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

In 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.


Introduction
9][10][11] For instance, PEHs can be incorporated in electronics located at remote and dangerous places where a recharging of battery is quite difficult, in wirelesscommunication modules that suffer from a battery problem that lots of sensors would cause, and in wearable devices for health care systems without batteries. [11,12]Yet, to catch up with the increasing electrical power requirements for an operation of those devices, it became necessary to integrate PEHs that have far better performances.In this context, a criterion for the comparison of PEHs' performances was needed, and the figure of merit (FOM) has been claimed as follows; [13,14]   0  r (1)   where |e 31,f |,  r , and  0 are the effective transverse piezoelectric coefficient, the relative permittivity of piezoelectric material, and the vacuum permittivity, respectively.Here, note that the |e 31,f | should be obtained under the occurrence of the direct piezoelectric effect by which the operation of PEHs are governed.According to the Equation (1), it is preferable to use a PZT thin film with a large direct |e 31,f | and a low  r to increase the FOM.The simplest way to achieve this in terms of material design is to modify the composition of the PZT thin film.Typically, the morphotropic phase boundary (MPB) composition has been a major concern due to better piezoelectric performances than other compositions. [15,16]Interestingly, the MPB composition of bulk ceramics, i.e., Pb(Zr 0.52 Ti 0.48 )O 3 did not guarantee a superior |e 31,f | value when the PZT was made into a thin film, though.[19][20][21][22] This implies that it is crucial to refrain from generalizing the selection of compositions.Moreover, epitaxial growth of PZT thin films were greatly effective for optimizing piezoelectric responses owing to the films' anisotropic nature. [8,23]Particularly, [001]-oriented epitaxial growth was favorable for a low  r , leading to a large FOM. [24]ased on the above, it can be concluded that PZT thin films should be prepared with modified compositions and grown epitaxially for high-performance PEHs.
30][31][32] However, Si substrate is the most compatible with conventional MEMS technique.Hence, investigations on PEHs that were fabricated from epitaxially grown PZT thin films on Si substrates (PZT/Si) have been implemented. [24,33,34]In particular, a microcantilever fabricated from an epitaxial PZT of Zr/[Zr+Ti] = 0.8 thin film demonstrated an excellent output power density of 560 μW −1 (cm −2 g −2 ) (where g denotes an gravitational acceleration, 1 g = 9.8 m −1 s −2 ) owing to a large FOM of ≈28 GPa. [24,33]n spite of such a high output performance, it is a bit open to question whether the PZT composition is an optimized one, as PZT thin films exhibited the highest FOM at the vicinity of conventional MPB in several cases. [17,18]Especially, PZT of Zr/[Zr+Ti] = 0.52 on MgO substrates possessed the highest FOM of 40 GPa within a series of PZT compositions, [18] which is superior to the value above.In this sense, it is essential to reevaluate the compositional dependence of FOM for epitaxial PZT/Si, utilizing a reliable evaluation approach.
In this study, we fabricated the epitaxial PZT/Si into unimorph cantilevers with various PZT compositions.The direct and converse |e 31,f | was evaluated, revealing that the optimal composition varied depending on the type of piezoelectric effect.Output performances of the PEHs were evaluated as well.It should be emphasized that the evaluation methodology for the output performance is exactly the same as that for the FOM measured by the direct piezoelectric effect, [34] allowing for an authentic correlation between them to be explored.

Crystal Structures of Epitaxial PZT/Si
Figure S1 (Supporting Information) displays out-of-plane X-ray diffraction (XRD) patterns of the epitaxial PZT thin films that were prepared for this study.Diffraction peaks assigned to (00l) pc (the subscript pc denotes pseudocubic coordinates) crystallographic planes are observed, suggesting that the PZT was grown with a preferred orientation.For a detailed insight for the crystal structure of the PZT, reciprocal space mapping (RSM) images around the PZT(204) pc node in the reciprocal lattice were taken, as shown in Figure 1.A big spot accompanied by three small spots was clearly observed from the PZT of Zr/[Zr+Ti] = 0.36, which is a typical pattern of the Ti-rich epitaxial PZT thin films with a tetragonal structure. [34,35]Lattice parameters were calculated as a = 0.3992 nm and c = 0.4177 nm from the big spot that indicates c-domain of the tetragonal PZT.On the other hand, the PZT of Zr/[Zr+Ti] = 0.64 had explicitly distinguishable two spots side by side.[38][39] When it comes to Zr/[Zr+Ti] = 0.44 and 0.51, the co-existence of tetragonal and rhombohedral phases was disclosed by out-of-plane XRD patterns obtained by synchrotron radiation XRD (SR-XRD).Diffraction patterns for (004) plane of PZT were comprised of several peaks that correspond to the tetragonal and rhombohedral phases.In case of the PZT of Zr/[Zr+Ti] = 0.44, the ratio of the tetragonal phase was dominant (MPB-T, Figure S2a, Supporting Information), while the rhombohedral phase mainly consisted of the Zr/[Zr+Ti] = 0.51 (MPB-R, Figure S2b, Supporting Information).It deserves attention that even though the tetragonal phase existed, very little amounts of a-domains were identified in the MPB-R.From the above, the composition range for the MPB was found to be quite broad compared to that of bulk PZT materials.Commonly, the crystal structure of epitaxial thin films exhibits a distinct behavior in response to compositional variations. [40,41]his can be attributed to the lattice constraints exerted by substrates, which induce strong internal stress within the PZT material.

Electrical Properties of Epitaxial PZT/Si
Ferroelectric properties of the epityaxial PZT thin films are shown in Figure 2. Rectangular-like polarization-electric field (P-E) hysteresis curves were obtained from the tetragonal and the MPB-T PZT, indicating that a swift reversal of c-domains and fast 90 o domain-wall motion occurred upon applying electric fields exceeding coercive electric field (E c ).In contrast, PZT with the rhombohedral phase exhibited a progressive polarization change, which is attributed to rotation and alignment of <111> pc domains.Also, the saturation polarization (P s ) decreased in a large extent because dipole moments were not aligned to the measurement direction.In case of the MPB-R, meanwhile, a hysteresis shape similar to that of the tetragonal and MPB-T was obtained although the rhombohedral phase was dominant.The exact reason for this phenomenon remains unclear, yet it is probable that domains were predominantly oriented toward the [001] pc direction owing to the very small interaxial angle  in the rhombohedral symmetry.Incidentally, the hysteresis curves are shifted toward the positive side throughout the whole compositions, implying that the upward polarization is stable within the epitaxial PZT thin films.This also let us know the suitable poling direction, i.e., a negative bias on the top electrodes.

Output Performance of PEH and its Relationship with Direct FOM
Output performances of the cantilever-type PEH are shown in Figure 4.It should be noted that they were tested on the basis of the same evaluation methodology applied for the measurement of direct |e 31,f |.The only thing we regulated was the load resistance (R L ) value.The R L dependence of output power (P out ) for various PZT compositions are depicted in Figure 4a.The epitaxial PZT/Si cantilever with the MPB-R produced an unrivaled P out of 14.5 μW when tested under the R L of 10 kΩ.For all the PZT compositions covered in this study, Figure S3 (Supporting Information) provides the P out and V out corresponding to different values of R L .Moreover, the influences of acceleration on the P out and resonance frequency (f r ) of the MPB-R cantilever were investigated by fixing the R L as 10 kΩ, as shown in Figure 4b.It seems like that even when subjected to high acceleration over the 1 g (9.8 m −1 s −2 ), the cantilever did not suffer significant damage.Otherwise, the experimental result would have been greatly deviated from the theoretical values that were derived following previous studies. [34,42]This result is consistent with the nearly constant |e 31,f | observed in terms of the acceleration (Figure 3a).Furthermore, we observed that the f r decreased from 427 to 421 Hz as the acceleration was raised, which is a trend very similar to what we observed in our previous studies. [34,42]This can be attributed to the nonlinear stiffness resulting from the geometric deformation of PEHs during vibration. [11]igure 5 describes direct FOMs that were derived using Equation (1), along with the corresponding output power densities.Figure 5a illustrates how the direct FOMs and output power densities depend on the PZT composition.The trend of FOM was very similar to that of direct |e 31,f | in Figure 3a, with the MPB-R achieving the highest FOM of 40 GPa.This is the highest value ever reported for the PZT thin films deposited on Si substrates owing to the large direct |e 31,f | and low  r .On top of that, the maximum output power density of 301.5 μW −1 (cm −2 g −2 ) was acquired at the same MPB-R composition, when the acceleration was 3 m −1 s −2 .Meanwhile, although it is generally accepted that a high FOM ensures a large output power density, it is important to emphasize that experimentally demonstrating this correlation can be quite challenging. [43]Nonetheless, it is noteworthy that variation in output power density closely paralleled the changes in FOM values, as depicted in Figure 5a.When the output power densities are plotted with respect to the FOMs, a linear relationship emerged, as shown in Figure 5b.This became possible by employing the same evaluation methodology for the FOM and output performance, highlighting the effectiveness of our approach in evaluating the performance of cantilever-type PEHs.

Conclusion
This study investigated the compositional dependence of crystal structure and piezoelectric properties of epitaxial PZT/Si cantilevers.XRD patterns and RSM images revealed an epitaxial growth along the [001] pc direction of the PZT crystal.As the Zr/[Zr+Ti] increased, the phase of PZT was transformed from the tetragonal to the rhombohedral phase.MPB was found within a broad composition range of 0.44 ≤ Zr/[Zr+Ti] ≤0.51, which was   maximum P out of 14.5 μW when the circuit was attached with the R L of 10 kΩ and the acceleration was 3 m −1 s −2 .At that point, the output power density was calculated to be 301.5 μW −1 (cm −2 g −2 ).In summary, we could make an epitaxial PZT/Si with an excellent FOM, therefore, a high-performance PEH could be realized.Moreover, we suggested a linear relationship between FOM and output power density, which demonstrates the validity of the approach we used.

Experimental Section
Fabrication of Cantilever-Shape Unimorph PEHs: Epitaxial PZT thin films with a thickness range of 2-3 μm were deposited on (001)SrRuO 3 (40 nm)/(100)Pt (150 nm)/ZrO 2 (50 nm)-coated Si substrate (725 μm) that was purchased from KRYSTAL Inc., with a size of 2 × 2 cm 2 by an RF magnetron sputtering.The key factor for the epitaxial growth of PZT thin films was the epitaxial growth of Pt bottom electrode.Previous studies have revealed that the use of a Y-doped ZrO 2 buffer layer facilitates the epitaxial growth of Pt on Si substrates. [44,45]Meanwhile, the Zr/[Zr+Ti] ratio of the thin films was manipulated by utilizing commercially available 3-inch sputtering targets of predetermined compositions (TOSHIMA, Pb(Zr 0.30 Ti 0.70 )O 3 , Pb(Zr 0.53 Ti 0.47 )O 3 , Pb(Zr 0.58 Ti 0.42 )O 3 , and Pb(Zr 0.70 Ti 0.30 )O 3 ), as well as a combination of them.To compensate evaporations of Pb elements during the deposition, excess PbO was added to the targets.The deposition of epitaxial PZT thin films followed the procedures outlined in our previous studies. [2,28]During the sputtering with an RF power of 90 W, the substrate temperature was maintained as 660 °C.The sputtering chamber was filled with a mixed gas of Ar/O 2 = 19.5/0.5 sccm under a working pressure of 0.5 Pa.The PZT/Si was diced into a cantilever shape after the 80 nm-thick Pt top electrode layer was formed by the sputtering.The length and the width of the cantilevers were aimed as ≈14 and 2 mm, respectively.
Structural and Dielectric/Ferroelectric Properties of PZT Thin Film: A high-resolution XRD (HR-XRD, RIGAKU, SmartLab) equipped with a fouraxis diffractometer was used to assess the crystal structure of the epitaxial PZT thin films, utilizing CuK radiation ( = 1.54 Å) at 40 kV and 30 mA.Detailed analyses were conducted by RSM images.In addition, SR-XRD was utilized to precisely evaluate crystal structures.Monochromatic X-rays (12.4 keV,  0 = 1.0 Å) were irradiated to the thin films at the synchrotron radiation facility SPring-8, beamline BL46XU.
To measure dielectric and ferroelectric properties, round-shaped Pt top electrodes were formed on PZT thin films by means of sputtering. r and dielectric loss (tan ) were measured at 1 kHz using an LCR meter (HIOKI, IM3536).Room-temperature P--E curves were taken by a ferroelectric tester (RADIANT Technologies, Precision Multiferroic II).
Evaluation of Piezoelectric Properties and PEH Output Performance: The piezoelectric properties, encompassing both direct and converse piezoelectric effects of the epitaxial PZT/Si, were assessed adopting the unimorph cantilever method, which was described in our earlier studies. [31,34]he direct |e 31,f | and PEH performances were characterized using the direct piezoelectric effect.A 0.96 g tungsten block as a proof mass was attached to the tip of the epitaxial PZT/Si cantilever.All the examined cantilevers were firmly fixed to clamping jig with a 5 cN m torque to prevent the movement of f r during the measurement.Prior to an operation of the PZT/Si cantilevers, a negative unipolar oscillation (1000 Hz, −20 V pp ) was applied to the top electrode for 1 min as a poling treatment using a function generator (NF Corporation, WF1943B) with an add-on power amplifier (NF Electronic Instruments, 4015).The cantilever was vibrated by a shaker equipped with an accelerometer and input tip displacements ( in ) were estimated by electric signal extracted from a laser Doppler vibrometer (ONOSOKKI, LV-1720A).During the vibration, a frequency response analyzer is used (NF Corporation, FRA5095) to check the f r .V out was measured using an oscilloscope (NF Corporation, GDS1062A) under various R L ranging from 10 −1 to 10 6 Ω.To calculate the direct |e 31,f |, the relationship was employed between  in and V out , which was amplified in our previous study. [34]From the direct |e 31,f | alongside the  r obtained from the cantilever, the direct FOM was calculated according to the Equation (1).P out was calculated as V out 2 /R L .The converse |e 31,f | was characterized using the converse piezoelectric effect.A negative unipolar sinusoidal oscillation was applied to the top electrode of the epitaxial PZT/Si cantilever using a function generator (NF Corporation, WF1946A), while the bottom electrode was grounded.The applied voltage was adjusted within the range of −5 -−20 V pp at a frequency of 600 Hz, far lower than the resonance frequency during the measurement of converse piezoelectricity, which was ≈2.3 kHz.The  out was measured using a laser Doppler vibrometer (Demodulator AT3700, GRAPHTEC, and Japan) and a digital oscilloscope (LT344, LeCroy, USA).To calculate the converse |e 31,f |, the relationship was utilized between the applied voltage and  out , as discussed in our previous studies. [31,34]

Figure 3
illustrates direct and converse |e 31,f | values of epitaxial PZT/Si cantilevers with various PZT compositions, while the dimensions of each cantilever are offered in Table S1 (Supporting Information).The direct |e 31,f | values along with output voltages out ) are plotted as a function of vibrational acceleration in Figure 3a.In a cantilever, the V out increased proportionally with the acceleration, leading to the almost constant |e 31,f | irrespective of the acceleration value.It is notable, that the cantilevers could endure the acceleration of up to 9 m −1 s −2 , retaining their piezoelectric responses.In addition, it was obvious that the direct |e 31,f | values became larger when the composition of PZT was located at the region of MPB.Within the MPB, we could observe that the MPB-R manifested the maximum direct |e 31,f | of 10.1 C m −2 .The relatively smaller direct |e 31,f | of 9.4 C m −2 was obtained from the MPB-T, which can be attributed to the existence of a nonnegligible amount of the a-domains that has a piezoelectric behavior opposite to that of the c-domains under the 31-mode operation.In Figure 3b, the converse |e 31,f | values along with output tip displacements ( out ) are exhibited.Interestingly, the maximum converse |e 31,f | of 14.0 C m −2 was yielded from the MPB-T, revealing that the compositions for the maximum direct and converse |e 31,f | values were not necessarily identical.This is because the converse |e 31,f | value incorporates extrinsic contributions that arise from the 90 o domain-wall motion.From the above findings, the epitaxial PZT/Si serves as a compelling example that highlights the importance of distinguishing between the direct and converse |e 31,f | when evaluating piezoelectric devices.

Figure 3 .
Figure 3. a) Direct |e 31,f | and V out versus acceleration and b) converse |e 31,f | and  out versus applied voltage of epitaxial PZT/Si cantilevers with various compositions.

Figure 4 .
Figure 4. a) P out versus R L acquired from epitaxial PZT/Si PEHs with various compositions.Acceleration was set at a constant value of 3 m −1 s −2 .b) P out and f versus acceleration obtained from the epitaxial PZT/Si PEH with the MPB-R.R L was set at a constant value of 10 kΩ.

Figure 5 .
Figure 5. a) FOMs and output power densities versus PZT composition.Error bars for the experimental standard deviation are presented, as well.b) A relationship between the output power densities and FOMs; white and black squares indicate measured and average values, respectively.The average values correspond to the data points in (a).