Boosting energy storage performance of low-temperature sputtered CaBi2Nb2O9 thin film capacitors via rapid thermal annealing

CaBi2Nb2O9 thin film capacitors were fabricated on SrRuO3-buffered Pt(111)/Ti/Si(100) substrates by adopting a two-step fabrication process. This process combines a low-temperature sputtering deposition with a rapid thermal annealing (RTA) to inhibit the grain growth, for the purposes of delaying the polarization saturation and reducing the ferroelectric hysteresis. By using this method, CaBi2Nb2O9 thin films with uniformly distributed nanograins were obtained, which display a large recyclable energy density Wrec ≈ 69 J/cm3 and a high energy efficiency η ≈ 82.4%. A superior fatigue-resistance (negligible energy performance degradation after 109 charge-discharge cycles) and a good thermal stability (from −170 to 150 °C) have also been achieved. This two-step method can be used to prepare other bismuth layer-structured ferroelectric film capacitors with enhanced energy storage performances.


Introduction 
Ferroelectric (FE) ceramic capacitors have been investigated intensively for applications in electric power systems and advanced pulsed-discharge devices, because of their fast charge/discharge rate, good thermal stability as well as a remarkable fatigue resistance [1][2][3][4][5][6][7]. where P m and P r are the maximum polarization and remnant polarization respectively, and W c is the stored energy density. Based on these equations, FE film capacitors with a slim P-E loop, featuring a large P m , a small P r , and a high maximum applicable/breakdown electric field, are desirable for capacitive energy storage applications. Recently, various methods have been proposed and investigated to improve the energy storage performance of FE film capacitors, such as domain engineering, interface engineering, chemical doping, and so on [2,6,[10][11][12]. In this work, we adopt a two-step process to fabricate nanocrystalline bismuth layer-structured ferroelectric (BLSF) CaBi 2 Nb 2 O 9 film capacitors, which display an excellent energy storage performance with a high energy density (W rec  69 J/cm 3 ) and charge-discharge efficiency (  82.4%). Compared to pseudo-equiaxial (a  b  c) perovskite ferroelectrics, such as BiFeO 3 and Pb(Zr,Ti)O 3 , the BLSFs have a highly anisotropic lattice (c  a  b). Except for a few rare cases of epitaxial c-axis or a/b-axis oriented BLSF films including Bi 4 Ti 3 O 12 [13,14] and CaBi 2 Nb 2 O 9 [15,16], most of the BLSF films were polycrystalline with (1, 1, 2n+1)-type textured grains which tilt a large angle from the c-or a/b-polar axis [17][18][19][20]. Therefore, compared with a perovskite FE film, it is more difficult to fully align the ferroelectric polarization in a BLSF film using an external electric field, leading to a relatively small remnant/saturated polarization [15,21]. On the other hand, with a large dielectric constant ( r  480) recently revealed in {1 1 2n+1} textured BLSF-type CaBi 2 Nb 2 O 9 films [20], it is possible to create a large maximum polarization, i.e., a large total electric displacement including both ferroelectric and linear dielectric parts, under a sizable electric field [22]. This will lead to a desirable energy storage performance (high W rec and ).
As a bismuth layer-structured ferroelectric, CaBi 2 Nb 2 O 9 (CBNO) has attracted a lot of research interests due to its high Curie temperature (~943 ℃), large ferroelectric polarization (P s  24.4 C/cm 2 ), and a remarkable fatigue resistance [15,20,[23][24][25]. Our previous investigations achieved W rec  21.5 J/cm 3 and   50% in CBNO films on SRO-buffered MgO substrates [15], and W rec  28 J/cm 3 and   62% in those grown on SRObuffered Pt/Ti/Si substrates [20]. In this work, we aim at further improving the energy storage performance of the CBNO film by significantly reducing its grain size. This is achieved by using a two-step process combining a low-temperature sputtering growth with a rapid thermal annealing (RTA) process. Firstly, CBNO films were grown by using a radio frequency magnetron sputtering process on SrRuO 3 -buffered Pt(111)/Ti/Si(100) substrates at 350 ℃ (denoted as "CBNO A350 "). Such a low-temperature deposition process facilitates nucleation of the (115) grains, which have a low activation energy and a high dielectric constant [20,27]. Then the samples were annealed in a rapid thermal annealing furnace at 700 ℃ for 15 min, which promotes a limited growth of the (115) grains and improves the overall crystalline quality of the as-grown film [25,26,28]. Compared with a single-step, high-temperature sputtering growth at 700 ℃, this two-step process not only reduces the thermal budget, but also decreases the average grain size. The latter is critical in improving the energy storage performance of the CBNO films. The resulted CBNO films (denoted as "CBNO 350-RTA ") showed a large P m (~51 microcoulombs/cm 2 ), a small P r (~3.4 microcoulombs/cm 2 ), and a small coercive field E c (~0.16 MV/cm) under a maximum applicable external field of 3.55 MV/cm. Such a slim P-E hysteresis loop corresponds to a large energy density (W rec  69 J/cm 3 ) and a high energy efficiency (  82.4%), which are much higher than those that we obtained earlier. Additionally, these films exhibited a remarkable fatigue resistance (fatigue-free after 10 9 charge-discharge cycles) and thermal stability (stable performance from -170 to 150 ℃).

Experimental procedure
CaBi 2 Nb 2 O 9 thin films were sputtered onto Pt(111)/Ti/ Si(100) substrates using a radio-frequency magnetron sputtering process at a substrate temperature of 350 ℃. A SrRuO 3 thin film of ~100 nm thick was used as a buffer layer due to its chemical and thermal stabilities as well as a good lattice matching with CBNO [20,24]. A commercially available ceramic target of SrRuO 3 and an in-house prepared CaBi 2 Nb 2 O 9 ceramic target were used for the sputtering deposition. A base pressure of 2.0×10 -4 Pa was achieved in a multi-target sputtering www.springer.com/journal/40145 chamber prior to the deposition of the SrRuO 3 layer. The detailed deposition parameters were summarized in Table 1. After the sputtering deposition, the samples were cooled down to room temperature at a cooling rate of 6-8 ℃/min in a pure O 2 atmosphere with a pressure of 7.5 Pa. Then the samples were annealed in a rapid thermal annealing furnace (RTP-400, Beijing East Star Applied Physics Research Institute) at 700 ℃ in an ambient atmosphere for 15 min. CaBi 2 Nb 2 O 9 thin films sputter-deposited at 700 ℃ without RTA, denoted as "CBNO A700 ", were prepared and characterized at the same time for the purpose of comparison. Its deposition parameters were the same as those of the CBNO A350 films ( Table 1) except for the deposition temperature.
Crystallographic characteristics of the CBNO films were analyzed by using standard X-ray diffraction (XRD) 2-scans in a Rigaku Dmax-2500PC equipped with a Ni-filtered Cu Kα radiation source. Crosssectional and surface morphologies were obtained using a high-resolution thermal field emission scanning electron microscope (SEM, Hitachi, SU-70). Circular Au electrode pads with a diameter of 200 μm were sputtered on the film surface for electrical measurements using a shadow mask. A standard ferroelectric tester (Precision Premium II, Radiant Technology, USA) was used to measure the ferroelectric properties. The roomtemperature relative dielectric constant (ε r ) and dielectric loss tangent (tanδ) were determined using an LCR meter (7600plus, QuadTech, USA) in the frequency range between 1 kHz and 1 MHz, using a small AC signal of 1 V p-p.

Results and discussion
The XRD 2θ-scan patterns and the surface and crosssectional SEM images of the CBNO films are displayed in Fig. 1. The CBNO A350 films were poorly crystallized, close to an amorphous state. After the RTA process, the overall crystalline quality of the as-grown film has been improved, mostly from growth of the (1, 1, 2n+1) grains driven by an interface energy [25,26,28]. Compared with those of the perovskite Pb(Zr , Ti)O 3 thin films [28], recrystallization and grain growth of the CBNO film during an RTA process are much more limited because of its strong anisotropic lattice. The CBNO 350-RTA film showed a preferred (115)-orientation, which is about 56° tilted away from the [100]/[010] polar axis (with a theoretical P s  24.4 C/cm 2 ) [23]. Therefore, a reduced ferroelectric polarization and an enhanced dielectric constant can be expected [20,24]. After RTA, the film's average grain size increased to ~22 nm (from analysis of Fig. 1(d)). Note that this number ("local average") was consistent with the result obtained by using the Scherrer formula ("global average", ~20 nm), suggesting a well-controlled grain size via the RTA process. For comparison, the XRD pattern and SEM images of the CBNO A700 film are shown in Fig. 1(a) and Fig. 1(c), respectively. The CBNO A700 film exhibited a wide range of grain size from ~28 to ~228 nm (from analysis of 50 grains in Fig. 1(c)). The difference between the global average grain size via Scherrer formula (~75 nm) and the local one via SEM analysis (~99 nm) can be partially attributed to the grain size nonuniformity. Figure 1(c) clearly reveals such a nonuniform and large-grain microstructure in the CBNO A700 film, which can be ascribed to an inhomogeneous nucleation and grain-growth at a high deposition temperature [27]. From Figs. 1(b) and 1(d), a smooth surface and a dense film morphology, as well as a nanograin structure, can be observed in both the CBNO A350 and CBNO 350-RTA films. The uniform and small grain size of the CBNO 350-RTA film is a natural consequence of the RTA process, which promotes a limited grain growth dictated by minimization of the interface energy [25,28]. A slim P-E hysteresis, a high breakdown field, and a low leakage current are expected for the CBNO 350-RTA film from its dense and nanograined microstructure. Figures 2(a) and 2(b) are the polarization-electric field (P-E) curves measured at 1 kHz. A slightly nonlinear P-E curve with a P r less than 1 C/cm 2 was observed for the CBNO A350 film. After the RTA process, a slim ferroelectric P-E hysteresis loop with a P r  3.4 C/cm 2 was displayed by the CBNO 350-RTA film. For the CBNO A350 film, its slightly nonlinear P-E curve with a sizable slope/ r yet lacking of a saturating trend/concave  curvature, plus a very small grain size, indicate the likelihood of a room temperature superparaelectric (SPE) state [29,30]. In such a state, the thermal energy of a nanograin is higher than the energy needed for its polarization switching, resulting in the absence of a long-range polar order, i.e., P r →0. The critical grain size 2R c for the SPE state can be estimated by using the relation: are the Boltzmann constant, the absolute temperature (K), the remnant polarization, the vacuum dielectric constant, the relative dielectric constant, and the correlation volume, respectively. With the known material parameters taken from the literature [23], the critical size 2R c for the appearance of an SPE state in a CBNO film was estimated to be between ~3.4 and ~4 nm, and our estimated grain size for the CBNO A350 film (2R avg < 10 nm) is in the same order as the 2R c , suggesting that it may exist in the SPE state. Compared with the www.springer.com/journal/40145 ferroelectric P-E loop of the CBNO 350-RTA film in Fig. 2(b), the CBNO A350 film showed about ~65%-75% of the polarization at a given electric field E, indicating an effective dielectric constant smaller yet on the same order as that of the latter. This is indicative of a strong local polar structure in the CBNO A350 film [30]. On the other hand, both the CBNO 350-RTA and CBNO A700 films showed ferroelectric characteristics in their P-E curves (Fig. 2(b)), including a small yet non-zero P r and a concave curvature indicative of polarization saturation. Their differences, however, lie on the shape of the P-E loops they present. The CBNO A700 film showed an earlier saturation at ~1 MV/cm, as well as a larger P r (~7.6 microcoulombs/cm 2 ) compared with those of the CBNO 350-RTA film. Meanwhile, the CBNO 350-RTA film showed an extended (E max  3.55 MV/cm) and slim P-E hysteresis loop, featuring a large maximum polarization P m (~51 microcoulombs/cm 2 ) and a small P r (~3.4 microcoulombs/cm 2 ). These differences can be attributed to those in their crystalline characteristics. As shown in Figs. 1(a), 1(c), and 1(d), the CBNO 350-RTA film has a narrowly-distributed nanograin structure (2R avg  20 nm) with a {115} texture, while the CBNO A700 film consists of nonuniform large grains (~28-228 nm) with random crystalline orientations. Compared with the dominant {115}-oriented grains in the CBNO 350-RTA film, the rich collection of ferroelectric domains with different orientations in the CBNO A700 film helps ease the polarization alignment under an external electric field, leading to an early polarization saturation and hence a poor energy performance. Specifically, under the same electric field of 1 MV/cm, the energy performance of the CBNO 350-RTA film (W rec  10.4 J/cm 3 and   93%) was much better than that of the CBNO A700 (W rec  6.4 J/cm 3 and   50.4%). In addition, a longer exposure to the high processing temperature of 700 ℃ helped generate more defects in the CBNO A700 film [28,31], leading to its lower breakdown strength/maximum applicable field, and hence contributes to a lower energy storage capability. The higher breakdown field of the CBNO 350-RTA film, as compared to that of CBNO A350 , is most likely due to the rapid thermal annealing process, which helped improve crystallinity of the film as well as reduce the number and volume ratio of poorly crystallized grain boundaries with many associated defects [32,33]. This has led to a delayed saturation of the overall electric polarization, and an  [15], and the CBNO film sputtered on SRO-buffered Pt/Ti/Si substrate at 500 ℃ (W rec  28 J/cm 3 and   62%) [20].
To further understand the energy storage performance of these thin film capacitors, the switching currentelectric field (I sw -E) loops were measured for the CBNO A350 and CBNO 350-RTA films, as shown in Figs. 3(a) and 3(b), respectively. It has been reported that the total accumulated charges of an FE film capacitor, as well as its contributions from the linear dielectric displacement, the electric conductivity, and the ferroelectric domain switching, can be quantitatively evaluated by analyzing the I sw -E loop [34][35][36]. In the present work, the corresponding contributions were schematically illustrated in Figs. 3(a) and 3(b), for the CBNO A350 and the CBNO 350-RTA films under the same applied electric field (2.76 MV/cm), respectively. In both cases, the central pseudo-trapezoid area in the I sw -E loop corresponds to the dominant contribution to the power density (energy density/time) of the film capacitor from the dielectric displacement, while the upper triangular area corresponds to the power density dissipated due to electrical conduction. For the CBNO 350-RTA film, there is another sizable contribution to its capacitive power coming from the ferroelectric domain switching (the top light blue, pseudo-triangular area in its I sw -E loop, Fig. 3(b)), which can be attributed to the size-driven transition to the ferroelectric state endowed by the RTA. The linear dielectric contribution to the recyclable energy density W rec can be obtained from the I sw -E loop, using the formula where W ln is the energy density from the contribution of the linear dielectric displacement,  JE is the green shaded area in the J-E diagram with J being the switching current density and E the applied electric field, and t is the duration time. The schematic illustrations of W ln and W bs for the CBNO 350-RTA film are also shown in Fig. 3(c). The W ln resulted from Fig. 3(b) is consistent with that obtained from the P-E loop in Fig. 3(c) (at the same applied field of 2.76 MV/cm). Moreover, as shown in Fig. 3(d), W ln consistently played a dominant role in W rec under an increasing electric field, indicating a major contribution to the capacitive energy of the CBNO 350-RTA film from its intrinsic linear dielectric response [22]. Furthermore, the ratio of W bs /W rec gradually increased with the electric field, which can be attributed to an enhanced domain backswitching in the fine-grained CBNO 350-RTA film at the removal of a higher electric field [37,38]. Overall, the nanometer-sized, uniformly distributed grains formed after the RTA process, not only endow the CBNO 350-RTA film an enhanced breakdown field, but also reduce its remnant polarization. Both features help improve the film's energy storage performance.
As important parameters to evaluate the energy storage performance of dielectric capacitors, the room temperature dielectric constant, loss tangent, and leakage current density were measured for the CBNO A350 , CBNO 350-RTA , and CBNO A700 films, and the results are displayed in Figs. 4(a)-4(d). From Fig. 4(a), we can see that both  r and tanδ exhibited a weak frequency dependence for the CBNO A350 and CBNO 350-RTA films.
With an increasing frequency,  r slightly decreased from 208.2 (at 1 kHz) to 203.8 (at 1 MHz) for the CBNO 350-RTA film, together with a frequency-insensitive low dielectric loss (≤ 0.016). However, for the CBNO A700 film, its  r decreased from 151 (at 1 kHz) to 97 (at 1 MHz) and dielectric loss jumped from ~0.11 (at 1 kHz) to ~0.23 (at 1 MHz). The high dielectric loss indicates a leaky dielectric while its further increase with the measuring frequency in the 100 kHz-1 MHz range can be attributed to bulk dielectric relaxation processes, which are induced by its large amount of defects generated under a long exposure to a high processing temperature [38]. The dielectric constant is affected by many factors, including structural defects, phase composition, domain structure, crystallographic orientation, and grain size [39]. In this work, the higher dielectric constant of the CBNO 350-RTA film compared to that of CBNO A350 , can be attributed to its preferred (115)-orientation [20,24], as well as its improved crystallinity via RTA. While the lower dielectric constant in the CBNO A700 film than CBNO 350-RTA can be ascribed to its random crystallographic orientation.
The temperature dependent leakage current densities of the CBNO A350 , CBNO 350-RTA , and CBNO A700 films were shown in Figs. 4(b), 4(c), and 4(d), respectively. It can be seen that, the leakage current densities of CBNO A350 and CBNO 350-RTA increased monotonically with an increasing temperature, and they were about ~1×10 -5 A/cm 2 and ~1×10 -4 A/cm 2 respectively under 1 MV/cm at the highest testing temperature of 120 ℃. For the CBNO A700 film, all the leakage currents are very high, and the temperature dependence is rather weak except for an electric breakdown at 100 ℃. This is likely due to its large amount of existing defects which can be readily driven by an external electric field. Based on the above discussion, it can be deduced that both the dielectric and leakage characteristics of the CBNO 350-RTA film are desirable for energy storage applications.
Fatigue-resistance of the CBNO 350-RTA film was analyzed through a charge-discharge cycling test (Fig. 4(e)). No noticeable losses can be observed for www.springer.com/journal/40145 maximum applicable field), indicating an excellent cycling stability (W rec /W rec-1st circle , / 1st circle )/fatigueresistance. Lastly, W c , W rec , and  were measured in the temperature range of (-170)-150 ℃ , and the results are shown in Fig. 4(f). Very small fluctuations were observed for these three parameters (~3.2% for W c , ~6.9% for W rec , and ~9 % for ) in this broad temperature range. The decreases in W rec and  near T = 150 ℃ can be attributed to an enhanced electrical conduction, which is associated with the thermallyinduced movement of charged defects [5,40].

Conclusions
In a summary, CaBi 2 Nb 2 O 9 (CBNO) thin film capacitors with an enhanced energy storage performance were successfully prepared on SRO-buffered Pt(111)/Ti/ Si(100) substrates by adopting a two-step fabrication process. A low temperature sputter-deposition leads to a small grain size and lowers the electrical conduction, enhancing the breakdown field as well as reducing the remnant polarization P r . On the other hand, the RTA process can improve the crystalline quality of the as-grown film without overgrowing the grains, thus increasing the maximum polarization P m and recyclable energy density W rec . The resulting dense and nanograined ferroelectric film not only shows an enhanced breakdown field, but also a much-reduced P r and a delayed polarization saturation, leading to a significantly improved energy storage performance (W rec  69 J/cm 3 ,   82.4% at 3.55 MV/cm). The linear dielectric contribution plays a dominant role in the total recyclable energy density. Excellent fatigueresistance and thermal stability were observed in these CBNO films. This work reveals a novel processing route to produce high performance BLSF FE film capacitors for energy storage applications.