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Lead zirconate titanate and barium titanate bi-layer ferroelectric films on Si
Ceramics International 45 (2019) 9032–9037
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Lead zirconate titanate and barium titanate bi-layer ferroelectric films on Si a,b,c
Yingying Wang Jun Ouyanga,c,∗
, Jing Yan
a,1
, Hongbo Cheng
a,d,1
b
b
T
e
, Ning Chen , Peng Yan , Feng Yang ,
a Key Laboratory for Liquid-Solid Structure Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan, 250061, China b School of Mechanical Engineering, Shandong University, Jinan, 250061, China c Suzhou Institute of Shandong University, Suzhou, 215123, China d College of Electronic and Optical Engineering, Nanjing University of Posts and Tele-communications, Nanjing, 210023, China e School of Materials Science and Engineering, University of Jinan, Jinan, 250022, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Lead zirconate titanate (PZT) Barium titanate (BaTiO3) Bi-layer Ferroelectric film Si Magnetron sputtering
In this work, a novel method is proposed to integrate ferroelectric lead zirconate titanate (PZT) films on Si with highly tunable functionalities, through bi-layering with a barium titanate (BTO) film. First of all, the BTO film acts as a growth-promotion template layer which has successfully lowered the in situ deposition temperature of a ferroelectric PZT film. The PZT/BTO bilayer film deposited at 350 °C on LaNiO3-buffered Si substrate displayed a room temperature remnant polarization ∼24 μC/cm2, and its quality can be further improved via a rapid thermal annealing (RTP) process. Furthermore, by changing their thickness ratio, various ferroelectric hysteresis loops (P-E loops) can be created in the PZT/BTO films, thereby enabling a broad range of applications for this simple bi-layer structure. Examples including high performance piezoelectric energy harvesters and high energy density dielectric capacitors are demonstrated for the PZT/BTO bi-layer films.
1. Introduction Since its discovery in the late 1950s, lead zirconate titanate [Pb (Zr,Ti)O3 or PZT] has amazed generations of researchers with its excellent electrical properties [1]. With the rapid development in Si-based microelectronics, PZT films are required to be integrated into the industrial-standard complementary metal-oxide semiconductor (CMOS) Si technology to achieve their functionalities in a miniaturized scale [2]. These films have been widely used in dielectric capacitors [3], ferroelectric memories [4,5], and piezoelectric microelectromechanical systems (piezo-MEMS) [3,6]. It usually requires a high processing temperature (> 500 °C) to deposit or anneal PZT films to create a high-quality ferroelectric phase [7]. However, there are some critical issues associated with a long-time exposure to the high temperature, including formation of the pyrochlore phase [8], loss of Pb [9], and inter-diffusion of elements across pre-defined interfaces. The latter has become a major issue during recent efforts in chasing the Moore's law in the microelectronics industry, resulting in a stringent requirement to reduce the thermal budget for semiconductor processing, i.e., a lower deposition/processing
temperature has been demanded for any functional layer being integrated into CMOS-Si. These issues have led to the development of a number of rapid annealing techniques, including microwave [10] and laser irradiations [11], and rapid thermal processing (RTP) [12]. For example, Wang et al. obtained perovskite PZT films at 450 °C with the aid of a magnetic field from a low power microwave irradiation, [10]. Using a second laser irradiation in their laser ablation depositions, Tabata et al. claimed that they had successfully lowered the preparation temperature of PbTiO3 films down to 350 °C. However, films showing a typical ferroelectric P-E hysteresis loop in their work were prepared at a higher temperature of 450 °C [11]. Velo et al. prepared amorphous PZT films at room temperature via a rf magnetron sputtering process. When annealed via RTP at 625 °C for 30s, ferroelectric PZT films with saturated polarization (Ps) ∼ 38 μC/cm2, coercive field (Ec) ∼ 53 kV/cm and remnant polarization (Pr) ∼ 20 μC/cm2 were obtained at an applied maximum voltage of 20 V [12]. Another issue associated with applications of the PZT film, or any other ferroelectric single layer film, is a weak tunability of its functionality, derived from its characteristic P-E hysteresis loop. Due to the composition-structure-property interrelationship, a single-layer PZT
∗
Corresponding author. Key Laboratory for Liquid-Solid Structure Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan, 250061, China. E-mail address: [email protected] (J. Ouyang). 1 These two authors contribute equally to this work. https://doi.org/10.1016/j.ceramint.2019.01.237 Received 24 November 2018; Received in revised form 27 January 2019; Accepted 28 January 2019 Available online 31 January 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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film with a fixed composition can only be used in certain applications. Tuning of the functionality of a PZT film is usually achieved via using different targets for film deposition [13,14], or in a graded multilayer film prepared with a special target containing different compositions [15]. The former is not a good cost-effective approach to address industrial demands. In the meanwhile, due to effect of a strong built-in potential, the latter usually displays a shifted ferroelectric hysteresis loop instead of a field-tunable polarization response [15]. Furthermore, even with a number of targets having different compositions, a continuous/semi-continuous tuning of the electrical property in a broad range is often difficult. Possible reasons include the following:
Table 1 Deposition parameters of the PZT/BTO/LNO/Pt/Ti heterostructures on SiO2/ (100)Si substrates during a multi-target rf magnetron sputtering process. Note that the sputtering times of BTO and PZT were determined by their thickness ratio. Parameters Base pressure (Pa) Target-substrate distance (mm) Substrate temperature (oC) Sputtering pressure (Pa) Sputtering power (W) Deposition time (min)
(1) An accurate composition control (down to 1%, for example) throughout the film thickness is rather difficult in some commonly used deposition processes, including sputtering [16] and chemical solution deposition [17]; (2) Even with an accurate composition control (for example, in CVD/ MBE/PLD depositions), sensitivity of the polarization response on chemical composition is rather flat in ferroelectric films, due to contributions from other factors, including effects of interfaces [18], substrate clamping [19] and a misfit strain [20]. (3) The electric polarization itself does not vary in a continuous fashion in a lot of ferroelectric solid solutions. For example, in the PZT material, the majority of the ferroelectric compositions corresponds to a square-shaped or close-to-square shaped P-E hysteresis loop, hence tuning of the properties usually has to go through doping with additional elements. However, from the point of view of industrial applications, doping not only adds up to the process complexity and cost, but may also bring some drawbacks in the film's property, for example, a higher leakage current due to proliferated oxygen vacancies [21].
Ti
Pt
LNO
BTO
PZT
1.2
1.2
–
–
4
2.0 × 10 40 300 0.3 0.3 55 5 15
350–650 0.3 100 25
were kept the same except the sputtering time, which was used to tune their thickness ratio. Prior to the deposition of the PZT/BTO bilayer, a LNO film of approximately 100 nm thick was deposited as a template layer to promote their oriented growth. The base pressure of the sputtering chamber was set at 2.0 × 10−4 Pa, and the substrate temperature remained constant during deposition. The Pt/Ti (∼130 nm thick) bottom electrode had been deposited on SiO2/(100) Si at 300 °C in a pure Ar atmosphere with a sputtering pressure of 0.3Pa [23]. The RTP annealing process started with a 2.5 °C/second ramping from room temperature, made a brief stay at 500 °C (100 s) for pre-annealing, and then ramped up to 700 °C and annealed for 15 min. Lastly, the annealed heterostructure was cooled down to room temperature after turning off the RTP furnace. The whole RTP process was carried out in an ambient atmosphere. The phase structures of the bilayer films and their crystallographic orientations were examined via XRD 2θ scans (Rigaku Dmax-2500 PC XRD diffractometer equipped with a Ni filtered Cu Ka radiation source, Japan). Thicknesses and morphology of the PZT films were measured via scanning electron microscopy (SEM) (SU-70, HITACHI, Hitachi, Japan). Chemical compositions of the bi-layer films were analyzed via Energy-Dispersive X-ray Spectroscopy (EDS, equipped in the SU-70 SEM). The room temperature ferroelectric hysteresis loops (P-E) and leakage currents (J-E) were measured by using a Radiant Precision Premium II ferroelectric tester (Radiant Technology, Albuquerque, NM, USA). The room temperature dielectric properties (dielectric constant εr and loss tgδ) were measured as functions of frequency by using a LCR meter (QuadTech 7600 plus). For the above electrical measurements, circular Au top electrodes (Φ = 200 mm) were sputter-coated in rarefied air at room temperature via a shadow mask. The transverse piezoelectric coefficients (e31,f) of the PZT films were measured from the tip displacements of rectangular prism-shaped thin film cantilevers (∼20 mm × 2.0mm × 0.5 mm), which were diced from the PZT/LNO/Pt/Ti/SiO2/Si and PZT/BTO/LNO/Pt/Ti/SiO2/Si specimens. A Pt top electrode layer of ∼10 nm thick was sputter-deposited to cover the whole surface of the PZT film, except for one end of the cantilever where the clamping was made. Using silver paste, the Pt bottom electrode was directly contacted at the clamping fixture after removing the local PZT layer, while the top Pt electrode was connected to a very fine gold wire to minimize disturbance to the piezoelectric vibration of the cantilever. Such a vibration was induced by the converse transverse piezoelectric effect of the ferroelectric film, as a consequence of an applied sine wave voltage. The e31,f coefficient of a film was determined using the tip displacement of the cantilever [1], which was measured using a laser Doppler vibrometer (OFV-5000 controller along with the OFV-505 sensor head, Polytec Corp., Karlsruhe, Germany). Note that the measuring frequency was set at 500 Hz, a value much smaller than the resonant frequency of the cantilever.
In this work, a simple yet very effective method to engineer the electrical properties of PZT films is proposed. PZT/BTO bi-layer films were deposited on LaNiO3(LNO)/(111)Pt/Ti/SiO2/(100)Si substrates via a rf magnetron sputtering process. Firstly, to ensure the formation of pure perovskites in the films, a systematic study on the deposition temperature window was carried out (350 °C - 650 °C) for both asgrown and RTP annealed films. This study led to the establishment of two low-thermal budget processing routes. One was deposition at 350 °C plus a RTP annealing, the other was an in situ deposition process at 500 °C. The 350 °C as-grown bi-layer film showed a remnant polarization (Pr) of ∼24 μC/cm2 at room temperature, as compared to that of zero in the single layer PZT film. This indicates the existence of a macroscopic ferroelectricity in the bi-layer film. On the other hand, the 500 °C as-grown bi-layer film showed a good Pr value of ∼30 μC/cm2, a piezoelectric e31, f coefficient up to 2.8 C/m2, and a much-reduced dielectric constant and loss tangent. The latter has endowed a better overall piezoelectric energy harvesting performance in the PZT/BTO bilayer than the single layer PZT film. Lastly, ferroelectric characteristics of RTP annealed bi-layer films with different thickness ratios were measured and compared. It was revealed that by controlling the PZT/ BTO thickness ratio, the ferroelectric behavior and hence the associated electrophysical properties of a bi-layer film can be engineered to meet the demand of a specific application. 2. Experimental Pb(Zr0.53, Ti0.47)O3 (PZT)/BaTiO3 (BTO) bi-layer films with a total thickness of ∼1 μm were deposited on LaNiO3(LNO)/(111)Pt/(100)Ti/ SiO2/(100)Si substrates (20 mm × 10 mm × 0.5 mm) in a multi-target rf-sputtering system. To compensate the loss of Pb during the vapor deposition, 20 mol% excessive lead oxide was added to the raw materials for the PZT ceramic target prior to its calcination [22]. On the other hand, the BaTiO3 target has a Ba/Ti ratio of 1 [23]. The deposition parameters of the BTO and PZT layers (summarized in Table 1)
3. Results and discussion Fig. 1 displays XRD 2θ scan patterns of as-grown and RTP annealed PZT/BTO (PZT:BTO = 7:1) bi-layer films deposited on LNO/(111)Pt/ 9033
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interlayer diffusions were very limited in the PZT/BTO/LaNiO3/Pt/Ti/ SiO2/(100)Si heterostructure. Moreover, nearly stoichiometric compositions in the bi-layer ferroelectric film were revealed, i.e., nPb (top layer): nBa (2nd layer): nTi (2nd layer)∼1: 1: 1. Overall, the PZT/BTO bi-layer film showed compositions close to the chemical stoichiometry in each of its component layers. This can be attributed to: (1) a low temperature deposition plus a single rapid thermal annealing step suppressed the interlayer diffusion; (2) the RTP annealing removed excessive PbO in the top layer; (3) BaTiO3 is a non-volatile perovskite, thus the composition of the sputtered BTO film well duplicated that of the stoichiometric target (Ba/Ti = 1:1). The room-temperature electrical properties, including polarization hysteresis loops (P-E), leakage currents (J-E), frequency-dependent dielectric properties (εr-f, tgδ-f) and transverse piezoelectric coefficients (e31,f) of single-layer PZT and PZT/BTO bi-layer films, were comparatively investigated. Typical P-E hysteresis loops of in situ prepared films are shown in Fig. 3(a). The inset is the P-E curve of the PZT single layer film deposited at 350 °C, which shows as a straight line and hence doesn't warrant a macroscopic ferroelectricity. For the PZT/BTO bilayer film deposited at the same temperature (film morphology shown in Fig. 2), however, a Pr of ∼24 μC/cm2 and a Ps of ∼44 μC/cm2 were observed in its P-E curve under an applied maximum field (Emax) of 1200 kV/cm. This observation validated our proposed idea that the in situ growth temperature of a ferroelectric PZT film can be significantly reduced via the addition of an underlying BTO buffer layer. Fig. 3(a) also shows the P-E loops of PZT and PZT/BTO films deposited at 500 °C, both measured at a maximum applied electric field of ∼2250 kV/cm. While the single layer PZT film shows a large Pr (∼29 μC/cm2) and Ec (∼360 kV/cm), the PZT/BTO bi-layer film shows a much smaller Pr (∼11.5 μC/cm2) and Ec (∼135 kV/cm). On the other hand, the two films displayed about the same Ps (∼57 μC/cm2 for the bi-layer film and ∼64 μC/cm2 for the monolayer film). Such different behaviors can not be explained without considering a coupling effect [27] between the bottom BTO layer, which has a slim P-E loop [28], and the top PZT layer, which shows a square-shaped P-E loop (Fig. 3(a)). The smaller Pr of the 500 °C deposited PZT/BTO bi-layer film, as compared to that of its 350 °C deposited counterpart, can be attributed to their different textures. The former showed a preferred orientation of (101), a nonpolar orientation, while the latter was highly (00l)-oriented and hence has a larger Pr. Fig. 3(b) shows the leakage current densities of both 350oC- and 500oC- deposited monolayer PZT and PZT/BTO bi-layer films. As expected, the bi-layer films showed much reduced leakage current as compared with their monolayer counterparts deposited at the same temperature. The improved insulation in the bi-layer films can be attributed to effects of space charges and interlayer charge coupling [29]. Having established the bi-layer process and an understanding on the fusing of properties, now we can switch to demonstrating the effects of bi-layering on tuning the electrical properties. The first case is on the piezoelectric energy harvesting performance of a 500 °C-deposited bilayer film. In Fig. 4(a), the dielectric properties of the PZT/BTO (with a thickness ratio of 7:1) and monolayer PZT films are displayed. The former shows a much lower dielectric constant than the latter in the whole range of measuring frequencies (1 kHz to 1 MHz). Specifically, in the low frequency end (1 kHz), the bi-layer film showed a dielectric constant about one fifth of that in the monolayer PZT film (∼450 vs. ∼2150), together with a lower dielectric loss tangent (∼3.6% vs. ∼8.0%). Interface charge coupling and smaller grains can account for the reduced dielectric constant in the bi-layer film, while its suppressed dielectric loss can be attributed to a denser film morphology. The latter is consistent with its improved resistivity shown in Fig. 3(b). Such a dielectric property is desirable for piezoelectric energy harvesting applications. Fig. 4(b) shows the figure of merit (FOM) and transverse piezoelectric coefficient (e31,f) as functions of an applied electrical voltage for the monolayer PZT and PZT/BTO bi-layer films deposited at 500 °C. The figure of merit (FOM) for the electric power generation of a
Fig. 1. XRD 2θ scan patterns of (a) as-grown and (b) RTP annealed PZT/BTO bilayer films grown on LNO buffered (111)Pt/Ti/SiO2/(100)Si substrates in the temperature range of 350oC-650 °C.
Ti/SiO2/(100)Si substrates in the temperature range of 350oC-650 °C, with an interval of 50 °C. It can be seen from Fig. 1(a) that 500 °C was the only in situ growth temperature to obtain pure perovskite phases (with a (101)-texture), while films grown at all other temperatures showed the existence of crystalline Pb oxide. For films post the RTP annealing, additional growth temperatures leading to the formation of pure perovskites appeared at 350 °C, 400 °C and 550 °C, with 350 °C being the lowest and promoting a (00l) texture. The (00l) texture is of great significance for improving the electrical properties of most perovskite ferroelectric films, including PZT [24] and BTO [25]. Based on these observations, an in situ growth temperature of 500 °C and a low deposition temperature of 350 °C followed by a RTP annealing were determined as the two processing routes for PZT/BTO bi-layer films in this work. Both routes are compatible with the concurrent CMOS-Si technology. Typical surface and cross-sectional morphologies of a PZT/BTO bilayer film are displayed in Fig. 2(a) and (b), respectively. In Fig. 2(a), the top PZT layer shows a densely-packed grain structure with an average size of ∼70 nm, as compared to that of 100–120 nm with larger voids in a single layer PZT film deposited at the same temperature [26]. The smaller size and denser packing of grains in the PZT top layer can be attributed to the buffering effects of the underlying BTO layer on promoting a better nucleation & growth of perovskite PZT. In Fig. 2(b), the cross-sectional SEM image shows clean-cut interfaces in the ferroelectric bi-layer heterostructure, especially those between LNO/BTO and BTO/PZT. In this bi-layer film, the thicknesses of the PZT and BTO layers were measured to be ∼700 nm and ∼100 nm, respectively, corresponding to a 7:1 thickness ratio. Chemical compositions of the bi-layer films were analyzed through surface EDS spectra and cross-sectional EDS line scans, as shown in Fig. 2(c) and (d), respectively. In Fig. 2(c), except Pb, Zr, Ti, O and some trace carbon (C), which might come from surface contamination during sample preparation for EDS, there were no other impurity elements observed. Semi-quantitative analysis yielded a surface atomic ratio of Pb/(Zr + Ti) of 1.01 ± 0.05. This ratio was substantially lower than that of the as-grown PZT film at 350 °C (Pb/(Zr + Ti)∼1.1) [26], due to the removal of excessive PbO in the high temperature annealing process. Fig. 2(d) presents multi-element EDS line scans along the thickness direction of the PZT/BTO bi-layer heterostructure (PZT/BTO thickness ratio ∼1:1). As shown by the elemental line scan profiles, the 9034
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Fig. 2. (a) Surface and (b) cross-sectional SEM images of a PZT/BTO bi-layer film deposited at 500 °C with a PZT/BTO thickness ratio of 7:1; (c) surface EDS spectrum of a 350oC-deposited, RTP annealed PZT/BTO bilayer film with a thickness ratio of 7:1; (d) cross-sectional elemental EDS line scan profiles in a 350oC-deposited, RTP annealed PZT/BTO bilayer film with a thickness ratio ∼ 1:1.
and signal-to-noise ratio [32]. Both parameters increase with a decreasing dielectric loss of the piezoelectric material. Based on the dielectric results shown in Fig. 4(a), the bi-layer film can outperform the monolayer PZT film in piezoelectric energy harvesting if its e31,f coef1 ficient is no less than 5 ∼ 45% of that of the latter. For the piezoelectric measurement, a 16 V bias voltage was firstly used to pole the monolayer or bi-layer thin film cantilever, then the piezoelectrically induced tip displacement was recorded under a sweeping unipolar voltage from 6 V to 20 V and vice versa in a stepwise manner. The e31,f values of the monolayer PZT film were in the range of 3.1–3.9 C/m2 under the applied bias voltage between 6 V and 20 V, while those of the PZT/BTO bi-layer film ranged between 1.0 and 2.8 C/m2 under the same bias voltages. The piezoelectric coefficient correlates with the electric polarization [32]. The bi-layer film showed higher e31,f and FOM values with an increasing bias voltage, which gradually poled the film towards the saturation of its polarization. On the other hand, the monolayer PZT film reached polarization saturation much faster (Fig. 3(a)), resulting in only a slight increase in its e31,f coefficient with the applied voltage. The FOM of the bi-layer film started to exceed that of the monolayer PZT film at a small bias voltage of 8 V, and became more than twice of the latter at a 20 V bias. These observations indicate that a fully poled or sufficiently biased PZT/BTO bi-layer film can outperform a single layer PZT film in piezoelectric energy harvesting applications. The second case study explores how bi-layering with a BTO film can change the polarization response of a PZT film, therefore creating a broader range of applications for this important ferroelectric oxide. Fig. 4(c) displays the P-E loops of PZT/BTO bi-layer films (∼1 μm total thickness) sputter-deposited with different thickness ratios and underwent the same RTP process. With an increasing BTO layer thickness, the hysteresis loop of the bi-layer film shrank and tilted gradually, changing from a square-shaped, “standing-up” P-E loop at the 9:1 thickness ratio to a slim, “lying-down” one at the 1:1 thickness ratio. The remnant polarization Pr underwent a 12-time reduction, decreasing from ∼36 μC/cm2 (PZT:BTO = 9:1) to ∼3 μC/cm2 (PZT:BTO = 1:1). On the
Fig. 3. (a) Polarization-electric field (P-E) loops and (b) leakage current density-electric field (J-E) curves of 350 °C and 500 °C as-grown PZT and PZT/BTO bi-layer films.
piezoelectric film working in its direct mode (mechanical energy→ electric energy) is determined using the equation,
FOM =
2 e31,f
ε0 ε33
(1)
where e31,f is the transverse piezoelectric coefficient, ε0 is the dielectric permittivity of free space, and ε33 is the (relative) dielectric constant [30]. From Eq. (1), a piezoelectric film with a large e31,f and a low ε33 is expected to have a high figure-of-merit for energy harvesting [31]. There are other parameters characterizing the efficiency of the piezoelectrically generated electric power, including the power efficiency 9035
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Fig. 4. (a) Dielectric constants and losses as functions of the measuring frequency, and (b) calculated transverse piezoelectric coefficients (e31,f) and figure-of-merit (FOM) as functions of the applied voltage, for 500 °C as-grown PZT/BTO bi-layer and single layer PZT films; (c) P-E loops of RTP annealed PZT/BTO bi-layer films deposited at 350 °C with different thickness ratios, and (d) P-E loops of the PZT/BTO bi-layer film with a 1:1 thickness ratio and the single layer PZT film processed under the same conditions (350 °C sputter + RTP). The green shaded areas represent the recyclable capacitive energies (per unit volume) during one charge-discharge cycle from the ferroelectric thin film capacitors. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
other hand, the saturated polarization only changed by ∼250%, reducing from ∼110 μC/cm2 (PZT:BTO = 9:1) to ∼44 μC/cm2 (PZT:BTO = 1:1). The stepwise tilting and shape-changing of the P-E loop with the thickness ratio indicate that the electrical property of the bi-layer film can be semi-continuously tuned to meet the demand of a given application. An example is given in Fig. 4(d) to validate this point. Fig. 4(d) display P-E loops of a PZT/BTO bi-layer film with a 1:1 thickness ratio and a PZT single layer film with the same thickness (∼1 μm). The P-E loop of the PZT film is rectangular-shaped with a large Pr (∼40 μC/cm2), while that of the PZT/BTO bilayer film is a slim, tilted one with a very small Pr (∼3 μC/cm2). The former saturated at a relatively small electric field, ∼900 kV/cm, while the latter showed a much higher saturating electric field exceeding 2000 kV/cm (corresponding to 200 V, the maximum voltage we could apply using our ferroelectric tester). The energy storage performances of the two films can be evaluated by integrating the E-P curves, which yielded a recyclable capacitive energy density of ∼29.9 J/cm3 and 10.7 J/cm3 for the PZT/BTO and PZT films, respectively. [33] The computed chargedischarge energy efficiencies were ∼79% and 38% for the bi-layer and monolayer PZT films, respectively. Compared with a single-layer PZT film, a PZT/BTO bi-layer film is a much better choice for applications in high energy density, high efficiency dielectric capacitors.
Acknowledgement The authors thank the National Key Research and Development Program of China (Grant #2017YFF0105903), the National Natural Science Foundation of China (NSFC) (Grant nos. 51772175, 51775319), the Nano Projects of Suzhou City (Grant no. ZXG201445), the Science and Technology Projects of Suzhou City (Grant # SYG201718), and the Program for New Century Excellent Talents in University (State Education Ministry), for providing financial support of this work. The support from the Independent Innovation Foundation of Shandong University (Grant nos. 2018JC045, 2017ZD008 and 2015JC034) is also acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.01.237. References [1] I. Kanno, H. Kotera, K. Wasa, Measurement dr of transverse piezoelectric properties of PZT thin films, Sens. Actuators, A 107 (2003) 68–74. [2] K. Kim, Y.J. Song, Integration technology for ferroelectric memory devices, Microelectron. Reliab. 43 (2003) 385–398. [3] A.I. Kingon, S. Srinivasan, Lead zirconate titanate thin films directly on copper electrodes for ferroelectric, dielectric and piezoelectric applications, Nat. Mater. 4 (3) (2005) 233–237. [4] J.F. Scott, Applications of modern ferroelectrics, Science 315 (2007) 954–959. [5] J.F. Scott, Carlos A. Paz De Araujo, Ferroelectric memories, Science 246 (1989) 1400–1405. [6] H. Kueppers, T. Leuerer, U. Schnakenberg, W. Mokwa, M. Hoffmann, T. Schneller, U. Boettger, R. Waser, PZT thin films for piezoelectric micro-actuator applications, Sens. Actuators, A 97–98 (2002) 680–684. [7] I. Kanno, S. Hayashi, T. Kamada, M. Kitagawa, T. Hirao, Low-temperature preparation of Pb(Zr, Ti)O3 thin films on (Pb, La)TiO3 buffer layer by multi-ion-beam sputtering, Jpn. J. Appl. 32 (1993) 4057–4060. [8] A.H. Carim, B.A. Tuttle, D.H. Doughty, S.L. Martinez, Microstructure of solutionprocessed lead zirconate titanate (PZT) thin films, J. Am. Ceram. Soc. 74 (6) (1991) 1455–1458. [9] D.S. Yoon, J.S. Roh, S.M. Lee, H.K. Baik, Alteration for a diffusion barrier design concept in future high-density dynamic and ferroelectric random access memory devices, Prog. Mater. Sci. 48 (4) (2003) 275–371. [10] Z.J. Wang, Z.P. Cao, Y. Otsuka, N. Yoshikawa, H. Kokawa, S. Taniguchi, Lowtemperature growth of ferroelectric lead zirconate titanate thin films using the magnetic field of low power 2.45 GHz microwave irradiation, Appl. Phys. Lett. 92 (2008) 222905 (1–3). [11] H. Tabata, O. Murata, T. Kawai, S. Kawai, M. Okuyama, Preparation of PbTiO3 thin films by anexcimer laser ablation technique with second laseri irradiation, Jpn. J. Appl. Phys. 31 (1992) 2968–2970.
4. Conclusions In this work, PZT/BTO bilayer films were sputter-deposited on Si substrates. The bi-layer films showed a macroscopic ferroelectricity at a deposition temperature as low as 350 °C, owing to the buffering effect of the BTO underlayer. At the deposition temperature of 500 °C, the PZT/BTO bi-layer film showed a pure perovskite phase structure with good electrical properties, including a large saturated polarization (Ps∼57 μC/cm2) and a much-reduced leakage current. Broadly tunable dielectric and ferroelectric responses were achieved in the bi-layer films through adjusting its PZT/BTO thickness ratio, enabling a wider spectrum of applications than those of the single-layer films. For example, high figure-of-merit piezoelectric energy harvesters and high energy density, high efficiency dielectric capacitors, with performances exceeding those of the same devices made from single-layer PZT films, were demonstrated for the PZT/BTO bi-layer films. It can be concluded that bi-layering is a simple and effective method for tuning of electrical properties in perovskite ferroelectric films. 9036
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Y. Wang et al. [12] G. Velu, D. Remiens, B. Thierry, Ferroelectric properties of PZT thin films prepared by sputtering with stoichiometric single oxide target: comparison between conventional and rapid thermal annealing, J. Eur. Ceram. Soc. 17 (1997) 1749–1755. [13] I. Kanno, H. Kotera, K. Wasa, D. Remiens, B. Thierry, Crystallographic characterization of epitaxial Pb(Zr,Ti)O3 films with different Zr/Ti ratio grown by radiofrequency-magnetron sputtering, J. Appl. Phys. 93 (7) (2003) 4091–4096. [14] Z.J. Wang, K. Kikuchi, R. Maeda, Effect of Pb content in target on electrical properties of laser ablation derived lead zirconate titanate thin films, Jpn. J. Appl. Phys. 39 (9S) (2000) 5413–5417. [15] Mark Brazier, M. McElfresh, Said Mansour, Unconventional hysteresis behavior in compositionally graded Pb(Zr,Ti)O3 thin films, Appl. Phys. Lett. 72 (9) (1998) 1121–1123. [16] H. Zhu, X. Sun, L. Kang, M. Hong, M. Liu, Z. Yu, J. Ouyang, Charge transport behaviors in epitaxial BiFeO3 thick films sputtered with different Ar/O2 flow ratios, Scripta Mater. 115 (2016) 62–65. [17] J.F. Scott, C.A. Araujo, B.M. Melnick, L.D. McMillan, R. Zuleeg, Quantitative measurement of space-charge effects in lead zirconate-titanate memories, J. Appl. Phys. 70 (1) (1991) 382–388. [18] Y. Masuda, T. Nozaka, Investigation into electrical conduction mechanisms of Pb (Zr,Ti)O3 thin-film capacitors with Pt, IrO2 and SrRuO3 top electrodes, Jpn. J. Appl. Phys. 43 (9S) (2004) 6576–6580. [19] J. Ouyang, R. Ramesh, A.L. Roytburd, Theoretical predictions for the intrinsic converse longitudinal piezoelectric constants of lead zirconate titanate epitaxial films, Adv. Eng. Mater. 7 (4) (2005) 229–232. [20] I. Maruyama, M. Saitoh, I. Sakai, T. Hidaka, Growth and characterization of 10-nmthick c-axis oriented epitaxial PbZr0.25Ti0.75O3PbZr0.25 Ti0.75O3 thin films on (100) Si substrate, Appl. Phys. Lett. 73 (24) (1998) 3524–3526. [21] Xiaoding Qi, Joonghoe Dho, Rumen Tomov, Mark G. Blamire, Judith L. MacManusDriscoll, Greatly reduced leakage current and conduction mechanism in aliovalention-doped BiFeO3, Appl. Phys. Lett. 86 (2005) 062903 (1–3). [22] C.C. Hsueh, M.L. Mecartney, Microstructural development and electrical properties of sol-gel prepared lead zirconate-titanate thin films, J. Mater. Sci. 6 (10) (1991) 2208–2217.
[23] Y. Gao, M. Yuan, X. Sun, J. Ouyang, In situ preparation of high quality BaTiO3 dielectric films on Si at 350–500 oC, J. Mater. Sci. Mater. Electron. 28 (1) (2017) 337–343. [24] J. Ouyang, S.-Y. Yang, L. Chen, R. Ramesh, A.L. Roytburd, Orientation dependence of the converse piezoelectric constants for epitaxial single domain ferroelectric films, Appl. Phys. Lett. 85 (2) (2004) 278–280. [25] J. Ouyang, R. Ramesh, A.L. Roytburd, Theoretical investigation of the intrinsic piezoelectric properties for tetragonal BaTiO3 epitaxial films, Appl. Surf. Sci. 252 (10) (2006) 3394–3400. [26] Y.Y. Wang, H. Cheng, J. Yan, N. Chen, P. Yan, F. Yang, J. Ouyang, Large piezoelectricity on Si from highly (001)-oriented PZT thick films via a CMOS- compatible sputtering/RTP process, Materialia (2019) 100228. [27] M. Liu, H. Zhu, Y. Zhang, C. Xue, J. Ouyang, Energy storage characteristics of BiFeO3/BaTiO3 Bi-layers integrated on Si, Materials 9 (2016) 935 (1–9). [28] W. Zhang, F. Hu, H. Zhang, J. Ouyang, Investigation of the electrical properties of RF sputtered BaTiO3 films grown on various substrates, Mater. Res. Bull. 95 (2017) 23–29. [29] A.K. Tagantsev, I.A. Stolichnov, Injection-controlled size effect on switching of ferroelectric thin films, Appl. Phys. Lett. 74 (9) (1999) 1326–1328. [30] X. Yan, W. Ren, Preparation and dielectric properties of Ba0.5Sr0.5Ti3/ Bi1.5Zn1.0Nb1.5O7 multilayer composite films, Electronic Compon. Mater. 30 (2011) 21–24. [31] H.A. Sodano, D.J. Inman, G. Park, Comparison of piezoelectric energy harvesting devices for recharging batteries, J. Intell. Mater. Syst. Struct. 16 (10) (2005) 799–807. [32] N. Ledermann, P. Muralt, J. Baborowski, S. Gentil, K. Mukati, M. Cantoni, A. Seifert, N. Setter, {100}-Textured, piezoelectric Pb(Zrx,Ti1-x)O3 thin films for MEMS: integration, deposition and properties, Sensor Actuator A Phys. 105 (2) (2003) 162–170. [33] H. Cheng, J. Ouyang, Y.-X. Zhang, D. Ascienzo, Y. Li, Y.-Y. Zhao, Y. Ren, Demonstration of ultra-high recyclable energy densities in domain-engineered ferroelectric films, Nat. Commun. 8 (2017) 1–6 1999.
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Lead zirconate titanate and barium titanate bi-layer ferroelectric films on Si a,b,c
Yingying Wang Jun Ouyanga,c,∗
, Jing Yan
a,1
, Hongbo Cheng
a,d,1
b
b
T
e
, Ning Chen , Peng Yan , Feng Yang ,
a Key Laboratory for Liquid-Solid Structure Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan, 250061, China b School of Mechanical Engineering, Shandong University, Jinan, 250061, China c Suzhou Institute of Shandong University, Suzhou, 215123, China d College of Electronic and Optical Engineering, Nanjing University of Posts and Tele-communications, Nanjing, 210023, China e School of Materials Science and Engineering, University of Jinan, Jinan, 250022, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Lead zirconate titanate (PZT) Barium titanate (BaTiO3) Bi-layer Ferroelectric film Si Magnetron sputtering
In this work, a novel method is proposed to integrate ferroelectric lead zirconate titanate (PZT) films on Si with highly tunable functionalities, through bi-layering with a barium titanate (BTO) film. First of all, the BTO film acts as a growth-promotion template layer which has successfully lowered the in situ deposition temperature of a ferroelectric PZT film. The PZT/BTO bilayer film deposited at 350 °C on LaNiO3-buffered Si substrate displayed a room temperature remnant polarization ∼24 μC/cm2, and its quality can be further improved via a rapid thermal annealing (RTP) process. Furthermore, by changing their thickness ratio, various ferroelectric hysteresis loops (P-E loops) can be created in the PZT/BTO films, thereby enabling a broad range of applications for this simple bi-layer structure. Examples including high performance piezoelectric energy harvesters and high energy density dielectric capacitors are demonstrated for the PZT/BTO bi-layer films.
1. Introduction Since its discovery in the late 1950s, lead zirconate titanate [Pb (Zr,Ti)O3 or PZT] has amazed generations of researchers with its excellent electrical properties [1]. With the rapid development in Si-based microelectronics, PZT films are required to be integrated into the industrial-standard complementary metal-oxide semiconductor (CMOS) Si technology to achieve their functionalities in a miniaturized scale [2]. These films have been widely used in dielectric capacitors [3], ferroelectric memories [4,5], and piezoelectric microelectromechanical systems (piezo-MEMS) [3,6]. It usually requires a high processing temperature (> 500 °C) to deposit or anneal PZT films to create a high-quality ferroelectric phase [7]. However, there are some critical issues associated with a long-time exposure to the high temperature, including formation of the pyrochlore phase [8], loss of Pb [9], and inter-diffusion of elements across pre-defined interfaces. The latter has become a major issue during recent efforts in chasing the Moore's law in the microelectronics industry, resulting in a stringent requirement to reduce the thermal budget for semiconductor processing, i.e., a lower deposition/processing
temperature has been demanded for any functional layer being integrated into CMOS-Si. These issues have led to the development of a number of rapid annealing techniques, including microwave [10] and laser irradiations [11], and rapid thermal processing (RTP) [12]. For example, Wang et al. obtained perovskite PZT films at 450 °C with the aid of a magnetic field from a low power microwave irradiation, [10]. Using a second laser irradiation in their laser ablation depositions, Tabata et al. claimed that they had successfully lowered the preparation temperature of PbTiO3 films down to 350 °C. However, films showing a typical ferroelectric P-E hysteresis loop in their work were prepared at a higher temperature of 450 °C [11]. Velo et al. prepared amorphous PZT films at room temperature via a rf magnetron sputtering process. When annealed via RTP at 625 °C for 30s, ferroelectric PZT films with saturated polarization (Ps) ∼ 38 μC/cm2, coercive field (Ec) ∼ 53 kV/cm and remnant polarization (Pr) ∼ 20 μC/cm2 were obtained at an applied maximum voltage of 20 V [12]. Another issue associated with applications of the PZT film, or any other ferroelectric single layer film, is a weak tunability of its functionality, derived from its characteristic P-E hysteresis loop. Due to the composition-structure-property interrelationship, a single-layer PZT
∗
Corresponding author. Key Laboratory for Liquid-Solid Structure Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan, 250061, China. E-mail address: [email protected] (J. Ouyang). 1 These two authors contribute equally to this work. https://doi.org/10.1016/j.ceramint.2019.01.237 Received 24 November 2018; Received in revised form 27 January 2019; Accepted 28 January 2019 Available online 31 January 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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film with a fixed composition can only be used in certain applications. Tuning of the functionality of a PZT film is usually achieved via using different targets for film deposition [13,14], or in a graded multilayer film prepared with a special target containing different compositions [15]. The former is not a good cost-effective approach to address industrial demands. In the meanwhile, due to effect of a strong built-in potential, the latter usually displays a shifted ferroelectric hysteresis loop instead of a field-tunable polarization response [15]. Furthermore, even with a number of targets having different compositions, a continuous/semi-continuous tuning of the electrical property in a broad range is often difficult. Possible reasons include the following:
Table 1 Deposition parameters of the PZT/BTO/LNO/Pt/Ti heterostructures on SiO2/ (100)Si substrates during a multi-target rf magnetron sputtering process. Note that the sputtering times of BTO and PZT were determined by their thickness ratio. Parameters Base pressure (Pa) Target-substrate distance (mm) Substrate temperature (oC) Sputtering pressure (Pa) Sputtering power (W) Deposition time (min)
(1) An accurate composition control (down to 1%, for example) throughout the film thickness is rather difficult in some commonly used deposition processes, including sputtering [16] and chemical solution deposition [17]; (2) Even with an accurate composition control (for example, in CVD/ MBE/PLD depositions), sensitivity of the polarization response on chemical composition is rather flat in ferroelectric films, due to contributions from other factors, including effects of interfaces [18], substrate clamping [19] and a misfit strain [20]. (3) The electric polarization itself does not vary in a continuous fashion in a lot of ferroelectric solid solutions. For example, in the PZT material, the majority of the ferroelectric compositions corresponds to a square-shaped or close-to-square shaped P-E hysteresis loop, hence tuning of the properties usually has to go through doping with additional elements. However, from the point of view of industrial applications, doping not only adds up to the process complexity and cost, but may also bring some drawbacks in the film's property, for example, a higher leakage current due to proliferated oxygen vacancies [21].
Ti
Pt
LNO
BTO
PZT
1.2
1.2
–
–
4
2.0 × 10 40 300 0.3 0.3 55 5 15
350–650 0.3 100 25
were kept the same except the sputtering time, which was used to tune their thickness ratio. Prior to the deposition of the PZT/BTO bilayer, a LNO film of approximately 100 nm thick was deposited as a template layer to promote their oriented growth. The base pressure of the sputtering chamber was set at 2.0 × 10−4 Pa, and the substrate temperature remained constant during deposition. The Pt/Ti (∼130 nm thick) bottom electrode had been deposited on SiO2/(100) Si at 300 °C in a pure Ar atmosphere with a sputtering pressure of 0.3Pa [23]. The RTP annealing process started with a 2.5 °C/second ramping from room temperature, made a brief stay at 500 °C (100 s) for pre-annealing, and then ramped up to 700 °C and annealed for 15 min. Lastly, the annealed heterostructure was cooled down to room temperature after turning off the RTP furnace. The whole RTP process was carried out in an ambient atmosphere. The phase structures of the bilayer films and their crystallographic orientations were examined via XRD 2θ scans (Rigaku Dmax-2500 PC XRD diffractometer equipped with a Ni filtered Cu Ka radiation source, Japan). Thicknesses and morphology of the PZT films were measured via scanning electron microscopy (SEM) (SU-70, HITACHI, Hitachi, Japan). Chemical compositions of the bi-layer films were analyzed via Energy-Dispersive X-ray Spectroscopy (EDS, equipped in the SU-70 SEM). The room temperature ferroelectric hysteresis loops (P-E) and leakage currents (J-E) were measured by using a Radiant Precision Premium II ferroelectric tester (Radiant Technology, Albuquerque, NM, USA). The room temperature dielectric properties (dielectric constant εr and loss tgδ) were measured as functions of frequency by using a LCR meter (QuadTech 7600 plus). For the above electrical measurements, circular Au top electrodes (Φ = 200 mm) were sputter-coated in rarefied air at room temperature via a shadow mask. The transverse piezoelectric coefficients (e31,f) of the PZT films were measured from the tip displacements of rectangular prism-shaped thin film cantilevers (∼20 mm × 2.0mm × 0.5 mm), which were diced from the PZT/LNO/Pt/Ti/SiO2/Si and PZT/BTO/LNO/Pt/Ti/SiO2/Si specimens. A Pt top electrode layer of ∼10 nm thick was sputter-deposited to cover the whole surface of the PZT film, except for one end of the cantilever where the clamping was made. Using silver paste, the Pt bottom electrode was directly contacted at the clamping fixture after removing the local PZT layer, while the top Pt electrode was connected to a very fine gold wire to minimize disturbance to the piezoelectric vibration of the cantilever. Such a vibration was induced by the converse transverse piezoelectric effect of the ferroelectric film, as a consequence of an applied sine wave voltage. The e31,f coefficient of a film was determined using the tip displacement of the cantilever [1], which was measured using a laser Doppler vibrometer (OFV-5000 controller along with the OFV-505 sensor head, Polytec Corp., Karlsruhe, Germany). Note that the measuring frequency was set at 500 Hz, a value much smaller than the resonant frequency of the cantilever.
In this work, a simple yet very effective method to engineer the electrical properties of PZT films is proposed. PZT/BTO bi-layer films were deposited on LaNiO3(LNO)/(111)Pt/Ti/SiO2/(100)Si substrates via a rf magnetron sputtering process. Firstly, to ensure the formation of pure perovskites in the films, a systematic study on the deposition temperature window was carried out (350 °C - 650 °C) for both asgrown and RTP annealed films. This study led to the establishment of two low-thermal budget processing routes. One was deposition at 350 °C plus a RTP annealing, the other was an in situ deposition process at 500 °C. The 350 °C as-grown bi-layer film showed a remnant polarization (Pr) of ∼24 μC/cm2 at room temperature, as compared to that of zero in the single layer PZT film. This indicates the existence of a macroscopic ferroelectricity in the bi-layer film. On the other hand, the 500 °C as-grown bi-layer film showed a good Pr value of ∼30 μC/cm2, a piezoelectric e31, f coefficient up to 2.8 C/m2, and a much-reduced dielectric constant and loss tangent. The latter has endowed a better overall piezoelectric energy harvesting performance in the PZT/BTO bilayer than the single layer PZT film. Lastly, ferroelectric characteristics of RTP annealed bi-layer films with different thickness ratios were measured and compared. It was revealed that by controlling the PZT/ BTO thickness ratio, the ferroelectric behavior and hence the associated electrophysical properties of a bi-layer film can be engineered to meet the demand of a specific application. 2. Experimental Pb(Zr0.53, Ti0.47)O3 (PZT)/BaTiO3 (BTO) bi-layer films with a total thickness of ∼1 μm were deposited on LaNiO3(LNO)/(111)Pt/(100)Ti/ SiO2/(100)Si substrates (20 mm × 10 mm × 0.5 mm) in a multi-target rf-sputtering system. To compensate the loss of Pb during the vapor deposition, 20 mol% excessive lead oxide was added to the raw materials for the PZT ceramic target prior to its calcination [22]. On the other hand, the BaTiO3 target has a Ba/Ti ratio of 1 [23]. The deposition parameters of the BTO and PZT layers (summarized in Table 1)
3. Results and discussion Fig. 1 displays XRD 2θ scan patterns of as-grown and RTP annealed PZT/BTO (PZT:BTO = 7:1) bi-layer films deposited on LNO/(111)Pt/ 9033
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interlayer diffusions were very limited in the PZT/BTO/LaNiO3/Pt/Ti/ SiO2/(100)Si heterostructure. Moreover, nearly stoichiometric compositions in the bi-layer ferroelectric film were revealed, i.e., nPb (top layer): nBa (2nd layer): nTi (2nd layer)∼1: 1: 1. Overall, the PZT/BTO bi-layer film showed compositions close to the chemical stoichiometry in each of its component layers. This can be attributed to: (1) a low temperature deposition plus a single rapid thermal annealing step suppressed the interlayer diffusion; (2) the RTP annealing removed excessive PbO in the top layer; (3) BaTiO3 is a non-volatile perovskite, thus the composition of the sputtered BTO film well duplicated that of the stoichiometric target (Ba/Ti = 1:1). The room-temperature electrical properties, including polarization hysteresis loops (P-E), leakage currents (J-E), frequency-dependent dielectric properties (εr-f, tgδ-f) and transverse piezoelectric coefficients (e31,f) of single-layer PZT and PZT/BTO bi-layer films, were comparatively investigated. Typical P-E hysteresis loops of in situ prepared films are shown in Fig. 3(a). The inset is the P-E curve of the PZT single layer film deposited at 350 °C, which shows as a straight line and hence doesn't warrant a macroscopic ferroelectricity. For the PZT/BTO bilayer film deposited at the same temperature (film morphology shown in Fig. 2), however, a Pr of ∼24 μC/cm2 and a Ps of ∼44 μC/cm2 were observed in its P-E curve under an applied maximum field (Emax) of 1200 kV/cm. This observation validated our proposed idea that the in situ growth temperature of a ferroelectric PZT film can be significantly reduced via the addition of an underlying BTO buffer layer. Fig. 3(a) also shows the P-E loops of PZT and PZT/BTO films deposited at 500 °C, both measured at a maximum applied electric field of ∼2250 kV/cm. While the single layer PZT film shows a large Pr (∼29 μC/cm2) and Ec (∼360 kV/cm), the PZT/BTO bi-layer film shows a much smaller Pr (∼11.5 μC/cm2) and Ec (∼135 kV/cm). On the other hand, the two films displayed about the same Ps (∼57 μC/cm2 for the bi-layer film and ∼64 μC/cm2 for the monolayer film). Such different behaviors can not be explained without considering a coupling effect [27] between the bottom BTO layer, which has a slim P-E loop [28], and the top PZT layer, which shows a square-shaped P-E loop (Fig. 3(a)). The smaller Pr of the 500 °C deposited PZT/BTO bi-layer film, as compared to that of its 350 °C deposited counterpart, can be attributed to their different textures. The former showed a preferred orientation of (101), a nonpolar orientation, while the latter was highly (00l)-oriented and hence has a larger Pr. Fig. 3(b) shows the leakage current densities of both 350oC- and 500oC- deposited monolayer PZT and PZT/BTO bi-layer films. As expected, the bi-layer films showed much reduced leakage current as compared with their monolayer counterparts deposited at the same temperature. The improved insulation in the bi-layer films can be attributed to effects of space charges and interlayer charge coupling [29]. Having established the bi-layer process and an understanding on the fusing of properties, now we can switch to demonstrating the effects of bi-layering on tuning the electrical properties. The first case is on the piezoelectric energy harvesting performance of a 500 °C-deposited bilayer film. In Fig. 4(a), the dielectric properties of the PZT/BTO (with a thickness ratio of 7:1) and monolayer PZT films are displayed. The former shows a much lower dielectric constant than the latter in the whole range of measuring frequencies (1 kHz to 1 MHz). Specifically, in the low frequency end (1 kHz), the bi-layer film showed a dielectric constant about one fifth of that in the monolayer PZT film (∼450 vs. ∼2150), together with a lower dielectric loss tangent (∼3.6% vs. ∼8.0%). Interface charge coupling and smaller grains can account for the reduced dielectric constant in the bi-layer film, while its suppressed dielectric loss can be attributed to a denser film morphology. The latter is consistent with its improved resistivity shown in Fig. 3(b). Such a dielectric property is desirable for piezoelectric energy harvesting applications. Fig. 4(b) shows the figure of merit (FOM) and transverse piezoelectric coefficient (e31,f) as functions of an applied electrical voltage for the monolayer PZT and PZT/BTO bi-layer films deposited at 500 °C. The figure of merit (FOM) for the electric power generation of a
Fig. 1. XRD 2θ scan patterns of (a) as-grown and (b) RTP annealed PZT/BTO bilayer films grown on LNO buffered (111)Pt/Ti/SiO2/(100)Si substrates in the temperature range of 350oC-650 °C.
Ti/SiO2/(100)Si substrates in the temperature range of 350oC-650 °C, with an interval of 50 °C. It can be seen from Fig. 1(a) that 500 °C was the only in situ growth temperature to obtain pure perovskite phases (with a (101)-texture), while films grown at all other temperatures showed the existence of crystalline Pb oxide. For films post the RTP annealing, additional growth temperatures leading to the formation of pure perovskites appeared at 350 °C, 400 °C and 550 °C, with 350 °C being the lowest and promoting a (00l) texture. The (00l) texture is of great significance for improving the electrical properties of most perovskite ferroelectric films, including PZT [24] and BTO [25]. Based on these observations, an in situ growth temperature of 500 °C and a low deposition temperature of 350 °C followed by a RTP annealing were determined as the two processing routes for PZT/BTO bi-layer films in this work. Both routes are compatible with the concurrent CMOS-Si technology. Typical surface and cross-sectional morphologies of a PZT/BTO bilayer film are displayed in Fig. 2(a) and (b), respectively. In Fig. 2(a), the top PZT layer shows a densely-packed grain structure with an average size of ∼70 nm, as compared to that of 100–120 nm with larger voids in a single layer PZT film deposited at the same temperature [26]. The smaller size and denser packing of grains in the PZT top layer can be attributed to the buffering effects of the underlying BTO layer on promoting a better nucleation & growth of perovskite PZT. In Fig. 2(b), the cross-sectional SEM image shows clean-cut interfaces in the ferroelectric bi-layer heterostructure, especially those between LNO/BTO and BTO/PZT. In this bi-layer film, the thicknesses of the PZT and BTO layers were measured to be ∼700 nm and ∼100 nm, respectively, corresponding to a 7:1 thickness ratio. Chemical compositions of the bi-layer films were analyzed through surface EDS spectra and cross-sectional EDS line scans, as shown in Fig. 2(c) and (d), respectively. In Fig. 2(c), except Pb, Zr, Ti, O and some trace carbon (C), which might come from surface contamination during sample preparation for EDS, there were no other impurity elements observed. Semi-quantitative analysis yielded a surface atomic ratio of Pb/(Zr + Ti) of 1.01 ± 0.05. This ratio was substantially lower than that of the as-grown PZT film at 350 °C (Pb/(Zr + Ti)∼1.1) [26], due to the removal of excessive PbO in the high temperature annealing process. Fig. 2(d) presents multi-element EDS line scans along the thickness direction of the PZT/BTO bi-layer heterostructure (PZT/BTO thickness ratio ∼1:1). As shown by the elemental line scan profiles, the 9034
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Fig. 2. (a) Surface and (b) cross-sectional SEM images of a PZT/BTO bi-layer film deposited at 500 °C with a PZT/BTO thickness ratio of 7:1; (c) surface EDS spectrum of a 350oC-deposited, RTP annealed PZT/BTO bilayer film with a thickness ratio of 7:1; (d) cross-sectional elemental EDS line scan profiles in a 350oC-deposited, RTP annealed PZT/BTO bilayer film with a thickness ratio ∼ 1:1.
and signal-to-noise ratio [32]. Both parameters increase with a decreasing dielectric loss of the piezoelectric material. Based on the dielectric results shown in Fig. 4(a), the bi-layer film can outperform the monolayer PZT film in piezoelectric energy harvesting if its e31,f coef1 ficient is no less than 5 ∼ 45% of that of the latter. For the piezoelectric measurement, a 16 V bias voltage was firstly used to pole the monolayer or bi-layer thin film cantilever, then the piezoelectrically induced tip displacement was recorded under a sweeping unipolar voltage from 6 V to 20 V and vice versa in a stepwise manner. The e31,f values of the monolayer PZT film were in the range of 3.1–3.9 C/m2 under the applied bias voltage between 6 V and 20 V, while those of the PZT/BTO bi-layer film ranged between 1.0 and 2.8 C/m2 under the same bias voltages. The piezoelectric coefficient correlates with the electric polarization [32]. The bi-layer film showed higher e31,f and FOM values with an increasing bias voltage, which gradually poled the film towards the saturation of its polarization. On the other hand, the monolayer PZT film reached polarization saturation much faster (Fig. 3(a)), resulting in only a slight increase in its e31,f coefficient with the applied voltage. The FOM of the bi-layer film started to exceed that of the monolayer PZT film at a small bias voltage of 8 V, and became more than twice of the latter at a 20 V bias. These observations indicate that a fully poled or sufficiently biased PZT/BTO bi-layer film can outperform a single layer PZT film in piezoelectric energy harvesting applications. The second case study explores how bi-layering with a BTO film can change the polarization response of a PZT film, therefore creating a broader range of applications for this important ferroelectric oxide. Fig. 4(c) displays the P-E loops of PZT/BTO bi-layer films (∼1 μm total thickness) sputter-deposited with different thickness ratios and underwent the same RTP process. With an increasing BTO layer thickness, the hysteresis loop of the bi-layer film shrank and tilted gradually, changing from a square-shaped, “standing-up” P-E loop at the 9:1 thickness ratio to a slim, “lying-down” one at the 1:1 thickness ratio. The remnant polarization Pr underwent a 12-time reduction, decreasing from ∼36 μC/cm2 (PZT:BTO = 9:1) to ∼3 μC/cm2 (PZT:BTO = 1:1). On the
Fig. 3. (a) Polarization-electric field (P-E) loops and (b) leakage current density-electric field (J-E) curves of 350 °C and 500 °C as-grown PZT and PZT/BTO bi-layer films.
piezoelectric film working in its direct mode (mechanical energy→ electric energy) is determined using the equation,
FOM =
2 e31,f
ε0 ε33
(1)
where e31,f is the transverse piezoelectric coefficient, ε0 is the dielectric permittivity of free space, and ε33 is the (relative) dielectric constant [30]. From Eq. (1), a piezoelectric film with a large e31,f and a low ε33 is expected to have a high figure-of-merit for energy harvesting [31]. There are other parameters characterizing the efficiency of the piezoelectrically generated electric power, including the power efficiency 9035
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Fig. 4. (a) Dielectric constants and losses as functions of the measuring frequency, and (b) calculated transverse piezoelectric coefficients (e31,f) and figure-of-merit (FOM) as functions of the applied voltage, for 500 °C as-grown PZT/BTO bi-layer and single layer PZT films; (c) P-E loops of RTP annealed PZT/BTO bi-layer films deposited at 350 °C with different thickness ratios, and (d) P-E loops of the PZT/BTO bi-layer film with a 1:1 thickness ratio and the single layer PZT film processed under the same conditions (350 °C sputter + RTP). The green shaded areas represent the recyclable capacitive energies (per unit volume) during one charge-discharge cycle from the ferroelectric thin film capacitors. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
other hand, the saturated polarization only changed by ∼250%, reducing from ∼110 μC/cm2 (PZT:BTO = 9:1) to ∼44 μC/cm2 (PZT:BTO = 1:1). The stepwise tilting and shape-changing of the P-E loop with the thickness ratio indicate that the electrical property of the bi-layer film can be semi-continuously tuned to meet the demand of a given application. An example is given in Fig. 4(d) to validate this point. Fig. 4(d) display P-E loops of a PZT/BTO bi-layer film with a 1:1 thickness ratio and a PZT single layer film with the same thickness (∼1 μm). The P-E loop of the PZT film is rectangular-shaped with a large Pr (∼40 μC/cm2), while that of the PZT/BTO bilayer film is a slim, tilted one with a very small Pr (∼3 μC/cm2). The former saturated at a relatively small electric field, ∼900 kV/cm, while the latter showed a much higher saturating electric field exceeding 2000 kV/cm (corresponding to 200 V, the maximum voltage we could apply using our ferroelectric tester). The energy storage performances of the two films can be evaluated by integrating the E-P curves, which yielded a recyclable capacitive energy density of ∼29.9 J/cm3 and 10.7 J/cm3 for the PZT/BTO and PZT films, respectively. [33] The computed chargedischarge energy efficiencies were ∼79% and 38% for the bi-layer and monolayer PZT films, respectively. Compared with a single-layer PZT film, a PZT/BTO bi-layer film is a much better choice for applications in high energy density, high efficiency dielectric capacitors.
Acknowledgement The authors thank the National Key Research and Development Program of China (Grant #2017YFF0105903), the National Natural Science Foundation of China (NSFC) (Grant nos. 51772175, 51775319), the Nano Projects of Suzhou City (Grant no. ZXG201445), the Science and Technology Projects of Suzhou City (Grant # SYG201718), and the Program for New Century Excellent Talents in University (State Education Ministry), for providing financial support of this work. The support from the Independent Innovation Foundation of Shandong University (Grant nos. 2018JC045, 2017ZD008 and 2015JC034) is also acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.01.237. References [1] I. Kanno, H. Kotera, K. Wasa, Measurement dr of transverse piezoelectric properties of PZT thin films, Sens. Actuators, A 107 (2003) 68–74. [2] K. Kim, Y.J. Song, Integration technology for ferroelectric memory devices, Microelectron. Reliab. 43 (2003) 385–398. [3] A.I. Kingon, S. Srinivasan, Lead zirconate titanate thin films directly on copper electrodes for ferroelectric, dielectric and piezoelectric applications, Nat. Mater. 4 (3) (2005) 233–237. [4] J.F. Scott, Applications of modern ferroelectrics, Science 315 (2007) 954–959. [5] J.F. Scott, Carlos A. Paz De Araujo, Ferroelectric memories, Science 246 (1989) 1400–1405. [6] H. Kueppers, T. Leuerer, U. Schnakenberg, W. Mokwa, M. Hoffmann, T. Schneller, U. Boettger, R. Waser, PZT thin films for piezoelectric micro-actuator applications, Sens. Actuators, A 97–98 (2002) 680–684. [7] I. Kanno, S. Hayashi, T. Kamada, M. Kitagawa, T. Hirao, Low-temperature preparation of Pb(Zr, Ti)O3 thin films on (Pb, La)TiO3 buffer layer by multi-ion-beam sputtering, Jpn. J. Appl. 32 (1993) 4057–4060. [8] A.H. Carim, B.A. Tuttle, D.H. Doughty, S.L. Martinez, Microstructure of solutionprocessed lead zirconate titanate (PZT) thin films, J. Am. Ceram. Soc. 74 (6) (1991) 1455–1458. [9] D.S. Yoon, J.S. Roh, S.M. Lee, H.K. Baik, Alteration for a diffusion barrier design concept in future high-density dynamic and ferroelectric random access memory devices, Prog. Mater. Sci. 48 (4) (2003) 275–371. [10] Z.J. Wang, Z.P. Cao, Y. Otsuka, N. Yoshikawa, H. Kokawa, S. Taniguchi, Lowtemperature growth of ferroelectric lead zirconate titanate thin films using the magnetic field of low power 2.45 GHz microwave irradiation, Appl. Phys. Lett. 92 (2008) 222905 (1–3). [11] H. Tabata, O. Murata, T. Kawai, S. Kawai, M. Okuyama, Preparation of PbTiO3 thin films by anexcimer laser ablation technique with second laseri irradiation, Jpn. J. Appl. Phys. 31 (1992) 2968–2970.
4. Conclusions In this work, PZT/BTO bilayer films were sputter-deposited on Si substrates. The bi-layer films showed a macroscopic ferroelectricity at a deposition temperature as low as 350 °C, owing to the buffering effect of the BTO underlayer. At the deposition temperature of 500 °C, the PZT/BTO bi-layer film showed a pure perovskite phase structure with good electrical properties, including a large saturated polarization (Ps∼57 μC/cm2) and a much-reduced leakage current. Broadly tunable dielectric and ferroelectric responses were achieved in the bi-layer films through adjusting its PZT/BTO thickness ratio, enabling a wider spectrum of applications than those of the single-layer films. For example, high figure-of-merit piezoelectric energy harvesters and high energy density, high efficiency dielectric capacitors, with performances exceeding those of the same devices made from single-layer PZT films, were demonstrated for the PZT/BTO bi-layer films. It can be concluded that bi-layering is a simple and effective method for tuning of electrical properties in perovskite ferroelectric films. 9036
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