Effect of substrate and electrode on the crystalline structure and energy storage performance of antiferroelectric PbZrO₃ films

Highlights

• Both polycrystalline and epitaxial antiferroelectric films grown in a columnar structure.

• Field-induced antiferroelectric-ferroelectric phase transition of about 600–625 kV/cm.

• Square-shaped double hysteresis loops achieved in all films.

• High energy density and breakdown strength obtained in epitaxial films on SrTiO₃/Si.

Abstract

We report on the correlated investigation between crystal structures, field-induced phase transition, and energy storage properties of both polycrystalline and epitaxial antiferroelectric PbZrO₃ (PZO) films grown by pulsed laser deposition on Si and SrTiO₃ substrates. The structural characterization revealed the polycrystalline structure of the PZO films on Pt/Si and the epitaxial relationship between the films and the SrTiO₃/Si and SrTiO₃ substrates. Different to normal ferroelectric fi

lms, the polycrystalline PZO films show similar polarization loops but with a higher maximum polarization, resulting in a larger energy storage density under the same conditions. Due to the larger electric breakdown strength (2800 kV/cm), however, the epitaxial PZO films grown on SrTiO₃/Si have a higher recoverable energy storage density (24.9 J/cm³) than those on Pt/Si (23.4 J/cm³ at 2500 kV/cm) and on SrTiO₃ (22.0 J/cm³ at 2550 kV/cm). Additionally, the introduction of SrRuO₃ oxide-electrode improves the endurance performance of energy storage properties of the films on STO/Si by suppressing the formation of the dead layer between the film and the electrode. In this way, applications based on PZO films would be more easily integrated on Si and open the way to develop high-power commercial energy storage systems.

Introduction

Antiferroelectric (AFE) materials have attracted considerable attention from both fundamental research and practical applications [1], [2], [3], [4], [5], [6]. One of the distinct characteristics of AFE materials is their crystal structure change during the AFE-FE phase transition, in which at sufficiently high applied electric fields (over a critical value), the orthorhombic (or tetragonal) AFE phase is transformed into the rhombohedral FE phase [6,7]. In the AFE state, the adjacent dipoles are aligned in opposite direction, therefore its spontaneous polarization is almost zero. The change in electric displacement during the phase transition is achieved due to the appearance of the spontaneous polarization of FE state [8]. Thus, AFE materials have great potential for use in some special applications such as pulse-power capacitors and digital displacement (actuator) transducers [1,6,9,10].

Perovskite PbZrO₃ (PZO) is the typical and one of the most widely studied AFE materials with a Curie temperature (T_c) of about 230 °C. In PbZrO₃, Pb²⁺ ions site at the A-site and Zr⁴⁺ ions site at the B-site. At room temperature and low electric fields, the orthorhombic structure of PZO have the lattice constants of a = 5.78 Å, b = 11.74 Å and c = 8.20 Å, meanwhile each unit cell of orthorhombic structure contains eight primitive cells, which have a tetragonal structure with the lattice constants of a₀=b₀=4.15 Å and c₀=4.10 Å [6,11]. Even though PZO has been extensively studied, the investigation of substrate effect on microstructure and energy storage properties of PZO films is still interesting in order to achieve low-cost and high-throughput production processes for high-performance energy storage applications.

The effect of the substrates, electrodes and buffer layers has been widely studied to control the crystalline structure of normal ferroelectric (FE) films (such as Pb(Zr,Ti)O₃ – PZT) and then the ferroelectric and piezoelectric properties can be tuned. As for PZT films, the dipoles are easily polarizable along <100> direction, therefore, the PZT films with (100)-orientation often showed higher maximum polarization (P_m) and remanent polarization (P_r) than those with other oriented films or random-oriented films [12], [13], [14]. For AFE films, the dipoles are anti-paralleled along <110> direction in AFE state and are paralleled along <111> direction in FE state, then the (111)-oriented AFE films (such as PZO) have been illustrated to show a higher P_m value [15].

Moreover, in the case of FE films, the selection of appropriate substrates can be used to control not only the crystalline orientation, but also the domain structure of the films. Previous studies of FE PZT films found that the (001)-oriented PZT films grown on single-crystal SrTiO₃ (STO) substrates have a compressive stress (with c-axis oriented domain), meanwhile the (001)-oriented PZT films grown on STO/Si substrates have a tensile stress (with a-axis oriented domain) [16], [17], [18], [19], [20]. In this case, the compressive stress causes the domains to orient along the longitudinal (out-of-plane) direction which results in an enhancement of polarization. However, the single-crystal substrates (such as STO) are not suitable for practical applications for various reasons, including complicated fabrication processes required, high cost and most importantly, the absence of scaling up to large scale (wafer scale) production. In contrary, silicon (Si) is well suited for wafer-scale fabricating microstructure devices due to the availability of well-established Si-micromachining techniques [21], [22], [23]. Several studies have been reported on the properties of AFE PZO films on various substrates and electrode/buffer-layers (such as, SrRuO₃/SrTiO₃, SrRuO₃/Si, Pt/Si and Indium Tin Oxide coated glass) [7,15,24,25], however, these films are mostly fabricated by different techniques (such as sputtering, pulsed laser deposition and chemical solution deposition). In this study, we report the structure, electrical and energy storage properties of the PZO films grown on SrRuO₃/SrTiO₃ (STO) SrRuO₃/SrTiO₃/Si (STO/Si) and Pt/Ti/SiO₂/Si (Pt/Si) substrates by pulsed laser deposition. In contrast to the case of FE PZT films, the (001)-oriented PZO films with higher crystalline quality will have smaller maximum polarization. On the other hand, the PZO film grown on Pt/Si exhibits the largest P_m value and the film on STO shows the smallest P_m value. However, the higher recoverable energy storage density is obtained in the films grown on STO/Si, due to the larger electric-breakdown strength. These findings could be important for the further integration of PZO film capacitors onto Si wafers to open the way for a cost-effective pulse-power energy-storage technology, especially in case of directly connected electronic circuits.