Barium Titanate Thin Films Research Articles

Relaxation of defects in amorphous barium titanate thin films

Relaxation of electrical defects in amorphous barium titanate thin films was studied by the thermally stimulated current (TSC) technique. We were able to detect at least three relaxation peaks in the TSC spectra. One is associated with activation energy of 0.65 eV and is possibly related to electronic trap levels below the conduction band. Another one is associated with activation energy close to 1 eV corresponding to the migration of positively charged oxygen vacancies within the films.

Preparation and characterization of Mn-doped BaTiO3 thin films by magnetron sputtering

Barium titanate (BaTiO3) thin films doped with Mn (0.1–1.0 at%) were prepared by r.f. magnetron sputtering technique. Oxygen/argon (O2/Ar) gas ratio is found to influence the sputtering rate of the films. The effects of Mn doping on the structural, microstructural and electrical properties of BaTiO3 thin films are studied. Mn-doped thin films annealed at high temperatures (700 °C) exhibited cubic perovskite structure. Mn doping is found to reduce the crystallization temperature and inhibit the grain growth in barium titanate thin films. The dielectric constant increases with Mn content and the dielectric loss (tan δ) reveals a minimum value of 0.0054 for 0.5% Mn-doped BaTiO3 films measured at 1 MHz. The leakage current density decreases with Mn doping and is 10−11 A/cm−2 at 6 kV/cm for 1% Mn-doped thin films.

Studies of pulsed laser deposition processes of BaTiO3 thin films

Pulsed laser deposition technique (PLD) has had a widespread use during the last years, but there is not a full understanding of the different processes involved in this technique. In the present paper a study of the ablation process of a BaTiO3 target and its deposition as a thin film is shown. Two interesting results were observed. First, an inversely proportional relation between the most probable kinetic energies (and most probable momenta) of ionic elements in the plume and their ionization potentials was observed. This result is related to the way the material is extracted and the formation of the spatial charge in the plume. Second, polycrystalline barium titanate thin film with preferential crystallographic orientation was deposited at room temperature and under high vacuum conditions. This result is related with the transference of kinetic energy of the species in the plume as they reach the substrate.

Effect of lanthanum substitution on the Raman spectra of barium titanate thin films

AbstractThin films of Ba1−xLaxTiO3 on platinum substrates were synthesized using the sol–gel method for x values of 0.0, 0.03, 0.05, and 0.10, and the effect of trivalent La3+ substitution on the structural and dielectric properties was studied. Using X‐ray diffraction, structural analysis of these compositions revealed a slight increase in the tetragonal distortion of the unit cell with increase in La content. Accordingly, an increase in the tetragonal to cubic transition temperature TT/C was detected by temperature‐dependent Raman spectroscopy in the range of 70–500 K. Unlike the results from Raman scattering for the La‐doped BaTiO3 films, the dielectric measurements showed broad and diffused dielectric maxima, making the estimation of the transition temperature merely qualitative. Copyright © 2006 John Wiley & Sons, Ltd.

Thermally stimulated currents in amorphous barium titanate thin films deposited by rf magnetron sputtering

Thermally stimulated currents (TSCs) are measured in amorphous barium titanate thin films deposited by the rf sputtering technique. The TSC global curve is composed of three overlapping peaks in the 0–200°C temperature range. At 50°C, a first TSC peak is observed that can be related to a shallow-trap level. A second peak due to a dipolar polarization process is observed at 95°C. Finally, a third peak appears at 140°C that is ascribed to the oxygen vacancy motion and their accumulation at electrodes. The different peaks constituting the global TSC spectrum are separately studied by the fractional polarization technique in order to analyze their fine structures and to determine their activation energies.

Barium titanate nanocrystals and nanocrystal thin films: Synthesis, ferroelectricity, and dielectric properties

Advanced applications for high k dielectric and ferroelectric materials in the electronics industry continues to demand an understanding of the underlying physics in decreasing dimensions into the nanoscale. We report the synthesis, processing, and electrical characterization of thin (<100nm thick) nanostructured thin films of barium titanate (BaTiO3) built from uniform nanoparticles (<20nm in diameter). We introduce a form of processing as a step toward the ability to prepare textured films based on assembly of nanoparticles. Essential to this approach is an understanding of the nanoparticle as a building block, combined with an ability to integrate them into thin films that have uniform and characteristic electrical properties. Our method offers a versatile means of preparing BaTiO3 nanocrystals, which can be used as a basis for micropatterned or continuous BaTiO3 nanocrystal thin films. We observe the BaTiO3 nanocrystals crystallize with evidence of tetragonality. We investigated the preparation of well-isolated BaTiO3 nanocrystals smaller than 10nm with control over aggregation and crystal densities on various substrates such as Si, Si∕SiO2, Si3N4∕Si, and Pt-coated Si substrates. BaTiO3 nanocrystal thin films were then prepared, resulting in films with a uniform nanocrystalline grain texture. Electric field dependent polarization measurements show spontaneous polarization and hysteresis, indicating ferroelectric behavior for the BaTiO3 nanocrystalline films with grain sizes in the range of 10–30nm. Dielectric measurements of the films show dielectic constants in the range of 85–90 over the 1KHz–100KHz, with low loss. We present nanocrystals as initial building blocks for the preparation of thin films which exhibit highly uniform nanostructured texture and grain sizes.

Structure–properties correlations for barium titanate thin films obtained by rf-sputtering

Stoichiometric thin films were deposited by rf-magnetron sputtering from a BaTiO 3 ceramic target on Pd foils used as substrates. Polycrystalline BaTiO 3 films with different grain sizes were obtained by post-deposition heat treatment in air, at 900 °C for 8 h. A multi-step deposition–annealing technique was used in order to improve the compactness and therefore, the dielectric behaviour. The structural characteristics and the surface topography varied obviously with both films thickness and deposition–annealing procedure. X-ray diffraction data pointed out that the so-called “pseudocubic” phase, generally reported for such fine-grained thin films, does not involve only a highly distorted unit cell, but consists obviously, in our case, from a mixture of crystalline phases, with tetragonal and cubic symmetry, respectively. The dielectric properties (relative permittivity and loss tangent) showed small frequency dispersion. The BaTiO 3 film of 0.6 μm thickness obtained by four deposition–annealing cycles exhibited a dielectric constant of ∼700 and a dissipation factor of 0.06 at 100 kHz.

Space charge limited transient currents and oxygen vacancy mobility in amorphous BaTiO3 thin films

Time-dependent transient currents were studied at elevated temperatures (200–220°C) in amorphous barium titanate thin film capacitors. Current transients display a peak whose time position varies with applied voltage and temperature. The response is analyzed through space charge limited current transient theories. Extracted drift mobilities are in the 10−11–10−12cm2V−1s−1 range and show an activation energy of 1eV. The phenomena are associated with oxygen vacancies migration in BaTiO3.

Structural, dielectric and electromechanical study of Hf-substituted BaTiO3 thin films fabricated by CSD

Hafnium-substituted barium titanate thin films were deposited on platinized silicon substrates. Two different concentrations of solutions (0.1 M and 0.3 M) were used to deposit films of various compositions Ba(Ti1-x,Hfx)O3 (x=0.03, 0.05, 0.07, 0.1, 0.2, 0.3 and 0.4). The microstructure of the films depended on the concentration of the solution. Lower concentration (0.1 M) solutions led to columnar films, but with higher hafnium percent compositions the columnar structure was lost. The films which were derived from the 0.1 M concentration solutions had better dielectric and electrical properties compared to the films derived from a higher concentration (0.3 M). All the films were found to be polycrystalline in nature. The dielectric constant of the films was found to decrease with higher amounts of hafnium substitution. From the I–V characteristics it is noticed that the leakage decreases by almost four orders of magnitude with a hafnium substitution of 40%. The d33 values of the films were between 23 and 5.6 pm/V for the different films.

Characterization and microstructure of highly preferred oriented lead barium titanate thin films on MgO (100) by sol-gel process

Highly preferred oriented lead barium titanate (Pb 1− x ,Ba x )TiO 3 thin film, with particular emphasis on (Pb 0.5,Ba 0.5)TiO 3, can be obtained by spin-coating on MgO (100) substrate by using the precursor sol, which was synthesized from acetylacetone chelating with titanium isopropoxide and ethylene glycol as a solvent, in the sol-gel process. Film thickness, pyrolysis temperature and heating rate were studied systemically to investigate their influences on the formation of preferred oriented thin films. The highly preferred (001)/(100) oriented thin film could be obtained by the pyrolysis of wet film at 500 °C and annealing at 600 °C at a slow heating rate of 5 °C/min. It is confirmed that the tetragonal perovskite structure of the titanate ceramic decreases with an increase of Ba content in (Pb 1− x ,Ba x )TiO 3. The (001)/(100) oriented films were synthesized from all compositions between x = 0.2 and x = 0.8, at a crystallization temperature of 600 °C. In particular, for the Ba content in the range of x = 0.5∼0.6, highly preferred (001)/(100) planes were observed.

Chemical mechanical polishing of BTO thin film for vertical sidewall patterning of high-density memory capacitor

Most high- k materials cannot to be etched easily. Problems such as low etch rate, poor sidewall angle, plasma damage, and process complexity have emerged in high-density DRAM fabrication. Chemical mechanical polishing (CMP) by the damascene process has been used to pattern high- k materials for high-density capacitor. Barium titanate (BTO) thin film, a typical high- k material, was polished with three types of silica slurry having different pH values. Sufficient removal rate with adequate selectivity to realize the pattern mask of tetra-ethyl ortho-silicate (TEOS) film for the vertical sidewall angle was obtained. The changes of X-ray diffraction pattern and dielectric constant by CMP process were negligible. Planarization was also achieved for the subsequent multilevel processes. Our new CMP approach will provide a guideline for effective patterning of high- k materials by CMP.

Electrical properties of low temperature deposited amorphous barium titanate thin films as dielectrics for integrated capacitors

Amorphous barium titanate (a-BaTiO 3) thin films were deposited by the RF magnetron sputtering technique. The films were grown directly on copper substrates which were maintained at low temperature during deposition (water-cooled substrates). We studied the permittivity (dielectric constant and loss) in the 0.1 Hz–100 kHz range, from − 100 to 225 °C. Leakage currents (DC conductivity) were also studied as a function of the applied field and temperature. At room temperature the dielectric constant is 18.5 (100 kHz), the dissipation factor is 4 × 10 − 3 and the conductivity is 6 × 10 − 16 S/cm (2.7 μm thick films). A dielectric dispersion is observed at low frequencies that becomes pronounced when the temperature is increased.

Ferroelectric properties of barium titanate thin films grown on nichrome substrates by RF sputtering

Ferroelectric thin films have attracted attention because of their possible applications in new microelectronic devices. In particular, the BaTiO3 thin films, a material with a high dielectric constant, low leakage current and the quality of the change of its ferroelectric properties by adding and modifying the concentration of a doppand [1], can be used for a wide range of functional purposes, from simple capacitors to complicated microwave devices [2–6]. BaTiO3 films have been grown by many deposition techniques [1, 4,7, 8] but RF sputtering is known to be one of the best because it preserves the stoichiometry under the right deposition conditions. A major disadvantage of BaTiO3 is that it possesses a mechanical fragility, therefore, one of the main technological challenges for this material is to find an electrode-substrate that could offer mechanical stability. Also, this electrode-substrate must have a good thermal stability, low electrical resistance, high resistance to oxidation and good adherence for the film. Pt/TiO2/SiO2/Si is commonly used because it posseses the mentioned characteristics [2]. Nevertheless, it has many problems because titanium can migrate to the platinum surface during the annealing process, adding electrical resistively to electrode [9–12]. It also generates cracking in by thermal stress in the BaTiO3 thin films. The nichrome is suggested for a suitable electrode because it fulfills the requirements mentioned above [13–17] and, in contrast to Pt/TiO2/SiO2/Si, its commercial availability and its low price. Therefore, in this work, BaTiO3 films were grown on nichrome substrates by RF-sputtering, and their structural and ferroelectric properties were measured and improved in comparison with those deposited on Pt/TiO2/SiO2/Si substrates using the same method and under same deposition conditions. BaTiO3 thin films on nichrome substrates were prepared by RF magnetron sputtering in an off-axis geometry at room temperature. A 50.8 mm diameter. BaTiO3 target, 99.9% pure, from SCI Engineered Materials, Inc., was used. The nichrome was obtained from a commercial strip, nichrome 80, with 0.127 mm thickness from H. Cross Company, Inc. The films were also grown over Pt/TiO2/SiO2/Si substrates in order to com-

Effect of Zr Substitution for Ti on the Dielectric and Ferroelectric Properties of Barium Titanate Thin Films

ABSTRACT The paper reports the study of the effect of Zr contents on the dielectric, ferroelectric, leakage current and fatigue properties of Ba (Zr x Ti1 − x ) O3 films. All these films exhibited high dielectric constant, which tends to be maximum for 5 at % (BZT5) Zr doped BaTiO3 film. The dielectric properties at the sub-switching ac oscillation field were analyzed following Rayleigh formalism. The reversible component of the switchable measured polarization was separated out by measuring both polarization hysteresis and capacitance bias voltage characteristics. This method was utilized to detect the defect-domain wall interaction present in the material. Current-voltage characteristics were found to be ohmic in nature. Fatigue free characteristics were observed for parent material (BaTiO3) as well as for 5% Zr substituted film.

Synthesis of thin films of barium titanate and barium strontium titanate nanotubes on titanium substrates

In this study, well-ordered nanotubes of titania were fabricated by anodic oxidation of pure titanium foils in HF aqueous solution. Fe-SEM images indicate the average nanotubes diameter is ∼100 nm with a thickness of about 200 nm. Nanotube arrays of barium titanate and barium strontium titanate were synthesized under hydrothermal condition at 200 °C taking above-oxidized titania nanotubes as templates. In contrast to porous alumina membranes obtained by others using similar approaches, a different morphology was obtained in the current study. The nanotube arrays of barium titanate and barium strontium titanate may have many potential applications in microelectronics.

Copper Compatible Barium Titanate Thin Films for Embedded Passives

Barium titanate thin films have been prepared by chemical solution deposition on 18 μ m thick, industry standard copper foils in the absence of chemical barrier layers. The final embodiment exhibits randomly oriented BaTiO3 grains with diameters between 0.1 and 0.3 μ m, and an equiaxed morphology. The average film thickness is 0.6 μ m and the microstructure is free from secondary or interfacial phases. The BaTiO3 films are sintered in a high temperature reductive atmosphere such that copper oxidation is avoided. Subsequent lower-temperature, higher oxygen pressure anneals are used to minimize oxygen point defects. Permittivities of 2500 are observed at zero bias and room temperature, with permittivities greater than 3000 at the coercive field. Loss tangents under 1.5% are demonstrated at high fields. The BaTiO3 phase exhibits pronounced ferroelectric switching and coercive field values near 10 kV/cm. Temperature dependent measurements indicate a ferroelectric transition near 100∘C with very diffuse character. Combining the approaches of the multilayer capacitor industry with traditional solution processed thin films has allowed pure barium titanate to be integrated with copper. The high sintering temperature—as compared to typical film processing—provides for large grained films and properties consistent with well-prepared ceramics. Integrating BaTiO3 films on copper foil represents an important step towards high capacitance density embedded passive components and elimination of economic constraints imparted by traditional noble metallization.

Contact mode potentiometric measurements with an atomic force microscope on high resistive perovskite thin films

We have investigated the potential distribution on barium titanate thin films with an atomic force microscope in contact mode to find answers to the important question of local electric conductivity. A detailed knowledge about the electrical transport mechanisms is very important to receive a sound operation for highly integrated circuits such as non-volatile memory cells. With this paper we present an advanced method to perform these potential scans in galvanic contact. Key element of the set-up is an optimized electrometer amplifier which has an electronically reduced input capacitance avoiding the work function influence on the surface potential scan. To demonstrate the capability of our set-up we present example measurements performed on thermally reduced BaTiO 3 thin films.

Structure dynamics of strongly reduced epitaxial BaTiO 3− x studied by Raman scattering

Raman scattering technique was used to study structure dynamics of strongly reduced epitaxial barium titanate (BaTiO 3− x ) thin films grown on MgO (1 0 0) substrates by laser molecular beam epitaxy under different oxygen pressures from 10 −2 to 10 −5 Pa. X-ray diffraction and asymmetric rocking curves indicate that lattice parameters c and c/ a ratio increase, and a slightly decreases with decreasing oxygen pressure, indicating increased lattice volume of BaTiO 3− x thin film. Raman spectra confirm that BaTiO 3− x thin films are in tetragonal phase with some deviated features, which maybe origin from tensile strain at film–substrate interface due to lattice parameter mismatch. Moreover, two weak peaks in Raman spectra of BaTiO 2.52 thin film grown under 3.0 × 10 −5 Pa may be induced by second-order two-phonon processes. Raman peaks shift towards lower frequencies with decreasing oxygen pressure during deposition, suggesting a decrease of the stress in BaTiO 3− x thin films. In the meantime, Raman peaks become broadened, which may be attributed to higher degree of structural disorder in strongly reduced BaTiO 3− x lattice structure.

Theoretical and experimental study of the relation between photoluminescence and structural disorder in barium and strontium titanate thin films

Thin films of barium and strontium titanate (BST), synthesized by the polymeric precursor solution and spin coated on [Pt (140 nm)/Ti (10 nm)/SiO 2(1000 nm)/Si] substrates were found to be photoluminescent at room temperature when heat treated below 973 K, i.e. before their crystallization. First principles quantum mechanical techniques, based on density functional theory (DFT) were employed to study the electronic structure of two periodic models: one is standing for the crystalline BST thin film and the other one for the structurally disordered thin film. The aim is to compare the photoluminescence (PL) spectra of the crystalline and disordered thin films with their UV–vis spectra and with their computed electronic structures. The calculations show that new localized states are created inside the band gap of the crystalline model, as predicted by the UV–vis spectra. The study of the charge repartition in the structure before and after deformation of the periodic model shows that a charge gradient appears among the titanate clusters. This charge gradient, together with the new localized levels, gives favorable conditions for the trapping of holes and electrons in the structure, and thus to a radiative recombination process. Our models are not only consistent with the experimental data, they also allow to explain the relations between structural disorder and photoluminescence at room temperature.

Synthesis and Properties of Barium Titanate Thin Films Deposited on Copper Foil Substrates

AbstractBarium titanate thin films have been deposited on copper foils in the absence of interfacial layers via a chemical solution process. The dielectric – base metal stacks have been processed in reductive atmospheres such that substrate oxidation is avoided while allowing the perovskite film phase to crystallize. This accomplishment has facilitated the pursuit of a new embedded capacitor technology offering compatibility with polymer printed wiring boards and capacitance densities in excess of 2.5 µF/cm2. This represents a distinct improvement beyond conventional foil-based capacitor strategies. Finally, two critical phenomena will be discussed: (1) the effect of grain size on the dielectric properties of barium titanate thin films and (2) the effect of the B-site substituent Zr on the lattice, microstructure, and dielectric properties. Most importantly, high processing temperatures have allowed for microstructural and dielectric properties similar to well-prepared bulk ceramics, including average grain diameters greater than 0.1 µm, relative permittivities in excess of 2000, and coercive fields below 10 kV/cm. These properties will be discussed in the context of bulk ceramic and thin film reference data and with regard to integration into printed wiring boards.