Epitaxial growth and dielectric characterization of atomically smooth 0.5Ba(Zr_0.2Ti_0.8)O₃–0.5(Ba_0.7Ca_0.3)TiO₃ thin films

Ferroelectric thin films have attracted attention for nonvolatile memories, actuators, transducers, and electro-optic devices. Large ferroelectric, piezoelectric, and electro-optic properties are characteristics of complex oxides with perovskite structures such as PbTiO₃, LiNbO₃, and BaTiO₃.^1–4 Recent investigations on solid solutions of BaTiO₃, specifically 0.5 Ba(Zr_0.2Ti_0.8)O₃–0.5 (Ba_0.7Ca_0.3)TiO₃ (BCZT-50)—a composition near the morphotropic phase boundary,⁵ have shown a high dielectric constant of 2800, a piezoelectric coefficient d₃₃ of 500 pC/N, and a high effective electro-optic coefficient r_c of 530 pm/V.^6–10 With the growing need for on-chip, lossless conversion of electrical to optical signals, especially for visible light, electro-optic modulation is widely explored as a viable solution. For electro-optic applications, one would use the electro-optic oxides as optical waveguides that can act as light modulators. Hence, epitaxial ferroelectric films with low propagation loss and large electro-optic coefficients are critical for achieving on-chip modulators.¹¹ 

Synthesis of BCZT thin films has been explored by a variety of methods such as sputtering, chemical solution deposition, screen printing, and pulsed laser deposition.^12–16 However, few reports have investigated the epitaxial growth of BCZT thin films. Among the epitaxial growth methods for such ferroelectric alloy thin films, pulsed laser deposition remains one of the facile and reliable approaches. As the performance of nonlinear optical materials is often determined by the defects, it is necessary to carefully study the processing-defect relationships for enabling a high performance electro-optic modulator.¹⁷ Past studies have explored the effect of growth conditions and thickness on the structure of BCZT thin films on platinized silicon or SrTiO₃ substrates.^18–20 Nevertheless, the large mismatch or lack of epitaxial matching has hindered the growth of high quality BCZT films.

Early studies of optical loss on thin film ferroelectric waveguides indicated that scattering by surfaces was the major loss mechanism.²¹ Optical losses by scattering and absorption should be low (less than 1 dB/cm) for most applications. A root mean square (rms) roughness of 1 nm can lead to significant scattering of 3 dB/cm.²² Thus, surface and interface smoothness at the atomic level is critical for low loss operation of electro-optic thin films. Nevertheless, the as-deposited films often display appreciable surface roughness mainly due to a large lattice mismatch of the films and substrates, and the uncontrolled domain structure. To overcome these issues, we selected GdScO₃ as a substrate to achieve epitaxial thin films of BCZT with low lattice mismatch. The lattice mismatch $m=(dfilm−dsubstrate)/dsubstrate$ between orthorhombic GdScO₃ (a_pseudocubic = 3.968 Å at 300 K) and tetragonal BCZT-50 (a = 4.003 Å, c = 4.021 Å at 300 K) is as small as 0.9%.⁸ 

In this paper, we report the growth of atomically smooth BCZT-50 thin films on the (110) GdScO₃ substrate. We used a conductive SrRuO₃ layer as a bottom electrode to screen the surface charges induced by the polarization in the BCZT films.²³ We carried out systematic studies by varying the substrate temperature and the oxygen partial pressure to understand their effects on the structure of BCZT-50 films and achieve the optimal conditions for high quality films. We performed in situ structural analyses using reflection high energy electron diffraction (RHEED), and ex situ studies using atomic force microscopy (AFM), high-resolution x-ray diffraction (HRXRD), and scanning transmission electron microscopy (STEM) to study the surface topography and crystal structure of the films. Finally, we studied the ferroelectric properties of BCZT-50 thin films in the out-of-plane geometry.

The BCZT-50 films were prepared by the pulsed laser deposition method using a 248 nm KrF excimer laser. We carried out the growth using a dense, the polycrystalline ceramic target of BCZT-50 prepared by the solid state reaction. GdScO₃ (110) single crystal substrates were preannealed in an atmosphere of high purity oxygen at 1100 °C for 3 h and then cleaned sequentially in acetone, and isopropanol alcohol prior to the deposition. The chamber was evacuated to a base pressure of 10⁻⁷Torr before the introduction of high-purity oxygen. The growth was preceded by a preablation of the target with 500 laser pulses under the deposition pressure. The laser fluence for the growth was fixed at 1.5 J/cm² for all the studies reported here. First, a thin layer of SrRuO₃ was deposited on the GdScO₃ substrate to serve as a bottom electrode at the same deposition temperature and oxygen pressure used for BCZT-50. The BCZT-50 films were subsequently deposited on SrRuO₃ coated GdScO₃ from a dense, polycrystalline, stoichiometric BCZT-50 target at various substrate temperatures of 650–850 °C and oxygen pressures of 1–50 mTorr. After the deposition, the thin films were slowly cooled to room temperature at a rate of 2 °C/min.

The growth rate and thickness of the films were determined in situ by RHEED. The lattice parameters of the films were characterized by HRXRD and reciprocal space map (RSM) with Cu K_α radiation (λ = 0.154 nm). The surface topography was investigated using AFM in the noncontact mode. STEM experiments were carried out using the aberration-corrected Nion UltraSTEM™200 (operating at 200 kV) microscope at Oak Ridge National Laboratory, which is equipped with a fifth-order aberration corrector and a cold field emission electron gun. Circular electrodes of diameter 10, 25, 50, 100, and 200 μm were patterned on the BCZT-50 films by photolithography, followed by metal deposition and lift-off. Electron beam evaporated 5 nm Ti and 100 nm Au was used as the electrode metal stack. The polarization versus electric field measurement was performed using an aixACCT TF-3000 Ferroelectric Parameter Analyzer at 1 kHz. The dielectric constant versus electric field measurement was performed using a Keysight E4990A Impedance Analyzer at 1 kHz.

We used structural properties derived from x-ray diffraction measurements as the criteria for the optimization of the growth parameters for BCZT-50 thin films. We varied the substrate temperature and the oxygen partial pressure to identify the optimized parameters. First, we varied the substrate temperature, T_s from 650 to 850 °C with fixed oxygen partial pressure at 10 mTorr. Figure 1(a) shows the XRD plot of ∼100 nm BCZT-50 films deposited at T_s= 650, 750, 800, and 850 °C. The BCZT-50 films exhibit a strong c-axis oriented texture with the [110] substrate crystallographic direction for all temperatures. It is worth noting that we observed thickness fringes around primary reflection in the XRD curve of 750 °C. This indicates that the surface of the samples grown at this condition tends to be smooth, also confirmed by AFM in Fig. 4(c). We observed a gradual shift in the position of reflections for BCZT-50 thin film as a function of the T_s. We show the evolution of the BCZT lattice parameter as a function of the growth temperature in Fig. 1(b). We observed a clear decrease in the out-of-plane lattice parameter from 4.2 to 4.06 Å as T_s increased from 650 to 850 °C. This change in the lattice parameter is ascribed to the relaxation of the strain induced by lattice mismatch of the substrate and film. We also performed rocking curve measurements to study the perfection of the out-of-plane texture of BCZT-50 002 reflections at different growth temperatures. The full-width at half-maximum values of those rocking curves are plotted against the substrate temperature in Fig. 1(c). The best FWHM value is 0.036° obtained at T_s = 750 °C, indicating good crystallinity with an excellent out-of-plane texture of the as-grown BCZT-50 films.

The oxygen partial pressure was varied from 1 to 50 mTorr with T_s fixed at 750 °C to investigate the effects of oxygen pressure on BCZT-50 film structures. XRD spectra of BCZT-50 films grown at 1, 10, 20, and 50 mTorr are shown in Fig. 2(a). Unlike the temperature series, oxygen pressure plays a lesser role in determining the relaxation state of the film. Instead, it gives us clear guidance to smooth surface. At lower pressure (1 and 10 mTorr), the presence of Pendellösung fringes in the θ–2θ scans indicates a flat surface, whereas the high pressures (20 and 50 mTorr) give a relatively rough surface. The rocking curve FWHM values of all samples were summarized in Fig. 2(b). The smallest FWHM value obtained at 10 mTorr was 0.036°.

In Fig. 3, we present a set of representative RHEED patterns and intensity oscillations of the specular spot at the optimal condition of 750 °C and 10 mTorr. At first, the annealed GdScO₃ substrates exhibited a clear 2D diffraction pattern, which attests to the presence of highly smooth GdScO₃ surface. During the deposition of SrRuO₃, well defined diffraction and specular spots with streaky pattern, and RHEED intensity oscillations were observed. We grew 12 nm thick SrRuO₃ layer and estimated the growth rate of SrRuO₃ to be 0.049 Å/pulse. We followed the SrRuO₃ deposition with the growth of BCZT-50. Initially, the 2D diffraction pattern was retained with the subtle change in the diffraction spots corresponding to the larger lattice parameter of BCZT-50. As the growth continued, the diffraction spots gradually transformed to become streaks. The streaky pattern remained till the end of growth, which again indicates a relatively smooth surface. Figure 3(c) shows the RHEED pattern at the end of the growth of a 100 nm thick BCZT-50 film. As an evidence of layer-by-layer growth, we show the oscillations in the intensity of the specular spot for BCZT-50 in Fig. 3(d). The growth rate of BCZT-50 was estimated as 0.135 Å/pulse. To quantitatively analyze the interface and surface roughness obtained upon the growth, we performed AFM studies on three different samples of the annealed GdScO₃ substrate, 15 nm SrRuO₃ on GdScO₃, and 100 nm BCZT-50 film on the 15 nm SrRuO₃/GdScO₃ substrate as shown in Fig. 4. Overall, all the surfaces have a root mean square roughness of ∼2 Å or less. SrRuO₃ has a relatively smooth surface while the step feature is not readily apparent. The annealed substrate and BCZT-50 film exhibit atomically smooth surface with step terraces. The height of the steps corresponds to single unit cell as shown in Fig. 4(d).

To establish the epitaxial relationship between the film and the substrate, we performed high-resolution XRD, RSM and STEM studies. Figure 5(a) shows a representative XRD pattern from a 50 nm BCZT-50 thin film grown at 750 °C and 10 mTorr, where only 00l family of reflections are visible for the BCZT-50 thin film with the ll0 reflections of the substrate, which correspond the pseudocubic 00l reflections. This indicates that the BCZT-50 is highly oriented along the out-of-plane direction of the GdScO₃ substrate. The SrRuO₃ layer reflections are not readily observed due to weak reflections arising from its ultrathin nature, but one could clearly observe them in Figs. 1 and 2. To further illustrate the in-plane epitaxial relationships, we performed RSM studies centered on 332 GdScO₃ reflection. We observed three reflections corresponding to SrRuO₃, GdScO₃, and BCZT-50 in the RSM. All of three reflections possess the same in-plane reciprocal lattice parameter (q_x), which indicates that all the layers are fully strained to the substrate with negligible relaxation. For the out-of-plane (q_z) direction, the lattice constants agree well with the out-of-plane lattice parameters obtained from 2θ − θ scans. Based on these results, we conclude that BCZT films were epitaxially grown on GdScO₃ (110) substrates with the following epitaxial relationship: BCZT(001)||GdScO₃(110) and BCZT[100]||GdScO₃[001].

To investigate the atomic structure of BCZT/SRO heterostructures, STEM experiments were performed on BCZT (109 nm)/SrRuO₃ (15 nm) thin films. Cross-sectional high-angle annular dark-field (HAADF) STEM images clearly revealed that films of BCZT and SrRuO₃ were fully strained on the GdScO₃ substrate as shown in Fig. 6(a). We did not observe any misfit dislocations at the film/substrate interface. The atomic resolution HAADF image in STEM is obtained by elastic scattering of the electron beam from different atomic columns (Ba, Ca, Zr, and Ti), where the degree of elastic scattering is approximately proportional to the squared atomic number (∼Z²).²⁴ Therefore, it is easy to distinguish the elements by the brightness of the atomic columns. As a result, atomic columns corresponding to (Ba, Ca) appear brighter than the (Ti, Zr) atomic columns. This is consistent with the theoretical structure of BCZT that is overlaid in Fig. 6(b) with negligible lattice parameter difference.^5,8 Sharp interfaces between SrRuO₃/GdScO₃ and BCZT/SrRuO₃ were observed and confirmed in Figs. 6(b) and 6(c).

Figure 7(a) shows the dielectric constant (⁠$εr$⁠) versus electric field (E) characteristics of a 200 nm BCZT-50 capacitor with SrRuO₃ and Au/Ti electrodes measured at 1 kHz for both upward and downward sweeps. A slight shift in the $εr−E$ characteristics between upward and downward sweeps is observed, and at their intersection, $εr$ = 1400 is measured. The corresponding phase angle of the impedance spectroscopy is shown in Fig. 7(b). This demonstrates that the sample has low loss with the phase angle close to 90° over the range of the applied electric field. Figure 7(c) depicts the measured polarization (P)-electric field (E) characteristics at f = 1 kHz of the same sample. Note in Fig. 7(c) that the hysteresis in the P-E curve has a narrow opening; such slim hysteresis loops are characteristic of ferroelectric relaxors.²⁵ Our measured remnant polarization (P_r) of 3.5 μC/cm² and coercive field (E_c) of 26 kV/cm are in the same range of BCZT-50 films as previously reported by Luo et al.²⁶ 

In addition, the P-E hysteresis loop is not symmetric with respect to the origin which is presumably due to our use of different metallic layers with different work functions as top and bottom electrodes (top: Au/Ti, bottom: SrRuO₃). The resulting built-in electric field is calculated to be −38 kV/cm. This built-in electric field has also been reported in many compositional ferroelectric films, such as (Ba,Sr)TiO_3, Pb(Zr,Ti)O₃, and BCZT superlattices.^27–29 The origin of the internal field is still not completely understood. Both intrinsic (polarization gradient, free space charge, etc.) and extrinsic (asymmetric contact, strain at interface, oxygen vacancies, etc.)³⁰ factors could play a role in the offset of the polarization or the built-in electric field.^31,32

In summary, we have shown high-quality epitaxial growth of BCZT-50 thin films on SrRuO₃/GdScO₃ substrates. The growth condition has been optimized based on crystallinity as deduced by XRD studies. The sustained layer-by-layer growth mode was achieved as monitored by RHEED. AFM studies reveal one-unit cell high steps with atomically flat terraces in as-grown BCZT-50 film surfaces. STEM studies did not reveal evidence of misfit dislocations and strain relaxation, confirming high quality interfaces between substrate/SrRuO₃ and BCZT/SrRuO₃. A relaxor ferroelectric behavior has been observed with a relatively high dielectric constant. These studies establish a pathway for further applications in electronic and photonic devices for BCZT thin films.

This work was supported by the Air Force Office of Scientific Research under Award No. FA9550-16-1-0335. The authors acknowledge the use of Center for Excellence in Nano Imaging for the characterization of the thin films. Z.W. and A.I.K. acknowledge support from the National Science Foundation (NSF; Grant No. 1718671). R.K. acknowledges support from the NSF (Award No. 1610604). A.S.T. and R.M. acknowledge support from the NSF (Grant No. DMR-1806147). A.Y.B. was supported by the Division of Materials Science and Engineering, U.S. DOE.