Periodicity Dependence of the Built-in Electric Field in (Ba 0.7 Ca 0.3 )

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Periodicity dependence of the built-in electric field in (Ba Ca )TiO/Ba(Zr Ti )O ferroelectric superlattices 0.7

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Qianru Lin, Danyang Wang, Zhi-Gang Chen, Wenfeng Liu, Sean Lim, and Sean Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08943 • Publication Date (Web): 10 Nov 2015 Downloaded from http://pubs.acs.org on November 14, 2015

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ACS Applied Materials & Interfaces

Periodicity dependence of the built-in electric field in (Ba0.7Ca0.3)TiO3/Ba(Zr0.2Ti0.8)O3 ferroelectric superlattices Qianru Lin1, Danyang Wang1*, Zhigang Chen2, Wenfeng Li3, Sean Lim4 and Sean Li1 1

School of Materials Science and Engineering, The University of New South Wales, Sydney,

NSW 2052, Australia 2

Materials Engineering, The University of Queensland, Brisbane, QLD 4072, Australia

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State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University,

Xi’an 710049, China 4

Electron Microscopy Unit, University of New South Wales, Sydney, NSW 2052, Australia

KEYWORDS: lead-free, ferroelectric superlattice thin films, periodicity dependence, built-in electric field, laser molecular beam epitaxy

Abstract

Symmetric ferroelectric superlattices consisting of (Ba0.7Ca0.3)TiO3 (BCT) and Ba(Zr0.2Ti0.8)O3 (BZT) layers were successfully grown on La0.7Sr0.3MnO3 electroded (001)-oriented SrTiO3

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substrates by laser molecular beam epitaxy. With the monitor of reflective high-energy electron diffraction (RHEED), the growth mode and rate were precisely controlled to realize the desired superlattice periodicity as confirmed by both X-ray diffraction (XRD) and transmission electron microscopy (TEM) results. The microscopic piezoelectric response and macroscopic ferroelectric properties were investigated as a function of periodicity of the BCTm/BZTm (m = 3, 5, 10 and 15 unit cells) superlattices. The existence of a built-in electric field was confirmed in all the superlattices and its strength was highly dependent on the periodicity. The excellent tunability of built-in electric field opens a path for designing microelectronic devices with various functionalities based on BCTm/BZTm superlattices.

1. INTRODUCTION Due to the potential of superlattices exerted on exploring new functionalities or enhancing physical properties of thin films,1-6 the structural geometry of an artificial superlattice structure (e.g. stacking sequence, layer thickness, periodicity etc.) is currently of great scientific interest. The influence of geometry on electrical and optical properties of superlattice thin films has been confirmed by numerous studies, ranging from semiconductor to ferroelectric materials.7-14 Recently, the geometry dependence of the built-in electric field in semiconductor superlattices GaN/AlN and InN/GaN was theoretical investigated by Cui et al. based on all-electron density functional theory calculations. Results indicated the strength of the built-in electric field was strongly dependent on the superlattice geometric factors, providing more flexibility in device optimization and design.15,16


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Analogous to semiconductors, the built-in electric field in ferroelectric thin films also dominates the applications in various devices. Graded ferroelectric devices (GFDs), characterizing by an offset in hysteresis loop along the polarization axis, give rise to a particular class of transcapacitive ferroelectric devices (or “transpacitors”), which have potential applications in infrared detection, actuation and energy storage.17-19 However, in the case where a symmetric switching between two opposite polarization states is required, such as ferroelectric random access memories, the built-in electric field induced imprint effect or asymmetric switching behaviour may result in a write failure.20,21 Consequently, it is of both scientific and technical significance to tune the strength of the built-in electric field in ferroelectric thin films for different applications. Nevertheless, the studies on tuning the built-in electric field in ferroelectric superlattice by geometric factors are still very limited. In this work, we investigated the periodicity dependence of built-in electric field in (Ba0.7Ca0.3)TiO3/Ba(Zr0.2Ti0.8)O3 (BCT/BCT) ferroelectric superlattice thin films. The relationship between the strength of the built-in electric field and superlattice periodicity was revealed, greatly expanding the functionality of BCT/BZT superlattices through periodicity engineering. 2. EXPERIMENTAL SECTION The BCTm/BZTm superlattice thin films (m represents the number of unit cells of BCT and BZT in one period) were deposited on (001)-oriented SrTiO3 (STO) single crystal by laser molecular beam epitaxy (LMBE). Conductive La0.7Sr0.3MnO3 (LSMO) was used as lattice-matched bottom electrodes. A KrF excimer laser (248 nm) with an energy density of 2 J/cm2 was adopted. The deposition was carried out at a substrate temperature of 800 ˚C under an oxygen partial pressure of 10 mTorr. The repetition rate of the laser was kept at 0.5 Hz and the distance between the targets and substrates was fixed at 5 cm. A 22 kV reflection high-energy electron diffraction


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(RHEED) system was used to real-time monitor the growth rate/mode of the superlattices. In order to facilitate epitaxial growth, the deposition began with the growth of BCT layer (a = 3.955 Å, tetragonal) due to its smaller lattice mismatch with STO substrate (a = 3.905 Å, cubic) and LSMO electrode (a = 3.876 Å, pseudocubic) compared with BZT (a = 4.044 Å, rhombohedral). BCTm/BZTm superlattices with different periodicities of m = 3, 5, 10, 15 were prepared and the total thickness was kept around 50 nm. Symmetric structure was selected due to the average composition is in the vicinity of morphotropic phase boundary (MPB) of BZT-BCT system, which possesses a giant piezoelectric and ferroelectric response at room temperature in both ceramics and films form.22-25 The crystallographic structure was characterised by four-circle Xray diffraction (XRD, Bruker AXS D8 Discover, Madison, WI) with Cu Kα radiation and 4bounce Ge(220) monochromators. The microstructure of BCT15/BZT15 superlattice thin film was performed by the transmission electron microscopy (TEM, JEOL2100). The piezoelectric responses were examined using piezoelectric force microscope (PFM, Asylum, Cypher, Asylum Research, Santa Barbara, CA). Ferroelectric hysteresis loops were measured by Radiant precision workstation (Radiant Technologies, Albuquerque, NW) using triangle signals of frequency 2 kHz at room temperature. 3. RESULTS AND DISCUSSION


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Figure 1 RHEED patterns (a) before BCT deposition; (b) after BCT deposition; (c) before BZT deposition and (d) after BZT deposition; Typical RHEED intensity oscillations for (e) BCT and (f) BZT layers. The RHEED patterns before and after the deposition of BCT and BZT are shown in Figure 1. Both constituent layers exhibit well-contrasted streaky patterns, indicating a layer-by-layer growth mode and smooth surfaces for both materials under the specific deposition conditions. Typical intensity oscillations for BCT and BZT are recorded as shown in Figure 1(c) and (f), respectively. It can be estimated that approximately 12 pulses are equivalent to one monolayer for both BCT and BZT. With the aid of RHEED oscillations, the periodicity of the BCTm/BZTm superlattice thin films can be precisely controlled.


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Figure 2 XRD θ-2θ diffraction patterns for the BCTm/BZTm superlattice thin films with different periodicities (m= 3, 5, 10, 15) Figure 2 shows the XRD theta-2theta scan patterns for the symmetric superlattices with different periodicities. The satellite peaks which are characteristics of a superlattice structure can be clearly identified in all structures, suggesting the (00l)-oriented superlattice structures are successfully grown on STO substrates. The correlation of the angular separation between satellite peaks and periodicity is described by the following equation, 26 /2Δ ∙ where L is the periodicity of the superlattice, ∆θ is the angular separation between two adjacent satellite peaks, θ0 is the Bragg angle of the superlattice (here adopting the position of the 0th order satellite peak) and λ is the wavelength of the incident x-ray. The equation indicates that the superlattice periodicity is in inverse proportion to ∆θ, i.e. the angular separation is bigger in short-period superlattice as shown in Figure 2. When the stacking periodicity is short, such as BCT3/BZT3, the diffraction intensities of the satellite peaks are rather weak.


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Figure 3 (a) XRD reciprocal space mapping around (103) diffraction for BCT3/BZT3 superlattice thin film; (b) X-ray reflectivity profiles for BCT3/BZT3 superlattice grown on LSMO and LSMO bottom electrode layer. XRD reciprocal space mapping (RSM) around the asymmetric (103) plane for BCT3/BZT3 superlattice is shown in Figure 3(a). The top peak in the figure is the LSMO bottom electrode, followed by a STO peak with high intensity underneath. The satellite peak shown on the bottom corresponds to the superlattice structure, indicating high quality of the superlattice. No peak corresponding to a-domains (or 90˚ domains) are visible, implying that BCT3/BZT3 superlattice is highly c-axis oriented.27 Figure 3(b) shows the X-ray reflectivity (XRR) results of the LSMO


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bottom electrode and BCT3/BZT3 superlattice grown on LSMO. The nominal thickness obtained through Fourier transformation for LSMO and LSMO+BCT3/BZT3 heterostructure is ~25 nm and ~73 nm, respectively. The thickness of BCT3/BCT3 superlattice was then determined to be ~50 nm, confirming the good control of the superlattice growth by RHEED. The thicknesses of other supperlattices were maintained the same values by counting the total number of ablation pulses.

Figure 4 (a) Cross-sectional TEM image of BCT15/BZT15 superlattice thin film grown on LSMO-electroded (001)-oriented STO substrate; Selected area electron diffraction (SAED) patterns for (b) BCT15/BZT superlattice and (c) STO substrate. The cross-sectional TEM image of the BCT15/BZT15 superlattice is shown in Figure 4(a). The thicknesses of the superlattice thin film and LSMO bottom electrode are determined to be approximately 53 nm and 24 nm, respectively, which are highly consistent with the values estimated from XRR. The alternate layers of BCT and BZT can be clearly identified in the TEM image, confirming a symmetric superlattice structure. The selected area electron diffraction


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(SAED) patterns of BCT15/BZT15 and STO substrate taken along [011] axis are shown in Figure 4(b) and (c), respectively. The diffraction spots of the superlattice coincide well with those of the substrate, further suggesting the highly epitaxial nature of the superlattice.

Figure 5 (a) Surface morphology; (b) PFM amplitude and (c) associated phase image after boxin-box switching experiment on the BCT3/BZT3 superlattice, clearly demonstrating the ferroelectric switching behaviour. The surface morphology of the BCT3/BZT3 superlattice is shown in Figure 5(a). A flocculent surface can be observed with a surface roughness Ra of ~ 180 pm. The switching behaviour was investigated by applying dc bias voltages between conducting PFM tip and the LSMO bottom electrode. As shown in Figure 5(b) and (c), a 3 × 3 µm2 box was first poled with the tip held at +6 V, and then a smaller box, 1 × 1 µm2, was subsequently scanned with the tip held at -6 V before the PFM phase image was captured. In the DART (Dual AC Resonance Tracking) mode, the positive voltage is applied from bottom electrode up to the film surface, domains with polarization pointing upwards present as dark regions, whereas domains oriented downwards appear bright in the phase image. The complete contrast in the box-in-box phase image illustrates a 180˚ polarization switching within BCT3/BZT3 superlattice. However, the unpoled region around the perimeter exhibited nearly the same phase contrast as that of negatively poled region


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in the centre, confirming the monodomain structures with polarization pointing downwards in BCT3/BZT3 superlattice before poling.

Figure 6 Local switching spectroscopy PFM amplitude-voltage butterfly loops and phase-voltage hysteresis loops for (a) BCT3/BZT3; (b) BCT5/BZT5; (c) BCT10/BZT10; (d) BCT15/BZT15 under ±6 V and (e) BCT10/BZT10; (f) BCT15/BZT15 under ±8 V. The local switching spectroscopy amplitude-voltage butterfly loops and phase-voltage hysteresis loops for the BCTm/BZTm superlattices with different periodicities under a voltage of ±6 V are shown in Figure 6(a)-(d). It is found that the out-of-plane piezoresponse phase can be reversibly switched from 0˚ to 180˚ in the short-period superlattices with m = 3, 5. However, the upward switching of polarization in response to a positive voltage is significantly restricted in the longperiod superlattice thin films (m = 10, 15). This pinning behaviour can be also observed from the asymmetric amplitude-voltage butterfly loops of all samples. The asymmetry of the butterfly loops becomes more evident in the long-period structures with near-zero piezoresponse at the positive voltage side as shown in Figure 6(c) and (d). This phenomenon can be attributed to a


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built-in electric field pointing to the bottom electrode, which facilitates the downward switching of polarization but hinders the upward polarization rotation.28, 29 To further verify the contribution of the built-in electric field in our superlattices, higher voltages (± 8 V) were applied to BCT10/BZT10 and BCT15/BZT15. In marked contrast to the switching behaviour under low voltages, complete phase reversal and weak piezoresponse are clearly seen at positive voltage side as illustrated in Figure 6(e) and (f), further confirming the existence of the downward builtin electric field in our samples. We can also infer that the internal bias in the long-period superlattice (m = 10, 15) is stronger than that in the short-period (m = 3, 5) ones. The piezoelectric coefficients extracted from the amplitude-voltage butterfly loops of BCTm/BZTm superlattices are summarized in Table 1. The d33+ and d33- correspond to the values estimated from the positive and negative sides of amplitude-voltage butterfly loopsby the slopes of the linear portions. Strong piezoresponses especially for d33- are obtained in shortperiod superlattices, which are superior to the other lead-free piezoelectric films and BZT0.5BCT homogeneous thin films.30-34 The excellent piezoelectric properties indicate the advantage of superlattice structure and the significance of tuning the built-in electric field. Table 1 Piezoelectric coefficients of BZTm/BCTm superlattices with different periodicities (d33+, d33- represent the piezoelectric coefficients estimated by the slopes of amplitude-voltage butterfly loops when positive and negative voltages were applied, respectively)

Sample

d33- (pm/V)

d33+ (pm/V)

BCT3/BZT3

150±5

90±5

BCT5/BZT5

142±5

77±5

BCT10/BZT10

63±5

~0

BCT15/BZT15

75±5

~0


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Figure 7 Ferroelectric hysteresis loops for (a) short-period BCT3/BZT3 and (b) long-period BCT15/BZT15 superlattice thin films. The macroscopic ferroelectric hysteresis loops for the short-period BCT3/BZT3 and long- period BCT15/BZT15 superlattice thin films are shown in Figure 7(a) and (b), respectively. Both loops exhibit significant shifts along the polarization axis, indicating the presence of built-in electric fields in the superlattices. It is also found the degree of the polarization offset in BCT15/BZT15 superlattice is considerably larger than that in BCT3/BZT3. Theoretical simulation indicated the polarization offset was proportional to the strength of the built-in electric field.35 Consequently, it can be concluded that the built-in electric field in long-period superlattices is larger than that in short-period ones, which is in good agreement with the microscopic PFM results. Nevertheless, the origins of the internal field are still not completely understood. Both extrinsic (asymmetric contact effects, film/substrate interface etc.) and intrinsic factors (polarization gradient, free space charge etc.) were considered to be responsible for the presence of the built-in electric field.36-40 However, those extrinsic factors are less likely to be the dominant contributors to the variation of built-in electric strength in our BCTm/BZTm superlattices. Considering the


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polarization offset is related to the composition gradient,41 the stronger built-in electric field in long-period BCTm/BZTm superlattices may be attributed to the smoother composition gradient. In addition to the aforementioned factors in homogeneous or compositionally graded thin films, the density of the interfaces, which is particularly critical to superlattices, should also be taken into consideration. As the space charges tend to accumulate near the interfaces in superlattices,42 both polarization distribution and the internal electric field may be significantly altered by the density of interfaces. 4. CONCLUSIONS Symmetric BCTm/BZTm superlattice thin films with different periodicities (m = 3, 5, 10, 15) were successfully grown on the LSMO electroded STO (001) substrates by laser MBE. The growth mode and rate of the superlattices were precisely controlled with the aid of RHEED as confirmed by XRD and TEM results. The asymmetric switching behaviours in both microscopic piezoelectric response and macroscopic ferroelectric hysteresis loops indicated the existence of built-in electric field in all superlattices. The strength of built-in electric field was highly dependent on the superlattice periodicity, i.e. the longer the periodicity, the larger the built-in electric field. Our results provide a feasible method to expand the functionalities of ferroelectric superlattice thin films through tuning the built-in electric field by periodicity engineering, paving the way to the development of a variety of microelectronic devices based on BCT/BZT superlattices. AUTHOR INFORMATION Corresponding Author *Email of corresponding author: [email protected].


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Australian Research Council Discovery Project (Grant No. DP110104629). The financial support from State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, China (Grand number EIPE15206) is also acknowledged. Q.R. Lin acknowledges the support from the CSC under Grant number 201206120021. This work was performed in part at the NSW node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano and micro-fabrication facilities for Australia’s researchers. REFERENCES 1.

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Periodicity Dependence of the Built-in Electric Field in (Ba 0.7 Ca 0.3 )

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