Ferroelectric thin films have attracted much attention because of their technological applications in memory devices due to their excellent ferroelectric and dielectric properties. To realize high properties of the memory devices, for example, nonvolatile ferroelectric random access memories (NV-FRAM) and dynamic random access memory (DRAM), it is required to overcome the reliability problems in the ferroelectric thin films such as the high leakage, the notable imprint and the high dielectric loss compared with the bulk counterparts. To improve these electrical properties of the ferroelectric thin films, the researchers found that forming ferroelectric superlattices with well-organized interfaces would be a flexible and efficient method. Generally, the theoretical works found that the improvement of electrical properties in the ferroelectric superlattices is mainly attributed to both lattice strain effect and interfacial electrostatic interaction. Most experimental works revealed that the strain effect is the determined factor to influence the electrical properties of ferroelectric superlattices because of the different crystalline structures of parent materials or a large lattice mismatch between the parent materials. Therefore, to understand the mechanism of the interface effects on the electrical properties in the ferroelectric superlattices, it is important to choose appropriate parent materials to weaken the strain effect. In this study, we select the BaTiO₃ (BTO) and PbZr_0.52Ti_0.48O₃(PZT) as the constituent materials for the ferroelectric superlattices, since both PZT and BTO are the practical ferroelectric materials, and have the same crystalline structure with a very similar lattice constant. The BTO/PZT superlattices were successfully grown on (001) SrTiO₃ substrates by the growth mode of layer by layer with the will-defined periodicity using by the pulsed laser deposition (Fig. 1). We found that the leakage current, ferroelectric and dielectric properties of the BTO/PZT superlattices could be enhanced strongly. Compared with the pure BTO and PZT films, the leakage current was reduced by almost 2～3 orders of magnitude (Fig. 2(a)). The BTO/PZT superlattices showed the remarkably improved dielectric properties with dielectric constant and loss of 684 and 0.02 measured at the frequency of 10 kHz, respectively (Fig. 2(b)). In addition, a more symmetric P-E hysteresis loop was observed in the BTO/PZT superlattices compared with the pure PZT and BTO films. The XPS analysis results for the chemical valence state show that oxygen vacancy is concentrated at the BTO/PZT interface due to the large difference between the ferroelectric polarization of BTO and PZT. Based on these experimental results, it can be considered that the BTO/PZT interfaces play a very important role for the enhanced electrical properties of the BTO/PZT superlattices. Related research results have been published in ACS Applied Materials & Interfaces, 8 (2016) 6736.

Fig. 2 Cross-sectional low-magnification and high-resolution TEM images of the BTO/PZT superlattices grown on the (001)STO substrates, and a schematic diagram of interface structure.

Fig. 2 (a) Leakage current density (J) as a function of the electric voltage (E) for the pure PZT and BTO films, and the BTO/PZT superlattices. (b) Frequency dependence of the dielectric constants (ε_r) and loss tangent (tanδ) for the pure PZT and BTO films, and the BTO/PZT superlattices.