Elasticity Modulation Due to Polarization Reversal and Ionic Motion in

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Functional Inorganic Materials and Devices

Elasticity Modulation due to Polarization Reversal and Ionic Motion in the Ferroelectric Superionic Conductor KTiOPO4 Gustav Lindgren, Anton V. Ievlev, Stephen Jesse, Olga S. Ovchinnikova, Sergei V. Kalinin, Rama Vasudevan, and Carlota Canalias ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07537 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on September 1, 2018

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ACS Applied Materials & Interfaces

Elasticity Modulation due to Polarization Reversal and Ionic Motion in the Ferroelectric Superionic Conductor KTiOPO4 Gustav Lindgren1, Anton Ievlev2,3, Stephen Jesse2,3, Olga S. Ovchinnikova2,3, Sergei V. Kalinin2,3, Rama K. Vasudevan2,3,* and Carlota Canalias1,* 1

Department of Applied Physics, Royal Institute of Technology, Roslagstullsbacken 21, 10691 Stockholm, Sweden 2Center for Nanophase Materials Sciences and 3Institute for Functional Imaging of Materials, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA, * E-mail: [email protected]; [email protected] KEYWORDS. KTiOPO4, Ferroelectricity, Ionic conductor, Surface-screening, Ionic transport, Ferroelectric switching

ABSTRACT: The coupling between ionic degrees of freedom and ferroelectricity has received renewed attention in recent years, given that surface electrochemical processes have been shown to be intrinsically linked to ferroelectric phase stability in ultra-thin ferroelectric films. However, the coupling between bulk ionic transport and local polarization switching has received less attention, as typically the bulk ionic mobilities are low for common ferroelectrics at room temperature. Here, we use a coupled bandexcitation method in conjunction with site-correlated time of flight secondary ion mass spectrometry, to determine the coupling between ferroelectric switching and ionic motion in single crystal KTiOPO4. The local scanning probe measurements indicate a substantial softening, as determined by resonant frequency changes, during reversal of polarization along one direction. These changes are correlated with the mass spectrometry measurements, showing a polarization-dependent accumulation of K ions at the polar surfaces, thus corroborating their role in the screening process. These studies shed light on the interplay between ionic dynamics and bulk ferroelectric switching, and have implications for studies on domain wall conductivity, chemical switching and bulk and surface screening phenomena.

INTRODUCTION The coupling between multiple ferroic order parameters and their modification by charge, electronic, structural and chemical disorder remains a major focal point of studies in condensed matter physics, not only due to rich intrinsic physics, but also due to the possibility of obtaining control over material properties via modification and control of surfaces, interfaces and defects in these systems.1, 2, 3, 4, 5 Some notable examples include the dependence of the metal-insulator transition of VO2 with oxygen vacancy concentration6, 7 or the conductivity of ferroelectric domain walls, which lies at the intersection of the topological defects, dynamics of defects under applied electric fields, and structural disorder. Recently, the strong interaction between chemical state and ferroelectric properties of thin-films, has been shown via experiment, first-principles and phenomenological theory.8, 9, 10 The primary functional property of ferroelectrics – the ability to reorient the polarization via applied electric fields11 – involves both atomic displacements, and charge compensation at the surface to screen the generated polarization. Since screening must occur for the polarization to be stable in standard ferroelectrics (though hyper-ferroelectrics do not ascribe to this constraint12), these processes can range anywhere from adsorption and diffusion of charged surface species (wellstudied by Kelvin Probe Force microscopy measurements13, 14) to band-bending at the interface, ionic motion or high-energy

process up to and including x-ray emission.15 For systems with high ionic mobilities, this presents an opportunity to investigate the coupling between ionic processes and ferroelectric switching16, which remains poorly understood, and yet critical for understanding processes such as ferroelectric fatigue.17 In this regard, KTiOPO4 (KTP) is an attractive choice, as it not a semiconductor/dielectric but is instead considered a quasi-one-dimensional superionic conductor.18, 19 The high conductivity is provided by the K ions that are loosely bound to the crystal network and can move through it along channels in the polar direction through a vacancy-hopping mechanism. Flux-grown KTP presents nonstoichiometric defects on the K and O sublattices, known to have a great impact on the ionic conductivity.20 The defect density depends on the range of temperatures used for crystal growth; at elevated temperatures there is a tendency for the O to be expulsed from the crystal. This, in turn, causes an increase in K vacancies, necessary to maintain charge neutrality. The motion of the K ions also plays an important role in the polarization reversal. The crystal structure has two non-equivalent K sites, one highly coordinated site and one with lower coordination. The polarization switching occurs through a displacement along the polar axis of the K ions, whereby the coordination numbers of the two sites are interchanged followed by a relaxation of the surrounding oxygen sub-lattice, resulting in an interchange of the long and short Ti-O bonds.21

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KTP a has high optical nonlinearity, a wide transparency window and a high optical damage threshold making it an attractive and commonly used material for nonlinear optical applications employing the quasi-phase matching technique, in which a periodic domain structure is used to compensate for the phase mismatch of the interacting waves.22 Furthermore, its domain-velocity anisotropy allows the fabrication of domain gratings with high-aspect ratio, enabling efficient frequency conversion over a broad range of frequencies. At the same time, the high ionic mobility screens the polarizationswitching, complicating the fabrication of periodic domain gratings.23 Therefore, the interplay between ion dynamics and polarization reversal is important from a device perspective. To gain further insight into the phenomena mediating ionic mobility and the ionic shifts during polarization switching, we explored the coupling between the polarization switching and ionic mobility of the system, using a combination of bandexcitation piezoresponse force spectroscopy (BEPS), along with site-correlated Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The BEPS measurements provide information on local polarization dynamics, as well as qualitative elastic modulus (via the resonant frequency of the cantilever) data as a function of applied voltage. The chemical modifications in both surface and bulk of the crystal are then explored via TOF-SIMS measurements, with ion sputtering enabling full 3D reconstructions of the chemical state of the system. Our results reveal a gradual change in the resonance frequency due to ionic motion, induced by an external bias. When the bias causes the polarization to switch, a sudden shift in the frequency is detected at the switching event, corresponding to a reorganization of the K ions in response to the polarization-reversal. The role of the K ions in the screening process is corroborated using ToF-SIMS showing a selective distribution of the ions on the polar faces, proving their importance for polarization screening. METHODS For our experiments, we used commercial flux-grown, c-cut 1 mm thick KTP crystals. The crystals were periodically poled with different periods ranging from 8.9 µm to 9.3 µm at room temperature using standard electric field poling techniques. First, a periodic aluminum-photoresist grating was deposited on the c- faces of the crystals. Afterwards, the crystals were contacted to an external circuit using liquid electrodes and were poled using electric field pulses in the high-field regime,24, 25 resulting in domain gratings extending over the whole crystal thickness. Subsequently, the aluminum gratings were removed and the crystals were selectively etched, to confirm the quality of the domain grating and chemomechanically polished down to a thickness of 150 µm from the un-patterned face, leaving the etched-relief on the patterned face for reference. Atomic Force Microscopy. The AFM measurements were performed using an Asylum Research, Cypher system, with a

modified tip holder connected to an external voltage amplifier to extend the range of available bias voltages. For the piezoforce microscopy (PFM) measurements 295 kHz drive frequency at 3 V amplitude was used. For the switching-spectroscopy PFM (SS-PFM), the measurement area was divided into a grid of 50x50 equally spaced points. At each point three consecutive bias waveforms were applied. Each waveform consisted of a linear ramp from 0 V to 15 V and then to -15 V and back to 0 V at a constant ramprate of 44 V/s. During each waveform a total of 64 BE-scans were acquired in the range 287 kHz to 327 kHz. Time-of-Flight Secondary Ion Mass Spectrometry. ToFSIMS measurements have been done using TOF.SIMS.5-NSC instrument, using bismuth liquid metal ion gun as a primary ion source and cesium ion gun as a sputter source. For the imaging ToF-SIMS experiments we used mode with high spatial resolution – Bi spot size ~120 nm, energy 30 keV, current 0.48 nA and spectral resolution 0.3 – 0.5 Da. Sputter cesium ion gun (energy 1 keV, current 60 nA, spot size ~20 µm) have been used for surface cleaning and depth profiling. RESULTS AND DISCUSSION Figure 1(a) shows the PFM-phase of a representative 15x15 µm2 region of one of our samples. The dark and bright contrast corresponds to Ps pointing down (Ps↓) and Ps pointing up (Ps↑), respectively. Figures 1(b and c) show the BEPS-phase registered at the peak negative and peak positive bias respectively. On the Ps↓ domains, the phase remains constant throughout the series of waveforms indicating stable polarization orientation. The scanned region was divided into four areas using k-means clustering, based on the phase-response. K-means clustering is a simple statistical technique that assigns each pixel (measurement point) to belong to a certain set (cluster), iterating on the members of the clusters until the within-cluster variance is minimized. The four clusters correspond to the Ps↓ domains, the non-switching parts of the Ps↑ domains, the switching parts of the Ps↑ domains and the boundary-regions separating these two areas. The average resonance frequency and phase response, averaged over the third voltage-cycle of the first three clusters are shown in figures 1(d-f), the final cluster showed intermediate characteristics and is omitted. Figures 1(g-i) show the bias waveform corresponding to figures (d-f). For both domains, the resonance frequency is modulated by the external bias; it increases slowly when the bias is positive and then drops as the bias becomes negative. The change in resonant frequency is sharp during the negative sweep, suggesting an activated process. The K ions are known to be loosely bound to the titanyl-phosphate structure and therefore can move under the action of external electric field18, 19; as the ions move to and from the surface the ionic distribution changes, causing a local change of the elastic modulus. This leads to the shift of the resonance frequency observed experimentally (Fig. 1d-f).26


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Figure 1: (a) PFM-phase of a representative region, (b) SS-PFM phase at peak negative bias, (c) SS-PFM phase at peak positive bias, (d-f), average phase (solid, black) and resonance frequency (dashed, red) in the final voltage cycle extracted from Ps↓ domains (d), non-switching parts of Ps↑ (e), and switching Ps↑ (f); the blue arrows in figure f mark fine-structure in the resonance frequency response, related to polarization-reversal. The bias waveform corresponding to figures d-f is shown in figures (g-i).

The resonance-frequency shift varies between different parts of the sample: At the Ps↓ domains, the total frequency shift is around 550 Hz peak-to-peak; as the bias increases there is a gradual increase of the frequency followed by a plateau as the bias starts declining and then a sudden drop, close to the peak negative bias. In the non-switching Ps↑ regions, the frequency shift is lower (280 kHz peak-to-peak) and the rate of change is also lower in both the positive and the negative bias range. In the switching regions intermediate characteristics are observed, both in terms of overall frequency-shift and change– rate. The differences between Ps↑ and Ps↓ domains can be attributed to the selective accumulation of K+ ions and VK- vacancies at Ps↓ and Ps↑ domains respectively. Indeed, the workfunction difference between the two domains observed using KPFM has been attributed to selective ion accumulation27. The K ions are expected to have a higher mobility than the vacancies thus explaining the relatively higher frequencyshifts occurring on Ps↓. The fact that the frequency increases slowly during the full range of positive bias indicates a gradual accumulation of ions; the subsequent drop, occurring just before the negative bias peak, shows that the accumulated ions suddenly relax and deplete. The fine-structure in the resonance frequency, observed at the points marked by blue arrows in figure 1(f) coincides with the switching events. It can be understood as a consequence of the ionic redistribution that occurs during polarization-switching; in which the highly coordinated K site changes into the low coordinated K site and vice versa.21 As the crystal structure reconfigures, the titanylephosphate network must adapt to these changes and can thus be expected to loosen to allow the motion of the large K ions, temporarily increasing the ionic mobility. The polarization

switching occurs only on parts of the original Ps↑ face, which can be attributed to variations in the nucleation site density across the surface, most likely as a consequence of changes in the local defect-density. However, switching was never observed on the Ps↓ domains during the experiment in sharp contrast to what is normally seen during switching with macroscopic electrodes. This discrepancy could be attributed to the effect of local screening due to the increased mobility of the K ions at the Ps↓ domains observed in these measurements. Further investigation will be necessary to fully understand this observation. To confirm the selective accumulation of ions and vacancies, we performed ToF-SIMS measurements on the surface of a periodically poled KTP sample (fig 2). For these measurements, the peak of the 41K+ isotope was used to track the K+ concentration, as the original K+ signal was too strong and led to saturation of the mass detector. We also used the detection of TiO+ clusters as a signature of the O- in order to map the concentration of oxygen vacancies. Figures 2(a, b) show maps of the 41K+ and TiO+ respectively, the white spots correspond to contaminated regions of the sample. Figure 2(c) shows normalized concentration profiles corresponding to figures a and b respectively, obtained by averaging the concentration maps along the vertical direction. As expected, the K+ concentration is around 5 percent higher in the Ps↓ domains than in the Ps↓ domains, thus explaining the difference in elastic modulus modification observed in the BEPS measurements. A somewhat lower selectivity is observed for the TiO+ measurement; the concentration is approximately 3 percent higher in the Ps↑ domains than in the Ps↓ domains. The findings indicate that, despite their lower mobility, oxygen vacancies also play a



role in the screening process alongside the more mobile K ions.

Figure 2: (a) K concentration map, (b) TiO concentration map, (c) normalized concentration profiles extracted from figures a and b through averaging along the vertical axis. In order to further explore the ion-selectivity, the sample was milled with Cs ions and the K+ and TiO+ concentrations were measured at several depths down to a maximum of 28 nm. Subsequently a PFM map was acquired, confirming that the domain structure was not damaged by the milling. The K selectivity gradually declined over the depth studied as expected for mobile screening charges. However, it should be noted that the selectivity did not disappear completely at the maximum depth of 28 nm, in good agreement with previous estimates of the screening depth.27 The TiO signal is much weaker and, as a result of the poorer signal to noise ratio, varies substantially from one scan to the next. Figure S1 of the supplementary information shows the selectivity as a function of milling depth. The BEPS measurement allows the ionic motion, resulting from the external bias, to be monitored through its effect on the resonance frequency. The fact that different responses are registered at Ps↑ and Ps↓ domains can be attributed to the corresponding difference in K+ concentration encountered at the different domains using ToF-SIMS. The ionic distribution and the changes induced by the external bias are illustrated in figure 3, in the non-switching regime, for a Ps↓ domain (a), a Ps↑ domain (b), and in the switching regime for a Ps↑ domain (c).

Figure 3: (a) Screening charge distribution and ionic motion induced by an external voltage on a Ps↓ domain, (b) corresponding charges for a Ps↑ domain, (c) redistribution of screening-charges in response to polarization-reversal The selective potassium distribution confirms the role of mobile K ions as the main screening mechanism in KTP. However, the distribution of oxygen vacancies follows the same pattern, indicating that they may also take part in the screening process. The fine-structure in the frequency response indicates a sudden structural change at the switching event. Since the reversed polarization will require a corresponding reversal of the screening charges, this shift could be attributed to the resulting motion of K+ and VK-. Such motion would occur in a direction contrary to that induced by the bias itself (fig 3c). Furthermore, there may also be a contribution from the re-coordination of the K that occurs during the polarization reversal and the subsequent relaxation of the surrounding lattice.21 This situation can be contrasted with the case of ferroelectric capacitors with top and bottom electrodes. In those cases, the screening is mostly electronic in nature, coming from the electrodes themselves, which will be associated with much smaller screening lengths than are observed for the ionically mediated process in KTP. It should be noted that injected charges from the tip, i.e. electrons or holes, may also participate in the screening process, and have previously been mentioned in the literature in the context of back-switching of ferroelectrics.28 Then, the polarization evolution becomes a complex process dictated by relaxation times of injected charges coupled with surface and bulk electrochemical screening processes. Future studies may be directed towards better understanding these interplays via more advanced Kelvinprobe force microscopy based studies in controlled environments.14, 29


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ACS Applied Materials & Interfaces These observations show direct evidence of the coupling between weak chemical disorder and ferroic properties during switching. While this study deals with KTP, a similar situation should arise in other ferroelectrics that are also ionic conductors, such as BaTiO3. Comparing the switching and nonswitching parts of the sample allows the decoupling of the shifts related to switching from those related to bias-induced ionic motion. It also provides a more complete picture of the screening mechanisms that are largely neglected to date, and confirms them via chemical and functional property (elastic and ferroelectric) mapping via SPM. This is relevant for understanding e.g. charge gradient microscopy, as well as ferroelectric fatigue.

ASSOCIATED CONTENT Supporting Information. The supporting information contains ToF-SIMS data, showing the preferential accumulation of K and TiO at Ps↓ domains as a function of the milling depth. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]; [email protected]

ACKNOWLEDGMENT A portion of this work was sponsored by the U.S. Department of Energy (DOE), Office of Science, Materials Sciences and Engineering Division (R.K.V. and S.V.K.). Research was conducted at the Center for Nanophase Materials Sciences (A.V.I, S.J., O.S.O.), which is a DOE Office of Science User Facility, and using instrumentation within ORNL's Materials Characterization Core provided by UT-Battelle, LLC under Contract No. DEAC05-00OR22725 with the U.S. Department of Energy. G.L. and C.C. acknowledge support and access to the Center for Nanophase Materials Sciences through the proposal CNMS2017-066. G.L. and C.C. also acknowledge support from the Swedish Foundation for Strategic Research and the Swedish Research Council (VR) through its Linné Center for Advanced Optics and Photonics (ADOPT).

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Elasticity Modulation Due to Polarization Reversal and Ionic Motion in

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