Lanthanum-substituted Bismuth Titanate Research Articles

Effect of dopant on piezoelectric and dielectric properties of thin films Bi-=SUB=-3.25-=/SUB=-La-=SUB=-0.75-=/SUB=-Ti-=SUB=-3-x-=/SUB=-A-=SUB=-x-=/SUB=-O-=SUB=-12-=/SUB=- (A --- Mn, Zr, Nb)

It is shown that microstructure, dielectric and piezoelectric properties change in films based on lanthanum-substituted bismuth titanate (BLT) depending on the alloying impurity material, which leads to changes in coercive voltage, internal displacement field and control coefficient. Keywords: lead-free ferroelectric films, bismuth titanate, electrophysical properties, capacitance-voltage characteristics, piezoresponse force microscopy.

Sol–gel hydrothermal synthesis and enhanced biosensing properties of nanoplated lanthanum-substituted bismuth titanate microspheres

Nanoplated lanthanum-substituted bismuth titanate (Bi3.25La0.75Ti3O12, BLTO) microspheres constructed with tens of BLTO nanoplates were synthesized by a sol–gel hydrothermal method. Using nanoplated BLTO microspheres, a novel third-generation H2O2 biosensor was fabricated with loading of myoglobin (Mb) and chitosan (Chi). Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) measurements reveal that partial Bi ions of bismuth titanate (BTO, Bi4Ti3O12) are successfully substituted with La ions by the sol–gel hydrothermal method. UV-visible (UV-vis) and Fourier-transform infrared (FTIR) spectra show that Mb encapsulated in the Mb–Chi–BLTO film can retain its bioactivity well. Comparative experiments witness that the Mb–Chi–BLTO biosensor, compared with the Mb–Chi–BTO biosensor, not only has enhanced direct electron-transfer capacity (e.g., stronger redox peak currents (approximately 3-fold) and a larger heterogeneous electron-transfer rate constant of 12.8 ± 3.3 s−1), but also exhibits a wider linear response to H2O2 in the concentration range of 2–490 μM, higher sensitivity (88 mA cm−2 M−1), a lower Michaelis–Menten constant (0.55 mM) and detection limit (0.14 μM), a shorter response time (2.8 s), and better reproducibility and stability. These results imply that La doping greatly improves electrochemical and electrocatalytic properties of the Mb–Chi–BLTO biosensor, which will open up a new idea for the design of third-generation electrochemical biosensors, and the BLTO-based biosensors are also expected to find potential applications in many areas such as clinical diagnosis and food and environmental detection.

Crystallization process of lanthanum-substituted bismuth titanate synthesized by a facile sol–gel method

We present a facile sol–gel route to synthesize lanthanum-substituted bismuth titanate (BLT). The chemical reactions and crystallization process of this method using the initial materials of bismuth subnitrate [4BiNO3(OH)2·BiO(OH)], lanthanum nitrate [La(NO3)3·6H2O] and tetrabutyl titanate [Ti(C4H9O)4] were investigated by thermogravimetric and differential thermal analysis, IR spectroscopy, gas chromatography/mass spectrometry, Raman spectroscopy and XRD. The evaporation of the dissolved CO2 in the amorphous BLT matrix is associated with the crystallization of BLT. The BLT gel is pure BLT perovskite when calcination temperature is higher than 500 °C. The grain size of the obtained nanoparticles ranges from 15 to 82 nm. The Arrhenius curve is obtained from the representation of the reduced sizes with respect to the calcination temperature. The activation energy of grain growth in BLT nanoparticles is 0.36 eV, which shows a rapidly growth process in the temperature range of 500–850 °C.

Electric and ferroelectric properties of PZT/BLT multilayer films prepared by photochemical metal-organic deposition

The electric and ferroelectric properties of lead zirconate titanate (PZT) and lanthanum-substituted bismuth titanate (BLT) multilayer films prepared using photosensitive precursors were characterized. The electric and ferroelectric properties were investigated by studying the effect of the stacking order of four ferroelectric layers of PZT or BLT in 4-PZT, PZT/2-BLT/PZT, BLT/2-PZT/BLT, and 4-BLT multilayer films. The remnant polarization values of the 4-BLT and BLT/2-PZT/BLT multilayer films were 12 and 17 μC/cm 2, respectively. Improved ferroelectric properties of the PZT/BLT multilayer films were obtained by using a PZT intermediate layer. The films which contained a BLT layer on the Pt substrate had improved leakage currents of approximately two orders of magnitude and enhanced fatigue resistances compared to the films with a PZT layer on the Pt substrate. These improvements are due to the reduced number of defects and space charges near the Pt electrodes. The PZT/BLT multilayer films prepared by photochemical metal-organic deposition (PMOD) possessed enhanced electric and ferroelectric properties, and allow direct patterning to fabricate micro-patterned systems without dry etching.

Electronic Structure and Ferroelectric Behavior of Lanthanum Substituted Bismuth Titanate

The effect of substituting La for Bi atoms in Bi 4 Ti 3 O 12 is investigated from first-principles calculations using the full-potential linearized augmented plane wave (LAPW) method. A comparative study of the electronic structure and lattice-dynamics properties of Bi 4 Ti 3 O 12 and La 2 Bi 2 Ti 3 O 12 shows the role of covalent bonding in the ferroelectric instabilities. The displacement patterns displayed by unstable zone-center phonon modes and a detailed analysis of the orbital contribution to the valence band illustrate the importance of Bi (La)-O interactions for the ferroelectric behavior of the doped material.

Growth and characterization of epitaxial ferroelectric lanthanum-substituted bismuth titanate nanostructures with three different orientations

Well-ordered large-area arrays of ferroelectric La-substituted Bi4Ti3O12 (BLT) nanostructures were prepared by pulsed laser deposition using gold nanotube membranes as shadow masks. By x-ray diffraction and transmission electron microscopy, it was found that well-defined (001)-, (118)∕(100)-, and (104)-oriented BLT nanostructures were obtained on (001)-, (011)-, and (111)-oriented SrTiO3 single crystal substrates covered with epitaxial SrRuO3 electrode layers, respectively, through ex situ crystallization at 700°C. Atomic force microscopy analyses revealed that the epitaxial BLT nanostructures maintain a height of about 100nm and a lateral size of about 150nm in spite of the postannealing process. The anisotropic ferroelectric properties of the BLT nanostructures of three different orientations were investigated by scanning force microscopy in piezoresponse mode. The highest effective remanent piezoresponse coefficient (2dzz) of about 38.0pm∕V was observed in (118)∕(100)-oriented BLT nanostructures, whereas (001)- and (104)-oriented nanostructures showed smaller 2dzz values of 5.0pm∕V and 1.4pm∕V, respectively.

Effects of oxygen-plasma treatment on lanthanum-substituted bismuth titanate ferroelectric thin films

Oxygen-plasma treatment has been done on Bi3.25La0.85Ti3O12 (BLT) films for lower crystallization temperatures and improved ferroelectric properties. The films, prepared by metal organic decomposition method on Pt and IrO2 electrodes sputtered on Al2O3/SiO2/Si substrates, were pre-annealed at the relatively low temperatures of 550 and 600 °C, followed by room temperature microwave oxygen-plasma treatment. The oxygen-plasma-treated BLT films showed very good ferroelectric characteristics after a final furnace annealing at 600 °C.

Retention characteristics of Au/Bi3.25La0.75Ti3O12/Si metal-ferro- electric-semiconductor structure

Lanthanum-substituted bismuth titanate (Bi3.25La0.75Ti3O12) (BLT) thin films were deposited on p-type Si(100) substrates using a chemical solution deposition process. The ferroelectric and dielectric properties of the films with an Au/BLT/Si structure were investigated. It was found that retention behaviors of the capacitors after polling with a negative and positive voltage were very different. The capacitor at an accumulation state exhibited a better retention characteristic than that at a depletion state. A rapid loss of memory for the capacitor at depletion state was found and attributed to the depolarization fields inside the ferroelectric film. It is proposed that the interaction between injected charges and ferroelectric polarization plays a role in the retention properties of the MFS capacitors.

Gel Structure, Gel Decomposition and Phase Formation Mechanisms in the Aqueous Solution–Gel Route to Lanthanum Substituted Bismuth Titanate

Phase pure lanthanum substituted bismuth titanate was prepared by an aqueous solution-gel route. In this route aqueous precursors are synthesized by stabilization of the metal ion in the aqueous medium through complexation with strong electron donor ligands. The chemical structure of metal-less, monometal ion as well as multimetal ion precursorgels was studied by means of FTIR and FT-Raman spectrometry. The precursors were transformed to phase pure oxides by heat treatment in an appropriate dynamic atmosphere. The decomposition reactions of the gels were studied by means of several hyphenated thermal analysis techniques (thermogravimetry coupled on-line to FTIR or MS (TGA-MS, TGA-FTIR) and high temperature—diffuse reflectance infrared spectrometry by fourier transform (HT-DRIFT)) and reactions are proposed. The phase formation was studied by means of X-ray diffraction. Phase segregation during gel calcination was correlated to the oxygen partial pressure. An explanation for this phenomenon was suggested based on thermal analysis and on SEM and TEM studies.

Influence of mobile space charges on electrical properties of ferroelectric Bi 3.5La 0.5Ti 3O 12 thin films

Lanthanum-substituted bismuth titanate, Bi 3.5La 0.5Ti 3O 12 (i.e., x=0.5 in Bi 4− x La x Ti 3O 12), thin films have been grown on Pt/Ti/SiO 2/Si substrates using pulsed laser deposition. The frequency dependence of the real part ε′( ω) and the imaginary part ε″( ω) of the dielectric constant has been studied. The ε′( ω) does not show any sudden change within the frequency range of 10 2–10 6 Hz. In contrast, the ε″( ω) shows a large dispersion as frequency decreases. The observed relaxation behavior in ε″( ω) can be explained in terms of a migration of oxygen vacancies in (Bi 2O 2) 2+ layers, not in Bi 2Ti 3O 10 perovskite layers.

Five-day-long ferroelectric memory effect in Pt∕(Bi,La)4Ti3O12∕HfO2∕Si structures

In order to form metal–ferroelectric–semiconductor–insulator–semiconductor (MFIS) structures, hafnium oxide (HfO2) films were first deposited on Si(100) substrates at room temperature using an electron-beam evaporation method. Then, they were annealed in an O2 atmosphere at 800°C for 1min. No hysteretic characteristics were observed in the capacitance–voltage (C–V) measurement for the metal–insulator–semiconductor capacitors. On these films, lanthanum-substituted bismuth titanate [(Bi,La)4Ti3O12:BLT] films with 380nm in thickness were deposited by a sol–gel spin-coating method and annealed in an O2 atmosphere at 750°C for 30min. The MFIS sample showed the hysteretic C–V characteristics and the memory window width was about 0.9V for the voltage sweep of ±7V. Furthermore, the high and low capacitance values biased in the hysteresis loop were clearly distinguishable for over 5 days. The origin of the excellent data retention characteristics is discussed from a viewpoint of the transient current across the film.

The Effect of Substrate Materials on Orientation Degree of Lanthanum-Substituted Bismuth Titanate Thin Films

The effect of different substrates (Si, Pt/Ti/Si and quartz) on the orientation degree of Bi3.4La0.6Ti3O12 thin films was studied. Bi3.4La0.6Ti3O12 thin films were prepared by sol-gel process. X-ray diffraction (XRD) studies indicate the formation of the lanthanum-substituted bismuth titanate thin films. It is found that the orientation ratio along c-axis direction of Bi3.4La0.6Ti3O12 thin films deposited on various substrates were different. Bi3.4La0.6Ti3O12 thin films deposited on quartz substrate has relatively higher c-axis orientation degree (about 90%) than that on Si (86%) and Pt/Ti/Si (88%). The results of XRD pole figures indicate that Bi3.4La0.6Ti3O12 thin films deposited on quartz show a well-defined (006)-orientation. It is found that surface morphology of Bi3.4La0.6Ti3O12 thin films deposited on quartz substrate and Pt/Ti/Si contain spheroidal grains and sheet-like grains, respectively. The grain size of the former is about 65 nm, and 200 nm in diameter for the latter.

Domain structure and polarization properties of lanthanum-substituted bismuth titanate single crystals

Single crystals of bismuth titanate (BiT) and lanthanum-substituted BiT [Bi4−xLaxTi3O12 (BLT)] were grown by a self-flux method. BLT (x=0.85) showed a smaller remanent polarization (2Pr) along the a(b) axis of 41 μC/cm2 than 98 μC/cm2 for BiT, which mainly originates in its smaller ionic displacements along the a axis. Striped 90° domain walls as well as head-to-head and tail-to-tail 180° domain walls were observed for both crystals, and La substitution led to a marked decrease in the 90° domain width from 8.5 μm (BiT) to 2.5 μm for BLT (x=1.20). Piezoelectric-force microscope observations suggested that antiphase domain boundary (ADB) is present only for BLT and the ADB plays an important role in the formation of the 90° domain.

Growth of uniformly a-axis-oriented ferroelectric lanthanum-substituted bismuth titanate films on silicon substrates

Uniformly a-axis-oriented, epitaxially twinned La-substituted Bi4Ti3O12 (BLT) thin films having the major spontaneous polarization entirely along the film normal were grown by pulsed laser deposition on yttria-stabilized zirconia-buffered Si(100) substrates covered with very thin SrRuO3 bottom electrodes. Using SrRuO3 bottom electrodes of a specific low thickness in combination with a relatively high growth rate and a high oxygen pressure, the volume fraction of the BLT (100) orientation, which is competing with the BLT (118) orientation, was increased up to 99%. In this way the growth of fully a-axis-oriented BLT films was achieved, attaining a remanent polarization of 32 μC/cm2. Initial fatigue experiments indicated hardly any fatigue after 109 switching cycles.

Effect of bismuth oxide as a buffer layer on metal–lanthanum-substituted bismuth titanate–insulator–semiconductor structures

We have investigated metal/ferroelectric/insulator/semiconductor (MFIS) structures with lanthanum-substituted bismuth titanate (BLT) as a ferroelectric layer and bismuth oxide as an insulating buffer layer between BLT and Si substrate. BLT films and Bi oxide films were prepared by the direct liquid injection metal organic chemical vapor deposition process. The morphology of the Bi oxide film was changed with the increase of its thickness and the annealing temperature. Bi oxide on silicon was converted into Bi silicate during annealing at 750 °C. The morphology of the BLT films deposited on Bi oxide depended on the morphology of the Bi oxide film and on the reaction with Bi oxide during the annealing process, which was confirmed by transmission electron microscopy and energy dispersive x-ray spectroscopy. The maximum memory window was 0.83 V at the sweep voltage of 5 V with the Bi oxide film annealed at 650 °C and with a thickness of 5 nm. With BLT/Bi oxide annealed at 750 °C, the window was decreased due ...

Crystal Growth and Characterization of Lanthanum Substituted Bismuth Titanate Single Crystals

The thermostability of lanthanum substituted bismuth titanate, Bi4-xLaxTi3O12 (BLT), with La-content up to x=0.75 was investigated using differential thermal analysis. Subsequently, BLT and Bi4Ti3O12 (BIT) single crystals were grown by a self-flux method. The BLT crystals with two different ranges of La content, x=1.0–1.5 and 0<x<0.1, were obtained using starting materials with mole ratio BLT(x=0.25):Bi2O3=1:9. Segregation phenomena of La during crystal growth process were observed. A comparison of dielectric and ferroelectric properties in BIT and BLT crystals was performed. The La incorporation led to a significant increase in the coercive field value, Ec, of BIT single crystals.

Why lanthanum-substituted bismuth titanate becomes fatigue free in a ferroelectric capacitor with platinum electrodes

Bi 4 Ti 3 O 12 , (BTO) a Bi-layered perovskite oxide, shows fatigue after repeated ferroelectric polarization reversals. On the other hand, Bi3.25La0.75Ti3O12 (BLT) is fatigue free. From an extensive transmission electron microscopy study, it was found that there is a high density of antiphase boundaries (APBs) in BLT like in the fatigue-free SrBi2Ta2O9 but not in BTO. It is proposed that the existence of APBs possibly plays a key role in the fatigue-free behavior of Bi-layered perovskite oxides.

Lanthanum-substituted bismuth titanate for use in non-volatile memories

Non-volatile memory devices are so named because they retain information when power is interrupted; thus they are important computer components. In this context, there has been considerable recent interest1,2 in developing non-volatile memories that use ferroelectric thin films—‘ferroelectric random access memories’, or FRAMs—in which information is stored in the polarization state of the ferroelectric material. To realize a practical FRAM, the thin films should satisfy the following criteria: compatibility with existing dynamic random access memory technologies, large remnant polarization (Pr) and reliable polarization-cycling characteristics. Early work focused on lead zirconate titanate (PZT) but, when films of this material were grown on metal electrodes, they generally suffered from a reduction of Pr (‘fatigue’) with polarity switching. Strontium bismuth tantalate (SBT) and related oxides have been proposed to overcome the fatigue problem3, but such materials have other shortcomings, such as a high deposition temperature. Here we show that lanthanum-substituted bismuth titanate thin films provide a promising alternative for FRAM applications. The films are fatigue-free on metal electrodes, they can be deposited at temperatures of ∼650 °C and their values of Pr are larger than those of the SBT films.