer the freestanding perovskite film of 0.8nm to the flexible substrate,which ensures the atomically flat surface of the single crystal oxide film and simplifies the transfer process.By stretching the flexible substrate,the strain tuning of oxide film can be realized,which is huge enough for physical manipulation.At the same time,the conductive graphite can be stuck to the back of the flexible target substrate to effectively apply the electric field for ferroelectric measurements.The measurement and manipulation of the ferroelectric domain and other properties can be realized by piezoresponse force microscopy and optical testing devices.(3)Based on the research of two-dimensional electron gas,a high quality Ti O₂-teminated SrTiO₃ film buffered by Sr₃Al₂O₆ sacrificial film can be obtained,and the SrTiO₃/LaAlO₃/SrTiO₃ heterostructure can be constructed.The structure of the heterostructure is improved to suppress the strain relaxation after transfer,avoiding the bending of the heterostructure and preserving the two-dimensional electron gas at the interface.Due to the high lattice mismatch between SrTiO₃ and LaAlO₃(?2.9%),the LaAlO₃ film shows lattice relaxation when being transferred from the substrate.It is showed in the freestanding SrTiO₃/LaAlO₃/SrTiO₃ thin films that,the upper and bottom SrTiO₃ film experience a 2.0%compressive strain under the in-plane strain.The out-of-plane ferroelectric polarization of the freestanding SrTiO₃/LaAlO₃/SrTiO₃heterostructure is also proved by piezoresponse force microscopy.