Ba0.6Sr0.4TiO3 Films Research Articles

Improvement of dielectric properties of Ba0.6Sr0.4TiO3 thin films by MgO doping

MgO-doped Ba0.6Sr0.4TiO3 (BST) thin films were synthesized by rf magnetron sputtering at substrate temperature of 750°C using single-phase targets with different MgO contents ranging from 0to5mol%. Microstructure, dielectric constant, and leakage current density of the MgO-doped BST films were characterized to understand the influence of the MgO dopant on film properties. Polycrystalline and perovskite phase solid solution films with a dense microstructure were obtained in all deposition conditions. The thickness of the 0,2,and5mol% MgO-doped BST films was measured to be 190, 140, and 150nm, respectively. The electrical and dielectric properties of the BST-containing capacitors are both found to be improved significantly by doping MgO in the BST films. The voltage-dependent capacitance C-V increases with increasing MgO doping up to 5mol%. The evidence for the asymmetric distribution of charge carriers in the MgO–BST film is derived from the interfacial layer. The leakage current density of the 5mol% MgO-doped BST capacitors is nearly two orders of magnitude lower and the dielectric constant is about 45% times higher than that of the undoped BST capacitors. Correlation of the material properties with dielectric properties suggests that the 5mol% MgO-doped BST films are the optimal choice for applications.

Structural Features and Microwave Properties of Ba<sub>0.5</sub>Sr<sub>0.5</sub>TiO<sub>3</sub> Films Grown on Sapphire Substrates

The change in dielectric constant of ferroelectric materials as a function of electric field is the key to wide range of microwave application such as tunable filter, impedance matching network, and phase shifter. In this study, ferroelectric Ba0.5Sr0.5TiO3 (BST) films were grown on r-cut sapphire and polycrystalline sapphire (poly-sapphire) substrates by RF sputtering. The results of comprehensive structural diagnostics of the films are correlated with the dielectric constant and dielectric loss of a co-planar BST varactor, measured at a frequency range of 1~3 GHz. Textured BST films approximately 500 nm thick, grown on r-cut sapphire substrates, are characterized by high dielectric constant ≥ 650. However, polycrystalline BST films, grown on poly-sapphire substrates, are less strained, having dielectric constant range of 430 ~ 640.

Preferred Orientation Growth of Ba<sub>0.6</sub>Sr<sub>0.4</sub>TiO<sub>3</sub> Thin Films with Epitaxial MgO Buffer Layer on Si(100) Substrate

Epitaxial MgO thin films were deposited on Si(100) substrate by atmospheric-pressure metalorganic chemical vapor deposition for using as buffer layers. Ba0.6Sr0.4TiO3 thin films were prepared on MgO/Si(100) substrate by sol-gel technique. The independence of crystallinity on annealing temperature was investigated. The Ba0.6Sr0.4TiO3 (BST) thin films are crystallized in preferential (100) orientation after post-deposition annealing at 850°C and 950°C for 2h in air, respectively. Rutherford backscattering spectroscopy analysis confirmed that both the BST and MgO films have stoichiometric composition.

Transient Response in Monolithic Mach–Zehnder Optical Modulator Using (Ba,Sr)TiO3 Film Sputtered at Low Temperature on Silicon

We have fabricated Mach–Zhender interferometers (MZIs) using the (Ba,Sr)TiO3 (BST) film sputter-deposited at 450 °C, which is a critical temperature for the process after metallization. An optical modulation of about 10% is achieved when 200 V is applied (electric field in BST is 1.2×104 V/cm). However, the response time of optical modulation to step function voltage is slow (1.0–6.3 s). We propose a model for the slow transient behavior based on movable ions and a long dielectric relaxation time for the BST film, and good qualitative agreement is obtained with experimental results.

The Interfacial Characteristics of Ba<sub>0.6</sub>Sr<sub>0.4</sub>TiO<sub>3</sub> Films Deposited by Radio Frequency Magnetron Sputtering

Ba0.6Sr0.4TiO3 (BST) thin films deposited on Pt/Ti/SiO2/Si substrates by radio frequency magnetron sputtering and crystallized by rapid thermal annealing (RTA) exhibit much thinner BST/Pt interfacial transition layer and higher dielectric properties than the films crystallized by conventional thermal annealing (CTA). HRTEM observations show that the transition layer is 2-3nm thick for RTA and 4-5nm thick for CTA. XPS investigations display that the transition layer is composed of perovskited BST phase and non-perovskited BST phase, and RTA corresponds to much less non-perovskited BST phase than CTA. The reason for non-perovskited BST phase and the dielectric properties of BST films are also presented.

Fabrication and characterization of epitaxial Ba0.6Sr0.4TiO3/LaNiO3 heterostructures

Epitaxial Ba0.6Sr0.4TiO3 (BST)/LaNiO3 (LNO) heterostructures were fabricated on LAO (100) substrates using pulsed laser deposition (PLD). Their structural properties were investigated by X-ray diffraction (XRD). The θ–2θ scans showed single crystalline BST and LNO layers with a (100) orientations perpendicular to the substrate plane. Phi scans (ϕ) on the (220) plane of BST layer indicated that the films have two in-plane orientations with respect to the substrate. The atomic force microscope (AFM) surface morphologies showed a smooth and crack-free surface with the average grain size of 55 nm and the root-mean-square (RMS) of 4.53 nm for BST films. Capacitance–voltage curves are measured. From the capacitance, a dielectric constant of 762, tunabilty of 82.81% and loss tangent of 0.032 are obtained. The current–voltage curve shows that the leakage current is 2.41 × 10−7 A/cm2 under an applied voltage of 2 V.

Structural and electro-optic properties of Ba0.7Sr0.3TiO3 thin films grown on various substrates using pulsed laser deposition

Epitaxial Ba0.7Sr0.3TiO3 (BST) thin films were deposited on various single crystal substrates, including LaAlO3 (LAO) (001), (LaAlO3)0.3(Sr2AlTaO6)0.35 (LSAT) (001), and SrTiO3 (STO) (001), using pulsed laser deposition in order to study their structural and electro-optic properties. All the films exhibit a good crystalline quality and a pure perovskite phase with a distorted lattice structure. The in-plane temperature dependence of relative permittivity of the films was measured on interdigital electrodes. The films grown on LAO and LSAT exhibited an obvious room-temperature ferroelectric state, while the film grown on STO showed a broad phase transition peak near room temperature. Correspondingly, large linear electro-optic effects were observed in the BST films grown on LAO and LSAT in a transverse geometry at a wavelength of 632.8nm using a modified Sénarmont method. The linear electro-optic coefficient rcof the BST films grown on LAO and LSAT was found to be 82.7×10−12 and 125.0×10−12m∕V, respectively. Nevertheless, a predominantly quadratic and slightly asymmetric electro-optic behavior is observed for the film grown on STO with the quadratic electro-optic coefficient Rc of 12.9×10−18m2∕V2.

Low frequency and microwave performances of Ba0.6Sr0.4TiO3 films on atomic layer deposited TiO2/high resistivity Si substrates

We report on high tunablity of Ba0.6Sr0.4TiO3 (BST) thin films realized through the use of atomic layer deposited TiO2 films as a microwave buffer layer between BST and a high resistivity (HR) Si substrate. Coplanar waveguide (CPW) meander-line phase shifters using BST/TiO2/HR-Si and BST/MgO structures exhibited a differential phase shift of 95∘ and 24.4∘, respectively, at 15 GHz under an electric field of 10 kV/cm. The figure of merit of the phase shifters at 15 GHz was 30.6∘/dB for BST film grown on a TiO2/HR-Si substrate and 12.2∘/dB for BST film grown on a MgO single crystal substrate. These results constitute significant progress in integrating BST films with conventional silicon technology.

STRESS CONTROLLED FERROELECTRIC (BA0.5Sr0.5)TiO3 THIN FILMS PREPARED ON GD2O3/SRTiO3(001) BY A PULSED LASER DEPOSITION

ABSTRACT Stress controlled epitaxial ferroelectric Ba0.5Sr0.5TiO3 (BST) films have been deposited on Gd2O3/SrTiO3 by pulsed laser deposition with oxygen background pressure of 200 mTorr at the deposition temperature of 750°C. In order to control the stress in BST films, oxygen pressures for Gd2O3 buffer layers have been varied from 0.1 to 100 mTorr, while that of BST films have been fixed at 200 mTorr. It has been found that the lattice parameters of the BST films deposited on Gd2O3 were changed. Furthermore, microwave properties of co-planar waveguide (CPW) fabricated on BST films were investigated by a HP 8510C vector network analyzer from 1–20 GHz. Large dielectric tunabilities were observed from the CPW's fabricated on BST films deposited on Gd2O3 layers deposited at low and high oxygen pressures, 0.1 and 100 mTorr, respectively.

Orientation effect on microwave dielectric properties of Si-integrated Ba0.6Sr0.4TiO3 thin films for frequency agile devices

The effect of texture with (100) and (110) preferred orientations on dielectric properties of Ba0.6Sr0.4TiO3 (BST) thin films grown on SrO (9nm) and CeO2 (70nm) buffered Si substrates, respectively, was investigated. The coplanar waveguide (CPW) phase shifter using (100) oriented BST films on SrO buffered Si exhibited a much-enhanced figure of merit of 24.7°∕dB, as compared to that (10.2°∕dB) of a CPW phase shifter using (110) oriented BST films on CeO2 buffered Si at 12GHz. This work demonstrates that the microwave properties of the Si-integrated BST thin films are highly correlated with crystal orientation.

Simulation of leakage characteristics in epitaxial (Ba, Sr)TiO3 thin film after hydrogen-containing forming gas anneal

A leakage current model that includes space charge effects is developed for (Ba,Sr)TiO3 (BST) thin films. Numerical simulation using this model has been performed to investigate the leakage mechanism in an epitaxial BST thin film after a hydrogen-containing forming gas anneal. The present work suggests that before the forming gas anneal the leakage mechanism in the BST film is probably the Schottky emission whereas after the forming gas anneal the leakage characteristics can be described by the modified Schottky equation of Simmons with effective barrier heights obtained from the model simulation.

Calculation of the intrinsic dead layers thicknesses from Au/Ba0.5Sr0.5TiO3/Pt thin film capacitors

Ferroelectric Ba0.5Sr0.5TiO3 (BST) films were prepared on Pt/Ti/SiO2/Si substrates by the sol-gel process. The films were spin-coated at 2000 rpm for 30 secs and then pyrolysed for 5 mins at the temperature of 350∘C. This coating procedure was repeated for 3, 4, 5 and 6 times to obtain BST films with different thicknesses. After coating the films with the desired repetition times, the films were finally annealed in a conventional furnace at temperatures ranging from 600∘C to 800∘C with a 50∘C interval in between. The films obtained with an annealing procedure of 750∘C were polycrystalline with the presence of an impurity BaCO3 phase. The capacitance and leakage current were measured and used to extract information on the metal-BST interface. With the series capacitance model and modified Schottky emission equation, the thickness of the dead layers for Au/BST and Pt/BST interfaces were calculated to be less than 6 nm and 5 nm, respectively.

Dielectric properties of Ba0.6Sr0.4TiO3 thin films deposited by mist plasma evaporation using aqueous solution precursor

Mist plasma evaporation (MPE) technique has been developed to deposit Ba0.6Sr0.4TiO3 (BST) thin films on SiO2∕Si and Pt∕Ti∕SiO2∕Si substrates at atmospheric pressure using metal nitrate aqueous solution as precursor. MPE is characterized by the injection of liquid reactants into thermal plasma where the source materials in the droplets are evaporated by the high temperature of the thermal plasma. Nanometer-scale clusters are formed in the tail flame of the plasma, and then deposited and rearranged on the substrate at a lower temperature. Due to the high temperature annealing process of the thermal plasma before deposition, well-crystallized BST films were deposited at substrate temperature of 630°C. The dielectric constant and dielectric loss of the film at 100kHz are 715 and 0.24, respectively. Due to the good crystallinity of the BST films deposited by MPE, high dielectric tunability up to 39.3% is achieved at low applied electric field of 100kVcm−1.

Repairing of Etching-Induced Damage of High-k Ba0.5Sr0.5TiO3 Thin Films by Oxygen Surface Plasma Treatment

Ba0.5Sr0.5TiO3 (BST) thin films were patterned for fabricating BST capacitors in a helicon-wave plasma system. The optimization conditions were an Ar (80%)/Cl2 (20%) gas mixture with a helicon-wave plasma power and a substrate bias rf power of 1500 and 90 W, respectively. From results of X-ray photoelectron spectroscopy, physical ion bombardment is more effective than chemical reaction for removing Sr, while Ti can be removed by the formation of volatile TiClx. Ba was primarily removed by chemically assisted physical etching (such as that using BaClx). Some etching residues consisting of Ba and Sr were found after the BST films were etched and increased leakage current density. Oxygen surface plasma treatment can effectively repair surface damage caused by etching, and it reduced the leakage current density of the BST capacitor from 4.0×10-7 to 3.0×10-8 A/cm2 at 1 MV/cm and increased the breakdown field to ∼2 MV/cm at 1.0×10-6 A/cm2.

Influence of oxygen pressure on the structural and dielectric properties of laser-ablated Ba0.5Sr0.5TiO3 thin films epitaxially grown on (001) MgO for microwave phase shifters

Highly oriented Ba0.5Sr0.5TiO3 (BST) thin films were grown on MgO (001) single-crystal substrates by pulsed-laser deposition at 800 °C in oxygen pressure ranging from 1.2 × 10−3 to 40 Pa. A strong correlation was observed between the growth process, structure and dielectric properties for the BST films. The dielectric properties in the low frequency range 10 k–1 MHz were measured in the interdigital capacitor configuration. The tetragonal distortion (ratio of in-plane and surface normal lattice parameters, D = a/c), surface morphology and dielectric response of the films are strongly dependent on the oxygen deposition pressure. With increase in oxygen pressure, the in-plane strain for the BST films changes from compressive to tensile. The BST film grown at 25 Pa exhibits the best overall dielectric properties. This corresponds to the film with a slight in-plane tensile strain (D = 1.0012). It is believed that a reasonable tensile strain, which increases the ionic displacement and thus promotes the in-plane polarization in the field direction, could enhance the tunability and dielectric constant. Based on the BST film grown at 25 Pa, distributed phase shifters were successfully fabricated with promising performance. The phase shifter shows a relatively low insertion loss of about 3.5 dB at zero bias and 10 GHz, a good return loss better than −15 dB for all phase states from dc to 16 GHz and a differential phase shift of about 43° with 120 V dc bias at 10 GHz.

The origin of the weak ferroelectric-like hysteresis effect in paraelectricBa0.5Sr0.5TiO3 thin films grownepitaxially on LaAlO3

Perovskite Ba0.5Sr0.5TiO3 (BST) thin films, with thickness of about 350 nm, have been epitaxially grown on (001)LaAlO3 substrates by pulsed-laser deposition. The good crystallography and epitaxy characteristicswere confirmed using x-ray diffraction and transmission electron microscopy (TEM). Thedielectric properties of the BST thin films were measured with a planar capacitorconfiguration in the temperature range of 77–300 K. The capacitance–temperaturecharacteristics, measured with no and several different DC biases, reveal thatthe BST films are in the paraelectric state at room temperature, with a verygood dielectric tunability. However, a butterfly-shaped curve, typical for aferroelectric material, was obtained from room temperature capacitance–voltage(C–V) measurements, suggesting a faint ferroelectric-like effect for the BST thin films. Careful dielectricproperty characterizations showed significant temperature and frequency dependence, probablyindicating a behaviour of Maxwell–Wagner-type dielectric relaxation. As a result, the weakC–V hysteresis effect in our paraelectric BST thin films is believed to be ascribable both tooxygen vacancies and to the presence of other space charges, trapped at the grainboundaries and/or at the substrate/dielectric film interface, which give rise tolocal polar regions in the thin-film samples. Furthermore, this explanation issupported by cross-sectional TEM and off-axis electron holographic observations.

Structural and Optical Properties of Electro-Optic Material: Sputtered (Ba,Sr)TiO3

In order to develop a novel ring resonator optical switch, we have studied the structural and optical properties of the electro-optic material (Ba,Sr)TiO3 (BST) deposited by RF sputtering on a SiO2 cladding layer (1.0 µm). The crystallinity of the BST films is evaluated by X-ray diffraction and the optical propagation loss of the waveguides is measured using a He–Ne laser. As a result, it is found that there is a strong relationship between the optical propagation loss and crystallinity of the sputtered film. It is suggested that the propagating light is influenced by the crystal property, for example, the grain size and density of the polycrystalline BST film.

Influence of annealing temperature on the band structure of sol-gel Ba0.65Sr0.35TiO3 thin films on n-type Si(100)

The annealing temperature effect on the band structure of Ba0.65Sr0.35TiO3 (BST) thin films coated on n-type Si(100) substrate was investigated by ellipsometry and x-ray photoelectron spectroscopy. The band energy shifts of sol-gel BST films annealed at different temperatures are dependent on their developed microstructure. Related to the amorphous BST films annealed at 600°C, the polycrystalline BST film annealed at 700°C exhibits narrow band gap, upwards-moved Fermi level, and downwards-moved conduction band minimum, which are believed to contribute the enhanced field emission of BST-coated Si field emitter arrays.

Integration of coplanar (Ba,Sr)TiO/sub 3/ microwave phase shifters onto Si wafers TiO/sub 2/ buffer layers

In this paper, a Ba0.6Sr0.4TiO3 (BST) tunable phase shifter with TiO2 films as microwave buffer layer between BST and silicon (Si) substrates is presented. The TiO2 buffer layer is grown by atomic layer deposition (ALD) onto Si substrate followed by pulsed laser deposition (PLD) of BST thin films onto the TiO2 buffer layer. The phase shifter fabricated on BST films grown on TiO2/Si substrate shows a good figure of merit (FOM) of 75.4 degrees/dB by exhibiting improved tunablity while retaining an appropriate dielectric Q as compared to 55.1 degrees/dB of BST/MgO structure. The TiO2 buffer layer grown by ALD enables successful integration of BST-based microwave tunable devices with high resistive Si wafer.

Ce-doping effects on electronic structures of Ba0.5Sr0.5TiO3 thin film

In order to clarify the basic reason why Ce doping can dramatically decrease the leakage current in Ba0.5Sr0.5TiO3 (BST) as reported in our previous work (Wang et al 2005 J. Phys. D: Appl. Phys. 38 2253), we have employed x-ray photoelectron spectroscopy (XPS) and the optical transmittance technique to study the electronic structure of undoped and 1.0 at% Ce-doped BST (CeBST) films fabricated by pulsed laser deposition. XPS results show that Ce doping has a strong influence on the valence band and core levels of BST films, and that the Fermi level is lowered by about 0.35 eV by Ce doping. Optical transmittance measurements demonstrate that the energy gap is expanded with Ce doping. These Ce-doping effects can induce an increase in the barrier height for the thermionic emission and eventually reduce leakage current in CeBST thin films.