Yueli Zhang

Effect of annealing temperatures on the structure and leakage mechanisms of BiFeO3 thin films prepared by the sol–gel method

In this paper, we report the fabrication of polycrystalline BiFeO3 (BFO) thin films on Pt(111)/Ti/SiO2/Si(100) substrates by the sol–gel method. The effect of annealing temperature (Ta) on the structural, optical, morphological, ferroelectric and leakage properties of the films were analyzed by X-ray diffraction (XRD), Raman spectroscopy, Atomic force microscope (AFM) and ferroelectric measurements. XRD patterns and micro-Raman spectra demonstrated that all films had a single perovskite-type rhombohedral structure. A distortion in crystal lattice constants and a contraction in unit cell volume were observed with the increase of annealing temperatures. AFM images showed that dense and uniform grains were obtained. This reveals that the film is well crystallized. The data of ferroelectric test indicated that the double remanent polarization (2Pr) value of the thin film at Taxa0=xa0600xa0°C was 22xa0μC/cm2 and the leakage current density was 9.0xa0×xa010−8xa0A/cm2 at an applied electric field of 1.0xa0×xa0105xa0V/cm. The leakage mechanisms of thin films have been studied in order to identify the cause of high leakage currents such as Ohmic conduction, the space-charge-limited current, the Schottky emission, the Fowler–Nordheim tunneling and the Poole–Frenkel emission. In low electric field region (<1.0xa0×xa0105xa0V/cm), the conduction behavior was found to be dominated by Ohmic conduction for the thin film annealed at 500xa0°C. The Ohmic conduction and space-charge-limited current dominated the leakage behavior for the thin films at Taxa0=xa0550 and 600xa0°C. Fowler–Nordheim tunneling was responsible for the leakage behavior of the thin films at Taxa0=xa0550, 600xa0°C in high electric field region (>1.0xa0×xa0105xa0V/cm). The results demonstrated that the microstructure, surface morphology and ferroelectric properties of BFO thin films have a strong dependence on annealing temperatures.

Structural, luminescence and magnetic properties of Yb3+-Er3+ codoped Gd2O3 hierarchical architectures

In this paper, Gd(OH)3 microcrystals (MCs), with hierarchical dumbbell-like structures, were prepared via a one-step hydrothermal route using KOH as the alkaline source and ethylenediaminetetraacetic acid as the chelating reagent. XRD patterns and SEM images showed the typical hexagonal phase and the three-dimensional micro-dumbbell structure with an average length of 9 μm and diameter of 7 μm. The time dependent morphogenesis, the growth kinetic mechanism and the intrinsic electric field were exploited to demonstrate the formation mechanism of the Gd(OH)3 micro-dumbbell hierarchical architectures. Gd2O3 hierarchical MCs and their lanthanide-doped counterparts could also be obtained via heat treatment. Gd2O3:Yb3+/Er3+ hierarchical MCs exhibited relatively pure visible emission under near-infrared (980 nm) and violet (379.4 nm) excitation. Besides, NIR emission from Yb3+ 2F5/2 and Er3+ 4I13/2 was also found, which may be of particular interest for telecommunications applications. The possible energy transfer mechanisms were discussed. Moreover, Gd2O3 samples also have considerable magnetic mass susceptibility with a value of 1.321 × 10−4 emu g−1 Oe−1 by magnetic property studies. This work provides another path for preparing Gd2O3 hierarchical MCs.

Effects of Y doping on multiferroic properties of sol–gel deposited BiFeO3 thin films

Polycrystalline BiFeO3 (BFO), Bi0.99Y0.01FeO3 (BYF1), Bi0.97Y0.03FeO3 (BYF3), Bi0.95Y0.05FeO3 (BYF5) and Bi0.90Y0.10FeO3 (BYF10) thin films have been fabricated on Pt(111)/Ti/SiO2/Si(100) substrates by sol–gel method. The effects of Y doping on the structural, optical, morphological, ferroelectric and magnetic properties of BFO thin film were analyzed by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscope (SEM), ferroelectric and magnetic measurement system, respectively. XRD, Rietveld refinement and Raman analysis demonstrated that the phase of all films was a single perovskite-type rhombohedral structure. A structural transition from the main R3c to the single phase R-3xa0m was obtained with the BYF10 thin film. SEM showed that the Y doping was beneficial to the crystal growth of BFO films. The ferroelectric test indicated that the Y dopingxa0in BYF1 and BYF3 thin films wasxa0effective toxa0reduce the leakage current density of the BFO films. The leakage current density of BYF1 thin film was reduced by two orders of magnitude compared to that of the BFO film. The BYF10 thin film had a superior remanent polarization compared to that of other BFO film due to the distorted deformation of FeO6 octahedra. The magnetic test indicated that a substantially enhanced magnetization was observed in the BYF3 thin film.

Optical Properties of Lead Lanthanum Zirconate Titanate Amorphous Ferroelectric-Like Thin Films

Lead lanthanum zirconate titanate (PLZT) amorphous thin films, whose composition is 5/50/50, have been prepared by the sol–gel technique. The optical transmission property of the thin film was measured in the wavelength range of 200–820 nm. The ellipsometric spectra of the PLZT thin film on a glass substrate were measured in the wavelength range of 200–670 nm, and their optical constant (refractive index n and extinction coefficient k) spectra have been determined. The dispersion of the refractive index is interpreted in terms of a single electric oscillator at 9.64 eV. The direct band energy is found to be 3.67 eV.

Optical functions of Ca-modified PbTiO3 thin films determined by spectroscopic ellipsometry

Abstract Calcium-modified lead titanate thin films deposited on Si(100) substrate have been prepared by sol–gel processing. The structural properties of the thin films have been studied by X-ray diffraction. The ellipsometric spectra of the Pb1−xCaxTiO3 (x=0.0–0.4) films have been obtained in the spectral range of 2.2–5.0xa0eV, and their optical constants (the refractive index n and the extinction coefficient k) determined. The results show that after doping Ca, the refractive indexes of the Pb1−xCaxTiO3 films increase, the maximum value of refractive indexes and the absorption edges shift to lower energy.

Epitaxial growth and optical properties of Sr2−xCaxNaNb5O15 thin films by pulsed laser deposition

Abstract High-quality Sr2−xCaxNaNb5O 15 (SCNN) thin films have been epitaxially grown on MgO(0xa00xa01) substrates by pulsed laser deposition (PLD) technique for the first time. The thickness of the films is in the range of 600–800 nm. Their structural qualities and surface morphology were studied by X-ray diffraction and scanning electron microscopy. θ–2θ scans indicate that single crystalline SCNN layers with (0xa00xa01) orientation perpendicular to the substrate plane were obtained. φ-scans on the (2xa02xa01) plane confirm that the unit cell of SCNN films rotates in the plane of the film by ±17.8° with respect to the MgO substrate unit cell. Optical studies by optical transmittance measurements were carried out in the spectral range of 200–900 nm. In the analysis of the measured optical transmittance spectra, a single electronic oscillator model was adopted to represent the refractive index and the extinction coefficient of the SCNN films. Our results suggest that PLD is a promising technique in preparing high-quality epitaxial SCNN films for electro-optic device applications.

Photorefractive properties and phase-conjugation of doped KNSBN crystals

Abstract Doped KNSBN single crystals were prepared by the Czochralski technique. The two-wave coupling experiments were performed and the gain coefficients as functions of the crossing angle of two beams in various samples were obtained. The experimental result was compared with the theoretical analysis. In addition, the intensity-dependent gain coefficient of two-wave coupling was measured and analyzed. Some results of self-pumped phase-conjugate reflectivities of doped KNSBN samples are given. Using the 50-75ps pulses, we have successfully observed the transient two-wave coupling and the time-reverse of the phase-conjugation.

Ferroelectric Properties in Multiferroic BiFeO3 Thin Films Codoped with La and Co

BiFeO3, LO2a-doped BiFeO3, and (La, Co)-codoped BiFeO3 thin films were fabricated on Pt/Ti/SiO2/Si(111) substrates by chemical solution deposition method. The structural and electrical properties of pure, La-doped, and (La, Co)-codoped BiFeO3 thin films were studied. X-ray diffraction analysis showed that all the films were single perovskite structure. Electrical measurements indicated that the ferroelectric properties of BiFeO3 thin films were significantly improved by La and Co codoping. Well saturated hysteresis characteristic with large remanent polarization (larger than 57 μC/cm2) were observed in Bi0.9La0.1Co0.1Fe0.9O3 thin film.

Ellipsometric spectra and optical constants of PLZT thin films

Abstract Lead lanthanum zirconate titanate (PLZT) amorphous films with composition 9/65/35 were prepared by the sol-gel technique. The ellipsometric spectra of the PLZT films on Si substrates were measured in the wavelength range of 200–700nm. By analyzing the ellipsometric spectra, the optical constant (refractive index n and extinction coefficient k) spectra of the PLZT amorphous films were obtained. We also measured the ellipsometric spectra of PLZT ceramics with the same composition and compared the results of the amorphous films with those of the ceramics. In comparison with the ceramics, the absorption edge of the PLZT imorphous films was found to be at a shorter wavelength.

Effect of La Substitution on Structural, Electrical and Magnetic Properties in BF-BCZT Ceramics

Lead-free 0.725Bi1-xLaxFeO3-0.275Ba0.85Ca0.15Zr0.1Ti0.9O3 (BLF-BCZT, x = 0, 2.5, 5.0, 7.5 and 10.0 mol%) multiferroic ceramics were fabricated via a conventional solid-state reaction method. Effect of La3+ substitution on the structural, electrical and magnetic properties has been studied. XRD patterns show a pure perovskite structure. The SEM images show that the ceramics are dense and pore-free, and the average grain size decreases slightly as La3+ content increases. The Neel temperature decreases with the increasing La content from 428.0°C to 404.0°C, whereas the Curie temperature increases from 610.2°C to 620.4°C for BLF-BCZT ceramics with 5.0 mol% La. Ferroelectric properties are improved and optimized at x = 2.5 mol%: Pr = 1.48 μC/cm2, Pmax = 4.56 μC/cm2, Ec = 1120V/mm. Maximum of Mr is obtained for 7.5 mol% La doped samples, which is 0.50 emu/g, showing a ferromagnetic nature.