Textured LaNiO3 Electrode Grown by Spin-Coating Technique

LaNiO₃ thin films were deposited by spin-coating technique on various substrates using metal naphthenates as starting materials. The highly oriented LaNiO₃ films with smooth and crack-free surfaces grown on SrTiO₃(100) and LaNiO₃(012) substrates were observed by X-ray diffraction –2 scans, while the film on sapphire(0001) substrate showed polycrystalline structure. Resistivity vs. temperature curves of the textured LaNiO₃ films showed that the film possessed a good metallic character.     

Effect of Poling on Photovoltaic Properties in Highly Oriented BiFeO3 Thin Films

We report enhancement of photovoltaic properties of highly oriented BiFeO₃ (BFO) thin films. The single phase rhombohedral R3c space group structure was confirmed by XRD and Raman spectra in the films grown on SrTiO₃ (111) substrates using RF magnetron sputtering. The films of BFO were characterized in planar geometry with top Pt electrodes having 200μm diameter, ～40nm thickness and at a regular spacing of ～150μm deposited by DC sputtering process. For photovoltaic studies the current voltage characteristic of as grown and poled samples were studied under the white light illumination having energy density of ～1kW/m². The as grown films showed open circuit voltage (Voc) of ～3.9V which increased up to ～17.8V on poling with 200V. This enhancement in Voc by poling was attributed to increase in net polarization due to alignment of domains in the direction of applied electric field and hence potential barriers between the domains.     

Structure of Ferroelectric (Ba,Sr)TiO3 Thin Films for Tunable Microwave Devices

Ferroelectric (Ba,Sr)TiO₃ films have been deposited on (001) MgO single crystals by a pulsed laser deposition with oxygen background while heating the substrates. Deposited BST films exhibit epitaxial growth along (001), which are confirmed by x-ray diffraction measurement. Structure of (Ba,Sr)TiO₃ along in-plane and surface normal direction have been investigated and found to have a tetragonal distortion depend on the deposition conditions, such as oxygen pressure. Lattice parameter decreases with increasing oxygen pressure, and tetragonallity (c/a) changes from 1.005 to 0.997 as oxygen pressure increase. Interestingly, energy gap measured by FTIR decreases with decreasing oxygen pressure until it reach a certain oxygen pressure, then increases again with increasing oxygen pressure. Furthermore, microwave properties of devices measured by a HP 8510C vector network analyzer from 0.045–20 GHz suggest that the least distorted films exhibit a larger dielectric constant changes with dc bias field.     

The Study of Ferroelectric La-Doped PbTiO3 Thin Films Prepared by RF Magnetron Sputtering

Ferroelectric La-modified lead titanate (PLT) thin film were grown on Pt/Ti/SiO2/Si by sputtering the Pb_0.93La_0.07TiO₃ targets containing an amounts of excess 8% PbO. The effects of sputtering and annealing conditions on the crystalline structures and the surface morphologies of the PLT thin films have been investigated. The remanent polarization (Pr) and the coercive field of the PLT film through rapid thermal annealing (RTA) was 10.2 μC/cm² and 45 kV/cm respectively. The maximum pyroelectric coefficient reached 19 nC/cm².K at 20°C.     

Annealing Effects on Structural and Dielectric Properties of Tunable BZT Thin Films

Ba(Zr, Ti)O₃ thin films have attracted great attention in recent years for their potential use in DRAMs and MCMs due to their high dielectric constant and relatively low leakage current. However, their tunable dielectric properties were rarely investigated and the corresponding potential for tunable microwave applications was seldom reported. In this paper, we present the tunable dielectric behavior of BZT thin films deposited by RF magnetron sputtering from a Ba(Zr_0.3Ti_0.7)O₃ ceramic target on MgO single crystal substrates. The composition, thickness and crystallinity of the thin films were analyzed by Rutherford backscattering (RBS), scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The dielectric constant and loss tangent were measured as a function of electric field (0–7 kV/mm) and temperature (–140 to +160°C) at frequencies up to 1 MHz, using interdigital capacitors (IDC) with Au electrodes on thin films. By optimizing the preparation process, a tunability {defined as = [ (0) – (E_max)]/ (0)} of 76% at E_max = 7 kV/mm and a low loss tangent of 0.0078 can be achieved. In addition, the influence of annealing temperature on the dielectric properties of the thin films is also discussed.     

Synthesis, Microstructure and Electrical Properties of Hydrothermally Prepared Ferroelectric BaTiO3 Thin Films

Thin films of polycrystalline, tetragonal BaTiO₃ on oxidized Ti metal substrates were synthesized at 240°C under hydrothermal conditions. Microstructure and electrical properties of the films generated over a four week period of synthesis formed the focus of this study. The films displayed a smooth and shiny surface with a relatively dense structure and no observable cracks. Film thickness reached 0.5 m after two weeks of synthesis and thereafter remained constant. Diameters of the grains on the film surface were in the range of 12 m. It is proposed that initial formation of the BaTiO₃ film occurs by reaction of Ba²⁺ with solubilized titanium oxide on the Ti metal surface followed at later stages by an in-situ growth via reaction of TiO_x with Ba²⁺ diffusing through the BaTiO₃ film. X-ray diffraction and Raman spectroscopy indicated that the BaTiO₃ films are tetragonal, and the films exhibited typical ferroelectric hysteresis loops at room temperature. However, no evidence of the dielectric anomaly (Curie transition) between 30 and 200°C was observed. Dielectric constant of the films at 1 kHz at room temperature was between 400–500. Both dielectric constant and tan exhibited low dispersion as a function of frequency at temperatures below 150°C, and the dispersion increased with temperature.     

Initial Stage Nucleation and Growth of Epitaxial SrRuO3 Thin Films on (0 0 1) SrTiO3 Substrates

We have investigated the initial stage nucleation and growth of epitaxial SrRuO₃ thin films grown on both polished (as received) and buffered HF (BHF) etched single crystal (0 0 1) SrTiO₃ substrates by 90° off-axis sputtering. Atomic force microscopy indicates a dramatic difference in the initial stage growth of SrRuO₃ films on the two substrates. The films on polished substrates nucleate as rectangular islands, which merge together to form a continuous film as the thickness increases. Complete coverage is obtained at film thickness of 20 nm. In contrast, the film on BHF etched substrate nucleates as finger-shaped islands at the step sites and continues to grow by adatom diffusion to the step sites. Complete coverage is obtained at a film thickness of 10 nm. This difference in the initial stage nucleation is attributed to the difference in surface morphology and termination layer of the two substrates. However, the thicker films on both as received and BHF etched substrates have identical surface morphologies. Such studies on the initial stage nucleation will also help us understanding the growth kinetics and development of surface morphology and interfaces in multilayered perovskite thin film heterostructures and devices.     

The Electrical Conductance of SrFeO2.5+x Thin Films

Thin films of the non-stoichiometric perovskite SrFeO_2.5+x have been grown by the pulsed excimer laser deposition technique onto sapphire substrates. The electrical conductance properties of the thin films have been determined in a series of experiments done both isothermally and with programmed temperature changes from ambient to 490°C and under O₂/N₂ atmospheres with oxygen concentrations in the range from 100 ppm to 100%. Over these ranges of temperature and oxygen partial pressure a wide range of oxygen stoichiometry in SrFeO_2.5+x occurs (approximately 0 < x < 0.5), which includes all four known phases in the SrFeO_{2.5 + x} + O₂ system. The experimentally measured values for the activation energy of conduction, _A, for SrFeO_2.5+x films at temperatures 100 < T < 200°C are in the range 0.30 < _A < 0.47 eV under oxygen at partial pressures 0.001 O ₂)< 0.05 atm and 0.18 < _A < 0.28 eV for 0.2 O ₂)< 1 atm. These values for _A are typical for compositions of SrFeO_2.5+x with stoichiometries in the range 0.25 < x < 0.45. For T < 300°C and for P(_{O 2})< 0.001 atm the films were essentially insulators. For T > 250^°C and P(_{O 2})> 0.001 atm, the oxygen stoichiometries of the films change during the programmed temperature ramps. For these conditions, the values _A/ T exhibit minima/maxima in the temperature range 250 < T < 320^°C which are interpreted as being due to the onset of the order-disorder phase transition from the cubic to the tetragonal and orthorhombic ordered phases of SrFeO_2.5+x with oxygen stoichiometry in the range 0.08 < x < 0.38. The SrFeO_2.5+x thin films have application as oxygen sensing materials, and a relationship between conductance and oxygen sensitivity, S _ox , has been derived. The values of S _ox for SrFeO_2.5+x thin films increases by more than an order of magnitude for compositions close to the lower stoichiometric limit where the principal phase conversion is between the cubic perovskite and the brownmillerite forms.     

Ferroelectric Properties of Bi3.25Nd0.75Ti3O12 Thin Films Prepared by MOD Process

Ferroelectric Bi_{4 – x}Nd_xTi₃O₁₂(BNdT) thin films with the composition (x = 0.75) were prepared on Pt/Ti/SiO₂/Si(100) substrate by metal-organic deposition. The films were annealed by various temperatures from 550 to 650C and then the electrical and structural characteristics were investigated for the application of FRAM. Electrical properties such as dielectric constant, 2Pr and capacitance were quite dependent on the thermal heat treatment. The measured 2Pr value on the BNdT capacitor annealed at 650C was 56 C/cm² at an applied voltage of 5 V. No fatigue was observed up to 8 × 10¹⁰ read/write switching cycles at a frequency of 1 MHz regardless of annealing temperatures.     

Ferroelectric Properties of (Bi, Sm)4Ti3O12 (BST) Thin Films Fabricated by a Metalorganic Solution Deposition Method

Ferroelectric properties of samarium substituted Bi₄Ti₃O₁₂ films, Bi_3.15Sm_0.85Ti₃O₁₂ (BST), were evaluated for use as lead-free thin film ferroelectrics for FeRAM applications. The BST films were fabricated on the Pt/Ti/SiO₂/Si(100) substrates by a metalorganic solution deposition method. The measured XRD patterns revealed that the BST films showed only a Bi₄Ti₃O₁₂-type phase with a random orientation. The BST film capacitors showed excellent ferroelectric properties. For the film capacitor annealed at 700C, 2P_r of 64.2 C/cm² and 2E_c of 101.7 kV/cm at applied electric field of 150 kV/cm were observed. The capacitor did not show any significant fatigue up to 1.5 × 10⁸ read/write switching cycles at a frequency of 1 MHz, which suggests that the samarium should be considered for a promising lanthanide elements to make a good thin ferroelectric film for memory applications.     

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.     

Dielectric Properties of Homoepitaxial SrTiO3SrTiO3SrTiO3 Thin Films Grown in the Step-Flow Mode

We have deposited SrTiO₃ thin films on Nb-doped SrTiO₃ substrates by pulsed laser deposition at temperatures of up to 1400°C. Reflection high energy electron diffraction was used to monitor the film growth mode at various temperatures and it was shown that growth proceeded in the step-flow mode at above 900°C. Capacitors were formed by evaporating platinum pads on the film surface and gold pads on the substrate. Films grown in the step-flow mode showed consistently higher dielectric constants below 200 K than films grown in the layer-by-layer mode. Films with the highest dielectric constant () were obtained using a stoichiometric ablation target at an oxygen pressure of 10SrTiO₃ ^–6SrTiO₃ Torr.     

Effect of a Second Annealing Process on the Properties of BaTiO3 Thin Films on Polycrystalline Nickel Substrates Grown by Polymer-Assisted Deposition

Ferroelectric BaTiO₃ thin films were successfully deposited on nickel foils by a chemical solution technique named polymer-assisted deposition. It is found that a second annealing in oxygen using a rapid thermal annealing furnace would strongly affect the electric properties of the as-prepared thin films. Dielectric and ferroelectric properties have been systematically studied. The correlation between the second annealing conditions and dielectric properties was established and discussed. Optimized second annealing condition was found.     

Phase Formations and Electrical Properties of Various (Bi, La)4Ti3O12 Thin Films by Chemical Solution Deposition

The phase formation and electrical properties of (Bi, La)₄Ti₃O₁₂ (BLT) thin film and V-, Sm-doped BLT thin films prepared by the chemical solution deposition method on Pt/TiO₂/SiO₂/Si substrates have been investigated. It was observed that the microstructure and electrical properties of BLT thin films dramatically varied with V- and Sm-doping. The crystallinity and grain size of BLT thin films were definitely increased by V- and Sm-doping into BLT films, which resulted in the enhancement of remanent polarization in doped BLT films. The remanent polarization (Pr) of Sm-doped BLT films annealed for 3 min by an RTA system was about 9 C/cm². The V- and Sm-doped BLT films also exhibited good fatigue characteristics under bipolar stressing to 10¹⁰ cycles.     

Study of Microwave Dielectric Properties of Perovskite Thin Films by Near-Field Microscopy

Perovskite thin film materials possess good dielectric properties, which vary with applied voltage, and have thus been thoroughly investigated for applications as thin film tunable microwave devices. However, the tunability of the thin film materials derived from the frequency response of the thin film devices suffers from ambiguity in extracting the true dielectric response of the thin film materials in microwave frequency regime. To circumvent such a difficulty, we investigated the dielectric properties of perovskite thin films by using a novel scanning evanescent microwave microscopy (SEMM). To extract the dielectric parameters from original microwave frequency response signal of SEMM probe, we perform a 3-dimensional (3D) finite element simulation to model the frequency behavior of the SEMM microwave probe. Dielectric images of the thin films with submicron resolution can be obtained by using such a near-field technique, which correlates very well with the morphology of the films examined by atomic force microscopy. Moreover, the dielectric images of dielectric thin films were compared to those of ferroelectric thin films in order to discuss the related dielectric mechanism of the materials.     

The Effect of La-Doped Bi4Ti3O12 Buffer Layer on Crystallinity and Ferroelectric Properties of PbZr0.58Ti0.42O3/Bi3.25La0.75Ti3O12 Multilayered Thin Films

PbZr_0.58Ti_0.42O₃ (PZT) ferroelectric thin films with Bi_3.25La_0.75Ti₃O₁₂ (BLT) buffer layer of various thickness were fabricated on Pt/TiO₂/SiO₂/p-Si(100) substrates by rf-magnetron sputtering method. The pure PZT film showed (111) preferential orientation in the XRD patterns, and the PZT/BLT films showed (110) preferential orientation with increasing thickness of the BLT layer. There were no obvious diffraction peaks for the BLT buffer layer, for its thin thickness in PZT/BLT multilayered films. There were the maximum number of largest-size grains in PZT/BLT(30 nm) film among all the samples from the surface images of FESEM. The growth direction and grain size had significant effects on ferroelectric properties of the multilayered films. The fatigue characteristics suggested that 30-nm-thick BLT was just an effective buffer layer enough to alleviate the accumulation of oxygen vacancies near the PZT/BLT interface. The comparison of these results suggests that the buffer layer with an appropriate thickness can improve the ferroelectric properties of multilayered films greatly.     

Low-Temperature Process of Ferroelectric (Y0.95,Bi0.05)MnO3 Thin Films and Their Structural and Electrical Properties

[(Y_0.95,Bi_0.05)MnO₃] (YBM) films have been grown on Y₂O₃ buffered Si (001) by pulsed-laser deposition (PLD). We have compared the structural and dielectric properties of YBM films with those of typical YMnO₃ films from the viewpoint of lowering the process temperature. The highly c-axis oriented YBM film have been obtained on Y₂O₃/Si (001) at 700°C, which is a significantly reduced growth temperature from that of typical YMnO₃ films (850°C). The Bi modification was effective for the low temperature processing of YBM films. These highly c-axis oriented YBM films was obtained only at high ambient oxygen pressures, for example above 100 mTorr, contrary to YMnO₃ films which requires low ambient oxygen pressure for the growth of c-axis preferred orientation. The dielectric constant and dissipation factor was 29 and 0.017 at 1 MHz, respectively. The memory window due to ferroelectric polarization switching was found in a capacitance-voltage (C-V) characteristic. The YBM/Y₂O₃/Si structure with above characteristics of YBM films exhibited the C-V memory window of 1.2 V at a sweep voltage of 5 V. The flat-band voltage shifted symmetrically with increasing the sweep voltage up to 8 V due to little charge injection from Si. As a result, the memory window increased progressively with increasing the sweep voltage and amounted to 2 V at a sweep voltage of 8 V. The leakage current density was below 5 × 10^?7 A/cm² at a bias voltage of 8 V.     

Microwave Properties of Thin BSTO Films Based Varactors for High Frequency Applications

The influence of film thickness on electrophysical properties of planar varactors based on ion-plasma deposited BaSrTiO₃ films is investigated. Planar capacitors with copper electrodes were used for tunability and loss factor investigation. Microwave parameters of varactors based on BaSrTiO₃ films were measured at (1 ÷ 8) GHz frequency range and (170 ÷ 370) K temperature range. The tunability and loss factor was found to be strongly depended on film thickness. The optimal thickness of BaSrTiO₃ film from high tunability and low loss factor point of view was estimated as 1 μm approximately. Dielectric losses at frequency range (1 ÷ 8) GHz (tan δ ～0,01 at 1 GHz, tan δ ～ 0,014 at 8 GHz and zero bias) and high tunability (Cmax/Cmin ～ 2) at room temperature were observed.     

Microwave Tunable Properties of Ni-Doped (Ba0.5Sr0.5)TiO3 Thin Films Grown by Pulsed Laser Deposition

(Ba_0.5Sr_0.5)TiO₃ (BST) thin films were deposited by pulsed laser deposition (PLD) and investigated as a function of Ni dopant concentration in low and high frequency regions. In low frequency region (<10 MHz), the Ni-dopant concentration in BST films has a strong influence on the material properties including dielectric and tunable properties. Ni-doped (≤3 mol%) BST films showed denser, smoother morphologies and smaller grain sizes than those with 6 and 12 mol% Ni. Dielectric constant and loss of 3 mol% Ni-doped BST films were about 980 and 0.3%, respectively. In addition, tunability and figure of merit of 3 mol% doped BST films showed maximum values of approximately 39% and 108, respectively. In high frequency region (>1 GHz), the frequency tunability range at center frequency of undoped BST and 3 mol% Ni-doped BST coplanar waveguide (CPW) resonators showed 102 and 152 MHz, respectively at 30 V dc bias. The Ni-doped BST thin films are possible in applications of microwave tunable capacitors.     

Pulsed Laser Deposited Ba(Mg1/3Ta2/3)O3 Microwave Dielectric Thin Films

Ba(Mg_1/3Ta_2/3)O₃ (BMT) microwave dielectric thin films were successfully synthesized by a modified pulsed laser deposition (PLD) process, which includes low temperature (200°C) deposition and high temperature (>500°C) annealing. Crystalline structured BMT thin films were obtained when the PLD-deposited films were post-annealed at a temperature higher than 500°C in oxygen atmosphere. The characteristics of BMT thin film, including crystallinity, grain size, film roughness, and dielectric properties were improved with annealing temperature, achieving dielectric constant K = 23.5 and dissipation factor tan δ = 0.015 (at 1 MHz) for the 800°C-annealed films.