Substrate effect on in-plane ferroelectric and dielectric properties of Ba0.7Sr0.3TiO3 thin films

Heteroepitaxial Ba_0.7Sr_0.3TiO₃ thin films were grown on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.35 (001) (LSAT) and SrTiO₃ (001) (STO) single crystal substrates using pulsed laser deposition (PLD). X-ray diffraction characterization revealed a good crystallinity and a pure perovskite structure for films grown on both LSAT and STO substrates. The in-plane ferroelectric and dielectric properties of the films were studied using interdigital electrodes (IDE). The film grown on LSAT substrate exhibited an enhanced in-plane ferroelectricity, including a well-defined P-E hysteresis loop with the remnant polarization P _r = 10.5 μC/cm² and a butterfly-shaped C-V curve. Nevertheless, only a slim hysteresis loop was observed in the film grown on STO substrate. Curie temperature T _c of the film grown on LSAT substrate was found to be ∼105^∘C, which is nearly 70^∘C higher than that of the bulk Ba_0.7Sr_0.3TiO₃ ceramics. T _c of the film grown on STO substrate has almost no change compared to the bulk Ba_0.7Sr_0.3TiO₃ ceramics. The dielectric tunabilities were found to be 64% and 52% at 1 MHz for the films grown on LSAT and STO substrates, respectively.     

Planar and Parallel Dielectric Properties of Compositionally Graded (Ba,Sr)TiO3 Thin Films for Tunable Microwave Applications

Compositionally graded (Ba_x,Sr_{1 ? x})TiO₃ [BST] ferroelectric thin films have been received much attention in graded ferroelectric devices due to their unique properties, such as large pyroelectric coefficients, large polarization offset and small temperature coefficient of dielectric constant for microwave tunable devices. Compositionally graded BST thin films were deposited epitaxially on LaAlO₃ [LAO] and Nb-doped SrTiO₃ [STO:Nb] substrates by pulsed laser deposition. The planar and parallel dielectric properties of compositionally graded BST epitaxial thin films ware investigated in the frequency ranges of 100 Hz ～ 1 MHz as a function of the direction of the composition gradient with respect to the substrate at room temperature. The dielectric properties of the graded BST films depended strongly on the direction of the composition gradient with respect to the substrate. The graded ST → BT films grown on LAO and STO:Nb substrates exhibited a excellent dielectric properties than the graded BT → ST films.     

Structural Investigation of Thin SrTiO3 Films Grown on MgO and LaAlO3 Substrates

Structural investigations of SrTiO₃ thin films deposited by pulsed laser ablation onto MgO and LaAlO₃ substrates are presented. The residual strain along the c-axis (the growth direction) was evaluated with conventional X-ray diffraction. Evaluation of the in-plane strain was accomplished with grazing-incidence X-ray diffraction. The unit cell of the STO film on LAO substrate had an orthorhombic structure with a coexistence of compressive strain along a-axis and tensile strain along b-axis. The film on an MgO substrate was tetragonal, stretched along the c-axis and compressed in-plane. The elemental composition of the samples was investigated using an INCA system for X-ray elemental analysis attached to a Hitachi S-4300 SEM. A comparison of the residual strain and stoichiometric ratio between as-deposited films and films annealed for four hours at 1100°C in flowing oxygen is presented.     

Performance of ferroelectric based tunable capacitors as a function of electrode geometry

Abstract

High quality Ba_0.4Sr_0.6TiO₃ and SrTiO₃ films were grown by Pulsed Laser Deposition on single crystal LaAlO₃ and MgO substrates. Temperature dependencies of the dielectric constant and loss tangent of the films were studied using planar interdigitated test capacitors with various electrode geometries. The temperature where the maximum or “peak” capacitance occurs (T_p and referred to as the “peak temperature”) is found to depend on the electrode geometry in these films. As much as 40 K difference in T_p was observed between the STO test capacitors with 5 μm and 40 μm gaps between electrodes. Interface built-in electric field and metal-ferroelectric thermal mismatch strain are considered as possible explanation of the effect of electrode geometry on peak temperature of the capacitors.     

Preparation of Y-Ba-Cu-O tapes by pyrolysis of organic acid salts

High-T_C superconducting Y-Ba-Cu-O films were prepared on certain ceramic and metal substrates by pyrolysis of 2-ethylhexanoates. Only the films on MgO, yttria-stabilized zirconia (YSZ), and Ag substrates exhibited the superconducting transition above liquid nitrogen temperature. Transition temperatures (T_C) of 82, 85, and 82 K were observed in films of MgO, YSZ, and Ag substrates. The important factors in preparing superconducting films on substrates are the reactivity of the films and the substrates and the difference in their thermal expansions. The Y-Ba-Cu-O films were well suited for use with MgO, YSZ, and Ag substrates. Long superconducting tapes were produced on Ag substrate; flexible tape with a critical current density J_C of 3 · 10³ A/cm² at 77 K has now been prepared.     

Heteroepitaxial Growth of High K Dielectric Thin Films by Pulsed Laser Deposition

High K dielectric [Pb(Mg_1/3Nb_2/3)O₃]_{1 ? x} -[PbTiO₃] _x (PMN_{1 ? x} -PT _x ) thin films and conducting La_0.7Sr_0.3MnO₃ (LSMO) oxide electrodes were deposited on LaAlO₃ (LAO) (100) single crystal substrates by pulsed laser deposition. A novel “split-target” technique was used to fabricate films with x = 0.00, 0.10, 0.35, 0.53 and 1.00. Conditions for depositing high quality films were studied. The structural characteristics of the PMN_{1 ? x} -PT _x /LSMO/LAO heterostructures were examined by X-ray diffraction and scanning electron microscopy. A cube-on-cube epitaxy was obtained for films grown at 650°C and under an oxygen ambient pressure of 200 mTorr. Electrical measurement was performed with 0.2 mm diameter patterned Au top electrodes. The dielectric constant and dielectric loss of the PMN_{1 ? x} -PT _x films were studied as functions of frequency and temperature for x = 0.10 and 0.35. All films grown at 650°C showed small leakage current density of below 10^?7 Acm^?2 at 1 V.     

IN-PLANE DIELECTRIC PROPERTIES OF EPITAXIAL Ba(Zr0.3Ti0.7)O3 THIN FILM GROWN ON LSAT (001) SINGLE CRYSTAL SUBSTRATE

ABSTRACT

Ba(Zr_0.3Ti_0.7)O₃ (BZT) thin film was deposited on (LaAlO₃)0.3(Sr₂AlTaO₆)_0.35 [LSAT] (001) single crystal substrate using pulsed laser deposition. The X-ray diffraction pattern reveals an epitaxial growth of the film with a pure perovskite phase. The in-plane dielectric properties of the Ba(Zr_0.3Ti_0.7)O₃ thin film was characterized as a function of frequency (1 kHz–500 MHz), temperature (125 K–373 K) and dc electric field (0–13.3 V/μm) using gold interdigital electrodes. The relative permittivity of the film exhibits strong dependence of the dc bias field over the whole frequency range. The relative permittivity has a high tunability of 73%–50% at room temperature in the frequency range of 1 kHz to 500 MHz, showing the potential of our Ba(Zr_0.3Ti_0.7)O₃ thin film to be used in microwave devices.     

The Effect of Stress on the Microwave Dielectric Properties of Ba0.5Sr0.5TiO3 Thin Films

Single phase, (1 0 0) epitaxial Ba_0.5Sr_0.5TiO₃ (BST) films have been deposited onto (1 0 0) LaAlO₃ and MgO substrates by pulsed laser deposition (PLD). The capacitance and dielectric losses of as-deposited and annealed films have been measured from 1–20 GHz as a function of electric field (0–80 kV/cm) at room temperature. The dielectric properties are strongly affected by the substrate type, post-deposition annealing time (6 h) and temperature (1200°C). For epitaxial BST films deposited onto MgO, it is observed that, after a post-deposition anneal the dielectric constant and the dielectric loss decreases. For epitaxial BST films deposited onto LAO, a post-deposition anneal (1000°C) results in an increase in the dielectric constant and an increase in the dielectric loss. The dc electric field induced change in the dielectric constant tends to increase with the dielectric constant and is largest for as-deposited films on MgO and post-deposited annealed films on LAO. In general, for epitaxial BST films, a large electric field effect is observed in films that have a large dielectric loss and a small electric field effect in films that have a low dielectric loss. High resolution X-ray diffraction measurements indicate that deposited film exhibit a significant tetragonal distortion which is strongly affected by a by a post deposition anneal. The observed differences in dielectric properties of the epitaxial BST films on MgO and LAO are attributed to the differences in film stress which arise as a consequence of the lattice mismatch between the film and the substrate and the differences in the thermal coefficient of expansion between the film and the substrate. A thin amorphous buffer layer of BST has been used to relieve stress induced by the lattice mismatch between the film and the substrate. Unlike epitaxial films, stress relieved films do not show an inverse relationship between dielectric tuning and Q (1/tan) and may be superior materials for tunable microwave devices.     

High quality BaxSr1-xTiO3 films grown by mocld and novel ferroelectric/ferrite structures for dual-tuning microwave devices

Abstract

Excellent single crystal Ba_xSr_1-xTiO₃ (BST) films were grown on LaAlO₃ substrates using the metal-organic chemical liquid deposition (MOCLD) method. Very low losses (tanδ ∽0.002-0.008) were measured from these films at 400 KHz. Biaxially oriented BST films were successfully grown on polycrystalline YIG substrates using both MOCLD and pulsed laser deposition methods with biaxially oriented MgO and YSZ buffer layers. The dielectric losses of the films range from 0.005 to 0.015 while 25% of dielectric constant change was observed with 40V bias voltage up to 10 MHz. Both the dissipation and dielectric constant of the films remained nearly constants over a wide temperature range (77 K to 380 K). A dual-tuning microwave coplanar phase shifter using a BST film grown on a MgO buffered polycrystalline YIG substrate was fabricated. A significant phase shift was observed in GHz frequency range when an electric bias or a magnetic field was applied to the device.     

Chemical Solution Deposition of Epitaxial La1−x (Ca, Sr) x MnO3 Thin Films

We report on the epitaxial growth of magnetoresistive La_0.7Ca_0.3MnO₃ and La_0.7Sr_0.3MnO₃ thin films by chemical solution deposition. Thin films were prepared by spin-coating of single-crystal LaAlO₃ (100) substrates with precursor solutions of different concentrations and crystallized at 850°C. The structure of the thin film was found to be influenced by the concentration of the spin-coating solution. The thin film structure and epitaxy was clearly improved by reducing the concentration of the precursor solution. All thin films displayed excellent electrical properties such as a low resistivity and very high metal-insulator transition temperatures T _MI .     

Conservation of Permittivity and Tunability of Ferroelectrics Over Temperature

The existence of an intersection point of electric field strength dependencies of permittivity of ferroelectrics at different temperatures is theoretically described by Ginzburg-Devonshire (GD) theory and experimentally confirmed by studying (Ba_0.3Sr_0.7)TiO₃ (BSTO) and SrTiO₃ (STO) thin film varactors. GD analysis enables one to propose a way to improve the thermal stability of ferroelectric microwave devices. In particular, parameters of a L-band digital (0°/180°) phase-shifter fabricated from thick BSTO film varactors were stabilized in the temperature range T = (210–310) K by application of variable to DC control voltages.     

PATTERNED HIGH FREQUENCY THICK FILM MEMS TRANSDUCER

ABSTRACT

Ferroelectric properties of BiFeO₃ (BFO) thin films epitaxially grown on SrRuO₃ (SRO)/(001)SrTiO₃ (STO) structure were investigated. First, bottom SRO electrodes were deposited on STO substrates by metalorganic chemical vapor deposition (MOCVD) or by sputtering. Then, BFO thin films were deposited on SRO/STO structures by chemical solution deposition. X-ray diffraction analysis showed that the SRO films deposited by both methods grew epitaxially on STO substrates as a single phase perovskite structure, but the out-of-plane lattice parameters of SRO were different, that is, they were 0.396 nm in MOCVD and 0.399 nm in sputtering. The leakage current densities were higher than 1 A/cm² at room temperature in BFO films on both MOCVD-and sputtered-SRO, but the current density in the film on sputtered SRO decreased to 2 × 10^?4 A/cm² at 80 K. The remanent polarization of approximately 50 μC/cm² was observed at 80 K in the BFO thin film on sputtered-SRO/STO.     

HTc superconducting films of bismuth-strontium cuprate prepared by modified chemical deposition techniques

Abstract

The homogeneous solutions of meta-acrylates of Bi, Pb, Sr, Ca, Cu were synthesized, and by spray-pyrolysis of the solutions and following annealing the high temperature superconducting Bi₂Sr₂CaCu₂O_x+Bi₂Sr₂Ca₂Cu₃O_x films were prepared on LaAlO₃ substrates. The connection was observed between annealing temperature, film thickness and type, quantity, and forms of impurity phases, such as strontium and calcium cuprates, calcium oxides, strontium oxides. The properties of these films (T_s ? 81 K, ΔT_s ? 1 K) were shown to take place in case if their microstructure consisted of scale-like plates forming [001] texture. Also, it was concluded that under optimum conditions the T_scould be raised to > 100 K in Bi–containing films     

Fabrication of MFIS diodes using sol-gel derived SBT films and LaAlO3 buffer layers

Abstract

We demonstrate the ferroelectric behavior of Sr_0.8Bi_2.2Ta₂O₉ (SBT) films grown on Si(100) substrates by using lanthanum aluminate (LaAlO₃) buffer layers. LaAlO₃ films were prepared by vacuum evaporation method. Then, they were subjected to ex situ dry N₂ annealing in a rapid thermal annealing (RTA) furnace. From the capacitance-voltage (C-V) measurement, the dielectric constant of LaAlO₃ was estimated to be 20～25. On these substrates, SBT films (210nm) were deposited by sol-gel method and they were characterized by XRD analysis after annealing under various conditions. It was found from C-V characteristics that the memory window of an SBT film annealed at 750°C for 30min in O₂ atmosphere was about 3.0V for the voltage sweep of ±10V. It was also found from the retention measurement that the capacitance values of the SBT film annealed at 750°C did not change over 12hours. It is concluded from these results that the SBT/LaAlO₃/Si(100) structure is one of the most promising structures for realizing MFISFETs (metal-ferroelectric-insulator-semiconductor field-effect-transistors).     

Ferroelectric switching and fatigue behavior for PZT/YBCO thin film heterostructures

Abstract

The dielectric and ferroelectric properties for Au/Pb(Zr,Ti)O₃/YBa₂Cu₃O_7?x heterostructures at low temperatures are reported. The fatigue behavior and the ferroelectric switching effect for the structures are also investigated. The PZT/YBCO thin film heterostructures were deposited on MgO(100) substrates by laser ablation. The ferroelectric and dielectric properties and optical response of the oriented PZT films with different thicknesses have been studied over the temperature range from 20 K to 300 K. The dielectric loss of the structure was found to decrease by an order of magnitude when the YBCO bottom electrode became superconducting. A very low fatigue rate of the structure has also been obtained below T _c of YBCO layer.     

Structural and Electrical Properties of Ba(ZrxTi1-x)O3 Films Prepared by RF-magnetron Sputtering Using Metal Targets

Ba(Zr_xTi_1-x)O₃ (BZT) thin films with different Zr contents were deposited on (100)MgO and (100)Pt/(100)MgO substrates by RF-magnetron sputtering using metal targets. The BZT thin films had a single perovskite phase with only (001)/(100) orientation. In all cases, the ratio of Ba/Ti was stoichiometry and BZT films possess a dense microstructure. The grain size was decreased and BZT thin films showed ferroelectric-to-paraelectric properties with increasing Zr content. At room temperature, the tunability of nearly 30% was achieved at 1 MHz; meanwhile, a relatively low dielectric loss was obtained. These results indicated that we succeeded in depositing high-quality and potential tunable ferroelectrics.     

Preparation of Epitaxial Bi4Ti3O12 Thin Films on LaAlO3(012) Substrates by a Spin Coating-Pyrolysis Process

Bismuth titanate Bi₄Ti₃O₁₂ thin films were prepared on LaAlO₃(012) substrates by a spin coating-pyrolysis process using metal naphthenates as starting materials. The c-axis oriented Bi₄Ti₃O₁₂ thin films, which contained no second phases as –2 scans, were obtained by heat-treatment in air at temperatures of 600°C and above. X-ray diffraction pole-figure analysis showed that the Bi₄Ti₃O₁₂ thin film has an epitaxial relationship with the LaAlO₃ substrate.     

Rf Sputtered Na0.5K0.5NbO3 Films on Oxide Substrates as Optical Waveguiding Material

Highly crystalline Na_0.5K_0.5NbO₃ (NKN) thin films of 1–2 μm thickness were deposited by rf-magnetron sputtering of a stoichiometric, ceramic target on single crystal LaAlO₃(001) and Al₂O₃(0112) substrates. X-ray diffraction measurements revealed epitaxial quality of NKN/LaAlO₃ film structures, whereas NKN films on sapphire substrates were found to be preferentially c-axis oriented. A prism-coupling technique was used to characterize optical and waveguiding properties. A bright-line spectrum at λ = 632.8 nm, revealed sharp peaks, corresponding to transverse magnetic (TM) and electric (TE) waveguide propagation modes in NKN/LaAlO₃ and NKN/Al₂O₃ thin films. Using a least mean square fit the refractive index for the films and film thickness were calculated. The extraordinary and ordinary refractive indices were determined to n _e = 2.207 ± 0.002 and n _o = 2.261 ± 0.002, and n _e = 2.216 ± 0.002 and n _o = 2.247 ± 0.002 at λ = 632.8 nm for 2.0 μm thick NKN films on LaAlO₃ and Al₂O₃, respectively. This corresponds to a birefringence Δn = n _e ? n _o = ?0.054 ± 0.003 and Δn = ?0.031 ± 0.003 in the films, where the larger Δn for the NKN/LaAlO₃ structure can be explained by the superior crystalline quality compared to NKN/Al₂O₃. Atomic force microscopy images of the film surfaces revealed rms roughnesses of 2.5 nm and 8.0 nm for 1.0-μm thick NKN/LaAlO₃ and NKN/Al₂O₃ films, respectively. We believe surface scattering is one of the main sources of waveguide losses in the thin films.     

Temperature dependence of mechanical deformations in strontium titanate thin film on the sapphire substrate

Abstract

The temperature dependence of the dielectric constant (ε) of SrTiO₃ films on the various dielectric substrates differs radically from ε(T) dependence for bulk single crystal. This difference may be connected with temperature dependence of mechanical stress in the film. XRD measurements suggest the high mechanical deformation (u) of STO film unit cells at room temperature. But there is no experimental data about dependence u(T) nowadays. Present work is devoted to investigation of temperature and electric field dependencies of capacitance (C(U,T), C(Q,T)) of capacitor on the structure STO film/sapphire substrate. It is shown that derivative of inverse capacitance on the temperature is not constant and has a maximum at temperature about 200 K. Position of maximum is not influenced by charge (Q_i) on the capacitor. The mechanical deformations play the dominant role in the formation of maximum. The temperature behavior of macroscopic mechanical deformations in STO thin film on sapphire substrate is discussed.     

Shift of Phase Transition Temperature in Strontium Titanate Thin Films

The results of structural and electrical characterizations of SrTiO₃ thin films deposited onto MgO and LaAlO₃ substrates by pulsed laser deposition technique are presented. The influence of substrate and annealing procedure on the crystalline structure and dielectric properties of these ferroelectric thin films are investigated. The obtained experimental data are analyzed in terms of the Landau theory taking into account the room-temperature lattice mismatch of the ferroelectric and substrates as well as the difference in their thermal expansion. It is shown that the behavior of the SrTiO₃ thin films could not be attributed to the effect of the film/substrate mechanical coupling. As a possible nature of the observed behavior one can consider the non-stoichiometry of the film composition caused by the chemical contact of the film with the substrate and by the annealing.