Enhanced ferroelectric and improved leakage of BFO-based thin films through increasing entropy strategy

BiFeO_(3)(BFO)has received considerable attention as a lead-free ferroelectric film due to its large theoretical remnant polariza-tion.However,BFO suffers from a large leakage current,resulting in poor ferroelectric properties.Herein,the sol-gel method was used to deposit a series of BFO-based thin films on fluorine-doped tin oxide substrates,and the effects of the substitution of the elements Co,Cu,Mn(B-site)and Sm,Eu,La(A-site)on the crystal structure,ferroelectricity,and leakage current of the BFO-based thin films were invest-igated.Results confirmed that lattice distortion by X-ray diffraction can be attributed to the substitution of individual elements in the BFO-based films.Sm and Eu substitutions contribute to the lattice distortion in a pseudo-cubic structure,while La is biased toward pseudo-tet-ragonal.Piezoelectric force microscopy confirmed that reversible switching of ferroelectric domains by nearly 180°can be realized through the prepared films.The ferroelectric hysteresis loops showed that the order for the polarization contribution is as follows:Cu>Co>Mn(B-site),Sm>La>Eu(A-site).The current density voltage curves indicated that the order for leakage contribution is as follows:Mn<Cu<Co(B-site),La<Eu<Sm(A-site).Scanning electron microscopy showed that the introduction of Cu elements facilitates the formation of dense grains,and the grain size distribution statistics proved that La element promotes the reduction of grain size,leading to the increase of grain boundaries and the reduction of leakage.Finally,a Bi_(0.985)Sm_(0.045)La_(0.03)Fe_(0.96)Co_(0.02)Cu_(0.02)O_(3)(SmLa-CoCu)thin film with a qualitative leap in the remnant polarization from 25.5(Bi_(0.985)Sm_(0.075)FeO_(3))to 98.8µC/cm^(2)(SmLa-CoCu)was prepared through the syner-gistic action of Sm,La,Co,and Cu elements.The leakage current is also drastically reduced from 160 to 8.4 mA/cm^(2)at a field strength of 150 kV/cm.Thus,based on the increasing entropy strategy of chemical engineering,this study focuses on enhancing ferroelectricity and decreasing leakage current,providing a promising path for the advancement of ferroelectric devices.

Structure,ferroelectric,and enhanced fatigue properties of sol–gel-processed new Bi-based perovskite thin films of Bi(Cu_(1/2)Ti_(1/2))O_(3)–PbTiO_(3)

Bi-based perovskite ferroelectric thin films have wide applications in electronic devices due to their excellent ferroelectric properties.New Bi-based perovskite thin films Bi(Cu_(1/2)Ti_(1/2))O_(3)–PbTiO_(3)(BCT–PT) are deposited on Pt(111)/Ti/SiO_(2)/Si substrates in the present study by the traditional sol–gel method.Their structures and related ferroelectric and fatigue characteristics are studied in-depth.The BCT–PT thin films exhibit good crystallization within the phase-pure perovskite structure,besides,they have a predominant(100) orientation together with a dense and homogeneous microstructure.The remnant polarization(2P_(r)) values at 30 μC/cm^(2) and 16 μC/cm^(2) are observed in 0.1BCT–0.9PT and 0.2BCT–0.8PT thin films,respectively.More intriguingly,although the polarization values are not so high,0.2BCT–0.8PT thin films show outstanding polarization fatigue properties,with a high switchable polarization of 93.6% of the starting values after 10^(8) cycles,indicating promising applications in ferroelectric memories.

Optimal parameter space for stabilizing the ferroelectric phase of Hf_(0.5)Zr_(0.5)O_(2) thin films under strain and electric fields

Hafnia-based ferroelectric materials, like Hf_(0.5)Zr_(0.5)O_(2)(HZO), have received tremendous attention owing to their potentials for building ultra-thin ferroelectric devices. The orthorhombic(O)-phase of HZO is ferroelectric but metastable in its bulk form under ambient conditions, which poses a considerable challenge to maintaining the operation performance of HZO-based ferroelectric devices. Here, we theoretically addressed this issue that provides parameter spaces for stabilizing the O-phase of HZO thin-films under various conditions. Three mechanisms were found to be capable of lowering the relative energy of the O-phase, namely, more significant surface-bulk portion of(111) surfaces, compressive c-axis strain,and positive electric fields. Considering these mechanisms, we plotted two ternary phase diagrams for HZO thin-films where the strain was applied along the in-plane uniaxial and biaxial, respectively. These diagrams indicate the O-phase could be stabilized by solely shrinking the film-thickness below 12.26 nm, ascribed to its lower surface energies. All these results shed considerable light on designing more robust and higher-performance ferroelectric devices.

Ferroelectricity in Layered Perovskites as a Model of Ultra-Thin Films

The instability of thin ferroelectric films is discussed based on the close similarity of dielectric properties between bulk Bi-layered perovskites and thin BaTiO3 films. The dielectric properties of pseudo-two-dimensional layered perovskites suggest that the bulk layered ferroelectric is a good model of ultra-thin ferroelectric film with a few perovskite units, free from any misfit lattice strain. It seems plausible that the ferroelectric interaction is still prominent but shows a crossover from ferroelectric to antiferroelectric along the unique c-axis (perpendicular to the film plane);with decreasing thickness, the ferroelectricity appears within the plane, which results in so-called “canted ferroelectricity”. An extra relaxation mode induced by surface effect of thin films correlates with soft mode, which results in a new intermediate phase between the paraelectric and ferroelectric phases. These evidences may indicate no critical thickness even for ferroelectric ultra- thin films.

Influence of pyrolysis temperature on ferroelectric properties of La and Mn co-doped BiFeO_3 thin films

Bi0.9La0.1Fe0.95Mn0.05O3 （BLFMO） ferroelectric thin films were fabricated on Pt/Ti/SiO2/Si/ substrates by the sol-gel process at different pyrolysis temperatures. The mass loss of BLFMO powder was investigated by thermo gravimetry analyser （TGA）, and the polycrystalline structure and smooth surface of BLFMO thin films were characterized by X-ray diffraction （XRD） and atomic force microscopy （AFM）, respectively. The remnant polarization （Pr） of the BLFMO films pyrolyzed at 420 ℃ is 21.2 μC/cm2 at the coercive field （Ec） of 99 kV/cm and the leakage current density is 7.1×10-3 A/cm2, which indicates that the BLFMO thin films display relatively good ferroelectric property at this temperature.

Combined effect of the transition layer and interfacial coupling on the properties of ferroelectric bilayer film

Within the framework of modified Ginzburg Landau-Devonshire phenomenological theory, a ferroelectric bilayer film with a transition layer within each constituent film and an interfacial coupling between two materials has been studied. Properties including the Curie temperature and the spontaneous polarization of a bilayer film composed of two equally thick ferroelectric constituent films are discussed. The results show that the combined effect of the transition layer and the interracial coupling plays an important role in explaining the interesting behaviour of ferroelectric multilayer structures consisting of two ferroelectrie materials.

Enhancement of photoelectrochemical performance in ferroelectric films via the introduction of an Au buffer layer

The inefficient separation of photogenerated carriers has become a serious problem that limits the photoelectrochemical(PEC)performance of semiconductors.Herein,a sol-gel method was used to prepare BiFeO_(3) ferroelectric thin films with FTO and FTO/Au as substrates,respectively.The polarization electric field of the ferroelectric can more effectively separate the carriers generated in the photoelectrode.Meanwhile,the introduction of an Au buffer layer can reduce the resistance in the process of charge transfer,accelerate the carrier migration,and enhance the efficiency of the charge separation.Under light irradiation,Au/BiFeO_(3) photoelectrode exhibited an extraordinary improvement in PEC water splitting compared with BiFeO_(3).In addition,the ferroelectric polarization electric field causes band bending,which further accelerates the separation of electrons and holes and improves the PEC performance of the photoelectrode.This work promotes the effective application of ferroelectric films in PEC water splitting.

Study of Depolarization Field Influence on Ferroelectric Films Within Transverse Ising Model

An improved transverse Ising model is proposed by taking the depolarization field effect into account. Within the framework of mean-held theory we investigate the behavior of the ferroelectric thin film. Our results show that the influence of the depolarization field is to flatten the spontaneous polarization profile and make the films more homogeneous, which is consistent with Ginzburg Landau theory. This fact shows that this model can be taken as an effective model to deal with the ferroelectric film and can be further extended to refer to quantum effect. The competition between quantum effect and depolarization field induces some interesting phenomena on ferroelectric thin films.

Three dimensional phase field study on the thickness effect of ferroelectric polymer thin film

The electromechanical behavior of poly(vinylidene fluoride-trifluoroethylene)[P(VDF -TrFE)]ferroelectric thin film was investigated using the three dimensional(3D) phase-field method. Various energetic contributions,including elastic,electrostatic,and domain wall energy were taken into account in the variational functional of the phase field model.Evolution of the microscopic domain structures of P(VDF-TrFE) polymer film was simulated.Effects of the in-plane residual stress,the film thickness and externally applied electric bias field on the electromechanical properties of the film were explored.The obtained numerical results showed that the macroscopic responses of the electric hysteresis loops are sensitive to the residual stress and electric bias field.It was also found that thickness has a great effect on the electric hysteresis loops and remanent polarization.

Effect of Annealing on Ferroelectric Properties of Bi_ (3.25)La_(0.75)Ti_3O_ (12) Thin Films Prepared by the Sol-gel Method

Bi3.25La0.75Ti3O12（BLT）thin films were prepared on Pt/Ti/SiO2/Si substrate by the sol-gel method.The effect of annealing on their structures and ferroelectric properties was investigated.The XRD patterns indicate that the BLT films annealed at different temperatures are randomly orientated and the single perovskite phase is obtained at 550℃.The remmant polarization increnses and the coercive field decreases with the annealing temperature increasing.The leakage current density of the BLT films annealed at 700℃ is about 5.8×10^-8A/cm^2 at the electrie field of 250kv/cm.

Ferroelectric properties of sol-gel derived Nd-doped SrBi_4Ti_4O_(15) thin films

Neodymium-doped strontium bismuth titanate （SrBi4-xNdxTi4O15） ferroelectric thin films were fabricated using the sol-gel method on Pt/Ti/SiO2/Si substrates. The influence of Nd content on the microstructure and ferroelectric properties of SrBi4-xNdxTi4O15 thin films were systematically studied. The results indicated that the SrBi3.88Nd0.12Ti4O15 （SBNT0.12） thin films had better ferroelectric properties, with a remanent polarization of （2Pr） of 34.3 μC/cm^2 and a coercive field （2Ec） of 220 kV/cm. This could be attributed to the fact that SBNT0.12 ferroelectric thin films consisted of more and larger ball-like grains, approximately 150-200 nm, with structure distortion, which greatly contributed to the improvement of the ferroelectric properties of the films. Furthermore, the film exhibited a good fatigue resistant property. The value of 2Pr after 10^10 switching cycles did not change significantly. The SrBi3.88Nd0.12Ti4O15 films were promising candidates for the application of FeRAMs.

ANALYTICAL SOLUTION FOR RESPONSE OF FERROELECTRIC THIN FILM TO INFRARED RADIATION

Micro-bolometer pixel is an essential element in the infrared focal plane array （IRFPA） of infrared detectors. Its response to infrared radiation is analyzed in this paper. The pixel structure is modeled as a composite laminate thin plate whose sides are measured with the thickness from 0.1-1 μm. Its middle ply is a ferroelectric thin film. Its top surface is covered with a gold or platinum infrared absorber, while the bottom surface is deposited with platinum or lanthanum-nickel. Meanwhile both surfaces are a pair of electrodes. The top surface receives infrared radiation pulses successively. For the very tiny micro bolometer pixel, it is assumed that the infrared radiation is uniformly distributed on the plate. Furthermore, as the ratio of the side length to the thickness of the plate is dramatically large, it is assumed that heat transfer only takes place across the thickness of the plate. The thermal-electric-mechanical coupling governing equations are solved in a form of Fourier series. Results of the displacement, temperature variation and electric output signals of the micro bolometer pixel structure under infrared radiation are obtained, analyzed and compared with experimental data.

Simulation of electric properties of MFIS capacitor with BNT ferroelectric thin film using Silvaco/Atlas

Metal-ferroelectric-insulator-silicon(MFIS) capacitors with Bi3.15Nd0.85Ti3O12(BNT) ferroelectric thin film were simulated using a commercial software Silvaco/Atlas,and the effects of applied voltage and insulator layer on capacitance-voltage(C-V) hysteresis loops and memory windows were investigated. For the MFIS capacitors with CeO2 insulator,with the increase of applied voltage from 2 V to 15 V,the C-V loops become wider and memory windows increase from 0.15 V to 1.27 V. When the thickness of CeO2 layer increases from 1 nm to 5 nm at the applied voltage of 5 V,the C-V loops become narrower and the memory windows decrease from 1.09 V to 0.36 V. For MFIS capacitors with different insulator layers(CeO2,HfO2,Y2O3,Si3N4 and SiO2),the high dielectric constants can make the C-V loops wider and improve the capacitor's memory window. The simulation results prove that Silvaco/Atlas is a powerful simulator for MFIS capacitor,and they are helpful to the fabrication of MFIS nonvolatile memory devices.

Effect of crystallization temperature on microstructure and ferroelectric property of Bi_(3.25)Eu_(0.75)Ti_3O_(12) thin films prepared by MOD method

Europium-substituted bismuth titanate (Bi3.25Eu0.75Ti3O12) thin films were deposited on the Pt/Ti/SiO2/Si(111) substrates by metal-organic decomposition (MOD) method using a repeated coating/drying cycle. Effect of crystallization temperature on microstructure of Bi3.25Eu0.75Ti3O12(BET) thin films was investigated by X-ray diffractometry(XRD), scanning electron microscopy (SEM) and Raman spectroscopy, and ferroelectric property was studied by Precision Workstation Ferroelectric Tester. The crystallinity of BET thin films is improved and the average grain size increases with the crystallization temperature from 600 to 750 ℃. Under 9 V applied voltage, the remnant polarization (2Pr) of BET thin films annealed at 700 ℃ is 50.7 μm/cm2, which is higher than that of the films annealed at 600, 650 and 750℃.

Effect of electromechanical the properties of epitaxial boundary conditions on ferroelectric thin films

The effects of internal stresses and depolarization fields on the properties of epitaxial ferroelectric perovskite thin films are discussed by employing the dynamic Ginzbur~Landau equation （DCLE）. The numerical solution for BaTiO3 film shows that internal stress and the depolarization field have the most effects on ferroelectric properties such as polarization, Curie temperature and susceptibility. With the increase of the thickness of the film, the polarization of epitaxia] ferroelectric thin film is enhanced rapidly under high internal compressively stress. With the thickness exceeding the critical thickness for dislocation formation, the polarization increases slowly and even weakens due to relaxed internal stresses and a weak electrical boundary condition. This indicates that the effects of mechanical and electrical boundary conditions both diminish for ferroelectric thick films. Consequently, our thermodynamic method is a full scale model that can predict the properties of ferroelectric perovskite films in a wide range of film thickness.

Remarkable Improvement of Ferroelectric Properties and Leakage Current in BiFeO_3 Thin Films by Nd Modification

Ferroelectric and leakage properties are important for ferroelectric applications. Pure and Nd-doped（x=0.05-0.20） BiFeO3 thin films were fabricated by sol-gel method on FTO substrates. The phase structure, surface morphology, leakage current, ferroelectric properties, and optical properties of BiFeO3-based thin films were investigated. The substitution of Nd^3＋ ions for the Bi^3＋ site converts the structure from rhombohedral to coexisting tetragonal and orthorhombic. Nd doping improves the crystallinity of BiFeO3 thin films. The leakage current of Nd-doped BiFeO3 decreases by two to three orders of magnitude compared with that of pure BiFeO3. Among the samples, 15% Nd-doped BiFeO3 exhibits the strongest ferroelectric polarization of 17.96 μC/cm^2. Furthermore, the absorption edges of Bi1-xNdxFeO3 thin films show a slight red-shift after Nd doping.

MINIMUM SIZE OF 180 DEGREE DOMAINS IN FERROELECTRIC THIN FILMS COVERED BY ELECTRODES

Ferroelectric domain switching under low voltage or short pulses is of interest for the development of high-density random access memory （FRAM） devices. Being necessarily very small in size, instability and back switching often occur when the external voltage is removed, which creates serious problems. In this investigation, a general approach to determine the minimum size of ferroelectric domain to avoid back switching was developed, and as an example, a 180° domain in a ferroelectric thin film covered by the upper and lower electrodes was considered in detail. We note that our approach is generally applicable to many other fields, including phase transformation, nucleation and expansion of dislocation loops in thin films, etc.

Effect of Annealing Temperature on the Ferroelectric Properties of BiFeO_3 Thin Films Prepared by Sol-gel Process

Sol-gel process was adopted to prepare BiFeO3 films.BiFeO3 films were deposited on LaNiO3 coated Si（100） substrates annealed at 500 and 550 ℃,respectively.The X-ray diffraction results reveal that BiFeO3 film has a rhombohedrally distorted perovskite structure with space group R3c.The film annealed at 500 ℃ has larger remnant polarization（Pr） of 35.3 μC/cm2.For the film annealed at 550 ℃,smaller remnant polarization of Pr=4.8 μC/cm^2 is observed for its low breakdown electric field.Lower leakage conduction is observed in the film annealed at 500 ℃ at low applied field.

Orientation of Bi_(3.2)La_(0.8)Ti_3O_(12) Ferroelectric Thin Films with Different Annealing Schedules

Fatigue-free Bi3.2La0.8Ti3O12 ferroelectric thin films were successfully prepared on p-Si （100） substrates using metalorganic solution deposition process. The orientation and formation of 5-layers thin films were studied under different processing conditions using XRD. Experimental results indicate that increase in annealing time at 700 ℃ after preannealing for 10 min at 400 ℃ can remarkably increase （200）-orientation of the films derived from the precursor solutions with two contents of citric acid. Meanwhile, high content of citric acid increases the film thickness and is conducive to the α-orientation of the films with the preannealing, and low concentration of the solution is conducive to the c-orientation of the films without the preannealing.