Barium Titanate Thin Films Research Articles

Highly efficient broadband second harmonic generation using polydomain epitaxial barium titanate thin film waveguides

Broad band second harmonic generation (SHG) over the spectral range of 530–700nm is reported using an epitaxial barium titanate (BaTiO3) thin film waveguide. The epitaxial layers are multidomain consisting of both a and c domain variants. The SHG conversion efficiency is nearly independent of wavelength over the visible spectra range. An enhanced broadband conversion efficiency was observed that is attributed to random quasi-phase matching in the polydomain epitaxial layers. In contrast to theory of SHG for disordered nonlinear media, the observed SHG shows a quadratic dependence on pump power.

Structural and Magnetic Properties of Nanogranular BaTiO3-CoFe2O4 Thin Films Deposited by Laser Ablation on Si/Pt Substrates

Thin film nanocomposites of cobalt ferrite (CoFe2O4) dispersed in barium titanate (BaTiO3) matrix, have been deposited with different cobalt ferrite concentrations (from 20% to 70% CoFe2O4), as well as pure barium titanate and cobalt ferrite thin films (end members). The films were prepared by pulsed laser ablation on platinum covered Si(001) substrates. The films structure was studied by X-ray diffraction and their surface was examined by scanning electron microscopy (SEM). The magnetic properties were measured in a SQUID magnetometer. The results show that the deposited films are polycrystalline with a slight (111) barium titanate phase orientation and (311) CoFe2O4 phase orientation. The grain sizes measured from the X-ray diffraction peak widths, for both phases, are in the range 40nm to 100nm. However, as the concentration of the cobalt ferrite increases, the grain size of the BaTiO3 phase decreases, from 100nm to 30nm, up to 40% CoFe2O4 concentration beyond which the BaTiO3 grain size has an approximately constant value near 30nm. On the other hand the cobalt ferrite grain size does not show a clear trend with increasing cobalt ferrite concentration, fluctuating in the range 20nm to 30nm. The magnetic measurements show an increase of the magnetic moment from the low concentration region where the magnetic grains are more isolated and their magnetic interaction is small, towards the bulk value at higher CoFe2O4 concentrations. Also, a strong reduction of the magnetization with increasing temperature was observed, due to the corresponding decrease of the magnetocristalline anisotropy of the cobalt ferrite.

Phase thickness approach for determination of thin film refractive index dispersion from transmittance spectra

A novel approach for determination of refractive index dispersion n (λ) and thickness d of thin films of negligible absorption and weak dispersion is proposed. The calculation procedure is based on determination of the phase thickness of the film in the spectral region of measured transmittance data. All points of measured spectra are included in the calculations. Barium titanate and titanium oxide thin films are investigated and their n (λ) and d are calculated. The approach is validated using Swanepoel's method and it is found to be applicable for relatively thinner films when measured transmittance spectra have one minimum and one maximum only.

Strain effect on coercive field of epitaxial barium titanate thin films

Strain is generally known to increase the coercive field of a ferroelectric thin film as compared to a stress-free single crystal or a strain-relaxed film. We studied the coercive fields and remanent polarizations of (001)-oriented epitaxial barium titanate thin films using the phase-field approach. It is demonstrated, while the remanent polarization decreases as in-plane strain changes from being compressive to tensile, the variation of coercive field with strain is complicated. We noted more than two times drop in coercive field with a reduction of compressive strain of only ∼0.05%, which we attribute to the existence of multiple ferroelectric phases.

Determination of thin film refractive index and thickness by means of film phase thickness

Abstract A new approach for determination of refractive index dispersion n(λ) (the real part of the complex refractive index) and thickness d of thin films of negligible absorption and weak dispersion is proposed. The calculation procedure is based on determination of the phase thickness of the film in the spectral region of measured transmittance data. All points of measured spectra are included in the calculations. Barium titanate thin films are investigated in the spectral region 0.38–0.78 μm and their n(λ) and d are calculated. The approach is validated using Swanepoel’s method and it is found to be applicable for relatively thin films when measured transmittance spectra have one minimum and one maximum only.

Enhanced dielectric tunability in barium titanate thin films with boron additions

Boron incorporation in 650 nm thick polycrystalline BaTiO3 films results in grain growth and dramatic dielectric response improvements. For levels between 0% and 3%, average grain size is increased from 150 to 200 nm and the maximum permittivity values are nearly doubled, with tunabilities increased to greater than 90% while maintaining high field loss tangents below 0.03. These results demonstrate a pathway for microstructure engineering and property optimization in refractory electroceramic oxide thin films without increasing the thermal budget.

Cascaded Bragg reflectors for a barium titanate thin film electro-optic modulator

An electro-optic modulator integrated with a Bragg reflector structure utilizing a barium titanate thin film platform is proposed and its optical properties are simulated. An electro-optic modulator with a drive voltage-length product of 27 mV cm is achievable with a cascaded Bragg reflector structure. Phase matching conditions can be obtained using a simple Bragg reflector structure, and a > 100 GHz bandwidth is predicted for a millimetre-long electro-optic modulator device.

Structural evolution of nanostructured barium titanate thin film sol–gel derived

Ferroelectric nanostructured barium titanate (BT) thin films derived by a modified sol–gel process were formed by a deep coating method on Si (100) substrate. In this work was investigated the influence of different types and amounts of water (free water directly added in process and crystalline water from barium precursors) and of different barium precursors (hydroxide and acetate) on the gel-structure evolution and the barium titanate thin film crystallization. The IR-spectroscopy and X-ray diffraction analysis were used for sample characterization. Experimental results show that crystalline water from barium hydroxide octahydrate stabilizes the cubic phase of barium titanate. Barium titanate crystallizes in the tetragonal form in thin films when sols were prepared with free water and Ba-acetate or Ba-hydroxide monohydrate as precursors. The type of Ba-precursor used in preparation of sols effects the rate of crystallization of the BT in thin films, making it slower in the case when as precursor was used Ba-acetate than Ba-hydroxides. The prepared BT-thin films demonstrated good adhesion towards substrate and were smooth and uniform.

Structural and optical properties of annealed W-dopedBaTiO3 thin films prepared by pulsed laser deposition

Tungsten-doped barium titanate thin films have been prepared on glass and silicon substratesby laser ablation. The films were compositionally, structurally and optically characterized.Rutherford backscattering spectroscopy simulation gave the composition to beBa0.85W0.05Ti0.94O3.2 with acationic ratio (Ba+W)/Ti of 0.95. All the films without annealing were amorphous. The calculated lattice constantsthrough the diffraction patterns on the silicon substrate indicated the high probability ofthe film having a cubic symmetry. Atomic force microscopy results showed the film surfaceto be smooth with a maximum roughness of 5 nm, varied grain size distributionand average grain size of 50 nm. Through the Swanepoel envelope method, therefractive index dispersion was obtained. Maximum and minimum refractive indices of2.6 and 2.32 at 475 and 812 nm, respectively, were calculated. Analysis of thedependence of the refractive index and the dielectric constant on wavelength led to thedetermination of the high-frequency dielectric constant and the carrier density effective mass ratio(Nc/m*). The single-oscillator model proved to be adequate for describing the dispersion behaviour.Through this model, the dispersion parameter, average oscillator parameter and strengthwere determined. The optical band gap obtained on annealed sample was 3.8 eV byassuming a direct transition.

Experimental evidence for the role of electrodes and oxygen vacancies in voltage nonlinearities observed in high-k metal-insulator-metal capacitors

This work reports on voltage nonlinearities in metal-insulator-metal (MIM) capacitors using amorphous barium titanate (a-BaTiO3) thin films. It is experimentally demonstrated that voltage nonlinearity is related to the formation of a double layer at electrodes (electrode polarization mechanism). The magnitude of nonlinearities is shown to be controlled by the nature of the metal contacts (Al, Cu, Au, and Ag), as well as by the presence of oxygen during film deposition. It is thought that oxygen vacancies are the defects responsible for the nonlinear character of high-k oxide-based MIM capacitors.

Electrical and optical properties of chemical solution deposited barium hafnate titanate thin films

We have investigated the electrical and optical properties of Ba(Hf x Ti 1 − x )O 3 ( x = 0, 0.1, 0.2, 0.3, 0.4) (BHT) thin films deposited on platinized silicon and fused quartz substrates. Analyses of the X-ray diffraction patterns reveal that with the increase in Hf contents there is a systematic increase of the lattice constants of BHT films. Irrespective of the measurement frequencies the dielectric constants was found to be systematically decreased, whereas their frequency dispersion was found to be reduced with increasing Hf contents. The leakage current data measured using a metal-insulator–metal configuration reveal that the Schottky emission is the dominant leakage current mechanism in these films. BHT films, deposited on transparent fused quartz substrates, were also characterized in terms of their optical properties. For this purpose the transmittance of the undoped as well as Hf doped barium titanate thin films was measured as a function of wavelength in the range of 290 nm to 800 nm. The transmission spectra were analysed to estimate the wavelength dependence of the refractive indices/extinction coefficients as well as the variation of optical band gap of these films. With the increase of Hf contents, a systematic increase of the band gap [from 3.65 eV (undoped film) to 4.15 eV (40 at.% Hf doped barium titanate film)] was observed. The reduction of the leakage current with increasing hafnium substitution is discussed on the basis of an increasing Schottky barrier height and due to a simultaneous increase in the band gap of the material.

Defect chemistry of nano-grained barium titanate films

Polycrystalline barium titanate thin films have been prepared with 100 ppm of B-site acceptor dopants Ca, Mg, and Mn via chemical solution deposition on base metal substrates. The films are fired in low pO2 atmospheres at 900 °C to prevent substrate oxidation. All dopant species produce low loss, space-charge free material without secondary reoxidation anneals. We note that the dopant concentrations required to compensate for oxygen non-stoichiometry are substantially greater than expected by equilibrium thermodynamic calculations. This observation is rationalized in the context of a composite oxygen defect model with differing reduction enthalpies for grain interiors and surfaces.

Nonlinear photonic crystal waveguide structures based on barium titanate thin films and their optical properties

Nonlinear photonic crystal waveguide structures were fabricated from barium titanate thin films using nanolithography. A cascaded Bragg reflector using a strip waveguide was designed and analyzed. Both simulation and experimental results show that there is sufficient refractive index contrast to form a stop band by only etching through the Si3N4 strip layer. The band gap of the Bragg reflector can be engineered through control of the Bragg spacing, thickness, and etching depth of the strip layer. The transmission spectrum of the Bragg reflector waveguide was measured over the spectral range of 1500–1580nm. A 27nm wide stop band was obtained for a millimeter long sample. The nonlinear photonic crystal waveguides are potentially suitable as tunable filters, optical switches, and ultrawide bandwidth modulators.

The preparatory optimal conditions of barium titanate thin film from a hydrothermal method at low temperature

The preparation of barium titanate thin films on Ti substrates at low temperature using the hydrothermal process was studied. In addition, the interface relationship between film and substrate was investigated, revealing that the Ba2+ ion diffused into the Ti(OH)4 gel formed by Ti in a strong alkaline solution, resulting in a homogeneous film. A 3-level orthogonal array design was utilized to optimize the experimental conditions. The effects of preparatory conditions such as the barium-concentration, NaOH concentration, reaction time and substrate surface treatment were systematically studied. Results indicate that the effects of the NaOH concentration and substrate surface treatment are the significant variables influencing the relative crystal intensity (intensity of (110) BaTiO3 peaks in XRD spectrum/intensity of (001) Ti peak in XRD spectrum) of the films obtained. Polycrystalline, BaTiO3-films were obtained, with improved crystallinity and a dielectric constant of about 915 at 1 MHz.

Barium titanate thin films plasma etch rate as a function of the applied RF power and Ar/CF4 mixture gas mixing ratio

AbstractBaTiO3 (BT) nanocrystalline thin films were plasma etched in the course of several experiments which varied in RF power as well as CF4/(CF4 + Ar) gas‐mixing ratio. The maximum etch rate of approximately 30 nm/min was observed for the maximum power (300 W) and pure Ar plasma atmosphere, which indicates that the process is controlled by the physical mechanism of Ar ion bombardment, while the increasing content of chemically active plasma component (CF4) in the gas mixture slows down barium titanate etch rate. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dielectric and optical properties of BaTiO3 thin films prepared by low-temperature process

Barium titanate (BaTiO3) thin films have been prepared by low temperature processing on Pt/Ti/SiO2/Si substrates using sol-gel-hydrothermal (SGHT) technique, which combined the conventional sol-gel process and hydrothermal method. X-ray diffraction analysis showed that the barium titanate thin films are polycrystalline. As-reacted barium titanate films grown on Pt(111)/Ti/SiO2/Si(100) substrates had a dielectric constant (e) and loss tangent (tanδ) of 80 and 0.05 at 1 MHz, respectively. The optical constants including refractive index n, extinction coefficient k, and absorption coefficient α of the barium titanate thin films in the wavelength range of 2.5–12.6 μm were obtained by infrared spectroscopic ellipsometry.

Simulation and Fabrication of Two Dimensional Nonlinear Photonic Crystals using Barium Titanate Thin Films

AbstractPhotonic crystal (PhC) can potentially result in enhanced optical properties around critical points in the photonic band gap. Of interest here are enhanced nonlinear optical effects. In this study, one and two dimensional nonlinear PhC were designed and fabricated using barium titanium oxide (BTO) thin films as the active medium. Nonlinear PhC made with barium titanate thin films potentially provide integrated devices with the advantages of wide tunability and high stability. Films 500 nm thick deposited on MgO substrates were utilized. Two dimensional PhC structures were defined by focused ion beams (FIB). Before patterning, a thin metal layer was deposited on the barium titanate layers in order to improve the conductivity of the samples. After writing the patterns, cylindrical air holes were generated in the thin film layers. The PhC lattice constant and the hole radius were selected in sub-micron region in order to satisfy the requirement of wave resonance. The PhCs with sub-micron features were characterized by the atomic force microscopy (AFM), scanning electron microscopy (SEM), and near field optical microscopy (NSOM). The transmission spectra of the PhC waveguides were calculated with a continuous wide band source that covered 1 to 2 micron wavelength. Simulations of the transmission characteristics were performed using the two dimensional finite difference time domain method (FDTD).

Growth and optical property characterization of textured barium titanate thin films for photonic applications

We have investigated the growth of barium titanate thin films on bulk crystalline and amorphous substrates utilizing biaxially oriented template layers. Ion beam-assisted deposition was used to grow thin, biaxially textured, magnesium oxide template layers on amorphous and silicon substrates. Growth of highly oriented barium titanate films on these template layers was achieved by molecular beam epitaxy using a layer-by-layer growth process. Barium titanate thin films were grown in molecular oxygen and in the presence of oxygen radicals produced by a 300 W radio frequency plasma. We used X-ray and in situ reflection high-energy electron diffraction (RHEED) to analyze the structural properties and show the predominantly c-oriented grains in the films. Variable angle spectroscopic ellipsometry was used to analyze and compare the optical properties of the thin films grown with and without oxygen plasma. We have shown that optical quality barium titanate thin films, which show bulk crystal-like properties, can be grown on any substrate through the use of biaxially oriented magnesium oxide template layers.

Structure and dielectric properties of barium titanate thin films grown by sol-gel-hydrothermal process

Barium titanate (BT) thin films have been prepared on Pt∕Ti∕SiO2∕Si substrates through the use of a low temperature process. This technique combines the conventional sol-gel process and hydrothermal methods. X-ray diffraction analysis showed that the BT thin films had a pure perovskite structure. Au∕BT∕Pt capacitors have been fabricated and the dielectric properties have been discussed for SGHT-prepared BT films treated at 180 and 500°C, respectively. It was found that the BT films annealed at 500°C had much better ferroelectric properties. This improvement of ferroelectric properties through a heat treatment is probably associated with the change of density of hydroxyl groups in the films.

Investigation of electrically active defects in amorphous barium titanate thin films

DC and AC electrical properties of amorphous barium titanate thin film capacitors have been investigated as a function of temperature. A clear correlation is found between the temperature dependence of DC leakage currents and the temperature variation of the AC loss peaks, showing that these measurement techniques are probing the same electrical defects. Using either of these two techniques in amorphous barium titanate, we were able to detect oxygen vacancies diffusion with activation energy around 1 eV, and electron traps at 0.3 and 0.4 eV.