Epitaxial PZT Research Articles

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.

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.

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.

Sputter deposition and characterization of “epi-poly” Pb(Zr, Ti)O3 thin film on (100) Si substrate for MEMS applications

Monocrystalline Pb(Zr, Ti)O3 (mono-PZT) thin films are suitable for specific microelectromechanical systems applications. However, these films are more brittle than general polycrystalline PZT thin films. Herein, we focus on an epitaxial PZT thin film with multiple variants in the in-plane direction. Such a unique crystalline structure is expected to have properties intermediate between the mono- and polycrystalline PZT thin films. Such a film is called as “epi-poly” thin film in this paper. We formed an LaNiO3 buffer layer with three variants as an underlayer. PZT was then sputter-deposited onto it. Thus, a (100)-oriented epi-poly PZT thin film with three variants was obtained. Its piezoelectricity, ∣e 31,f ∣, dielectric constant, ε r33, dielectric loss, and tanδ were measured to be approximately 9 C m−2, 750–800, and 0.02, respectively. Nano-indentation tests on the epi-poly and mono-PZT thin films indicated that the former had superior mechanical robustness than that of the latter.

Revealing intrinsic electro-optic effect in single domain Pb(Zr, Ti)O3 thin films

We deposited polar-axis-oriented tetragonal and rhombohedral single domain Pb(Zr, Ti)O3 (PZT) films on CaF2(100) substrates by inserting SrRuO3 (SRO)/LaNiO3 and SRO/SrTiO3/TiO2/CeO2 buffer layers. Both PZT films grew epitaxially and had a (001)- and (111)-domain with the remnant polarization and piezoelectric constant comparable to the theoretical values of PZT single crystals having the same compositions. The electro-optic (EO) response of the fabricated PZT films was constant with respect to the DC electric field and increased linearly with an increasing AC electric field, thus representing a typical linear EO response in single domain ferroelectrics. The measured EO coefficients were larger than the value for a single crystal of PbTiO3, i.e., one of the end members of PZT, but smaller than the values reported for polycrystalline and epitaxial PZT films with multiple domains. These findings show that the intrinsic EO effect is enhanced in PZT, which is similar to the enhancement seen in the dielectric and piezoelectric constants. Moreover, most of the reported EO response in PZT films is supported by additional extrinsic contributions.

Domain structure transition in compressively strained (100)/(001) epitaxial tetragonal PZT film

A 30-nm-thick epitaxial tetragonal (100)/(001) Pb(Zr0.22Ti0.78)O3 (PZT) thin film was grown at 600 °C on (001) KTaO3 (KTO) single-crystal substrate by metalorganic chemical vapor deposition. The a/c domain structure in the PZT film was analyzed in detail at room temperature by synchrotron x-ray diffraction (XRD) and piezoresponse force microscopy techniques. The temperature dependence of the crystal structure was investigated by XRD reciprocal space mapping and in-plane grazing incidence XRD. The PZT films were grown on KTO substrates under compressive strains from 700 °C to room temperature. This compressive strain gave rise to a perfect (001) orientation below a Curie temperature (TC) of ∼520 °C. An in-plane a-axis lattice parameter for the c domain kept the same value as that of the substrate lattice up to 350 °C despite the ferroelectric transition. Nucleation of the a domain started at around 350 °C. The formation of the a domain released the strain for the in-plane a-axis lattice parameter, as confirmed by in-plane analysis of the crystal structure. The results revealed that the in-plane average surface area of the PZT unit cell continued to match that of the KTaO3 substrate from 700 °C to room temperature, regardless of the domain structure evolution.

Feasibility study of ultrasonic biometrics based on finger vessel imaging by epitaxial-PZT/Si piezoelectric micromachined ultrasonic transducer

In this study, we demonstrated a potential usability of a piezoelectric micromachined ultrasonic transducer (pMUT) for a high-performance body-coupled ultrasonic imager based on an epitaxial lead zirconate titanate (Epi-PZT) thin film. A finger vessel biometrics was proposed as one of its applications. At first, transmitting abilities and receiving sensitivities of the single-element pMUT based on Epitaxial-PZT (Epi-PZT) and polycrystalline-PZT (Poly-PZT) were evaluated as a fundamental study. It was experimentally verified that the Epi-PZT was a preferable material for a higher performance pMUT. Then, the linear array driven by a commercially-available pulser board was constructed, and successfully formed a focused ultrasonic beam. Finally, we demonstrated the ultrasonic imaging of a polydimethylsiloxane-based phantom by using the Epi-PZT pMUT array. We believe this achievement is a great step to realize the finger vessel biometric by pMUT.

Low thermal budget lead zirconate titanate thick films integrated on Si for piezo-MEMS applications

Piezoelectric cantilevers made from ferroelectric thick films are commonly used as key components in many prototypical piezo-MEMS devices, including sensors, micro-actuators and piezoelectric energy harvesters. In this work, transverse piezoelectric performance of rapid thermal annealed Pb(Zr0.53Ti0.47)O3 (PZT) thick film (~1-2 μm) were characterized. These films were sputtered on LaNiO3 buffered Pt/Ti/SiO2/(100) Si substrates at 350 °C followed by a rapid thermal process (RTP) with different annealing time (30 s to 15 min) at the high temperature end (700 °C), and then patterned into rectangular prism-shaped cantilevers. Such a two-step process yields highly (001)-oriented perovskite PZT films with good overall electrical properties (ferroelectric, dielectric and piezoelectric) close to those of an epitaxial PZT film, yet on a much reduced thermal budget. With the elementary and chemical valence information from EDS and XPS analysis, an optimal annealing time was determined for the PZT thick film cantilevers to have both an outstanding piezoelectric performance (actuating, sensing or energy harvesting), and a good compatibility with the Si technology (a brief exposure to a high temperature). This study promotes the integration of large piezoelectricity into Si-based MEMS.

Controlled piezotronic properties on recoverable energy storage density in rare-earth ions doped epitaxial PZT thin films

The present study describes the influence of rare-earth (RE = La, Eu, Dy and Ho) ions on recoverable energy storage density in PLD grown epitaxial Pb(ZrxTi1−x)O3 (PZT) thin films on SRO/LSAT (0 0 1) hetrostructures. Special attention has been paid to remove the pyrochlore phase, which is a prominent unwanted phase in ferroelectric specimens with rare earth dopants. The evidence of single crystallinity of the hetrostructures was confirmed from x-ray diffraction. The presence of single zone points in the Kikuchi patterns in electron back scattering diffraction reveals the epitaxial nature of rare earth doped PZT thin films. In addition, an x-ray photoelectron spectroscopy experiment was carried out to determine the oxidation states after doping the rare-earth ions in the PZT sample. The enhancement of spontaneous polarization in the donor (La, Eu) doped PZT films compared to the pure one is attributed to easy orientation and mobility of domain walls. It is found that the piezoelectric coefficient (d33) is more (130 pm V−1) for La doped PZT films (La:PZT), compared to the other rare earth doped PZT thin films. In our results, we found asymmetric polarization versus electric field hysteresis loop and energy storage efficiency () values of 30% were recorded in the case of pure PZT film and the value decreased to 28% with Ho doping, however, a high value of 46% could be realized in La:PZT.

Polarization induced switching in PZT back gated multilayer MoS2 FETs for low power non-volatile memory

We demonstrated polarization induced switching in lead zirconium titanate (PZT) back gated multilayer MoS2 ferroelectric field effect transistors (Fe-FETs) for low power non-volatile memory devices. In this work, we investigate the influence of the interface between ferroelectric and 2D (MoS2) materials on the transistor characteristics for non-volatile memory applications. We fabricated Fe-FET devices with multilayer MoS2 as a channel material and polycrystalline and single crystal PZT thin films as the ferroelectric gate materials. The Fe-FET with polycrystalline PZT as a gate shows good memory characteristics; however, the hysteresis in transfer characteristics of the Fe-FET is in the clockwise direction (anti-hysteresis), which indicates that the transistor characteristics are not purely controlled by the ferroelectric charge. On the other hand, the Fe-FET devices with a single crystal PZT as the gate shows stable memory characteristics with a clear anti-clockwise hysteresis, which shows that the transistor characteristics are purely controlled by the ferroelectric charge. To understand the source of the anti-hysteresis and the interface between the ferroelectric and 2D materials, we have performed polarization switching at nanoscale using piezo force microscopy (PFM). The local piezoelectric hysteresis loop measured by PFM reveal that the polarization reversal in polycrystalline MoS2/PZT heterostructures is due to inhomogeneous polarization distribution, oxygen vacancies and high surface roughness of the polycrystalline PZT thin films. Our MoS2 Fe-FET devices with epitaxial PZT as a gate exhibit low switching voltages ≤2 V (much lower than the reported 2D-FET devices (8–20 V)), high ON-OFF ratios ≥104 (comparable to state of the art Fe-FET devices) and reproducible hysteresis behaviour with good sustainability over 200 cycles of switching operations. This study helps with the understanding of the fundamental phenomenon for anti-hysteresis, and the realization of low power and reliable non-volatile memory devices.

Large piezoelectricity on Si from highly (001)-oriented PZT thick films via a CMOS-compatible sputtering/RTP process

Integration of lead zirconate titanate (PZT) ferroelectrics into CMOS-Si technology has become a perennial challenge due to the continuously shrinking thermal budget in semiconductor processing. In this work, Pb(Zr0.53Ti0.47)O3 (PZT) thick films (∼1 µm) were prepared on LaNiO3 buffered (111)Pt/Ti/SiO2/(100) Si substrates via a low temperature (350 °C) sputtering deposition followed by a rapid thermal process (RTP). This two-step fabrication process resulted in a highly (001)-oriented perovskite PZT film with a dense, fine-grain morphology and reduced strains, hence an optimal electrical performance intrinsic to the chemical composition was achieved. EDS and XPS analyses verified a desirable evolution of the chemical stoichiometry in the RTP, as well as the expected chemical bonding states of the elements in the annealed films. Moreover, prototypical piezoelectric thin film cantilevers fabricated from the PZT/LaNiO3/Pt/Ti/SiO2/(100)Si heterostructure yielded a large transverse piezoelectric coefficient (e31,f) up to 15.7 C/m2, together with a high coupling coefficient k2∼0.23 for energy conversion efficiency. The PZT films showed high e31,f coefficients (∼10 C/m2) even when the RTP annealing time was reduced down to 2 min. Such a piezoelectric performance is close to the highest ones reported in the literature from epitaxial or highly-oriented PZT films, which require a much higher thermal budget for processing. These high quality PZT films open up many possibilities for the integration of piezoelectricity into Si-based micro-electro-mechanical systems (MEMS).

Domain structure transition from two to three dimensions in tensile strained (100)/(001)-oriented epitaxial tetragonal PZT film

Pb(Zr0.05Ti0.95)O3 thin films of 30 nm thickness were epitaxially grown at 600 °C on (001) KTaO3 single crystal substrates by metal organic chemical vapor deposition. Films consist of the mixture of the a/c-domain composed of the c-domain with out-of-plane polarization and the a-domain with in-plane polarization and the a1/a2-domain formed by the combination of a-domains whose in-plane polar axes are orthogonal to each other. Films above the Curie temperature (Tc) of 550 °C were found to be under the in-plane tensile strain and transformed to a1/a2-domains at Curie temperature when the temperature decreased. This a1/a2-domain further changed to the mixture of the a/c-domain and a1/a2-domain below 350 °C. This indicated that the two-dimensional domain structures of the a1/a2-domain changed to three dimensional ones below Tc. Although the a/c-domain was induced to relax the strain, the a1/a2-domain was still unrelaxed. This is different from the widely investigated in-plane compressive strained films that have a one to three dimensional domain structure.

Enhanced electrical properties of epitaxial PZT films deposited by sol-gel method and crystallized by microwave irradiation

Lead zirconate titanate Pb(Zr0.2Ti0.8)O3 (PZT) thin films were deposited on Nb-doped SrTiO3 (NSTO) substrates by a sol-gel method and then crystallized by microwave irradiation. The effects of microwave irradiation on the electrical properties of PZT films were investigated in comparison with conventional annealing. The PZT films crystallized by microwave irradiation at 550 °C for only 10 min exhibited better electrical properties than those obtained by conventional heating at 550 °C for 45 min. The theoretical analysis based on the Schottky emission model reveals that microwave irradiation can increase the concentration of oxygen vacancies at the PZT/NSTO interface, thus reducing the effective built-in potential and depletion layer width at the PZT/NSTO interface to make PZT films exhibit better ferroelectric properties.

Enhanced Piezoelectric Response in Hybrid Lead Halide Perovskite Thin Films via Interfacing with Ferroelectric PbZr0.2Ti0.8O3.

We report a more than 10-fold enhancement of the piezoelectric coefficient d33 of polycrystalline CH3NH3PbI3 (MAPbI3) films when interfacing them with ferroelectric PbZr0.2Ti0.8O3 (PZT). Piezoresponse force microscopy (PFM) studies reveal [Formula: see text] values of 0.3-0.4 pm/V for MAPbI3 deposited on Au, indium tin oxide, and SrTiO3 surfaces, with small phase angle fluctuating at length scales smaller than the grain size. In sharp contrast, on samples prepared on epitaxial PZT films, we observe large-scale polar domains exhibiting clear, close to 180° PFM phase contrasts, pointing to polar axes along the film normal. By separating the piezoresponse contributions from the MAPbI3 and PZT layers, we extract a significantly higher [Formula: see text] of ∼4 pm/V, which is attributed to the enhanced alignment of the MA molecular dipoles promoted by the unbalanced surface potential of PZT. We also discuss the effect of the interfacial screening layer on the preferred polar direction.

Epitaxial Stress‐Free Growth of High Crystallinity Ferroelectric PbZr0.52Ti0.48O3 on GaN/AlGaN/Si(111) Substrate

AbstractDue to its physical properties gallium‐nitride (GaN) is gaining a lot of attention as an emerging semiconductor material in the field of high‐power and high‐frequency electronics applications. Therefore, the improvement in the performance and/or perhaps even extension in functionality of GaN based devices would be highly desirable. The integration of ferroelectric materials such as lead–zirconate–titanate (PbZrxTi1‐xO3) with GaN has a strong potential to offer such an improvement. However, the large lattice mismatch between PZT and GaN makes the epitaxial growth of Pb(Zr1‐xTix)O3 on GaN a formidable challenge. This work discusses a novel strain relaxation mechanism observed when MgO is used as a buffer layer, with thicknesses down to a single unit cell, inducing epitaxial growth of high crystallinity Pb(Zr0.52Ti0.48)O3 (PZT) thin films. The epitaxial PZT films exhibit good ferroelectric properties, showing great promise for future GaN device applications.

In-plane orientation and composition dependences of crystal structure and electrical properties of {100}-oriented Pb(Zr,Ti)O3 films grown on (100) Si substrates by metal organic chemical vapor deposition

In-plane orientation-controlled Pb(Zrx,Ti1−x)O3 (PZT) films with a thickness of approximately 2 µm and a Zr/(Zr + Ti) ratio of 0.39–0.65 were grown on (100) Si substrates by pulsed metal–organic chemical vapor deposition (MOCVD). In-plane-oriented epitaxial PZT films and in-plane random fiber-textured PZT films with {100} out-of-plane orientation were grown on (100)c SrRuO3//(100)c LaNiO3//(100) CeO2//(100) YSZ//(100) Si and (100)c SrRuO3/(100)c LaNiO3/(111) Pt/TiO2/SiO2/(100) Si substrates, respectively. The effects of Zr/(Zr + Ti) ratio and in-plane orientation on the crystal structure, dielectric, ferroelectric, and piezoelectric properties of the films were systematically investigated. The X-ray diffraction measurement showed that the epitaxial PZT films had a higher volume fraction of (100) orientation than the fiber-textured PZT films in the tetragonal Zr/(Zr + Ti) ratio region. A large difference was not detected between the epitaxial films and the fiber-textured films for Zr/(Zr + Ti) ratio dependence of the dielectric constant, and remanent polarization. However, in the rhombohedral phase region [Zr/(Zr + Ti) = 0.65], coercive field was found to be 1.5-fold different between the epitaxial and fiber-textured PZT films. The maximum field-induced strains measured at 0–100 kV/cm by scanning atomic force microscopy were obtained at approximately Zr/(Zr + Ti) = 0.50 and were about 0.5 and 0.3% for the epitaxial and fiber-textured PZT films, respectively.

Polarization switching behavior of one-axis-oriented lead zirconate titanate films fabricated on metal oxide nanosheet layer

For the application of electronic devices using ferroelectric/piezoelectric components, one-axis-oriented tetragonal Pb(Zr0.40Ti0.60)O3 (PZT) films with thicknesses of up to 1 µm were fabricated with the aid of a Ca2Nb3O10 nanosheet (ns-CN) template for preferential crystal growth for evaluating their polarization switching behavior. The ns-CN template was supported on ubiquitous silicon (Si) wafer by a simple dip coating technique, followed by the repetitive chemical solution deposition (CSD) of PZT films. The PZT films were grown successfully with preferential crystal orientation of PZT(100) up to the thickness of 1020 nm. The (100)-oriented PZT film with ∼1 µm thickness exhibited unique polarization behavior of ferroelectric polarization, i.e., a marked increase in remanent polarization (Pr) up to approximately 40 µC/cm2 induced by domain switching under high electric field, whereas the film with a lower thickness showed only a lower Pr of approximately 11 µC/cm2 even under a high electric field. The ferroelectric property of the (100)-oriented PZT film after domain switching on ns-CN/Pt/Si can be comparable to those of (001)/(100)-oriented epitaxial PZT films.

Tuning electro-optic susceptibity via strain engineering in artificial PZT multilayer films for high-performance broadband modulator

A series of Pb(Zr1-xTix)O3 multilayer films alternatively stacked by Pb(Zr0.52Ti0.48)O3 and Pb(Zr0.35Ti0.65)O3 layers have been deposited on corning glass by magnetron sputtering. The films demonstrate pure perovskite structure and good crystallinity. A large tetragonality (c/a) of ∼1.061 and a shift of ∼0.08eV for optical bandgap were investigated at layer engineered films. In addition, these samples exhibited a wild tunable electro-optic behavior from tens to ∼250.2 pm/V, as well as fast switching time of down to a few microseconds. The giant EO coefficient was attribute the strain-polarization coupling effect and also comparable to that of epitaxial (001) single crystal PZT thin films. The combination of high transparency, large EO effect, fast switching time, and huge phase transition temperature in PZT-based thin films show the potential on electro-optics from laser to information telecommunication.

Angular dependence of Raman spectrum for Pb(Zr,Ti)O3 epitaxial films

Herein, we investigate the angular dependence of the Raman spectra of (001)/(100)-oriented tetragonal and (111)/-oriented rhombohedral epitaxial lead zirconate titanate (PZT) films. The A1 and E modes have the same angular dependence for the (111)/-oriented rhombohedral epitaxial PZT film, but not for the (001)/(100)-oriented tetragonal epitaxial PZT film. These results agree well with theoretical predictions. On the basis of the fact that the angular dependence of the phonon mode in PZT films is linked to the crystal orientation and structure, the angular dependence of the Raman modes can be used to investigate the orientation and structure of PZT films.