Ba0.6Sr0.4TiO3 Films Research Articles

Dielectric properties of ⟨001⟩-oriented Ba0.6Sr0.4TiO3 thin films on polycrystalline metal tapes using biaxially oriented MgO∕γ-Al2O3 buffer layers

We report the growth of ⟨001⟩-oriented Ba0.6Sr0.4TiO3 (BST) thin films on polycrystalline Ni-alloy tapes by pulsed laser deposition using biaxially oriented, ion-beam-assisted deposited (IBAD) MgO and γ-Al2O3 buffer layers. Dielectric constant values of our BST films were up to ∼85% of those in the epitaxial films prepared under similar conditions on single-crystal MgO substrates. No significant dispersion of the dielectric constant was observed for frequencies from 100Hz to 1MHz. These results demonstrate the versatility of using IBAD-textured MgO and γ-Al2O3 buffer layers to integrate highly oriented good-quality BST films with nonsingle-crystalline substrates.

Comparison of temperature-dependent dielectric characteristic in two different (Ba,Sr)TiO3 films

A high dielectric constant of 2500 near room temperature was observed in (Ba０.５Sr０.５)TiO３ (BSTO) film prepared by pulsed-laser deposition (PLD) on Pt/Ti/SiO２/Si (100) substrate at 550℃ in N2 atmosphere. The dielectric constant is weakly temperature dependent above 200 K. The dielectric behavior of this film is different from the BSTO film deposited in the O2 atmosphere, but very similar to that reported for the so-called “colossal” dielectric constant materials, such as CaCu3Ti4O12. The film prepared in O2 atmosphere at 650℃ shows normal ferroelectric phase transition, which are fitted with Curie-Weiss law. However, for the film prepared in N2 atmosphere at 550℃, the temperature dependence of dielectric relaxation can be characterized by a thermally excited relaxation process. Such anomalous dielectric response of the BST films is ascribed to the formation of the Schottky barrier between the metallic electrode and the film surface.

ENHANCED TUNABILITY AND ELECTRICAL PROPERTIES OF Ba0.7Sr0.3TiO3 THIN FILMS BY CU DOPING

ABSTRACT Cu-doped Ba0.7Sr0.3TiO3 (BST) thin films were prepared on Pt(111)/Ti/SiO2/Si(100) substrates by a sol-gel spin coating process. The coated thin films were annealed at 750°C for 3 min in oxygen, air and nitrogen atmospheres by a rapid thermal annealing process. All of the Cu-doped BST thin films were consisted of a single phase perovskite structure and exhibited smooth crack-free surfaces. We investigated the effects of the annealing atmosphere on the microstructure and the electrical properties of the Cu-doped BST films. The Cu-doped BST film by annealing in oxygen exhibits a dielectric constant of 923 with electric field dependent tunability of 48.6% at electric field 250 kV/cm and a leakage current density of 1.82 × 10− 5A/cm2 at electric field of 150 kV/cm.

FERROELECTRIC THIN-FILM BASED TWO-DIMENSIONAL ELECTROMAGNETIC BANDGAP STRUCTURES

ABSTRACT A two-dimensional electromagnetic bandgap structure has been developed using ferroelectric thin-films. Using lumped inductors and parallel plate capacitors a planar structure is developed which creates a bandgap in the 20–40 GHz range, as demonstrated by experimental results. The ferroelectric material Ba0.6Sr0.4TiO3 (BST) is used in the form of a thin-film between patterned layers of gold to form capacitors on a 500 μm thick high-resistivity Si substrate. A meander line based design is used to form the two-dimensional surface which provides biasing pathways to the varactors. The concept is simulated and experimentally verified using a coplanar waveguide based device in which the EBG surface is used as the ground plane. Designs are also presented for the development of a biasing network to take advantage of the non-linear tunability of the BST dielectric constant in order to make the surface tunable. Experiments used both sputtered and PLD BST films with a maximum bandgap depth of 35 dB. Potential applications are in the area of frequency selective surfaces, and high-impedance ground planes.

Nonlinear optical absorption and refraction of epitaxial Ba0.6Sr0.4TiO3 thin films on (001) MgO substrates

Highly epitaxial Ba0.6Sr0.4TiO3 (BST) ferroelectric thin films were fabricated on (001) MgOsubstrates by pulsed laser deposition. The nonlinear optical absorption coefficients (β) and refraction indices (γ) of the BST thin films on (001) MgO substrates were investigated using the single beam Z-scan technique with femtosecond laser pulses at the wavelengths of 790 nm and 395 nm, respectively, at room temperature. The nonlinear absorption coefficients of BST thin films were measured to be ∼0.087 cm/GW and ∼0.77 cm/GW at 790 nm and 395 nm, respectively. The nonlinear refraction indices of BST thin films exhibit a strong dispersion from a positive value of 6.1×10-5 cm2/GW at 790 nm to a negative value of -4.0×10-5 cm2/GW at 395 nm near band gap. The dispersion of γ is roughly consistent with Sheik-Bahae’s theory for the bound electronic nonlinear refraction resulting from the two-photon resonance. These results show that the BST film is a promising material as a candidate for nonlinear optical applications.

Mn-doped Ba0.6Sr0.4TiO3 high-K gate dielectrics for low voltage organic transistor on polymer substrate

In this letter, we report on the role of Mn doping in markedly reducing leakage currents in Ba0.6Sr0.4TiO3 (BST) high-K gate dielectrics utilized in organic thin film transistors (OTFTs) fabricated on plastic substrates. Undoped and 3% Mn-doped BST thin films, deposited by rf magnetron sputtering at room temperature on Pt∕Ti∕SiO2∕Si substrates, exhibited relative dielectric constants of ∼24–28. At an applied electric field of 250kV∕cm, the 3% Mn-doped BST films exhibited leakage current densities below 2×10−8A∕cm2 compared to the much higher value of 5×10−4A∕cm2 characteristic of undoped BST films. Pentacene based OTFTs using 3% Mn-doped BST gate dielectrics exhibited low voltage operation of <10V. This demonstrates the potential use of Mn-doped BST films as high-K gate dielectrics for stable and low operating voltage OTFTs.

X-Band Phased Array Antenna Using Ferroelectric (Ba,Sr)TiO3 Coplanar Waveguide Phase Shifter

A phased array antenna was fabricated using fourelement ferroelectric phase shifters with a coplanar waveguide (CPW) transmission line structure based on a Ba0.6Sr0.4TiO3(BST)/MgO structure. Epitaxial BST films were deposited on MgO (001) substrates by pulsed laser deposition. To attain the large differential phase shift and small losses for a ferroelectric CPW phase shifter, an impedance-matching-part adding technique between the effective transmission line and connecting cable was used. The return loss and insertion loss for this techniqueadapted BST CPW device were improved with respect to those for a normal BST CPW device. For an X-band phased array antenna system consisting of ferroelectric BST CPW phase shifters, power divider, dc block, patch antenna, and programmed dc power, the steering beam could be tilted by 15 ° in either direction.

Enhanced dielectric properties of Mn doped Ba0.6Sr0.4TiO3 thin films fabricated by pulsed laser deposition

Pure and 2 mol% Mn doped Ba0.6Sr0.4TiO3 (BST) thin films have been deposited on La0.67Sr0.33MnO3 (LSMO) coated single-crystal (001) oriented LaAlO3 substrates using pulsed-laser deposition technique. The bilayer films of BST and LSMO were epitaxially grown in pure single-oriented perovskite phases for both samples, and an enhanced crystallization effect in the BST film was obtained by the addition of Mn, which were confirmed by X-ray diffraction (XRD) and in situ reflective high energy electron diffraction (RHEED) analyses. The dielectric properties of the BST thin films were measured at 100 kHz and 300 K with a parallel-plate capacitor configuration. The results have revealed that an appropriate concentration acceptor doping is very effective to increase dielectric tunability, and to reduce loss tangent and leakage current of BST thin films. The figure-of-merit (FOM) factor value increases from 11 (undoped) to 40 (Mn doped) under an applied electric field of 200 kV/cm. The leakage current density of the BST thin films at a negative bias field of 200 kV/cm decreases from 2.5×10−4 A/cm2 to 1.1×10−6 A/cm2 by Mn doping. Furthermore, a scanning-tip microwave near-field microscope has been employed to study the local microwave dielectric properties of the BST thin films at 2.48 GHz. The Mn doped BST film is more homogeneous, demonstrating its more potential applications in tunable microwave devices.

High-tunability and low-microwave-loss Ba0.6Sr0.4TiO3 thin films grown on high-resistivity Si substrates using TiO2 buffer layers

In this Letter, we report on high-tunability and low-microwave-loss properties of Ba0.6Sr0.4TiO3 (BST) thin films by use of atomic-layer-deposited TiO2 films as the microwave buffer layer between BST and high-resistivity Si substrate. The interdigital capacitor fabricated on BST films grown on TiO2∕high resistivity Si (2kΩcm) substrates showed the much enhanced tunability value of 33.2% while retaining an appropriate Q factor, as compared to the tunability values of BST (21%) films grown on MgO single-crystal substrates and BST (8.2%) films grown on TiO2∕normal Si (10Ωcm) substrates. The coplanar waveguide BST phase shifter fabricated on TiO2∕high resistivity Si exhibited a phase shift of 95° and insertion loss of 3.09 dB at 15 GHz and an applied voltage of 50 V. ALD-grown TiO2 buffer layers enable the successful integration of BST-based microwave tunable devices onto high-resistivity Si wafers.

Dielectric properties of epitaxial Ba0.5Sr0.5TiO3 films on amorphous SiO2 on sapphire

The strain-relieved crystalline Ba0.5Sr0.5TiO3 (BST) film on SiO2∕Al2O3 was achieved by combining a molecular-beam epitaxy of BST on Si∕Al2O3 and a post-growth anneal in oxygen at elevated temperatures. The oxidation anneal not only converted the thin Si interlayer into amorphous SiO2 and eliminated the dielectric loss from the Si, but also relieved local strain in the film. The resulting BST film showed promising dielectric properties with 66% tunability and 0.016 dielectric loss, respectively. Additionally, temperature-dependent permittivity of the BST film resembled that of the bulk BST ceramics.

Direct observation of a fully strained dead layer at Ba0.7Sr0.3TiO3∕SrRuO3 interface

Epitaxial SrRuO3∕Ba0.7Sr0.3TiO3 (BST)/SrRuO3 thin film capacitors were prepared on SrTiO3 substrates by pulsed laser deposition. The structures of stacked BST films with different thicknesses were investigated by transmission electron microscopy. A distinctive layer of about 3nm of thickness was identified within BST films thicker than 9nm at the interface with the SrRuO3 bottom electrode. The distinctive layer is misfit dislocation-free showing pseudoconstant lattice parameters. Misfit dislocations are formed at the interface between the distinctive layer and the BST film bulk layer relaxing the latter as the film thickness increases. The effect of the distinctive layer on the system dielectric response is discussed within the framework of an interfacial dead-layer model.

Effect of annealing on leakage current in Ba0.5Sr0.5TiO3 and Ba0.96Ca0.04Ti0.84Zr0.16O3 thin films with Pt electrodes

Ba 0.96 Ca 0.04 Ti 0.84 Zr 0.16 O 3 (BCTZ) and Ba0.5Sr0.5TiO3 (BST) thin films were deposited at a substrate temperature of 450°C via rf magnetron sputtering to form Pt∕BCTZ∕Pt and Pt∕BST∕Pt capacitors. BCTZ thin films hold promise as an alternative to BST in capacitor applications due to the resistance of BCTZ to reducing atmospheres. In order to produce BST films with low dc leakage current, oxygen is routinely used during film growth and often afterwards during anneals. The effect of postannealing the capacitors in either forming gas or oxygen at temperatures up to 700°C (BST) and 800°C (BCTZ) were studied. The leakage mechanism is shown to be dominated by Schottky emission and the Schottky barrier height is reported as a function of anneal gas composition and anneal temperature.

Influence of Ce doping on leakage current in Ba0.5Sr0.5TiO3 films

Undoped and Ce-doped Ba0.5Sr0.5TiO3 (BST) thin films were prepared by pulsed-laser deposition onto a Nb-doped SrTiO3 (STON) substrate. The Ce concentration, ranging from 0.5 to 1.0 at.%, was found to have a strong influence on the electric properties of films at room temperature. We find that, with a positively biased Pt electrode, the leakage current controlled by BST/STON interface can be described by a space-charge-limited-current model. When the Pt electrode is negatively biased, the leakage current controlled by the BST/Pt interface can be explained by the Schottky emission mechanism. In both cases the Ce-doped BST thin films exhibited a lower leakage current (1.2 × 10−4 and 5.0 × 10−5 versus 3.4 × 10−2 A cm−2 at 450 kV cm−1; 4.0 × 10−4 and 4.0 × 10−5 versus 6.2 × 10−3 A cm−2 at −450 kV cm−1) than undoped BST films. The reduction of the leakage current is ascribed to the effect of acceptor Ce3+ doping, determined by x-ray photoelectron spectroscopy measurement.

Simulation of leakage currents in (Ba,Sr)TiO3 thin films with extracted charge profile

Though many researchers have realized the importance of charge density and distribution to the leakage currents of (Ba,Sr)TiO3 (BST) thin films, till now, there has been no effective method of determining the charge profile of high-permittivity thin films directly. Most studies of the leakage currents of BST thin films are simply based on the assumption that the charge density is identical throughout the films. This is not an exact way to understand the nature of leakage currents. In this paper, we have extracted the charge profile from capacitance–voltage measurements and then used the combined model of thermionic emission and drift diffusion to simulate the leakage currents. Using the extracted charge profile, the thermionic emission and drift diffusion model, with almost all the parameters consistent with separate measurements or theoretical results, can fit the leakage characteristic of the BST film very well at medium–high fields and sufficiently high temperature. Based on the simulation results, the leakage conduction mechanism is determined reliably.

Crystallization Temperature Limit of (Ba,Sr)TiO3 Thin Films Prepared by a Non Oxocarbonate Phase Forming CSD Route

A new chemical solution deposition (CSD) route for the fabrication of Ba0.7Sr0.3TiO3 (BST) thin films has been developed which completely prevents the formation of an intermediate oxo-carbonate phase. The latter has been reported previously by several authors to be responsible for increased crystallization temperatures. Barium and strontium diaminoethoxides were synthesized starting from pure barium and strontium metal and aminoethanol. These alkoxides were found to be readily soluble in a wide range of solvents and thus were excellent candidates for the CSD process. To prepare a stable precursor solution the aminoalkoxides were dissolved in 2-butoxyethanol and then used for the deposition of BST thin films. We conclude that the minimum crystallization temperature of 600∘C to be independent of the formation of the oxo-carbonate phase. DTA-TGA were performed on the precursors and their solutions to study their decomposition behaviour. All films annealed at different temperatures were physically characterized by XRD, IR, and SEM. The films prepared by this route at 650∘C were found to have high dielectric constant and the leakage currents were comparable to BST films prepared by normal carboxylate based routes at 750∘C.

The effects of PBB on the dielectric properties of BST film

AbstractThe mixture of PbO-Bi2O3-B2O3 (PBB) were formed on the Ba0.5Sr0.5TiO3 (BST) film and the effects of PBB phase on the dielectric properties of the BST film were investigated. The amorphous PBB layer was deposited on the BST film using rf magnetron sputtering and diffused into the film by heat treatment. The PBB phase in BST film was identified by energy dispersive spectrometer (EDS). Dielectric properties of BST film were significantly improved after the diffusion of PBB. The leakage current density of BST film decreased from 3.24×10-5 A/cm2 to 9.45×10-8 A/cm2 at 1.5 V and the dielectric constant increased from 238 to 533. These results show that the diffusion of insulating metal oxide into the ferroelectric thin film can improve the dielectric properties of the film.

Role of stress on the phase control and dielectric properties of (1-x) BiFeO3 - xBa0.5Sr0.5TiO3 solid solution thin films

AbstractDielectric properties and structure of (1-x) BiFeO3 (BFO) - x Ba0.5Sr0.5TiO3 (BST) (x = 0 ∼1) solid solution thin films were investigated. All films were prepared at 600 oC on (111) oriented Pt / TiO2 / SiO2 / Si substrates by pulsed laser deposition (PLD) technique. Solid solution could be achieved in all composition ranges, evidenced by X-ray diffraction (XRD) and field emission scanning electric microscope (FE-SEM). The intermediate compositions (0.4 = x = 0.8) exhibited a distinct (111) oriented cubic perovskite structure, while rhombohedra symmetry was found in the x < 0.4 range. Dielectric constant and tunability of the (1-x) BFO – x BST films within this composition region (0.4 = x = 0.8) decreased from 1110 to 920 at 1 MHz, and increased from 28.34 % to 32.42 % at 200 kV/cm, respectively, while loss tangent remains constant. A systematic decrease in lattice parameter with BST addition reduced stress due to reduction of lattice parameter mismatch between film and the substrate. In that range, the improvement of the dielectric properties without a degradation of loss tangent is attributed to the presence of the stress relaxation, which was quantitatively confirmed by a surface profiler based on Stoney's equation.

Temperature dependence of the ferroelectric and dielectric properties of the Ba0.5Sr0.5TiO3/La0.67Sr0.33MnO3 heterostructure

A ferroelectric and colossal-magnetoresistant material with a Ba0.5Sr0.5TiO3 (BST)/La0.67Sr0.33 MnO3 (LSMO) heterostructure was deposited epitaxially on (001) LaAlO3 substrates through pulsed laser deposition. Measurements of the magnetic and ferroelectric properties show no obvious degradation of the LSMO layer after BST film deposition. For the BST film, as the temperature decreases from 300 to 20 K, both the remanent polarization and the coercive field increase monotonously up to 5.7 µC cm−2 and 63.7 kV cm−1, respectively. Meanwhile, the temperature dependence of the spontaneous polarization shows a second-order ferroelectric phase transition, and the Curie temperature is estimated to be about 220 K. Furthermore, in the temperature range from 20 to 300 K, the dielectric constant (E = 0) and the dielectric tunability (E = 200 kV cm−1) exhibit similar temperature dependences, and show maximum values of 1252 and 45.3% at 200 K, respectively.

Enhanced dielectric properties of Ba1−xSrxTiO3 thin film grown on La1−xSrxMnO3 bottom layer

The Ba0.7Sr0.3TiO3(BST)∕La0.67Sr0.33MnO3(LSMO) heterostructure has been fabricated by pulsed-laser deposition on (001) SrTiO3 single crystal substrate. The crystallization and surface morphology of the heterostructure have been characterized by x-ray diffraction and atomic force microscopy. The composition distribution along the depth of the heterostructure is analyzed by Auger electron spectroscopy. It is founded that the LSMO bottom layer shows a marked effect on the dielectric properties of the BST films. Comparing with BST∕YBCO (YBCO—YBa2Cu3O7) heterostructure grown by similar process, the dielectric loss exhibits much lower and less frequency dispersive, especially in the high frequency region. The loss tangent at 100kHz is about 0.012 and the figure of merit factor is about 25 under the condition of 200kV∕cm. Ferroelectric hysteresis measurement shows that the remanent polarization and coercive field of the heterostructure are 3.4μC∕cm2 and 39kV∕cm, respectively. These parameters are all better than BST∕YBCO heterostructure. The good surface morphology and the element diffusion between BST and LSMO layers may be suggested to be responsible for the above-improved parameters of BST∕LSMO heterostructure.

High tunability (Ba,Sr)TiO3 thin films grown on atomic layer deposited TiO2 and Ta2O5 buffer layers

In this letter, we report on increased tunability of Ba0.6Sr0.4TiO3 (BST) thin films by use of Ta2O5 and TiO2 films as buffer layers between BST and Si substrates. Ta2O5 and TiO2 buffer layers were grown by atomic layer deposition (ALD) onto Si substrates followed by pulsed laser deposition of Ba0.6Sr0.4TiO3 thin films onto the buffer layers. The randomly oriented BST films deposited on TiO2∕Si substrates exhibited a broader grain size distribution than the (110) textured BST films grown on Ta2O5∕Si substrates. At an applied voltage of 10 V, the BST films grown on Ta2O5∕Si and TiO2∕Si substrates showed much enhanced tunability values of 53.1% and 72.9%, respectively, as compared to the 20.7% value obtained with BST films grown on MgO single crystal substrates. Successful integration of BST low voltage microwave tunable devices onto Si substrates thus appears possible with the aid of ALD grown Ta2O5 or TiO2 buffer layers.