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High Permittivity (1 – x)Ba(Zr0.2Ti0.8)O3 – x(Ba0.7Ca0.3)TiO3 (x = 0.45) Epitaxial Thin Films with Nanoscale Phase Fluctuations
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High Permittivity (1 – x)Ba(Zr0.2Ti0.8)O3 – x(Ba0.7Ca0.3)TiO3 (x = 0.45) Epitaxial Thin Films with Nanoscale Phase Fluctuations
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† National Institute for Laser, Plasma and Radiation Physics, 409 Atomistilor, 077125 Magurele, Romania
‡ CNR-ISC, Istituto dei Sistemi Complessi, Area della Ricerca di Roma-Tor Vergata, Via del Fosso del Cavaliere 100, I-00133, Rome, Italy
§ National Institute of Materials Physics, 105 bis Atomistilor, 077125 Magurele, Romania
*E-mail: [email protected]. Fax: 0040214574467.
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Cite this: ACS Appl. Mater. Interfaces 2015, 7, 43, 23984–23992
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https://doi.org/10.1021/acsami.5b06745
Published October 13, 2015

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Abstract

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Epitaxial (1 – x)Ba(Ti0.8Zr0.2)TiO3 – x(Ba0.7Ca0.3)TiO3, x = 0.45 (BCZT 45), thin films have been deposited on (001) SrTiO3 (STO) and (001/100) SrLaAlO4 (SLAO) substrates by pulsed laser deposition. X-ray diffraction and high-resolution transmission electron microscopy (HRTEM) confirmed the epitaxial growth of the films. A high structural quality has been evidenced for the BCZT/STO films. Geometric phase analysis (GPA) associated with the HRTEM enabled us to obtain microstrain analysis and the in-plane and out-of-plane lattice parameter variation on different areas. Tetragonality ratio fluctuations at nanoscale level which are relevant for the existence of nanodomains have been evidenced on the BCZT/STO films. The in-plane dielectric constant has been measured on interdigital electrodes deposited by lift-off technique on the top of the films. High values of dielectric permittivity (>3000) combined with low dielectric loss (<0.01) are obtained for BCZT 45 film deposited on STO substrate, showing nearly constant values between 1 kHz and 10 MHz. The high dielectric permittivity of BCZT thin films was attributed to their high structural quality and to the loss of rotation stability of the polarization associated with the presence of nanodomains. This results into a divergence of fluctuations of polarization direction and a peak of dielectric susceptibility. The enhanced switching of such nanodomain configuration was probed by piezoforce microscopy, by writing and reading domains during topography scanning.

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1 Introduction

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Due to its large piezoelectric response, BaTiO3 (BT) ferroelectric perovskite is commonly used to develop new lead-free piezoelectric materials. The ferroelectricity in this compound originates from the Ti4+ ion shift in the BO6 octahedron, giving rise to three ferroelectric phases: tetragonal (T) with space group P4mm (stable between 390 and 265 K) with spontaneous polarization Ps ∥ (001), orthorhombic (O) with space group Amm2 (stable between 265 and 180 K) with Ps ∥ (011), and rhombohedral (R) with space group R3m (stable below 180 K) with Ps ∥ (111). (1) However, in pure BT, these three phases are very different in free energy and separated by large temperature intervals. Moreover the T–O and O–R phase transitions occur below room temperature. Therefore, A- and/or B- site substitutions have been employed in order to bring these phase transitions closer to each other and near room temperature. The most interesting results have been obtained by substituting Zr4+ for Ti4+ and Ca2+ for Ba2+. (1-9) Thus, it has been shown that polymorphic phase boundaries and enhanced piezoelectric response can be obtained by substituting larger ionic radius Zr4+ (0.72 Å) in place of Ti4+ (0.605 Å), which favors oxygen octahedra tilting. (1) The ferroelectric transition temperature Tc decreases while TT–O and TR–O increase with Zr concentration, and they all converge at a temperature of 340 K for Zr content 15 mol %. The detailed structural analysis showed that the composition range where enhanced dielectric and piezoelectric properties are found is characterized by coexistence of two (R+O or O+T) ferroelectric phases. The response can be further enhanced by substituting smaller Ca2+ ions for Ba2+ in Ba(Zr,Ti)O3 as shown in refs., (2, 9) since this substitution brings the different phase transitions of Ba(Zr,Ti)O3 in the room temperature range. The mechanism is related to the large difference between the ionic radii of Ba2+ (1.61 Å) and Ca2+ (1.34 Å); the chemical pressure due to unit cell reduction in (Ba,Ca) (Zr,Ti)O3 influences the phase transition temperature similarly to hydrostatic pressure, that is TT–O and TR–O show a continuous decrease down to the lowest temperatures. (10) However, Tc is nearly independent, which means that a new polarization component acts in these compounds to compensate for the pressure-induced instability. Indeed, first-principles calculations showed that the small Ca2+ cations incorporated in the host lattice tend to locate at an off-centering position. (9) Thus, the additional polarization originates from this off-center displacement of smaller Ca ions at the Ba site, which works cooperatively with Ti–O distortion.
The ferroelectric distortions of the T, O, and R phases in (Ba,Ca)(Zr,Ti)O3 compounds are very small, and moreover, the difference of R–O and O–R transition temperatures is about 10 K, indicating that the free energy difference between these ferroelectric phases is very small. The stabilization of the two polymorphic phase boundary regions near room temperature in these compounds, which is the origin of their high dielectric and piezoelectric response, is favored by three factors: the first-order nature of the phase transitions, the increased coexistence range due to random elastic strain associated with substitutions, and the shift of transition temperatures toward room temperature with Zr and Ca substitutions.
The phase diagram of (Ba,Ca)(Zr,Ti)O3 compounds has been rationalized in terms of two end-members, Zr-doped BT and Ca-doped BT, and the most investigated compositions belong to the pseudobinary phase diagram (1 – x)Ba(Zr0.2Ti0.8)O3 – x(Ba0.7Ca0.3)TiO3 (BCZT 100×). (2-8) The symmetry of the O phase in BCZT has been found to be Amm2, like in BT. (3) Rietveld refinements confirmed that the phase symmetry sequence observed in BCZT with temperature decreasing is Pm3m–P4mm–Amm2–R3m, similar to that observed in BTO. (3)
Thus, maximizing the number of coexisting phases at room temperature was found to be the main route to enhance the dielectric and piezoelectric response in BCZT solid solutions. However, this can be achieved not only by substitutions but also by tailoring the space of constraints (epitaxial strain, pressure, electric field etc.), (11) so that phase diagrams with multiple dimensions, as e.g. T–σ1–σ2–Ei–x(Ba,Ca)TiO3–yBa(Zr,Ti)O3 can be considered. Indeed, it has been shown that a large contribution to the mechanical or electrical response comes from the entropy increase due to coexistence of paraelectric cubic, R, O, T, and eventually other low symmetry phases, like monoclinic, with local polarization directions randomly distributed along allowed symmetry directions. (11) Thus, by engineering the material system so that it can be operated near the invariant critical point to maximize the number of coexisting phases and, consequently, the entropy, a very large dielectric and electromechanical response may be realized. (11) The ways open by strain engineering in thin films can thus be fruitfully applied in BCZT thin films, to maximize the number of coexistent competing phases.
A remarkable feature of these systems is the potential to transform their ground states under an applied mechanical constraint. For example, when a large compressive strain occurs in a thin film due to the epitaxial constraint from the underlying substrate, the ground state R transforms into a T-like (or O-like) phase. (12) Moreover, when the epitaxial constraint is partially relaxed by increasing the film thickness, this T phase evolves into a state consisting of a nanoscale mixture of the R-like phase embedded into the T-like phase. (12) Similar effects have been reported in SrRuO3 thin films grown in epitaxial perovskite heterostructures like SrTiO3/SrRuO3/Pb(Zr,Ti)O3 or SrRuO3/GdScO3, where the epitaxial strain induces local tilt/distortion of the RuO6 octahedra or the formation of monoclinic distorted nanodomains. (13, 14)
In the present study, we make a careful analysis of strain and local lattice distortions in BCZT thin films with high dielectric and piezoelectric response and identify in the peculiar nanoscale features the cause of these enhancements. The results can be of more general relevance also for other materials with nanoscale fluctuations in constrained conditions.

2 Experimental Section

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The ceramic targets used for the thin films deposition were fabricated by conventional solid-state method. Precise amounts of reagent-grade starting materials of BaCO3, CaCO3, TiO2 and ZrO2 were mixed for obtaining the stoichiometric composition of (1 – x)Ba(Ti0.8Zr0.2)O3 – x(Ba0.7Ca0.3)TiO3, x = 0.45 (BCZT 45). The resulting targets were characterized using X-ray diffraction (XRD) technique. The XRD measurements were performed with a Panalytical X’Pert MRD system equipped with Pixcel detector, by using a monochromated beam (λ Cu Kα1 = 1.540598 Å) provided by a hybrid asymmetric monochromator 2xGe(220). The BCZT 45 targets show pure phase perovskite structure with rhombohedral R3m symmetry, as expected for this composition, and the pseudocubic lattice parameter was found to be a = 4.0176 Å.
Epitaxial thin films of BCZT 45 were obtained by pulsed laser deposition (PLD) technique. The deposition of the thin films has been carried out in oxygen atmosphere with partial pressure of 10 Pa, on substrates heated at 700 °C. For achieving the desired strain conditions two different single-crystal substrates, (001)SrTiO3 (STO) and (001/100)SrLaAlO4 (SLAO), have been selected. The thickness of the films was measured by spectrometric ellipsometry and confirmed by transmission electron microscopy (TEM) in cross section, being in the range of 160–175 nm.
The structural characterization of the BCZT thin films has been made by XRD on the same equipment used for the target characterization and by high-resolution transmission electron microscopy technique (HRTEM). Cross-section specimens for HRTEM investigations have been prepared by mechanical grinding and ion milling using a Gatan PIPS installation at 4 kV acceleration voltage and 7° beam incidence angle. The samples have been investigated on a JEOL ARM 200 F transmission electron microscope operated at 200 kV. For dielectric measurements Au interdigital electrodes (IDE) have been deposited on top surfaces of the films by employing the lift-off technique. The capacitance and dielectric loss have been measured at different temperatures and frequencies by using an HP4194A impedance analyzer.
The local piezoelectric properties, polarization dynamics and switching characteristics of the samples were investigated by Piezoresponse Force Microscopy (PFM), on a commercial AFM (XE-100, Park Systems). Two types of cantilevers were used: titanium–platinum coated NSC36 series (Mikromasch) for lower stiffness (k < 1 N/m), and bulk platinum cantilevers (Rocky Mountain Nanotechnology) with higher stiffness (k ∼ 18 N/m). The samples were glued to stainless steel disks with silver paint in order to achieve a good electrical contact. The conductive Nb-doped STO substrate acted as bottom electrode and the conductive AFM tip acted as top electrode. The DC bias was applied to the substrate and the AC (testing) bias was applied to the tip, both referenced to a common ground. The local vertical (out-of-plane) mechanical response of the material to the AC bias was recorded during topography scanning (PFM imaging) and during ramping of the DC bias (hysteresis loops). The amplitude and phase of the piezoelectric out-of-plane response were extracted from the AFM vertical deflection signal by a lock-in amplifier (Stanford Research Systems SR830).

3 Results and Discussion

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3.1 Structural Results

The XRD patterns of the BCZT/STO and BCZT/SLAO heterostructures are presented in Figure 1. Highly oriented (00l) growth of the BCZT films is observed on both substrates, in spite of the large lattice misfit between the pseudocubic lattice parameters of the BCZT 45 target (a = 4.0176 Å) and the STO (a = 3.905 Å) and SLAO (a = 3.756 Å) lattice parameters. The only peaks observed in the diffractograms are from the (00l) reflections indicating the absence of impurity phases or domains with other orientations, as can be better seen in Figure 1b, where only half of the recorded angular range is presented.

Figure 1

Figure 1. (a) XRD patterns of the BCZT/STO and BCZT/SLAO heterostructures; (b) detailed XRD patterns in the first half of the recorded angular range; (c) 360° φ-scans of BCZT (101) and STO (101) asymmetric reflections; (d) 360° φ-scans of BCZT (101) and SLAO(103) reflections; and (e) ω-scans of symmetric reflections.

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The 360° φ-scans of BCZT (101) and STO (101) asymmetric reflections (Figure 1c) and BCZT (101) and SLAO (103) reflections, respectively (Figure 1d) evidence perfectly matched 4-fold symmetries, suggesting a cube-on-cube epitaxial growth.
The in-plane and out-of-plane lattice parameters were determined by conventional and off-axis diffraction. The out-of-plane (an) lattice parameters were obtained in normal θ–2θ scans using the formula an = 2d002, while the in-plane (ap) lattice parameters have been obtained by tilting the samples at 45° in a θ–2θ scan from the equation(1)where d002 and d202 are the lattice spacing of (002) and (202) planes, respectively. (15)
To separate the coherently diffracting domains effect from the lattice strain effect, we used the Williamson-Hall method on the (00l) reflections. (16) The mathematical relation between the X-ray wavelength λ, integral breadth β (instrumental corrected FWHM), θ Bragg angle, the domain size (namely, the short coherence length perpendicular to the substrate surface) D⊥ and the vertical inhomogeneous strain (ε⊥) is(2)
The plot of (βcos θ/λ) versus (sin θ/λ) gives the value of the strain from the slope and the vertical coherence length from the ordinate intercept.
The mosaicity of the films is described by the ω-scans of symmetric reflections. Figure 1e presents the superimposed ω-scans (so-called rocking curves) of the (002) reflection of the BCZT films deposited onto STO and SLAO substrates. The broadening of the ω-scans symmetric reflections is determined by the tilt angle αtilt (rotation of the mosaic blocks around the axis lying in the substrate surface) and to the coherence length parallel to the substrate surface (D∥). Due to the different dependences of the two broadening mechanisms on the reflections orders, a similar Williamson–Hall approach proposed by Mentzger et al. (17) allows the separation of the two effects. By plotting βω(sin θ)/λ against (sin θ)/λ where βω is the FWHM of the rocking curve, θ the Bragg angle and λ the X-ray wavelength, the tilt angle is obtained from the slope of the linear dependence and the lateral coherence length from the inverse of the intercept of the fitting line with the ordinate. The structural data extracted from the different X-ray diffraction results are presented in Table 1 and will be commented in the following.
The low magnification cross-sectional TEM images (Figure 2a,b) reveal the compact growth of the BCZT thin films deposited on the two different substrates (STO and SLAO). The BCZT layer grown on STO substrate shows a more compact structure and better interface than the film grown on SLAO, which contains structural defects (grain boundaries, threading and misfit dislocations), due to the higher misfit of the in-plane lattice parameters.

Figure 2

Figure 2. Low-magnification cross-section TEM images showing the BCZT layers grown on STO(001) (a) and SLAO(001) (b) single crystal substrates; (c) and (d) corresponding SAED patterns revealing epitaxial relationship between the as grown layers and the single crystal substrates; (e) and (f) corresponding HRTEM images at the film–substrate interface for the two substrates. The interface is marked by the white arrow.

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The selected area electron diffraction (SAED) patterns in Figure 2c,d indicate an epitaxial or highly orientated growth of the BCZT films with respect to the substrate in both analyzed cases. The main Miller index corresponding to the BCZT layer and the substrate (subscript s) are indicated on the diffraction patterns. The cube-on-cube growth can be noticed, with the following relationship between the layer and substrate main axes: [001] BCZT ∥[001]s and [100] BCZT ∥[100]s. The density of defects in the BCZT layers and the associated strain field revealed by the nanometric dark/bright stripes determines the slight elongation of the diffraction spots corresponding to the BCZT layer along a direction perpendicular to the one of the defects (parallel to the interface), unlike the diffraction spots of the substrates which do not exhibit any particular deformation.
The epitaxial growth is also confirmed by the HRTEM images at the film–substrate interface for both substrates, as can be seen in Figure 2e,f.

3.2 Microstrain Analysis

The residual strain in the as-grown layers has been measured on HRTEM micrographs using the geometrical phase analysis (GPA) software. (18) A typical result concerning the strain mapping on a large area HRTEM image next to the BCZT/STO interface is presented in Figure 3 (a–c). The in-plane and out-of-plane lattice parameter variation, εxx and εyy, respectively, have been measured with respect to a reference area chosen inside the STO substrate. The εxx and εyy strain maps are represented in false colors in Figure 3b,c.

Figure 3

Figure 3. (a) Large-area HRTEM image at the BCZT/SRO interface. (b and c) 2D maps of in-plane and out-of-plane lattice parameter variation and residual strain, εxx and εyy, respectively. (d) Line profiles of the εxx and εyy maps along the arrow crossing the interface. (e) Tetragonality c/a ratio variation along the same line. (f) Large-area HRTEM image at the BCZT/SLAO interface. (g and h) 2D maps of in-plane and out-of-plane lattice parameter variation and residual strain, εxx and εyy, respectively. (i) Line profiles of the εxx and εyy maps along the arrow crossing the interface.

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The corresponding quantitative values can be read out on the line profiles obtained along the black arrows across the interface, as indicated on the colored maps. In order to smooth out the intensity variations due to noise or presence of defects (strain field around dislocations cores), the line profile has been averaged on a lateral range around the black arrow (about 80 nm wide), as indicated by the dashed black line on the strain maps.
The εxx and εyy in-plane and out-of-plane strain profiles for the BCZT/STO film are shown in Figure 3d, in the upper and lower plots, respectively. In the analyzed micrographs, the line profiles have been averaged on a lateral range on the widest band allowed by the micrograph, such as a significant region of the substrate is included as reference in the measured (averaged) area. Small noise oscillations around zero are observed in the STO substrate region chosen as reference area. In the BCZT film region the line profile shows a jump to a plateau around the average value of 2.8 ± 0.4% for εxx and 2.9 ± 0.3% for εyy. Using these values and considering that the substrate lattice is fully relaxed (quasi-infinite thickness as compared to the BCZT thin film), we have calculated the average in-plane (a) and out-of-plane (c) lattice parameters inside the BCZT thin film, starting from the bulk values of the STO lattice parameter (aSTO = 3.905 Å). This method was originally applied to study the displacement and strain fields associated with domain walls in PbTiO3, which are defined by the relative directions of the a and c axis. (18) The elastic and electric properties of the material are strongly influenced by the structure of the interfaces between domains, so it is important to determine how the tetragonality (c/a ratio) varies across the interface and how abruptly the distortion of the lattice occurs. In Figure 3 e we have represented the tetragonality c/a variation along the BCZT film thickness. We recall that the lattice distortions in all the phases of BCZT are very small, as discussed in the Introduction, with c/a ∼ 1.005 in T phase. (2) Thus, given the film thickness (175 nm), the lattice in the BCZT/STO film is almost completely relaxed to the bulk value. However, the tetragonality ratio fluctuations, clearly visible in Figure 3e, may be assimilated with a and c-axis interchanging in tetragonal nanodomains, with coexisting R or O nanodomains, or both. We presume that, as previously found in other epitaxial ferroelectric films (12) and discussed in the Introduction, the tetragonal T phase, originally induced by the epitaxial strain in very thin BCZT 45 films, evolve into a mixture of nanodomains of different phases close in energy when the film thickness increases.
Similar image processing has been applied to the HRTEM images obtained on the BCZT/SLAO films (Figure 3f–i). The in-plane and out-of-plane lattice parameters of BCZT have been calculated starting from the bulk values of the SLAO lattice parameters and using the quantitative data read out of the 2D strain maps of the cross-section HR micrographs. In the case of the SLAO substrate, we calculated the strain maps by comparing the (200)BCZT and (002)BCZT film lattice fringes with the (200)s and (006)s substrate lattice fringes spaced at 0.186 and 0.210 nm, respectively. Unlike the film on the STO substrate, for the GPA calculation of the εyy strain field in BCTZ/SLAO we have chosen as reference for the SLAO substrate the interplanar distance between the (006)s planes parallel to the interface, as this set of planes exhibits the nearest value to the (002)BCZT interplanar distance. We remember here that the two substrates, SrTiO3 (STO) and SrLaAlO4 (SLAO), have different crystalline structures. While STO is a cubic perovskite (a = 3.905 Å, S.G. Pm3m), SLAO has a tetragonal structure (a = b = 3.755 Å, c = 12.62 Å, S.G. I4/mmm). The pseudocubic lattice parameter of the deposited BCZT 45 layers is a = 4.0176 Å, about 3 times smaller than the c parameter of the SLAO substrate. For the out-of-plane strain in BCZT we have chosen to compare the interplanar distance between planes oriented parallel to the interface, both in BCZT and in the substrates. In addition, we have selected those families of planes having close values for the interplanar distance. Therefore, for the BCZT/STO sample we have chosen the (002)BCZT planes (measured interplanar distance ca. 2.01 A) and (002)STO sets of planes (interplanar distance 1.95 A), while for the BCZT/SLAO sample we have chosen the same (002)BCZT planes (measured interplanar distance 2.01 A) for the film, but the (006)SLAO planes (interplanar distance 2.10 A) for the SLAO substrate. On the SAED patterns the corresponding diffraction spots appear like slightly separated doublets, both in the STO and SLAO case.
Negative values of εyy have been measured inside the film area with respect to the SLAO substrate, as a result of the larger [006]s interplanar distance in the SLAO substrate with respect to the [002]BCZT interplanar distance in the thin film. The SLAO-BCZT interface exhibits a high density of dislocations characterized by their own strain field, generating a narrow band of tensile strain along the interface as indicated by the red stripe at the interface on the εyy map (positive values on the εyy line profile). Extended structural defects (threading dislocations and grain boundaries) are also present inside the BCZT/SLAO films, at a relatively higher density compared to the film deposited onto STO (Figures 2b and 3f). These defects modify the local stoichiometry and induce a local strain field, with likely consequences on the dielectric losses and the lower dielectric permittivity, as will be shown in the following.
The structural data extracted from XRD and HRTEM analysis are gathered in Table1. An inspection of the data evidence the high crystallographic quality of the BCZT film deposited on STO, with a vertical coherence length D⊥ of 152 nm, close to the film thickness (175 nm) as determined by TEM. Extremely low mosaicity, small broadening of the rocking curves (high parallel coherence length D∥) and, in particular, an extremely small value of the tilt angle can also be observed. The out-of-plane and in-plane lattice parameters are quite close to the values provided by TEM, which stands for the high homogeneity of the film. Instead, for the film deposited on SLAO substrate, much higher values for the broadening of the rocking curves, shorter vertical and parallel coherence lengths, and substantially higher value of the tilting angle are measured, due to the large misfit with the substrate. Therefore, the structural quality of the BCZT/SLAO film is much lower, and we expect it to reflect in the dielectric properties.
Table 1. Structural Data Extracted from XRD and HRTEM Analysis
TEM Results
film on substrateεxx in BCZT with respect to substrate (%)εyy in BCZT with respect to substrate (%)calcd ap (nm) in-plane in BCZTcalcd an (nm) out-of-plane in BCZT
STO2.8 ± 0.42.9 ± 0.30.4014 ± 0.00160.4018 ± 0.0012
SLAO5.3 ± 0.6–3.1 ± 0.80.3955 ± 0.00240.4081 ± 0.032
XRD results
 lattice parametersstrain due to misfit with the substraterocking Williamson–Hall analysis
film on substrateap(nm) in planean (nm) out of planeεxx in plane (%)εyy out of plane (%)curve (002) (deg)D⊥(nm)ε⊥ (10–3)D∥(nm)αtilt (deg)
STO0.40150.40212.83.00.052215246.51960.012
SLAO0.40410.40047.6–4.80.20035012.7550.447

3.3 Dielectric Permittivity and Piezoelectric Response

The dielectric permittivity of the film samples has been evaluated by using gold interdigital electrodes (IDE) deposited on the film surface. Dielectric spectroscopy measurements have been carried out between 1 kHz and 10 MHz on IDE structures consisting of N = 21 finger pairs of length L = 464 μm and width 10 μm. The interspace between fingers is 10 μm, and thus, the distance D between finger centers is 20 μm. The measurements yield the capacitance and the dielectric loss tan δ.
The in-plane dielectric constants of the Au/BCZT/STO and Au/BCZT/SLAO thin film capacitors with IDE configuration have been calculated by using the analytical model proposed in refs 20 and 21, which give for the dielectric constant of the thin film ε the following formula(3)where εs is the dielectric constant of the substrate, h is the film thickness, Cn is the measured capacitance of the IDE structure normalized to the finger length (L) and to the number of fingers (2N – 1), while CK is a constant depending on IDE geometry. In the case of IDE patterns with equal finger width and spacing, CK = 4.53 pF/m. (19, 20) The dielectric constant of the STO substrate is ∼300 and that of the SLAO substrate is ∼17. The BCZT/STO and BCZT/SLAO dielectric constants and loss values dependence on frequency at room temperature are plotted in Figure 4a.

Figure 4

Figure 4. (a) BCZT/STO and BCZT/SLAO dielectric permittivity and loss values dependence on frequency at room temperature. Continuous curves represent the dielectric permittivity and loss values for the bulk BCZT sample. (b) Dielectric permittivity and loss variation with temperature of (○) BCZT/STO films and (□) bulk BCZT 45 samples. Hatched rectangles approximately indicate the R/O and O/T phase transformation regions in the bulk sample. (c) Out-of-plane PFM image for BCZT/STO thin films.

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For comparison, the bulk dielectric permittivity and loss values are plotted, too. Very high values of dielectric permittivity (>3000) combined with very low dielectric loss (<0.01) are obtained for BCZT 45 film deposited on STO substrate, while lower dielectric permittivity value, comparable with that of the bulk material, has been obtained for BCZT/SLAO film. The differences in the dielectric permittivity values of the two films are attributed to their different structural quality. Thus, as it has been shown in the previous sections, the BCZT/SLAO interface shows a high density of dislocations, due to the high lattice misfit of the two materials. Moreover, a higher amount of extended structural defects (threading dislocations and grain boundaries) are present inside the BCZT/SLAO films in comparison with BCZT/STO films. The lower structural quality of BCZT/SLAO films was evidenced also by the higher values of the broadening of the rocking curves, shorter vertical and parallel coherence lengths, as well as substantially higher value of the tilting angle (Table 1), and it is associated with the higher dielectric losses and the lower dielectric permittivity values of these samples. The occurrence of many defects and dislocations has a detrimental effect on the dielectric response because they pin the nanodomain walls and do not allow the easy movement of walls. This is manifested also in the higher losses for BCZT/SLAO film.
We mention that the in-plane dielectric constant values have been measured on a large number of capacitors on each sample; therefore, they can be considered as representative for each configuration. Moreover, to verify the experimental and calculation methods, we repeated the measurements on IDE electrodes directly deposited on the STO and SLAO substrates. The same model has been used to calculate their dielectric constants. Perfect agreement has been obtained between the measured and the data sheet values of the STO and SLAO dielectric constants.
We mention also that, although dielectric results have been occasionally reported on some BCZT films, (21-25) they show lower values or they are accompanied by substantial variation with frequency, which is a fingerprint of some relaxation mechanism contribution to permittivity, while the values reported here on BCZT/STO film show neglectable variation up to 10 MHz. A comparison with other BCZT films as well as with other lead-free ferroelectric films is given in a table in the Supporting Information.
The dielectric permittivity and loss variation with temperature of BCZT/STO films have been measured and compared with similar curves obtained on bulk BCZT 45 samples (Figure 4b).
In the calculation, we have taken into account the variation with temperature of the dielectric constant of the substrate (Supporting Information). The fact that the film values do not follow the increasing with temperature of bulk values can be explained in the following way: the increasing with T of bulk values is due to the increased mobility of polar domains on approaching ferroelectric-paraelectric transition, which for bulk BCZT is about 365 K. (4) However, in epitaxial thin films, Tc is usually higher than in bulk, therefore this increasing on approaching the transition temperature will occur at higher temperatures than in bulk. However, we could not measure the in-plane dielectric constant at higher T due to the peculiar holder type for contacting interdigital electrodes which does not support higher temperature.
It can be observed that the dielectric constant of the film preserves a high value in all the temperature region. It does not show the anomalies associated with R–O and O–T transitions in bulk. Instead ε conserves a high value (∼3000) in a large temperature range, well above room temperature. This behavior is associated with the broad coexistence range of T, O and R phases, which probably extends up to higher temperatures in thin films. As discussed in the Introduction, when the epitaxial strain is partially released by increasing film thickness, the induced tetragonal phase evolves into a nanoscale mixture of T, O, and R-like nanodomains. Therefore, we propose as possible origin of the high dielectric response the intrinsic contribution of the nanoscale boundaries. (8) Indeed, as confirmed by a rich phenomenology, phase boundaries in solids have been found to influence critically the macroscopic properties of materials. (8, 12) Near the phase boundary the free energies of different competing phases are almost degenerate, producing a nearly continuous variation in the local structure from phase to phase. As a consequence, a small external signal can induce a high response in such a material. (12)
It is well established also that in polar materials the difference between the free energies of the competing phases near the phase boundary decreases and vanishes when approaching it. (26) In this limit, the ferroelectric material can behave as a dipolar glass, (26) where the polarization is only weakly coupled with the lattice. The difference between the free energies of the phases strongly reduces so that small perturbations like structural defects, chemical fluctuations, or small strains in films can lead to phase instabilities. This finally leads to the deviation of polarization from the easy directions within the ferroelectric domains of T, R, and O phases and to the formation of frustrated nanodomains. (26) Thus, the high dielectric permittivity of BCZT thin films can be seen as a loss of rotation stability of the polarization associated with the presence of nanodomains. This results in a divergence of fluctuations of polarization direction and a peak of dielectric susceptibility. (26)
Moreover, an enhanced field-induced rearrangement of nanodomains under applied field, caused by a drastic reduction of the polar anisotropy and leading to a high piezoresponse, should be also measured on such configurations. This is indeed what we have observed in BCZT/STO thin films, from the out-of-plane piezoelectric response (PFM image) and switching kinetics tested at different applied electric DC fields. An example of PFM imaging is shown in Figure 4c. Square shaped domains were first “written” on the material by applying a DC bias between the substrate and the tip held at ground potential, during topography scanning. The piezoelectric response was subsequently “read” by applying a 3 V, 10 kHz AC bias on the tip while holding the substrate at ground potential. The switching behavior has been tested at different DC bias values and orientation. The domains in Figure 4c correspond to the following values of the DC bias: upper left and lower right (10 × 10 μm), + 10 V; center (15 × 15 μm, dark), + 10 V; center (7 × 7 μm, light), – 10 V; upper right and lower left (10 × 10 μm), + 12 V and +14 V, respectively. In the inset image, a square shaped domain written on the BCZT thin film by applying +15 V DC bias is presented. The 0–180° reversibility of the out-of-plane piezoelectric response evidences a net out-of-plane switchable polarization which is correlated with the applied DC bias values and orientation, the upward polarization state being obtained for positive electric field and easily reversed by changing the field orientation.

4 Conclusions

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In summary, lead-free ferroelectric BCZT 45 thin films have been grown epitaxially on (001) (STO) and (001/100) (SLAO) single-crystalline substrates by pulsed laser deposition. The epitaxial growth of the films has been confirmed by X-ray diffraction and HRTEM. A microstrain analysis and the variation of the in-plane and out-of-plane lattice parameters on different areas has been evidenced by GPA performed on HRTEM images. A nanoscale level tetragonality ratio variations, associated with the existence of polar nanodomains was evidenced on the BCZT/STO films which have a high structural quality. The in-plane dielectric permittivity showed very high values (>3000) for the BCZT film deposited on STO substrate, in a wide frequency range. This has been attributed to the loss of rotation stability of the polarization associated with the presence of nanodomains, which results into a divergence of fluctuations of polarization direction and a peak of dielectric susceptibility.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b06745

  • Dielectric constant and loss of used substrates, temperature dependence of SrTiO3 dielectric constant, comparison with similar results reported on lead-free ferroelectric films, and references. (PDF)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
    • Nicu D. Scarisoreanu - National Institute for Laser, Plasma and Radiation Physics, 409 Atomistilor, 077125 Magurele, Romania;  Email: [email protected]
  • Authors
    • Floriana Craciun - CNR-ISC, Istituto dei Sistemi Complessi, Area della Ricerca di Roma-Tor Vergata, Via del Fosso del Cavaliere 100, I-00133, Rome, Italy
    • Antoniu Moldovan - National Institute for Laser, Plasma and Radiation Physics, 409 Atomistilor, 077125 Magurele, Romania
    • Valentin Ion - National Institute for Laser, Plasma and Radiation Physics, 409 Atomistilor, 077125 Magurele, Romania
    • Ruxandra Birjega - National Institute for Laser, Plasma and Radiation Physics, 409 Atomistilor, 077125 Magurele, Romania
    • Corneliu Ghica - National Institute of Materials Physics, 105 bis Atomistilor, 077125 Magurele, Romania
    • Raluca F. Negrea - National Institute of Materials Physics, 105 bis Atomistilor, 077125 Magurele, Romania
    • Maria Dinescu - National Institute for Laser, Plasma and Radiation Physics, 409 Atomistilor, 077125 Magurele, Romania
  • Notes
    The authors declare no competing financial interest.

Acknowledgment

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N.D.S. and V.I. gratefully acknowledge the financial support from UEFISCDI in the frame of the TE14/2013 PNII-RU-TE-2012-3-0467 project. C.G. and R.F.N. gratefully acknowledge the financial support from UEFISCDI in the frame of the PN-II-ID-PCE-2012-4-0362 project.

References

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This article references 26 other publications.

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    The voltage dependence of the dielec. const. of ferroelec. materials makes them attractive for use as tuning elements in microwave circuits. In this study, capacitance tuning and loss measurements were performed on ferroelec. materials prepd. by the sol-gel process and radio-frequency magnetron sputtering. The authors find that Pb(Zr,Ti)O3 (PZT) thin films with interdigital electrodes can be fabricated with reasonably low loss to make them useful for room temp. tuning applications. In addn., it is found that high temp. post-deposition annealing of both sputtered SrTiO3 (ST) and sol-gel derived BaTiO3 (BT) films markedly improves their tuning and loss factor characteristics. By annealing the samples in the temp. range 1000-1100°, the tunability was increased by as much as a factor of seven, while the dissipation factors were decreased to values of 0.3-0.5%. In addn., it is shown that the permittivity of the films in these interdigitated capacitor structures can be calcd. using an anal. model previously described by G. Farnell et al. (1970).
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    Ferroelec. solid solns. with different symmetry phases at opposite sides of the perovskite-based pseudo-binary phase diagram are considered. An anal. of the concn. dependence of polar anisotropy shows conditions under which the anisotropy changes sign, assuming zero value. The orientational stability of a polarization was investigated near this point, which is also a position of so-called morphotropic boundary (MB). The symmetry conditions resulting in the high-symmetry ferroelec. phases with finite temp.-compn. stability field as well as the thermodn. conditions providing their metastable two-phase coexistence are formulated. A loss of orientation stability around MB results in a divergence of fluctuations of polarization direction and a peak of dielec. susceptibility. The calcd. energy of a 90° domain wall demonstrates that this energy is drastically reduced or even vanishes at the MB. The latter leads to a miniaturization of the domain structure to the nano-scale level and the formation of a mixed adaptive state. The miniaturization explains the obsd. special properties of ferroelecs. near MB that are extrinsic for the nanodomain adaptive state. It also explains the obsd. diffraction patterns generated by nanodomains of the mixed state. A coherent scattering generated by nanodomains mimics diffraction patterns that could be confused with a pattern generated by a homogeneous monoclinic phase.
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  • Abstract

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    Figure 1

    Figure 1. (a) XRD patterns of the BCZT/STO and BCZT/SLAO heterostructures; (b) detailed XRD patterns in the first half of the recorded angular range; (c) 360° φ-scans of BCZT (101) and STO (101) asymmetric reflections; (d) 360° φ-scans of BCZT (101) and SLAO(103) reflections; and (e) ω-scans of symmetric reflections.

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    Figure 2

    Figure 2. Low-magnification cross-section TEM images showing the BCZT layers grown on STO(001) (a) and SLAO(001) (b) single crystal substrates; (c) and (d) corresponding SAED patterns revealing epitaxial relationship between the as grown layers and the single crystal substrates; (e) and (f) corresponding HRTEM images at the film–substrate interface for the two substrates. The interface is marked by the white arrow.

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    Figure 3

    Figure 3. (a) Large-area HRTEM image at the BCZT/SRO interface. (b and c) 2D maps of in-plane and out-of-plane lattice parameter variation and residual strain, εxx and εyy, respectively. (d) Line profiles of the εxx and εyy maps along the arrow crossing the interface. (e) Tetragonality c/a ratio variation along the same line. (f) Large-area HRTEM image at the BCZT/SLAO interface. (g and h) 2D maps of in-plane and out-of-plane lattice parameter variation and residual strain, εxx and εyy, respectively. (i) Line profiles of the εxx and εyy maps along the arrow crossing the interface.

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    Figure 4

    Figure 4. (a) BCZT/STO and BCZT/SLAO dielectric permittivity and loss values dependence on frequency at room temperature. Continuous curves represent the dielectric permittivity and loss values for the bulk BCZT sample. (b) Dielectric permittivity and loss variation with temperature of (○) BCZT/STO films and (□) bulk BCZT 45 samples. Hatched rectangles approximately indicate the R/O and O/T phase transformation regions in the bulk sample. (c) Out-of-plane PFM image for BCZT/STO thin films.

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  • References

    This article references 26 other publications.

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      There is no corresponding record for this reference.
    11. 11
      Liu, Z. K.; Li, X.; Zhang, Q. M. Maximizing the Number of Coexisting Phases near Invariant Critical Points for Giant Electrocaloric and Electromechanical Responses in Ferroelectrics Appl. Phys. Lett. 2012, 101, 082904– 082904–4 DOI: 10.1063/1.4747275
      11
      Maximizing the number of coexisting phases near invariant critical points for giant electrocaloric and electromechanical responses in ferroelectrics
      Liu, Z. K.; Li, Xinyu; Zhang, Q. M.
      Applied Physics Letters (2012), 101 (8), 082904/1-082904/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Ferroelec. materials directly convert elec. energy to mech. or thermal work and are crit. to applications such as sensors, transducers, actuators, and cooling devices. Numerous efforts have been undertaken to develop materials with high electrocaloric (EC) and electromech. (EM) responses. Here, we present a theor. anal., based on thermodn. fundamentals, for developing ferroelec. materials with high EC and EM responses, i.e., searching for and operating the material near an invariant crit. point (ICP). We show that by tailoring the constraints to maximize the no. of coexisting phases near ICPs, large EC and EM responses may be realized. (c) 2012 American Institute of Physics.
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    12. 12
      Zhang, J. X.; Zeches, R. J.; He, Q.; Chu, Y. H.; Ramesh, R. Nanoscale Phase Boundaries: a New Twist to Novel Functionalities Nanoscale 2012, 4, 6196– 6204 DOI: 10.1039/c2nr31174g
      12
      Nanoscale phase boundaries: a new twist to novel functionalities
      Zhang, J. X.; Zeches, R. J.; He, Q.; Chu, Y.-H.; Ramesh, R.
      Nanoscale (2012), 4 (20), 6196-6204CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      A review. In functional materials, nanoscale phase boundaries exhibit exotic phenomena that are notably absent in their parent phases. Over the past two decades, much of the research into complex oxides (such as cuprate superconductors, CMR manganites and relaxor ferroelecs.) has demonstrated the key role that nanoscale inhomogeneities play in controlling the electronic and/or ionic structure of these materials. One of the key characteristics in such systems is the strong susceptibility to external perturbations, such as magnetic, elec. and mech. fields. A direct consequence of the accommodation of a large no. of cationic substitutions in complex oxides is the emergence of a no. of phys. phenomena from essentially the same crystal framework. Recently, multiferroic behavior, which is characterized by the co-existence and potential coupling of multiple ferroic order parameters, has captured considerable worldwide research interest. The perovskite, BiFeO3, exhibits robust ferroelectricity coupled with antiferromagnetism at room temp. A rather unique feature of this material system is its ability to "morph" its ground state when an external mech. constraint is imposed on it. A particularly striking example is obsd. when a large (∼4 to 5%) compressive strain is imposed on a thin film through the epitaxial constraint from the underlying substrate. Under these conditions, the ground state rhombohedral phase transforms into a tetragonal-like (or a deriv. thereof) phase with a rather large unit cell (c/a ratio of ∼1.26). When the epitaxial constraint is partially relaxed by increasing the film thickness, this tetragonal-like phase evolves into a "mixed-phase" state, consisting of a nanoscale admixt. of the rhombohedral-like phase embedded in the tetragonal-like phase. Such a system gives us a new pathway to explore a variety of mech., magnetic and transport phenomena in constrained dimensions. This article reviews our progress to date in this direction and also captures some possible areas of future research. We use the electromech. response and the magnetic properties as examples to illustrate that its novel functionalities are intrinsically due to the phase boundaries and not the constituent phases. The possible origin of the giant piezoelec. response and enhanced magnetic moment across the boundaries is proposed based on the flexoelec. and flexomagnetic effects.
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    13. 13
      Aso, R.; Kan, D.; Shimakawa, Y.; Kurata, H. Atomic Level Observation of Octahedral Distortions at the Perovskite Oxide Heterointerface Sci. Rep. 2013, 3, 2214– 2220 DOI: 10.1038/srep02214
      13
      Atomic level observation of octahedral distortions at the perovskite oxide heterointerface
      Aso Ryotaro; Kan Daisuke; Shimakawa Yuichi; Kurata Hiroki
      Scientific reports (2013), 3 (), 2214 ISSN:.
      For perovskite oxides, ABO3, slight octahedral distortions have close links to functional properties. While perovskite oxide heterostructures offer a good platform for controlling functionalities, atomistic understanding of octahedral distortion at the interface has been a challenge as it requires precise measurements of the oxygen atomic positions. Here we demonstrate an approach to clarify distortions at an atomic level using annular bright-field imaging in aberration-corrected scanning transmission electron microscopy, which provides precise mappings of cation and oxygen atomic positions from distortion-minimized images. This technique revealed significant distortions of RuO6 and ScO6 octahedra at the heterointerface between a SrRuO3 film and a GdScO3 substrate. We also found that structural mismatch was relieved within only four unit cells near the interface by shifting the oxygen atomic positions to accommodate octahedral tilt angle mismatch. The present results underscore the critical role of the oxygen atom in the octahedral connectivity at the perovskite oxide heterointerface.
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    14. 14
      Ghica, C.; Negrea, R. F.; Nistor, L. C.; Chirila, C. F.; Pintilie, L. Nanoscale Monoclinic Domains in Epitaxial SrRuO3 Thin Films Deposited by Pulsed Laser Deposition J. Appl. Phys. 2014, 116, 023516– 023516–11 DOI: 10.1063/1.4889932
      There is no corresponding record for this reference.
    15. 15
      Specht, E. D.; Christen, H. M.; Norton, D. P.; Boatner, L. A. X-Ray Diffraction Measurement of the Effect of Layer Thickness on the Ferroelectric Transition in Epitaxial KTaO3/KNbO3Multilayers Phys. Rev. Lett. 1998, 80, 4317– 4320 DOI: 10.1103/PhysRevLett.80.4317
      There is no corresponding record for this reference.
    16. 16
      Williamson, G. K.; Hall, W. H. X-Ray Line Broadening from Filed Aluminium and Wolfram Acta Metall. 1953, 1, 22– 31 DOI: 10.1016/0001-6160(53)90006-6
      16
      X-ray line broadening from filed aluminum and tungsten
      Williamson, G. K.; Hall, W. H.
      Acta Metallurgica (1953), 1 (No. 1), 22-31CODEN: AMETAR; ISSN:0001-6160.
      Methods of analysis previously used in the interpretation of line broadening are discussed and are shown to be inadequate; more reliable methods being outlined. An analysis of published results by one of these methods suggests that the observed effects can be attributed to simultaneous small particle size and strain broadening. Measurements of the changes in intensity distribution have been made, with a Geiger counter spectrometer, in the spectra of cold-worked Al and W. The line breadths may be attributed to simultaneous small particle size and strain broadening, the latter predominating, particularly at the higher Bragg angles, and it is shown that the observed effects are produced by dislocations or some similar structural fault. The observed rise in the breadths of the high angle lines from annealed materials suggests that some dislocations remain after annealing. Fourier analysis of the line shapes in general merely confirm the results of the analysis of the line breadths, but in the case of the recovered specimens it suggests that the dislocations form into walls ("polygonization").
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    17. 17
      Metzger, T.; Hopler, R.; Born, E.; Ambacher, O.; Stutzmann, M.; Stömmer, R.; Schuster, M.; Göbel, H.; Christiansen, S.; Albrecht, M.; Strunk, H. P. Defect Structure of Epitaxial GaN Films Determined by Transmission Electron Microscopy and Triple-Axis X-Ray Diffractometry Philos. Mag. A 1998, 77, 1013– 1025 DOI: 10.1080/01418619808221225
      17
      Defect structure of epitaxial GaN films determined by transmission electron microscopy and triple-axis X-ray diffractometry
      Metzger, T.; Hopler, R.; Born, E.; Ambacher, O.; Stutzmann, M.; Stommer, R.; Schuster, M.; Gobel, H.; Christiansen, S.; Albrecht, M.; Strunk, H. P.
      Philosophical Magazine A: Physics of Condensed Matter: Structure, Defects and Mechanical Properties (1998), 77 (4), 1013-1025CODEN: PMAADG; ISSN:0141-8610. (Taylor & Francis Ltd.)
      Important structural characteristics (correlation lengths of columnar crystallites, dislocation densities, angles of rotational disorder) of hexagonal GaN grown by metalorg. CVD on c-plane sapphire are detd. by TEM and triple-axis x-ray diffractometry. GaN films exhibit an edge dislocation d. in the range of 1011 cm-2, a tilt and twist angle of 0.1° and 1.3° and a columnar structure with a lateral and vertical correlation length of 150 and 1000nm resp. The detn. of correlation lengths and dislocation densities from x-ray patterns was undertaken using two independent evaluation methods which are discussed. Also triple-axis x-ray diffractometry is a highly suitable technique for the sepn. of different kinds of structural defects such as edge and screw dislocations that lead to a characteristic broadening of sym. and asym. Bragg reflections. The correlation lengths and dislocation densities were obtained addnl. by electron transmission microscope studies which are in good agreement with the x-ray diffraction results.
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    18. 18
      Hÿtch, M. J.; Snoeck, E.; Kilaas, R. Quantitative Measurement of Displacement and Strain Fields from HREM Micrographs Ultramicroscopy 1998, 74, 131– 146 DOI: 10.1016/S0304-3991(98)00035-7
      18
      Quantitative measurement of displacement and strain fields from HREM micrographs
      Hytch, M. J.; Snoeck, E.; Kilaas, R.
      Ultramicroscopy (1998), 74 (3), 131-146CODEN: ULTRD6; ISSN:0304-3991. (Elsevier Science B.V.)
      A method for measuring and mapping displacement fields and strain fields from high-resoln. electron microscope (HREM) images has been developed. The method is based upon centering a small aperture around a strong reflection in the Fourier transform of an HREM lattice image and performing an inverse Fourier transform. The phase component of the resulting complex image is shown to give information about local displacements of at. planes and the two-dimensional displacement field can be derived by applying the method to two non-colinear Fourier components. Local strain components can be found by analyzing the deriv. of the displacement field. The details of the technique are outlined and applied to an exptl. HREM image of a domain wall in ferroelec.-ferroelastic PbTiO3.
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    19. 19
      Farnell, G. W.; Cermak, I. A.; Silverster, P.; Wong, S. K. Capacitance and Field Distributions for Interdigital Surface-Wave Transducers IEEE Trans. Sonics Ultrason. 1970, SU-17, 188– 195 DOI: 10.1109/T-SU.1970.29560
      There is no corresponding record for this reference.
    20. 20
      Al-Shareef, H. N.; Dimos, D.; Raymond, M. V.; Schwartz, R. W.; Mueller, C. H. Tunability and Calculation of the Dielectric Constant of Capacitor Structures With Interdigital Electrodes J. Electroceram. 1997, 1, 145– 153 DOI: 10.1023/A:1009924716633
      20
      Tunability and calculation of the dielectric constant of capacitor structures with interdigital electrodes
      Al-Shareef, H. N.; Dimos, D.; Raymond, M. V.; Schwartz, R. W.; Mueller, C. H.
      Journal of Electroceramics (1997), 1 (2), 145-153CODEN: JOELFJ; ISSN:1385-3449. (Kluwer Academic Publishers)
      The voltage dependence of the dielec. const. of ferroelec. materials makes them attractive for use as tuning elements in microwave circuits. In this study, capacitance tuning and loss measurements were performed on ferroelec. materials prepd. by the sol-gel process and radio-frequency magnetron sputtering. The authors find that Pb(Zr,Ti)O3 (PZT) thin films with interdigital electrodes can be fabricated with reasonably low loss to make them useful for room temp. tuning applications. In addn., it is found that high temp. post-deposition annealing of both sputtered SrTiO3 (ST) and sol-gel derived BaTiO3 (BT) films markedly improves their tuning and loss factor characteristics. By annealing the samples in the temp. range 1000-1100°, the tunability was increased by as much as a factor of seven, while the dissipation factors were decreased to values of 0.3-0.5%. In addn., it is shown that the permittivity of the films in these interdigitated capacitor structures can be calcd. using an anal. model previously described by G. Farnell et al. (1970).
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    21. 21
      Piorra, A.; Petraru, A.; Kohlstedt, H.; Wuttig, M.; Quandt, E. Piezoelectric Properties of 0.5(Ba0.7Ca0.3TiO3) – 0.5[Ba(Zr0.2Ti0.8)O3] Ferroelectric Lead-Free Laser Deposited Thin Films J. Appl. Phys. 2011, 109, 104101– 104101–4 DOI: 10.1063/1.3572056
      21
      Piezoelectric properties of 0.5(Ba0.7Ca0.3TiO3) - 0.5[Ba(Zr0.2Ti0.8)O3] ferroelectric lead-free laser deposited thin films
      Piorra, A.; Petraru, A.; Kohlstedt, H.; Wuttig, M.; Quandt, E.
      Journal of Applied Physics (2011), 109 (10), 104101/1-104101/4CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Ferroelec. lead-free thin films of 0.5(Ba0.7Ca0.3TiO3) - 0.5Ba(Zr0.2Ti0.8)O3 (BCZT) were successfully deposited by pulsed laser deposition on Pt/TiO2/SiO2/Si substrates using a ceramic BCZT target prepd. by conventional solid state reaction. The in (111) direction oriented 600 nm thick films shows a clamped piezoelec. response of approx. d33,f = 80 pm/V and a dielec. coeff. of about εr = 1010; these are close to values obtained for lead zirconate titanate (PZT) films. (c) 2011 American Institute of Physics.
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    22. 22
      Lin, Y.; Wu, G.; Qin, N.; Bao, D. Structure, Dielectric, Ferroelectric, and Optical Properties of (1 – x)Ba(Zr0.2Ti0.8)O3 – x(Ba0.7Ca0.3)TiO3 Thin Films Prepared by Sol–Gel Method Thin Solid Films 2012, 520, 2800– 2804 DOI: 10.1016/j.tsf.2011.12.030
      There is no corresponding record for this reference.
    23. 23
      Kang, G.; Yao, K.; Wang, J. (1 – x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 Ferroelectric Thin Films Prepared from Chemical Solutions J. Am. Ceram. Soc. 2012, 95, 986– 991 DOI: 10.1111/j.1551-2916.2011.04877.x
      There is no corresponding record for this reference.
    24. 24
      Luo, B. C.; Wang, D. Y.; Duan, M. M.; Li, S. Growth and Characterization of Lead-Free Piezoelectric BaZr0.2Ti0.8O3–Ba0.7Ca0.3TiO3 Thin Films on Si Substrates Appl. Surf. Sci. 2013, 270, 377– 381 DOI: 10.1016/j.apsusc.2013.01.033
      There is no corresponding record for this reference.
    25. 25
      Luo, B. C.; Wang, D. Y.; Duan, M. M.; Li, S. Orientation-Dependent Piezoelectric Properties in Lead-Free Epitaxial 0.5BaZr0.2Ti0.8O3–0.5Ba0.7Ca0.3TiO3 Thin Films Appl. Phys. Lett. 2013, 103, 122903– 122903–5 DOI: 10.1063/1.4821918
      There is no corresponding record for this reference.
    26. 26
      Khachaturyan, A. G. Ferroelectric Solid Solutions with Morphotropic Boundary: Rotational Instability of Polarization, Metastable Coexistence of Phases and Nanodomain Adaptive States Philos. Mag. 2010, 90, 37– 60 DOI: 10.1080/14786430903074789
      26
      Ferroelectric solid solutions with morphotropic boundary. Rotational instability of polarization, metastable coexistence of phases and nanodomain adaptive states
      Khachaturyan, A. G.
      Philosophical Magazine (2010), 90 (1-4), 37-60CODEN: PMHABF; ISSN:1478-6435. (Taylor & Francis Ltd.)
      Ferroelec. solid solns. with different symmetry phases at opposite sides of the perovskite-based pseudo-binary phase diagram are considered. An anal. of the concn. dependence of polar anisotropy shows conditions under which the anisotropy changes sign, assuming zero value. The orientational stability of a polarization was investigated near this point, which is also a position of so-called morphotropic boundary (MB). The symmetry conditions resulting in the high-symmetry ferroelec. phases with finite temp.-compn. stability field as well as the thermodn. conditions providing their metastable two-phase coexistence are formulated. A loss of orientation stability around MB results in a divergence of fluctuations of polarization direction and a peak of dielec. susceptibility. The calcd. energy of a 90° domain wall demonstrates that this energy is drastically reduced or even vanishes at the MB. The latter leads to a miniaturization of the domain structure to the nano-scale level and the formation of a mixed adaptive state. The miniaturization explains the obsd. special properties of ferroelecs. near MB that are extrinsic for the nanodomain adaptive state. It also explains the obsd. diffraction patterns generated by nanodomains of the mixed state. A coherent scattering generated by nanodomains mimics diffraction patterns that could be confused with a pattern generated by a homogeneous monoclinic phase.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b06745

    • Dielectric constant and loss of used substrates, temperature dependence of SrTiO3 dielectric constant, comparison with similar results reported on lead-free ferroelectric films, and references. (PDF)


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