Bi3.15Nd0.85Ti3O12 Thin Films Research Articles

Significantly enhanced energy storage density and efficiency in flexible Bi3.15Nd0.85Ti3O12 thin film via periodic dielectric layers

Flexible energy storage based on ferroelectric capacitors enjoys high power density and rapid respond time, but the polarization fatigue problems limit its long-term reliability. Bi3.15Nd0.85Ti3O12 (BNT) is a lead-free ferroelectric material with fatigue-free properties, which is promising for applications in long-term ferroelectric devices. Nevertheless, the applications of BNT thin films for energy storage are restricted by their poor energy efficiency and low energy density. In this work, flexible BNT thin films with different numbers of SrTiO3 (STO) insert layers are fabricated on flexible mica substrates by an exquisitely designed dual-target pulsed laser deposition process, and the energy storage performances are effectively optimized via interface engineering. By inserting periodic STO dielectric layers with repetition periods of 100, the energy storage density (Wrec) and efficiency (η) are improved to ∼24.26 J cm−3 and ∼71.93%, which are, respectively, increased to 287% and 132% compared with the pure phase BNT thin film. Importantly, the improved Wrec and η can be well maintained under large bending deformation (bending radius as small as r = 4 mm) and within a wide temperature range (25–175 °C), suggesting its good stability and reliability. These results show that the involvement of periodic dielectric layers in BNT thin films can significantly enhance energy storage density and efficiency and effectively promote its applications in future flexible energy storage devices.

Effects of deposition temperatures of Nd-doped Bi4Ti3O12 thin films prepared by pulsed laser deposition

Recently the deleterious environmental impact of the lead used in many ferroelectric and piezoelectric devices has stimulated research into lead-free alternatives with comparable properties. Bismuth titanate-based materials are considered to be candidates for use as lead-free piezoelectrics. In this study, we prepared the Bi3.15Nd0.85Ti3O12 (BNdT) thin films by using Pulsed Laser Deposition (PLD), which is a relatively easy method for the fabrication of thin films. The samples were grown on Pt/Ti/SiO2/Si substrates using a KrF laser (248 nm). To find an optimal fabrication condition of this sample, we varied the PLD deposition conditions, including the substrate temperature, oxygen gas pressure, repetition frequency, target-substrate distance and pulse energy density. We found that the deposition temperature is an important parameter which should be optimized to get high-quality BNdT films. The BNdT thin films were studied by X-ray diffraction (XRD), atomic force microscopy (AFM) surface images, measurements of frequency-dependent dielectric constant and dielectric loss, polarization–electric field (P–E) curves, and fatigue tests. With increasing the deposition temperature the (117) peak of XRD increased, and the intensity ratio of (117)/{(117) + (006) + (200)} peak increased and its value was 68.3% at 650 °C and 44.3% at 700 °C. We note that the phase formation was strongly dependent on the deposition temperature in our BNdT films.

Unipolar resistive switching in porous perovskite-like Bi3.15Nd0.85Ti3O12 thin films

In this study, the unipolar resistive switching (RS) effect is exhibited in porous perovskite-like Bi3.15Nd0.85Ti3O12 (BNT) thin films fabricated on Pt/Ti/SiO2/Si substrates through chemical solution deposition technique employed PEG template. The unipolar RS characteristics can be adjusted by the size and density of the pores in the porous perovskite-like Bi3.15Nd0.85Ti3O12 thin films. The size and density of the pores are controlled by the processing parameters of the annealing temperature or the dwell time or the both during the preparation of the porous perovskite-like Bi3.15Nd0.85Ti3O12 thin films. The porous BNT thin films annealed at 720 °C for 10 min show large current ratio of low/high resistance state of four orders and low operating voltage of 5 V for SET and 1.5 V for RESET. Moreover, the LRS/HRS can be reversed repeatedly without large decay. After the retention test of 104 s for low/high resistance state, the devices still show as large current ratio of low/high resistance state as about four orders. The present study should have guide to develop a unipolar RS memory using the perovskite oxides.

Mixed grains and orientation-dependent piezoelectricity of polycrystalline Nd-substituted Bi4Ti3O12 thin films

Polycrystalline Bi3.15Nd0.85Ti3O12 (BNT) thin films were prepared on Pt/Ta/glass substrates by a pulsed laser deposition method. X-ray diffraction measurements revealed that the BNT thin films were preferentially oriented along the (117) direction although they possessed a polycrystalline structure. Good ferroelectric properties of the BNT thin film were observed with a remnant polarization of 13 μC/cm2 (2Pr ~26μC/cm2). The fatigue resistance test exhibited that the ferroelectric polarization of the BNT thin film degraded significantly after around 109 switching cycles, which can be attributed to its crystal structure. We investigated the surface morphology and ferroelectric domain structure by atomic force microscopy (AFM) and piezoresponse force microscopy (PFM), respectively. Interestingly, mixed grains consisting of long and circular shapes were observed on the BNT film surface, which corresponded to a- and c-axes orientations of crystal growth, respectively. The PFM study revealed that the piezoelectric coefficient (d33) of the long grains was much larger than that of the circular grains.

Deposition of Bi3.15Nd0.85Ti3O12 ferroelectric thin films on 5-inch diameter Si wafers by a modified pulsed laser deposition method

5-inch diameter Bi3.15Nd0.85Ti3O12 (BNT) ferroelectric thin films were prepared on Si wafers by a modified pulsed laser deposition method, in which two laser scanning rates and 6-inch diameter target were used. The investigation on the distribution of the film thickness reveals that the laser scanning technique used in conjunction with the large-diameter target can improve the uniformity of film thickness. Especially the reasonable setup of the laser scanning rate ablating the fringe area of the large target has a prominent effect on weakening the edge deviation of thickness distribution. Such phenomenon was fundamentally explained by the simulation results on the ablation tracks of the laser beam on target. Through optimizing the laser scanning rates, BNT thin film with uniform thickness was obtained on 5-inch diameter Si wafer and the maximum variation in film thickness was found to be less than ±2.5%. The results of X-ray photoelectron spectroscopy and X-ray diffraction show that the prepared 5-inch BNT thin film has great uniformity in stoichiometry and crystallinity. In addition, the measurements of electrical properties indicated that the prepared BNT thin film exhibits good ferroelectric property and I–V characteristic.

Nonlinear optical absorption tuning in Bi3.15Nd0.85Ti3O12 ferroelectric thin films by thickness

The tunability of nonlinear optical (NLO) absorption in Bi3.15Nd0.85Ti3O12 (BNT) ferroelectric thin films was investigated through the open aperture Z-scan method with femtosecond laser pulses at the wavelength of 800 nm. NLO absorption responses of the BNT films were observed to be highly sensitive to the film thickness. As the film thickness increases from 106.8 to 139.7 nm, the NLO absorption changes from saturable absorption (SA) to reverse saturable absorption (RSA). When the film thickness further increases to 312.9 nm, the RSA effect is enhanced. A band-gap-related competition between the ground-state excitation and the two-photon absorption is responsible for the absorption switching behavior. Such a tunable NLO absorption can widen the photonic application of the BNT thin films.

Switchable diode effect in polycrystalline Bi3.15Nd0.85Ti3O12 thin films for resistive memories

The switchable diode effect is found in the Bi3.15Nd0.85Ti3O12 (BNT) polycrystalline thin films with a residual polarization (2Pr) of 55 μC/cm2 fabricated on Pt/Ti/SiO2/Si substrates by chemical solution deposition. The consistencies of P-V and I-V curves demonstrate that the switchable diode effect is mainly triggered by polarization modulated Schottky-like barriers. The ON/OFF ratio of resistive switching based on these switchable diodes is more than 3 orders during the retention capacity measurement, which indicates that the polycrystalline BNT thin films are promising for the resistive memories applications.

Effects of SrTiO3 buffer layer on structural and electrical properties of Bi3.15Nd0.85Ti3O12 thin films prepared by a chemical method

Ferroelectric Bi3.15Nd0.85Ti3O12 (BNT) thin films have been grown on Pt/Ti/SiO2/Si substrates at 750°C by a chemical solution deposition method using SrTiO3 (STO) as a buffer layer. The influence of STO buffer layer on the phase and microstructure of BNT thin films was examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrical properties were investigated both for BNT thin films with and without STO buffer layer. The results showed that STO buffer layer strongly influenced the microstructure and electric properties of BNT thin films. BNT ferroelectric thin films with STO buffer layer exhibited the good crystallization behavior, the enhanced fatigue characteristics and excellent leakage current properties. This indicates that the introduction of the STO buffer layer prevents the interfacial diffusion and charge injection between BNT thin films and the substrate effectively and improves the interface quality.

Effects of Film Thickness on Microstructure and Ferroelectric Properties of Bi<sub>3.15</sub>Nd<sub>0.85</sub>Ti<sub>3</sub>O<sub>12</sub> Thin Films Prepared by Sol-Gel Method

Bi3.15Nd0.85Ti3O12 (BNT) thin films with different thicknesses (200, 270, 360, 450 and 540 nm) were prepared on Pt/Ti/SiO2/Si substrates by sol–gel method. The effect of film thickness on the microstructure and ferroelectric properties of BNT thin films was investigated. All BNT thin films were consisted of a single phase of bismuth-layered perovskite structure. With increasing film thickness, grains gradually became larger, the remanent polarization (2Pr) firstly increased and then decreased, and the leakage current density showed opposite trend. The 360 nm-thick BNT film exhibited better electrical properties with 2Pr 26 µC/cm2, coercive field (2Ec) 220 kV/cm, dielectric constant 345 (at 1 MHz) and low leakage current density.

Annealing temperature‐dependent piezoelectric properties of Bi3.15Nd0.85Ti3O12 ferroelectric thin films

AbstractAnnealing temperature‐dependent (ATD) microstructure, ferroelectric, dielectric, and piezoelectric properties of Bi3.15Nd0.85Ti3O12 (BNT) thin films by metal‐organic decomposition method were investigated in detail. The relative permittivity εr (702) at 1 kHz, the spontaneous polarization 2Ps (72 µC/cm2), and the effective piezoelectric coefficient d33 (62 pm/V) under the amplitudes of the bipolar driving field 320 and 280 kV/cm are the best for BNT thin film annealed at 700 °C. The 2Ps and εr moderated by annealing temperature are responsible for ATD piezoelectric properties according to phenomenologic equation. The results show BNT is a promising candidate for piezoelectric thin film devices.

The ferroelectric properties and residual stress analysis of Bi3.15Nd0.85Ti3O12 thin films with a LaNiO3 buffer layer

Bi3.15Nd0.85Ti3O12 (BNT) thin film with a thin LaNiO3 film as buffer layer was fabricated by sol–gel method on Pt/TiO2/SiO2/Si substrate. The BNT thin films have a perovskite phase with a dense microstructure. The P r and V c value are 25.5 μc/cm2 and 3.7 V, respectively under the applied voltage of 15 V. After the switching of 2 × 109 cycles, the P r value decreases to 86% of its pre-fatigue value. The leakage current density of the BNT thin films with LaNiO3 buffer layer were generally in the order of 10−8 to 10−6 A/cm2. The fatigue and leakage current properties were improved dramatically compared with the BNT film without a LaNiO3 buffer layer that we prepared before. The measured residual stress was tensile stress and its value was 176 MPa.

INTERFACE STUDIES AND ELECTRONIC PROPERTIES OF SILICON BASED Nd-DOPED BISMUTH TITANATE

ABSTRACT The metal-ferroelectric-insulator-semiconductor (MFIS) structure were fabricated and investigated. The Bi3.15Nd0.85Ti3O12 (BNdT) thin films were deposited on the Si substrates by sol-gel process with HfO2, SiO2 and Si3N4 as the buffer layers. Perovskite crystalline was obtained and the ferroelectric polarization-voltage (P-V) hysteresis was studied for the Pt/BNdT/Pt/Ti/SiO2/Si(100) and Pt/BNdT/HfO2/Si(100) capacitors. Good interfacial properties were examined between the BNdT thin film and Si substrate. Memory windows of the Pt/BNdT/HfO2/Si(100) structure were in the range of 0.9v-1.2v when the thickness of the HfO2 buffer layer varied from 3 nm to 5 nm, exhibiting a considerable memory effect in the capacitors.

Sol–gel derived ferroelectric Bi3.15Nd0.85Ti3O12 thin films of predominant 100/010/119 orientation deposited both on Nb-doped (011)SrTiO3 and on (011)SrRuO3/(011)SrTiO3

Ferroelectric Bi3.15Nd0.85Ti3O12 (BNdT) thin films of predominant 100/010/119 orientation were grown through a cheap and simple sol–gel process both on Nb-doped (011)SrTiO3 and on (011)SrRuO3/(011)SrTiO3. Using rapid heating rates during crystallization, films containing 28% (100)/(010)-oriented grains plus 19% (119)-oriented grains were obtained on SrRuO3/SrTiO3, while 30% (100)/(010)- and 18% (119)-oriented grains were obtained on Nb:SrTiO3. The films consist of columnar grains and 90° a–b domains exist in large BNdT grains. The BNdT thin films exhibit excellent ferroelectric and dielectric properties with a remanent polarization 2Pr=39.2 μC/cm2 and a dielectric constant er=184.5.

Effect of precursor solution concentration on the grain orientation of Bi3.15Nd0.85Ti3O12 thin films

Polycrystalline Bi3.15Nd0.85Ti3O12 (BNT) thin films were fabricated on Pt/Ti/SiO2/Si by the sol–gel method. The dependence of precursor solution concentration (PSC) on the grain orientation of the films was observed, and a model was proposed to explain it by taking into account the effect of bulk nucleation. With increasing PSC, the BNT thin film turned from c-axis-preferential-oriented to random-oriented. It is suggested that a competition exists between the nucleation and grain growth on the grain orientation. When the PSC is low, the former plays a predominant role, while in the opposite case, the latter is dominant. The bulk nucleation inhibits the homoepitaxial grain growth, and it gradually replaces the heterogeneous nucleation as the main nucleation mechanism with the increase in PSC.

Effects of TiO2 Seeding Layer on Crystalline Orientation and Ferroelectric Properties of Bi3.15Nd0.85Ti3O12 Thin Films Fabricated by a Sol–Gel Method

The effect of a 20‐nm thick TiO2 seeding layer on the growth of a Bi3.15Nd0.85Ti3O12 (BNT) thin film on Pt(111) thin‐film substrates has been studied. Under otherwise identical deposition process conditions, the BNT film could be turned from a highly random orientation to a (200) preference orientation by adding the seeding layer. Field‐emission scanning electron microscope result reveals that the BNT thin film with the TiO2 seeding layer is composed of fine grains with smaller sizes about 80–150 nm in diameter. The Pr and Ec values of the BNT thin film and BNT film with the TiO2 seeding layer were 36 and 16 μC/cm2, and 96.9 and 92 kV/cm at a voltage of 12 V, respectively. The fatigue test exhibited a very strong fatigue endurance up to 109 cycles for both films. The leakage current densities were generally in the order of 10−6–10−5 A/cm2 for both samples.

The effects of annealing temperature on the properties of Bi 3.15Nd 0.85Ti 3O 12 thin films

The effects of annealing temperature on microstructure, remnant polarization (2Pr), leakage current and residual stress in Bi3.15Nd0.85Ti3O12 (BNT) thin films prepared by metal-organic decomposition were studied. The best surface features was observed, and the largest 2Pr (63.2 μC cm−2 under 530 kV cm−1) and lowest leakage current (1.32 × 10−5 A cm−2 under 125 kV cm−1) of the film were obtained with annealing at 700 °C. Moreover, the strong residual tensile stress in BNT thin film causes surface fracture and ferroelectric degradation.

The influence of the thickness of TiO2 seeding layer on structural and electrical properties of Bi3.15Nd0.85Ti3O12 thin films

Thin films of Bi3.15Nd0.85Ti3O12(BNT) and BNT with various TiO2 seeding layer thicknesses BNT-Tx (x = 10, 20, 30 nm) were fabricated on Pt/Ti/SiO2/Si substrates by the sol–gel method. The influence of the TiO2 seeding layer thickness on the structural and the electrical properties of BNT thin films was investigated. The x-ray diffraction pattern indicated that the BNT thin film with a TiO2 seeding layer showed a-axis preference orientation. The Pr value was a maximum for the BNT-T20 film and decreased with both decreasing and increasing thickness. The BNT-T20 film had the largest εr and the lowest tanδ. The leakage current density of the BNT thin films with various TiO2 seeding layer thicknesses was generally in the order of 10−6–10−5 A cm−2. The surface micrograph of BNT-Tx films was more homogeneous and dense than that without a seeding layer.

Effective thickness and dielectric constant of interfacial layers of Pt∕Bi3.15Nd0.85Ti3O12∕SrRuO3 capacitors

Epitaxial c-axis-oriented Bi3.15Nd0.85Ti3O12 (BNT) thin films with thickness ranging from 150to350nm were deposited on conductive SrRuO3 (SRO) on (001) SrTiO3 substrates by pulsed laser deposition. The top Pt electrode was deposited by sputtering to construct a capacitor Pt/BNT/SRO. The authors have evaluated the effective thickness (ti) and dielectric constant (εi) of interfacial layers at the Pt/BNT and BNT/SRO interfaces based on the optical refractive index of the BNT layer and the capacitance frequency as well as the current-voltage characteristics of the capacitors. Using a series capacitor model, they have found that the dielectric constant of bulk BNT and the ti∕εi ratio are 586 and 1.46nm, respectively. Knowing the optical dielectric constant (εopt) and the product of εoptti of BNT thin films, the authors have estimated that the effective thickness and dielectric constant of the interfacial layers are 20.1nm and 13.7, respectively.

PROPERTIES OF NEODYMIUM-DOPED Bi 4 Ti 3 O 12 THIN FILMS FOR FERROELECTRIC RANDOM ACCESS MEMORY

ABSTRACT The microstructure and ferroelectric properties of Neodymium-doped Bi4Ti3O12 (Bi1-xNd x )4Ti3O12 were studied in the paper. Bi3.15Nd0.85Ti3O12 (BNT) polycrystalline thin films were successfully prepared on Pt/Ti/SiO2/Si (100) substrates using Sol-Gel technique. The 6-layer BNT thin films with only one phase were produced at 750°C. BNT thin films showed certain c-axis orientation growth and the c-orientation ratio was about 72.5%. The thickness of one layer film was about 20 ∼ 22 nm. BNT thin films was composed of grains of 250 nm in diameter and the surface roughness (Ra) of the film was about 1.25. Pt/BNT/Pt capacitor annealed at 750°C exhibited larger remament polarization (Pr) and lower coercive field (Ec). At the applied voltage of 5V, 2Pr and Ec values was 45 μ C/cm2 and 80 kV/cm, respectively. The fatigue characteristics of Pt/BNT/Pt capacitor indicated that BNT thin films exhibited little change in polarization up to 1011 switching cycles. It suggested that BNT films showed excellent polarization and fatigue properties, which could be used for FeRAM applications.

Ferroelectric Properties of Both Nd Substituted and W Doped Bi4Ti3O12 Thin Films Deposited by Chemical Solution Deposition Technique

Both Nd substituted and W doped bismuth titanate thin films were deposited by chemical solution deposition (CSD) technique on the (111)Pt/Ti/SiO2/(100)Si substrates. B-site doping with W6+, enhanced the remnant polarization and improved the fatigue property. Remnant polarization of Bi3.15Nd0.85Ti2.98W0.02O12 (BNTW) is 27 μ C/cm2, which is higher than that of Bi3.15Nd0.85Ti3O12 (BNT) deposited in the same condition. Good fatigue endurance was confirmed at 1 M Hz frequency up to 1010 switching cycles in both BNTW and BNT thin films. However, at the frequency of 50 kHz, the BNTW thin films show an improved fatigue property. W-doped effect is discussed.