Epitaxial PZT Thin Films 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.

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.

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.

Differential phase-contrast dark-field electron holography for strain mapping

Strain mapping is an active area of research in transmission electron microscopy. Here we introduce a dark-field electron holographic technique that shares several aspects in common with both off-axis and in-line holography. Two incident and convergent plane waves are produced in front of the specimen thanks to an electrostatic biprism in the condenser system of a transmission electron microscope. The interference of electron beams diffracted by the illuminated crystal is then recorded in a defocused plane. The differential phase recovered from the hologram is directly proportional to the strain in the sample. The strain can be quantified if the separation of the images due to the defocus is precisely determined. The present technique has the advantage that the derivative of the phase is measured directly which allows us to avoid numerical differentiation. The distribution of the noise in the reconstructed strain maps is isotropic and more homogeneous. This technique was used to investigate different samples: a Si/SiGe superlattice, transistors with SiGe source/drain and epitaxial PZT thin films.

Comparison of effective transverse piezoelectric coefficients e31,f of Pb(Zr,Ti)O3 thin films between direct and converse piezoelectric effects

We evaluated the effective transverse piezoelectric coefficients (e31,f) of Pb(Zr,Ti)O3 (PZT) thin films from both the direct and converse piezoelectric effects of unimorph cantilevers. (001) preferentially oriented polycrystalline PZT thin films and (001)/(100) epitaxial PZT thin films were deposited on (111)Pt/Ti/Si and (001)Pt/MgO substrates, respectively, by rf-magnetron sputtering, and their piezoelectric responses owing to intrinsic and extrinsic effects were examined. The direct and converse |e31,f| values of the polycrystalline PZT thin films were calculated as 6.4 and 11.5–15.0 C/m2, respectively, whereas those of the epitaxial PZT thin films were calculated as 3.4 and 4.6–4.8 C/m2, respectively. The large |e31,f| of the converse piezoelectric property of the polycrystalline PZT thin films is attributed to extrinsic piezoelectric effects. Furthermore, the polycrystalline PZT thin films show a clear nonlinear piezoelectric contribution, which is the same as the Rayleigh-like behavior reported in bulk PZT. In contrast, the epitaxial PZT thin films on the MgO substrate show a piezoelectric response owing to the intrinsic and linear extrinsic effects, and no nonlinear contribution was observed.

Fabrication of High-Efficiency Piezoelectric Energy Harvesters of Epitaxial Pb(Zr,Ti)O3 Thin Films by Laser Lift-off

Abstract We fabricated piezoelectric vibration energy harvesters of c-axis-oriented epitaxial Pb(Zr,Ti)O3 (PZT) thin films on stainless steel (SS304) cantilevers in an effort to improve their power-generation efficiency and toughness. Using radio-frequency magnetron sputtering, we deposited the epitaxial PZT thin films on the MgO substrates, and then transferred the PZT films onto microfabricated SS304 cantilevers using laser lift-off (LLO). LLO did not degrade the transferred epitaxial PZT thin films, which exhibited a high piezoelectric coefficient (e 31,f =–4.8 C/m2) and a low relative dielectric constant (ε r=340), comparable to those of the original PZT thin film on MgO. At a resonance frequency of 143 Hz, the energy harvesters generated large output power of 1.8 μW at an acceleration of 1.0 m/s2, and the output power reached a maximum of 49 μW at an acceleration of 7.5 m/s2.

Process dependence of the piezoelectric response of membrane actuators based on Pb(Zr0.45Ti0.55)O3 thin films

A process for the fabrication of piezoelectric membranes with Pb(Zr0.45Ti0.55)O3 (PZT) thin films for micrometer-range actuation and sensing applications has been developed. The films grown by sol-gel on the Pt/Ti/SiO2/SOI (silicon-on-insulator) substrates have similar randomly oriented perovskite phases, but the nearly epitaxial film made by the pulsed laser deposition exhibits a more compact and flat morphology. It is observed that a denser microstructure produced by pulsed laser deposition leads to a significantly higher remanent polarization and piezoelectric coefficient in the capacitor structure, as well as in a much higher piezoelectric membrane displacement, but only a slightly enhancement of the quality factor of the membrane actuator is obtained. The membrane actuator with the epitaxial PZT thin film grown on a SrRuO3/Yttria-Stabilized Zirconia/SOI substrate using pulsed laser deposition shows significantly enhanced piezoelectric membrane displacement and quality factor. The results and discussion in our paper provide a better understanding of the relationship between the microstructure and the thin film properties, which may lead to significant improvements in device performance.

Optimized electrode coverage of membrane actuators based on epitaxial PZT thin films

This research presents an optimization of piezoelectric membrane actuators by maximizing the actuator displacement. Membrane actuators based on epitaxial Pb(Zr,Ti)O3 thin films grown on all-oxide electrodes and buffer layers using silicon technology were fabricated. Electrode coverage was found to be an important factor in the actuation displacement of the piezoelectric membranes. The optimum electrode coverage for maximum displacement was theoretically determined to be 39%, which is in good agreement with the experimental results. Dependences of membrane displacement and optimum electrode coverage on membrane diameter and PZT-film/Si-device-layer thickness ratio have also been investigated.

Film-thickness and composition dependence of epitaxial thin-film PZT-based mass-sensors

The transverse piezoelectric coefficient e31,f and mass-sensitivity were measured on piezoelectric cantilevers based on epitaxial PZT thin-films with film-thicknesses ranging from 100 to 2000nm. The highest values of e31,f and mass-sensitivity were observed at a film thickness of 500–750nm, while the observed remnant polarization Pr and longitudinal piezoelectric coefficient d33,f values become saturated with a film thickness of 750–1000nm. To obtain high performance by making use of its optimal film thickness, PZT thin films with various Zr/Ti ratios from 20/80 to 80/20 were studied. The experimental results indicated that the ferroelectric property reached a highest remnant polarization Pr at a Zr/Ti ratio of 20/80, while the longitudinal piezoelectric coefficient d33,f increased with increasing Zr content and reaches a maximum at a Zr/Ti ratio of 52/48. The findings suggest that the optimal composition for mass-sensitivity and transverse piezoelectric coefficient e31,f was shifted to the tetragonal part of the phase diagram with the Zr/Ti ratios of 45/55 and 40/60, respectively.

Epitaxial PZT thin films on YSZ-buffered Si (001) substrates for piezoelectric MEMS or NEMS applications

We report the growth of epitaxial Pb(Zr0.54Ti0.46)O3 (PZT) thin films on yttria-stabilized zirconia buffered silicon substrates by pulsed laser deposition. We demonstrate a full in plane epitaxy of the buffer layer, showing a RMS roughness of less than 0.3 nm for a 120 nm thick layer. This buffer layer allows the growth of fully (110) textured oxide conducting SrRuO3 and subsequent functional oxide layers. Here the Pb(Zr,Ti)O3 oxide was chosen to demonstrate its possible integration in piezoelectric microelectromechanical systems on silicon.

Shift of morphotropic phase boundary in high-performance [111]-oriented epitaxial Pb (Zr, Ti) O3 thin films

High-performance epitaxial niobium-doped lead zirconate titanate thin films [Pb(ZrxTi1−x)0.98Nb0.02O3] with a fixed thickness were deposited on [111]-cut single-crystalline SrTiO3 substrates by a sol-gel process as a function of Zr/Ti ratio ranging from 20/80 to 80/20. An obvious shift of morphotropic phase boundary (MPB) was observed in the [111]-oriented epitaxial films. Deviating from the Zr/Ti ratio of 52/48 in bulk materials, the MPB composition in the as-deposited films was found to move to a lower Zr/Ti ratio between 30/70 and 40/60, which agrees with the theoretical analysis on the basis of lattice mismatch between films and substrates and is further confirmed by the XRD patterns and Raman spectrum as well as ferroelectric test. Furthermore, a superior remnant polarization (Pr) value among all the compositions was obtained at a Zr/Ti ratio of 30/70, indicating that the extrinsic ferroelectric polarization near the new morphotropic phase boundary will lead to outstanding dielectric and piezoelectric performance in the epitaxial PZT thin film system.

Taper PbZr0.2Ti0.8O3 Nanowire Arrays: From Controlled Growth by Pulsed Laser Deposition to Piezopotential Measurements

Single crystalline PbZr(0.2)Ti(0.8) (PZT) nanowires arrays (NWAs) with taper morphology were epitaxially grown on SrTiO(3) (STO) substrate using pulse laser deposition. The taper morphology was attributed to the overcoating of PZT layer via a lateral growth of PZT clusters/adatoms during PZT NW growth. The growth window for PZT film or nanowire was systematically studied at varied temperatures and pressures. The proposed growth mechanism of the taper PZT NWAs was investigated from a layer by layer growth via Frank-Van Der Merwe growth, followed by a formation of three-dimensional islands via Stranski-Krastanow growth, and then axial growth on the lowest energy (001) plane with growth direction of [001] via vapor-solid growth mechanism. However, under certain conditions such as at higher or lower pressure (>400 or <200 mTorr) or substrate temperatures (>850 °C and <725 °C), formation of the PZT NWs is suppressed while the epitaxial PZT thin film via the layer-by-layer growth remains. The controllable growth directions of the PZT NWAs on (001), (110), and (111) STO substrates were demonstrated. The piezopotential of the taper PZT NWAs using a conducting atomic force microscope with the average voltage output of ~18 mV was measured. The theoretical piezopotential of a PZT NW was calculated to compare with the measured outputs, providing a comprehensively experimental and theoretical understanding of the piezoelectricity for the PZT NW.

Determination of the Young's modulus of pulsed laser deposited epitaxial PZT thin films

We determined the Young's modulus of pulsed laser deposited epitaxially grown PbZr0.52Ti0.48O3 (PZT) thin films on microcantilevers by measuring the difference in cantilever resonance frequency before and after deposition. By carefully optimizing the accuracy of this technique, we were able to show that the Young's modulus of PZT thin films deposited on silicon is dependent on the in-plane orientation, by using cantilevers oriented along the ⟨1 1 0⟩ and ⟨1 0 0⟩ silicon directions. Deposition of thin films on cantilevers affects their flexural rigidity and increases their mass, which results in a change in the resonance frequency. An analytical relation was developed to determine the effective Young's modulus of the PZT thin films from the shift in the resonance frequency of the cantilevers, measured both before and after the deposition. In addition, the appropriate effective Young's modulus valid for our cantilevers’ dimensions was used in the calculations that were determined by a combined analytical and finite-element (FE) simulations approach. We took extra care to eliminate the errors in the determination of the effective Young's modulus of the PZT thin film, by accurately determining the dimensions of the cantilevers and by measuring many cantilevers of different lengths. Over-etching during the release of cantilevers from the handle wafer caused an undercut. Since this undercut cannot be avoided, the effective length was determined and used in the calculations. The Young's modulus of PZT, deposited by pulsed laser deposition, was determined to be 103.0 GPa with a standard error of ± 1.4 GPa for the ⟨1 1 0⟩ crystal direction of silicon. For the ⟨1 0 0⟩ silicon direction, we measured 95.2 GPa with a standard error of ± 2.0 GPa.

Scaling Behavior of Amplitude-dependent Ferroelectric Hysteresis Loops in an Epitaxial PbZr0.2Ti0.8O3 Thin Film

We investigated the scaling behavior of ferroelectric (FE) hysteresis loops as a function of the applied field amplitude (E{sub 0}) in a high-quality epitaxial PbZr{sub 0.2}Ti{sub 0.8}O{sub 3} (PZT) thin film. We observed that the areas of the polarization-electric field hysteresis loops (A) followed the scaling law A {proportional_to} E{sub 0}{sup {alpha}}, with the exponent {alpha} = 0.45 {+-} 0.01. This result is in excellent agreement with the theoretical prediction of {alpha} by the two-dimensional Ising model. In addition, we found that the coercive field (E{sub C}) showed E{sub C} {proportional_to} E{sub 0}{sup {gamma}} with the exponent {gamma} = 0.28 {+-} 0.01. We attribute this relationship to the difference in the sweep rate of the field amplitude E{sub 0}. From the obtained {gamma} value, the growth dimension of FE domains is found to be about 1.68 in our epitaxial PZT thin film.

The realization and performance of vibration energy harvesting MEMS devices based on anepitaxial piezoelectric thin film

This paper focuses on the fabrication and evaluation ofvibration energy harvesting devices by utilizing an epitaxialPb(Zr0.2Ti0.8)O3 (PZT) thin film. Thehigh quality of the c-axis oriented PZT layer results in a high piezoelectric coefficient and a low dielectricconstant, which are key parameters for realizing high performance piezoelectric energyharvesters. Different cantilever structures, with and without a Si proof mass, are realizedusing micro-patterning techniques optimized for the epitaxial oxide layers, to maintain thepiezoelectric properties throughout the process. The characteristics and the energyharvesting performances of the fabricated devices are experimentally investigatedand compared against analytical calculations. The optimized device based on a0.5 µm thick epitaxial PZT film, a cantilever beam of1 mm × 2.5 mm × 0.015 mm, with a Siproof mass of 1 mm × 0.5 mm × 0.23 mm, generates an output power, current and voltage of, respectively,13 µW g − 2,48 µA g − 1 and0.27 V g − 1 (g = 9.81 m s − 2) at the resonant frequency of 2.3 kHz for an optimal resistive load of5.6 kΩ. The epitaxial PZT harvester exhibits higher power and current with usable voltage,while maintaining lower optimal resistive load as compared with other examplespresent in the literature. These results indicate the potential of epitaxial PZTthin films for the improvement of the performances of energy harvesting devices.

Finite element analysis and experiments on a silicon membrane actuated by an epitaxial PZT thin film for localized-mass sensing applications

In this paper, we investigate the performance of a piezoelectric membrane actuated by an epitaxial piezoelectric Pb(Zr 0.2Ti 0.8)O 3 (PZT) thin film for localized-mass sensing applications. The fabrication and characterization of piezoelectric circular membranes based on epitaxial thin films prepared on a silicon wafer are presented. The dynamic behavior and mass sensing performance of the proposed structure are experimentally investigated and compared to numerical analyses. A 1500 μm diameter silicon membrane actuated by a 150 nm thick epitaxial PZT film exhibits a strong harmonic oscillation response with a high quality factor of 110–144 depending on the resonant mode at atmospheric pressure. Different aspects related to the effect of the mass position and of the resonant mode on the mass sensitivity as well as the minimum detectable mass are evaluated. The operation of the epitaxial PZT membrane as a mass sensor is determined by loading polystyrene microspheres. The mass sensitivity is a function of the mass position, which is the highest at the antinodal points. The epitaxial PZT membrane exhibits a mass sensitivity in the order of 10 −12 g/Hz with a minimum detectable mass of 5 ng. The results reveal that the mass sensor realized with the epitaxial PZT thin film, which is capable of generating a high actuating force, is a promising candidate for the development of high performance mass sensors. Such devices can be applied for various biological and chemical sensing applications.