Temperature-dependent Evolutions Of Surface Charge States In BaTiO₃ Thin Film

Ferroelectrics have discovered pyroelectric properties in Roy salt since 1824 year to the present time of ferroelectric thin films and devices,due to their fascinating properties,such as ferroelectricity,piezoelectricity,and photoelectricity,etc.,widely used in nonvolatile memory,micro-electro mechanical systems,microwave ceramics,resistivity devices,and the positive temperature coefficient of sensors,etc.Particularly,due to the phase transition occurring near the Curie temperature,the properties of ferroelectric materials are more attractive and unique.As a typical ferroelectric material,BaTiO₃?BTO?has attracted more attention which can be used in memory devices,multilayer capacitors,electro-optic devices,infrared detectors,and other applications due to its ferroelectricity,excellent electro-optic,and nonlinear optical properties.In addition,there is a unique property of BTO with different crystal structures at different temperatures,in which the phase transition from the tetragonal ferroelectric phase to the cubic paraelectric phase occurs at Curie temperature 120?.This temperature is close to room temperature and is easier to study than other ferroelectric materials.On the other hand,in recent years,scanning probe microscopy?SPM?techniques such as piezoresponse force microscopy?PFM?and Kelvin probe force microscopy?KPFM?have been extensively used for the imaging and quantification of local domain structure,local ferroelectric properties,and local surface charge state of the ferroelectric materials.It is crucial to understand the dynamics of tip-injected surface screening charges and their relaxation behavior.Surface charge screening determines the stabilization of the ferroelectric domains and further influences the data storage density,signal reliability,and bit readout.Therefore,the surface charge screening plays a significant role in the polarization reversal processes and determines the stability of domain structures in ferroelectrics to a large extent.In the past years,both experimental and theoretical methods have been used to deeply understand the structural,ferroelectric,and electrical properties of BTO.Yuan et al.investigated the evolution of ferroelectric domains from 20–120?of the BTO film.They found that the in-plane domains are very stable at 20?,and piezoelectric response and coercive voltage increase largely when mono-domain appears in most regions at 60–80?.Kalinin et al.investigated the domain polarity and temperature induced potential inversion on the BTO?100?surface as well as the temperature dependence of polarization and charge dynamics.The screening charge relaxation kinetics is found to be weakly dependent on temperature with activation energy Ea?4 kJ/mole.The ferroelectric-domain induced surface corrugations and piezoelectric response decrease with temperature and disappear at Curie temperature.The temperature dependence of the surface potential contrast is ascribed to the interplay between the release of the screening charges with temperature and their slow relaxation.Though the effect of temperature on ferroelectric property and surface charge state of BTO has been investigated,a deep and systematic investigation of the temperature-dependent evolution of ferroelectric property and surface charge states is still rare.In this work,the epitaxial BTO ferroelectric films on?001?SrTiO₃?STO?substrates with La_0.5Ca_0.5MnO₃?LCMO?buffered layer as bottom electrode by pulsed laser deposition was fabricated in order to gain a deeper understanding of the temperature-dependent evolution of ferroelectric property and surface charge states.The BTO thin films were written as rectangular-ambulatory-plane of reversed ferroelectric domains by scanning probe microscopy technique.The surface potential?SP?on the area with pre-written domains was read out by Kelvin Probe Force Microscopy?KPFM?.We investigated the temperature-dependent evolutions of the polarization reversal,surface potential,and surface potential decay process.These experimental results provide a pathway to improve the BTO film performance at an optimized temperature,and could enhance the stability when which are used in the data storage.The main results are as follows:1.BTO and LCMO ceramics have been successfully prepared by a conventional solid state reaction method with powder embedded sintering;2.The LCMO films were deposited on?001?SrTiO3?STO?substrate by pulsed laser deposition?PLD?;LSMO is not only as a buffered layer,but also as a bottom electrode;there is also a great influence on the as-grown polarization state of BTO ferroelectrical layer;3.By PLD,the BFO film was grown on the LCMO/?001?STO;4.The growth of LCMO and BTO films were tested by X ray diffractometer?XRD?and X ray reflectometer?XRR?.The results showed that the thickness of LCMO and BTO was 12nm and 35nm,respectively;5.The XRD of the double layer membrane at different temperatures was tested.6.Various atomic force microscopes were applied in our work,such as piezoresponse force microscopy?PFM?and surface Kelvin probe force microscopy?KPFM?,with these imaging technologies,the double layer membrane on the ferroelectric properties?the domain structure and the pizeoresponse?,the electrical characters?surface potential?,and the surface charge states were obtained;1.BTO and LCMO thin films are of good qualities,and no impurity phase is produced and the surface is smooth and dense.2.BTO thin film showed spontaneous polarization downward,With the increase of temperature,the spontaneous polarization and piezoelectric response of BTO film also increase.The maximum temperature is about 80?,and as the temperature continues to rise,the ferroelectric property slowly disappears because of the Curie temperature.3.The surface charge density and surface potential of the BTO film gradually increase with the temperature,which is due to the complete screening of polarization bound charge resulting in the formation of an electric double layer.However,when the temperature is close to Curie,the ferroelectric property gradually disappears,the intensity of the bound charge decreases,and the surface charge density and surface potential decrease.4.At different temperatures,the surface charge stability of BTO films is different,which is most stable at about 80?.