Precise Preparation Of Oxide Interface And Exploration Of Oxide Two-dimensional Materials

With the rapid development of electronic information industry,the current size of semiconductor device in the integration has been close to the limit of Moore's Law,and because of the limitations of material research and production process,semiconductor devices become more and more unable to meet the needs of high-performance devices.At this time,"beyond silicon materials"research comes into being,which mainly studies the surface and interface of the transition metal oxide.The destination is to create a new generation of low-loss high-performance electronic devices through the synthesis of new materials and artificial microstructure design.Perovskite(ABO3)structure is the most common structure in transition metal oxide materials,and it is also our main research object.Considering the existence of charge,spin,orbit and lattice-four strong correlation degrees of freedom in perovskite,we can intentionally carry out artificial designs and accurate epitaxial growth of low-dimensional structures on atomic scales,such as thin films,heterointerfaces and superlattices.Under the effect of quantum confinement,the correlation between four degrees of freedom is further enhanced,and it is expected to obtain novel phenomena and properties.In this paper,we have discussed how to prepare the precise interface of complex perovskite oxides such as SrTiO3 and how to separate the film from the substrate by using oxide molecular beam epitaxial technology and in-situ reflection high-energy electron diffraction(RHEED)in detail.The microscopic crystal structure of the stripped ultrathin freestanding perovskite film and the critical thickness of maintaining single-crystal state were studied.The main results obtained are as follows;a)In-situ growth of SrTiO3for pure termination control as an exampleAlthough the current researches on transition metal oxide films and heterointerfaces are in full swing,how to prepare the precise configuration of the surface and interface is still the key factor which limits the in-depth study of two-dimensional strong correlated intriguing electron phases.Taking the typical perovskite SrTiO3 as an example,we mon-itor the change of the film surface composition in real-time growth using the in-situ re-flection high-energy electron diffraction.Moreover,the mean inner potential of complex oxide is measured in situ by fitting the Kikuchi lines,and the results show that the mean inner potential is periodically oscillating with the growth.The maximum and minimum values of oscillation provide important chemical information for the surface termination during real-time growth.In addition,according to the observed phenomena of RHEED intensity oscillations which show the phase inversion and frequency doubling,we have proposed a model including the effects of mean inner potential and periodic surface roughness to reveal the potential mechanism of complex RHEED intensity oscillation during oxide growth.On this basis,we summarized the general guiding rules for the preparation of the oxide interface with precise chemical composition,and successfully fabricated pure TiO2-terminated SrTiO3 epitaxial films with arbitrary thickness.Our guidelines also provide a reliable method to solve a range of problems associated with such surfaces and interfaces of oxides,creating more possibilities for fine structural mod-ulation(including thickness and termination)at the atomic scale.b)Preparation of the atomically smooth surface of Sr3Al2O6thin filmThe separation of perovskite films and substrates using water-soluble Sr3Al2O6 films as sacrificial layers was reported by Di Lu et al.in 2016,which greatly contributed to reducing dimensions of the strong correlation system.Therefore,the preparation of atomically smooth Sr3Al2O6 is the primary task,but there is still a lack of understanding of the growth mechanism and practical guidance for synthesizing smooth Sr3Al2O6 film.In this paper,we systematically discuss the growth mechanism of Sr3Al2O6films.By comparing the diffraction patterns,oscillations and surface morphology of Sr3Al2O6 beam films growing under different Sr/Al beam flux ratios,the effects of the Sr/Al ratios on crystal quality and surface smoothness were investigated.It is found that the film quality is very sensitive to the Sr/Al ratio,and only the Sr3Al2O6 with the optimal flux ratio has an atomically smooth surface.A four-fold reconstructed RHEED pattern and 4 peri-ods of RHEED oscillations in the growth of one unit-cell Sr3Al2O6 are the key signatures of the optimal growth conditions.Following these criteria,the Sr/Al beam ratio can be optimized in real time,and we successfully prepared atomically smooth Sr3AlO6 films.Our research provides a practical in-situ judgment standard for the growth of Sr3Al2O6 films and makes the calibration of the optimal growth conditions easy and reproducible.Moreover,it lays a foundation for further use of Sr3Al2O6 to separate large-size perov-skite films.c)Fabrication and characterization of freestanding monolayer perovskiteIn order to draw an analogy with two-dimensional graphene and its related devices in the strong correlation system,we use molecular beam epitaxial technology to prepare an atomically smooth Sr3Al2O6 film as a sacrificial layer for the separation of perovskite films and substrates.Result shows that the freestanding SrTiO3 film has no critical thick-ness limit,even if the thickness is down to monolayer,it can still stabilizing the crystal-line order.This breakthrough in stripping technology is of great significance to the ex-ploration of strong correlation two-dimensional materials.At the same time,the free-standing conductive LaAlO3/SrTiO3 heterostructure has been successfully prepared for the measurement of thermal properties.From the view of the application,the freestand-ing perovskite films can be transferred to the semiconductor silicon wafer so that the traditional semiconductors can directly combines superior strong correlation character-istics,paving the way for a new generation of multifunctional electronic devices.Our work is a major technological breakthrough in the successful separation of strong corre-lated oxides and pioneers the exploration and application of monolayer oxide films.