BaTiO3 Thin Films Research Articles

Electron transport enhancement in perovskite solar cell via the polarized BaTiO3 thin film

Abstract

Skewed electronic band structure induced by electric polarization in ferroelectric BaTiO3

Skewed band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction (FTJs). Nonvolatile ferroelectric random access memory (FeRAM) and the artificial neural network device based on the FTJ system are rapidly developing. However, because the actual ferroelectric band structure has not been elucidated, precise designing of devices has to be advanced through appropriate heuristics. Here, we perform angle-resolved hard X-ray photoemission spectroscopy of ferroelectric BaTiO3 thin films for the direct observation of ferroelectric band skewing structure as the depth profiles of atomic orbitals. The depth-resolved electronic band structure consists of three depth regions: a potential slope along the electric polarization in the core, the surface and interface exhibiting slight changes. We also demonstrate that the direction of the energy shift is controlled by the polarization reversal. In the ferroelectric skewed band structure, we found that the difference in energy shifts of the atomic orbitals is correlated with the atomic configuration of the soft phonon mode reflecting the Born effective charges. These findings lead to a better understanding of the origin of electric polarization.

Preparation and characterization of a flexible ferroelectric tunnel junction

In this work, we propose a flexible ferroelectric tunnel junction (FTJ) with a nanometer-thick single-crystalline BaTiO3 barrier prepared by exfoliating and transferring epitaxial BaTiO3 thin films onto flexible poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) conductive electrodes using a water-soluble Sr3Al2O6 sacrificial layer. The transferred freestanding BaTiO3 films remain single crystalline and exhibit clear ferroelectric hysteresis, no matter being flat or bent. A combined piezoelectric force microscopy and conductive atomic force microscopy measurement reveals that the Pt/BaTiO3/PEDOT:PSS FTJ shows a clear polarization direction modulated tunneling resistance. By using x-ray photoelectron spectroscopy, the polarization direction-dependent electrostatic potential profile of this flexible FTJ has been reconstructed, consistent with the observed resistance modulation.

The enhanced ferroelectricity in Sr1-δTiO3/BaTiO3 superlattices with Sr deficiency

Here we report the tunable ferroelectricity in superlattices of (Sr1-δTiO3)n/(BaTiO3)8 with δ≈ 5% and the maximally enhanced ferroelectricity is found in the n= 6 superlattice. The room-temperature remnant polarization of n =6 is enhanced by 70% and 1120% compared to those of BaTiO3 and Sr1-δTiO3 thin films, respectively. The ferroelectric transition temperature of n =6 is 40 K higher than that of the BaTiO3 film, and is independent of the measurement frequency. The dependence of remnant field 2Pr on n coincides well with the n dependence of γ/α, which is ratio between out-of-plane and in-plane octahedral rotation angles. It thus suggests a strong correlation between the octahedral rotation pattern and the ferroelectric distortion. Our study demonstrates the importance of constructing superlattices to enhance the ferroelectric properties of the defect-engineered materials to be better than the pristine materials.

Role of ferroelectric polarization during growth of highly strained ferroelectric materials

In ferroelectric thin films and superlattices, the polarization is intricately linked to crystal structure. Here we show that it can also play an important role in the growth process, influencing growth rates, relaxation mechanisms, electrical properties and domain structures. This is studied by focusing on the properties of BaTiO3 thin films grown on very thin layers of PbTiO3 using x-ray diffraction, piezoforce microscopy, electrical characterization and rapid in-situ x-ray diffraction reciprocal space maps during the growth using synchrotron radiation. Using a simple model we show that the changes in growth are driven by the energy cost for the top material to sustain the polarization imposed upon it by the underlying layer, and these effects may be expected to occur in other multilayer systems where polarization is present during growth. This motivates the concept of polarization engineering as a complementary approach to strain engineering.

Polarization behavior of` lead-free 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 thin films with enhanced ferroelectric properties

(Bi0.5Na0.5)TiO3 based ferroelectric lead-free thin films have great potential for modern micro-devices. However, the multicomponent feature and volatile nature of Bi/Na makes the achievement of high quality films challenging. In this work, the morphotropic phase boundary composition, 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 thin films were successfully prepared by CSD method. Dense films with low dielectric loss and low leakage current density were obtained. A well-defined polarization hysteresis loop with a high remnant polarization was observed in the thin films. Moreover, the polarization behavior of the film at original state, under electric field and upon heating was investigated by PFM. A self-polarization and asymmetric domain switching behavior were observed. High temperature induced depolarization and the self-polarization recovered upon cooling. The thin films with good quality show a promising potential for the application in electrical devices, and the in-depth investigation of the polarization behavior improves the understanding of ferroelectric and piezoelectric properties of thin films.

Mechanically induced ferroelectric switching in BaTiO3 thin films

The ability to reverse or switch the polarization of a ferroelectric thin film through a mechanical force under an atomic force microscopy (AFM) tip offers the exciting possibility of a voltage-free control of ferroelectricity. One of the important metrics for characterizing such a switching process is the critical force Fc required to reverse a polarization. However, the experimentally measured values of Fc display a large uncertainty and vary significantly even for the same ferroelectric film. Here, using BaTiO3 thin films as a model system, we systematically evaluate Fc using a combination of AFM-based experiments and phase-field simulations. In particular, we study the influence of the AFM tip radius, misfit strain, and film thickness on Fc as well as the interplay between the flexoelectric and piezoelectric effects. This work provides a deeper understanding on the mechanism and control of mechanically induced ferroelectric switching and thus guidance for exploring potential ferroelectric-based nanodevices based on mechanical switching.

Polarization Rotation by Barium Titanate (BaTiO3) Thin Film with an External Electric Field

Barium Titanate (BaTiO3) thin films are suitable for many applications especially photonic due to its large electro-optic (EO) coefficient where the EO effect causes a change of optical refractive index in a film due presence of an electric field. The first step towards the development of thin film based EO modulators is to study its EO properties. The thin film BaTiO3 was prepared by a sol gel process and annealed at 900°C and then subjected to EO interrogation via the Sénarmont technique. It shows that the technique affected the electro-optic properties of the thin film and the electro-optic behaviour. The Sénarmont technique also utilized the EO measurement of BaTiO3 thin films from negative 20V up to positive 20V by connecting the electrical and optical domain with different angles of polarizer that are from 0° up to 180°. The results show that the power measurement is much higher between 30° to 50° with positive 20V. It is found that electric field disrupts the EO behaviour of the film.

Control of Oxygen Vacancies in TiO6 Octahedra of Amorphous BaTiO3 Thin Films with Tunable Built‐In Electric Field in a‐BaTiO3/p‐Si Heterojunction for Metal–Oxide–Semiconductor Applications

Amorphous BaTiO3 (a‐BTO) thin films are deposited on p‐Si(111) substrates by radio frequency (RF) magnetron sputtering without extra substrate heating. The microstructure of a‐BTO thin films is modified by postannealing at low temperatures (260–600 °C) in air. The results show that distorted TiO6 octahedra exist in a‐BTO thin films, and the degree of distortion decreases when the annealing temperature rises. The concentration of O vacancies in the TiO6 of the thin films remains high when postannealed between 370 and 460 °C, and a built‐in electric field across the a‐BTO/p‐Si heterojunction is established in this annealing temperature range. This built‐in electric field is regulated by controlling the concentration of O vacancies from the TiO6. The tunable C–V characteristic of the Au/a‐BTO/p‐Si metal–oxide–semiconductor (MOS) capacitor is obtained by modulating the built‐in electric field at the heterojunction, and the flat band voltage of the MOS capacitor shifts from about −3 V to at least −13 V when the postannealing temperature increases from 260 to 420 °C. This kind of tunable built‐in electric field at the a‐BTO/p‐Si heterojunction, controlled by modifying the microstructure of amorphous oxide thin films, has never been observed before, which will be useful for MOS applications.

Crystallization behavior of amorphous BaTiO3 thin films

The crystallization behavior of amorphous barium titanate (BaTiO3) thin films was studied as a function of annealing temperature from 500 to 1000 °C. Quantitative phase analysis by grazing incidence X-ray diffractometry revealed that the metastable hexagonal phase preferentially nucleates at lower temperatures with a transition to stable cubic nuclei at higher temperatures. The predominance of the metastable hexagonal-phase nucleation at lower temperatures suggests that it has the lowest nucleation barrier, in accordance with the Ostwald’s step rule. To help induce the nucleation of the cubic phase at lower temperatures, we explored the effects of electric fields on the crystallization behavior and found that direct-current fields moderately enhanced the cubic phase fraction in the 500–700 °C temperature range. Although the nucleation barrier of the cubic phase in the presence of an electric should be lowered more significantly with respect to that of the hexagonal phase because of the former’s higher relative permittivity, the effect arising from the change in volume free energy should only become significant at electric field strengths in the range of MV cm−1, an order of magnitude higher than the experimentally accessible fields in the present study. This suggests alternative, perhaps interface-mediated, mechanisms by which the electric field modifies the nucleation behavior of BaTiO3.

Domain evolution in bended freestanding BaTiO3 ultrathin films: A phase-field simulation

Perovskite ferroelectric oxides are usually considered to be brittle materials; however, recent work [Dong et al., Science 366, 475 (2019)] demonstrated the super-elasticity in the freestanding BaTiO3 thin films. This property may originate from the ferroelectric domain evolution during the bending, which is difficult to observe in experiments. Therefore, understanding the relation among the bending deformation, thickness of the films, and domain dynamics is critical for their potential applications in flexible ferroelectric devices. Here, we reported the dynamics of ferroelectric polarization in the freestanding BaTiO3 ultrathin films in the presence of large bending deformation up to 40° using phase-field simulation. The ferroelectric domain evolution reveals the transition from the flux-closure to a/c domains with “vortex-like” structures, which is caused by the increase in out-of-plane ferroelectric polarization. Additionally, by varying the film thickness in the identical bending situation, we found that the a/c phase with a vortex-like structure emerges only as the film thickness reached 12 nm or higher. The results from our investigations provide instructive information for the microstructure evolution of bending ferroelectric perovskite oxide films, which could serve as a guide for the future application of ferroelectric films on flexible electronic devices.

Ferroelectric and piezoelectric response in (100)-oriented Mn-doped Bi0.5Na0.5TiO3–BaTiO3 thin films

In this work, high-quality Mn-doped 0.935Bi0.5Na0.5TiO3–0.065BaTiO3 (Mn-BNBT) ferroelectric film was prepared on the SrRuO3-buffered (100)-oriented SrTiO3 single-crystal substrate using pulsed laser deposition method. The phase structure, morphology, domain and electrical properties were studied. Large remnant polarization Pr up to 35 μC/cm2, moderate dielectric constant ɛ33 /ɛ0 of 800 and low dielectric loss tanδ of 0.037 at 1 kHz was obtained at room temperature along with excellent local piezoelectric response. The temperature dependence of the ferroelectric hysteresis loop measurements indicates the ferroelectric long-range order can be sustain as high as 150 °C. The excellent global properties make the present Mn-BNBT thin film quite potential in environmental-friendly piezoelectric integrated device applications.

Effects of Mn doping on BaTiO3 thin films grown on highly oriented pyrolytic graphite substrates

We report multiferroelectric properties of Mn-doped BaTiO3 (MBTO) thin films on highly oriented pyrolytic graphite (HOPG) substrates. The MBTO thin films were grown on the HOPG substrate by pulse laser deposition. For comparison purpose, undoped BaTiO3 (BTO) thin films were also prepared under same experimental conditions. The BTO and MBTO thin films were polycrystalline, indicating that the MBTO thin film has better crystallinity than the BTO thin film. The leakage current of the MBTO thin film was reduced due to the Mn doping substitution. In addition, the MBTO thin film exhibited better than the BTO thin film in ferroelectric and magnetic behaviors. We suggest that the Mn doping bring about the improvements of ferroelectric and ferromagnetic properties of the BTO thin films. Based on atomic force microscopy (AFM) and conducting AFM (CAFM) studies, the grain size of MBTO thin film was much larger than that of BTO thin film.

BaTiO3 Nanosheets and Caps Grown on TiO2 Nanorod Arrays as Thin-Film Catalysts for Piezocatalytic Applications.

Powder-form piezocatalysts suffer from poor recyclability and pose a potential threat of creating serious secondary pollution, which restrict their practical applications. Thin-film piezocatalysts, which not only exhibit good recyclability but also fully contact with solution, are believed to be one of the solutions to address these problems. In this work, the nanostructured BaTiO3 (BTO) thin films were fabricated by a facile hydrothermal method for their potential applications in piezocatalysis. The vertically standing BTO nanosheets grown on the top of TiO2 nanorod arrays exhibited superior piezocatalytic performance as well as piezo-electrochemical property. Given the different strain states between thin-film piezocatalyst and powder-form piezocatalyst, both the impact force of water and isostatic pressure are taken into consideration in finite element method (FEM) simulation. The FEM simulation shows that a stronger piezoelectric filed can be built in BTO nanosheets because of their easier deformation, and thus can lead to a higher piezocatalytic degradation efficiency. Our work presented here is expected to provide a potential route for the nanoengineering of thin-film piezocatalysts and clarify the catalytic mechanism for substrate-fixed piezocatalysts.

Surface Stress and Lattice Dynamics in Oxide Ultrathin Films

The lattice misfit between the substrate and an epitaxial film leads in general to static forces, which define the interface stress, and dynamic responses that modify the thin‐film lattice dynamics. Although these are both fundamental concepts that are important for film growth and thin‐film properties, they have not been investigated in a combined way so far. Therefore, herein, surface stress experiments in combination with surface phonon studies for three different, cubic oxide ultrathin film systems are reviewed. Within the class of binary oxides, NiO(001) grown on Ag(001) is chosen, which exhibits a −2.2% lattice mismatch, and BaO(001) on Pt(001), a system with a negligible lattice mismatch. For the ternary oxides, perovskite thin films of BaTiO3 grown epitaxially on Pt(001) with a lattice mismatch of −2.3% are focused upon. The surface stress experiments are conducted with an optical two‐beam curvature technique under in situ growth conditions. Surface and thin‐film phonons are determined by high‐resolution electron energy loss spectroscopy. Surface stress and lattice dynamics are discussed in the range from the oxide monolayer to thin films of about 20 unit cell in thickness.

The effects of oxygen pressure on the structural, magnetic, and ferroelectric properties of 0.55BiFe0.95Ga0.03Mn0.02O3–0.45BaTiO3 thin films

0.55BiFe0.95Ga0.03Mn0.02O3–0.45BaTiO3 (BFGMO–BTO) thin films have been epitaxially grown on (0 0 1)-oriented Nb-doped SrTiO3 (NSTO) single-crystal substrates by the pulsed laser deposition. X-ray diffraction, selected area electron diffraction, magnetic hysteresis loop, and piezoresponse force microscopy results show that the structural and magnetic properties of the BFGMO–BTO epitaxial films can be progressively tuned by the oxygen pressure (PO2) during film deposition. The films prepared atPO2 = 5 Pa phase separated into BFGMO and BTO phases, however, with increasing PO2 to 20 Pa the films evolve from two phases to single phase with the best crystalline quality, largest saturation magnetization, lowest coercive field and relatively easy-switchable ferroelectric domains.

Lead-free relaxor thin films with huge energy density and low loss for high temperature applications

We report record energy storage density (>80 J cm−3) in Pb-free relaxor ferroelectrics based on Mn-doped BiFeO3–BaTiO3 thin films. Rapid interval deposition was used to impose layer-by-layer growth improving crystallinity and lowering unwanted defects concentration. The growth and Mn doping produced an order of magnitude lower leakage, with strongly reduced dielectric loss (from room temperature to >300 °C, and 100 Hz to 1 MHz), e.g. by a factor of 5 at 225 °C and 25 kHz. At room temperature (RT), the dielectric breakdown strength increased by a factor of 1.5 to >3000 kV cm−1 while the dielectric constant remained flat, at ~1000 from RT to 350 °C. The films perform better than competing materials (e.g. PZT and SrTiO3-based) while being Pb-free and while operating up to 350 °C, which SrTiO3-based systems do not. Our work gives considerable promise for high energy and power density capacitors for harsh environments.

Temperature-independent giant dielectric response in transitional BaTiO3 thin films

Ferroelectric materials exhibit the largest dielectric permittivities and piezoelectric responses in nature, making them invaluable in applications from supercapacitors or sensors to actuators or electromechanical transducers. The origin of this behavior is their proximity to phase transitions. However, the largest possible responses are most often not utilized due to the impracticality of using temperature as a control parameter and to operate at phase transitions. This has motivated the design of solid solutions with morphotropic phase boundaries between different polar phases that are tuned by composition and that are weakly dependent on temperature. Thus far, the best piezoelectrics have been achieved in materials with intermediate (bridging or adaptive) phases. But so far, complex chemistry or an intricate microstructure has been required to achieve temperature-independent phase-transition boundaries. Here, we report such a temperature-independent bridging state in thin films of chemically simple BaTiO3. A coexistence among tetragonal, orthorhombic, and their bridging low-symmetry phases are shown to induce continuous vertical polarization rotation, which recreates a smear in-transition state and leads to a giant temperature-independent dielectric response. The current material contains a ferroelectric state that is distinct from those at morphotropic phase boundaries and cannot be considered as ferroelectric crystals. We believe that other materials can be engineered in a similar way to contain a ferroelectric state with gradual change of structure, forming a class of transitional ferroelectrics. Similar mechanisms could be utilized in other materials to design low-power ferroelectrics, piezoelectrics, dielectrics, or shape-memory alloys, as well as efficient electro- and magnetocalorics.

Effects of substrate-controlled-orientation on the electrical performance of sputtered BaTiO3 thin films

In this work, the relationships between bottom electrode/substrate configuration, crystalline microstructure, and electrical performances of BaTiO3 (BTO) thin films were investigated. The films were fabricated via RF magnetron sputtering on (Sr0.5La0.5)CoO3 (LSCO) buffered (110)-, (111)-SrTiO3 (STO) and SrRuO3 (SRO) buffered (110)-, (111)-MgO (MGO) substrates. The x-ray diffractometer results show that the LSCO/STO substrate resulted in films with high-quality epitaxial orientation, whereas the SRO/MGO substrate resulted in films having a strong (110) texture. The electrode/substrate configurations were designed to control the crystalline microstructure, which in turn affects the electrical performances of the films. The electrical performances were studied by employing a metal/ferroelectric/metal model. The J-V characteristics show obvious asymmetry with bias, which is mainly due to the varying transport state of oxygen vacancies. BTO films grown on LSCO/STO substrates exhibit large dielectric frequency dispersion, while those grown on SRO/MGO substrates display a nearly frequency independent response. All electrical parameters of these films were strongly affected by the polarization-tilting angle and the preferred orientation degree. Epitaxial and textured (110)-oriented films show higher permittivity but lower loss tangent, free-carrier concentration, built-in voltage, and leakage current density as compared to the epitaxial (111)-oriented film.

Comparison of Epitaxial and Textured Ferroelectric BaTiO<sub>3</sub> Thin Films

The properties of BaTiO3 (BTO) thin films deposited on different substrates by RF magnetron sputtering were investigated. Two representative substrates were selected and different heterostructures were studied. 1) SrTiO3 (STO) single crystals as a bulk oxide reference material, and 2) silicon as a semiconductor. SrRuO3 (SRO) and Pt bottom electrodes were deposited on the silicon substrate. The BTO structural characterizations show that all the films have (001) crystallographic orientation. We have compared the electrical properties of the different samples: the same dielectric constant and polarization values were obtained independently of the nature of the substrate.