Lanthanum Titanate Thin Films Research Articles

Elucidating Dynamic Conductive State Changes in Amorphous LLTO for Resistive Switching Devices

Resistive random-access memory (ReRAM) is one of the promising memory devices to achieve neuromorphic computing that is a novel system to mimic human brain. ReRAM has a simple metal-insulator-metal structure and has advantages such as low power consumption and low manufacturing cost. Recently, our group has developed amorphous lithium lanthanum titanate (Li3xLa2/3-xTiO3, LLTO) thin film as a solid-state electrolyte for all solid-state thin-film batteries by using pulsed laser deposition, which has high ionic conductivity (≈10-3 S/cm) [1]. Beyond its application to energy storage devices, we found that the amorphous LLTO (a-LLTO) demonstrates a resistive switching response under the stimuli of an external electric field, where LLTO becomes high resistance state (HRS) at pristine state and switches to low resistance state (LRS) by external electric field. However, despite its unique feature, the resistive switching mechanism of a-LLTO is poorly understood.Thus, in this work, we demonstrate successful observation of LLTO resistive switching behavior and optimize voltage range to achieve both resistive switching and cycling stability. Furthermore, electronic conductivity changes that provoke the LLTO switching are investigated through the comparison of thin films with different oxygen composition, under an hypothesis that oxygen vacancy plays a role in changing local LLTO conductivity. First-principles analysis also supports such conductivity change mechanism due to the local composition changes under electrical bias. Based on these experimental and computational results, the switching mechanisms are discussed with a simplified LLTO model. At last, in-situ testing of LLTO nano device under focused ion beam (FIB) setup, which aims for scanning transmission electron microscopy (STEM)-electron energy loss spectroscopy (EELS) to fully elucidate the switching mechanisms of LLTO at nanoscale.Reference[1] Lee, J. Z.; Wang, Z.; Xin, H. L.; Wynn, T. A.; Meng, Y. S. Amorphous Lithium Lanthanum Titanate for Solid-State Microbatteries. J. Electrochem. Soc. 2016, 164 (1), A6268. Figure 1

Lithium-preserved sintering method for perovskite-based solid electrolyte thin films via flash light sintering for all-solid-state lithium-ion batteries

A novel rapid sintering method, namely flash-light sintering, was applied as a post-treatment for spin-coated lithium lanthanum titanate thin films to prevent lithium evaporation and obtain fully dense thin films with perovskite structures.

Thin Film Processing of Amorphous Lithium Lanthanum Titanate Based Solid-State Microbatteries

Next generation batteries will require a broad range of energies to meet the challenges of portable electronic storage from electric vehicles to microelectromechanical systems (MEMS). Solid state electrolytes are researched heavily because they have the potential to improve capacity loss, cycle lifetime, operation temperature, and safety [1]. Lithium lanthanum titanate (LLTO), with the perovskite structure (ABO3), has gained interest due to its high bulk conductivity (10-3 S cm-1) at room temperature [2]. LLTO has many advantages including negligible electronic conductivity, high voltage stability (> 8 V), atmospheric stability, and temperature stability [3]. Furthermore, LLTO is transferable to thin film structures for fabricating microbatteries for powering portable electronics on-chip [4]. However, crystalline LLTO has a relatively low grain boundary conductivity (<10-5 S cm-1), lowering the overall material conductivity [3]. Therefore, in this work, we investigate amorphous LLTO thin films grown by pulsed laser deposition (PLD). By controlling the laser fluence, background pressure, and temperature we are able to grow dense films with an ionic conductivity of 8x10-4 S/cm and electronic conductivity of 5x10-11 S/cm. Secondly, we explore LiNi0.5Mn1.5O4 (LNMO) spinel thin films deposited via PLD as a potential high voltage cathode for our battery. We report on LNMO/LLTO half cells and novel full LLTO based thin film microbatteries. [1] A. Hayashi et. al.“ Superionic glass-ceramic electrolytes for room-temperature rechargeable sodium batteries,” Nature Communications. 3 (2012) 856–860. [2] O. Bohnke. “The fast lithium-ion conducting oxides Li3xLa2/3 − xTiO3from fundamentals to application,” Solid State Ionics. 179 (2008) 9-15. [3] C. Cao et. al.“Recent advances in inorganic solid electrolytes for lithium batteries,” Frontiers in Energy Research. 2 (2014) 1-10. [4] A. Furusawa et. al. “Ionic conductivity of amorphous lithium lanthanum titanate thin film,” Solid State Ionics. 176 (2005) 553-558.

Amorphous LiLaTiO3 As Solid Electrolyte for Lithium Ion Batteries

Amorphous lithium lanthanum titanate (LLTO) thin films were successfully prepared by a sol-gel method with an all-alkoxide based route. The thin film resistance was determined from the complex spectra by fitting experimental data to the equivalent circuit. The ionic conductivities of amorphous LLTO thin films were 4.5×10-6, 6.9×10-6, 1.3×10-5, and 3.8×10-5 S/cm at 30°C, 50°C, 70°C, and 90°C, respectively. The activation energy of lithium ion conduction in the thin film was evaluated to be 0.36 eV in the temperature range of 30-90°C. The amorphous structure of LLTO was predicted by ab-initio molecular dynamics (AIMD) silumations and analyzed by partial pair distribution functions and coordination numbers. It is observed that the local atomic environment of Ti in amorphous LLTO is quite similar to that in the crystal state but the atomic packing is much less dense. The ionic diffusivities were derived from the mean square displacements obtained from the AIMD simulation results, and the ionic conductivities were evaluated through the Einstein’s relation, showing good agreement with experimental data. The good ionic conductivity of amorphous LLTO is attributed to its open and disordered structure with large excessive volume.

Amorphous LiLaTiO3 as Solid Electrolyte Material

Amorphous lithium lanthanum titanate (LLTO) thin films were successfully prepared by a sol-gel method with an all-alkoxide based route. The thin film resistance was determined from the complex spectra by fitting experimental data to the equivalent circuit. The ionic conductivities of amorphous LLTO thin films were 4.5 × 10−6, 6.9 × 10−6, 1.3 × 10−5, and 3.8 × 10−5 S/cm at 30°C, 50°C, 70°C, and 90°C, respectively. The activation energy of lithium ion conduction in the thin film was evaluated to be 0.36 eV in the temperature range of 30–90°C. The structure of amorphous LLTO was predicted by the ab-initio molecular dynamics (AIMD) simulations and analyzed by partial pair distribution functions, coordination numbers and Voronoi tessellation. It is observed that the local environment of Ti in amorphous LLTO is quite similar to that in the crystal state but the atomic packing is much less dense. The ionic diffusivities were derived from the mean square displacement curves and the conductivities were evaluated from the Nernst-Einstein's relation, showing good agreement with experimental data. The good ionic conductivity of amorphous LLTO is attributed to its open and disordered structure with large excessive volumes.

Preparation and performance of novel LLTO thin film electrolytes for thin film lithium batteries

We prepared a series of lithium lanthanum titanate (LLTO) thin film electrolytes by radio frequency (RF) magnetron sputtering using LLTO targets in a N2 atmosphere. We also deposited the LLTO thin films in an Ar atmosphere under a same condition as references for comparison. The microstructure morphology and the composition of the thin films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively. Results show that the thin film has an amorphous structure with a uniform surface and it is free of pinholes and cracks. Impedance measurements reveal that the ionic conductivity of the electrolytes is beneficial for all solid lithium batteries dependent on the lithium content at room temperature. We found that the amorphous LLTO thin film performs well and it has potential application in microbatteries for use in microelectronic devices.

Growth and Transport Properties of Sr-Doped Lanthanum Titanate Thin Films on LaAlO&lt;sub&gt;3&lt;/sub&gt;

Sr-doped LaTiO3 (SLTO) thin epitaxial films on LaAlO3 (100) have been fabricated by the off-axis rf magnetron co-sputtering system. The dopant Sr2+ ions were expected substituting La3+ ions in the films to introduce the hole carriers. The X-ray diffraction shows that the [001] direction of the SLTO films is perpendicular to the substrate surface. The in-situ grown specimens were measured the temperature dependence of resistivity and the Hall coefficients to study the transport properties. The Hall measurements show that the carrier is electron for Sr-doping over 16%. The temperature dependence of the resistance shows that the n-type films are metallic and deviate from free electron gas model obviously. We found the behaviour could be well described by the small-polaron coherent motion model.

Fabrication and Characterization of Bismuth Lanthanum Titanate (Bi3.25La0.75Ti3O12) Thin Films for FeRAM Devices

Thin films of Lanthanum doped Bismuth Titanate (Bi3.25La0.75Ti3O12) (BLT) have been prepared and characterized for their applications in ferroelectric random access memory (FeRAM) devices. The films have been characterized via X-ray diffraction (XRD), atomic force microscopy (AFM), capacitance-voltage (C-V), ferroelectric (P-E) and current-voltage (J-V) characteristics. The effect of thickness on these characteristics has also been investigated. Thickness variation shows significant effect on the morphology and ferroelectric properties of the films. The nature of the J-V characteristics was observed to be independent of the thickness of the films. Three distinct regions, i.e., ohmic, trap filled limited and Child's law were explicitly observed in J-V characteristics. The J-V behavior of BLT thin films was found to follow the Lampert's theory of space charge limited conduction in an insulator with traps.

Effects of annealing temperature on structure and opt-electric properties of ion-conducting LLTO thin films prepared by RF magnetron sputtering

Highly transparent and ion-conducting lithium lanthanum titanate (LLTO) thin films were deposited on ITO/glass substrates by RF magnetron sputtering. The results of X-ray diffraction indicate that the as-prepared LLTO thin films and the annealed ones at temperatures up to 300 °C are amorphous; however, crystal phases, including La 0.56Li 0.33TiO 3 and other unexpected ones, appear on the annealed films at 400 °C. SEM and AFM results show that the prepared films turn to be denser, smoother and more uniform up to 300 °C while the deteriorative results come out following the further enhancement of annealing temperature. Further characterization of opt-electric properties of the prepared films reveals that the annealed ones at 300 °C own the best optical transmittance of 85% and the highest room temperature ionic conductivity of 5.25 × 10 −5 S cm −1, which are suitable for the application as an electrolyte of all-solid-state electrochromic devices.

Lithium Lanthanum Titanate Thin Films Grown by Atomic Layer Deposition for All-Solid-State Lithium-Ion Battery Applications

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Lanthanum titanate and lithium lanthanum titanate thin films grown by atomic layer deposition

Thin films of lanthanum titanate and lithium lanthanum titanate (LLT) have been grown by atomic layer deposition (ALD). Studies on the growth of lanthanum titanates showed that the lanthanum deposition rate is reduced when the titanium oxide and lanthanum oxide processes are combined, leading to higher titanium contents in the films. The precursor systems used for deposition of lanthanum titanates were TiCl4 + water and La(thd)3 (thd = 2,2,6,6-tetramethyl-3,5-heptanedione) + ozone. Lithium was introduced into the material in order to deposit LLT by using lithium tert-butoxide (LiOtBu) and water as precursors. The deposited films were analyzed by time-of-flight elastic recoil detection analysis (TOF-ERDA), secondary ion mass spectrometry (SIMS), X-ray fluorescence (XRF), X-ray reflectivity (XRR) and X-ray diffraction (XRD). TOF-ERDA gave the film composition of Li0.32La0.30TiOz at saturation conditions.

Study on the LLT solid electrolyte thin film with LiPON interlayer intervening between LLT and electrodes

In this study, a lithium lanthanum titanate (LLT) thin film electrolyte was prepared by RF magnetron sputtering, in order to assess its potential use in solid state thin film batteries. Even though the LLT has high ionic conductivity, it cannot be used alone as a thin film electrolyte since it is chemically unstable when it comes into contact with Li metal and it has a high electronic conductivity. Lithium phosphorous oxynitride (LiPON) is stable when in contact with Li and has an extremely low electronic conductivity. We expected that the LiPON/LLT/LiPON structure would make it possible to use a LLT thin film as a thin film solid electrolyte. In order to prepare this structure, a LiPON thin film was also deposited by RF magnetron sputtering and was deposited for various times (30, 60, 90 and 120 min), in order to determine the optimum thickness ratio between LLT and LiPON. In linear sweep voltammetry measurements, the current hardly flowed in the potential range from 0 to 5.5 V in the blocking electrode and ac impedance was measured for measuring the resistance at LiPON/LLT/LiPON. When only the LLT thin film was deposited, a current of scores of mA flowed in the operating potential range, but when an interlayer of LiPON thin film was deposited for more than 30 min on both sides of the LLT thin film, the current was less than 1 μA. Ionic conductivities of 1.11, 0.82 and 0.48 × 10 −7 S cm −1 were observed for the deposition times of the LiPON thin film of 60, 90 and 120 min, respectively. This result suggests that the LiPON/LLT/LiPON structure might be able to be used as a thin film solid electrolyte if its ionic conductivity could be improved.

Structural, Dielectric and Ferroelectric Properties of La Doped PbTiO3 Sol Gel Derived Thin Films

Ferroelectric lead lanthanum titanate (Pb1−x La x Ti1−x/4O3) thin films (x = 0.04 & 0.08) have been fabricated by sol-gel spin coating process on ITO coated 7059 glass substrates. Characterization of these films has been done by x-ray diffraction and atomic force microscope images. Tetragonality decrease with addition of La content. While dielectric constant and pyrocoefficient increase, Curie temperature decreases with La content. Also, there is decrease in remanent polarization and coercive field with addition of La content.

Microwave characterization of (Pb,La)TiO3 thin films integrated on ZrO2∕SiO2∕Si wafers by sol-gel techniques

Polycrystalline perovskite lead lanthanum titanate (PLT) thin films were prepared by a sol-gel method on ZrO2∕SiO2∕Si substrates. The structure of the films was studied by x-ray diffraction and scanning electron microscopy, and the microwave dielectric properties characterized on a network analyzer. A strong dependence of the dielectric constant of PLT films and, correspondingly, the resonance frequency of PLT-based interdigital capacitor on the sample preparation conditions were observed. They resulted from the structural transformation of PLT from a layered structure to a uniform film as the annealing temperature was raised from 550to700°C, suggesting a possible way to modify the device performance by controlling the layered structure of the ferroelectric film.

Ionic conductivity of amorphous lithium lanthanum titanate thin film

Amorphous lithium lanthanum titanate (LLT) thin films were prepared by the pulsed laser deposition (PLD) method. By X-ray diffraction measurement of the films, it was confirmed that the crystal structure of the as-prepared thin films was amorphous. The temperature dependence of the ionic conductivity of the amorphous LLT thin film was measured from 300 to 475 K over the frequency region 100 Hz to 10 MHz. The value 0.35 eV was estimated as the activation energy of Li + ionic conduction in the film in the temperature region 300–475 K. It was revealed that the transfer ratio of Li + ion for the ionic conduction of the amorphous LLT thin film is over 94%, and the amorphous LLT thin film shows super-ionic conduction at and above room temperature. The conductivity of the LLT thin film was an order of magnitude higher than that of its polycrystalline precursor, mainly due to absence of grain boundary elements and its structural disorder.

Phase stabilization and microstructural studies of lead lanthanum titanate thin films

Thin ferroelectric films of PLT x (Pb 1− x La x Ti 1− x/4 O 3) have been prepared by a sol–gel spin coating process. As deposited films were thermally treated for crystallization and formation of perovskite structure. Characterization of these films by X-ray diffraction (XRD) have been carried out for various concentrations of La ( x = 0.04, 0.08 and 0.12) on ITO coated corning glass substrates. For a better understanding of the crystallization mechanism, the investigations were carried out on films annealed at temperatures (350, 450, 550 and 650 °C). Characterization of these films by X-ray diffraction shows that the films annealed at 650 °C exhibit tetragonal phase with perovskite structure. Atomic force microscope (AFM) images are characterized by slight surface roughness with a uniform crack free, densely packed structure. Fourier transform infrared spectra (FTIR) studies of PLT x thin films ( x = 0.08) deposited on Si substrates have been carried out to get more information about the phase stabilization.

Fabrication of robust Bi 3.25La 0.75Ti 3O 12 thin film resistant to hydrogen damage

Ferroelectric bismuth lanthanum titanate (Bi 3.25La 0.75Ti 3O 12; BLT) thin films were deposited on Pt/TiO 2/SiO 2/Si substrate by chemical solution deposition method. The films were crystallized in the temperature range of 600–700 °C. The spontaneous polarization ( P s) and the switching polarization (2 P r) of BLT film annealed at 700 °C for 30 min were 22.6 μC/cm 2 and 29.1 μC/cm 2, respectively. Moreover, the BLT capacitor did not show any significant reduction of hysteresis for 90 min at 300 °C in the forming gas atmosphere.

Effect of bismuth excess on the crystallization of Bi 3.25La 0.75Ti 3O 12 thin films on Pt/Ti/SiO 2/Si substrates

Bismuth lanthanum titanate (BLT) thin films with excess Bi contents were prepared onto a Pt/Ti/SiO 2/Si substrate by a metalorganic decomposition technique (MOD). The effect of Bi excess on the microstructure and ferroelectric properties was investigated. When a 10% of Bi excess was added, the BLT thin films showed a polycrystalline structure. The remanent polarization and dielectric constant decreased with more than 10% of Bi excess. Bi deficiency or Bi excess over 10% in the BLT films resulted in poor fatigue properties. This was attributed to the structure defects and a presence of a secondary phase, which coexists with a layered perovskite phase. The BLT thin films with 10% of Bi excess have the remanent polarization (2 P r) value of 25.66 μC/cm 2 and a coercive field of 84.75 kV/cm. The BLT thin films with 10% of Bi excess showed no fatigue even after 1 × 10 9 bipolar switching cycles.

Raman scattering of ferroelectric lead lanthanum titanate thin films grown on fused quartz by metalorganic chemical vapor deposition

Highly textured lead lanthanum titanate (Pb 1− x La x )TiO 3 (PLT) thin films have been grown on fused quartz substrates by metalorganic chemical deposition (MOCVD). A series of PLT with different x between 0 and 0.32 were prepared and studied by Raman scattering. Raman spectra, measured at 300 K and 80 K, showed the features from the PLT film and quartz substrate. By using a “difference Raman” technique, more PLT modes are shown. The variations of the PLT Raman modes with the La composition and the measurement temperature are studied, and related physical phenomena and problems are discussed.

The Study of Temperature Dependence of Second Harmonic Generation in Lead Lanthanum Titanate Thin Film by Corona Poling

Ferroelectric materials possess certain properties such as spontaneous polarization, domain structure, the hysteres is effect, and optical nonlinearity which attract much interest for research. Lead zirconate titanate (PZT) is one of the most researched materials for piezoelectric and electro optical applications. Recently, lanthanum modified lead titanate (PLT) has become popular because it possesses interesting properties such as a lower Curie temperature, a lower coercive field, and smaller remanent polarizations than PZT and has great potential for nonlinear optical and electro optical applications. PLT thin films were sputter deposited on fused silica substrate. We studied the second-order nonlinear optical properties in these thin films. The second harmonic generation intensities as a function of temperature were obtained. The optimum temperatures for obtaining the largest nonlinear optical coefficient have been found and the results are presented.