Microstructural influence on ferroelectric domain pattern and piezoelectric properties of Na0.5Bi0.5TiO3 thin films

Ceramics International 44 (2018) 14556–14562

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Microstructural influence on ferroelectric domain pattern and piezoelectric properties of Na0.5Bi0.5TiO3 thin films Kumaraswamy Miriyala, Ranjith Ramadurai

T

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Department of Materials Science and Metallurgical Engineering, Indian Institute of Technology, Hyderabad, Kandi, Sangareddy, Telangana 502285, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Lead-free piezoelectrics PFM NBT Ferroelectricity, Piezoelectricity

Sodium bismuth titanate (Na0.5Bi0.5TiO3: NBT) thin films were fabricated under various growth conditions (substrate temperature from 400 °C to 650 °C and oxygen pressures from 50 to 200 mTorr) by using pulsed laser deposition technique. The films grown at low partial pressures (< 100 mTorr) exhibited a preferred orientation along the < 220 > direction and at higher partial pressures (> 100 mTorr) exhibited polycrystalline nature. The microstructures were tuned from coarse faceted grains to fine spherical grains by varying the ambient pressures and the growth temperatures. The ferroelectric domain studies reveal that in case of fine spherical grains, the domain pattern was dominated by the surface morphological features and in the case of coarse faceted grain structure, domain features were independent of its morphology. Fast Fourier Transform (FFT) spectrum analysis of the domain patterns confirmed that only highly oriented films possessed periodic domain pattern and the periodicity is in the range of 140–240 nm. Further, the estimated piezocoefficient value (d33) increased from 16 to 31 pm/V with increasing the oxygen partial pressures (50–200 mTorr) and substrate temperatures (400–650 °C). The leakage current density measurements confirm that films grown at low partial pressures possess relatively larger leakage current density at room temperature.

1. Introduction Sodium bismuth titanate (Na0.5Bi0.5TiO3: NBT) is one among the promising lead-free piezoelectric with a piezocoefficient (d33) of 79 pC/ N and exhibits a room temperature ferroelectric behavior (remnant polarization (Pr) ~ 38 µC/cm2) [1]. NBT possess mixed occupation of heterovalent cations at the A-site consisting of Na+ and Bi3+ [2]. The isoelectronic character of Bi3+ with Pb2+ has motivated scientific community to explore the structural and electrical properties of NBT and its solid solutions as it could be a potential replacement for the lead-based piezoelectrics [3–5]. Though NBT undergoes several structural phase transitions with temperature [6], the in-situ TEM and structural refinement studies confirm that it stabilizes in rhombohedral symmetry at room temperature [7,8]. NBT thin films have been fabricated by various deposition techniques such as sol-gel [9–12] or metalorganic decomposition [13], sputtering [14,15] and pulsed laser deposition (PLD) [16–18]. However, PLD is known for its versatility in the fabrication of complex oxide thin films. The growth conditions facilitate to achieve the target stoichiometry, aspired texture, and microstructural features in the fabricated thin film [19–21]. Oxygen partial pressure is one among the process parameter, which facilitates tunable microstructure in PLD. Moreover

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Corresponding author. E-mail address: [email protected] (R. Ramadurai).

https://doi.org/10.1016/j.ceramint.2018.05.074 Received 20 March 2018; Received in revised form 20 April 2018; Accepted 9 May 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

lowering the partial pressures could increase the oxygen vacancies that can deteriorate the physical properties of the film. Thus attaining the optimized growth parameters is a crucial step in PLD technique. Moreover, the microstructural features influence the nano-scale ferroelectric (FE) domain distribution and piezoelectric properties. Thus, it is crucial to understand the role of grain and grain boundary influences on the FE domain distribution. There have been limited studies in the literature that correlates the FE domain pattern with the microstructural features of the thin films [22–26]. It is conceived that the grain boundaries could pin the domain wall and act as a physical boundary of the domains [27,28] But recent studies confirm that local strain and the electric field generated at grain boundaries could also favor the interaction of domains between the adjacent grains. This interaction favors the domain wall motion across the grain boundaries. However, the nature of grain boundaries will also decide the domain wall motion across the boundaries. Marincel et al. [29] confirm that low angle grain boundaries (≤ 15°) are more feasible for the domain wall motion in PZT epilayers grown on specific substrates. Further S. Mantri et al. [30] studied the requirements for domain walls to cross various grain boundaries by using mathematical modeling. Therefore, tuning the microstructure and studying its influence on the domain distribution is crucial for applications.

Ceramics International 44 (2018) 14556–14562

K. Miriyala, R. Ramadurai

In this study, we fabricated NBT thin films under different oxygen partial pressures and substrate temperatures. The effect of these growth parameters on microstructural features, crystallographic orientations and the resultant ferroelectric domain pattern were studied. Piezoresponse force microscopy (PFM) studies were employed to study the microstructural influence on FE domain distribution and role of grain boundaries on domain wall motion. Further, the influence of microstructure on macroscopic electrical properties such as leakage current was studied. 2. Experimental procedure NBT target was synthesized via conventional solid state reaction route. The achieved ceramic pellet density was around 90–92% of theoretical density and later used as a target material for ablation. A KrF excimer laser (COMPex Pro 205, Coherent Lambda Physik) of 248 nm wavelength was used for the ablation, and a commercially available platinum coated silicon wafer with the following sequence Pt (111)/TiO2/SiO2/Si (100) (make: MTI Corporation, USA) was used as a substrate. The depositions were carried out at different substrate temperatures (400–650 °C) and oxygen partial pressures (50–200 mTorr) to tune the microstructural features of the fabricated thin films. The crystal structure of the films was studied by using X-ray diffractometer (Bruker AXS D8 Discover), with Cu Kα radiation in grazing incidence mode to eliminate the substrate effects. The morphology and cross-sectional features of the films were studied by using a Fieldemission scanning electron microscopy (FE-SEM, Carl Zeiss). 500 µm diameter circular platinum dots were deposited by physical masking in electron beam evaporation. These circular dots served as a top electrode and the Pt layer on the substrate was masked during the deposition, which acted as the bottom electrode. The room temperature, currentvoltage (I-V) characteristics were measured by using Keithley source meter (2450). Piezoresponse force microscopy (PFM) was performed using a Nanoscope V controller (make; Bruker, Model: Dimension ICON). Commercially available Pt/Ir coated conductive tips were used to record the PFM images with a drive frequency of 15 kHz and spring constant of 1–5 N/m. 3. Results and discussion 3.1. Crystallographic phase and microstructural analysis The phase purity and crystallinity of the NBT films were examined by using grazing incidence x-ray diffraction (GI-XRD) studies. Fig. 1(a) shows the diffraction pattern of the films grown at various oxygen partial pressures (50, 100, 150 and 200 mTorr) and at a substrate temperature of 650 °C. From the pattern, it is observed that films were

grown at 50 and 100 mTorr of ambient pressure possessed an impure pyrochlore phase of Bi2Ti2O7 (ICDD NO: 01-076-8225) and the corresponding volume fractions are 6.7% and 14% respectively. Further, the films that were grown at 150 and 200 mTorr were phase pure and polycrystalline in nature. The film grown at 50 mTorr exhibited a preferred orientation along < 220 > direction and a further increase in the oxygen partial pressure stabilizes the film as polycrystalline. Fig. 1(b) shows the growth diagram of NBT films constructed by using various substrate temperatures and oxygen partial pressures. The intensity ratio was estimated from the ratio of two consecutive highest intense peaks using Eq. (1). (110), (100) reflections were chosen in the case of polycrystalline films. (220) and (100) reflections were chosen in the case of oriented films.

Intensity ratio = Ihk0 /(Ihk0 + Ih00)

(1)

ICDD NO: 01-074-9525 data of NBT sample was used as a reference and thus the films that possessed a calculated (Eq. (1)) intensity ratio of above 80% was considered as highly oriented and if the ratio is less than 80%, those films were considered as polycrystalline in nature [31]. Thus, from our growth diagram it is confirmed that for the substrate temperatures 575–650 °C films grown at low oxygen partial pressure (50 mTorr) exhibits a preferred orientation feature along < 220 > direction and other films are showing polycrystalline nature. However, for relatively lower growth temperature (400 °C) the preferred orientation was achievable up to 100 mTorr. The films grown in the range of 575–650 °C reveals an empirical threshold of 100 mTorr and above to transform from a textured film to a polycrystalline film. It is known that films grow into a specific orientation or a plane whose surface energy is minimized. In ABO3 type perovskite ceramics (110) is thermodynamically more stable plane than others due to the larger packing fraction [32]. At low partial pressures (50 mTorr), the ablated species would possess higher mean free path due to less collision with oxygen molecules. This helps the ablated species to acquire higher ad-atom mobilities on the substrate surface [32]. Thus, the adsorbed species on the substrate surface could be mobile and nucleate into the planes of relatively larger packing fractions. On the other hand, the films grown at high partial pressures the ablated species are expected to possess relatively low kinetic energy and eventually a low adatom mobility. This could result in a randomly oriented growth in the film that contains all possible crystallographic planes. Thus, in our case, we have seen a highly oriented growth in the films grown at pressures < 100 mTorr and a polycrystalline nature of the films grown at pressures > 100 mTorr. Fig. 2(a), (b) and (c), (d) shows the microstructural features of the NBT films grown at 50 and 150 mTorr respectively. From the micrographs, it is evident that microstructural tunability of NBT films is plausible by varying the growth parameters. At low partial pressures

Fig. 1. (a) X-ray Diffraction pattern of NBT thin films grown at 650 °C. (b) Growth diagram of NBT films constructed by using various substrate temperatures and O2 pressures. 14557

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Fig. 2. (a) - (b) FE-SEM Micrograph of NBT films grown at 50 mTorr pressure. (c)- (d) FE-SEM Micrograph of NBT films grown at 150 mTorr pressure. (e)- (f) Kikuchi pattern of NBT film grown at 650 °C and 150 mTorr.

Fig. 3. Cross-sectional FE-SEM images of NBT film grown at 650 °C.

(50 mTorr) when you increase the temperature, the microstructural features are changed from a fine spherical grain structure to an elongated needle-shaped grain structure (see Fig. 2(a), (b)). However, at high partial pressures (150 mTorr) when you increase the growth temperature the morphological features transformed from a fine spherical grain structure to coarse faceted columnar grain structure (see Fig. 2(c), (d)). During the deposition, the increase in the substrate temperature can cause the Ostwald ripening and allows the grains to grow bigger [33]. Further, at higher temperatures these grains will grow into columns along the thickness axis and exhibit columnar structure as shown in the inset of Fig. 2(d). Fig. 3 shows the cross-

sectional SEM of the films grown at various oxygen partial pressures from 50 to 200 mTorr at a substrate temperature of 650 °C 3.2. Piezoresponse force microscopy Piezoresponse Force Microscopy (PFM) is a versatile technique to study the ferroelectric domain structures of thin films [34]. It facilitates imaging the piezoresponse (in-plane and out of plane) along with the morphology, from which the correlation between the grain structure and the ferroelectric (FE) domain patterns can be studied. Fig. 4 represents the morphology and corresponding PFM images of NBT films

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Fig. 4. (a) Morphology, (b)-(c) Out of plane amplitude and phase images, (d)-(e) in-plane amplitude and phase images of NBT films grown at 625 °C and 150 mTorr.

Fig. 5. (a)-(c) Morphology NBT film grown at 650 °C, 625 °C, 600 °C and 150 mTorr pressure. (d)-(f) In-plane piezoresponse phase images of NBT films grown at 650 °C, 625 °C, 600 °C and 150 mTorr pressure.

grown at 625 °C and 150 mTorr oxygen partial pressure. Fig. 4(a) shows the morphology and Fig. 4(b), (c), (d) and (e) are the corresponding out of plane amplitude, out of plane phase, in-plane amplitude, and inplane phase images respectively. The measured piezoresponse amplitude image is proportional to the local piezoelectric coefficient (dzz) and piezoresponse phase image represents the possible orientations of the

polarization vector [35]. Fig. 5(a), (b) and (c) shows the morphology of NBT films grown at 650 °C, 625 °C and 600 °C at 150 mTorr respectively. Fig. 5(d), (e) and (f) shows the corresponding in-plane (IP) phase images. The films grown at 650 °C possessed a faceted grain structure, whereas, films grown at 625 °C and 600 °C exhibited a fine spherical grain structure. In case of the faceted grain structure, the obtained FE

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Table 1 Angular relationship of crystallographic planes with the family of polarization vectors in NBT rhombohedral crystal system. Planes having polarization components in the NBT Rhombohedral system

Planes having low angle grain boundaries (≤ 15°) with polarization plane

Family of planes showing high angle grain boundaries (> 15°) with polarization planes

(-211),(-311) (1-21),(1-31) (11-2),(11-3) (-1-12),(-1-13) (2-1-1),(3-1-1) (-12-1),(-13-1)

{100},{200}{110},{220}, {211},{311} {100},{200}{110},{220} {100},{200}{110},{220} {100},{200}{110},{220} {100},{200}{110},{220} {100},{200}{110},{220} {100},{200}{110},{220} {100},{200}{110},{220}, {211},{311}

(111) (-111) (1-11) (11-1) (-1-11) (1-1-1) (-11-1) (-1-1-1)

domain features are independent of its surface morphological features. However, in case of spherical grained structure, the FE domain patterns are dominated by its morphological features, which is evident from the identical nature of grain and it's corresponding domain features as shown in Fig. 5. Further, in case of faceted grain structure, we have observed the extension of domains beyond the grain boundaries (see region 1, 2, 3 in Fig. 5(d)). However, in case of spherical grain structure, the majority of the domains tends to have the grain boundary as the domain boundary. In addition to the identification of various grain orientations by xray diffraction, electron backscatter diffraction (EBSD) study was performed to confirm the presence of various grain orientation on the surface where the PFM studies were carried out. The EBSD pattern shown in Fig. 2(e), (f) confirms the presence of various grain orientations. The polycrystalline NBT film with faceted microstructure revealed the extension of FE domains across the grain boundaries (see

Fig. 5(a) & (d)). Such an extension could plausibly arise due to the close crystallographic relation between the adjacent grains. Table 1 shows the calculated grain boundary angles between the family of planes and polarization components (i.e. between {111} in rhombohedral NBT system and crystallographic planes observed in the NBT films). In the case of grains with {100}, {200}, {110} and {220} orientations they possess a high angle grain boundary (> 15°) with the eight equivalent polarization invariants along {111}. However, {112} and {113} orientations exhibited (see Table 1 for details) a low angle grain boundary (< 15°) with the polarization invariants of NBT. The ferroelectric interaction among the dipoles can overcome the grain boundary pinning of domain walls in the case of low angle grain boundaries [29]. Thus, a small electric field enables the domain wall motion across the grain boundaries. However, at high angle grain boundaries, the domain walls are pinned very strongly and need a relatively larger field to enable the domain wall crossing the grain boundary. Thus, in NBT thin films with rhombohedral symmetry the grain orientations belonging to {112} and {113} families (see Table 1) will form low angle grain boundary with planes that belong to {111} orientation and facilitates dipolar interaction overcome the grain boundary pinning and effectively domain spreading across the grains. Moreover, recent studies performed in the tetragonal ferroelectric system confirm that among the several grain boundaries Σ3 (60° rotation around [111]), Σ7 (38.2° rotation around [111]) are the two CSL (coincidence-site lattice) grain boundaries whose grain boundary energies are low [36]. This helps the domain walls to meet continuously and satisfy the polarization charge minimization at the grain boundary [30]. In addition, in our previous studies we reported the influence of surface roughness on defining the physical boundary for the domain wall propagation [37]. Further, the FE domain periodicity was analyzed by performing a 2D Fast Fourier Transform (FFT) spectrum and its power spectral distribution data. Fig. 6(a) (b) shows the IP-Phase images and its corresponding FFT plots (see inset) for NBT films grown at 625 °C and at a

Fig. 6. (a)-(b) In-plane phase images of NBT films grown at 625 °C and at a pressure of 50 mTorr and 150 mTorr respectively (inset shows the FFT spectrum of domain pattern). (c) Power spectral density graph (d) Correlation between domain size and grain size for NBT film grown at 50 mTorr pressure. 14560

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Fig. 7. (a) Local piezoresponse phase and d33 hysteresis curves. (b) Piezocoefficient (d33) Variation with growth Parameters.

decreases, which is in accordance with the dielectric loss tangent values. This is an indicative of the role of oxygen vacancies on leakage currents in NBT thin films [40]. Hence, the optimized oxygen partial pressures could result in improved electrical properties in NBT thin films. 4. Conclusions

Fig. 8. (a) Leakage current density plot of NBT films grown at 650 °C and various oxygen partial pressures. (Inset shows the variation in dielectric constant and dissipation at 1 MHz as a function of oxygen partial pressures).

partial pressure of 50 and 150 mTorr. The FFT pattern of the film grown at low partial pressure (50 mTorr) exhibits two distinguishable Fourier Transform peaks, whereas the films grown at high partial pressure (150 mTorr) show a randomly distributed peak. Hence, the FFT analysis of the domain patterns confirm that films grown at low partial pressures exhibits periodic nature, in its domain distribution, whereas films grown at high partial pressures possess a random distribution of domain structure [38]. The domain periodicity of all the highly oriented NBT thin films were found to be in the range of 140–240 nm for various growth temperatures. In addition, as grain the size increased the corresponding ferroelectric domain sizes were found increasing (see Fig. 6(d)). Moreover, in case of polycrystalline films, we have observed the fractal-like domain pattern as observed in various FE thin films with rhombohedral symmetry [39]. The local piezoresponse studies was performed under the PFM tip by applying a forward and reverse voltage ramp from −10 V to +10 V, with the combination of an ac voltage. Fig. 7(a) shows the phase switching curve along with d33 (piezo coefficient) hysteresis loop. This feature confirms the ferroelectric nature of the sample. Fig. 7(b) shows the variation of the piezocoefficient with the morphology of NBT thin films. The polycrystalline thin films possessed relatively larger piezocoefficient, which could be due to the combination of domains and domain wall motion. In highly oriented samples with smaller grain, the anisotropy energy and the movement of domain walls hindered by the grain boundary, could be attributed to relatively lower piezo-response behavior. Fig. 8 shows the variation of dielectric constant, dissipation (inset) and leakage current density under an applied electric field for the films grown at 650 °C with various oxygen partial pressures. At a given field as the oxygen partial pressures increases, the leakage current density

In summary, NBT thin films were fabricated under various oxygen partial pressures (50–200 mTorr) and substrate temperatures (400–650 °C) by pulsed laser deposition technique. Irrespective of the substrate temperatures studied, the films grown at low oxygen partial pressures (< 100 mTorr) tend to possess a high orientated growth with a texturing along < 220 > direction and for high pressures (100 mTorr and above) films are stabilized with polycrystalline features. The piezoresponse force microscopy studies evidently showed that the periodic domain pattern was observed for highly oriented films and domains extending beyond the grain boundaries. However, in the case of polycrystalline films with various orientations, the grain boundary angle seems to be the key parameter in deciding the domain crossover across the grain boundaries. The estimated grain boundary angles between the planes (-211), (-111) and (-311), (-111) were showing that there exist some of the planes that form a low angle grain boundary (≤ 15°) with polarization components which would favor the domain walls to cross over the adjacent grain boundaries. Further, the increment in the local piezocoefficient value (d33) was observed with increasing the partial pressures and growth temperature, which could be due to the polycrystalline nature of the films with more isotropic piezo properties. In addition, the macroscopic leakage current density values decreases with increasing the oxygen partial pressures which can be attributed to a reduction in the oxygen vacancies in the film. Acknowledgment One of the author Kumaraswamy Miriyala would like to thank the Ministry of Human Resource Development (MHRD), Government of India for providing fellowship. References

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