Study On Transport Properties And Modulation Of Metal-insulator Transition In Correlated Electronic Oxide VO₂/Nb:TiO₂ Epitaxial Heterojunctions

Vanadium dioxide as a proptyope of correlated electron system,a reversible Metal—insulator transition(MIT)near 340 K has been observed with a first-order phase tansformation.In vinciniyt of MIT,its light absorption,electrical resistivity,magnetic susceptibility,refractive index,crystal structure and the other physical properties correspondigly change,making it as a promising candidate for novel transistor,phase change storage,photoelectric switch,and infrared detection,and so on.Moreover,large-scale VO₂thin films can be better compatibile with modern microelectronic techniques.In the field of infrared detector,the VO₂thin films gradually begin to be applied in pratical devices.On the other hand,the MIT is susceptible to external fields(e.g.electric fields,strain,temperature,etc.),and in situ control of MIT in VO₂has become one of hot topics in the field of condensed matter physics.However,electric field,doping,strain,temperature,and other means are all contact-type controls for modulating MIT in VO₂.It is difficult to apply in situ dynamic strain,temperature and other external field conditions,and it is not conducive to device integration,which hinders VO₂based the electronic devices moving toward practical applicaitons.The advantage of optical control of MIT in VO₂can overcome the static,contact-type and difficult-to-integrate deficiencies of the traditional adjustment methods mentioned above.As a consequence,we can achieve dynamic and non-contact control of MIT in VO₂using optical scheme.Moreover,the research on the optical control of MIT mainly focused on the in-plane devices with lateral geometry,but the research of controlling MIT in vertical thin film devices and heterojunctions is still in the early stage,and further exploration is urgently needed for downsizing the VO₂-based devices to the nanoscale.In this work,we use high-vacuum magnetron sputtering technology to grow a correlated electronic oxide VO₂thin film,which is on a Nb-doped TiO₂(Nb:TiO₂)crystal substrate to construct a vertical VO₂/Nb:TiO₂epitaxial heterojunction.Firstly,we investigated electrical transport properties of the vertical VO₂/Nb:TiO₂epitaxial heterojunction in the vicinity of MIT.Next,the optical control of MIT in the correlation electronic oxide VO₂/Nb:TiO₂heterojunction was studied,and the mechanism of optical control of MIT was also discussed.The main results in this paper were as follows:(1)Study on the microstructure of VO₂/Nb:TiO₂ epitaxial heterojunction and its electrical transport characteristics across the MIT.As a prototype of correlated electron system,vanadium dioxide(VO₂)is much more attractive due to a giant metal-insulator transition(MIT)near room temperature and potential applications in the photo-and electronic devices.In this work,we first constructed a vertical heterojunction VO₂/Nb:TiO₂,which is made of the VO₂epitaxial thin film and 0.05 wt%-doped Nb:TiO₂semiconductor substrate using magnetron sputteringtechnique and symmetrically investigated the its electrical transport properties.We observed that this vertical heterojunction device has a pronounced MIT behavior as similar as in the single-layer VO₂thin film.A good rectifying characteristic with a phase-controllable possibility is also achieved.At 24 ^oC,the rectification ratio(RR)is up to 7000 with a bias±1 V.Such a larger RR could be attributed to the formation of the staggered n-n-type heterostructure between the VO₂thin film and Nb:TiO₂substrate.As increasing temperature above the transition temperature,the RR is reduced to 29.4 owing to the formation of the Schottky junction at 114 ^oC.Additionally,both the barrier height and ideal factor reversibly increase and decrease in the heating and cooling process with a hysteretic feature across the MIT.The barrier height is approximately 0.825 e V in the VO₂M1 phase and increases to 0.9 e V in the VO₂rutile phase with a decrement in the phase-coexisted region in the vicinity of the MIT,respectively.The ideal factor is approximately 1.024 and slightly increases to 1.030 as increasing temperature.The voltage-dependent RR is attributed to the voltage-modulated energy band structure of the VO₂/Nb:TiO₂heterojunction.Moreover,the RR,barrier height,and ideal factor is affected by temperature with a hysteretic behavior,which is driven by the metal-insulator transition in the VO₂thin films.This work indicates that the transport properties of the heterojunction device based on the correlated electron system of VO₂can be artificial engineered through utilizing the MIT characteristics.(2)The photoresponse characteristics of VO₂/Nb:TiO₂epitaxial heterojunction and optical control mechniasm of metalinsulator transition are investigated.We demonstrate a vertical heterojunction made of a correlated electronic oxide thin film VO₂and a conductive 0.05 wt%Nb-doped TiO₂single crystal,whose metal–insulator transition(MIT)across the nanoscale heterointerface can be efficiently modulated by visible light irradiation.The magnitude of the MIT decreases from 350 in the dark state to 7 in the illuminated state,obeying a power law with respect to the light power density.The junction resistance is switched in a reversible and synchronous manner by turning light on and off.The optical tunability of it is also exponentially proportional to the light power density,and a 320-fold on/off ratio is achieved with an irradiance of 65.6m W/cm²below the MIT temperature.While the VO₂thin film is metallic above the MIT temperature,the optical tunability remarkably weakened,with a one-fold change remaining under light illumination.These results are co-attributed from a net reduction(?15 me V)in the apparent barrier height and the photocarrier-injection-induced metallization of the VO₂heterointerface through a photovoltaic effect,which is induced by deep defect level transition upon the visible light irradiance at low temperature.Additionally,the optical tunability is minimal,resulting from the quite weak modulation of the already metallic band structure in the Schottky-type junction above the MIT temperature.This work enables a remotely optical scheme to manipulate the MIT,implying potential uncooled photodetection and photoswitch applications.