All the Pb(Zr0.52Ti0.48)O3 thin films with Pb(ZrxTi1−x)O3 seed islands show pure perovskite phase with no other impurity phases [21], as demonstrated by the XRD patterns shown in Fig. 2a. From these X-ray diffraction patterns, we learn pure Pb(Zr0.52Ti0.48)O3 thin films and the Pb(Zr0.52Ti0.48)O3 thin films with Pb(ZrxTi1−x)O3 seed islands except for PTO prefer to performing textured (111) orientation along out of the plane with weak (110) peak [22]. The PTO is mainly textured along the (001) direction with a weak (110) peak and the crystal structure tends to be tetragonal phase since the PTO itself grown on Pt prefers to grow along (001). Although the PZT thin films modified by the seed islands grow along (100) to a small extent, there is no impurity phase. In addition, all PZT films grow in the (111) directional texture to ensure that the films own the expected crystal structure. Figure 2b-f are the cross-section of SEM for PZT(52/48) ferroelectric thin films with seed islands of different Zr/Ti ratios. All of the films we obtained are 150 nm. It can be seen from the figures that the crystal structures of the films changed after adding the seed islands. When the PZT film has no seed islands, there are some small particles inside the film, and the size of the crystal grains is distinct. After the seed islands are added, the grain size becomes uniform, and the films become more compact. This indicates that seed islands play a certain role in promoting the crystallization of PZT films [23]. This is because the seed islands can quickly form seeds with the coincident orientation at a slightly lower temperature, which provides nucleation points for the subsequent crystallization of PZT films and decreases the activation energy of film nucleation and growth, especially the Fig. 2e and f.
The polarization-electric field hysteresis loops (P-E) and the capacitance-electric field curves (C-E) on all the Pb(Zr0.52Ti0.48)O3 thin films with Pb(ZrxTi1−x)O3 seed islands were measured at a frequency of 1kHz and 1MHz at room temperature, which are shown in Fig. 3a and c, respectively. All films exhibit ferroelectric switching loop behavior and representative ‘‘butterfly’’ curves (C-E) which indicate excellent ferroelectric properties. Obviously, the P-E loops and the C-E curves for these PZT thin films modified by seed islands are better than the ones without seed islands, including spontaneous polarization, remnant polarization, capacitance in Fig. 3a and c. The spontaneous polarization, remnant polarization, and coercive field of the films are summarized in Fig. 3b, and the 2Pr of these films modified by the PZT seed islands with different ratios of Zr/Ti (0/100, 25/75, 52/48, 75/25) are 55.9, 53, 56.5 and 49.5 µC/cm2 respectively, and the remnant polarization is enhanced a lot compared with pure PZT thin film 42 µC/cm2. When the Zr/Ti ratio of the seed islands is 52/48, the regulation of the seed islands on PZT ferroelectric thin films reaches the maximum, while the coercive field is the smallest (2Ec = 138 kV/cm), and the spontaneous polarization reaches the maximum(2Ps = 161.4 µC/cm2). Thus, the seed islands with the same ratio of PZT sol-gel can enhance the ferroelectric and dielectric properties of the PZT films. This is because the addition of the seed islands optimizes the thin film microstructure that is crucial to get access to high performance [24]. Furthermore, the polarization fatigues of all the Pb(Zr0.52Ti0.48)O3 thin films with Pb(ZrxTi1−x)O3 seed islands are displayed in Fig. 3d. It can be seen that the seed islands have a great influence on the fatigue resistance of the the films. When the ratio of the seed islands is 52/48, the thin film shows the most superior fatigue resistance, can reach 109 cycles without attenuation, which is attributed to the high quality and stability of the film structure. The improvement of ferroelectric properties of these seed-films is since PZT seeds provide nucleation points for subsequent crystallization of PZT precursor solution, thus reducing the nucleation barrier, shown in Fig. 1. What’s more, the reason why the best ferroelectric and dielectric properties appear in the ones whose ratio of Zr/Ti get to 52/48 is due to this ratio of the seed is the same with the following covered PZT (Zr/Ti = 52/48) thin film, as a result, it can induce the directional crystallization of PZT film to the greatest extent, and greatly reduce the defects, strain and dead layer caused by interface mismatch, making the interface between seed islands and the thin film better than others [25, 26].
In order to further investigate the quality of PZT films, dielectric properties testing and leakage current density testing are essential. Figure 4a is a dielectric spectrum (Ɛ-F) of PZT thin films with seed islands in different Zr/Ti ratios. As it can be seen from the figure, they were measured at room temperature with a frequency range from 1 kHz to 1 MHz function varies. The dielectric constants of all the Pb(Zr0.52Ti0.48)O3 thin films slowly decrease linearly as the logarithm of the frequency increases. And the dielectric constants of films modified by seed islands are all higher than no modified one [27]. When the Zr/Ti ratio of seed islands is 52/48, the dielectric constant reaches the maximum, and the dielectric constant is 960 at the frequency of 1 kHz, which is about twice as high as that of pure PZT film. Adding seed islands to the interface between the bottom electrodes and the thin films can reduce the overall activation energy of the PbZr0.52Ti0.48O3 thin films for crystallization, and the interfacial crystallization is improved. At the same time, the diffusion is reduced between thin films and substrates, which decrease the deed layer, and the grain size is more uniform and the defects are less [28]. Therefore seed islands can amplify the dielectric constant [29, 30]. Figure 4b shows the leakage current densities as a function of the electric field with the Pt top electrodes for the above PZT thin films. It is obvious that the leakage current densities of PZT thin films modified by seed islands are smaller than the one without seed islands. This is because the addition of the extremely low concentration of PZT(Zr/Ti = 52/48) as the seed islands provides nucleation points for subsequent crystallization of PZT films, which reduces the activation energy of film nucleation and growth. Besides, it can also maintain high surface energy, which is conducive to the diffusion of thin films surface at low temperature and allows the films and oxygen to be in full contact. Thus, the crystallization transition speed of the thin film is increased, the oxygen vacancy concentration in the thin film is decreased, which increases the density of the thin film and reduces the defects in the thin film. And finally, the leakage current density of the thin film is reduced, which corresponds to the experimental results of fatigue resistance in Fig. 3d [31, 32].
In order to characterize the mesoscopic domain switching of the thin films, piezoresponse force microscopy (PFM) is an ideal tool for both probing and switching the local ferroelectric polarization at the nanoscale. The box-in-box switched patterns were written on the five samples with the application of the electric field via a conducting tip. A tip bias of -15 V was applied to pole the 12 µm × 12 µm square region followed by another poling with a tip bias of + 15 V in the central area of 6 µm × 6 µm. The out-of-plane polarization signals of these five samples are shown in Fig. 5a-e, which show clear bright and dark contrast regions. As can be seen from the figures, the switching contrasts of the films with the seed islands are larger than that of the film without the seed islands. In particular, the PZT thin films with Zr/Ti ratio of 52/48 of the seed islands can be switched when the tip voltage is 12 V. This result suggests that adding PZT seeds into the PZT thin films can promote the domain switching. This is because the addition of PZT seed islands regulates the interface of PZT film, which reduces the internal defects of the film, and the pinning effect on the domain wall is weakened due to fewer defects, so the domain is easier to be switched [33, 34]. This result can also be reflected in Fig. 3a and Fig. 4b. Figure 5f shows representative local PFM phase hysteresis loops. The square loops demonstrating a 180° change in the PFM phase confirm the excellent ferroelectric switching nature of these five samples. From the PFM loops, the Vc of the one modified by PZT(Zr/Ti = 52/48) is the lowest, which also confirms the film modified by PZT(Zr/Ti = 52/48) can accelerate the domain switch [35, 36] and enhance the ferroelectric properties, which is consistent with the previous macro results.