Congbing Tan

Improved ferroelectric properties of bismuth titanate films by Nd and Mn cosubstitution

Thin films of Nd and Mn cosubstituted bismuth titanate, i.e., Bi3.15Nd0.85(Ti3−xMnx)O12 (BNTM) (x=0, 0.005, 0.01, 0.03, 0.05, and 0.1), were fabricated on Pt∕Ti∕SiO2∕Si(100) substrates at 700°C by a chemical solution deposition technique. The structures of the films were analyzed using x-ray diffraction and Raman spectroscopy. These films possessed preferred (117) and (00l)-oriented polycrystalline structures. The ferroelectric properties of BNTM films were systematically investigated as a function of the Mn content. It is found that a low concentration substitution with manganese ions in Bi3.15Nd0.85Ti3O12 greatly enhances the remnant polarization (2Pr) and reduces the coercive field (2Ec) of the film. The 2Pr and 2Ec are 78μC∕cm2 and 205kV∕cm, respectively. No fatigue phenomenon is also observed for the BNTM film with x=0.01 up to 1.5×1010 switching cycles.

Ferroelectric and dielectric properties of Nd3+∕Zr4+ cosubstituted Bi4Ti3O12 thin films

Thin films of Nd3+∕Zr4+ cosubstituted Bi4Ti3O12 (BIT), i.e., Bi3.15Nd0.85Ti2.8Zr0.2O12 (BNTZ), were fabricated on Pt∕Ti∕SiO2∕Si(100) substrates by chemical solution deposition and annealed at different temperatures of 600, 650, 700, and 800°C. The effects of annealing temperature on the microstructure, leakage current, ferroelectric, and dielectric properties of the BNTZ films were investigated in detail. Significantly, compared with the Bi3.15Nd0.85Ti3O12 film, the BNTZ thin film has a lower coercive field (2Ec) and leakage current density and a slightly larger remnant polarization (2Pr). It shows that Nd3+∕Zr4+ cosubstitution in BIT film might be an effective way to improve ferroelectric properties of BIT.

Large femtosecond third-order optical nonlinearity of Bi3.15Nd0.85Ti3O12 ferroelectric thin films

Both the linear and nonlinear optical properties of Bi3.15Nd0.85Ti3O12 (BNT0.85) ferroelectric thin films deposited on quartz substrates were investigated. The fundamental optical constants were determined as a function of light wavelength by optical transmittance measurements. By performing single-beam Z-scan experiments with femtosecond laser pulses at a wavelength of 800 nm, the two-photon absorption (TPA) coefficient β and third-order nonlinear refraction index γ were measured to be 1.15 × 102 cm/GW and −8.15 × 10−3 cm2/GW, respectively. The large TPA is attributed to an indirect transition process via the intermediate energy levels and the large refractive nonlinearity is the result of the electronic polarization and ferroelectric polarization arisen from the femtosecond midinfrared radiation. The results indicate that the BNT0.85 thin film is a promising candidate for applications in nonlinear photonic devices.

Ferroelectric-gate thin-film transistors with Bi3.15Nd0.85Ti3O12 gate insulators on LaNiO3-buffered Si substrates

Ferroelectric-gate thin-film transistors (FGTs) with a stacked oxide structure of ZnO/Bi3.15Nd0.85Ti3O12 (BNT)/LaNiO3 (LNO) on Si substrates have been prepared and characterized. The FGT devices show good electrical properties, such as a large “on” current of 2.5 × 10−4 A and low threshold voltage of 1.1 V. These are mainly attributed to the coupling enhancement of the gate electric field to the channel layer due to a-axis preferential orientation of BNT ferroelectric-gate insulator thin films obtained by using the LNO buffer layer and the relatively good interface properties. The results suggest that ZnO/BNT/LNO/Si structures are well suited for thin-film transistors for future nonvolatile memory applications.

Nonlinear optical absorption tuning in Bi3.15Nd0.85Ti3O12 ferroelectric thin films by thickness

The tunability of nonlinear optical (NLO) absorption in Bi3.15Nd0.85Ti3O12 (BNT) ferroelectric thin films was investigated through the open aperture Z-scan method with femtosecond laser pulses at the wavelength of 800 nm. NLO absorption responses of the BNT films were observed to be highly sensitive to the film thickness. As the film thickness increases from 106.8 to 139.7 nm, the NLO absorption changes from saturable absorption (SA) to reverse saturable absorption (RSA). When the film thickness further increases to 312.9 nm, the RSA effect is enhanced. A band-gap-related competition between the ground-state excitation and the two-photon absorption is responsible for the absorption switching behavior. Such a tunable NLO absorption can widen the photonic application of the BNT thin films.

Characterization of domain distributions by second harmonic generation in ferroelectrics

Domain orientations and their volume ratios in ferroelectrics are recognized as a compelling topic recently for domain switching dynamics and domain stability in devices application. Here, an optimized second harmonic generation method has been explored for ferroelectric domain characterization. Combing a unique theoretical model with azimuth-polarization-dependent second harmonic generation response, the complex domain components and their distributions can be rigidly determined in ferroelectric thin films. Using the proposed model, the domain structures of rhombohedral BiFeO3 films with 71° and 109° domain wall, and, tetragonal BiFeO3, Pb(Zr0.2Ti0.8)O3, and BaTiO3 ferroelectric thin films are analyzed and the corresponding polarization variants are determined. This work could provide a powerful and all-optical method to track and evaluate the evolution of ferroelectric domains in the ferroelectric-based devices.Ferroelectrics: optical method characterizes domain structuresAn optimized optical method is developed to determine the ferroelectric domain structure and their distributions in several ferroelectric films. A team led by Xiangli Zhong and Yichun Zhou from Xiangtan University, and Jianmin Yuan from National University of Defense Technology in China, combined optimized second harmonic generation (SHG) measurements and theoretical analysis to determine the polarization components of domains and their distribution in rhobohedral BiFeO3, tetragonal BiFeO3, Pb(Zr0.2Ti0.8)O3 and BaTiO3 thin films. They first collect azimuth-polarization-dependent SHG signals with respect to both the polarization angle of the incident beam and the azimuth angle of sample relative to the beam. Then they fit the SHG signals with a theoretical model with good agreement. This work provides an effective and all-optical method to determine the domain structures and distributions for ferroelectric thin films.

Voltage pulse controlling multilevel data ferroelectric storage memory with a nonepitaxial ultrathin film

Multilevel data ferroelectric storage memory is a breakthrough for addressing low density in ferroelectric random access memories. However, the application of the multilevel data ferroelectric storage memory is limited by high cost and difficulty to prepare high quality epitaxial films. Herein, we create a multilevel data ferroelectric storage memory with a non-epitaxial ferroelectric ultrathin film to overcome these issues. Through controlling the polarization switching and oxygen vacancy migration with the voltage pulses, we demonstrated that voltage-controlled barrier heights yield a memristive behavior with resistance variations. Moreover, we achieved eight logic states, which are written and read easily. Our results suggest new opportunities for ferroelectrics as high density non-volatile memories.

Surface-Step-Terrace Tuned Second-Order Nonlinear Optical Coefficients of Epitaxial Ferroelectric BaTiO3 Films

Ferroelectric films epitaxially grown on vicinal substrates have been widely researched in recent decades for their unprecedented performance tuned by the surface-step-terrace (SST) induced strains of substrates. Here, BaTiO3 films are epitaxially grown on miscut (001) SrTiO3 substrates with different miscut angles of 0°, 0.2°, 1°, and 2°, and the impacts of SST-induced strains on the second-order nonlinear optical coefficients are discussed by fitting the measured second harmonic generation (SHG) signals from the films. The results show that different miscut angles of the substrates can bring about different SST strain states in the BTO films, which make different contributions on the second-order nonlinear optical coefficients. These results disclose that the second-order nonlinear optical coefficients can be effectively controlled by SST induced strains, which provides a new way to tune the optical properties of functional oxide thin films.

Deterministic, Reversible, and Nonvolatile Low-Voltage Writing of Magnetic Domains in Epitaxial BaTiO3/Fe3O4 Heterostructure

The ability to electrically write magnetic bits is highly desirable for future magnetic memories and spintronic devices, though fully deterministic, reversible, and nonvolatile switching of magnetic moments by electric field remains elusive despite extensive research. In this work, we develop a concept to electrically switch magnetization via polarization modulated oxygen vacancies, and we demonstrate the idea in a multiferroic epitaxial heterostructure of BaTiO3/Fe3O4 fabricated by pulsed laser deposition. The piezoelectricity and ferroelectricity of BaTiO3 have been confirmed by macro- and microscale measurements, for which Fe3O4 serves as the top electrode for switching the polarization. X-ray absorption spectroscopy and X-ray magnetic circular dichroism spectra indicate a mixture of Fe2+ and Fe3+ at O h sites and Fe3+ at T d sites in Fe3O4, while the room-temperature magnetic domains of Fe3O4 are revealed by microscopic magnetic force microscopy measurements. It is demonstrated that the magnetic domains of Fe3O4 can be switched by not only magnetic fields but also electric fields in a deterministic, reversible, and nonvolatile manner, wherein polarization reversal by electric field modulates the oxygen vacancy distribution in Fe3O4, and thus its magnetic state, making it attractive for electrically written magnetic memories.