CaBi4Ti4O15 Thin Films Research Articles

Effect of oxygen partial pressure on structural and optical properties of pulsed laser deposited CaBi4Ti4O15 thin films

The influence of oxygen partial pressure (OPP) on the structural and optical properties of CaBi4Ti4O15 (CBTi) thin films deposited by pulsed laser deposition have been investigated in the range of 0.1mbar to 7.8×10−3mbar. The structural properties show all the films are polycystlline in nature with orthorombic structure. The optical transmission of the films is in the range of 60–90%. A slight shift in transmission threshold towards higher wavelength region with an increase in O2 pressure reveals the systematic reduction in the optical band gap energy (3.69 to 3.59eV) of the films. Raman studies confirm the phase formation and presence of stresses in the films. It is suggested that the OPP played a key role in controlling crystallinity, morphology, chemical composition and optical properties in CBTi thin films.

Effect of substrate temperature on the optical properties of CaBi4Ti4O15 thin films deposited by pulsed laser ablation

Thin films of CaBi4Ti4O15 (CBTi) were deposited at different substrate temperatures (550–700 °C) using pulsed laser ablation technique. Structural, morphological and linear optical properties of the same were investigated. The CBTi thin films crystallize above 550 °C and forms single phase is confirmed by XRD and Raman spectroscopy. The lattice strain induced during deposition, controls the unit cell volume, grain size and crystallite size. The refractive index, real and imaginary parts of optical dielectric constant and optical band gap were extracted from transmission spectra (190–2500 nm). Tauc’s plot confirmed the direct band gap nature ranging from 3.4 to 3.6 eV. The study on CBTi thin films and their optical properties opens up a new window for optical applications.

Bipolar resistive switching behaviours of perovskite CaBi4Ti4O15 thin films

The perovskite CaBi4Ti4O15 thin film for resistive random access memory device application has been investigated in this work. Experimentally, the CaBi4Ti4O15 thin films were deposited on the indium-tin oxide/glass substrates by radio-frequency (RF) magnetron sputtering and then annealed by rapid thermal annealing. The annealing effects on the bipolar resistive switching behaviours and the conduction mechanisms of the Al/CaBi4Ti4O15/indium-tin oxide resistive random access memory structure with a compliance current of 0·1 A were discussed in this work. The results show a stable bipolar resistive switching with a retention time of over 1 × 104 s and a high resistance state/low resistance state ratio larger than 5 × 102.

The Ferroelectric and Electrical Properties of CaBi<sub>4</sub>Ti<sub>4</sub>O<sub>15</sub> Thin Films Prepared by Sol-Gel Technology

In this study, thin films of CaBi4Ti4O15 with preferential crystal orientation were prepared by the chemical solution deposition (CSD) technique on a SiO2/Si substrate. The films consisted of a crystalline phase of bismuth-layer-structured dielectric. The as-deposited CaBi4Ti4O15 thin films were crystallized in a conventional furnace annealing (RTA) under the temperature of 700 to 800°C for 1min. Structural and morphological characterization of the CBT thin films were investigated by X-ray diffraction (XRD) and field-emission scanning electron microscope (FE-SEM). The impedance analyzer HP4294A and HP4156C semiconductor parameters analyzer were used to measurement capacitance voltage (C-V) characteristics and leakage current density of electric field (J-E) characteristics by metal-ferroelectric-insulator- semiconductor (MFIS) structure. By the experimental result the CBT thin film in electrical field 20V, annealing temperature in 750°C the CBT thin film leaks the electric current is 1.88x10-7 A/cm2 and the memory window is 1.2V. In addition, we found the strongest (119) peak of as-deposited thin films as the annealed temperature of 750°C

Crystal Structure and Dielectric Property of Bismuth Layer-Structured Dielectric Films with c-Axis Preferential Crystal Orientation

Thin films of bismuth layer-structured dielectrics (BLSDs), CaBi4Ti4O15, and SrBi4Ti4O15, were prepared by a chemical solution deposition (CSD) technique on various substrates, such as (111)Pt/TiO2/(100)Si, (100)LaNiO3/(111)Pt/TiO2/(100)Si, and (100)SrRuO3∥(100)SrTiO3 substrates. Conductive perovskite oxide LaNiO3 with (100) preferential crystal orientation was introduced into the interface between the BLSD film and the (111)Pt/TiO2/(100)Si substrate to control the crystal orientation of BLSD by lattice matching between pseudo-perovskite blocks in the BLSD crystal and the (100)LaNiO3 plane with the perovskite structure. The (00l) planes of BLSD crystals were preferentially oriented on the substrate surface of the (100)LaNiO3/(111)Pt/TiO2/(100)Si, whereas randomly-oriented BLSD crystals with lower crystallinity were only obtained on the surface of (111)Pt/TiO2/(100)Si substrate. The (001)-oriented BLSD films exhibited the leakage current densities below 10-7 A/cm2 at ±50 kV/cm, which is significantly lower than those for randomly-oriented films, above 10-6 A/cm2, The room-temperature dielectric constants (εr) of CaBi4Ti4O15 and SrBi4Ti4O15 thin films on the (100)LaNiO3/(111)Pt/TiO2/(100)Si substrate were both approximately 250, while those on the (100)SrRuO3∥(100)SrTiO3 substrate were approximately 220. The temperature dependence of the capacitances for the CaBi4Ti4O15 and SrBi4Ti4O15 films on the (100)LaNiO3/(111)Pt/TiO2/(100)Si substrate were approximately +17 and +10%, respectively, in the temperature range from 25 to 400 °C. These values were slightly larger than those of epitaxial BLSD films, but smaller than those of (Ba,Sr)TiO3 films.

Retention Characteristics of CBTi144 Thin Films Explained by Means of X-Ray Photoemission Spectroscopy

CaBi4Ti4O15(CBTi144) thin films were grown on Pt/Ti/SiO2/Si substrates using a soft chemical solution and spin-coating method. Structure and morphology of the films were characterized by the X-ray Diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), Raman analysis, X-ray photoemission spectroscopy (XPS), and transmission electron microscopy (TEM). The films present a single phase of layered-structured perovskite with polar axis orient. Thea/b-axis orientation of the ferroelectric film is considered to be associated with the preferred orientation of the Pt bottom electrode. XPS measurements were employed to understand the nature of defects on the retention behavior of CBTi144 films. We have observed that the main source of retention-free characteristic of the capacitors is the oxygen environment in the CBTi144 lattice.

Preparation of (001)-Oriented CaBi4Ti4O15 and SrBi4Ti4O15 Films Using LaNiO3 Nucleation Layer on Pt-passivated Si Wafer

Thin films of CaBi4Ti4O15 and SrBi4Ti4O15 with preferential crystal orientation were prepared by the chemical solution deposition (CSD) technique on a (100)LaNiO3/(111)Pt/(100)Si substrate. The films consisted of a crystalline phase of bismuth-layer-structured dielectric (BLSDs). The (00l) planes of a BLSD crystal were preferentially oriented to the surface of the (100)LaNiO3/(111)Pt/(100)Si substrate, which is ascribed to the lattice matching between pseudo perovskite blocks in the BLSD crystal and the (100)LaNiO3 plane with a perovskite structure. Frequent heat treatment for crystallization, i.e., once after each coating, also enhanced the degree of crystallinity and crystal orientation of the BLSD films. The dielectric constants (εr) of SrBi4Ti4O15 and CaBi4Ti4O15 thin films were approximately 250 at room temperature. The εr values of both films increased with ambient temperature. The variations in capacitance in the temperature range from 25 to 200 °C were approximately 3 and 5% respectively, which were significantly smaller than that in the (Ba,Sr)TiO3 thin film.

ORIENTATION DEPENDENCE OF FERROELECTRIC AND DIELECTRIC PROPERTIES IN CaBi4Ti4O15 THIN FILMS

ABSTRACT Ferroelectric CaBi4Ti4O15 (CBTi) thin films with random orientation and high c-axis orientation were prepared on a Pt(111)/Ti/SiO2/Si(100) substrate by a chemical solution deposition method. We observed a strong dependence of ferroelectric and dielectric properties on the film orientation. The CBTi thin film with random orientation exhibited remnant polarization (2P r ) of 54 μ C/cm2 at an applied electric field of 250 kV/cm and dielectric constant (ϵ) of 412 at a frequency of 1 kHz. The values of 2P r and ϵ were larger than those observed in the CBTi thin film with high c-axis orientation. In addition, the randomly oriented thin film showed lower leakage current density than the highly c-axis-oriented thin film.

Conduction Behaviors of CaBi4Ti4O15 Thin Films Prepared by Using Chemical Solution Deposition

Conduction Behaviors of CaBi4Ti4O15 Thin Films Prepared by Using Chemical Solution Deposition

Enhanced ferroelectric properties of predominantly (100)-oriented CaBi4Ti4O15 thin films on Pt∕Ti∕SiO2∕Si substrates

Predominantly (100)-oriented CaBi4Ti4O15 (CBTi) films were fabricated on Pt (111)∕Ti∕SiO2∕Si substrates using a metal organic decomposition method at annealing temperatures ranging from 600to800°C. The growth mode of the predominantly (100)-oriented CBTi films fabricated by the sequential layer annealing method was discussed based on the structure evolution with the annealing temperature. The remanent polarization and coercive field of the CBTi film annealed at 750°C are 38.1μC∕cm2 and 216kV∕cm, respectively. No fatigue can be observed after 109 switching cycles. The remanent polarization of the purely a-axis-oriented CBTi film should be higher than 50μC∕cm2.

Nature of defects for bismuth layered thin films grown on Pt electrodes

The authors investigated the influence of defects on the piezoelectric and dielectric properties of Bi4Ti3O12 (BIT), SrBi4Ti4O15 (SBTi) and CaBi4Ti4O15 (CBTi144) thin films by x-ray photoemission spectroscopy measurements. In the SBTi film, Sr which is a nonpolarizable ion restricting the movement of Ti4+ ions and thus leads to a low piezoresponse. Meanwhile, the oxygen environment is quite different in the BIT and CBTi144 films exhibiting excellent piezoelectric properties. The piezoelectric coefficient and the dielectric behavior were larger for a-b axis oriented than for c axis-oriented films due to the defects created during the films crystallization.

Effect of amorphous TiO2 buffer layer on the phase formation of CaBi4Ti4O15 ferroelectric thin films

Amorphous buffer layers of TiO2 were introduced between CaBi4Ti4O15 thin films and Pt bottom electrodes through chemical solution deposition techniques. Several tens of nanometers thick alkoxy-derived TiO2 layers were found to be critical for acceleration of the phase transition in the thin films. Unlike thin film crystallized directly onto a highly (111) oriented Pt bottom electrode, the thin film on an amorphous TiO2 layer was almost single phase perovskite and showed random orientation. The crystallinity of the CaBi4Ti4O15 thin film was much higher than that crystallized directly onto the Pt bottom electrode. The dielectric and ferroelectric properties of the CaBi4Ti4O15/TiO2 stacked thin films on Pt coated Si substrates are evaluated, leading to the potential of the amorphous buffer layer for the integrated devices being clarified.

Crystal Phase and Orientation Control in Integrated Ferroelectric CaBi4Ti4O15 Using a Tailored Liquid of Alkoxides

We used a Ca–Bi–Ti complex alkoxide, in which metal–oxygen bonding was confirmed by spectroscopic analysis, to deposit CaBi4Ti4O15 (CBTi144) thin films in various configurations. The phase transition of non‐ferroelectric pyrochlore to ferroelectric perovskite in the complex‐alkoxy‐derived CBTi144 thin films was found to depend on the Pt bottom electrodes. Matching of the atomic arrangement to the Ca–Bi–Ti–O thin films was predominant rather than the strain and crystallinity of the bottom electrode. The thin films crystallized at 650°C on (111)‐oriented Pt showed random orientation and ferroelectric P–V hysteresis loops. The endurance property was excellent against a number of switchings. For this reason, CBTi144 thin films would be expected to be excellent for application to ferroelectric random access memories (FeRAM). Polar‐axis‐oriented CBTi144 films were fabricated on Pt foils using the complex metal alkoxide solution. The 500‐nm‐thick film had a columnar structure comprising well‐developed grains. The a/b‐axis orientation of the ferroelectric films is considered to be associated with the preferred orientation of Pt foil. The film showed improved ferro‐ and piezoelectric properties. The Pr, Ec, and d33 values were enhanced to become twice those of CBTi144 thin films with random orientation. These polar‐axis‐oriented CBTi144 films are eminently useful in devices as Pb‐free piezoelectric materials.

Ferroelectric Properties of MBi4Ti4O15 (M = Sr, Pb, Ca) Thin Films

Among the ferroelectric bismuth layered perovskites belonging to Aurivillius family, the MBi4Ti4O15 (MBT15, M = Sr, Ca, Pb) thin films have been fabricated by a chemical solution deposition technique, and their structures and ferroelectric properties were investigated in this work. The SrBi4Ti4O15 (SBT15) film fabricated on IrO2 presented the highest remanent polarization (Pr) among the films in this series. It demonstrated a saturated hysteresis loop at 5 V with Pr of 19 μ C/cm2 and coercive field (Ps) of 116 kV/cm. The PbBi4Ti4O15 (PBT15) thin films were selectively controlled in c-axis and off-c-axis orientation on Pt layer by adjusting the annealing condition. The off-c-axis oriented PbBi4Ti4O15 film demonstrated considerably higher Pr (8.7 μ C/cm2) than that of c-axis oriented film (3.7 μ C/cm2). Regardless of grain orientation, however, PBT15 films were not fatigued up to 1010 cycles under 9V application. The CaBi4Ti4O15 (CBT15) thin films were also produced on Pt and IrO2 electrode, respectively, by controlling the annealing condition. The CBT15 film on IrO2 presented higher Pr (∼10.8 μ C/cm2) than that of film (∼ 6.7 μ C/cm2) on Pt. † Present address: Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, Korea

Novel Ferroelectric Candidates in a Series of ABi4Ti4O15 (A: Alkaline Earth Metals) Thin Films

ABi4Ti4O15 (A: alkaline earth metals) thin films were prepared on Pt-passivated Si substrates by using the precursor solution of metal alkoxides. As-deposited thin films initiated crystallization below 550°C and enhanced the crystallinity at higher temperature via rapid thermal annealing in oxygen. The crystallinity of the ABi4Ti4O15 thin films depended on the A-site cation of the alkaline earth metals but the content of the pyrochlore phase as a second phase was independent of the cation. The phase purity of BLSFs strongly depended on the crystallinity and crystallographic orientation of the Pt bottoms. The 650°C-annealed CaBi4Ti4O15 thin films on the relatively low-crystalline Pt was a single phase of the BLSFs and exhibited better ferroelectric properties.

Platinum-assisted phase transition in bismuth-based layer-structured ferroelectric CaBi4Ti4O15 thin films

The phase transition of nonferroelectric pyrochlore to ferroelectric perovskite in CaBi4Ti4O15 thin films depends on platinum bottom electrodes. Rather than the strain and crystallinity of the bottom electrode, matching of the atomic arrangement to the Ca–Bi–Ti–O thin films is predominant. CaBi4Ti4O15 thin films crystallized on (200)-oriented platinum at 650 °C showed c-axis orientation. In contrast, thin films crystallized on highly crystalline (111)-oriented platinum at the same temperature contained pyrochlore grains which were about several tens of nanometers in diameter and located in the interface region. They showed P–V hysteresis loops. The remanent polarization and coercive electric field depended on platinum top electrode size.

Platinum-Accelerated Phase Transition in Bismuth-Based Layer-Structured Ferroelectric Thin Films

ABSTRACTThe phase transition of non ferroelectric pyrochlore to ferroelectric perovskite in CaBi4Ti4O15 thin films depended on matching of the atomic arrangements in platinum bottom electrodes to the Ca-Bi-Ti-O thin films. CaBi4Ti4O15 thin films crystallized on (200)-oriented platinum at 650°C showed c-axis orientation. In contrast, thin films randomly crystallized on highly crystalline (111)-oriented platinum at the same temperature contained pyrochlore phase and showed P-V hysteresis loops. The ferroelectric properties improved with the degrees of (h00) orientation.

Comparison of Microstructure and Ferroelectric Properties of Alkoxy-Derived MBi4Ti4O15 (M: Ca or Sr) Thin Films

CaBi4Ti4O15 (CBTi144) and SrBi4Ti4O15 (SBTi144) thin films were prepared on Pt-passivated silicon substrate using complex metal alkoxide solutions. The CBTi144 thin films crystallized to a single phase of perovskite at 650°C via a mixture of perovskite and pyrochlore phases. The 650°C-annealed CBTi144 thin films consisted of uniform and isotropic grains and had a closely packed columnar structure. In contrast, the SBTi144 thin films crystallized to the perovskite phase at relatively lower temperature and consisted of smaller and non-uniform grains. The dielectric constants and loss factors of the 650°C-annealed CBTi144 and SBTi144 thin films were 300 and 0.03, and 330 and 0.04, respectively, at 100 kHz. The remanent polarizations and coercive electric fields were 6.0 µC/cm2 and 79 kV/cm, and 2.9 µC/cm2 and 55 kV/cm, respectively, at 9 V. The dielectric and ferroelectric properties depended on both the cation size in the A site and the microstructure.

Ferroelectric properties of alkoxy-derived CaBi4Ti4O15 thin films on Pt-passivated Si

CaBi 4 Ti 4 O 15 (CBTi144) thin films were prepared by spin coating a precursor solution of metal alkoxides. As-deposited thin films began crystallization below 550 °C and reached full crystallinity of a single phase of layered perovskite at 650 °C via rapid thermal annealing in oxygen. The 650 °C annealed CBTi144 thin film showed random orientation on Pt-passivated Si substrate and exhibited P-E hysteresis loops. The remanent polarization (Pr) and coercive electric field (Ec) were 9.4 μC/cm2 and 106 kV/cm, respectively, at 11 V. The dielectric constant and loss factor were 300 and 0.033, respectively, at 100 kHz.

Chemical processing and characterization of ferroelectric thin films of bismuth-based layer-structured perovsktte cabi4ti4o15 with the octahedron number of 4

CaBi4Ti4O15 (CBTi144) thin films were prepared by spin-coating a precursor solution of metal alkoxides. As-deposited thin films began crystallization below 550°C and reached full crystallinity of a single phase of layered-perovskite at 650°C via rapid thermal annealing in oxygen. 650°C-annealed CBTi144 thin film showed random orientation and had a columnar structure on Pt-passivated Si substrate. The dielectric constant and loss factor were 300 and 0.033, respectively, at 100 kHz. The thin film exhibited P-E hysteresis loops. The remanent polarization (Pr) and coercive electric field (Ec) were 6.0 μC/cm2 and 78.7 kV/cm, respectively, at 9 V. The thin film showed good endurance properties against number of switching cycles.