BaxSr1-xTiO3 Films Research Articles

Structural and optical properties of BaxSr1−xTiO3 thin films on indium tin oxide/quartz substrates prepared by radio-frequency magnetron sputtering

Ba x Sr 1 − x Ti O 3 (x=0.6 and 0.8) thin films have been prepared on indium-doped tin oxide (ITO) coated quartz substrates using radio-frequency magnetron sputtering. Their structural properties and surface morphologies were examined by x-ray diffraction and atomic force microscopy, respectively. The BaxSr1−xTiO3 (BST) thin films with x=0.6 and 0.8 annealed at 650°C for 20min exhibit good surface morphology and well-crystallized perovskite structure. High quality BST ferroelectric thin films were further investigated by electrical measurements, showing the remnant polarization (Pr) of 6.75μC∕cm2 and the coercive field (Ec) of 43.2kV∕cm. Optical transmittance measurement indicated that the Ba concentration has an effect on the band gap energy (Eg) structure of the BaxSr1−xTiO3 thin films. The Eg decreases linearly with the increase of the Ba content. The refractive index (n) and extinction coefficient (k) of the BST films with x=0.6 and 0.8 were obtained by fitting the spectroscopic ellipsometric data using a parametric dielectric function model. The refractive index (n) and extinction coefficient (k) increase with increasing wavelength. All of the results show that the BST/ITO/quartz heterostructure is promising for applications in integrated optical waveguide devices.

Low Pressure RF Plasma Jet Sputtering Technique Applied to Ferroelectric Films: Ba<sub>1-x</sub> Sr<sub>x</sub>TiO<sub>3</sub>

Technology aspects and characterization of BaxSr1-xTiO3 (BST) films fabricated with low pressure plasma jet technique are presented. BST films were deposited on silicon coated with Pt/TiO2/SiO2 and on bare Si substrates. The nozzles-type RF hollow cathode has been fabricated from hot pressed BaTiO3, SrTiO3, and BST ceramics. Controlling of RF voltage, RF current and substrate temperature allowed us to deposit reproducible films with controlled grain size. Hysteresis loops, ellipsometric and micro-Raman investigation results are presented and discussed.

Electrical Characteristics of BaxSr1-xTiO3(BST) Capacitors Implemented with Ti-Al Electrodes

AbstractTi-Al was used as the electrode for RF magnetron sputtered BST film at a substrate temperature of 450°C for decoupling capacitor applications. X-ray diffraction analysis shows that the deposited film BST film is amorphous. Electrical characterizations of the devices performed by capacitance versus voltage measurements show a dielectric constant of about 55. With increase in annealing temperature above 450°C, the capacitance was found to decrease significantly.

Dielectric Properties of the Compositionally Graded (Ba,Sr)TiO3 Thin Film

The compositionally graded Ba1–xSrxTiO3 films with a fine compositional gradient from BaTiO3 to Ba0.70Sr0.30TiO3 were fabricated on Pt/Ti/SiO2/Si substrates by a sol-gel deposition method. The graded films crystallized into a pure perovskite structure after post-deposition annealing. Dielectric constant peaks, common to a ferroelectric transition, were not observed in the temperature range from -35°C to 190°C, within which the dielectric constant and dielectric loss showed negligible temperature dependence. The improved temperature stability from this type of compositionally graded material is beneficial to applications requiring a wide range of operating temperatures, thereby eliminating the need for environmental controls.

Stress-induced domain structure formation in nanodimensional barium strontium titanate films

Thin heteroepitaxial films of BaxSr1−x TiO3 (BST) with x = 0.80 possessing high structural perfection have been grown by RF magnetron sputtering on MgO(001) substrates. The microstructure and crystallographic characteristics of the BST films of various thicknesses have been studied using electron microscopy and X-ray diffraction. It is established that there is a threshold thickness at which the BST crystal lattice parameter in the direction perpendicular to the substrate exhibits a sharp change.

MICROWAVE PROPERTIES OF BST AND BST/BMT THIN FILMS GROWN ON SAPPHIRE SUBSTRATE BY EVANESCENT MICROWAVE PROBE

ABSTRACT Thin films of BaxSr1−xTiO3 (BST) serial materials have the advantages of adjustable tunability and are good candidates for the application in DRAM and microwave devices. However, these films usually have loss tangent higher than the order of 0.01 at microwave frequencies. To improve the crystal structures and suppress the microwave losses, an interlayer material with good microwave properties can be used. In this present work, a low loss Ba(Mg1/3Ta2/3)O3 (BMT) thin buffer layer with varying thickness and its effect on the microwave properties of Ba0.4Sr0.6TiO3 thin films is investigated. Moreover, to overcome the spatial limit in the traditional microwave measurement, a novel technique, evanescent microwave probe (EMP) method, is used to directly probe the microwave dielectric properties of the films. This technique also provides the capability to study the dielectric mechanism in micro-scale region. Pulsed laser deposition technique was used to synthesize thin films. The films shows (111) preferably oriented growth of BST films with the introduction of BMT layer for the films grown on sapphire substrates. As the thickness of BMT increases, this behavior is more obvious. The microwave dielectric constants (ϵ) and dielectric losses (tan δ) of the films grown on sapphire substrates have been measured by EMP. The dielectric constants of BST thin films decrease monotonously with the increase of BMT thickness. In contrast, the tan δ shows a discontinuity variation when the BMT buffer layer is deposited for 10–20 minutes.

Ferroelectric properties of barium strontium titanate thin films grown by RF co‐sputtering

In this work, we present the variation of the ferroelectric properties of Ba1–xSrxTiO3 films deposited on Pt/TiO2/SiO2/Si substrates by RF co-sputtering with 0 ≤ x ≤ 1. The co-sputtering was done using a single magnetron with BaTiO3/SrTiO3 targets in a pie mosaics configuration. Smooth and uniform films were obtained using the same conditions of growth and annealing temperature. The X-ray diffraction and EDS results show that the processes were managed to obtain crystalline materials with x from 0 to 1. The behaviour of P-E loops suggests that the ferroelectric properties of the films were tuned by changing the concentration of the cation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Microstructure of compositionally-graded (Ba1−xSrx)TiO3 thin films epitaxially grown on La0.5Sr0.5CoO3-covered (100) LaAlO3 substrates by pulsed laser deposition

Compositionally-graded (Ba1−xSrx)TiO3 (BST) epitaxial thin films (with x decreasing from 0.25 to 0.0) were deposited by pulsed laser deposition on (100)LaAlO3 (LAO) single-crystal substrates covered with a conductive La0.5Sr0.5CoO3 (LSCO) layer as a bottom electrode. X-ray and electron diffraction patterns demonstrate that the entire graded film has a single-crystal cubic structure. The epitaxial relationship between BST, LSCO, and LAO can be described as (100)BST‖(100)LSCO‖(100)LAO; [001]BST‖[001]LSCO‖[001]LAO. Cross-sectional transmission electron microscopy (TEM) images reveal that both the BST films and the LSCO bottom electrodes have sharp interfaces and overall uniform thickness across the entire specimen, and that they grow with a columnar structure. Planar TEM images show that the graded films exhibit granular and/or polyhedral morphologies with an average grain size of 50nm. High-resolution TEM images reveal aligned rectangular-shaped voids in the graded BST film, with length size of 12–17nm, and width of 5–8nm along the ⟨001⟩ direction in the (100) plane.

Influence of ambient oxygen pressure on the preferred orientation, microstructures, and dielectric properties of (Ba1-xSrx)TiO3 thin films with compositionally graded structures

Compositionally graded (Ba1-xSrx)TiO3 (BST) thin films, with x decreasing from 0.25 to 0.0, were deposited on Pt(111)/Ti/SiO2/Si(100) substrates by pulsed-laser ablation at 600 °C and under ambient oxygen pressures ranging from 50 to 400 mTorr. The influence of the ambient gas pressure on the preferred orientation, microstructures, and dielectric properties of compositionally graded BST films was investigated by X-ray diffraction, scanning electron microscopy, and dielectric frequency spectra, respectively. As the ambient oxygen pressure was increased, the preferred orientation evolved in the order: (100)+(110)→(110)+(111)→ random orientation, and the surface roughness of the graded BST films also increased. The graded BST films deposited at high ambient oxygen pressures (300∼400 mTorr) exhibited a grainy structure with polycrystalline grains throughout the film thickness, whereas the graded films deposited at low ambient oxygen pressures (50∼200 mTorr) possessed a columnar structure. The evolution of the microstructure was ascribed to the different physical and chemical properties of the species that were incident onto the substrates at the various oxygen pressures. The dielectric properties of the graded BST films were dependent upon the ambient oxygen pressures. The graded BST films deposited at 200 mTorr exhibited the highest dielectric constant.

Structural and dielectric properties of epitaxial Ba1−xSrxTiO3 films grown on LaAlO3 substrates by polymer-assisted deposition

Epitaxial Ba1−xSrxTiO3 (BST) thin films with different Ba∕Sr ratio (x=0.1–0.9 with an interval of 0.1) have been grown on (001) LaAlO3 substrates using polymer-assisted deposition. Dielectric measurements show that the films have dielectric properties comparable to the BST films grown by the pulsed laser deposition. Systematic changes in the lattice parameters and dielectric behavior with x values have been measured. The highest dielectric constant (∼1010) and tunability (∼69%) at 1MHz and room temperature have been obtained at x=0.3, which is at the phase boundary of tetragonal and cubic structures.

Characterization of Thin-Film Decoupling and High-Frequency (Ba,Sr)TiO3 Capacitors on Al2O3 Ceramic Substrates

In this paper we present the results of the characterization of parallel-plate thin-film (Ba1-x,Srx)TiO3 (BST) capacitors, to demonstrate their suitability for use as decoupling capacitors (a capacitance as high as 0.34 µF and a capacitance density of up to 70 fF/µm2) and as tunable RF components (a small capacitance from 0.5 pF to 16 pF, a high tunability of 4.22:1 at 10 V and a capacitance density of up to 34 fF/µm2). BST films of different compositions, (Ba0.7Sr0.3)TiO3 and (Ba0.5Sr0.5)TiO3, were grown by metal-organic decomposition (MOD) and RF magnetron reactive sputtering on Pt/TiOx/SiO2/Al2O3 ceramic substrates. For large capacitors (2.25 mm2), capacitance and tan δ were measured at low frequencies (1 kHz) using an LCR meter. Smaller capacitors (16 µm2 to 961 µm2) were characterized in the frequency range of 0.01–20 GHz. Capacitance, tan δ and equivalent series resistance (ESR) were extracted from two port scattering parameters obtained using a vector network analyzer (VNA). The relationships between dielectric loss, tunability and commutation quality factor (CQF) vs BST composition and deposition conditions were outlined.

Frequency and Voltage Dependent Dielectric Properties of Ni-doped Ba0.6Sr0.4TiO3 Thin Films

Highly (100) preferred undoped and 1–5% Ni-doped Ba1−xSr x TiO3 (BST) thin films were deposited onto MgO (100) single crystal substrate at 750°C using pulsed laser deposition. BST thin film-based interdigital capacitors (IDC) were prepared by standard photolithography process. The microwave properties of BST films were measured at 10 GHz. Ni-doped BST films showed better dielectric properties by exhibiting improved dielectric Q while retaining an appropriate capacitance tuning compared to undoped BST films. 1% Ni-doped BST film showed the maximum figure of merit of 2896.1. It is suggested that 1 mol% Ni doped BST film is an effective candidate for high performance tunable device applications.

Characteristics of Ba x Sr 1–x TiO 3 thin films grown by pulsed laser ablation of rotating split targets of BaTiO 3 and SrTiO 3

We have grown heterostructures of BaxSr1-xTiO3(BST)/La0.7Sr0.3MnO3(LSMO) on LaAlO3(LAO) substrates by the pulsed laser deposition method. BST films with x=0.2,0.5,0.7 and 0.8 have been prepared using a novel rotating split-target arrangement. The lattice constant of the BST films is found to vary linearly with Ba/Sr ratio. An excellent cube-on-cube epitaxial relationship of (100)BST||(100)LSMO||(100)LAO has also been obtained. Scanning electron microscopy studies have revealed smooth and crack-free BST films with a uniform grain size of about 100 nm. Dielectric measurements, made with patterned Au top electrodes and conducting LSMO bottom layers, have shown a maximum permittivity of approximately 280 at 5 kHz in BST films with x=0.7. P–E loop analyses of the Ba0.7Sr0.3TiO3 and Ba0.8Sr0.2TiO3 films have yielded remnant polarization values of 1.66 and 1.81μC/cm2, respectively.

High-K BST films deposited on MgO by PLD with and without buffer-layer

Epitaxial thin films of BaxSr1−xTiO3 (BSTO) are grown on (100) MgO single crystal using pulsed laser deposition (PLD). XRD, SEM, and RHEED ‘in situ’ are used to investigate microstructural growth phenomena. Microwave dielectric properties are measured by a resonant cavity method. Correlations between film structure and microwave dielectric properties are studied as function of film thickness and oxygen pressure during deposition. The results show that BSTO films are c-axis (00l) oriented, with a weak (111) disorientation, which appears with increasing the thickness and oxygen pressure and a direct influence of crystallographic structure and stress state of the film on the microwave dielectric properties is pointed out. Dielectric constant values larger than 1000 are obtained for 900 nm thickness weakly disoriented BSTO films and we show that the insertion of an suboxidized BSTO layer as buffer layer at the substrate/film interface, improves the epitaxy and microwave dielectric properties, leading to dielectric constants larger than 2000.

Materials science and integration bases for fabrication of (BaxSr1−x)TiO3 thin film capacitors with layered Cu-based electrodes

The synthesis and fundamental material properties of layered TiAl/Cu/Ta electrodes were investigated to achieve the integration of Cu electrodes with high-dielectric constant (κ) oxide thin films for application to the fabrication of high-frequency devices. The Ta layer is an excellent diffusion barrier to inhibit deleterious Cu diffusion into the Si substrate, while the TiAl layer provides an excellent barrier against oxygen diffusion into the Cu layer to inhibit Cu oxidation during the growth of the high-κ layer in an oxygen atmosphere. Polycrystalline (BaxSr1−x)TiO3 (BST) thin films were grown on the Cu-based bottom electrode by rf magnetron sputtering at temperatures in the range 400–600 °C in oxygen, to investigate the performance of BST/Cu-based capacitors. Characterization of the Cu-based layered structure using surface analytical methods showed that two amorphous oxide layers were formed on both sides of the TiAl barrier, such that the oxide layer on the free surface of the TiAl layer correlates with TiAlOx, while the oxide layer at the TiAl/Cu interface is an Al2O3-rich layer. This double amorphous barrier layer structure effectively prevents oxygen penetration towards the underlying Cu and Ta layers. The TiAlOx interfacial layer, which has a relatively low dielectric constant compared with BST, reduced the total capacitance of the BST thin film capacitors. In addition, the layered electrode-oxide interface roughening observed during the growth of BST films at high temperature, due to copper grain growth, resulted in large dielectric loss on the fabricated BST capacitors. These problems were solved by growing the BST layer at 450 °C followed by a rapid thermal annealing at 700 °C. This process significantly reduced the thickness of the TiAlOx layer and interface roughness resulting in BST capacitors exhibiting properties suitable for the fabrication of high-performance high-frequency devices. In summary, relatively high dielectric constant (280), low dielectric loss (0.007), and low leakage current (<2×10−8 A/cm2 at 100 kV/cm) were achieved for BST thin film capacitors with Cu-based electrodes.

Recent progress of (Ba,Sr)TiO3 thin films for tunable microwave devices

The (Ba1−xSrx)TiO3 (BST) ferroelectric thin films exhibit outstanding dielectric properties, even at high frequencies (>1 GHz), and large, electric-field dielectric tunability. This feature makes them suitable for developing a new class of tunable microwave devices. The dielectric properties and dielectric tuning property of BST thin films are closely related to the film compositions, substrate types, and post-deposition process. The successful implementation of BST films as high-frequency dielectrics in electrically tunable microwave devices requires a detailed understanding of both their processing and material properties. This paper will review the recent progress of BST thin films as active dielectrics for tunable microwave devices. The technical aspects of BST thin films, such as processing methods, post-annealing process, film compositions, film stress, oxygen defects, and interfacial structures between film and substrate, are briefly reviewed and discussed with specific samples from the recent literature. The major issues requiring additional investigations to improve the dielectric properties of BST thin films for tunable microwave applications are also discussed.

Chemical solution deposition of biaxially oriented (Ba,Sr)TiO3 thin films on 〈100〉 Ni

Biaxially oriented Ba1−xSrxTiO3 (BST) thin films were fabricated under a highly reducing atmosphere (oxygen partial pressure ∼10−18 atm) on base metal substrates (〈100〉 Ni) using a fluorinated chemical solution deposition method. The degree of film orientation was investigated with respect to film annealing temperature and composition (x = 0, 0.33, 0.5, 0.67, 1). The solution synthesis route included fluorinated solvents, donor-dopants, and chelating agents for control of orientation, defect chemistry, and morphology. Free-energy and phase diagrams guided solution development using halide addition to avoid intermediate BaCO3 formation and instead produce direct crystallization of BaTiO3-type materials from BaF2. The degree of (200) orientation of BST films deposited on 〈100〉 Ni substrates displayed a compositional dependence for 0 < x < 1, with a maximum orientation Lotgering factor approaching unity. Low-temperature (500 °C) crystallization of highly oriented films was demonstrated for solution-derived BST on Ni, which may have enabling materials integration implications for capacitors and other applications.

Orientation dependent microwave dielectric properties of ferroelectric Ba1−xSrxTiO3 thin films

The effects of anisotropic dielectric properties of ferroelectric Ba1−xSrxTiO3 (BST) films on the characteristics of the interdigital (IDT) capacitors have been studied in microwave regions at room temperature. Ferroelectric BST films with (001), (011), and (111) orientation were epitaxially grown on (001), (011), and (111) MgO substrates, respectively, by the pulsed laser deposition method. The microwave properties of orientation engineered BST films were investigated using interdigital capacitors. The calculated dielectric constant tunability with 40 V dc bias variation and the calculated dielectric quality factor values for IDT capacitors based on (001), (011), and (111) oriented BST films at 9 GHz with no dc bias were about 47%, 55%, 43%, and 12, 14, 21, respectively.

Preparation of epitaxial compositionally graded (Ba1-xSrx)TiO3 thin films with enhanced dielectric properties

Epitaxial compositionally graded (Ba1-xSrx)TiO3 (BST) (0.0≤x≤0.25) thin films were deposited on (100) LaAlO3 substrates by pulsed laser ablation, the substrates having bottom electrodes made of 100-nm-thick conductive La0.5Sr0.5CoO3 (LSCO). Extensive X-ray diffraction, rocking-curve, and φ-scan studies indicate that the graded films are (100)-oriented and exhibit good in-plane relationships of [010]BST//[010]LAO and [001]BST//[001]LAO. For the up-graded films with barium concentration (1-x) increasing across the film thickness in the direction from the film/substrate interface to the film surface, the full width at half maximum of the BST film (200) rocking curve and the surface roughness, examined by atomic force microscopy, were larger than those of the down-graded films with barium concentration decreasing from the film/substrate interface to the film surface. The dielectric properties of the graded films, measured using vertical structures, show that at room temperature, the dielectric constant (er) and dissipation factor (cosδ) at 100 kHz were 380 and 0.013 for the up-graded films, and 650 and 0.010 for the down-graded films, respectively. The dielectric behavior was enhanced in the down-graded films, which was attributed to the fact that the pure BaTiO3 layer in the down-graded BST films not only serves as a bottom layer but also acts as an excellent seeding layer for enhancing subsequent film growth, leading to better film crystallinity and larger grain sizes in the down-graded films. The graded BST films undergo a diffuse phase transition, giving a broad, flat capacitance-versus-temperature profile. With such a graded structure, it is possible to build a dielectric thin-film capacitor having a capacitance which has a low temperature dependence over a broad temperature regime.

Improved Performance in BST-Based Tunable Circuits Employing Low-Loss Nontunable Dielectric

BaxSr1 − xTiO3 (BST) materials are being developed for tunable microwave applications. A figure of merit “K” defined as K = is frequently used to evaluate tunable dielectrics. Based on a simple equivalent-circuit model, multilayer structures consisting of BST films and low-loss non-tunable dielectric should have a higher K than BST alone. The maximum improvement achievable by this approach is predicted to increase with tunability of the BST material. For a BST film with 80% tunability and maximum loss tangent of 0.07, such as those typically deposited by pulsed laser deposition, the maximum predicted improvement is close to a factor of five for K. A four-electrode chip capacitor was fabricated for experimental validation of the modeling predictions. A 25% increase in K was demonstrated for a multilayer four-electrode capacitor using low tuning voltages. An alternative figure of merit the Commutation Quality Factor (CQF) was also examined, it is reported to be invariable with the respect to series and parallel secondary low loss inclusions and is therefore potentially more universal quality parameter for characterization of tunable dielectrics than the widely-used K. We present experimental results for a multilayer structure and evaluate its impact on the two alternative figures of merits.