Studies On Defect Mechanism And Control Of Two-dimensional Electron Gas On Oxide Surface/Interface

Transition metal oxides have always been the research focus in condensed matter physics due to the interaction of their d-electron charge,spin and orbit,which can re-sult in many novel physical phenomena.With the progress made in the heterojunction preparation technique,people can combine different transition metal oxides to realize the design of quantum evolution phenomena.Therefore,the interface of transition metal oxide heterojunction has become a research hotspot.As the most representative crystal structure of transition metal oxides,the perovskite has attracted great attention.Among those,SrTiO₃is one of the most systematically and intensively studied one.The role it plays in oxide electronics is comparable to that of the silicon in traditional semiconduc-tor electronics.The most typical SrTiO₃-based heterojunction is the two-dimensional electron gas(2DEG)at the La Al O₃/SrTiO₃interface.Abundant physical properties have been dis-played such as interfacial superconductivity,magnetism and strong Rashba coupling,which make the 2DEG system process a great research potential.In this paper,we focus on the defect mechanism of surface/interface 2DEG,and introduce oxygen vacan-cies into SrTiO₃or TiO₂surface/interface by different means.We not only study the relationship between surface atomic structure,electronic structure,and transport prop-erties,but also expand the scope of defects beyond oxygen vacancies.On the basis of understanding the effect of defects on the 2DEG properties,we use subatomic layer growth,control ion etching parameters,gate voltage and light irradiation to modulate defects,so as to achieve the purpose of modulating electronic properties.Specifically,there are the following three research results:1.Ultra-thin single-layer films with different terminations were grown on SrTiO₃substrates by the in-situ oxide molecular beam epitaxy(MBE)system combined with synchrotron radiation source.The staggered two TiO₂layers(Double Layer)structure of the SrTiO₃substrate surface after the buffered HF process was discovered,and the island growth mode of a layer(two layers)of Sr O on this substrate was found.Interest-ingly,the growth of a layer of TiO₂on top of a layer of Sr O will smooth the islands and restore the TiO₂Double Layer structure.Using ARPES,it was found that the SrTiO₃substrate has the similar surface 2DEG as reported in the literatures,and a single layer of Sr O eliminates 2DEG.But following another single layer of TiO₂makes 2DEG as well as the Double Layer come back.So that we are able to relate the 2DEG to TiO₂Double Layer and turn”on”and”off”the 2DEG on the SrTiO₃surface,thereby real-izing”interface engineering”at the subatomic level.Spectroscopic data show that the surface oxygen vacancies will release electrons,which makes Ti valence change from the original tetravalent to trivalent.The covalent bond change of Ti ion causes the e_gor-bital of its conduction band to move down below the Fermi surface,and some electrons trapped by oxygen vacancies were released to form a characteristic local in-gap state at-1.2 e V.The remaining electrons can gather on t_2genergy level on the Fermi surface to form a mobile 2DEG.2.For the first time,using Ar ion bombardment method to prepare surface 2DEG on two crystalline TiO₂(anatase and rutile).It was found that the surface of anatase TiO₂has a one-dimensional to two-dimensional transport behavior varied with different irradiation time.The temperature dependence of its negative magnetoresistance and resistance also told us that there existed magnetic ions(Ti³⁺)on the surface.Although the surface of the rutile TiO₂substrate has not been bombarded to form typical 2DEG,it still possessed high mobility of 50 cm²/Vs at 2K under the irradiation time of 0.5minute,which is higher than the mobilities of anatase under all irradiation conditions.Comparing the transport data of the two crystal types,it was found that the introduction of oxygen vacancies by physical method to prepare surface 2DEG is only a necessary condition.To form a surface 2DEG will further depend on the crystal configuration,bond length,bond angle and other structural factors.3.The high-quality 2DEG at the interface of Sr Nb O₃/SrTiO₃was successfully prepared by pulsed laser deposition(PLD).It was found that the R_S(T)curve has a unique”hysteresis”phenomenon,that is,the cooling curve is monotonous and metallic(with some weak localization),and the warm-up curve has two resistance peaks at 84K and 179 K.The reason is that the structural domains caused by the structural phase transformation of SrTiO₃itself lead the electrons to gather in the domain walls.Such that whenever the temperature going up across the phase transition point,the electrons will be released from the walls in a sudden to cause a resistivity peak.Through the modulation of the gate voltage and light illumination on the resistance curve,it was found that the resistance peak at 84 K originates from the cubic-tetragonal phase transi-tion of the SrTiO₃bulk material,while the resistance peak at 179 K corresponds to the cubic-tetragonal phase of the SrTiO₃thin layer with 2DEG near the surface.Our find-ings suggest that in addition to the point defect-oxygen vacancies that play a role in the mechanism,the line defects caused by the phase transition of SrTiO₃itself-domain walls have a non-negligible effect on the electronic properties of the SrTiO₃-based 2DEG.