BiFeO₃-based Ferroelectric Heterostructures And Their Electrical Properties

Mutiferroic materials have attracted a great deal of attention due to the coexistence of ferroelectricity,ferromagnetism and ferroelasticity,which has become a leading hotspot in the field of material science.Among many single-phase multiferroic materials,BiFeO3(BFO)is the only one that can exhibit both ferroelectricity(TC-1100 K)and antiferromagnetism(TN-640 K)at room temperature and above,and it has a great application prospect in sensors,information storages,spintronic devices and microelectromechanica]systems(MEMS)because of its excellent magnetoelectric coupling effect.As is known to all,the difficulties in preparation,high leakage current and weak magnetism of pure-phase BFO materials have been a bottleneck restricting its developments and applications.The BFO films,as compared with their bulk materials,show better insulativity,as well as ferroelectric and ferromagnetic properties.However,the processing temperature of BFO film is usually high(600-700 ?),which not only makes its leakage current more serious,but also makes its compatibility with large-scale integrated circuits face a great challenge.Therefore,how to produce BFO films with excellent performances by optimizing the fabrication process and while reducing the deposition temperature by feasible methods has become a hot topic in the field of BFO films.In order to solve the above fatal issues of BFO films,the high-quality epitaxial and polycrystalline BFO films were fabricated by a radio-frequency(rf)magnetron sputtering technique.The microstructures and electrical properties of BFO films were characterized by means of a series of testing methods so as to improve and enhance the films,performances.Specifically,the main contents and results of this work can be summarized as the following aspects:(1)A series of epitaxial BFO thin films were deposited on the LAO single-crystal substrates by using rf magnetron sputtering,the effect of sputtering atmosphere(Ar/O2 flow ratio)on the microstructures and electrical properties of BFO films were studied.It is found that all the BFO films show the coexistence of rhombohedral(R)and tetragonal(T)phases near the bottom electrode.With the increase of thickness,a single R phase is observed in the region away from the interface due to the relaxation of misfit stress in the film.While the sputtering atmosphere dose not change the film's epitaxial growth and phase structure,it shows significant influences on the dielectric,ferroelectric and leakage current characteristics of BFO films.The film with a 4:1 flow ratio has the best overall electrical properties and it shows a remnant polarization(2P,)of 150 ?C/cm2 and a low leakage current density of 3.6×10-4 A/cm2.(2)On the basis of the findings of(1),we focus on the influence of Ar/O2 flow ratio on the leakage currents of BFO films.It is found that there are three leakage mechnisms for these BFO films deposited at different Ar/O2 flow ratios:ionic conduction,Schottky emission and space charge limited current.By analyzing the films' piezoelectric topographies and the Auger depth profile of elements in films,we revealed the interrelations between the film morphology,growth defects and the sputtering atmosphere,which in turn successfully explained the various charge transport behaviors in these BFO films.(3)A series of epitaxial BFO thin films were deposited on SRO-buffered YAO,LSAT,STO and LAO single crystal substrates by rf magnetron sputtering and the effects of these substrates on the properties of BFO films were investigated.It is found that the phase compositions and electrical properties of these BFO films are significantly different.Among all the films,the BFO film deposited on SRO-buffered LAO substrate exhibits the lowest dielectric loss(tan ?<0.05)and leakage current density(J<8×10-4 A/cm2),as well as a remnant polarization(2Pr)of 150 ?C/cm2.(4)Based on the studies of(3),a series of BFO thin films were deposited on SRO-,LSCO-and LNO-buffered LAO(100)substrates and the effects of these oxide electrodes on the properties of BFO films were also studied.Simialr to the results of(3),it is found that the bottom electrode show significant influences on the phase compositions and electrical properites of BFO films.The film deposited on SRO bottom electrode has the best overall electrical properties,it shows a reduced dielectric loss(tan?<0.08)and a low leakage current density(J?3.6×10-4 A/cm2),as well as a remnant polarization of 2Pr-150 ?C/cm2.(5)In order to improve the energy storage characteristics of BFO thin films,the BFO/BTO bilayers were grown on STO(100)substrate via rf magnetron sputtering.The microstructures and electrical properties of BFO/BTO bilayers were studied,focusing on their energy storage characteristics.It is found that,as compared with the single-layered BFO and BTO films,the BFO/BTO bilayer shows enhanced dielectric and ferroelectric properties,as well as excellent energy storage characteristics with a recoverable electric energy density of 51.2 J/cm3.Furthermore,based on the space charge model and a modified LGD thermodynamic approach,the enhanced energy storage characteristics of BFO/BTO bilayer are explained by a combination of space charge effect and the interlayer charge coupling.(6)The polycrystalline BFO thin films were successfully deposited on Pt/Ti/SiO2/Si substrates at middle-to-low temperature(? 500 ?)via an optimal rf magnetron sputtering technique.The results show that the BFO films deposited at 4500C and 500 ? have been fully crystallized into pure perovskite phase and exhibits good electrical properties.The corresponding remnant polarizations(2Pr)are-170?C/cm2 and-280 pC/cm,respectively.As compared with that deposited at 500 0C,the BFO film grown at 450? shows lower leakage current and dielectric loss.Moreover,both BFO thin films exhibit a significant piezoelectric response.In order to further reduce the leakage current of BFO thin film and enhance its ferroelectric and piezoelectric properties,the polycrystalline BFO thick films(>1 p?m)with excellent properties were prepared by using the same deposition process.It is found that the BFO thick films exhibit much lower dielectric losses(tan?<0.02 @ 450 ?,tan?<0.03 @ 500 ?)and leakage current densities(J<6.1 ×10-6 A/cm2 @ 450 ?,J<2.9×1 0-5 A/cm2 @ 500 ?)than those of the aforementioned BFO thin films.Furthermore,both BFO thick films show more excellent ferroelectric and piezoelectric properties,the corresponding remnant polarizations(2Pr)and piezoelectric coefficients(d33)are as high as 231?C/cm2 and 269 ?C/cm,66 pm/V and 120 pm/V,respectively.