Relaxor Ferroelectric AgNbO₃ Film Fabricated on (110) SrTiO₃ Substrates via Pulsed Laser Deposition

Abstract

AgNbO₃-based materials have attracted extensive attention in energy storage due to their double hysteresis loops, but they suffer from low breakdown strength (E_b). AgNbO₃ films with few defects and small thickness exhibit high E_b, which helps to improve the energy storage performance. In this work, we successfully prepared AgNbO₃ thin films on (110) SrTiO₃ substrate using pulsed laser deposition technology. The AgNbO₃ film shows good crystalline and relaxor ferroelectric behavior. A high E_b up to 1200 kV/cm is obtained in AgNbO₃ film, which contributes to good recoverable energy storage density W_rec up to 10.9 J/cm³ and energy efficiency η of 75.3%. Furthermore, the W_rec remains above 2.9 J/cm³ and the η varies between 72.5% and 82.5% in a wide temperature range of 30–150 °C. This work reveals the great potential of relaxor ferroelectric AgNbO₃ film for energy storage.

1. Introduction

Dielectric capacitors with advantages of ultra-fast charge and discharge rates, excellent cycle stability, and long service life have been widely used in electric vehicles, pulse power systems and portable electronic devices [1,2]. Linear dielectrics, ferroelectric (FE) materials, antiferroelectric (AFE) materials, and relaxor ferroelectric (RFE) materials are commonly used materials in high-power capacitors. However, the energy storage density and energy efficiency of electronics capacitors are limited, and they cannot meet the requirements of miniaturization, nor can they meet the integration of advanced electrical and electronic systems. Therefore, many tasks have been performed to improve the energy storage density and energy efficiency of the capacitor [1,2,3,4,5].

Energy storage can be realized via dielectric polarization P in an external electric field E (P-E loops) in dielectrics, with recoverable energy storage density W_rec (determined by ${\int }_{P_{\mathrm{r}}}^{P_{\mathrm{m}\mathrm{a}\mathrm{x}}}E\mathrm{d}P$) and energy efficiency η (defined as W_rec/(W_rec + W_loss)). P_max and P_r are the maximum polarization and remnant polarization. P_r would generate energy loss W_loss and cause an energy (heat) dissipation problem in practical uses [6]. High W_rec and η are thus to be achieved by realizing large polarization (P_max) and small hysteresis (P_r and W_loss) [7]. In addition, the large applied electric field E, which is limited by the breakdown strength E_b, is also responsible for the high energy storage performance (ESP). AFE and REF materials with large ΔP caused by high P_max and small P_r are very promising for high-performance dielectric capacitors [8,9]. AgNbO₃-based AFE ceramics with phase transitions from M₁ phase, M₂ phase, M₃ phase, O phase, T phase to C phase have received increasing attention for energy storage because of their typical double P-E loops with high P_max and low P_r [10,11,12,13,14]. The W_rec has been increased to more than 5.0 J/cm³ via ions doping at the A- or B-site [15,16,17]. In addition, increasing E_b is another effective strategy to optimizing the energy storage performance of AgNbO₃-based ceramics, which can be realized by either regulating the preparation process or reducing the sample thickness. For example, a high E_b of 250 kV/cm is obtained in AgNbO₃ ceramic using hydrothermal-synthesized nano AgNbO₃ powders [18]. The E_b of AgNbO₃ ceramic reaches to 307 kV/cm by using the tape casting process, which contributes to an increased W_rec of 2.8 J/cm³ [19]. It is also an effective way to enhance E_b by reducing the sample thickness since E_b follows an exponential decay relationship with the sample thickness [7]. Compared with ceramic bulks, multilayer ceramic capacitors (MLCCs) with a thin monolayer (i.e., 10–20 μm) usually contribute to high E_b and high W_rec. For example, an ultra-high E_b of 1500 kV/cm was obtained in (Ag_0.91Sm_0.03)(Nb_0.80Ta_0.20)O₃ MLCC with a dielectric layer of 12 μm, which contributes to an ultra-high W_rec of 15.0 J/cm³ [20]. Furthermore, films with a thickness of several microns or less normally have higher E_b than their bulk counterparts. For example, AgNbO₃ film with a thickness of 2.6 μm shows E_b of 352 kV/cm [12]. An amount of 400 nm AgNbO₃ film shows an E_b of 624 kV/cm, which displays double P-E loops [13]. A further high E_b of 1200 kV/cm is obtained in 550 nm RFE AgNbO₃ film, which results in a W_rec of 10.3 J/cm³ and a η of 72.2% [21].

It is found that the AFE or FE property of the AgNbO₃ films depends on the orientation direction and the applied electric field [12]. AgNbO₃ films deposited on (001)SrTiO₃ is AFE performance at the E of 100–350 kV/cm. On the (110)SrTiO₃ deposition, AgNbO₃ film has an AFE performance at 150 kV/cm and FE property as the E is higher than 200 kV/cm. The AgNbO₃ films deposited on (111)SrTiO₃ have FE property at 100–350 kV/cm [12]_.

In the present work, AgNbO₃ thin films are deposited on (110) SrTiO₃ substrate via pulse laser deposition. The crystal structure, the performance of the dielectric, and the performance of the ferroelectric were studied. The results show that the AgNbO₃ thin film has an extension and typical RFE nature which can contribute to a large ΔP. It is important to obtain a high E_b with more than 1200 kV/cm, which leads to a high W_rec up to 10.9 J/cm³.

2. Experimental Procedure

Pulse laser deposition technology (PLD) was used to prepare the AgNbO₃ (ANO) film. First, 40 nm SrRuO₃ (SRO) layer was deposited on (110) SrTiO₃ (STO) substrate as bottom electrode through magnetic sputtering with the conditions of 700 °C, 50 W and 3 Pa. Second, 500 nm ANO film was grown on the SRO bottom electrode at a temperature of 650 °C. The laser sputtering frequency is 3 Hz and the energy density is approximately 250 mJ/cm². The obtained ANO film was annealed in high-purity oxygen at 550 °C for 1 h. Finally, the symmetrical SRO electrode (i.e., 40 nm, 50 W and 3 Pa) and Pt upper electrode (i.e., 40 nm, 60 W and 3 Pa) were prepared via magnetron sputtering.

The crystal structure and orientation of ANO films were detected via X-ray diffractometer (XRD, D8 Advance, Saarbruken, Germany) in which non-monochromatic Cu radiation was used. The angle range of XRD is 20~80° with a rate of 1.2°/min and a step size of 0.02°/s. Its microstructure and morphology were characterized using an atomic force microscope (AFM, MFP-3D, Asylum Research, Oxford, UK). The chemical valence of ANO films was evaluated on ESCALAB 250Xi X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, Oxford, UK) with the penetration depth of 4–10 nm. The relative dielectric constant and dielectric loss of ANO thin films were measured using an LCR tester (Radiant Technologies, Albuquerque, NM, USA). The frequency range of the dielectric spectrum is 10³–10⁶ Hz, and the oscillation voltage is 1 V. The ferroelectric characteristics of ANO thin films were studied using a ferroelectric tester (Precision LC II, Radiant Technologies, Albuquerque, NM, USA) at a frequency of 10⁴ Hz.

3. Results and Discussion

Figure 1a shows the XRD patterns of Pt/SRO/ANO/SRO/STO heterostructure. In addition to the (ll0) diffraction peaks of STO substrate, the (ll0) diffraction peaks of ANO film are also observed, which means good crystallization of ANO film. The insets in Figure 1a show the rocking curves of the ANO film and STO substrate. The FWHM of the ANO film and STO substrate are 0.423° and 0.168°, respectively.

The morphology of ANO films is characterized by AFM, as shown in Figure 1b. The ANO thin films exhibit a relatively dense microstructure with root-mean-square roughness of 1.54 nm. The frequency dependence of the relative dielectric constant (ε_r) and dielectric loss (tanθ) of ANO film are shown in Figure 1c. At room temperature, ε_r and tanθ are 410 and 0.017 at 10³ Hz in the ANO film. During the process of increasing the testing frequency from 10³ Hz to 10⁴ Hz, both ε_r and tanθ show little dependence on frequency, while ε_r decreases and tanθ increases as the frequency is greater than 10⁴ Hz. The main conduction mode changes from carrier oscillation to ion oscillation at high frequency, resulting in a change in tanθ. Figure 1d shows the leakage current density of ANO film. It can be seen that the leakage current density of ANO film increased from 2.9 × 10⁻⁸ A/cm² to 9.4 × 10⁻³ A/cm² when E increased from 50 kV/cm to 300 kV/cm.

The chemical valence states of Ag, Nb and O elements in ANO films are investigated via the XPS spectra, as shown in Figure 2. Two peaks centered at 368.5 eV and 374.4 eV are observed in Figure 2a, which correspond to Ag 3d_5/2 and Ag 3d_3/2.

The FWHM for Ag 3d_5/2 and Ag 3d_3/2 are 0.7 eV and 0.6 eV. There are two peaks centered at 210.5 eV and 208.6 eV with FWHM of 1.4 eV and 1.6 eV in Figure 2b, which correspond to Nb 3d_5/2 and Nb 3d_3/2. The only one spin orbit of Ag and Nb elements indicates that they have only one chemical valence state, i.e., Ag⁺ and Nb⁵⁺. Therefore, it can be concluded that there is no metallic Ag in the ANO film obtained in this work. The Lorentz–Gauss function was used to perform the O 1s spectrum peak partial fitting, as shown in Figure 2c. Two peaks corresponding to oxygen vacancy (~532.0 eV, O_V) and lattice oxygen (~531.0 eV, O_L) [14] are fitted in the O 1s spectrum. The relative area of O_V is calculated to be 13.4%, which maybe the reason for the high leakage current density of ANO thin films at a higher E.

The P-E loops of the obtained ANO film were measured at a testing frequency of 10 kHz with varied E from 100 kV/cm to 1200 kV/cm. The results are shown in Figure 3a.

The ANO film exhibits a slender and almost hysteresis-free P-E loops as E is less than 300 kV/cm. When E increases from 400 kV/cm to 1200 kV/cm, the ANO film exhibits P-E loops with an FE-like feature. A power law proportional relation (<A>∝E^α) is used to produce a curve of the changes in the hysteresis area A of the ANO film with E [22], as shown in Figure 3b. Two linear stages are observed in the ln<A>-lnE curve of the ANO film, indicating that the ANO film exhibits REF behavior. With the increase in E, the change in ln<A> is divided into two stages, with a rapid increase in the first stage and a slow increase in the second stage, corresponding to α values of 3.83 and 0.57, respectively. The E-induced RFE-FE state transition is the reason for the rapid increase in polarization in the first stage, and the domain growth and switching are also associated with this process [23,24,25]. As for the slow increase in polarization in the second stage, it may be mainly attributed to the induced polarization. At this stage, the polarization of ANO film has been nearly saturated with the increase in E, which results in small α of 0.57. The variation trends of P_r, P_max and ΔP (ΔP = P_max − P_r) of the ANO thin film with E are shown in Figure 3c. Both P_r and P_max increases as E increases. ΔP increases rapidly as increasing E from 100 to 500 kV/cm and keeps almost unchanged as E increases from 500 to 1200 kV/cm. The increased ΔP is conducive to the improvement in ESP. When E reaches 1200 kV/cm, the P_max, P_r and ΔP of the ANO film are 33.3 μC/cm², 6.1 μC/cm² and 27.2 μC/cm², respectively. The W_rec and η of the ANO film are shown in Figure 3c. W_rec increases from 0.19 to 10.9 J/cm³ as E increases from 100 to 1200 kV/cm. η decreases from 95.0% to 65.5% when E increases from 100 to 400 kV/cm, and gradually increases from 67.6% to 75.3% as E increases from 500 to 1200 kV/cm. Both the E_b and W_rec of the ANO film are much better than those of the ANO ceramics (E_b = 140–307 kV/cm and W_rec = 1.6–2.8 J/cm³) prepared via the solid-state reaction method [18,19] and the pure-phase ANO film (E_b = 110 kV/cm) grown on Pt/Ti/SiO₂/Si substrate by using a chemical solution deposition method [14], but not as good as BiFeO₃-BaTiO₃-based and (Na,Bi)TiO₃-based RFE films [26,27]. High W_rec values of 152 J/cm³ and 106 J/cm³ have been obtained in the BiFeO₃-BaTiO₃-based and (Na,Bi)TiO₃-based RFE films due to their high E_b of 3500 kV/cm and 3683 kV/cm [26,27]. Therefore, if E_b can be further increased by optimizing thickness and quality, a further high W_rec can be expected in the ANO film.

Figure 4a shows the P-E loops of the ANO film at 30–150 °C. ANO film maintains slender P-E loops at a high temperature of 150 °C.

The changes in P_max, P_r and ΔP with temperature are shown in Figure 4b. Both P_r and P_max are found to show a decreased trend as elevating temperature from 100 °C to 150 °C. The P_r and P_max of the dielectric materials can be influenced by their domain wall motion, leakage current density, and phase structure [21]. Domain wall motion generally becomes easier at high temperatures, which is beneficial for high P_r and P_max [28]. When the temperature increases from 30 °C to 150 °C, ΔP increases from 16.7 μC/cm² to 18.4 μC/cm², which is conducive to the improvement in ESP. With the increase in temperature, W_rec increases from 2.9 J/cm³ to 3.3 J/cm³, and η varies between 72.5% and 82.5%, as shown in Figure 4c.

4. Conclusions

ANO film was fabricated on SRO/(110)STO substrate by using PLD. The ANO film has good crystalline with relatively flat surface. When E is less than 300 kV/cm, the ANO film show narrow P-E loops. When E is higher than 400 kV/cm, the P-E loops show some hysteresis. All of these conform to the RFE nature, as confirmed by the relation of <A>∝E^α. A high E_b of up to 1200 kV/cm was obtained, which contributes to a good W_rec of 10.9 J/cm³ and η of 75.3% in ANO film. In addition, the W_rec of the ANO film remains above 2.9 J/cm³ at 30–150 °C. This work reveals the great potential of RFE ANO film for energy storage.