Improved Tunability and Energy Storage Density Properties of Low-Loss, Lead-Free (Ba0.50Sr0.50)TiO3 and Ba(Zr0.15Ti0.85)O3 Bilayer Thin Film Stacks

Multilayer thin films of (Ba0.50Sr0.50)TiO3 (BST) and Ba(Zr0.15Ti0.85)O3 (BZT) were designed and grown using pulsed laser deposition technology. The periodic (BST/BZT)n thin films were deposited on Pt‹111›/SiO2/Si substrates. X-ray diffraction revealed the presence of a polycrystalline, perovskite structure corresponding to the bilayer thin film stacks. Scanning electron microscopy confirmed the multilayer structure without any interdiffusion across layers. It was also found that the dielectric and ferroelectric properties of the thin films were strongly influenced by the periodic heterostructures. The thin film stacks exhibited significantly higher tunability as compared with multilayer thin films grown on various single-crystal substrates such as LaAlO3, MgO and SrTiO3. Possible mechanisms explaining the other observed attributes such as improved dielectric properties and reduced leakage current are discussed. The effect of incorporating a comparatively lower-permittivity thin film in the multilayer stacks is presented. The observed properties of such multilayer structured films will aid in realizing low-loss and highly tunable applications.


Introduction
Dielectric capacitors play an important role in the development of the electrical engineering industry due to their high power density (up to 100 W/kg), faster charge-discharge capabilities, longer lifetime, and temperature-stable electrical properties, especially in advanced electronics and electrical power systems such as hybrid electric vehicles, high-frequency inverters, and power grids. 1,2 However, the achieved energy storage density of the dielectric capacitors is much lower than their electrochemical counterparts, such as Li-ion batteries and double layer supercapacitors. 3 Researchers are trying to improve the energy storage density of dielectric capacitors coupled with minimum dielectric loss over the past decades.
Tunable devices exhibit a decrease in capacitance with an increase in the applied electric field and thus find application in numerous areas including tunable filters, phase shifters, and voltage-controlled oscillators. (Ba 1−x Sr x )TiO 3 is an extensively studied material system for numerous highfrequency applications. 4 Several studies have been reported related to domain engineering, doping, modification of stress and strain in the films, and growing films of different Ba/Sr & (Ba + Sr)/Ti ratios to improve the film material properties. 5 Energy storage devices at different working temperatures can be fabricated using these materials owing to the materials' ability to attain high dielectric constant at desired temperatures by controlling the composition of the constituents. However, the relatively low breakdown strength as well as poor temperature stability near T C limits its application in dielectric capacitors. 6 Growing multilayered dielectric/ferroelectric thin films and superlattices is an effective way to improve properties of dielectric/ferroelectric thin films. A multilayer approach, in which the film is composed of alternating layers of different compositions or even different materials, appears to be very promising in optimizing the properties of materials. Pulsed laser deposition (PLD) is the most preferred method for deposition of ferroelectric multilayered thin films and superlattices to maintain stoichiometry. Bilayer thin film structures consisting of one layer of (Ba 0.50 Sr 0.50 )TiO (BST, a paraelectric composition) and another layer of (Ba 1−x Sr x )TiO 3 with a composition gradient, 7-9 multiferroic, 10 ferroelectric, 11,12 non-ferroelectric, 13,14 conducting perovskite, 13,15,16 and doped multilayers of BST thin films 17 have been reported. Another interesting sandwich structure of ferroelectric thin films gained much attention due to its improved electrical properties. 11,18-21 Zhao et al. studied stress at interfaces in a trilayered structure. 22 Multilayer structures resulted in better dielectric properties, decreased loss tangents, enhanced tunability, and improved fatigue-resistant properties. 10,23,24 Organic-inorganic (ferroelectric-polyvinylidene fluoride) composites displayed a low dielectric constant (~ 30) and were not resistant to high temperatures. 25 Reports on BST with another layer of material near the morphotropic phase boundary (MPB) are not reported in the literature. Compositions near the MPB exhibit superior properties due to contribution of all polar vectors of various coexisting phases. Hence, barium zirconium titanate Ba(Zr 0.15 Ti 0.85 )O 3 (BZT) is chosen as another layer in this bilayer pattern of the multilayer structure. The BZT material system exhibits several interesting features among the dielectric behaviour of doped BaTiO 3 materials. 26 BZT has attracted immense attention because of its potential applications in microwave technology, due to improved chemical stability, low dielectric loss and large tunability. 27 The substitution of Zr 4+ for Ti 4+ reduces average grain size and maintains the leakage current low and stable. In this report, the electric properties of the multilayer structure of (Ba 0.50 Sr 0.50 )TiO 3 and Ba(Zr 0.15 Ti 0.85 )O 3 thin films are studied.
The motivation of this article is to develop and characterize a multilayer (BST/BZT) n thin film structure which can exhibit high tunability coupled with low loss due to the incorporation of Zr in the lattice. The authors anticipate improving the efficiency of the energy storage performance of the multilayer structure by combining the high permittivity of the BST material system and the low-loss, highbreakdown characteristics of BZT materials.

Experimental Procedure
A multilayer structure consisting of bi-layers of (Ba 0.50 Sr 0.50 ) TiO 3 (BST) and Ba(Zr 0.15 Ti 0.85 )O 3 (BZT) was designed as shown in Fig. 1 and realized with the help of pulsed laser deposition (PLD) using two individual BZT and BST targets of 1-inch diameter. Preparation of targets was carried out by adopting solid state reaction method using a high-energy planetary ball mill. The calcined powders were compacted and sintered at 1350°C to obtain a density of > 98% of theoretical density. The platinized silicon substrates were loaded parallel to the surface of the targets in a PLD chamber at a distance of 5 cm. The substrate temperature is maintained at 650°C and the targets are ablated sequentially with an energy density of 1.5 J/cm 3 . The oxygen gas partial pressure is maintained at 200 mT throughout the deposition. The thickness is maintained equal for all different layers. The samples were annealed in an oxygen-rich atmosphere at 500°C. Full details of the preparation of targets 28 and film growth 29,30 were reported in our previous study. An x-ray diffractometer (Philips X'Pert PW-3020) with a monochromatic Cu-K α radiation (λ = 1.542 Å) was used over a 2θ range from 20° to 60° to characterize the crystalline phase of the (BST/BZT) n bilayer stack. Gold electrodes of 400 µm diameter are deposited on the multilayers by a tabletop sputtering system (Denton Vacuum Desk IV) using a grid mask for carrying out electrical characterization. The polarization versus electric field (P-E) hysteresis loops, leakage current characteristics and voltage-dependent capacitance curves of the multilayer thin films were recorded using a ferroelectric test system (M/s. aixAcct Systems, GmbH, Germany) which works in the virtual ground mode using a triangular waveform, at room temperature.

Results and Discussion
Crystal Structure Figure 2 shows the room-temperature x-ray diffractograms (XRD) of multilayer structure comprising of (BST/BZT) n bi-layers with n = 2. The multilayer thin films exhibit pure perovskite phase and no secondary phase is observed in the entire range of investigation. The characteristic single phase, randomly oriented polycrystalline growth can be observed from Fig. 2. The presence of both BST as well as BZT phases can be observed from the x-ray diffractograms. The phase purity and crystalline nature were confirmed from the measured intensities and the values of the full-width at half maximum (FWHM) of the diffraction peaks. We could not detect any reflection corresponding to any impurity phase in the XRD spectra, suggesting impurity free single phase growth of the multilayer thin films. The diffraction pattern also confirms that there was no measurable reaction between BST and BZT multilayers.
The cross section SEM images of multi layered (BST/ BZT) n stack with n = 2 is shown in the inset of Fig. 2. The total thickness of the multilayer structure as found from SEM, matches with the surface profiler results. It is also observed from Fig

Dielectric Properties
The electric-field-dependent dielectric constant and dielectric loss characteristics measured at 1 MHz at room temperature corresponding to the multilayer structure of (BST/ BZT) n with n = 2 fabricated by PLD are shown in Fig. 3. The contribution of the dielectric constant corresponding to the BST/BZT multilayer structure is due not only to the contribution as predicted by the Maxwell-Wagner model, 7 but also from other contributions including the strain-induced polarization, mismatch of lattice parameters of individual layers, 31,32 and the contribution of co-existent phases present in the MPB composition corresponding to BZT thin films. 27 The effective capacitance of a multilayer structure can be obtained by considering the capacitances of all constituent thin films. Hence, the effective capacitance, C eff , can be derived by considering the series combination of BST and BZT films using Eq. 1 where t BST and t BZT are thicknesses of the BST and BZT thin films, respectively. In this study the thickness of BST and BZT thin films is kept constant. The dielectric constant of BST thin films is greater than the dielectric constant of BZT thin films, ε BST > ε BZT . 33 Hence, Eq. 1 demonstrates that the effective dielectric constant, ε eff lies between the dielectric constant of individual multi layers BST and BZT thin films  i.e., ε BST > ε eff > ε BZT . Similar results were reported for compositionally graded multilayer thin films. 7 The electric field dependence of the dielectric constant of the bilayer structure shown in Fig. 3 highlights the paraelectric nature of the stacked (BST/BZT) n thin films. In addition, dielectric loss (tanδ) is also an important factor for capacitors. The dielectric loss of the multilayer structure is found to be < 0.05 throughout the measured range of applied electric fields, i.e., from -300 kV/cm to 300 kV/cm. The uniform low dielectric loss is another notable feature of the stacked structure of thin films. However, at relatively higher fields (> 200 kV/cm), non-uniform dielectric loss with respect to the direction of applied fields can be due to parasitic capacitance, unusual charge accumulation near the top BZT layer and electrode interface. The improved dielectric loss is due to defect trapping at interfaces or immobilization of defects to compensate polarization difference between thin film layers, 8 incorporation of chemically stable Zr +4 ion in BZT thin film and also due to minimised lattice mismatch between different layers of the fabricated structure such as BST and the Pt (111) substrate, BZT and BST thin films. Several challenges involved in the process optimization including obtaining a sharp interface between different layers, uniform growth of thin films, and sequential and periodic selection of target materials were addressed, thereby realizing a uniform multilayered stack. All of these require careful optimization of the PLD deposition parameters like laser energy fluence, repetition rate, target-substrate distance, oxygen partial pressure, deposition temperature, and thickness control. Each of these process parameters was carefully taken into account in this research. The uniform layer thicknesses in the stack led to consistent electrical properties across the sample.

Tunability Studies
The ability of a material to change its capacitance with increase in applied voltage is termed tunability and is determined by the ratio of change in capacitance on application of applied voltage to the capacitance at zero bias voltage i.e., (C 0 − C V )/C 0 where C 0 and C V are capacitance at zero voltage and applied voltage, respectively. Tunability is an essential parameter in understanding the behaviour of a material at high voltages. The capacitance versus electric field loops corresponding to (BST/BZT) n thin films are shown in Fig. 4. The zero-field capacitance at 1 MHz is found to be > 1.3 nF and is found to decrease with rise in applied field. Such high capacitance values are ascribed to inhomogeneous dielectric structure and resistivity of the bilayer thin film stack. The tunability of multilayer thin films is found to be 70% at an applied electric field of 300 kV/cm and there was also no evidence of saturation in tunability up to the maximum measured bias voltage. Therefore, higher tunability can be expected by further increasing the bias voltage. The observed tunability values are better than epitaxial films of lead-free (Ba 1−x Sr x )TiO 3 and/or Ba(Zr 1−y Ti y )O 3 multilayer structures grown on single-crystal substrates such as LaAlO 3 , 16 MgO 34 and SrTiO 3 35 and other polycrystalline heterostructure multilayer thin films. 7,9,[11][12][13][14][15][18][19][20][21]36 The ability to withstand high voltages without breaking down indicates the appearance of dielectric enhancement in multilayer heterogeneous structures. The enhancement can be explained by considering the Maxwell-Wagner series capacitor model 7 and is ascribed to stress between different layers caused by lattice mismatch, which is often discussed in super-lattice structures. 37 Layered structures have a strong influence on the electric field distribution. The relationship between field distribution and permittivity in a layered composite system 38 consisting of different layers possessing different permittivities, including ε BST , ε BZT and thicknesses, d BST and d BZT , respectively, can be evaluated using Eq. 2: where E BST and E BZT are the theoretical electric fields of BST and BZT films, respectively. It can be seen from Eq. 2 that the dielectric layers with lower permittivity (BZT thin films in this case) will experience higher electric fields than the applied field. The higher electric field assists in obtaining enhanced tunability. Hence, the BZT thin film in the multilayer structure studied is responsible for obtaining low loss and improved tunability. Space charges accumulate at the interface between the BST and BZT layers, forming low-resistance interfacial regions 39 resulting in enhanced properties.
(2) The C-V loops recorded at room temperature illustrates that the multilayer stack of BST/BZT thin films is in the paraelectric state with a weak hysteresis behaviour. Johnson reported phenomenological theory 40 explaining the explicit relationship between the dielectric constant and the applied electric field as below: where ε r(0) and ε r(E) are the dielectric constants at zero electric field and applied electric field E, respectively, and α is the temperature-dependent constant, which provides information on the degree of anharmonic contributions of the polarization to the free energy. 41 As shown in Fig. 4, the experimental data is fitted using Eq. 3. It can be seen that the multilayer films exhibit a paraelectric state, and the ε versus E characteristics can be fitted exactly over the whole electric field range from zero to the maximum applied positive voltage and back to zero (positive cycle), and similarly in the negative cycle from zero to the maximum negative voltage. The deviation of the experimental data from the fitting curve is found to be less than 1%. The agreement between the experimental data and the fitting lines of the BST/BZT multilayer structure is quite good, indicating the self-consistency of that model. The tunability values for the multilayer thin films grown on different substrates using different methods are compiled and shown in Table I. It can be seen that the obtained tunability values of the (BST/BZT) n bilayer stack of multilayer thin films are higher than the reported values for various multilayer thin films, 7,9,[11][12][13][14]17,20 including the heterostructure thin films grown on single-crystal substrates. 34,36 The tunability of multilayer thin films is found to be superior to that of the individual BST films grown on (100)-oriented STO substrates 42 and BZT films grown on La 0.7 Ca 0.3 MnO 3 , 43 and other substrates. 44,45 A tunability of 59% corresponding to textured BZT thin films was reported. 46 The gradual change in the lattice parameters of the bilayer thin film stack, composition near MPB, and low dielectric loss resulting from the incorporation of the BZT layer are considered as factors contributing to the observed enhanced tunability.

Field-Dependent Polarization Curves
The room-temperature polarization-electric field (P-E) curves of BST/BZT multilayer thin film structure measured at 100 kHz frequency is shown in Fig. 5. The onset of saturation in polarization at high fields can be observed from P-E loops of bilayer film stacks consisting of a paraelectric layer. The multilayer structure influences not only on the ionic polarization but also the electronic structure or chemical bonding nature of the bilayer stack of BST/BZT thin films. The results suggest that the multilayer thin films exhibit linear dependence on applied voltage to a considerable range. Ferroelectric saturation can be observed from Fig. 5 at high electric fields measured at room temperature. Such behaviour is attributed to B-site cation movements contributed by different phases of polar vectors that are present at MPB composition in BZT thin films, due to the interfacial coupling of electrostatic and elastic interactions. 23

Energy Storage Properties
The maximum polarization (P max ) of (BST/BZT) n multilayer thin films show strong dependence on the order of layers. The P-E loops illustrate the improved energy storage density (ESD) properties of multilayer structures. As shown in the Fig. 6, the area enclosed by the discharge curve with respect to the polarization axis represents the recoverable energy density, U rec , and the area enclosed between the charge and discharge curves represents energy loss, U loss are shown in Fig. 6. It is obvious that high P max and narrow P-E loops will be favourable for energy storage. Generally, the value of energy storage density, U C can be calculated from P-E loops using the following equation: where E max is the maximum applied field. It is also found that with an increase in number of bilayer stacks, maximum polarization also increases and thus results in improved ESD properties of the multilayer thin films structure. The calculated energy storage density of the multilayer thin films stack using Eq. 4 is found to be 2.67 J/cm 3 at relatively low electric field of 300 kV/cm. The (BST/BZT) n multilayer films were found to exhibit high breakdown strength and moderate maximum polarization, which is favourable for high energy storage density.
In actual applications, energy storage efficiency (η) is another very important parameter which can be defined as the ratio of the recoverable energy density, U rec and the total energy density, U as follows : The energy storage efficiencies obtained using Eq. 5 in this study (> 75% at an electric field of 300 kV/cm) is higher than the reported values. 47 Absence of saturation in P-E loops indicates that enhanced energy storage density values can be realized with further rise in electric field. Improved efficiency, electrical break down strength and dielectric loss were reported with increase in number of multilayers. 35,48 The heterostructure multilayer thin films of BST/BZT bilayer stack resulted in improved efficiency and low dielectric loss by restricting flow of leakage current.
The propagation of electric trees is shown in Fig. 7 when the multilayer thin film stack is subjected to high voltages. At such high voltages, the leakage current path propagates and thus leads to electric breakdown of the film. In this study, the bilayers of (BST/BZT) n thin films are repeated so as to obtain low loss and also to terminate the current carrying path by BZT layers. With the same total thickness, the breakdown electric field, E B and (5) = U rec. U rec. + U loss Fig. 5 Evolution of ferroelectric hysteresis loops with respect to the electric field corresponding to lead-free multilayer thin films. Fig. 6 Polarization versus bias voltage loops highlighting the recoverable energy density, U rec and energy loss, U loss . energy storage density of the multilayers can be significantly improved by increasing the number of interfaces. 48 From Fig. 7a it can be seen that the leakage current propagates due to the absence of layers that terminate the conduction path whereas in Fig. 7b the BZT layers exhibit an obstruction behaviour at the interfaces to the electrical trees. It is reported that BST thin film layers create a low oxygen vacancy concentration and high energy barrier for oxygen vacancies, 10 both of which are known to deteriorate the films' properties. In this study, the deposited multilayer thin films are subjected to post-annealing in an oxygen-rich atmosphere. In situ oxygen annealing of the as-grown multilayer thin films is also believed to reduce the oxygen vacancy concentration. 30 The MPB composition of the BZT thin films combined with the paraelectric BST thin films resulted in enhanced tunability for realizing tunable applications.

Conclusions
The effects of a bilayer stack of (Ba 0.50 Sr 0.50 )TiO 3 (BST) and Ba(Zr 0.15 Ti 0.85 )O 3 (BZT) thin films deposited on Pt‹111›/ SiO 2 /Si substrates using pulsed laser deposition technology on the dielectric properties and energy storage behaviour have been investigated. X-ray diffraction revealed the presence of a polycrystalline, perovskite structure corresponding to multilayers of BST/BZT thin films. SEM analysis confirmed the multilayer structure without any interdiffusion across layers. It was also found that the dielectric and ferroelectric properties of the thin films were strongly influenced by the periodic heterostructures. (BST/BZT) n films exhibited significantly high tunability of ~ 70% as compared with multilayer thin films grown on different single-crystal substrates such as LaAlO 3 , MgO and SrTiO 3 . Possible mechanisms explaining the other observed attributes, such as improved dielectric properties and reduced leakage current, were also explained. The heterostructure multilayers with compositions at the MPB assisted in realizing enhanced energy storage density efficiency of > 75%. The observed properties of such multilayer structured films help in realizing low-loss and tunable applications.