Epitaxial Pb Research Articles

Breakdown strength and energy storage properties of epitaxial lead-based relaxor-ferroelectric films over a wide range of film thickness

Herein, we report the effect of film-thickness, ranging from 0.1 μm to 7.0 μm, on the energy storage performance of epitaxial Pb0.91La0.09Zr0.7Ti0.3O3 (PLZT) films grown on silicon substrates. As the PLZT film-thickness increases, polarization is enhanced and reaches a maximum value at a film-thickness of 1.0 μm, while the breakdown-strength reaches its maximum value at 0.5 μm. A film-thickness of 1.0 μm achieves an optimal volumetric recoverable energy-storage density (Ur,V) of 114.4 J/cm3 and an energy-efficiency of 87.3% at 4.7 MV/cm. Contrastingly, a film-thickness of 4.0 μm exhibits the largest stored recoverable-energy capacity over surface-area (Ur,A) of 34.2 mJ/cm2 (Ur,V = 85.4 J/cm3 at 4.05 MV/cm). Moreover, a film-thickness of 4.0 μm displays a large Ur,V value of 66.8 J/cm3, good thermal stability and excellent fatigue-endurance even at an operational temperature of 200 °C. These results suggest that thick PLZT films hold promise as high-performance energy-storage capacitors capable of operating effectively in harsh conditions.

A Time-Domain Perspective on the Structural and Electronic Response in Epitaxial Ferroelectric Thin Films on Silicon.

This operando study of epitaxial ferroelectric Pb(Zr0.48Ti0.52)O3 capacitors on silicon substrates studies their structural response via synchrotron-based time-resolved X-ray diffraction during hysteresis-loop measurements in the 2-200 kHz range. At high frequencies, the polarization hysteresis loop is rounded and the classical butterfly-like strain hysteresis acquires a flat dumbbell shape. We explain these observations from a time-domain perspective: The polarization and structural motion within the unit cell are coupled to the strain by the piezoelectric effect and limited by domain wall velocity. The solution of this coupled oscillator system is derived experimentally from the simultaneously measured electronic and structural data. The driving stress σFE(t) is calculated as the product of the measured voltage U(t) and polarization P(t). Unlike the electrical variables, σFE(t) and η(t) of the ferroelectric oscillate at twice the frequency of the applied electrical field. We model the measured frequency-dependent phase shift between η(t) and σFE(t).

In-situ study on piezoelectric responses of sol-gel derived epitaxial Pb[Zr,Ti]O3 thin films on Si substrate

Pb[Zr,Ti]O3 (PZT) thin films with the Zr/Ti ratio of 45/55, 52/48, and 60/40 (PZT(45/55), PZT(52/48), and PZT(60/40), respectively) are deposited on (001)SrRuO3/(001)Pt/ZrO2/Si by sol-gel method. We confirm that the epitaxial (001)Pt electrode facilitate the epitaxial growth of the PZT thin films along the c-axis direction. Basic characteristics such as structural, dielectric, and ferroelectric properties are examined. Furthermore, direct and converse piezoelectric properties are evaluated using effective transverse piezoelectric coefficients, |e31,f|. In particular, the epitaxial PZT thin films manifest a dependence of the converse |e31,f| on applied voltages with the values as follows; 9.7–11.5 C/m2, 11.2–12.5 C/m2, and 11.7–12.3 C/m2 for the epitaxial PZT(45/55), PZT(52/48), and PZT(60/40), respectively. Moreover, by analyzing bias-resolved in-situ reciprocal space map (RSM) obtained from synchrotron radiation X-ray diffraction (SR-XRD) which allows for the understanding on the transition of crystal structures, we explore the voltage-induced contributions for the converse |e31,f|.

Compositional Modification of Epitaxial Pb(Zr,Ti)O3 Thin Films for High‐Performance Piezoelectric Energy Harvesters

AbstractIn this study, Piezoelectric energy harvesters (PEHs) are fabricated based on epitaxial Pb(Zr,Ti)O3 (PZT) thin films deposited by an RF magnetron sputtering with varied Zr/[Zr+Ti] ratio. For a compatibility with micro‐electromechanical systems, the epitaxial PZT thin films are deposited on Si substrates (PZT/Si). The morphotropic phase boundary (MPB) are formed in the compositional range of 0.44 ≤ Zr/[Zr+Ti] ≤ 0.51 of the epitaxial PZT/Si, which is far broader than that of the bulk PZT. Meanwhile, effective transverse piezoelectric coefficient (|e31,f|) values are evaluated by the direct and converse piezoelectric effects using the unimorph cantilever method. Among the compositions, the rhombohedral‐dominant MPB (MPB‐R) of Zr/[Zr+Ti] = 0.51 exhibits a direct |e31,f| of 10.1 C m−2 and a relative permittivity (εr) of 285, leading to the maximum figure of merit of 40 GPa in this study. On the other hand, the maximum converse |e31,f| of 14.0 C m−2 is measured from the tetragonal‐dominant MPB (MPB‐T) of Zr/[Zr+Ti] = 0.44. At a resonance frequency, the MPB‐T presents a high output power density of 301.5 µW−1/(cm2 g2) under an acceleration of 3 m−1 s−2, which is very promising for the high‐performance PEH applications.

Improving resistive switching effect by embedding gold nanoparticles into ferroelectric thin films

The ferroelectric memristor, as a new type of nonvolatile memory, has a broad prospect in the fields of information storage, exchange, and neural computing. Nowadays, it is still a challenge to achieve ferroelectric memristor with high resistive switching effect. In this work, epitaxial Ca-doped Pb(Zr0.40Ti0.60)O3 films embedded various concentrations Au nanoparticles (NPs) (Abbreviated as Au-PCZT) were deposited on the Nb:SrTiO3 (NSTO) substrate to form Au-PCZT/NSTO heterostructures. On the one hand, adding appropriate Au NPs into the PCZT films can improve the leakage current. When Au NPs capture electrons, the current in the high resistance state will be decreased due to the coulomb blocking effect. On the other hand, the ferroelectric polarization still maintains at a good level. Eventually, the resistive switching (RS) on/off ratio can reach 106 by embedding 3 mol% Au NPs. Compared with the pure PCZT thin films, the resistance-variable switching ratio is improved by two orders of magnitude. In addition, multi-level data storage can be realized under different bias voltages. Our results provide a feasible way to achieve high-on/off-ratio ferroelectric memristors with multi-level data storage capabilities.

Enhanced memristor performance via coupling effect of oxygen vacancy and ferroelectric polarization

As a new type of nonvolatile memory, the resistive memristor has broad application prospects in information storage and neural computing based on its excellent resistive switching (RS) performance. At present, it is still a great challenge to improve both ferroelectric polarization and leakage current to achieve a high RS on/off ratio of ferroelectric memristors. Herein, epitaxial Pb(Zr0.40Ti0.60)O3 (PZT) thin films with low content Ca doping were deposited on the Nb:SrTiO3 substrate to prepare PCZT/NSTO heterostructures and their RS behaviors were studied. The research findings show that compared with pure PZT film, the ferroelectric polarization of 1-mol%-Ca-doped PZT film is slightly improved, while the leakage current is increased by three orders of magnitude. Therefore, the RS on/off ratio reaches 2.5 × 105, about three orders of magnitude higher than pure PZT films. The theoretical analysis reveals that the RS behavior of PCZT/NSTO heterostructures is controlled by the PCZT/NSTO interfacial barrier and the space charge-limited current mechanism. Our results demonstrate that the ferroelectricity and electricity of ferroelectric thin films can be improved simultaneously by doping low-content Ca ions to increase the RS performance, which provides a good reference for the development of high-performance ferroelectric memristor devices.

On the importance of the SrTiO3 template and the electronic contact layer for the integration of phase-pure low hysteretic Pb(Mg0.33Nb0.67)O3-PbTiO3 layers with Si

The rapid advent of the piezoelectric microelectromechanical systems (PiezoMEMS) field has created a tremendous demand for low hysteretic piezoelectric thin films on Si. In this work, we present the integration of epitaxial Pb(Mg0.33Nb0.67)O3-PbTiO3 (PMN-PT) thin films with Si to enable device fabrication using state of the art methods. With optimized buffer layers and electronic contacts, high-quality low hysteretic PMN-PT thin films are integrated with Si, which is a significant stride towards employing PMN-PT thin film for PiezoMEMS devices. It is found that the processing of the necessary SrTiO3 buffer layer is crucial to achieve the growth of phase-pure perovskite PMN-PT layers on Si. Furthermore, we propose the engineering of the electronic contact for the PMN-PT-on-Si capacitors to obtain low hysteretic polarization and displacement responses.

Piezoelectric properties of epitaxial Pb(Zr,Ti)O3 thin films grown on Si substrates by the sol–gel method

In this study, we investigated the piezoelectric characteristics of the sol-gel Pb(Zr0.52Ti0.48)O3 (PZT) films deposited on two different types of substrates. Epitaxial PZT piezoelectric thin films were grown on ZrO2-buffered Si substrates, while polycrystalline PZT thin films were grown on conventional Pt/Ti/Si substrates by the sol–gel method. X-ray diffraction measurements revealed a cube-on-cube epitaxy on the buffered Si substrate. The piezoelectric coefficient (e31,f) of the epitaxial PZT thin films was investigated by measuring the tip displacement of PZT cantilevers, and the |e31,f | values were in the range of 8.9–12 C/m2. The voltage dependence of the |e31,f | values was more constant than that of the polycrystalline sol–gel PZT/Si cantilever. The epitaxial sol–gel PZT films thus exhibited good piezoelectric properties for applications in microelectromechanical systems devices.

Indirect Evaluation of the Electrocaloric Effect in PbZrTiO3 (20/80)-Based Epitaxial Thin Film Structures

Electrocaloric effect is the adiabatic temperature change in a dielectric material when an electric field is applied or removed, and it can be considered as an alternative refrigeration method. Materials with ferroelectric order exhibit large temperature variations in the vicinity of a phase transition, while antiferroelectrics and relaxors may exhibit a negative electrocaloric effect. In this study, the temperature variation in polarization was investigated for epitaxial ferroelectric thin film structures based on PbZrTiO3 materials in simple or complex multilayered structures. We propose the intriguing possibility of a giant negative electrocaloric effect (ΔT = −3.7 K at room temperature and ΔT = −5.5 K at 370 K) in a simple epitaxial Pb(ZrTi)O3 capacitor. Furthermore, it was shown that abnormal temperature variation in polarization is dependent on the non-FE component introduced in a multilayered structure. No significant variation in polarization with temperature was obtained for PZT/STON multilayered structures around room temperature. However, for PZT/BST or PZT/Nb2O5 multilayers, an abnormal temperature variation in polarization was revealed, which was similar to a simple PZT layer. The giant and negative ∆T values were attributed to internal fields and defects formed due to the large depolarization fields when the high polarization of the FE component was not fully compensated either by the electrodes or by the interface with an insulator layer. The presented results make Pb(ZrTi)O3-based structures promising for cooling applications operating near room temperature.

High output performance of piezoelectric energy harvesters using epitaxial Pb(Zr, Ti)O3 thin film grown on Si substrate

For a high power density in piezoelectric energy harvesters, both a large direct piezoelectric coefficient (e31,f) and a small relative permittivity constant (εr,33) are required. This study proposed an energy harvesting device made of an epitaxial Pb(Zr, Ti)O3 (PZT) thin film grown on a Si substrate. The epitaxial PZT thin film is deposited on the Si substrate by RF magnetron sputtering. The epitaxial PZT thin film grown on Si substrate has a εr,33 constant of 318. The output voltage as a function of input displacement was measured using a shaker to evaluate the direct e31,f coefficients and energy harvester output characteristics. According to the figure of merit defined as (e31,f)2/ε0εr,33, the epitaxial PZT/Si cantilever is 32 GPa. At a resonant frequency of 373 Hz under an acceleration of 11 m/s2, the epitaxial PZT/Si cantilever has a high output power of 40.93 μW and power density of 108.3 μW/cm2/g2 without any damage, which is very promising for high power energy harvester applications.

Piezoelectric, dielectric and ferromagnetic properties in epitaxial multiferroic Pb(Zr0.45,Ti0.55)O3/Co0.8Fe2.2O4 thin film treatment by nanosecond pulsed laser deposition

Recent years have seen a considerable research interest in multiferroic materials for their potential application as sensors and actuators. However, the scarcity of these materials in a single phase and the weak coupling of their ferroic components have directed research toward multiferroic heterostructures. These systems operate by coupling the magnetic and electric properties of two materials, generally ferromagnetic and ferroelectric, via strain. In this paper, thin films have been grown by chemical solution deposition (CSD) using spin coating technique using rotations peed of 4500 rpm and time of 50 s and dried at 120○C for 20 min. The optical, dielectric, magnetic, ferroelectric, ferromagnetic, and piezoelectric properties of thin films have been investigated by Uv- Visible spectrophotometer (300-2500) nm, X-Ray Diffraction (XRD), Vibrating Sample Magnetometer (VSM), Transmission Electron Microscope (TEM), and Scanning Electron Microscope (SEM) respectively. PZT/CFO thin films deposited at different temperatures from 400 °C, 500 °C and 600 °C. The as-deposited PZT/CFO show week piezoelectricity and the effective piezoelectric coefficient d33 because the materials deposited on the films not only the PZT layer but also the CFO layer, which decreases the piezoelectricity of these films. PZT/CFO Films were then recorded to enhance all physical properties (dielectric, optical, structural, surface morphology, and magnetic properties) as the sintering temperature increased. The images of a thin film before and after treatment with Nd-YAG, which have Power from 10 mw to 5300 mw, Pulse width: 7 ns pulsed laser, Repetition Rate: 10 HZ, and wavelength spectrum in (IR, Vis) laser beam indicate that sample prepared before treated has a lot of porous and agglomeration which decreases after treatment.

Ferroelectric photosensor network: an advanced hardware solution to real-time machine vision

Nowadays the development of machine vision is oriented toward real-time applications such as autonomous driving. This demands a hardware solution with low latency, high energy efficiency, and good reliability. Here, we demonstrate a robust and self-powered in-sensor computing paradigm with a ferroelectric photosensor network (FE-PS-NET). The FE-PS-NET, constituted by ferroelectric photosensors (FE-PSs) with tunable photoresponsivities, is capable of simultaneously capturing and processing images. In each FE-PS, self-powered photovoltaic responses, modulated by remanent polarization of an epitaxial ferroelectric Pb(Zr0.2Ti0.8)O3 layer, show not only multiple nonvolatile levels but also sign reversibility, enabling the representation of a signed weight in a single device and hence reducing the hardware overhead for network construction. With multiple FE-PSs wired together, the FE-PS-NET acts on its own as an artificial neural network. In situ multiply-accumulate operation between an input image and a stored photoresponsivity matrix is demonstrated in the FE-PS-NET. Moreover, the FE-PS-NET is faultlessly competent for real-time image processing functionalities, including binary classification between ‘X’ and ‘T’ patterns with 100% accuracy and edge detection for an arrow sign with an F-Measure of 1 (under 365 nm ultraviolet light). This study highlights the great potential of ferroelectric photovoltaics as the hardware basis of real-time machine vision.

Controlling polarization direction in epitaxial Pb(Zr0.2Ti0.8)O3 films through Nb (n-type) and Fe (p-type) doping

Fe (acceptor) and Nb (donor) doped epitaxial Pb(Zr0.2Ti0.8)O3 (PZT) films were grown on single crystal SrTiO3 substrates and their electric properties were compared to those of un-doped PZT layers deposited in similar conditions. All the films were grown from targets produced from high purity precursor oxides and the doping was in the limit of 1% atomic in both cases. The remnant polarization, the coercive field and the potential barriers at electrode interfaces are different, with lowest values for Fe doping and highest values for Nb doping, with un-doped PZT in between. The dielectric constant is larger in the doped films, while the effective density of charge carriers is of the same order of magnitude. An interesting result was obtained from piezoelectric force microscopy (PFM) investigations. It was found that the as-grown Nb-doped PZT has polarization orientated upward, while the Fe-doped PZT has polarization oriented mostly downward. This difference is explained by the change in the conduction type, thus in the sign of the carriers involved in the compensation of the depolarization field during the growth. In the Nb-doped film the majority carriers are electrons, which tend to accumulate to the growing surface, leaving positively charged ions at the interface with the bottom SrRuO3 electrode, thus favouring an upward orientation of polarization. For Fe-doped film the dominant carriers are holes, thus the sign of charges is opposite at the growing surface and the bottom electrode interface, favouring downward orientation of polarization. These findings open the way to obtain p-n ferroelectric homojunctions and suggest that PFM can be used to identify the type of conduction in PZT upon the dominant direction of polarization in the as-grown films.

Polar-axis-oriented epitaxial tetragonal (Bi,K)TiO3 films with large remanent polarization deposited below Curie temperature by a hydrothermal method

A hydrothermal method was used to epitaxially grow 70-nm-thick tetragonal (Bi,K)TiO3 films with stoichiometric compositions at 240 °C on (001)cSrRuO3//(001)SrTiO3 substrates. Crystal structure analysis revealed that the obtained (Bi,K)TiO3 films had a polar-axis orientation attributable to the matching of the in-plane lattice with the SrRuO3 underlayer. Large coherent displacement of A-site ions along the polar direction observed by transmission electron microscopy may have induced the large tetragonal distortion of c/a = 1.046. The dielectric constant was about 100 and was almost frequency independent in the range of 103–105 Hz, while the dielectric loss, tan δ, was below 5%. Well-saturated ferroelectric polarization−electric field (P−E) hysteresis loops were observed, and the remanent polarization (Pr) was 84 μC/cm2. In addition, the effective piezoelectric constant, d33,eff., was estimated to be 85 pm/V by time-resolved x-ray diffraction measured under an applied electric field. The Pr and d33,eff. values were larger than those reported for polar-axis-oriented epitaxial tetragonal Pb(Zr0.35Ti0.65)O3 films, indicating that the polar-axis-oriented tetragonal (Bi,K)TiO3 film has high potential as a lead-free material for various applications. Specifically, the small εr and large d33,eff. led to an improvement in the figures of merit [d33,eff./εr and (d33,eff.)2/εr], the most important index in sensor and energy-harvester applications.

Quantitative investigation of polarization-dependent photocurrent in ferroelectric thin films

Ferroelectric thin films are investigated for their potential in photovoltaic (PV) applications, owing to their high open-circuit voltage and switchable photovoltaic effect. The direction of the ferroelectric polarization can control the sign of the photocurrent through the ferroelectric layer, theoretically allowing for 100% switchability of the photocurrent with the polarization, which is particularly interesting for photo-ferroelectric memories. However, the quantitative relationship between photocurrent and polarization remains little studied. In this work, a careful investigation of the polarization-dependent photocurrent of epitaxial Pb(Zr, Ti)O3 thin films has been carried out, and has provided a quantitative determination of the unswitchable part of ferroelectric polarization. These results represent a systematic approach to study and optimize the switchability of photocurrent, and more broadly to get important insights on the ferroelectric behavior in all types of ferroelectric layers in which pinned polarization is difficult to investigate.

Dynamic response and roughening of ferroelectric domain walls driven at planar electrode edges

Understanding and controlling the motion, stability, and equilibrium configuration of ferroelectric domain walls is key for their integration into potential nanoelectronic applications, such as ferroelectric racetrack memories. Using piezoresponse force microscopy, we analyze the growth and roughness of ferroelectric domains in epitaxial thin film Pb(Zr0.2Ti0.8)O3, driven by the electric fields at straight edges of planar electrodes at two different temperatures. This device relevant geometry allows us to confirm that the domain walls are well described as one-dimensional monoaffine elastic interfaces driven in random-bond disorder. However, we observe a progressive increase in roughness as initially flat domain walls move through the disorder landscape, which could prove a significant limiting factor for racetrack-type memories using ferroelectrics.

Accidental Impurities in Epitaxial Pb(Zr0.2Ti0.8)O3 Thin Films Grown by Pulsed Laser Deposition and Their Impact on the Macroscopic Electric Properties.

Structural and electrical properties of epitaxial Pb(Zr0.2Ti0.8)O3 films grown by pulsed laser deposition from targets with different purities are investigated in this study. One target was produced in-house by using high purity precursor oxides (at least 99.99%), and the other target was a commercial product (99.9% purity). It was found that the out-of-plane lattice constant is about 0.15% larger and the a domains amount is lower for the film grown from the commercial target. The polarization value is slightly lower, the dielectric constant is larger, and the height of the potential barrier at the electrode interfaces is larger for the film deposited from the pure target. The differences are attributed to the accidental impurities, with a larger amount in the commercial target as revealed by composition analysis using inductive coupling plasma-mass spectrometry. The heterovalent impurities can act as donors or acceptors, modifying the electronic characteristics. Thus, mastering impurities is a prerequisite for obtaining reliable and reproducible properties and advancing towards all ferroelectric devices.

Effect of strain and stoichiometry on the ferroelectric and pyroelectric properties of the epitaxial Pb(Zr0.2Ti0.8)O3 films deposited on Si wafers

Properties of epitaxial PbZr0.2Ti0.8O3 (PZT) films deposited on Si substrates were investigated for integration in the present CMOS technology. Polarization is downward oriented, in association with the presence of an internal electric field, and has a lower value compared to the PZT films deposited on single crystal perovskite SrTiO3 (STO) substrates (40 µC/cm2 versus 80 µC/cm2), while the dielectric constant is larger (180 versus 120). Large value for the pyroelectric coefficient was also found, 1.22 × 10−3C/m2K, as for PZT grown on single crystal STO. The macroscopic ferroelectric and pyroelectric properties appear to be affected by the structural quality and stoichiometry of the PZT film. The changes in the electric properties are an effect of the strain gradients induced by the large difference between the thermal expansion coefficients of PZT and Si substrate, leading in turn to Pb oxidation and antisite defect formation compared to PZT films deposited on STO substrates.

Critical role of parallel momentum in quantum well state couplings in multi-stacked nanofilms: An angle resolved photoemission study

We use angle resolved photoemission spectroscopy to investigate the coupling of electron quantum well states (QWSs) in epitaxial thin Pb and Ag films. More specifically, we investigate the Ag/Si, Pb/Si, and Pb/Ag/Si systems. We found that the parallel momentum plays a very profound role in determining how two adjacent quantum wells are coupled electronically across the interface. We revealed that in the Pb/Ag bimetallic system, there exist two distinctly different regimes in the energy vs momentum (E vs k) space. In one regime, the electronic states in Ag and Pb are strongly coupled, resulting in a new set of QWSs for the bi-metallic system. In the other regime, the electronic states in individual metallic layers are retained in their respective regions, as if they are totally decoupled. This result is corroborated by calculations using density functional theory. We further unravel the underlying mechanism associated with the electron refraction and total internal reflection across the interface.

Hysteresis, Loss and Nonlinearity in Epitaxial PbZr0.55Ti0.45O3 Films: A Polarization Rotation Model

AbstractThe phenomena of hysteresis, ferroelectric loss, and nonlinearity are investigated for the strain and polarization of a monoclinic, epitaxial Pb(Zr,Ti)O3 film over the 70 Hz to 5 kHz frequency range at sub‐coercive excitation fields and zero electrical bias. For the strain, a linear hysteretic behavior is found, whereas the polarization shows a strongly nonlinear hysteretic behavior. In contrast to polycrystalline structures (for instance in ceramics or chemical solution deposited thin films), the commonly referred Rayleigh model cannot explain the observed behavior. A new model is presented, based on the rotation of the polarization vector within the monoclinic or rhombohedral unit cell under an applied electric field, with the viscous interaction of domains accompanying the unit cell deformation. The model explains the amplitude and frequency scaling of the strain, polarization, and loss tangent as well as the observed higher harmonics of polarization in the measured epitaxial Pb(Zr,Ti)O3 films. It is concluded that the nonlinear response and the hysteretic loss originate from two separate physical processes. The nonlinear response is attributed to the nonlinear angular rotation of the polarization vector, whereas the hysteresis and ferroelectric loss are due to a viscous interaction of domains while the polarization vector is rotating.