Ba0.6Sr0.4TiO3 Thin Films Research Articles

Optimized Process of Mn2+-Doped Ba0.5Sr0.5TiO3Thin Films on Platinum Coated Sapphire Substrates

Mn2+-doped Ba0.5Sr0.5TiO3 (BST) thin films were prepared on Pt-coated sapphire substrates by RF magnetron sputtering. By analyzing the energy state of particles during the sputtering, deposition parameters (substrate temperature, sputtering power, and atmosphere) were optimized for superior dielectric properties. Our results indicated that a compromise of relatively high tunability and low loss could be achieved. Resultant BST thin films has a tunability of 50% and dielectric loss of 0.64% at an applied field of 1.2 MV/cm, under optimized sputtering conditions of substrate temperature at 750 °C, pressure at 4 Pa with an O2/(O2+ Ar) mixing ratio of 50%, and sputter RF power density at 6.8 W/cm2.

Optimized Process of Mn2+-Doped Ba0.5Sr0.5TiO3 Thin Films on Platinum Coated Sapphire Substrates

Mn2+-doped Ba0.5Sr0.5TiO3 (BST) thin films were prepared on Pt-coated sapphire substrates by RF magnetron sputtering. By analyzing the energy state of particles during the sputtering, deposition parameters (substrate temperature, sputtering power, and atmosphere) were optimized for superior dielectric properties. Our results indicated that a compromise of relatively high tunability and low loss could be achieved. Resultant BST thin films has a tunability of 50% and dielectric loss of 0.64% at an applied field of 1.2 MV/cm, under optimized sputtering conditions of substrate temperature at 750 °C, pressure at 4 Pa with an O2/(O2+ Ar) mixing ratio of 50%, and sputter RF power density at 6.8 W/cm2.

Ionic Polarization Occurrence in BaSrTiO3 Thin Film by THz-Time Domain Spectroscopy

BaSrTiO3 thin films in a paraelectric phase were characterized on the one hand from 1 GHz to 200 GHz by microwave measurements on interdigitated capacitance and coplanar waveguides and on the other hand up to 3 THz by Time Domain Spectroscopy. An overlap of the polar nanoregion relaxation mechanism with a characteristic frequency around 800 GHz and of the ionic polarization with a relaxation frequency around 3 THz is directly evidenced experimentally.

ホイスラー/ペロブスカイト積層構造のマルチフェロイック特性

Layered structures composed of ferromagnetic, half-metallic Heusler Fe2CrSi (FCS) and ferroelectric perovskite Ba0.7Sr0.3TiO3 (BSTO) have been grown on LaAlO3 (001) (LAO) substrates by magnetron sputtering. Their magnetic and ferroelectric properties have been investigated. BSTO/FCS bilayer structures are epitaxially grown on LaAlO3 substrates with BSTO (001)[100]//FCS (001)[110] relationship. The remanent polarization of 40 μC/cm2 was observed at RT in a single-layer BSTO thin film. The layered structure exhibited saturation magnetization of 400 emu/cc at RT, which is equivalent to the bulk one. Therefore, the BSTO/FCS epitaxial heterostructure is a promising candidate for fabricating multiferroic devices.

Three-Step Deposition Method for Improvement of the Dielectric Properties of BST Thin Films

ABSTRACTEpitaixal Ba0.5Sr0.5TiO3 (BST) thin films were grown on SrTiO3 (STO) and DyScO3 substrates by radio-frequency magnetron sputtering system using three-step method which involves a relatively low-temperature (573-773 K) growth of a BST interlayer sandwiched between two BST layers deposited at a high substrate temperature of 1068 K. X-ray diffraction measurement showed different strains on the films with interlayers grown at different temperatures. Post-growth thermal treatment reduced film strain to a great extent (the film strain of a tri-layer film with a 773 K grown interlayer is only -0.001). Comparing with the control films grown at high temperature, three-step technique improved the dielectric properties, especially increased dielectric constant by 60% for BST/STO and 31% for BST/DyScO3, respectively. High dielectric constant of 1631.4 and its tuning of 36.7% were achieved on the BST/STO with an interlayer grown on 773 K.

Influence of oxygen pressure on the lattice constants of Ba0.6Sr0.4TiO3 thin films and preparation of BaTiO3/Ba0.6Sr0.4TiO3 superlattices

Using pulsed laser deposition (PLD) technology, BaTiO3 (BT) and Ba0.6Sr0.4TiO3 (BST) single-layer films were successfully prepared through a series of optimization experiments. X-ray diffraction investigations indicated that BT and BST thin films were nearly (001)-epitaxially grown on LaAlO3 (001) substrates. And functional relation between the lattice constants of BST thin films and the oxygen pressure was close to Boltzmann function from 10-3 to 25 Pa. Then the high-quality BT/BST superlattices were also prepared under these optimal conditions. The surface topography measurement by atomic force microscopy (AFM) demonstrated that the root-mean-square (RMS) roughness of BT/BST superlattice was only 0.3048 nm. The surface smoothness has achieved the atomic scale. In addition, satellite peaks were clearly observed in the XRD spectrum. According to the position of satellite peaks that belong to (002) diffraction peak, the superperiod of superlattices was calculated and the deposition rates of BT and BST thin films were deduced. Finally, laser induced thermoelectric voltage (LITV) effect was measured for the first time in BT/BST superlattices grown on vicinal-cut single crystal substrates, which was not found in BT and BST single-layer films.

Dielectric dispersion of BaSrTiO3 thin film from centimeter to submillimeter wavelengths

The dielectric dispersion of ferroelectric BaxSr1−xTiO3 (BST) thin film in a paraelectric phase was characterized from centimeter to submillimeter wavelengths. To this aim, interdigitated capacitors were patterned on a micrometer scale onto a BST layer with a barium concentration of 0.5 and were subsequently integrated by using a coplanar waveguide technology. The retrieval of the complex permittivity of BST was performed by vectorial scattering parameter measurements up to 190 GHz for various controlling dc field up to 300 kV/cm. At higher frequency, submillimeter wavelength measurements were performed by time domain spectroscopy under free space condition. On this basis, the dispersion of the real part of the permittivity along with the loss tangent are retrieved in agreement with a distributed interaction of ac-field with soft phonons vibration modes, and overlapping between dipole polarization and ionic polarization is observed, around 700 GHz. It is also shown that dipole polarization can be attributed to the presence of small polar nanoregions in the BST film which act as in ferroelectric material with diffuse phase transition.

Effects of HfO2 buffer layers on the dielectric property and leakage current of Ba0.6Sr0.4TiO3 thin films by pulsed laser deposition

Ba0.6Sr0.4TiO3 (BST) thin films with and without HfO2 buffer layer were fabricated on Pt/Ti/SiO2/Si substrates by pulsed laser deposition. Dependences of HfO2 thickness on the dielectric property and leakage current of BST thin films were focused. The dielectric constant of BST thin films increased and then decreased with the increase of HfO2 thickness, while the dielectric relaxation was gradually improved. The loss tangent and leakage current under positive bias decreased with the HfO2 thickness increasing. The leakage current analysis based on the Schottky emission indicated an improvement of the BST/Pt interface with HfO2 buffer layer. The loss tangent, tunability and figure of merit of optimized HfO2 buffered BST thin film achieved 0.009 8, 21.91% (E max= 200 kV/cm), 22.40 at 106 Hz, respectively.

Parallel Plate Varactors Employing Bi1.5Zn1.0Nb1.5O7/Ba0.5Sr0.5TiO3 Bilayered Thin Films

Parallel plate capacitors based on Bi1.5Zn1.0Nb1.5O7 (BZN) / Ba0.5Sr0.5TiO3 (BST) bilayered thin films were fabricated on sapphire substrates. The BZN/BST bilayered thin films were deposited by radio frequency magnetron sputtering, from two individual Bi1.5Zn1.0Nb1.5O7 and Ba0.5Sr0.5TiO3 stoichiometric ceramic targets. Dielectric measurements indicated that the capacitors exhibited device quality-factor of up to ∼ 200, with a relative tunability of ∼ 40% at a bias field of 1.6 MV/cm.The figure of merit was about 80. Compared with the parallel plate capacitors with single BST thin film configuration, the leakage current of the BZN/BST capacitors was observed to be much lower. It was also found that the capacitance was frequency independent in the range from 100 kHz to 100 MHz, while the device quality-factor dropped down as frequency increasing. The decrease of device quality-factor accompanied with frequency independent capacitance could be attributed to the conductor loss of electrodes.

A Phased Array Antenna Using Ferroelectric CPW Phase Shifter Active Modules

A phased array antenna using ferroelectric coplanar waveguide (CPW) phase shifter active modules is presented based on a Ba0.6Sr0.4TiO3 (BST) thin film. The ferroelectric BST thin film was deposited on an MgO substrate by using pulsed laser deposition. The active module amplifies an RF signal with low noise and changes the phase of a received RF signal to control the main beam of the array antenna. It is composed of low noise amplifier (LNA), dc block, and ferroelectric CPW phase shifter. The prototype of 6×12 phased array patch antenna with the active modules using a ferroelectric phase shifter is proposed. The proposed array antenna continuously scanned its main beam in elevation plane to 32° from its broadside direction at 11.85 GHz by applying the continuous dc voltage to the ferroelectric phase shifter.

Effect of Interlayer Thickness on Stress and Dielectric Properties of MgTiO3 Modified (Ba,Sr)TiO<sub>3</sub> Multilayer Thin Films

Ba0.6Sr0.4TiO3 (BST)/MgTiO3 (MT)/ Ba0.6Sr0.4TiO3 multilayer thin films were deposited on LaNiO3 (100)/Pt/Ti/SiO2/Si (100) substrates by pulsed laser deposition. It was found that the film orientation and dielectric properties of BST/MT/BST multilayer thin films are strongly dependent on MT interlayer thickness. Pure BST thin film exhibits a (100) preferred orientation, while BST thin films with a MT interlayer exhibit a random orientation. Residual stress is relaxed dramatically due to a closer match of thermal expansion coefficients between the BST and MT interlayers. The largest figure of merit of 18.7 is achieved in the multilayer thin film with a 50-nm-thick MT interlayer, which exhibits a tunability of 30% and a lost tangent of 0.016. Dielectric constant and loss tangent decrease with increasing MT interlayer thickness due to a series dielectric dilution effect.

MICROWAVE-INDUCED NANOCRYSTAL LINES ON THE Ba0.6Sr0.4TiO3 THIN FILMS SURFACE

ABSTRACT Ba0.6Sr0.4TiO3 (BST) thin films were deposited on Pt/Ti/SiO2/Si (100) substrates by radio-frequency magnetron sputtering. Nanocrystal lines on the BST films surface were obtained by microwave electrical field irradiation annealing. Atomic force microscopy (AFM) results reveal that microwave annealing can give rise to certain self-assembled nanocrystal lines on the BST films surface, and the length and the width of the nanocrystal lines are dependent on annealing time. After the films further annealed for 20 min or more, the nanocrystal lines broke up into spherical nanocrystals of about 25 nm in diameter. The directions of BST nanocrystal lines are determined by microwave electrical field. X-ray diffraction (XRD) shows that the microwave-annealed BST films are (110) preferred orientation. Specular X-ray reflectivity (XRR) measurement shows that the surfaces of the microwave-annealed BST thin films exhibit anisotropy.

THICKNESS DEPENDENT PROPERTIES OF BARIUM STRONTIUM TITANATE THIN-FILM VARACTORS

ABSTRACT Ba0.6Sr0.4TiO3 (BST) thin-films with large dielectric tunability as high as 5.8:1 were obtained using a large area pulsed laser deposition process, with low loss-tangents below 0.01 at zero-bias and 10 GHz. This paper summarizes experimental results obtained on large area processed BST thin film varactors. Varactors with 0.6 μm thick BST films exhibited large dielectric tunability, the relative dielectric permittivity at zero bias of 1280 tuned to 220 at 20 V, at an electric field of 333 kV/cm. The leakage current through the BST film was below 1 nA up to 10 V dc bias. The Quality factor (Q) exceeded 500 at relatively low 10 V dc bias for both 0.6 and 1 μm thick BST varactors.

Effects of Sol Viscosity on Properties of BST Thin Films Prepared by Sol-Gel Method

A series of BST(Ba0.65Sr0.35TiO3) sol with different viscosity were prepared by using Ba(CH3COOH)2, Sr(CH3COOH)2 and Ti(OC4H9)4 as raw materials. The BST thin films were fabricated by sol-gel method and spin-coating process on Si(100) substrates. The effect of sol viscosity on crystallization, microstructure and dielectric properties of BST thin films were analyzed by using X-ray diffractometry, scanning electron microscopy and impedance phase analyzer. The results show that the BST films coated by sol with viscosity of 3.0cp and annealed at 750°C for 1h are basically non-crystalline. The crystallization degree increases with the increase of sol viscosity at the same annealing temperature. The ABO3 perovskite structure is formed when the sol viscosity increases to 3.5cp. With further increasing sol viscosity to 6.0cp, the crystal grains of the film grow well and range very compact, the surface become smooth, and the films have optimal dielectric properties.

Dielectric Tunable Properties of Ba<sub>0.6</sub>Sr<sub>0.4</sub>TiO<sub>3</sub> Thin Films with and without LSCO Buffer Layer

Barium strontium titanate Ba0.6Sr0.4TiO3 (BST) thin films were fabricated by pulsed laser deposition (PLD) technique on Pt/TiO2/SiO2/Si substrate with and without high (100) oriented LSCO buffer layer. Crystal structure and surface morphology of BST thin films with and without LSCO buffer layer were characterized by X-ray diffraction (XRD) and atom force microscope (AFM). The dielectric measurements were conducted on metal-insulator-metal capacitors at the frequency from 100 Hz to 1M Hz and at room temperature. It was found that the LSCO buffer layer was beneficial for BST films to decrease surface roughness, dielectric loss and to increase grain size, dielectric constant, tunability and figure of merit (FOM). The influence of LSCO buffer layer on the microstructure, dielectric and tunable properties of BST thin films were analyzed. At 1M Hz, the dielectric constants of BST films with and without LSCO buffer layer are 684 and 592, respectively. The tunability of BST thin film with LSCO buffer layer was about 65.48%, which was higher than that (about 41.84%) of BST thin film without LSCO buffer layer.

Crystallization and electrical properties of Ba0.7Sr0.3TiO3 thin films on SrRuO3/Pt hybrid bottom electrode

Ba0.7Sr0.3TiO3 (BST) thin films were deposited on Pt and SrRuO3(SRO)/Pt hybrid bottom electrodes by radio frequency magnetron sputtering. X-ray analysis indicated that both films were polycrystalline. Dielectric measurements showed that the films on SRO/Pt hybrid bottom electrode had lower dielectric constant and loss than the films on single Pt and the dielectric properties were frequency-independent. The leakage current density of Ba0.7Sr0.3TiO3 thin films on hybrid bottom electrode was also lower. Leakage mechanism investigations showed that the contact between the electrode-film interfaces of thin films on SRO/Pt hybrid bottom electrode was ohmic. Based on the results, the effects of SRO/Pt hybrid bottom electrode on the crystallization and electrical properties of BST thin films were discussed.

Behavior of Hydrogen Atoms in Perovskite-Type Oxide Thin Films under Electric Fields

The behavior of hydrogen in (Ba,Sr)TiO3 (BST) thin film capacitors under electric fields was investigated by performing secondary ion mass spectroscopy (SIMS) analyses. It was clearly observed that the ingress of atmospheric hydrogen into BST thin film capacitors occurred through the anode and that it diffused toward the cathode under electric fields. In addition, it was found that the deterioration of the I-V properties of the BST thin film capacitors can be interpreted in terms of the distribution of hydrogen concentration in the BST thin films.

Enhanced Dielectric Properties of Nanocrystalline (Ba<SUB>0.65</SUB>Sr<SUB>0.35</SUB>)TiO<SUB>3</SUB> Thin Films Grown on CaRuO<SUB>3</SUB> Buffer Layers

Highly (100)-oriented and (110)-oriented (Ba0.65Sr0.35)TiO3 (BSTO) and (Ba0.65Sr0.35)TiO3/CaRuO3 (BSTO/CRO) heterostructure thin films, have been grown on Pt/Ti/SiO2/Si substrates prepared by pulsed laser deposition (PLD). The structure and surface morphology of the films have been characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The dielectric constants of the films changes significantly with applied dc bias field and have high tunability of 76.3% and 78.1% at an applied field of 256.3 kV/cm, respectively for BSTO and BSTO/CRO thin films on Pt/Ti/SiO2/Si substrates. The tunability of the BSTO/CRO heterostructure thin films on Pt/Ti/SiO2/Si substrate was higher than that of the BSTO thin films on Pt/Ti/SiO2/Si substrate. The high tunability has been attributed to the (110) texture of the films and lager grain sizes.

The Effect of Annealing Atmosphere on Electrical Properties of BST Thin Films Prepared on Copper Foils

Ba0.7Sr0.3TiO3 (BST) thin films have been prepared on copper foils via sol-gel method in almost inert atmosphere containing different level of oxygen which was achieved by flowing high purified argon with the varied rate from 0.3 L/min to 2.0 L/min. X-ray diffraction pattern of the BST thin film exhibits dominant perovskite phase. SEM images reveal good crystallization and much dense structure of the thin film. The BST thin film annealed at almost inert atmosphere by flowing the high purified argon at the rate of 0.5 L/min, exhibits highest dielectric constants which are 1549.65 and 1350.86 at the frequency of 10 kHz and 1 MHz, respectively. Additionally, the film also shows optimized ferroelectric behavior and low leakage current density. The mechanism of the annealing atmosphere on properties of BST thin films was investigated.

RELIABILITY OF BARIUM STRONTIUM TITANATE (BST) THIN FILMS

ABSTRACT Ba0.6Sr0.4TiO3 (BST) thin films were deposited on Pt/Ti/LaAlO3 (100) substrates by radio-frequency magnetron sputtering. Not only the fatigue characteristic of BST thin film but also the temperature and frequency dependences of dielectric constant (ϵ r ), loss tangent (tanδ) and tunability (at 300 kV/cm) were investigated. With the temperature increasing, the dielectric constant of BST thin film increases slowly and the fluctuation range is about 10%. From 20 Hz to 1 MHz, dielectric constant, loss tangent and tunability of BST thin films have a little change. After a hundred million reversals, the tunability of BST thin film slightly decreased, and the dielectric loss increased synchronously. The results reveal that BST thin films have high reliability including temperature, frequency stability and anti-fatigue characteristic.