Amorphous Barium Titanate Research Articles

Effect of annealing upon bipolar resistive switching in amorphous barium titanate (am-BTO) in Ag/am-BTO/ITO capacitor structure

The influence of annealing upon the resistive switching nature of amorphous barium titanate (am-BTO) is systematically investigated. The am-BTO thin film is fabricated on top of ITO coated glass substrate using RF-DC sputtering systems in the form of Ag/am-BTO/ITO dot point geometry. The room temperature electrical characterization shows the bipolar resistive switching characteristics. The resistive switching behaviour is diminished by increasing the temperature from RT to 600 °C. The conduction mechanism analysis is also been carried out. The ohmic and space change limited conduction mechanisms are dominated in lower and higher resistance regions respectively. The schematic representation of the proposed conduction mechanism is also drawn. The restricted movement of silver cations due to the local crystallization of BTO is presumed to be the reason for the observed reduction in switching behaviour.

Write-once-read-many-times resistive switching behavior of amorphous barium titanate based device with very high on-off ratio and stability

Write once read many times (WORM) memory devices based on the resistive switching mechanism of a sputtered amorphous BaTiO3 (am-BTO) thin film in a metal–insulator–metal structure is fabricated on a FTO coated glass substrate with a silver top contact. Fabricated devices show the switching from a low-conductance state to a high-conductance state with the formation of conductive filament(s) in the am-BTO layer. The memory characteristics are investigated as a function of thickness of am-BTO layer, which is determined by varying the deposition time. Devices with all deposited thicknesses show data retention for more than 4000 s and 300 reading cycle. Devices with 180 nm thickness show a high on-off ratio on the order of 106. The fabricated WORM devices exhibit good reading-endurance and data-retention characteristics.

Bipolar resistive switching studies in amorphous barium titanate thin films in Ag/am-BTO/ITO capacitor structures

The electroforming free bipolar resistive switching characteristics of amorphous barium titanate (am-BTO) thin films in Ag/am-BTO/ITO capacitor structures were systematically evaluated. The dot point capacitor structures were fabricated by using RF-DC sputtering deposition on ITO coated commercial glass substrates. Varied thicknesses of 142, 100, and 70 nm am-BTO thin films were utilized as a switching medium for exploring the resistive switching phenomena. The amorphous nature and topographical surface were probed with X-ray diffraction and atomic force microscopy techniques. The on/off resistance ratios, SET, RESET voltages, retention, and endurance parameters extracted from current-voltage (I-V) measurements demonstrate stable resistive switching. Space charge limited current conduction and ohmic conduction phenomena were found to be dominant in high resistance and low states of the junctions. The rupture formation upon the Ag active electrode and reduction in capacitance after the application of voltage specify the silver Ag+ ion mediated filament conduction mechanism for the observed bipolar switching phenomena.

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.

Hydrothermal synthesis and formation mechanism of tetragonal barium titanate in a highly concentrated alkaline solution

Tetragonal cube-shaped barium titanate (BaTiO3) was produced by the hydrothermal treatment of a peroxo-hydroxide precursor, a single-source amorphous barium titanate precursor, in a highly concentrated sodium hydroxide solution. Phase pure barium titanate with cube-shaped morphology and particle-sizes in the 0.2–0.5µm range were formed at temperatures above 80°C. Also, the cube-shaped morphology of the BaTiO3 product was preceded by spherical- and plate-like morphologies with, respectively, a Ti-excess and Ba-excess. Coinciding with these morphological observations, changes in the reaction product were also observed. The formation of crystalline BaTiO3 proceeded alongside secondary BaTi2O5 and Ba2TiO4 phases. These secondary phases disappeared as the reaction time was increased leaving only BaTiO3 as the sole reaction product. Kinetic analysis of the formation of hydrothermal BaTiO3 crystallization by the Johnson-Mehl-Avrami method showed that BaTiO3 crystallization is a homogeneous dissolution-precipitation reaction. The mechanism is governed by nucleation and growth in the beginning of the reaction and dissolution-precipitation dominating throughout the hydrothermal reaction process.

An amorphous barium titanate thin film improves light trapping in Si solar cells

A new anti-reflection coating based on amorphous barium titanate (a-BTO) was developed using RF magnetron sputtering technique.

Visible photoluminescence from amorphous barium titanate nanofibers

Broad visible emission at the wavelength range of 400–800nm was obtained at room temperature in amorphous barium titanate (BaTiO3) nanofibers, which were prepared by electrospinning and calcined at low temperatures. Structure and morphology of the BaTiO3 nanofibers were investigated by X-ray diffraction (XRD), transmission electron microscope (TEM), thermo-analysis and Raman spectroscopy. XRD data shows that the BaTiO3 nanofibers calcined at temperatures below 650°C are amorphous, in which intense photoluminescence (PL) emission bands at 600nm have been detected. More importantly, PL performance of the BaTiO3 nanofibers exhibited a strong dependence on the calcination temperature. Observable emission was only found in the low-temperature treated fibers. Mechanism of the observed photoluminescence has been discussed. The achievements shown herein may be help to find new light-emitting materials.

Luminescence enhancement of Eu-doped amorphous barium titanate films with crystalline BaTiO3 nanoparticle incorporation

Crystalline barium titanate (BT) nanoparticles were incorporated into Eu-doped amorphous BT films to examine the influence of local permittivity around Eu ions on their luminescent properties in the BT films. The BT nanoparticles were synthesized by hydrolysis of a BT complex alkoxide in 2-methoxyethanol/ethanol cosolvent, and then incorporated into the Eu-doped films at Eu/(Eu+BT) ratio of 10mol%. The emission intensities of Eu3+ in the film were linearly raised with volume of the nanoparticles incorporated, while dielectric constant of the film also increased with the nanoparticle volume. This indicates that the incorporation of crystalline nanoparticles is effective to enhance the luminescence of Eu ions in the amorphous film.

Microstructural and optical investigations of Ce-doped barium titanate thin films by FTIR and spectroscopic ellipsometry

(BaTiO3)1− x (CeO2) x (with x = 0, 1, 3, 5.7 and 16 at.%) thin films were prepared by electron beam evaporation method. According to X-ray diffraction, nanosized and amorphous Ce-doped barium titanate thin films were obtained. The crystallinity diminished as Ce content increased. The increase in the Ce slightly shifts the absorption peaks at 433–450 and 416–424 cm−1 to higher wavenumbers. A two-layer model was used to describe the experimental ellipsometric data. The Bruggeman effective medium approximation (BEMA) was used to describe the surface roughness layer and the Cauchy dispersion relation was used to describe the main Ce-doped barium titanate layer. The refractive index and the extinction coefficient were found to increase with increasing Ce content; however, the optical band gap decreased with increasing Ce content. The accurate determination of the optical constants of Ce-doped barium titanate thin films may favor them in the applications of optical thin film devices.

Hydrothermal synthesis of carbonate-free submicron-sized barium titanate from an amorphous precursor: Synthesis and characterization

In this paper, the amorphous barium titanate precursor was prepared by the peroxo-hydroxide method and post-treated by various drying procedures, such as: room temperature drying, room temperature vacuum drying and vacuum drying at 50 °C. The objective in the latter two treatments was to increase the Ti–O–Ba bonds of the precursor. The post-treated precursors were compared with the untreated (i.e., ‘wet’) precursor. Also, a barium titanate precursor was prepared by an alkoxide route. Afterwards, the precursors were hydrothermally treated at 200 °C in a 10 M NaOH solution. Vacuum drying of the precursor seemingly promoted the formation of Ti–O–Ti bonds in the hydrothermal end-product. The low Ba:Ti ratio (0.66) of the alkoxide-route prepared precursor lead to a multi-phase hydrothermal product with BaTiO 3 as the main phase. In contrast, phase pure BaTiO 3, i.e. without BaCO 3 contamination, was obtained for the precursor which was dried at room temperature. Cube-shaped and highly crystalline BaTiO 3 particles were observed by electron microscopy for the hydrothermally treated peroxo-hydroxide-route prepared precursor.

Characterization of optical properties of amorphous BaTiO 3 nanothin films

Amorphous barium titanate nanothin films were prepared by sol–gel dip-coating method. According to transmission spectrum, the refractive index and optical band gap of nanothin films have been determined. High transmission spectrum without any fluctuation in visible wavelength region was recorded. Experimental results indicated that the surface morphology of prepared nanothin films were improved and as a result of that, a better optical properties, less optical losses and higher band gap width were obtained in contrast with other reported data. It was found that optical propagation loss of BaTiO 3 nanothin film was much lower than normal polycrystalline BaTiO 3 thin film. It seems to us that, amorphous barium titanate nanothin films would be suitable for several applications.

X-ray initiation of nonthermal growth of single crystal pyramids in amorphous barium titanate

Nonthermal crystallization of amorphous barium titanate (BTO) was initiated by a monochromatic x-ray microbeam from a synchrotron radiation source. Following x-ray exposure, micron-sized BTO single crystals with a true square pyramid shape appeared on the surface of an amorphous BTO substrate-free film. The location and size of the areas in which crystal growth appeared fit well the x-ray microbeam path and size, respectively. A plausible mechanism of x-ray induced initiation of nonthermal crystallization in BTO is proposed. The observed phenomenon holds promise for materials engineering at nanoscale and crystallization of amorphous materials in situations where heating must be avoided.

Electrical properties of amorphous barium titanate films sputter deposited under hydrogen containing atmosphere

Hydrogenated barium titanate films (a-BaTiO3:H) were grown by rf magnetron sputtering under hydrogen containing atmospheres. X-ray photoelectron spectroscopy and Fourier transform infrared analyses were carried out in order to get insight on composition and chemical bonds. Impedance spectroscopy was used to investigate the dielectric behavior at low-temperature range. The strong O–H stretch band (at around 3500 cm−1) evidenced in the infrared spectrum clearly manifests the presence of protons in the hydrogenated material. The incorporation of protons during the deposition of a-BaTiO3:H films hugely modifies their electrical properties. These films exhibit large capacitance that is hundred times higher than the one measured when the film is grown without hydrogen. On the other hand, the presence of positively charged protons can contribute to the conduction process by introducing two main features. The first is related to a significant migration of proton between adjacent oxygen sites with an activation energy of 0.37 eV. This behavior might be responsible for the high protonic conductivity depicted in our material at temperature higher than −30 °C. Furthermore, protons were shown to engender hydrogen bonds in the structure. As a result, the dielectric relaxation observed at temperature below −30 °C could be ascribed to the dissociation of the hydrogen bonds. Activation energy for this process is around 0.26 eV.

Observation of negative capacitances in metal-insulator-metal devices based on a-BaTiO3:H

We study capacitance dispersion in hydrogen-doped amorphous barium titanate (a-BaTiO3:H). Negative values of capacitance are observed at very low frequencies (<10 Hz). This behavior is shown to be related to proton and oxygen vacancy conductions. A qualitative model is proposed to explain negative capacitances. It involves the modification of the Schottky barrier at the metal-dielectric interface upon accumulation of mobile protons and oxygen vacancies at electrodes.

EFFECT OF THE OXYGEN ADDITION IN THE SPUTTERING GAS ON THE DIELECTRIC PROPERTIES OF a-BaTiO3 FILMS

ABSTRACT Amorphous barium titanate films (a-BTO) were grown by the RF magnetron sputtering technique at low temperature and under various oxygen/argon mixing ratios (OMR). Their dielectric properties (dielectric constant, loss and ac conductivity in the 0.1–105 Hz range), dc leakage currents, and thermally stimulated currents were systematically studied as a function of OMR in the sputtering gas (0% to 20 %). We demonstrate that the presence of oxygen plays a significant role in improving the dielectric properties of a-BTO thin films. The oxygen incorporation successfully suppressed some electrical defects. The bulk conductivity and leakage currents are reduced although a small decrease of dielectric constant has been observed. Films prepared at higher oxygen ratio (20%) have a small leakage current density (3.6 × 10− 7 A/m2) and lower tangent loss (2 × 10− 3).

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.

Atomic Layer Deposition of BaTiO3 Thin Films—Effect of Barium Hydroxide Formation

AbstractBarium titanate thin films are grown by atomic layer deposition (ALD) at 340 °C from barium cyclopentienyl and titanium methoxide precursors. H2O is used as the oxygen source. Binary reactions of Ba(tBu3C5H2)2 and H2O are first studied separately in BaO deposition and are found to result in a hydration/dehydration cycle, which is strongly influenced by the process temperature. Self‐limiting growth of amorphous barium titanate films becomes possible when Ti(OMe)4 – H2O growth cycles are mixed as well as possible with Ba(tBu3C5H2)2 – H2O cycles. The as‐deposited amorphous films are crystallized by post‐deposition annealing at 600 °C. Permittivities of 15 and 70 are measured for as‐deposited and post‐deposition annealed films, respectively. A charge density of 1.9 μC cm–2 (equivalent oxide thickness of 1.8 nm) and leakage current density ≤ 1 × 10–7 A cm–2 were achieved at 1 V bias with a 32 nm thick Ba–Ti–O film in a Pt electrode stack annealed at 600 °C.

Dielectric response of Cu∕amorphous BaTiO3∕Cu capacitors

Cu∕amorphous BaTiO3∕Cu capacitors were tested for their dielectric properties in the 0.1Hz–100kHz range, from room temperature to 350°C. The amorphous barium titanate dielectric layer was deposited by rf sputtering on water-cooled copper electrodes. The room-temperature dielectric constant is around 18 and the dissipation factor is 3×10−3 at 100kHz. When increasing temperature the dielectric response displays an important frequency dispersion with the appearance of a marked loss peak at low frequencies. This dispersion is ascribed to electrode polarization effects, as evidenced by measurements performed on various film thicknesses and by using different electrodes. The electrode polarization phenomenon is discussed using a simple space charge model and is shown to be related to mobile oxygen vacancies.

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.

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.