Structure Tuning And Dielectric Energy Storage Properties Of(Ba,Sr)(Zr,Ti)O₃ Films Integrated On Si

With the rapid developments in microelectronics and microelectromechanical systems,there is a great demand for integrated energy devices at a miniaturized scale.Compared with energy storage components such as batteries,supercapacitors and electrochemical capacitors,dielectric capacitors possess high power density,ultrafast charge-discharge speed,light weight and high efficiency,excellent fatigue and mechanical stability and long work life.These abilities makes it have broad application prospects in many core fields,such as medical equipment(surgical lasers,pacemakers,etc.),military field(missiles,guns and high-power microwave devices,etc.),energy industry(high speed energy conversion and power recovery systems,back-up power sources,etc.)and transportation industry(new energy hybrid vehicles,small electric bicycles,etc.).However,the energy density of dielectric capacitors are relatively low,which seriously restricting its wide application in microelectronic and microelectromechanical industries.Therefore,improving its energy density is a momentous and difficult point in this field.BaTiO3-based ferroelectric thin films have the potential to achieve a high energy density and efficiency due to its intrinsic high dielectric constant,large breakdown strength,excellent dielectric,ferroelectric and insulating performances.In order to achieve excellent dielectric energy storage properties,BaTiO3-based ferroelectric films integrated on Si substrate were usually processed at a high temperature(generally>500?)reported in the literature.A processing temperature exceeding 500? is a show stopper for the integration of thin film functional layers into Si-based microelectronics,as doing so will introduce irremediable damages or performance degradation in the very large scale-integrated electronic components on Si.In summary,it is of great scientific significance and applied value to investigate film capacitors integrated on Si with excellent dielectric energy storage properties processed at temperatures below 500?.In this paper,perovskite ferroelectric(Ba0.95,Sr0.05)(Zr0.2,Ti0.8)O3(BSZT)materials,i.e.,an A,B-site co-doped BaTiO3,were used as a target to prepare the thin films.These thin films were deposited on Si substrate between 25?(unheated substrate)and 500? by multi-target radio frequency magnetron sputtering technology.The microstructure evolution of these thin films were studied and the correlation between microstructures and dielectric energy storage properties were also revealed.The main research contents are summarized as follows:(1)The effect of sputtering temperatures on the structures and dielectric energy storage properties of BSZT ferroelectric films were studied at 350-500?.The results showed that 450? and 500?-deposited thin films possess(111)-oriented grains and large grain sizes,which reducing the BSZT films preferred orientation,breakdown strength Eb and polarization P,leading to a decreased energy storage performances.At the same time,due to an increased in-plane compressive strain existed in 350?-deposited BSZT film,resulting in its(polarization-electric field)P-E loop become"fat",as well as remnant polarization Pr and coercive electric field Ec increased,so the recyclable energy density Wrec and efficiency ? were also degraded.The thin films deposited at 400? possessed best energy storage performances can be attributed to a subtle balance between preferred orientation,grain size and in-plane compressive strain competing factors.(2)We proposed a novel material design in ferroelectric film capacitors.The high-density grain boundary arrays formed by self-assembly is used as "distributed dead layer" to improve its capacitor energy storage performance.The micromorphology of the 400?-deposited films were columnar nanograin and grain boundary arrays,which can be built in a well-distributed way throughout the film's thickness.The periodic polarization gradient inside in ferroelectric films were created by the dispersed non-ferroelectric grain boundaries layers with low dielectric constant and high breakdown strength,contributing to a reduced Pr,a delayed spontaneous polarization Ps and an increased breakdown strength Eb.Therefore,the 400?-deposited films possessed superior energy performances(Wrec?148 J/cm3 and ??91±3%),as well as outstanding temperature(-175 ?-300 ?),frequency(1 Hz-20 kHz)and cycling(up to 2×109)stabilities.Finally,the effect of film thicknesses on BSZT ferroelectric films were also investigated.The results indicated that Wrec decreasing while ? increasing with the film thicknesses increasing.(3)The superparaelectric BSZT thin films were successfully prepared by turning the grain size at 25?(unheated substrate)and 150?.The superparaelectric films were macroscopically amorphous revealed by X-ray diffraction(XRD)patterns while high resolution transmission electron microscope(HRTEM)and selected area electron diffraction(SAED)images clearly displayed {110}-oriented well-dispersed nano clusters.Polarization-electric field(P-E),compensation current-electric field(I-E),dielectric constant-electric field(?r-E),second harmonic generation(SHG)and dielectric constant-frequency(?r-f)results strictly proved that the thin films deposited at 25? and 150? were short range order superparaelectric state.The evolution of nano polar clusters(NPCs)with temperature was also revealed by temperature dependent dielectric constant and Raman spectra,which further demonstrating the films were superparaelectric state and displaying how NPCs transforms from superparaelectric into paraelectric state.Capacitors based on these superparaelectric films possessed well-dispersed NPCs,which displaying high recyclable energy density Wrec?100 J/cm3 and efficiency ??90%together with outstanding thickness(up to 900 nm),electric field(Eb?6 MV/cm,Weibull modulus ??29),frequency(up to 15 GHz),temperature(-175?-300?)and fatigue(up to 2×109)stabilities.Finally,the source of these excellent electric performances were analyzed by the series impedance model,demonstrating that they were mainly dominated by the upper BSZT layer,not from the LaNiO3 buffer layer.(4)A series of superparaelectric BSZT high k ultrathin films with different thicknesses were deposited at 25?.All these ultrathin films possessed well-dispersed NPCs revealed by HRTEM.In the thinnest 4 nm thick films,the NPCs size,polarization,switching current and dielectric constant were all decreased,while the other BSZT films possessed similar NPCs size,polarization,switching current and dielectric constant.Furthermore,the transition of temperature dependent dielectric constant and Raman spectra for the thinnest 4 nm thick films were also weak,indicating it is a weak superparaelectric state.These excellent dielectric and insulating properties,indicate a great potential of this new dielectric material in high dielectric layer field.