Effects of LaNiO3 Seed Layer on the Microstructure and Electrical Properties of Ferroelectric BZT/PZT/BZT Thin Films

Ferroelectric multilayer films attract great attention for a wide variation of applications. The synergistic effect by combining different functional layers induces distinctive electrical properties. In this study, ferroelectric BaZr0.2Ti0.8O3/PbZr0.52Ti0.48O3/BaZr0.2Ti0.8O3 (BZT/PZT/BZT) multilayer thin films are designed and fabricated by using the magnetron sputtering method, and a LaNiO3 (LNO) seed layer is introduced. The microstructures and electrical properties of the BZT/PZT/BZT films with and without the LNO seed layer are systematically studied. The results show that the BZT/PZT/BZT/LNO thin film exhibits much lower surface roughness and a preferred (100)-orientation growth, with the growth template and tensile stress provided by the LNO layer. Moreover, an enhanced dielectric constant, decreased dielectric loss, and improved ferroelectric properties are achieved in BZT/PZT/BZT/LNO thin films. This work reveals that the seed layer can play an important role in improving the microstructure and properties of ferroelectric multilayer films.

Constructing multilayer thin films may be a compromise strategy to reduce the usage of lead and retain the outstanding characteristics of lead materials. Moreover, a novel physical phenomenon and exceptional properties may be achieved by combining the lead-free films with lead-based ones. For example, the BiFeO 3 /PbZr 0.5 Ti 0.5 O 3 multilayer films exhibit not only reduced leakage current but also enhanced dielectric properties compared with pure BiFeO 3 films (Li et al., 2005). Dielectric anomaly was also observed in PbZr 0.2 Ti 0.8 O 3 /SrTiO 3 (ferroelectric/paraelectric) bilayer films due to the polarization mismatch. A maximum dielectric constant could be obtained when the layer thickness ratio reached a critical value (Khassaf et al., 2016). Besides, compositionally multilayer thin films (Damodaran et al., 2017a) and heterostructure thin films (Damodaran et al., 2017b;Zhang et al., 2017) also exhibit outstanding electrical properties. In addition, in bilayer films composed of a ferromagnetic NiFe or CoFe 2 O 4 layer and ferroelectric Pb(Zr,Ti)O 3 layer, a remarkable magnetoelectric effect was obtained Ramírez-Camacho et al., 2017).
It is well-known that the growth orientations and electrical properties of ferroelectric thin films are very sensitive to the stress from the substrate, seed layer, buffer layer, or interfacial layer. For example, BaZr 0.2 Ti 0.8 O 3 (BZT) films with a preferred (100) orientation can be obtained by using a La 0.7 Sr 0.3 MnO 3 bottom layer (Tang et al., 2006) or LaNiO 3 (LNO) seed layer (LiZhang et al., 2015) on Pt/Ti/SiO 2 /Si substrates in which the dielectric and piezoelectric properties were substantially improved. Specifically, LNO was widely used to regulate the microstructure and growth orientation Zhu et al., 2016) and to optimize the piezoelectric coefficient (LiZhang et al., 2014;LiZhang et al., 2015), polarization behavior , and photovoltaic current (Cheng, et al., 2020) of ferroelectric films. In this study, BaZr 0.2 Ti 0.8 O 3 (BZT)/PbZr 0.52 Ti 0.48 O 3 (PZT)/BZT multilayer thin films are constructed for performance enhancement, and the LNO is chosen as a seed layer for the film growth. The effects of LNO seed layer on the microstructure and electrical properties are systematically studied in this work.

EXPERIMENTAL SECTION
An LNO seed layer of ∼10 nm in thickness was grown on Pt (111)/Ti/SiO 2 /Si substrates by a sol-gel method, as reported in our previous study (LiZhang et al., 2015). The raw materials were lanthanum nitrate [La(NO 3 ) 3 ] and nickel acetate [Ni(CH 3 COO) 2 ]. La(NO 3 ) 3 and Ni(CH 3 COO) 2 were dissolved and refluxed for 30 min in the heated 2methoxyethanol solvent, and the concentration of the solution was adjusted to 0.05 mol/L. After the precursor solution was aged for 24 h, the LNO layer was spin-coated on the Pt (111)/Ti/SiO 2 /Si substrates, followed by pyrolysis at 400°C for 3 min, and then annealing at 750°C for 30 min. BZT and PZT thin films were then grown on the LNO/Pt (111)/Ti/ SiO 2 /Si substrates with a magnetron sputtering method. A base pressure of 5 × 10 −5 Pa was achieved before sputtering, and the Ar-to-O 2 flow ratio was 3:1 during film growth. The top and bottom BZT layers are ∼150 nm in thickness, and the PZT interlayer is ∼200 nm in thickness. Finally, the prepared BZT/ PZT/BZT and BZT/PZT/BZT/LNO thin films were annealed at 750°C for 30 min to enhance crystallization.
The crystalline structures of the thin films were analyzed by a Philips X′ pert X-ray diffractometer (XRD) with Cu Kɑ radiation generated at 40 kV and 40 mA. The cross-section micrographs of the thin films were obtained by using scanning electron microscopy (SEM, Helios Nanolab600i), and the film surface morphology was characterized by an atomic force microscope (AFM). Piezoelectric response was detected by a piezoelectric force microscope (PFM). For further electrical property measurements, platinum electrodes with a diameter of 200 μm were deposited by DC magnetron sputtering. The ferroelectric properties and leakage current were characterized by a Radiant Precision Workstation Ferroelectric Measurement System. The frequency and temperature dependent dielectric properties were measured on a precision impedance analyzer (Agilent 4294A), with an AC voltage of 500 mV.

RESULTS AND DISCUSSION
The XRD measurement shows a pure perovskite phase of the films ( Figure 1A). It is found that the BZT/PZT/BZT/LNO thin films possess preferred (100)-oriented growth, while the BZT/ PZT/BZT films without the LNO seed layer do not exhibit an obvious crystal orientation preference. The XRD rocking curve (ω  Figure 1A). This small FWHM value reflects the strong orientation preference induced by the LNO seed layer. There are no diffraction peaks of the LNO seed layer due to its small thickness, consistent with our previous studies (LiZhang et al., 2015). To study the stress state in the BZT/PZT/BZT thin films, the grazing incidence X-ray diffraction (GIXD) method  was employed, and the results are shown in Figures 1B,C. According to the equation of ψ θ-ω (θ is incident angle, ω is the grazing incident angle, and ψ is the included angle between the normal direction of films and diffraction crystal plane), ψ can be obtained by changing the ω value. Furthermore, the slightly changed θ value means the variation of the interplanar crystal spacing d i , which can be calculated by the Bragg's equation 2d i sinθ λ, [here, d 0 is the interpalne spacing of stress-free specimen and (d i -d 0 )/d 0 corresponds the crystal lattice strain]. It should be noted that there is a linear relationship between the (d i -d 0 )/d 0 and sin 2 ψ.
(For detailed derivation process, refer the previous reports (Wang et al., 2010).) The increased incident angle ω corresponds to the increasing penetration depth of X-ray. Therefore, the strain distribution behavior along the normal direction in the films can be revealed by the GIXD method with different ω values. In this work, the GIXD peaks of the (200) planes are shown in Figures 1B,C, respectively, for the BZT/PZT/BZT and BZT/PZT/ BZT/LNO thin films. It can be seen that the diffraction peak is almost unchanged for the BZT/PZT/BZT films, which, as a comparison, exhibits an increased two theta with the increasing ω in the BZT/PZT/BZT/LNO films. It means that the LNO seed layer induces tensile stress in the BZT/PZT/BZT thin films, leading to an increased interplanar spacing d i , as shown in Figure 1D. Figure 2 shows the surface morphology of the films. The films fabricated by magnetron sputtering exhibit island growth characteristics. SEM images show that the grain size is about 1 μm, with some fine grains of ∼100 nm distributed sporadically between the adjacent large grains (Figures  2A,B). In order to obtain more detailed surface information, AFM characterization was employed, and the results are given in Figures 2C,D. It can be clearly found that a more compact and smoother surface structure is obtained in the BZT/PZT/BZT/LNO films. The film roughness was evaluated by the root-mean-square (RMS) values derived from the AFM measurement, which are 16.7 and 7.4 nm for the BZT/PZT/BZT and BZT/PZT/BZT/LNO thin films, respectively. The results indicate that the LNO seed layer is beneficial to improve the film quality and to reduce the surface roughness. Cross-sectional SEM images of the films are shown in Figure 3. It can be seen that the BZT/PZT/BZT/LNO thin film possesses a much denser microstructure than that of the BZT/PZT/BZT thin film without the LNO seed layer. Some voids between adjacent grains can be seen in the BZT/PZT/BZT thin film, which is much more obviously seen in the backscattered electron (BSE) images, as given in Figures  3B,D. The total thickness of the films is verified to be ∼500 nm, with the thickness of the top and bottom BZT layers of about 150 nm, and the PZT interlayer of about 200 nm. The interface between BZT and PZT can be clearly observed due to the different atomic numbers of Ba and Pb, and thus the varied contrast in the BSE image. The LNO seed layer is too thin to observe from the SEM images, whose thickness was estimated to be ∼10 nm via the X-ray reflectivity method (LiZhang et al., 2015).
Since dielectric properties are very important for ferroelectric thin films, the dependence of dielectric properties on the measurement frequency and temperature is investigated, and the results are given in Figure 4. It can be found that the dielectric constant is notably improved by introducing an LNO seed layer into the BZT/PZT/BZT film. The dielectric constants are 451.7 and 598.6, respectively, for the BZT/PZT/BZT and BZT/PZT/BZT/LNO thin films at 1 kHz. As shown in Figure 4B, the dielectric loss of the BZT/PZT/BZT/LNO thin film at higher frequencies is lower than that of BZT/PZT/BZT/LNO thin films. The enhanced dielectric constant and the decreased dielectric loss can be ascribed to the improved film quality and polarization capability by inserting the LNO seed layer, which is consistent with the results of previous reports (Wu and Shy, 2000;Gao et al., 2008). The temperature-dependent dielectric properties of the films are presented in Figures 4C,D. It can be observed that both the BZT/PZT/BZT and the BZT/PZT/BZT/ LNO thin films possess a wide peak of dielectric constant, which may correspond to the transition from the rhombohedral phase to the cubic phase of BZT (Lee et al., 2005;Xu et al., 2015). Such broad peaks could also link to the diffuse phase transitions due to the substrate clamping effect (Setter, et al., 2006;Tong, et al., 2011). Besides, the BZT/PZT/ BZT/LNO film maintains lower and more stable dielectric loss values as the temperature increases, which is also associated with the higher quality and less defective microstructure of the film with the LNO seed layer. Ferroelectric properties of the films are investigated by measuring hysteresis polarization-voltage loops. As given in Figure 5A, the hysteresis loops of the BZT/PZT/BZT thin film are not completely closed, and an obvious gap can be seen at the negative branch as the applied voltage decreases to zero. This phenomenon may be correlated with the leakage current or conduction loss, which further leads to the asymmetric coercive voltage (V c ) values. As a comparison, it can be noticed that the hysteresis loops of the BZT/PZT/BZT/LNO film is almost closed after the voltage cycle, indicating a lower leakage current. It should be noted that the obviously asymmetric coercive voltage values still exist in the BZT/PZT/BZT/LNO thin film, with the + V c of 16.4 V and-V c of -8.4 V. It was reported that the loop offset along the voltage axis can be induced by the asymmetric electrode structure of thin films (Lee et al., 2005;Wong and Shin, 2005). In addition, the interfacial charge in the multilayer films and the existence of built-in electric field may also lead to the asymmetrical characteristics of hysteresis loops (Karthik et al., 2013;Agar et al., 2015). Figures 5C,D show the leakage current of the thin films at different voltages. The leakage current of the BZT/PZT/BZT/ LNO film is generally lower than that of the BZT/PZT/BZT film. Besides, in the BZT/PZT/BZT/LNO thin film, the leakage current exhibits an unsymmetrical characteristic, which is higher at positive bias voltages. A possible reason may be the asymmetric electrodes that lead to different interfacial Schottky barrier heights and induce the unsymmetrical characteristics of IV curve (Shen et al., 2014;Fan et al., 2015;Chen et al., 2020). To further study the effects of the LNO seed layer on the polarization behavior of thin films, piezoelectric force microscopy (PFM) was employed to directly observe the polarization reversal behavior, based on the evolution of piezoelectric response. A positive voltage and a negative voltage were applied sequentially on the BZT/PZT/BZT/LNO film. A 3,000-nm × 3,000-nm area at the center of the selected region was first poled by +20 V voltage ( Figure 6A), and then a 1,000-nm × 1,000-nm area at the center was poled by an inversed -20 V voltage to study the polarization reversal ( Figure 6B). It should be pointed out that the voltage was applied between the bottom electrode and PFM tip, and the tip was grounded. Finally, the PFM signals of the 5,000 nm × 5,000 nm area were scanned after removing the voltage ( Figure 6C). Because the piezoelectric response signal is proportional to the out-of-plane polarization of the film, the bright regions correspond to upward polarization, and dark regions to downward polarization. In Figure 6D, one can find that the intensity of the positive piezoelectric response is stronger than that of the negative piezoelectric response, meaning that thin films are easier to be polarized when a positive polarization voltage is applied.

CONCLUSIONS
In this study, BZT/PZT/BZT/LNO thin films were fabricated by combining magnetron sputtering and sol-gel methods. The results show that the (100) preferred orientation growth of the BZT/PZT/ BZT thin film is induced by introducing an LNO seed layer, with which a denser microstructure and lower surface roughness are achieved. More importantly, the dielectric constant increases and the dielectric loss decreases with the insertion of the LNO layer, accompanied by a diffuse phase transition behavior. Besides, asymmetric hysteresis loops and leakage current curves are found in the BZT/PZT/BZT/LNO thin film, which can be attributed to the LNO seed layer that may cause different polarization abilities under different electric field directions. The PFM measurement also supports the above deduction. This work provides an effective way to improve the microstructure and electrical properties of ferroelectric multilayer thin films by introducing an LNO seed layer.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding authors.