Leakage current and pyroelectric properties of compositionally graded (Pb,Ca)TiO<Subscript>3</Subscript> films

Highly (100)-oriented, compositionally graded (Pb,Ca)TiO₃ (PCT) thin films with a Ca content from 0 to 24 mol% on Pt/Ti/SiO₂/Si substrates were prepared by a sol-gel process. The graded structure of the Au/PCT/Pt film capacitor showed a well-saturated hysteresis loop at an applied field of 500 kV/cm with remanent polarization (P_r), and coercive electric field (E_c) values of 9.35 C/cm² and 130 kV/cm, respectively. At 100 kHz, the dielectric constant and dielectric loss of the film were 129 and 0.024, respectively. The leakage current density of the graded PCT film was less than 1.0×10^-7 A/cm² over a voltage range from 0 to 4 V. The conduction current depended on the voltage polarity. At low electric field (110 and 180 kV/cm, respectively, for Pt and Au electrodes biased negatively), the Au/PCT and PCT/Pt interfaces form a Schottky barrier. At high electric field (>110 kV/cm), the Au electrode biased negatively shows space-charge-limited current (SCLC) behavior. The temperature dependencies of the pyroelectric coefficients of the graded PCT film were measured by a dynamic technique. From 20 to 82 °C, the pyroelectric coefficients of graded PCT film remain steady in the range 106 to 118 C/m²K. The detectivity figure of merit (F_D) of the graded PCT film was 6.7×10^-6 Pa^-0.5. PACS 77.80.-s; 77.70.+a; 77.22.-d; 51.50.+v; 68.37.-d     

Highly (100)-oriented Eu-doped PZT thin films on sol-gel derived (100)-textured LaNiO<Subscript>3</Subscript>/Si substrates

(100)-oriented LaNiO₃ (LNO) thin films were grown on Si substrates by a sol-gel method followed by a rapid thermal process at temperatures ranging from 650 °C to 800 °C. The films produced at 700 °C had a resistivity of 1.79 mcm and could be used as bottom electrodes in the fabrication of ferroelectric capacitors on Si. Subsequently, a sol-gel derived Eu-doped Pb(Zr_0.52,Ti_0.48)O₃ (PEZT) thin film with a thickness of 130 nm prepared on the LNO electrode was found to have a (100)-oriented texture. Possible reasons for the high degree of (100) orientation in PEZT thin films are given. Good ferroelectric performance was obtained for Au/PEZT/LNO capacitors. The remnant polarization (2P_r) was found to be 22 C/cm² at a coercive electric field (E_c) of 134 kV/cm. After 10¹¹ polarization reversals, P_r decreased by only 15%. PACS 68.37.Yz; 68.37.Hk     

Electrical properties of Sr<Subscript>3</Subscript>Bi<Subscript>4</Subscript>Ti<Subscript>6</Subscript>O<Subscript>21</Subscript> thin films

Highly c-axis-oriented Sr₃Bi₄Ti₆O₂₁ (SBTi) thin films were fabricated on Pt-coated Si substrates by pulsed laser deposition (PLD). The structures were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). No peaks of SrTiO₃ (STO) could be detected in the XRD pattern, indicating the existence of the SBTi single phase. Good ferroelectric hysteresis loops of the films with Pt electrodes were obtained. With an applied field of 400 kV/cm, the measured remanent polarization (P_r) and coercive field (E_c) values were 4.1 C/cm² and 75 kV/cm respectively. The films showed little fatigue after 2.22×10⁹ switching cycles: the nonvolatile polarizations decreased by less than 5% of the initial values. The dielectric constant and the loss tangent of the films were measured to be 363 and 0.04 at 100 kHz. These results might be advantageous for nonvolatile ferroelectric random access memory (NVFRAM) and dynamic random access memory (DRAM). PACS 77.84.Dy; 77.22.-d; 68.55.Jk     

Effect of substitution of vanadium on the structure and electrical properties of Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> thin films

The ferroelectric and dielectric properties of Bi_4-xLa_xTi₃O₁₂ (BLT) and Bi_4-xLa_xTi_2.97V_0.03O₁₂ (BLTV) thin films deposited on (111)Pt/Ti/SiO₂/Si substrates using a chemical solution method were investigated. The BLTV thin films showed a larger remanent polarization (9.6 C/cm²) than the BLT thin films (6.5 C/cm²), while the coercive field for both thin films was nearly the same. The capacitance of the films as a function of a small ac driving field was measured, and the data were processed using Rayleighs law. The results show that the Rayleigh constant of the BLT films was smaller than that of the BLTV films, indicating that the defect concentration was lower in the latter case. The superior ferroelectricity of the BLTV films was attributed to a decrease of both the (001) orientation and the defect concentration. PACS 77.80.Bh; 77.55.+f     

Structural and optical properties of Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ferroelectric thin films prepared by chemical solution methods

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films have been grown on Pt/Ti/SiO₂/Si substrates by chemical solution methods. X-ray diffraction analysis shows that BLT thin films are polycrystalline with (171)-preferential orientation. Atomic force microscopy investigation shows that they have large grains about 120 nm in size. A Pt/BLT/Pt capacitor has been fabricated and showed excellent ferroelectricity, with a remnant polarization and coercive field of 24 μC/cm² and 116 kV/cm, respectively. The capacitor shows no polarization fatigue up to 10⁹ switching cycles. The optical constants (n,k) of the BLT thin films in the wavelength range 0.35–1.7 μm were obtained by spectroscopic ellipsometry measurements, and the band-gap energy was found to be about 3.25 eV. Received: 16 October 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/65830-734, E-mail: gswang@mail.sitp.ac.cn     

Highly (100) oriented Pb(Zr<Subscript>0.52</Subscript>Ti<Subscript>0.48</Subscript>)O<Subscript>3</Subscript>/LaNiO<Subscript>3</Subscript> films grown on amorphous substrates by pulsed laser deposition

Pb(Zr_0.52Ti_0.48)O₃ (PZT)/LaNiO₃ (LNO) thin films with highly (100) out of plane orientation were produced on SiO₂/Si(100) and alkaline earth aluminosilicate glass substrates by pulsed laser deposition (PLD). Orientations of both PZT and LNO films were evaluated using X-ray diffraction. The pure (100)-oriented PZT/LNO films were obtained under optimized deposition conditions. Time of flight-secondary ion mass spectrometry analysis showed that LNO could effectively block interdiffusion between the PZT films and the substrates. Fairly smooth surfaces of the PZT films with roughness of about 4 nm were observed using an atomic force microscope. Cross sectional examination revealed that the films grew in columnar grains. The PZT films grown on both SiO₂/Si and glass substrates demonstrated very good ferroelectric characteristic at room temperature with remnant polarization of up to 26 μC/cm². PACS 79.20.DS; 77.84.DY; 78.70.Ck     

Preparation and electrical properties of Bi<Subscript>3.25</Subscript>Pr<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ferroelectric thin films

Bi_3.25Pr_0.75Ti₃O₁₂ (BPT) ferroelectric thin films have been prepared by chemical solution deposition on platinized Si substrates. Well-crystallized BPT films can be achieved by 600 °C rapid thermal annealing. The film surface is smooth and crack-free, composed of uniform spherical grains around 90–100 nm in diameter. The electrical properties of Pt/BPT/Pt thin film capacitors were characterized by hysteresis and impedance measurements. The remanent polarization of 700 °C annealed BPT films is around 20 C/cm² at 120-kV/cm stimulus field. The dielectric constant is around 380 at 10 kHz, 100-mV amplitude. The remanent polarization of BPT film showed a slight reduction, 10% of its original value, after 2.8×10⁹ cycles, while a 30% reduction of non-volatile polarization was observed. PACS 81.15.-z; 77.55.+f; 77.22.Gm     

Texture control and its impact on the properties of SrBi2Ta2O9 thin films prepared by pulsed laser deposition

SrBi₂Ta₂O₉ (SBT) ferroelectric thin films with different preferred orientations were deposited by pulsed laser deposition (PLD). Several methods have been developed to control the preferred orientation of SBT thin films. For SBT films deposited directly on Pt/TiO₂/SiO₂/Si substrates and in situ crystallized at the deposition temperature, the substrate temperature has a significant impact on the orientation and the remnant polarization (Pr) of the films; a higher substrate temperature benefits the formation of (115) texture and larger grain size. The films deposited on Pt/TiO₂/SiO₂/Si substrates at 830 °C are (115)-oriented and exhibit 2Pr of 6 μC/cm². (115)- and (200)-predominant films can be formed by using a La_0.85Sr_0.15CoO₃ (LSCO) buffer layer or by annealing amorphous SBT films deposited on Pt/TiO₂/SiO₂/Si substrates at 450 °C using rapid thermal annealing (RTA). These films exhibit good electric properties; 2Pr of the films are up to 12 μC/cm² and 17 μC/cm², respectively. The much larger 2Pr of the films deposited on the LSCO buffer layer and of the films obtained by RTA than 2Pr of the films deposited on Pt/TiO₂/SiO₂/Si substrates at 830 °C is attributed to a stronger (200) texture. Received: 30 January 2001 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Interaction of running gratings of the space charge and conductivity in photorefractive Bi<Subscript>12</Subscript>Si(Ti)O<Subscript>20</Subscript> crystals

The simultaneous excitation and nonlinear interaction of the space-charge and photoconductivity gratings are studied experimentally in photorefractive Bi₁₂SiO₂₀ and Bi₁₂TiO₂₀. The measurements are performed using the diffraction technique, which implies the illumination of the crystal by an oscillating interference pattern (_r=532 nm) along with the application of dc and ac electric fields and detection of the diffracted probe beam (_p=650 nm). Such illumination excites the running photoconductivity grating, which interacts with the ac component of the applied field giving rise to the space-charge wave. Being the eigenmode of the space-charge oscillations, this wave reveals itself as a low-frequency resonant maximum at the frequency-transfer function of the detected signal. The drift mobilities of electrons are estimated using the developed technique: =(1.1–1.4)×10^-2 cm²/Vs (Bi₁₂SiO₂₀, T=296–298 K) and =2.8×10^-3 cm²/Vs (Bi₁₂TiO₂₀, T=293 K). PACS 42.65.Sf; 42.70.Nq     

Effects of thickness on the infrared optical properties of Ba<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>TiO<Subscript>3</Subscript> ferroelectric thin films

Using infrared spectroscopic ellipsometry (IRSE), the optical properties of the Ba_0.9Sr_0.1TiO₃ (BST) ferroelectric thin films with different film thicknesses on Pt/Ti/SiO₂/Si substrates prepared by a modified sol-gel method have been investigated in the 2.5–12.6 m wavelength range. By fitting the measured ellipsometric parameter ( and ) data with a three-phase model (Air/BST/Pt) and the classical dispersion relation for the BST thin films, the optical constants and thicknesses of the thin films have been obtained. The average thickness of the single layer decreases with increasing film thickness. The refractive index of the BST films decreases with increasing thickness in the wavelength range 2.5–11 m, and increases with increasing thickness in the wavelength range 11–12.6 m. However, the extinction coefficient of the BST films monotonously decreases with increasing thickness. It is closely associated with the crystallinity of the thin films, the crystalline size effect and the influence of the interface layer. The absorption coefficient of the BST films with different thicknesses decreases with increasing thickness. PACS 77.55.+f; 78.20.Ci; 78.30.Am; 81.70.Fy; 81.40.Tv     

Ferroelectric and dielectric properties of SrBi<Subscript>4</Subscript>Ti<Subscript>4</Subscript>O<Subscript>15</Subscript> thin films grown on Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> film layer

Ferroelectric and dielectric properties of bilayered ferroelectric thin films, SrBi₄Ti₄O₁₅ grown on Bi₄Ti₃O₁₂, were investigated. The thin films were annealed at 700°C under oxygen atmosphere. The bilayered thin films were prepared on a Pt(111)/Ti/SiO₂/Si substrate by a chemical solution deposition method. The dielectric constant and dielectric loss of the bilayered thin films were 645 and 0.09, respectively, at 100 kHz. The value of remnant polarization (2P _r) measured from the ferroelectric thin film capacitors was 60.5 μC/cm² at electric field of 200 kV/cm. The remnant polarization was reduced by 22% of the initial value after 10¹⁰ switching cycles. The results showed that the ferroelectric and dielectric properties of the SrBi₄Ti₄O₁₅ on Bi₄Ti₃O₁₂ ferroelectric thin films were better than those of the SrBi₄Ti₄O₁₅ grown on a Pt-coated Si substrate suggesting that the improved properties may be due to the different nucleation and growth kinetics of SrBi₄Ti₄O₁₅ on the c-axis-oriented Bi₄Ti₃O₁₂ layer or on the Pt-coated Si substrate.     

Structural and electrical properties of (Na0.85K0.15)0.5Bi0.5TiO3 thin films deposited on LaNiO3 and Pt bottom electrodes

(Na_0.85K_0.15)_0.5Bi_0.5TiO₃ thin films were deposited on LaNiO₃(LNO)/SiO₂/Si(1 0 0) and Pt/Ti/SiO₂/Si(1 0 0) substrates by metal-organic decomposition, and the effects of bottom electrodes LNO and Pt on the ferroelectric, dielectric and piezoelectric properties were investigated by ferroelectric tester, impedance analyzer and scanning probe microscopy, respectively. For the thin films deposited on LNO and Pt electrodes, the remnant polarization 2P_r are about 22.6 and 8.8 μC/cm² under 375 kV/cm, the dielectric constants 238 and 579 at 10 kHz, the dielectric losses 0.06 and 0.30 at 10 kHz, the statistic d_33eff values 95 and 81 pm/V. The improved piezoelectric properties could make (Na_1−xK_x)_0.5Bi_0.5TiO₃ thin film as a promising candidate for piezoelectric thin film devices.     

Infrared Optical Characterization of PLT Thin Films for Applications in Uncooled Infrared Detectors

Lanthanum-modified lead titanate (PLT) thin films have been grown directly on Pt/Ti/SiO₂/Si (100) and LaNiO₃/Si (100) substrates by a modified sol-gel method. X-ray diffraction analysis shows that the PLT thin films are polycrystalline. The infrared optical properties of the thin films were investigated using infrared spectroscopic ellipsometry (IRSE) in the spectral range of 2.5–12.5 m. By fitting the measured ellipsometric parameter (tan and cos) data with a three phase model (air/PLT/Pt) for the PLT thin films on Pt/Ti/SiO₂/Si (100) and a four phase model (air/PLT/LNO/Si) for the PLT thin films on LaNiO₃/Si (100) substrates, and a derived classical dispersion relation for the thin films, the optical constants and thicknesses of the thin films have been simultaneously obtained. The refractive index and extinction coefficient of the PLT thin films on Pt/Ti/SiO₂/Si (100) substrates are slightly larger than those on LaNiO₃/Si (100) substrates. Given the infrared semitransparent metal of Nickel currently used, the absorption of the Ni/PLT/Pt and Ni/PLT/LNO/Si multilayer thin films in this study is very large around 3.0 m and 5.7 m wavelength range and decrease to 15% or 20% in the 8–12.5 m wavelength region.     

Asymmetric capacitors based on TiO<Subscript>2</Subscript> and mesoporous MnO<Subscript>2</Subscript> electrodes using neutral aqueous electrolyte

Asymmetric capacitor based on TiO₂ with the size range from 90 to 410 nm and mesoporous MnO₂ (ca. 200–380 nm) electrodes has been successfully constructed and characterized in LiClO₄ aqueous electrolyte. The samples of both metal oxides were fully characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray analysis (EDX), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), N₂ adsorption-desorption, and so on. The electrochemical capacitive performances of both electrode materials were evaluated by cyclic voltammetry and galvanostatic charge-discharge in 1 mol/L LiClO₄ with a working voltage of 2.0 V. The discharge profile of the asymmetric capacitor exhibited an excellent capacitive behavior and good cycling stability after 2000 cycles. Moreover, the TiO₂//MnO₂ asymmetric capacitor possesses both higher energy density and power density (7.7 Wh/kg, 762.5 W/kg) than that of Maxsorb//Maxsorb symmetrical capacitor (7.0 Wh/kg, 400.0 W/kg).

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Graphical abstract A novel asymmetric capacitor based on TiO₂ and mesoporous MnO₂ electrodes has been successfully constructed and characterized in LiClO₄ aqueous electrolyte.

     

The electrical properties and phase transformation of PLZST 2/85/13/2 antiferroelectric thin films on different bottom electrode

Pb_0.97La_0.02(Zr_0.85Sn_0.13Ti_0.02)O₃ (PLZST 2/85/13/2) antiferroelectric thin films were deposited on Pt(111)/Ti/SiO₂/Si and LaNiO₃(LNO)/SiO₂/Si substrates through a modified sol-gel process. The phase structure and microstructure of PLZST 2/85/13/2 antiferroelectric thin films were analysed by x-ray diffraction (XRD), scanning electron microcopy (SEM) and field-emission SEM (FE-SEM). The antiferroelectric nature of the PLZST 2/85/13/2 thin films on two electrodes was demonstrated by the C-V (capacitance-voltage) and P-E (polarization-electric field) measurement. The maximum polarizations for PLZST 2/85/13/2 films on Pt and LNO electrodes were 42 and 18 μC/cm², respectively. The temperature dependence of the dielectric property of the PLZST 2/85/13/2 films was measured under different dc electric fields. Also, the phase transformation of the PLZST 2/85/13/2 films was studied in detail as a function of temperature and dc electric field.     

Effect of thermal strain on structure and polarization fatigue of CSD-derived PbZr<Subscript>0.53</Subscript>Ti<Subscript>0.47</Subscript>O<Subscript>3</Subscript>/LaNiO<Subscript>3</Subscript> hetero-structures

PbZr_0.53Ti_0.47O₃/LaNiO₃ (PZT/LNO) hetero-structures have been successfully deposited on MgO, SrTiO₃, Al₂O₃ and Si substrate by chemical solution routes, respectively. The X-ray diffraction measurements show that out-of-plane lattice parameters of PZT increase as increase of thermal expansion coefficient of substrate. Polarization fatigues of Pt/PZT/LNO capacitors are strongly affected by the thermal strain caused by difference of thermal expansion coefficient between PZT and substrate materials. High fatigue resistance of Pt/PZT/LNO can be obtained by using substrate with similar thermal expansion coefficient as PZT. PACS 77.84.Dy; 78.20.Ci; 81.20.Fw     

High quality,high-<Emphasis Type="Italic">k</Emphasis> gate dielectric: amorphous LaAlO<Subscript>3</Subscript> thin films grown on Si(100) without Si interfacial layer

High-k dielectric amorphous LaAlO₃ thin films have been grown on n-type Si(100) substrates by laser molecular beam epitaxy. The interfacial characteristics of the LaAlO₃ films on Si were measured by high-resolution transmission electron micrography and X-ray photoemission spectroscopy. A sharp interface without a silica inter-layer between the LaAlO₃ film and Si substrate was observed. Atomic force microscopy measurements indicated that the root-mean-square surface roughness within a 2m×2m area of the LAO films was 0.12 nm. The flatband voltage and fixed charge density of the 8 nm thickness LaAlO₃ films were about 0.355 V and 1.8×10¹²/cm², respectively. The leakage currents of the LaAlO₃/Si(100) samples with different film thicknesses of 5, 8 and 12 nm were 0.45, 0.23 and 0.05 mA/cm², respectively, at a +1 V dc bias voltage. LaAlO₃ appears to be one of the most promising high dielectric constant materials for use in future ultra large scale integrated devices. PACS 77.55+f; 77.84.Bw; 73.40.Qv     

Barium silicate modified strontium bismuth tantalate ferroelectric thin films

Novel strontium bismuth tantalate (Sr_0.8Bi_2.2Ta₂O₉ (SBT)) modified with 3 and 5 mol% ratio barium silicate (Ba₂SiO₄) thin films were grown on Pt(100?nm)/Ti(50?nm)/SiO₂/Si(100) substrates by spin coating technique. The influence of barium silicate doping in SBT was studied from the view point of changing dielectric and ferroelectric properties like dielectric constant (ε_r) and remnant polarization (P_r). Well crystallized thin films showed convenient ferroelectric properties with comparatively lower P_r in the range between 1.52 and 0.44 µC/cm² and smaller ε_r value of 163. Thus, with such reduced values of P_r and ε_r barium silicate modified SBT offers a useful potential to be used in Ferroelectric Field Effect Transistor (FeFET) type (1T-type) Ferroelectric Random Access Memories (FeRAMs) upon improving insulation properties.     

Ferroelectric properties of bilayer structured Pb(Zr0.52Ti0.48)O3/SrBi2Ta2O9 (PZT/SBT) thin films on Pt/TiO2/SiO2/Si substrates

Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin films with large remanent polarization and SrBi₂Ta₂O₉ (SBT) thin films with excellent fatigue-resisting characteristic have been widely studied for non-volatile random access memories, respectively. To combine these two advantages_, bilayered Pb(Zr_0.52Ti_0.48)O₃/SrBi₂Ta₂O₉ (PZT/SBT) thin films were fabricated on Pt/TiO₂/SiO₂/Si substrates by chemical solution deposition method. X-ray diffraction patterns revealed that the diffraction peaks of PZT/SBT thin films were completely composed of PZT and SBT, and no other secondary phase was observed. The electrical properties of the bilayered structure PZT/SBT films have been investigated in comparison with pure PZT and SBT films. PZT/SBT bilayered thin films showed larger remanent polarization (2P_r) of 18.37 μC/cm² than pure SBT and less polarization fatigue up to 1 × 10⁹ switching cycles than pure PZT. These results indicated that this bilayered structure of PZT/SBT is a promising material combination for ferroelectric memory applications.     

Effects of weathering and rainfall conditions on the release of SiO<Subscript>2</Subscript>, Ag,and TiO<Subscript>2</Subscript> engineered nanoparticles from paints

Previous studies on the fate of engineered nanoparticles (ENPs) incorporated in paints mainly focused on the release of the particles as affected by a limited number of factors or monitoring their release from natural sources. In this study, the effects of four factors (i.e., weathering duration, water pH, rainfall duration and intensity) were investigated on the release of SiO₂-ENPs, Ag-ENPs, and TiO₂-ENPs from paints applied on panels. The static water immersion test showed that the concentrations of studied particles all increased with weathering duration. At low and high pH, SiO₂-ENPs and Ag-ENPs showed a higher release, while the release of TiO₂-ENPs was relatively high at low pH. With increased simulated rainfall duration, the concentration released decreased for Si, and the opposite was observed for Ag, while no obvious correlation was noted for Ti. With greater rainfall intensity, there was increasing release of all particles. In total, the releases of Ag-ENPs and TiO₂-ENPs were extremely low and within the level of 21.32–42.16 μg L^?1and 0.6–2.3 μg L^?1, respectively, while the values for SiO₂-ENPs were in the range of 7.5–12 mg L^?1. Additionally, microscopic results highlighted that SiO₂-ENPs were mainly released in the form of agglomerates, and only a small fraction was below 0.1 μm. Considering these influence factors together, conclusions may be made that weathering time and rainfall duration are more important in controlling release than water pH.

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Graphical abstract ?