Tuning the energy storage performance, piezoelectric strain and strain hysteresis of relaxor PLZT thin films through controlled microstructure by changing the ablation rate

Journal of the European Ceramic Society 39 (2019) 2076–2081

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Original Article

Tuning the energy storage performance, piezoelectric strain and strain hysteresis of relaxor PLZT thin films through controlled microstructure by changing the ablation rate Minh D. Nguyena,b, a b

T

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Division of Computational Mechatronics, Institute for Computational Science, Ton Duc Thang University, Ho Chi Minh City, Viet Nam Faculty of Electrical & Electronics Engineering, Ton Duc Thang University, Ho Chi Minh City, Viet Nam

A R T I C LE I N FO

A B S T R A C T

Keywords: Pulsed laser deposition Relaxor ferroelectrics Thin film Pulse-power energy storage Piezoelectric strain

Relaxor-ferroelectric Pb0.9La0.1(Zr0.52Ti0.48)O3 (PLZT) films with a thickness of 1.2 μm were deposited on LaNiO3-buffered Ca2Nb3O10-nanosheet/Si. It was revealed how structural modification of a PLZT film, fabricated using pulsed laser deposition under various ablation rates, can be used to tune its energy-storage performance and piezoelectric-strain. A highest unipolar piezoelectric-strain of 0.71% with extremely low strain-hysteresis of 1.9% and corresponding normalized-strain of 142 pm/V under an electric field of 500 kV/cm were observed in the film deposited at an ablation rate of 50 Hz, and such film consists of vertical columnar-structure. Whereas, the film deposited at a low ablation rate of 10 Hz with dense-structure had the higher recoverable energy-storage density (50.2 J/cm3) and energy-storage efficiency (82.2%) due to the larger electric-breakdown strength (3150 kV/cm). The strongly improved performance by choosing an appropriate film structure is important for practical applications in pulse-power energy-storage as well as for the development of piezo-driven microelectromechanical-systems.

1. Introduction Relaxor ferroelectrics have attracted much attention both for fundamental research and for advanced technological applications [1–6]. Owing to the high electric breakdown and the slim polarization hysteresis loop, the relaxor ferroelectric thin films hold special promise for advanced energy storage applications [7,8]. The coexistence of relaxor ferroelectric (RFE) and antiferroelectric (AFE) phases is used to explain P a high recoverable energy storage density (Ureco = ∫P m EdP ) in RFE due r+ to a high maximum polarization Pmax and low positive remanent polarization Pr+ [9–13]. Moreover, the low Pr+ value in RFE is also caused by the presence of polar nano-regions (PNRs) [14–17]. Various techniques have been used to deposit relaxor ferroelectric films, such as sol-gel [9,18–21], sputtering [22,23] and pulsed laser deposition (PLD) [8,12,13,24]. Even though each growth technique has its advantages and disadvantages, the PLD technique has been demonstrated to be a versatile method for the fabrication of high-quality thin films [25]. The main advantage of PLD is the possibility to transfer stoichiometrically multicomponent target material, especially containing volatile components such as lead, to the layer. Moreover, PLD is a powerful method which allows a thicker film to be fabricated in a

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short time due to the high deposition rate and the possibility to incorporate the process directly into a Si production line [26–28], for example in microelectromechanical systems (MEMS). Beyond the energy storage properties, the other important consideration is the presence of piezoelectric properties in RFE films [2,29,30]. The film has high piezoelectric coefficient (strain) and low piezoelectric strain hysteresis making it a good candidate for practical applications in precisely controlled piezoelectric devices and also piezoelectric vibration energy harvesters [31,32]. The previous study on ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) films also indicated that the piezoelectric strain and strain hysteresis were significantly enhanced for higher ablation rate, and it could be understood from the decreasing interconnection between the columnar grains and therefore less effective clamping of the film [33]. In the present work, we have outlined a change in the ablation rate (in the range of 10–50 Hz or 10–50 pulses/second) during the PLD deposition process that can be varied to tune the microstructure and then the energy storage performance, piezoelectric strain and strain hysteresis of relaxor ferroelectric Pb0.9La0.1(Zr0.52Ti0.48)O3 (PLZT) thin films. In this study, highly textured conductive oxide LaNiO3 layers grown on Ca2Nb3O10 nanosheet/Si substrates were used as the bottom

Correspondence address: Division of Computational Mechatronics, Institute for Computational Science, Ton Duc Thang University, Ho Chi Minh City, Viet Nam. E-mail address: [email protected].

https://doi.org/10.1016/j.jeurceramsoc.2019.02.006 Received 27 December 2018; Received in revised form 1 February 2019; Accepted 2 February 2019 Available online 04 February 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.

Journal of the European Ceramic Society 39 (2019) 2076–2081

M.D. Nguyen

Fig. 1. (a–c) XRD θ-2θ scans and (d) omega scans of PLZT films deposited on LNO/CNO/Si as a function of laser ablation rate.

etching of the LNO top-electrodes and wet-chemical etching (HF-HCl solution) of the PLZT films.

electrodes. Conductive oxide electrodes serve as a source/sink of mobile oxygen vacancies, thus preventing the degradation of the polarization and then energy storage performance during cycling [34]. The results indicate that the higher recoverable energy storage density of 50.2 J/cm3 (energy storage efficiency of 82.2%) and larger electric breakdown strength of 3150 kV/cm are concurrently achieved in the film deposited at 10 Hz. Whereas, the film deposited at 50 Hz exhibits * larger piezoelectric strain of 0.71% (normalized strain d33 of 142 pm/V) with extremely low strain hysteresis of 1.9%. These results demonstrate that suitable PLZT films obtained by tuning the ablation rate are great candidates for both actuator and energy storage applications.

2.4. Analysis and characterization Crystallographic properties of the thin films were analyzed by X-ray θ–2θ scans (XRD) and omega scans (rocking curves) using a PANalytical X-ray diffractometer (Malvern PANalytical) with Cu-Kα radiation (wavelength: 1.5405 Å). Normal operating power is 1.8 kW (45 kV and 40 mA). Atomic force microscopy (AFM: Bruker Dimension Icon) and cross-sectional high-resolution scanning electron microscopy (HRSEM) (Zeiss-1550, Carl Zeiss Microscopy GmbH) were performed to investigate the surface morphology, microstructure and thickness of the as-grown thin films. The polarization-electric field (P-E) hysteresis loops and switching current-electric field (IS–E) curves were performed with the dynamic hysteresis measurement (DHM) in ferroelectric module of the aixACCT TF-2000 Analyzer (aixACCT Systems GmbH). A Keithley 4200 Semiconductor Characterization System (Tektronix) was used for the leakage current measurement. The normalized strain piezoelectric * coefficient (d33 ) of the thin film capacitors was defined from the piezoelectric strain (S–E) measurement with a unipolar triangular-shaped ac amplitude (500 kV/cm and 50 Hz) applied to the thin film capacitors. * * = S (Emax )/ Emax , (The normalized strain d33 value is calculated as d33 where S(Emax) is the maximum strain obtained at maximum electric field Emax). All measurements were performed at room temperature.

2. Experimental procedure 2.1. Nanosheet deposition Ca2Nb3O10 (CNO) nanosheets were fabricated on Si substrates by the exfoliation of layered protonated calcium niobate (HCa2Nb3O10·1.5H2O) using the Langmuir-Blodgett deposition method [34]. The thickness of the CNO monolayers was approximately 2.7 nm. 2.2. Pulsed laser deposition Relaxor ferroelectric Pb0.9La0.1(Zr0.52Ti0.48)O3 (PLZT) thin films were deposited on LaNiO3 (LNO)-buffered Ca2Nb3O10 nanosheet/Si substrates using a pulsed laser deposition (PLD) method with a KrF excimer laser source (Lambda Physik, 248 nm wavelength). The optimal conditions for the deposition of PLZT films were: substrate temperature 600 °C, energy density 2.5 J/cm2 and O2 pressure 0.1 mbar. To investigate the effect of the ablation rate, PLZT films with a thickness of about 1.2 μm were deposited at the ablation (pulse) rate in the range of 10–50 Hz (or 10–50 pulses/second). The deposition conditions for 200nm-thick LNO top- and bottom-electrodes were 8 Hz, 600 °C, 2.25 J/ cm2 and 0.1 mbar O2. All layers were deposited successively without breaking the vacuum. After deposition, the films were cooled down to room temperature in a 1 bar oxygen atmosphere and at a ramp rate of 8 °C/min.

3. Results and discussion Fig. 1(a)–(c) shows the XRD θ-2θ patterns of PLZT thin films deposited at the laser ablation rate of 10, 20 and 50 Hz. All films are predominantly (001)-oriented with only minor fraction of (110) orientation, and the intensity of minor peak is increased slightly with the higher ablation rate. The (002) reflection peak of the PLZT films was further examined with an X-ray omega (ω)-scan (rocking curve), as shown in Fig. 1(d). The full-width-at-half-maximum (FWHM) values of the PLZT films increases slightly with increasing laser ablation rate (values in Table 1). A lower FWHM value corresponds to a higher thin film quality, implying that the grains are more aligned [35]. The film surface morphology and microstructure were investigated by AFM and cross-sectional SEM as shown in Fig. 2. The film deposited at low ablation rate of 10 Hz shows a very compact film structure

2.3. Fabrication of thin film capacitors The capacitor structures (size: 200 × 200 μm2) were patterned by a standard photolithography process and structured by argon-ion beam 2077

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Table 1 Properties of PLZT thin films as a function of ablation rate. Laser ablation rate (Hz)

Surface roughness, Rq (nm)a

Columnar grain size, dcol (nm)

Smax (%)

FWHM (degree) of PLZT(002)

at Emax 10 25 50 a

13.6 22.1 35.6

– 82 130

0.66 0.75 0.87

0.53 0.66 0.71

H (%)

* d33 (pm/ V) = 500 kV/cm

2.4 2.2 1.9

106 132 142

Ureco (J/cm3)

η (%)

EBD (kV/ cm)

at Emax = 1200 kV/cm 14.8 14.5 14.5

94.4 91.5 85.0

Ureco (J/ cm3)

η (%)

at EBD 3150 2600 2350

50.2 39.1 32.4

82.2 82.1 76.9

Defined in whole AFM scan area of 10 × 10 μm2.

(Fig. 2(a)). Similar to the Pb(Zr0.52Ti0.48)O3 (PZT) thin films [34], the observed columnar grain structure in the PLZT films formed along lines arises from the edges of the CNO nanosheets, due to structure being ascribed to a different nucleation density and growth at the edges of the nanosheets. Our previous study described that the columnar grains in PZT films grown along the CNO edges and in the uncovered Si substrate areas have (110) orientation [34] that could be confirmed by using selected area electron diffraction (SAED) inside a transmission electron microscope (TEM) measurement [33]. For higher laser ablation rates, the structure of the PLZT thin films is columnar (Fig. 2(c) and (e). The XRD data in Fig. 1(b)–(c) indicates that these films have a predominantly (001) orientation and only minor fraction of (110) orientation is observed. The columnar grain structures on the nanosheets with (001) orientation are very similar to those on the edges of the nanosheets with (110) orientation, which makes it impossible to distinguish the two orientations from the SEM images. However, these grain structures can be also confirmed by using SAED which was taken from TEM. The change from the dense structure to the columnar structure, as also described in the previous work [33], is attributed to a drastic increase in the nucleation density with increasing laser frequency, due to strongly reduced diffusion time for particles arriving on the film surface. It can be assumed that if the deposition rate (or laser deposition rate) is high, the arriving adatoms have a low surface mobility and then the islands will be formed during deposition. On the other hand, the low surface mobility of adatoms in high ablation rate results in a columnar grain growth [36,37]. The change in the structure is also shown in the surface roughness of PLZT thin films. Fig. 2(b) indicates that for the films deposited at 10 Hz

the root-mean-square surface roughness (Rq) is of about 2.2 nm for the films grown on top of the nanosheets (dashed box). However, in the entire scanned area of 10 × 10 μm2, the Rq has much higher value (13.6 nm), which should be related to the presence of the columnar grains of the films grown on the nanosheet edges (arrow in Fig. 2(a) and (b)). The Rq value rapidly increases from 22.1 to 35.6 nm for the films deposited at the ablation rates of 25 and 50 Hz (Fig. 2(d) and (f)), respectively. The increased Rq is correlated to the larger average columnar grain diameter (dcol) with increasing ablation rate (Table 1). Fig. 3 shows the unipolar piezoelectric strain (S–E), the normalized * strain d33 (= Smax / Emax , where Smax is the maximum strain at maximum applied electric field Emax) and the strain hysteresis that is related to the piezoelectric loss (H,=(Sforw − Sret ) Emax /2 / Smax with Sforw (Sret ) the strain on the rising (falling) branch of the hysteresis loop at half the maximum applied field Emax), of the PLZT films as a function of laser * ablation rate. The d33 values first increase significantly for the films deposited at 10 and 25 Hz and then slightly for the film deposited at higher ablation rate (50 Hz), whereas H values slightly decline when the ablation rate increases. Smax can reach a maximum value of 0.71% * (d33 = 142 pm/V) and a low H value of 1.9% is obtained at the maximum ablation rate of 50 Hz. The increase of the piezoelectric strain with increasing ablation rate can be understood from the decreasing interconnection between the grains and therefore less effective clamping of the film [33]. On the other hand, the resulting increase in piezoelectric properties was attributed to lower potential energy barriers in the less clamped films which allowed enhanced irreversible domain wall motion [38,39]. The high piezoelectric strain and very low strain hysteresis obtained in this study expand the possibilities for the application of such relaxor ferroelectric PLZT films in low voltage

Fig. 2. Cross-sectional SEM and corresponding AFM surface morphological images of PLZT films deposited on LNO/CNO/Si at (a,b) 10 Hz, (c,d) 25 Hz and (e,f) 50 Hz. 2078

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Fig. 3. (a) Unipolar piezoelectric strain versus E-field (S–E) loops of PLZT films deposited at various ablation rates. (b) Laser ablation rate dependence of normalized * ) and strain hysteresis (H). strain (d33

Fig. 4. (a) P-E ferroelectric hysteresis loops and (b) switching current (IS–E) curves, of PLZT films deposited at various ablation rates. The measurements were performed at ± 1200 kV/cm, 1 kHz frequency and room temperature (24 °C).

rate (Fig. 5(b)). The Ureco and η reach the maximum values of 14.8 J/ cm3 and 94.4% at the ablation rate of 10 Hz (under the same applied electric field of 1200 kV/cm). The increased Ustore value at higher ablation rate is caused by the lower rising branch of the hysteresis loop, as shown in Fig. 4(a). Fig. 6 illustrates energy storage density and energy storage efficiency calculated from the P-E loops of PLZT thin films as a function of the applied electric field, measured from low applied electric field (such as 500 kV/cm) until a critical electric breakdown strength, EBD (just below the electric field where the capacitor is broken completely) [13]. It can be seen that both Ustore and Ureco values increase gradually with the increase of electric field, as shown in Fig. 6(a), (c) and (e). However, the rate at which Ureco increases is less than that of Ustore, therefore the efficiency η gradually decreases with increasing applied electric field (Fig. 6(b), (d) and (f)). Further, it is found that the decrease of Ureco value with laser ablation rate is mainly due to the reduction in the breakdown strength. The EBD and thus Ureco values in the dense structure PLZT films deposited at low ablation rate (10 Hz) are higher than those deposited at higher ablation rate with the columnar grain structures. Owing to the high breakdown strength EBD of ˜3150 kV/cm, an ultrahigh Ureco(EBD) of ˜50.2 J/cm3 and a large η of ˜82.2% can be achieved in the 10 Hz PLZT thin films. This is one of the few thin film capacitors have both excellent recoverable energy storage density and energy storage efficiency, which makes a great impact on the pulsepower energy storage applications [44]. Fig. 7 shows the leakage current plotted as a function of the electric field (J-E) of PLZT thin films. The completely broken values in the J-E measurements are about 2670, 2115 and 1740 kV/cm, respectively, for the thin films deposited at 10, 25 and 50 Hz. They are smaller than the corresponding values obtained from the P-E measurements (3150, 2600

MEMS actuator systems. The polarization hysteresis (P-E) loops of PLZT thin films deposited at various ablation rates are presented in Fig. 4(a). It is obvious that all samples show slim hysteresis loops, which are typical of relaxor ferroelectrics [40]. Moreover, Fig. 4(a) also indicates that the while the falling branches of the hysteresis loops (discharging process) almost coincide with each other, the rising branch of the hysteresis loops (charging process) is lower with increasing ablation rate. In this case, the ablation rate dependence of the polarization in the rising branch can be influenced by the extrinsic switching [41], such as the change in volume fraction of polarization domains as a function of ablation rate, since a decreasing volume fraction of polarization domains due to the low density of the films deposited at high ablation rate will decrease the polarization with increasing applied electric field. On the other hand, the P-E hysteresis loops become slightly wider with increasing ablation rate, which influences the energy storage performance of thin film capacitors as discussed in the next section. Fig. 4(b) shows the switching current (IS–E) measurement of PLZT thin films deposited at various ablation rates. It indicates that four switching peaks in the IS–E curves were detected in the low electric field region (−150 to +150 kV/cm), demonstrating their AFE-like behaviors [42,43]. According to calculation methods of energy stored per unit volume P P Ustore = ∫0 m EdP , recoverable energy storage density Ureco = ∫P m EdP , r+

and energy storage efficiency η (%) = 100 × Ureco/ Ustore , where Pr+ and Pm are, respectively, the positive remanent polarization and maximum polarization [13], the energy storage properties of PLZT thin film capacitors with different ablation rates are calculated from P-E loops in Fig. 4(a) and given in Fig. 5. It is obviously seen that Ustore increases but Ureco is almost constant with the increase of ablation rate (Fig. 5(a)), and consequently the lower η value is obtained at the higher ablation 2079

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Fig. 5. (a) Energy storage density (Ustore and Ureco) and (b) energy storage efficiency (η), of PLZT films deposited at various ablation rates. The data were obtained and calculated from the P-E loops in Fig. 4(a).

and 2350 kV/cm) but show the same trend. The difference between these values was explained in the previous study, which could be related to the condition of the measurements [13]. In the J-E measurements, the dc electric fields are continuously applied to the capacitors from the low electric field until the electric field where the capacitors are fully broken, whereas the P-E loops are measured from the low acapplied electric field to the ac-applied electric field where the capacitors are broken but with a discontinuous process. Moreover, increase of the grain boundary density in the columnar structure PLZT thin films deposited at high ablation rate leads to a decrease of the electric breakdown field. Lee et al. also indicated that the leakage current through grain boundaries is much higher than that through bulk PZT films [45], and the current concentration was assumed to localize around the defect points in the films. 4. Conclusions

Fig. 7. Leakage current of PLZT films deposited at various ablation rates. Figure insets show the optical images of PLZT film capacitors: (a) before and (b) after broken.

We present a controlled microstructure and growth technique via pulsed laser deposition that enables broader tuning of the piezoelectric strain response and energy storage performance in the relaxor

Fig. 6. Dependence of energy storage density (Ustore and Ureco) and energy storage efficiency (η) on applied electric field for PLZT films deposited at (a,b) 10 Hz, (c,d) 25 Hz and (e,f) 50 Hz. The data were calculated from the corresponding P-E hysteresis loops performed at 1 kHz and room temperature (24 °C). 2080

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ferroelectric thin film capacitors. Ultrahigh energy storage performance with a recoverable energy-storage density of 50.2 J/cm3 and an energystorage efficiency of 82.2% is observed in dense structure PLZT thin films deposited at low ablation rate of 10 Hz. The high energy storage properties are the contribution of the slim hysteresis loop due to the existence of antiferroelectric-like behavior in the relaxor ferroelectric PLZT thin films and of the large electric breakdown strength (EBD = 3150 kV/cm). Whereas, the excellent piezoelectric response * with a normalized strain d33 of 142 pm/V and strain hysteresis H of 1.9% is obtained in columnar grain structure PLZT thin films deposited at high ablation rate of 50 Hz. The results suggest that relaxor ferroelectric PLZT thin films can be used as thin film capacitors in pulsepower energy storage systems and also in piezoelectric actuators for ultraprecision position and motion control.

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Acknowledgements

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The authors thank Mr. Phu Le and Prof. Johan E. ten Elshof for the nanosheet deposition, and Mr. Mark Smithers for performing the HRSEM measurement.

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