Energy Storage Characteristics of BiFeO₃/BaTiO₃ Bi-Layers Integrated on Si

Abstract

BiFeO₃/BaTiO₃ bi-layer thick films (~1 μm) were deposited on Pt/Ti/SiO₂/(100) Si substrates with LaNiO₃ buffer layers at 500 °C via a rf magnetron sputtering process. X-ray diffraction (XRD) analysis revealed that both BiFeO₃ and BaTiO₃ layers have a (00l) preferred orientation. The films showed a small remnant polarization (P_r ~ 7.8 μC/cm²) and a large saturated polarization (Ps ~ 65 μC/cm²), resulting in a slim polarization-electric field (P-E) hysteresis loop with improved energy storage characteristics (W_c = 71 J/cm³, η = 61%). The successful “slim-down” of the P-E loop from that of the pure BiFeO₃ film can be attributed to the competing effects of space charges and the interlayer charge coupling on charge transport of the bi-layer film. The accompanying electrical properties of the bi-layer films were measured and the results confirmed their good quality.

1. Introduction

Densities of electrical energy stored in or released from a dielectric can be calculated from their characteristic dielectric displacement-electric field (D-E) curves by the formula [1]:

$$W_c={\displaystyle {\int }_0^{D_s}EdD},W_d={\displaystyle {\int }_{D_r}^{D_s}EdD}$$

(1a)

$$\eta =\frac{W_d}{W_c},$$

(1b)

where W_c is the charged (stored) energy density, W_d is the discharged (released) energy density and η the energy efficiency. D_s and D_r are the saturated/maximum and remnant electrical displacements, respectively. For an ideal linear dielectric with a relative dielectric constant ε_r, D = ε₀ε_rE and D_r = 0, the energy densities are given by [1]:

$$W_c=\frac{1}{2}{\epsilon }_0{\epsilon }_r{E}_{\max }^2,$$

(2a)

$$W_d=W_c=\frac{1}{2}{\epsilon }_0{\epsilon }_r{E}_{\max }^2,$$

(2b)

and the efficiency η equals 1. Here ε₀ is the vacuum dielectric constant, E_max is the maximum applicable electric field which increases with the breakdown field E_b (E_max ≈ E_b for an ideal dielectric). On the other hand, non-linear dielectrics like ferroelectrics usually don’t have D ∝ E or D_r = 0, hence W_c > W_d and η < 1. Nevertheless, Equation (2a) can still be used for the estimation of energy densities W_c in non-linear dielectrics, wherein ε_r is the average or effective dielectric constant.

To meet the demands in developing portable and integrable power electronics, thin film ceramic capacitors with a high energy storage density W_c and a high efficiency η have been intensively investigated [2,3,4]. The previous studies on thin film ceramic capacitors have been focused on lead-containing perovskite ferroelectrics, including BNZ-PT, BNT-PT and BNH-PT [5,6,7]. P-E (*) curve of a typical ferroelectric is presented in Figure 1. In this figure, the green area represents the discharged energy density (W_d), the yellow area encircled by the hysteresis loop is the energy loss (W_loss), and the charged energy density is the sum of the two, W_c = W_d + W_loss. It’s obvious that the energy storage characteristics of a ferroelectric strongly depend on the shape of its P-E loop.

In the past decade, large energy storage densities (40 J/cm³–60 J/cm³) have been reported in lead-containing ferroelectric films together with good energy efficiencies (40%–60%) [6,7,8,9]. However, the growing environment concerns on lead and the usually complex compositions of the solid solutions being used are the two major drawbacks of the lead-containing ferroelectric capacitors. In order to minimize the environmental impact and promote a manufacturing-friendly preparation process, single component lead-free perovskites have been investigated by several research groups for applications in thin film dielectric capacitors [10,11,12,13,14]. Among them, BiFeO₃ and BaTiO₃ films are most popular candidates [13,14]. The former has a giant saturated polarization and tunable electrical properties via strain or chemical doping, while the latter has excellent dielectric properties (large ε_r and high E_b) and hence has been broadly used in multi-layer ceramic capacitors (MLCC). However, large leakage currents and poor energy efficiencies associated with BiFeO₃ thin films have hindered its further development for energy storage applications. On the other hand, a small saturated polarization of BaTiO₃ has capped its energy densities (see Equation (1a)) and limited most of its capacitor applications in MLCC.

According to Equation 1, dielectrics with high E_b and ε_r, a small remnant polarization and a large saturated polarization are ideal for high efficiency energy-storage applications. In this work, we demonstrate a novel approach to compensate for the lack of a single material with the above properties. We prepared ferroelectric bilayers of BiFeO₃/BaTiO₃ on Si substrates via an in-situ rf magnetron sputtering process at 500 °C. Textured growth of the bi-layer films at this moderate temperature was promoted by using a LaNiO₃ template layer. The resulted thin film capacitors of Au/BiFeO₃/BaTiO₃/LaNiO₃/Pt/Ti not only showed good dielectric properties (high ε_r and low leakage current), but also a slim P-E hysteresis featuring an enhanced energy storage density and efficiency, as compared with those of single layers of BiFeO₃ and BaTiO₃ on Si. The improved energy storage characteristics are interpreted based on our understanding of the charge transport process, which is dominated by the effect of space charges at low electric fields and by the interface charge coupling at high electric fields.

2. Experimental Section

BiFeO₃/BaTiO₃ bi-layer thick films were fabricated on Pt/Ti/SiO₂/(100)Si substrates buffered with a LaNiO₃ layer via a rf magnetron sputtering process. Commercially available ceramic targets of BaTiO₃, Bi_1.05FeO₃ and LaNiO₃ (4N purity, Ф = 50 mm, L = 5 mm) were used in the deposition process, which was carried out in a multi-target magnetron sputtering system with a base pressure of 2.0 × 10⁻⁴ Pa. Firstly, a 120-nm-thick Pt/Ti layer was deposited as the bottom electrode at 300 °C in a pure argon atmosphere. Then the LaNiO₃, BaTiO₃ and BiFeO₃ layers were sequentially sputtered at 500 °C in a mixed gas of Ar and O₂. The thickness of the BiFeO₃/BaTiO₃ bi-layer was about 1 μm with a 1:1 thickness ratio. Lastly, the as-deposited multi-layer thick film was cooled down at a rate of 6 °C/min–8 °C/min in pure oxygen. The deposition parameters of the sputtering process were summarized in

Table 1. Deposition Parameters of the Sputtering Process for BiFeO₃/BaTiO₃ Bi-layer Films on SiO₂/(100) Si Substrates.

Sputtering Parameters	BFO	BTO	LNO
Sputtering power (W)	100
Substrate temperature (°C)	500
Sputtering pressure (Pa)	1.2		0.3
Sputtering atmosphere	Ar + O2 (4:1 flow ratio)
Cooling atmosphere and pressure	Pure oxygen, 2.5 Pa
Pt/Ti sputtering parameters	300 °C, 0.3 Pa, 55 W, pure Ar

and the schematics of experimental procedures was shown in Figure 2. For electrical characterizations, circular Au top electrodes with a diameter of 200 μm were deposited at room temperature by using a shadow mask.

The phase structures and crystallographic orientations of the bilayer films were characterized by using X-ray diffraction (XRD) θ-2θ scans with a Ni-filtered Cu-Kα radiation resource (Dmax-rc, Monaghan, Ireland) and pole figures (R-156 Axis Spider, SmartLab^® Rigaku, Tokyo, Japan, 40 kV, 200 mA). A commercially available MicroNano D-5A Scanning Probe Microscope (SPM) (MicroNano, Shanghai, China) was used to analyze the surface morphology while the cross-sectional thin film morphology was analyzed by using a thermal field emission scanning electron microscope (SEM) (SU-70, HITACHI, Hitachi, Japan) The room temperature ferroelectric hysteresis loops (P-V) and leakage currents (I-V) were measured by using a Radiant Precision Premium II ferroelectric tester (Radiant Technology, Albuquerque, NM, USA). The dielectric properties were measured by using a high precision digital bridge (QuadTech 7600 Plus Precision LCR Meter, IET LABS, Inc., West Roxbury, MA, USA).

3. Results

3.1. Microstructures and Crystallographic Orientations

Figure 3a shows the XRD 2θ scan spectrum of the BiFeO₃/BaTiO₃ bilayer film grown on LaNiO₃/Pt/Ti/SiO₂/(100) Si, which is dominated by the preferred (00l) diffraction peaks of the bulk perovskite structures and does not show any crystalline impurities or secondary phases. A tiny amount of (110)-oriented BaTiO₃ grains and (100)-oriented tetragonal BiFeO₃ grains were detected, which can be attributed to the moderate growth temperature and the effect of residual stress [15,16]. In our previous work [17], it was revealed that a highly (00l)-oriented BaTiO₃ thin film can be grown on Pt/Ti/SiO₂/(100) Si substrate by using a LaNiO₃ buffer layer at 500 °C. Here it is confirmed that the presence of a (l00)-oriented LaNiO₃ buffer layer promoted the growth of a (00l) oriented BiFeO₃/BaTiO₃ bi-layer film.

To further investigate the crystallographic characteristics of the bi-layer film, pole figure analysis was carried out on the {001} BiFeO₃ reflection (2θ = 22.56°). During collection of the XRD signals, the sample was rotated by varying the tilt angle (0° < ψ < 70°) and the azimuthal angle (0° < φ < 360°) with respect to the scattering vector. The pole figure of (001)_BFO, as shown in Figure 3b, revealed a strong diffraction peak at the center (tilt angle ψ = 0°), and a set of 4-fold diffraction peaks at ψ ≈ 46°, corresponding to the {101} BiFeO₃ plane. This result confirmed the preferred (00l) orientation of the BiFeO₃ layer, as suggested by the result of the XRD 2θ scans.

Surface morphology of the BiFeO₃/BaTiO₃ bi-layer film is displayed in Figure 3c. It can be seen that the grains of the top BiFeO₃ layer are densely packed with an average size of ~200–300 nm. The surface roughness Ra was measured to be ~4.1 nm, which shows a significant reduction as compared with those of the single layer BiFeO₃ films (Ra is on the order of ~10 nm or above) [18,19,20,21]. The dense and smooth growth of the BiFeO₃ layer can be attributed to the good quality perovskite underlayers of LaNiO₃/BaTiO₃. In Figure 3d, the cross-sectional SEM image shows clean and sharp interfaces in the multi-layer thin film. The BiFeO₃ and BaTiO₃ layers were measured to be ~480 nm and ~500 nm, respectively.

3.2. Energy Storage Characteristics from the P-E Hysteresis Loop

The poling and depoling process of a dielectric under an external electric field simulates the charge-discharge process of a capacitor. Therefore, from the polarization–electric field (P-E) hysteresis loop, the energy storage density W_c and the energy efficiency η of the Au/BiFeO₃/BaTiO₃/LaNiO₃/Pt/Ti capacitors can be calculated by using Equation (1). Figure 4a displays the P-E hysteresis loop measured at a pseudo-static frequency of 1 kHz for the bi-layer film (under an applied electric field of 1940 kV/cm). A large maximum polarization (P_m ~ 65 μC/cm²) and a small remnant polarization (P_r ~ 7.8 μC/cm²) are simultaneously obtained, together with a reduced coercive field E_c (E_c ~ 152 kV/cm) as compared with that of pure BiFeO₃ film [22]. These features of the P-E curve ensure excellent energy storage characteristics of the film. When compared with pure BaTiO₃ films deposited under similar conditions [23], the energy storage density W_c of the bi-layer film reached 71 J/cm³ (E_max = 1940 kV/cm), a 100%–110% improvement, while the energy efficiency η stayed about the same level (~61% vs. 60%–70%). This improvement in W_c can be attributed to a much improved maximum polarization and a high dielectric strength in par with that of a pure BaTiO₃ film.

The “slim-down”of the P-E loop can be explained by the competition between the space charge effect and interlayer charge coupling in the bi-layer film. As demonstrated in our previous study, a space charge layer in BaTiO₃ near its interface with LaNiO₃ dominates the film’s electrical characteristics at a low electric field [24]. Basically, a depletion region with width ω forms at the BaTiO₃/LaNiO₃ interface upon the application of a small electric field. The width ω can be computed by [24]:

$$w=\sqrt{\frac{2\epsilon (V+V_{bi}^{*})}{q{N}_{eff}}},$$

(3)

where $V_{bi}^{*}$ is the modified built-in voltage, ε is the dielectric constant (ε = ε₀ε_r), N_eff is the space charge concentration, V is the applied electric voltage and q the electronic charge. The voltage V is concentrated across the depletion region and drives its expansion until the BaTiO₃ layer has been fully depleted. It has been shown that the existence and evolution of a depletion layer in a ferroelectric film will cause shrinking and tilting of its P-E loop, leading to reduced P_r and E_c [25]. On the other hand, after the BaTiO₃ layer has been completely depleted, a substantial amount of bound charges appear at the BiFeO₃/BaTiO₃ interface owing to the large difference in polarizations between the two layers. Hence the charge transport of the bi-layer film under a high electric field will be dominated by the interface charge coupling, which can be described by an added energy term $\frac{1}{2{\epsilon }_0}$α(1 − α)($\overrightarrow{P_1}$ − $\overrightarrow{P_2}$)² in the Landau-Ginzburg-Devonshire (LGD) thermodynamic potential (free energy density F) of the bilayer thick film [26]:

$$\mathrm{F}=(1-\alpha {\left)\right[\mathrm{F}}_1(\overrightarrow{P_1})-\overrightarrow{E}\cdot \overrightarrow{P_1}]+\alpha {[\mathrm{F}}_2(\overrightarrow{P_2})-\overrightarrow{E}\cdot \overrightarrow{P_2}]+\frac{1}{2{\epsilon }_0}\alpha (1-\alpha ){(\overrightarrow{P_1}-\overrightarrow{P_2})}^2$$

(4)

here α = h₂/h (h = h₁ + h₂) is the relative thickness of the second layer, $F_i$, $\overrightarrow{P_i}$ (i = 1, 2) are the bulk free energy density and polarization of the ith layer, and $\overrightarrow{E}$ is the applied electric field (see Figure 4b). The third term of the free energy density with the coefficient α(1 − α) expresses the energy of electrostatic interaction between the two layers, and it becomes dominant when the field is large enough to allow both layers become fully poled. In this case, the difference between the two polarizations diminishes and the film shows an “average” polarization. In our case, the polarization of the fully poled bi-layer film P_bi is close to that of BiFeO₃ due to the fact that P_BFO >> P_BTO. The computed P_bi by solving the equilibrium state of Equation (4) is close to 50 μC/cm² [21], fairly consistent with the observed P_m value if the linear contribution from dielectric susceptibility to the total polarization is deducted. Therefore, the enhanced energy storage capability of the BiFeO₃/BaTiO₃ bi-layer film can be attributed to a combination of space charge effect (dominant at low field, leading to small P_r and E_c) and effect of interlayer charge coupling (dominant at high field, leading to a large P_m).

Figure 5 a shows the leakage current density versus electric field (J-E) curve. At an electric field of 100 kV/cm (bias voltage of 10 V), the leakage current density of the bi-layer film is 5.0 × 10⁻⁶ A/cm². This is about an order of magnitude lower than those reported for single layer BiFeO₃ films grown on Si substrates [27,28]. The reduction in leakage current is in good agreement with the observed morphology change shown in AFM and SEM images, i.e., a densely and smoothly grown BiFeO₃ film was endowed with a much improved electrical resistivity [29]. In addition, an interface energy barrier φ_B between the BiFeO₃ (work function W ~ 4.7 eV) and BaTiO₃ (W ~ 4.0 eV) layers can also reduce the leakage current and hence allow the film to be exposed to a large electric field [30,31]. As a result, a high energy density is achieved in the BiFeO₃/BaTiO₃ bi-layer film.

Figure 5b displays frequency-dependent relative dielectric constant ε_r and loss tangent tanδ of the bi-layer film. It can be seen that the pseudo static ε_r is about 425 (@1 kHz), which is about 33% and 70% higher than those of the single layer BaTiO₃ and BFO films grown on Si, respectively (ε_r ~ 320 for BTO, ε_r ~ 250 for BFO) [22,32]. When the frequency increases from 1 kHz to 1 MHz, ε_r decreases from ~425 to ~380. On the other hand, tan δ varies between 2.4% and 8.4% in the same frequency range, similar to that of pure BTO films grown on Si. The enhanced dielectric constant ε_r contributes to the improved maximum polarization P_m, based on the relation P_m = D_m = P + ε_r × E_max, where P is the self-polarization and P ≈ P_r for ferroelectrics.

4. Conclusions

In this study, BiFeO₃/BaTiO₃ bi-layer thick films deposited on the LaNiO₃ -buffered Pt/Ti/SiO₂/(100) Si substrates display an enhanced energy density and charge-discharge efficiency (W_c = 71 J/cm³, η = 61%) as compared with those of the single layer films. This enhancement can be attributed to combined effects of space charges and the interlayer charge coupling. The bi-layer film exhibits a low leakage current density (5.0 × 10⁻⁶ A/cm² at 100 kV/cm) as a result of the dense and smooth film morphology achieved in the BFO layer, which is induced by the high quality underlayer of BaTiO₃/LaNiO₃. The relative dielectric constant ε_r is about 33% higher than that of the pure BaTiO₃ film and 70% higher than that of the pure BFO film. In conclusion, the BiFeO₃/BaTiO₃ bi-layer film shows excellent dielectric performance and energy storage characteristics, making this structure a promising candidate for applications in microelectronics as lead-free thin film ceramic capacitors.