Ultrahigh capacitive energy storage in highly oriented Ba(Zr_xTi_1-x)O₃ thin films prepared by pulsed laser deposition Free

Future use of electricity generated from any renewable energy source will benefit greatly from advanced approaches to scalable capacitive energy storage module with a favorable figure for the power-to-mass ratio (specific power) including its usage in personal electronics, entertainment, and homeland security. The capacitive energy storage in dielectrics has been in existence for several decades allowing rapid charge storage and discharge.^1–3 Utilizing right ferroelectric material compositions with appropriate architecture, we can achieve large energy densities reducing overall weight and volume. To achieve this goal, we need to design novel energy materials and their thin film geometries that will withstand high electric fields (≥1 MV/cm) and maintain a high dielectric constant (>1000 s). The maximum amount of energy (⁠$UST$⁠) that can be stored per unit volume in ideal linear dielectric materials configured in a metal-insulator-metal capacitor structure is given by the relation, $UST=CV2/2=ε0εrEBD2/2$⁠, where $C$ is the capacitance, $V$ is the voltage, $EBD$ is the breakdown electric field strength (⁠$E=V/d$⁠, where $d$ is the thickness of the dielectric), $ε0$ is the vacuum permittivity, and $εr$ is the relative permittivity of the dielectric material being investigated. From the above equation, it is evident that the development of materials with extremely high dielectric constant and high breakdown field are the two primary requirements toward achieving high energy storage performances.

The maximum energy density that can be stored in a capacitor is not only a function of material composition but also a function of the geometry utilized in the device. A higher energy density has largely been achieved by increasing the electric field (⁠$E=V/d$⁠) in the dielectric.^4,5 In the thin film form, the dielectric strength increases considerably, partially due to one dimension being at nanoscale which allows a better heat dissipation, keeps the trapping rate below the detrapping rate, and decreases the probability of finding critical defect concentrations.^6,7 The nanoscale dimension may increase Young's modulus (⁠$Y$⁠) as the layer becomes thinner and results in the elevated breakdown electric field as this parameter follows the relation, $EBD∝Yn$⁠, where the exponent $n$ varies with thickness.⁸ Basically, dielectric breakdown can be caused by a number of successive physical processes such as avalanche and field emission break, thermal breakdown produced by increasing conductance from Joule heating, an electromechanical collapse brought on by the attractive forces between electrodes, electrochemical deterioration, dendrite formation, etc. Out of all these, a major breakdown initiator is the avalanche emission (caused by the electronic charge injection from the cathode to the conduction band of the dielectric material and subsequent ionization of atoms to produce more and more electrons until conduction occurs through the bulk),⁹ wherein an inverse exponential theoretical dependence on the dielectric thickness is expected, $EBD$ ∝ $e−kd$⁠, where $k$ is a constant. Decreasing the thickness (⁠$d$⁠) of the active dielectric layer can serve to both increase capacitance and miniaturize device dimension and is therefore a very attractive approach for low-voltage electronic applications.

Recently, energy storage capacitor and power generation research attention has turned towards lead-free and environmentally friendly materials. Barium titanate (BaTiO₃) based ferroelectric materials are attractive because of their high dielectric constant, polarization, and high piezoelectric properties.^10–12 Among the doped BaTiO₃ systems, BaZr_xTi_1−xO₃ (BZT)^13–18 solid solutions have received renewed attention due to their high-strain level and high piezoelectric properties in both bulk and thin film forms. BaZr_xTi_1−xO₃ exhibits a pinched phase transition at x ∼ 0.15, i.e., all the three phase transitions that correspond to pure BaTiO₃ are merged or pinched into one broad peak. With a further increase in the Zr concentration, a typical ferroelectric relaxor behavior has been observed.¹⁷ Previous reports have shown the polar-cluster-like behavior in BZT solid solution having a higher concentration of Zr, 0.85 ≤ x ≤ 1.00.^19,20 BZT compositions with higher concentrations of the non-ferroactive dopant ions Zr⁴⁺ form disordered ferroelectric materials, in which polar-nanoregions (PNRs) are stabilized on cooling by random electric fields due to either charge or structural disorder.²¹ 

BaZr_xTi_1−xO₃ (BZT30, BZT40, and BZT50 for x = 0.3, 0.4, and 0.5, respectively) thin films of about 400 nm thickness were prepared by the pulsed laser deposition (PLD) technique. Barium carbonate (BaCO₃) (99%), zirconium (IV) oxide (ZrO₂) (99.7%), and titanium (IV) oxide (TiO₂) (99.8%) were used as precursors to synthesize PLD targets by solid state reaction. We used an excimer laser (KrF, 248 nm) operating at a pulse frequency of 10 Hz for BaZr_xTi_1-xO₃/La_0.7Sr_0.3MnO₃ (LSMO) heterostructure thin film fabrication on MgO (100) substrates kept at 650 °C. The LSMO layer was crystallized in situ at 725 °C in 300 mTorr of oxygen for 60 min and cooled down slowly to room temperature to form a highly oriented bottom electrode. During laser ablation of BZT on LSMO coated MgO at 650 °C, a laser energy of 250 mJ was used and an oxygen process pressure of ∼150 mTorr was preserved inside the chamber. The BZT thin films were crystallized by in situ post deposition anneal at 650 °C for 60 min in 300 mTorr of oxygen and were cooled down slowly to room temperature to form preferentially (100)-oriented BZT/LSMO heterostructures. The phase formation and crystallographic structure of the samples were confirmed with X-ray diffraction patterns, using an X-ray diffractometer (Rigaku Ultima III) operating in the Bragg-Brentano geometry outputting CuKα radiation at a wavelength of λ = 1.5404 Å. The Raman measurements were performed using an ISA T64000 triple monochromator. For electrical measurements, platinum (Pt) top electrodes of ∼80 μm diameter were sputter deposited through a shadow mask onto the BZT thin films to form Pt/BZT/LSMO metal-insulator-metal (MIM) capacitors. The dielectric constant of the samples was measured using an HP4294 inductance capacitance resistance (LCR) meter from 100 Hz to 1 MHz in the temperature range of 100–450 K at a 5 K/min cooling/heating rate. The remanent polarization and coercive field of the ferroelectric capacitors were measured using a Sawyer Tower test configuration (Radiant Technologies).

X-ray diffraction (XRD) patterns (Fig. S1 in the supplementary material) showed that BZT thin films have a highly (100) textured perovskite tetragonal crystal structure with reduced tetragonality, c/a, that is characterized by peak (002)/(200) splitting at 2θ ∼ 45° as shown in the inset of Fig. S1 (supplementary material).^22,23 It can be seen that the Bragg peak positions shift towards lower angles with an increase in % Zr content suggestive of an increase in the crystal unit cell size with the increase in Zr content incorporation as Zr is having an ionic radius of 0.98 Å greater than that of Ti, 0.72 Å. Raman-active optical modes for BaTiO₃ in the tetragonal phase with the $P4mm$ spacegroup are 3 A₁(TO) + 3 A₁(LO) + 4 E(TO) + 4 E(LO) + 1 B₁.²⁴ The room temperature Raman spectrum of all BZT compositions displayed in Fig. S3 (supplementary material) shows broad and mixed phonon modes at 160 cm⁻¹ [E(TO₂) + E(LO₁) + A₁(TO₁) + A₁(LO₁)], 295 cm⁻¹ [E(TO₃) + E(LO₂) + B₁], 515 cm⁻¹ [E(TO₄) + A₁(TO₃)], and 728 cm⁻¹ [E(LO₄) + A₁(LO₃)]. The observed decrease in phonon mode energies and weakening of mode intensities as the Zr content increases (force constant decreases) may be considered as an indication of the incorporation of chemically more stable Zr⁴⁺ at the Ti⁴⁺ site of the polar BaTiO₃ matrix and subsequent disruption of the long-range ferroelectric ordering and disordered cubic structure formation as evident from too weak Raman scattering displayed by BZT50. The dip observed at 125 cm⁻¹ represents B-site cation ordering and is an indication of the expected relaxor behavior.²⁵ 

The room temperature frequency (10²–10⁶ Hz) dependence of the dielectric constant and loss tangent of Pt/BZT/LSMO capacitors is shown in Fig. 1. At low frequencies (<10 kHz), changes in the dielectric constant are small for BZT30, whereas for BZT40 and BZT50, the dielectric constant remains almost constant. The observed decrease in the dielectric constant (>10 kHz) occurs due to the fact that the polarization does not arise instantaneously with the application of the electric field as mainly the relaxation of the dipole polarization commences. This type of frequency dependent dielectric behavior has been found in many ferroelectric ceramics.^26,27 We obtained a higher dielectric constant for BZT30 than for BZT40 and BZT50; this may be because of the micro-regions in BZT30 contributing to the rise in permittivity due to internal electric field distribution, while in BZT40 and BZT50, these micro-regions are substantially reduced when more disorder is introduced due to the higher concentration of Zr ions. The obtained high permittivity of the BZT attributed to the dipole polarization of PNRs makes it suitable for multilayer ceramic capacitor (MLCC) applications.²⁸ This enhanced permittivity might be correlated with the fact that the ionic polarizability of Zr⁴⁺ ions is higher than that of Ti⁴⁺ ions based on the dielectric polarizability theory.²⁹ On the other hand, dielectric loss increases as the frequency increases. For low frequencies (<1 kHz), we can see low dielectric losses (∼0.025) for all three compositions. The values of tan δ were heavily dependent on the Zr content for frequencies >1 kHz. For example, the ε_r values at 10 kHz are 3446, 1971, and 1406, while the tan δ values are 0.115, 0.033, and 0.017 for BZT30, BZT40, and BZT50 thin films, respectively.

A diffuse phase transition (DPT) is present in materials where the composition fluctuations lead to large fluctuations in the Curie temperature. A DPT is generally characterized by (a) broadening in the dielectric constant (ε) versus temperature (T) curve, (b) a relatively large separation (in temperature) between the maximum of the real (dielectric constant) and imaginary (dielectric loss) parts of the dielectric spectrum, (c) a deviation from the Curie–Weiss law in the vicinity of T_m (transition temperature), and (d) frequency dispersion of both ε and tan δ in the transition region, thereby implying a frequency dependence of T_m.^23,30,31 It is known that above the Curie temperature, the dielectric permittivity of a normal ferroelectric follows the Curie–Weiss law formulated as $1ε=(T−Tm)C,T≥Tm$⁠, where T_m is the Curie–Weiss temperature and C is the Curie–Weiss constant. For BZT thin films, the dielectric constant (ε) was fitted to the Curie–Weiss law (solid line in Fig. 2) by plotting the inverse dielectric constant (at 10 kHz) against temperature. It can be seen that a deviation from the Curie–Weiss law starting at T_cw becomes more evident as the Zr content decreases. The Curie–Weiss law parameters determined by modeling the experimental data are listed in Table I. The parameter ΔT_m, which is used often to describe the degree of the deviation from the Curie–Weiss law, is defined as ΔT_m= T_cw − T_m, where T_cw describes the temperature from which the permittivity starts to deviate from the Curie–Weiss law and T_m is the temperature that corresponds to the dielectric constant maxima. For BZT30, BZT40, and BZT50, the values of ΔT_m are 60, 170, and 220 K, respectively, as shown in Table I. The values of ΔT_m increase when the Zr concentration increases, which provides evidence of a composition-induced diffuse phase transition behavior in the BZT thin films with 0.30 ≤ x ≤ 0.50. Uchino and Nomura³² proposed a modified empirical expression to describe the diffuseness of ferroelectric phase transition as $1ε−1εm=(T−Tm)γC′$⁠, where γ is the degree of relaxation and C′ is assumed to be constant. The parameter γ gives information on the character of the phase transition: for γ = 1, a normal Curie–Weiss law is obtained which is valid for a perfect ferroelectric, γ = 2 describes a complete diffuse phase transition for a perfect relaxor material,^23,33 and the γ value between 1 and 2 corresponds to the incomplete diffuse phase transition where the correlated ferroelectric clusters are hypothesized. The inset of Fig. 2 shows the linear relationship between ln (1/ε − 1/ε_m) as a function of ln (T − T_m) for the three samples. The slope of the linear fit of the curves is used to determine the γ values as 1.83, 2.02, and 1.85 for BZT30, BZT40, and BZT50 thin films, respectively, and shows a strong diffuse phase transition or relaxor behavior.

Figure 3 shows the non-saturated slim room temperature hysteresis loops for all BZT thin films measured in Pt/BZT/LSMO capacitor geometry that sustained high electric fields of ∼3 MV/cm at a frequency of 10 kHz. The observed behavior in this slimmer P-E loop possibly indicates that the size of the polar regions is smaller than the dimension of the micron sized domains in the thin films, but it is large enough to have significant dipolar cooperation among neighboring unit cells to present polarization hysteresis.³⁴ As zirconium (Zr) and titanium (Ti) have the same valency and only differ in their ionic radius (R_Zr⁴⁺= 0.98 Å, R_Ti⁴⁺= 0.72 Å), it is expected that the addition of small amounts of zirconium in barium titanate does not generate sufficient disorder. However, when adequate quantities of zirconium (0.30 ≤x ≤0.50 in this study) are added, an interaction of polar and non-polar regions is generated resulting in the relaxor behavior with low coercive fields (∼0.28 MV/cm) indicating that the thin films are soft electrically. The remanent polarization (P_r) decreases gradually from 39 μC/cm² to 33 μC/cm² and then to 31 μC/cm² with the increase in the Zr content (see Table II). This reduction in the remanent polarization of BZT thin films can be explained by the difference of the radius of Zr⁴⁺ and Ti⁴⁺.

The stored electrostatic energy densities (⁠$UST=∫0PMAXEdP$⁠, where $PMAX$ is the charge density or the polarization measured at the maximum electric field $EMAX$ represented by regions I + II in the inset of Fig. 3), recoverable energy densities (⁠$URE=∫PrPMAXEdP$⁠, region I in the inset of Fig. 3), and charge-discharge efficiency (⁠$η=(URE/UST)×100$⁠) determined for Pt/BZT/LSMO capacitors are given in Table II. It was found that energy density increased with the increase in the electric field and decrease in the Zr content for all BaZr_xTi_1−xO₃ (0.30 ≤ x ≤ 0.50) thin films. In particular, $URE$ was found to increase with the decreasing Zr content from 144 ± 0.7 J/cm³ at x = 0.50 to 156 ± 0.7 J/cm³ at x = 0.30. These ultrahigh energy density values suggest the extremely high potential of BZT thin films to store electrical energy. The charge-discharge efficiency of these thin films varied from 72.8 ± 0.6% to 82.4 ± 1.0% at an electric field around 3.0 MV/cm, and these figures are very high compared to reports on different compositions.^35–37 In our current work, the successive evolution of relaxation in the BaZr_xTi_1-xO₃ (BZT) thin films has been hypothesized as being due to the increasing amount of ordering and density of nanosize Ti⁴⁺ rich polar regions in the Zr⁴⁺ rich matrix as Ti⁴⁺ is progressively incorporated in the BaZrO₃ lattice. It is also seen that remanent polarization decreases with the increase in the Zr content. This could be due to the fact that the Zr content can form a BaZrO₃ cubic structure, and therefore, less polar regions can be found in the film having low remanent polarization.

We have fabricated (100) highly textured BaZr_xTi_1−xO₃ (0.30 ≤ x ≤ 0.50) thin films on LSMO/MgO substrates utilizing the optimized PLD process and demonstrated them as potential lead-free capacitive energy storage materials for scalable electronic devices in terms of their structural, optical, temperature and frequency dependent dielectric, and ferroelectric charge storage properties. Ultrahigh stored and recoverable electrostatic energy densities as high as 214 ± 1 and 156 ± 1 J/cm³, respectively, were achieved at a sustained high electric field of ∼3 MV/cm with an efficiency of 72.8 ± 0.6% in an optimum 30% Zr substituted BaTiO₃ composition.

See supplementary material for XRD, AFM, and Raman data of the studied BZT samples.

Financial support from DOD Grant No. FA9550-16-1-0295 is acknowledged. S.P.P. gratefully acknowledges the financial assistance from IFN-NSF under the Grant No. EPS-1002410.