Preparation of Bi4Ti3O12 thin films by electron cyclotron resonance sputtering

Abstract

Ferroelectric Bi₄Ti₃O₁₂ thin films were deposited on Pt-coated oxidized Si substrate by electron cyclotron resonance (ECR) sputtering using ceramic targets. Crystal structure and dielectric properties of the films were investigated as functions of sputtering conditions such as substrate temperature and sputtering gas. Using a target with excess Bi content compared to stoichiometric composition was required to compensate Bi re-evaporation from the substrate and to obtain a perovskite single phase at 600°C. (117)-oriented films exhibited ferroelectric hysteresis loops. The remanent polarization and coercive field of the films were 9.8 μC/cm² and 180 kV/cm, respectively.     

Fatigue Melioration of Neodymium-Modified Bi4Ti3O12 Ceramics

Pure and Nd-modified Bi₄Ti₃O₁₂ ceramics were prepared using the conventional solid state reaction method and their dielectric properties and mechanical properties are investigated. It shows that the activation energy of oxygen vacancies is enhanced whereas the concentration of oxygen vacancies is reduced when Bi³⁺ ions are partially substituted by Nd³⁺ ions. The Cole-Cole fitting to the dielectric loss reveals a strong correlation among oxygen vacancies. The strong correlation reduces the activation energy of oxygen vacancies efficiently. Therefore, we conclude that the diluted oxygen vacancies concentration is the origin of the excellent fatigue resistance of Nd-modified Bi₄Ti₃O₁₂ materials.     

Dielectric behavior of Bi2/3Cu3Ti4O12 ceramic and thick films

The paper reports on synthesis, sintering and microstructure of Bi_2/3Cu₃Ti₄O₁₂, a lead-free, high-permittivity material with internal barrier layer capacitor behavior. Complex impedance and capacitance of the ceramic and thick films were studied as a function of frequency (10 Hz–2 MHz) and temperature (−170 to 400°C). Dc electrical conductivity of the samples was measured in the temperature range 20–400°C. Broad and high maxima of dielectric permittivity versus temperature plots were observed reaching 60,000 for ceramic and 5,000 for thick films. The maxima decrease and shift to higher temperatures with increasing frequency. Two arcs ascribed to grains and grain boundaries were found in the plots of imaginary part versus real part of impedance. Analysis of the impedance spectra indicates that Bi_2/3Cu₃Ti₄O₁₂ ceramic could be regarded as electrically heterogeneous system composed of semiconducting grains and less conducting grain boundaries. The developed thick film capacitors with dielectric layers based on Bi_2/3Cu₃Ti₄O₁₂ exhibit dense microstructure, good cooperation with Ag electrodes, high permittivity up to 5,000 and relatively low temperature coefficient of capacitance in the temperature range 100–300°C. Broad maxima in the dielectric permittivity versus temperature curves may be attributed to Maxwell–Wagner relaxation.     

Asymmetric capacitance-voltage characteristics of (Bi3.25, La0.75)Ti3O12 thin films grown on Si

Abstract

Electrical properties of Lamodified bismuth titanate Bi_3.25La_0.75Ti₃O₁₂) thin films for a metal-ferroelectric-insulator-semiconductor (MFIS) structure were investigated with capacitance-voltage (C-V). The MFIS structure exhibits progressively increasing C-V memory window with a sweep voltage due to ferroelectric polarization with suppressed charge injection. Moreover, the asymmetric shift of threshold voltage with a sweep voltage was observed. The flat-band voltage (V_fb2) at the negative sweep was gradually increased with a sweep voltage. The flatband voltage (V_fb1) at the positive sweep decreased at low sweep voltages and then increased at further high voltages (i.e., V_fb1 shift toward the positive direction rather than the negative direction). The asymmetric behavior of C-V characteristics was attributed to negative trapped charges by electron injection from Si.     

Formation Mechanism of Plate-like Bi4Ti3O12 Particles in Molten Salt Fluxes

The plate-like Bi₄Ti₃O₁₂ particles were prepared by molten salt synthesis method. The influence of sintering temperature and cooling process on the microstructure of Bi₄Ti₃O₁₂ powders was studied. Much larger particles were formed at higher temperatures. The particles could grow larger in slow cooling process. The formation mechanism of plate-like Bi₄Ti₃O₁₂ particles in Na₂SO₄-K₂SO₄ system could be viewed as four processes: (1) solid reaction and nucleation, (2) plate-like structure formation, (3) diffusion and edge nucleation, (4) diffusion and epitaxial growth.     

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.     

Flux-mediated epitaxy for ferroelectric Bi4Ti3O12 single crystal film growth

We propose the “Flux-mediated epitaxy” as a novel concept for the growth of single crystalline films of incongruent, volatile, and high-temperature-melting compounds. In flux-mediated eptitaxy, by supplying materials precursors from the gas phase through the liquid flux films pre-deposited on the substrate, a quasi-thermodynamic equilibrium condition is obtained at the interface between the growing films and the flux films. This process has been demonstrated in this paper by fabricating ferroelectric Bi₄Ti₃O₁₂ films, which has volatile Bi oxide. The most important step in this process is the selection of the right flux material, which is hard to predict due to the lack of an appropriate phase diagram. In order to overcome this problem, we have selected the combinatorial approach. A series of ternary flux libraries composed of two self-fluxes (Bi₂O₃ and Bi₄Ti₃O₁₂) and a third impurity flux were fabricated on SrTiO₃ (001) substrates. After that, stoichiometric Bi₄Ti₃O₁₂ films were grown on each of these flux libraries at a temperature presumed to melt the flux. High-throughput characterization with the concurrent X-ray diffraction method resulted in the identification of CuO containing Bi₂O₃ as the flux material for the growth of single crystalline Bi₄Ti₃O₁₂ films. Stoichiometric Bi₄Ti₃O₁₂ films fabricated by using a novel CuO containing Bi₂O₃ are qualified to be single crystalline judging from their large grain size and the electrical properties equivalent to bulk single crystal’s.     

Properties of Chemical Solution Deposited Polycrystalline Neodymium-Modified Bi4Ti3O12

Thin films of neodymium-modified bismuth titanate Bi_3.44Nd_0.56Ti₃O₁₂ (BNT) were grown on Pt/TiO₂/SiO₂/Si substrates using chemical solution deposition method. The capacitors made by applying top Au electrodes on BNT films showed significantly improved values of the remanent polarization as compared to that using bismuth titanate Bi₄Ti₃O₁₂ (BT) films. The 2P _r value for the BNT capacitors was determined to be equal to 38 C/cm² at an applied voltage of 24 V, whereas, for Bi₄Ti₃O₁₂ (BT) capacitors a value of 20 C/cm² was measured at the same applied voltage. The maximum piezoelectric and pyroelectric coefficients of 22 pm/V and 112 C/m² K respectively, were measured for the BNT thin films.     

Laser ablation preparation and property of bismuth-layer-structured SrBi2Ta2Ta2O9 and Bi4Ti3O12 ferroelectric thin films

Abstract

Bismuth-layer-structured ferroelectric thin films, SrBi₂Ta₂O₉ and Bi₄Ti₃O₁₂, have been prepared by laser ablation method on both Pt sheets and Si wafers at low temperatures of 400 ～ 500°C. These thin films have been characterized by XRD, XPS, AFM, C-V, D-E hysteresis and J-V measurement. SrBi₂Ta₂O₉ thin films have a good (105) preferential orientation, and Bi₄Ti₃O₁₂ thin films have (117) and c-axis orientation on these substrates. Ferroelectric film-SiO₂-Si structures show good C-V hysteresis curve owing to Si surface potential controlled by the D-E hysteresis. D-E hysteresis is obtained in Bi₄Ti₃O₁₂ thin film prepared on Pt sheet, and the remnant polarization and the coercive force are 7.5 μC/cm² and 72 kV/cm, respectively.     

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.     

Synthesis and properties of ferroelectric Si-doped (Bi, Nd)4Ti3O12 thin films by chemical solution deposition

Ferroelectric Si-doped (Bi,Nd)₄Ti₃O₁₂ thin films have been prepared on Pt/TiO_x/SiO₂/Si substrates through metal-organic compounds by the chemical solution deposition. The Bi_3.25Nd_0.75Ti_2.9Si_0.1O₁₂ (BNTS) precursor films were found to crystallize into the Bi-layered perovskite Bi₄Ti₃O₁₂ single-phase above 600^∘C. The synthesized BNTS films revealed a random orientation having a strong 117 reflection. The BNTS thin films prepared between 600^∘C and 700^∘C showed well-saturated P-E hysteresis loops with P _r of 13–14 μ C/cm² and E _c of 100–110 kV/cm at an applied voltage of 5 V. The surface roughness of the BNTS thin films was improved by Si doping compared with that of undoped Bi_3.35Nd_0.75Ti₃O₁₂ films.     

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.     

Structural and Multiferroic Properties of Bi3.4La0.6Ti3O12/CoFe2O4 Composite Thin Films

Bi_3.4La_0.6Ti₃O₁₂ and CoFe₂O₄ were synthesized by chemical solution route, and Bi_3.4La_0.6Ti₃O₁₂/CoFe₂O₄ multilayers were deposited by spin coating on Pt substrate. X-ray diffraction of multilayer structures reveals composite-like polycrystalline film. Leakage current is less than 10^?5 A at electric field < 90 KV/cm and follows the Ohmic behavior. Dielectric response shows relaxation and the loss (tan δ) is below 3% at 10⁶ Hz. Room temperature ferrroelectric polarization (P_r) = 20.2 μC/cm² and ferromagnetic memory (M_r) = 46.5 emu/cm³ has been obtained. Co-existence of FE and FM response can be attributed to stress and different permeability and permittivity involved in multilayer structures.     

Atomic Force Microscopy Examination of the Evolution of the Surface Morphology of Bi4Ti3O12 grown by Molecular Beam Epitaxy

The surface morphology of (0 0 1) Bi₄Ti₃O₁₂ grown on (0 0 1) SrTiO₃ by reactive molecular beam epitaxy (MBE) has been examined using atomic force microscopy (AFM). Initial nucleation of a 1/4 unit cell thick layer is followed by growth of 1/2 unit cell thick layers. Between 9 and 16 layers, a transition to 3-dimensional growth occurs, leading to well-defined mounds. This implies a Stranski-Krastonov growth mode. During growth, the morphology follows a behavior consistent with the dynamic scaling hypothesis and we extract values for the scaling exponents and from the AFM data. A thickness variation in is observed and reflects the strain relief associated with the Stranski-Krastonov growth.     

Ferroelectric properties of (Bi3.15 Nd 0.85 )Ti 3 O 12 with decreasing film thickness

Film texture and ferroelectric behaviors of (Bi_3.15Nd_0.85)Ti₃O₁₂ (BNdT) of layered-perovskite structure deposited on Pt/TiO₂/Si substrate are dependent on the film thickness. When the film thickness is reduced from ∼240 to ∼120 nm, BNdT grains evolve from a rod-like morphology to a spherical morphology, accompanied by a decrease in average grain size. At the same time, P-E hysteresis transforms from a square-shaped hysteresis loop (2Pr ∼24.1 μC/cm² at 240 nm) to a relative slimmer hysteresis loop (with a lower 2Pr = 19.8 μC/cm² at 120 nm). The nonvolatile polarization (Δ P) shows a maximum at the film thickness of 160 nm, where Δ P was measured to be 14.7 μC/cm² and 6.8 μC/cm² at 5 V and 3 V, respectively. A small amount of excess bismuth in the film thickness of 130 nm, introduced by co-sputtering, can lead to a much enhanced remanent polarization (2Pr of 21.3 μC/cm² at 5 V and 15.2 μC/cm²at 3 V), which is promising for low voltage FRAM applications.     

Inverse Capacitance-Voltage Characteristics of Bi3.25La0.75Ti3O12 Thin Film Pulsed Laser Deposited on Thermally Oxidized n-Type Si Substrates

Comparative studies on the electrical properties of a metal-ferroelectric-insulator-semiconductor field effect transistor were conducted using pulsed laser ablated ferroelectric Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films deposited on SiO₂/Si substrates with different SiO₂ thicknesses. The SiO₂ layer was prepared on n-type Si substrates by dry oxidation at a temperature of 800°C. Small angle x-ray reflectivity studies were used to measure the SiO₂ thickness. The capacitance-voltage (C-V) measurements revealed that the films showed good interfacial properties. Shifts in flatband voltages were observable, but were effectively reduced by deposition of the ferroelectric films. Au/BLT/SiO₂/Si diodes with 8 nm SiO₂ layer showed to be stable with relatively large memory window values of about 0.3 V, 2.5 V, 5.0 V, and 7.0 V, at increasing bias voltages of ±5 V, ±7 V, ±10 V, and ±12 V, respectively.     

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.     

Sol-Gel Synthesis and Sensing Study of Perovskite CaCu3Ti4O12 Nanopowders

CaCu₃Ti₄O₁₂ nanopowders were fabricated by a facile effective sol-gel route using tetrabutyl titanate, calcium acetate and cupric nitrate as precursors. The intermediate xerogel was characterized by thermal analyzer and the products with different calcination temperature were then studied by IR spectrum. Further X-ray diffraction, transmission electronic microscopy, high-resolution transmission electron microscopy and energy dispersive X-ray spectrum were used to characterize the prepared CaCu₃Ti₄O₁₂ nanopowders. Additionally, the obtained CaCu₃Ti₄O₁₂ nanopowders were investigated as a gas sensitive material responding to ethanol atmosphere, and it showed high sensitivity for ethanol gas.     

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.     

Characterization of the CeO2 thin films for insulation layer and Pt/SrBi2Ta2O9/CeO2/Si MFISFET structure

Abstract

CeO₂ and SrBi₂Ta₂O₉ (SBT) thin films for MFISFET (metal-fcrroelectrics-insulator-semiconductor field effect transistor) were deposited by rf sputtering and pulsed laser deposition method, respectively. The effects of oxygen partial pressure during deposition for CeO₂ films were investigated. The oxygen partial pressure significantly affected the preferred orientation, grain size and electrical properties of CeO₂ films. The CeO₂ thin films with a (200) preferred orientation were deposited on Si(100) substrates at 600°C. The films deposited under the oxygen partial pressure of 50 % showed the best C-V characteristics among those under various conditions. The leakage current density of films showed order of the 10^?7～10^?8 A/cm² at 100 kV/cm. The SBT thin films on CeO₂/Si substrate showed dense microstructure of polycrystalline phase. From the C-V characteristics of MFIS structure composed of the SBT film annealed at 800°C, the memory window width was 0.9 V at ±5 V. The leakage current density of Pt/SBT/CeO₂/Si structure annealed at 800°C was 4×10^?7 A/cm² at 5 V.