Study On The Resistance Behavior Of LaAlO₃/SrTiO₃ Heterointerface By Switching The Polarization Of The BifeO₃

In recent years,the interface properties of strongly correlated oxides has attracted much attentionin a variety of fields.Specifically,perovskite oxide heterointerfaces,as represented LaAlO₃/SrTiO₃?LAO/STO?,show novel characteristics.The regulation of two-dimensional electron gas?2DEG?at LAO/STO interface has attracted extensive interests.In 2004,it was reported that the interfaces of two band insulators,LAO and STO,has conductive properties,high carrier concentration and mobility.Since then,a large variety of novel characteristics,such as magnetoresistance characteristics,superconductivity,spin-orbit coupling,electromechanical coupling effects and Hall effect have been reported in this heterointerface.The formation origin of 2DEG at the oxide heterointerfaces can be related to polar nature of the LAO atomic planes,or to oxygen vacancies,or to interdiffusion phenomena.Subsequently,the interface was proposed for use in sensors,nonvolatile memories,field-effect transistors.In addition,the electronic state at the interface can be modulated by various external stimuli,such as electric field,magnetic field,force,and light illumination.For example,Brown et al.discovered a fully reversible conductance change at the LAO/STO interface regulated by LAO surface protonation.However,the unstable resistance switching properties of the structure cannot be widely used in devices.Kim et al.reported the control of the LAO/STO interfacial conductivity depends on polarization of epitaxial Pb(Zr_0.2Ti_0.8)O₃ films at the dark,and the large resistance switching ratio with high power consumption.However,Pb is harm to the environment.In addition,the polarization-controlled interfacial conductivity has not fully explained.As is well known,BiFeO₃?BFO?films exhibit multiferroic properties at room temperature with the high Curie temperature?T_C¹100 K?and Neel temperature?T_N⁶40 K?.BFO is an environmentally friendly material without Pb element and have a good chemical stability.It has great potential application in the information technology,such as sensors and spintronics devices.Mix et al.investigated the conductance control at the LAO/STO interface by switching the polarization of the BFO layer,showing a hysteretic behavior with the poling voltage and realizing the control of interface conductivity.However,resistance switching ratio is just 1.15,which is easy to cause the misoperation in using,since the high and low resistance states are difficult to be distinguished.In this work,we investigated the mechanism of the polarization-controlled reversible resistance of 2DEG by switching the polarization of ferroelectric material,and improved the resistance characteristics via precise process parameters,and improved sensitivity of the device to external factors and expanded the application range of the device.This work used pulsed laser deposition system,magnetron sputtering and UV lithography to make devices,used atomic force microscopy,four-probe testing methods,physical property measurement system?PPMS?to observe ferroelectric and electrical properties.we have performed systematic experimental works and have had the following achievements:1.In order to obtain atomically smooth TiO₂-terminated?001?STO substrates with a step-terrace morphology,etching and annealing procedures were optimized in respect to?001?STO substrates.A kind of treatments,“chemical etching-high temperature annealing”.Single-terminated?001?STO substrates with an atomically smooth surface have been prepared.Influences of etching time,concentration of buffered-HF has been studied systematically.2.The BFO/LAO/STO device structure was fabricated using UV lithography.a.Solid Bi?1.125?FeO3 ceramic targets were successfully prepared using the solid phase reaction method.b.The effects of pre-baking,exposure,development,post-baking and stripping times on the lithographic pattern were studied,and the optimum lithography conditions were obtained.c.Films were grown by Pulsed Laser Deposition?PLD?and magnetron sputtering.The oscillatory curves of LAO growth were monitored by high energy electron diffractometer?HREED?.The thickness of LAO is 10 u.c.d.the 150 nm thick Pt electrode was prepared by sputtering and the thickness was closer to the bulk.3.The microscopic ferroelectric and electrical properties of the structure were studied by PFM and KPFM.It shown that the LAO layer improve the flatness of the BFO surface,and make the BFO spontaneous polarization down,and reduce the BFO coercive voltage by PFM measurement.we calculated the work function of BFO to be approximately 4.7 eV by KPFM.4.Comparing the electrical properties of the BFO/STO,LAO/STO,and BFO/LAO/STO structures,A large reversible resistance was found that the LAO/STO interface in the BFO/LAO/STO structure.The ratio of the 2DEG resistance switching is as large as 102,and the photo-dark resistance ratio>20 times in the BFO/LAO/STO device,the largest switching ratio in free-lead ferroelectric devices so far.This reversible resistance change and light resistance characteristics at the LAO/STO interface result from the polarization of BFO films.The mechanisms of the modulations of the 2DEG resistance at the BFO/LAO/STO devices by switching the polarization of the BFO;ayer were discussed.Our results show that the novel properties in oxide heterojunction.