Imprint Control of BaTiO3 Thin Films via Chemically Induced Surface

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Imprint Control of BaTiO3 Thin films via Chemically-induced Surface Polarization Pinning Hyungwoo Lee, Tae Heon Kim, Jacob Patzner, Haidong Lu, Jungwoo Lee, Hua Zhou, Wansoo Chang, Mahesh Mahanthappa , Evgeny Y Tsymbal, Alexei Gruverman, and Chang-Beom Eom Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b05188 • Publication Date (Web): 22 Feb 2016 Downloaded from http://pubs.acs.org on February 25, 2016

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Imprint Control of BaTiO3 Thin films via Chemically-induced Surface Polarization Pinning Hyungwoo Leea, Tae Heon Kima, Jacob J. Patznera, Haidong Lub, Jung-Woo Leea, Hua Zhouc, Wansoo Changd, Mahesh K. Mahanthappad §, Evgeny Y. Tsymbalb, Alexei Gruvermanb and Chang-Beom Eoma* a

Department of Materials Science and Engineering, University of Wisconsin-Madison,

Madison, WI 53706, USA b

Department of Physics and Astronomy, Nebraska Center for Materials and Nanoscience,

University of Nebraska, Lincoln, NE 68588, USA c

X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne,

IL 60439, USA d

Department of Chemistry, University of Wisconsin-Madison, Madison, WI 53706, USA

§

Current address: Department of Chemical Engineering and Materials Science, University of

Minnesota, Minneapolis, MN 55455, USA

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ABSTRACT: Surface-adsorbed polar molecules can significantly alter the ferroelectric properties of oxide thin films. Thus, fundamental understanding and controlling the effect of surface adsorbates is crucial for the implementation of ferroelectric thin film devices, such as ferroelectric tunnel junctions. Herein, we report an imprint control of BaTiO3 (BTO) thin films by chemically-induced surface polarization pinning in the top few atomic layers of the water-exposed BTO films. Our studies based on synchrotron X-ray scattering and coherent Bragg rod analysis demonstrate that the chemically-induced surface polarization is not switchable but reduces the polarization imprint and improves the bistability of ferroelectric phase in BTO tunnel junctions. We conclude that the chemical treatment of ferroelectric thin films with polar molecules may serve as a simple yet powerful strategy to enhance functional properties of ferroelectric tunnel junctions for their practical applications.

KEYWORDS: Imprint Control, Ferroelectric thin films, BaTiO3, Ferroelectric tunnel junctions, Surface chemistry, Water adsorption


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The materials characteristics of ferroelectric oxide thin films sensitively depend on their surface chemistries. Since the surface effects can dominate over the physics of the thin film interior, chemical environments and the resulting surface modification can play a dominant role in governing the overall performance of the thin films. Previous reports have shown experimental and theoretical evidence that the physical properties of ferroelectric oxide thin films can be affected by chemical environments1-9. For example, it was shown that a monodomain polar state in ultrathin ferroelectric films could be more stable in the presence of polar adsorbates on surface1, 8. Ab-initio calculations have also shown that the surfaceadsorbed ions can provide an effective charge compensation and reduce depolarization field in ultrathin ferroelectric systems1, 2. Recently, it was reported that a ferroelectric polarization state can even be switched by controlling the oxygen partial pressure5. These results show that chemically-induced surface effects can significantly influence the equilibrium polarization states in ferroelectric oxide thin films. Barium titanate, BaTiO3 (BTO) is one of the most important ferroelectric perovskite materials10-13. Epitaxial BTO thin films have extensively been studied for tunable electronic applications, such as ferroelectric tunnel junctions (FTJs) and memristors14-20. However, the major challenges for the implementation of practical devices are to stabilize ferroelectric polarization states and to achieve a symmetric switching behavior21, 22. Previous studies have shown that the stability of ferroelectricity can be easily affected by an external chemical environment. It was reported that, for example, a humidity-controlled environment could alter the surface dipole orientation, and resulted in polarization reversal throughout the BTO thin films23, 24. This effect was mainly attributed to the structural distortion of the top BTO layer, so-called rippling. Such chemically-induced structural changes are usually inadvertent and detrimental to the reliable device functionality. Therefore, a fundamental understanding of and control over this chemical effect is essential for the implementation of thin film 3 ACS Paragon Plus Environment

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electronic devices. However, there is still a lack of insightful research directly describing the major influence of the surface chemistry on the internal atomic structure and the resulting ferroelectric properties of BTO thin films. Herein, we report an imprint control of BTO thin films by chemically-induced surface polarization pinning. We demonstrate that when a BTO thin film is exposed to water, the top few unit cells of BTO form a surface polarization opposite to that of the as-grown film. The chemically-induced structural change in BTO is evident from our studies based on synchrotron X-ray scattering and coherent Bragg rod analysis (COBRA). Furthermore, we find that the chemically-induced surface polarization is not switchable but reduces the polarization imprint to render the switching behavior more symmetric and to improve the bistability of polarization states in ferroelectric tunnel junction (FTJ) structures. Thus, this chemical method may serve as a simple strategy to implement practical FTJ devices. Figure 1 shows a hypothetical mechanism of the chemically-induced polarization pinning effect. An as-grown (001) BTO epitaxial thin film grown on SrRuO3/SrTiO3 (SRO/STO) substrate is schematically depicted in Figure 1(a). Spontaneous polarization in a singledomain BTO thin film points to the SRO bottom electrode as depicted by a grey arrow in Figure 1(a)25. It is known that oxygen vacancies are easily formed at the top of the BTO for the polarization charge screening26, 27. In this configuration, the downward polarization state is quite stable, while the upward polarization state is comparatively unstable21. Such asymmetric polarization states can be described by an energy diagram as shown in Figure 1(b). Since the upward polarization state is energetically less favorable, the stability and retention of two polarization states are asymmetric as well. In addition, the asymmetric energy barrier between the two polarization states is directly related to the imprint behavior in ferroelectric hysteresis.


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Figure 1(c) shows a schematic of a BTO thin film that has been exposed to water. Water molecules are known to adsorb on BTO surface either physically or chemically4, 23, 24, 28-30. The physisorption is ignored here since the chemisorption of water is more energetically favorable in the case of our BaO-terminated BTO surfaces4, 23. When water molecules adsorb on the polarized BTO surface, they dissociate into hydroxyls (OH-) and protons (H+). The surface-adsorbed hydroxyls can reverse the polarization direction of BTO from the downward to the upward direction (blue arrows in Figure 1(c))23,

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. However, we

hypothesize that the polarization reversal will not occur throughout the whole film due to possibly-existing electropositive elements or defects inside the film. If the electropositive elements or defects can diffuse inside the film, they will migrate to the surface region to screen polarization charges. Following charge entrapment, the reversed polarization will be pinned in the top few unit cells, referred to as a top passive layer. In this hypothetic mechanism, note that the polarization in the passive layer can be pinned by any kinds of positive elements or defects such as oxygen vacancies. However, based on the following structural investigations, we suggest that electropositive protons dissociated from water is probably one of the most plausible candidates for the positive elements inside BTO. Figure 1(d) shows an energy diagrams for the water-treated BTO thin film. In the top passive layer, only the upward polarization state is energetically favorable (blue curve). However, the lower region of the BTO film will exhibit switchable polarization states. Since the passive layer decreases the internal built-in field in BTO (Figure S1 in Supporting Information), the switchable polarization states will be more symmetric (red curve) than those of the as-grown BTO. The expected ferroelectric hysteresis behavior of BTO thin films before and after the water treatment is depicted as the schematic P-E loops in Figure 1(e). The as-grown BTO thin film will exhibit a ferroelectric hysteresis loop with a strong imprint (black curve) with 5 ACS Paragon Plus Environment

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asymmetric coercive fields. However, as mentioned above, the imprint will be decreased or even removed after the water treatment (red curve). The diminished remnant polarization is due to the pinned polarization in the passive layer, which decreases the overall switchable polarization in the BTO. To verify this hypothetic mechanism, we have grown a BTO/SRO heterostructure by PLD and investigated it before and after a water treatment. Sample preparation and water treatment process is shown in Figure S2 in Supporting Information. In this study, unless otherwise noted, we used BTO ultrathin films (12 u.c.) deposited on 30 nm of SRO. Figure 2(a) and 2(b) shows atomic force microscopy (AFM) topography images for the BTO/SRO heterostructure before and after the water treatment, respectively. “The surface topography does not exhibit any change after the water treatment, indicating that there was no surface degradation derived from the water treatment at room temperature.” “To confirm the surface hydroxylation by water, the surface chemical functionality of the BTO surface was interrogated by X-ray photoelectron spectroscopy (XPS) measurements. Figures 2(c) and 2(d) show the O1s peak of the BTO sample before and after the water treatment, respectively. The largest green peak with a binding energy of ~529.1 eV indicates the lattice oxygen in perovskite BTO31. The red and blue peaks at higher binding energies represent the presence of terminal hydroxyls. The red peak with a binding energy of ~530.3 eV corresponds to the hydroxyls that filled oxygen vacancies24. The peak area as a fraction of the total O 1s signal increased from 7 % to 19 %, indicating that many surface oxygen vacancies were filled by the dissociative water adsorption. The blue peak with a binding energy of ~531.4 eV corresponds to the hydroxyls bonded onto surface metal atoms24. The peak area as a fraction of the total O 1s signal increased from 8 % to 9 %. The increase of both peaks clearly shows that the BTO surface is well hydroxylated by the water treatment.


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The already-existing hydroxyl peaks in Figure 2(c) stem from the water adsorption from ambient air24, 32.” The effect of water treatment on average lattice structure of BTO was determined by Xray diffraction (XRD) with a Cu Kα source (λ=1.5405 Å). For the XRD study, a thicker BTO film (25 u.c.) with a thinner SRO layer (8 u.c.) was utilized in order to clearly observe BTO peaks with minimized thickness fringes of SRO. Figure 2(e) and 2(f) show XRD reciprocal space maps (RSMs) of the BTO around the (-103) peak before and after the water treatment, respectively. The BTO (-103) peak remains fully coherent to STO substrate even after the water-treatment. The c-lattice parameter has decreased from 4.176±0.003 Å to 4.172±0.003 Å after the water treatment (dotted line in Figure 2(e) and 2(f)). The out-of-plane θ-2θ XRD pattern around (002) BTO peak is also shown in Supporting Information Figure S3. After the water treatment, (002) BTO peak shifts to a higher angle, which indicates the decrease of the c-lattice parameter in agreement with the RSM result. Therefore, we confirmed that the water exposure is sufficient to induce a structural distortion of BTO thin films. The physical meaning of the decrease of the c-lattice parameter will be discussed in what follows. To better understand the effect of water treatment, we have investigated the microscopic atomic structure of BTO thin films. The expected atomic structure near the BTO surface before and after the water treatment is shown in Figures 3(a) and 3(b), respectively. Before the water treatment, all B-site atoms (Ti) have a downward off-center displacement, leading to an overall downward polarization as depicted by black arrows in Figure 3(a). This configuration is quite stable due to the presence of surface oxygen vacancies for charge screening26,

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. On the other hand, the water-treated BTO has a quite different atomic

structure. In particular, the off-center displacement is reversed in a few unit cells at the top surface as depicted by red arrows in Figure 3(b), resulting in the upward polarization. Due to the charge screening, electropositive protons are accumulated near the top surface. 7 ACS Paragon Plus Environment

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To verify the atomic structure, synchrotron X-ray crystal truncation rod (CTR) measurements were conducted at the sector 33-ID-D in the Advanced Photon Source (APS) with an X-ray energy of 17.5 KeV, and subsequently analyzed by the coherent Bragg rod analysis (COBRA) method33. Experimentally measured CTR data and COBRA-calculated CTR spectra are shown in COBRA Analysis section of Supporting Information. Figure 3(c) shows the resulting B-B lattice spacing profiles along the (001) direction. B-site here represents the Ti atoms in BaTiO3. The lattice spacing of the as-grown BTO (black circles) increases rapidly near the top surface unlike the water-treated BTO. This large lattice spacing near the top surface clearly shows that there are a large number of oxygen vacancies on the top surface of as-grown BTO. “However, after the water treatment, the lattice spacing of the top two unit cells is decreased as shown by the red squares in Figure 3(c). This result implies that the surface oxygen vacancies are filled up by the hydroxyls, which is also consistent with our XPS and XRD results shown in Figure 2.” Figure 3(d) shows the B-site off-center displacement profiles. In the case of as-grown BTO, all 12 unit cells show the negative off-center displacement, which indicates that the spontaneous polarization direction is downward. However, after the water treatment, the offcenter displacement of the top 3 layers clearly increased as shown by red squares. Particularly, the top 2 unit cells showed positive off-center displacement values, indicating the pinned polarization consistent with our model. Therefore, these COBRA results have directly demonstrated that the upward polarization has been pinned at the top few layers by the water treatment. To identify the effect of the chemically-induced surface polarization pinning on ferroelectric performance of BTO devices, we fabricated FTJs with an as-grown and a watertreated BTO thin film. Schematic diagrams depicting the structure of the FTJs using an asgrown and a water-treated BTO thin film is shown in figure 4(a) and 4(d), respectively. The 8 ACS Paragon Plus Environment

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top metal electrodes of Au/Co were fabricated by a conventional lift-off process. Then, the piezoresponse signals of the samples were measured by piezoresponse force microscopy (PFM). The tunneling currents were also measured through the Au/Co top electrodes using a conducting AFM. The tunneling currents data measured at on/off states is given in Supporting Information Figure S7. Figures 4(b) and 4(c) show the PFM amplitude and phase hysteresis loops obtained from the FTJ based on the as-grown BTO. As expected, the as-grown BTO exhibited an asymmetric hysteresis behavior. The hysteresis loops are off-centered toward the negative direction, indicating the large polarization imprint. In addition, the upward polarization state (at negative bias) was hardly detectable because the retention time was too short. This asymmetric hysteresis behavior is mainly attributed to the surface oxygen vacancies and the strong built-in field associated with energy band bending25, 34. On the other hand, in the case of the water-treated BTO, the PFM hysteresis loops were found to be quite symmetric as shown in Figures 4(e) and 4(f). The polarization imprint is clearly reduced compared to that of the as-grown BTO. Since the built-in field is reduced by the passive layer (Figure S1 in Supporting Information), the polarization imprint becomes smaller. This result can be also supported by the structural change confirmed by our COBRA study. The retention of the upward polarization state is also remarkably enhanced. The enhanced retention is presumably due to the amount of surface oxygen vacancies decreased by the dissociative water adsorption on the surface. We could observe similar symmetric hysteresis behaviors by measuring PFM signals through the bare surface of BTO samples without metal electrodes (Figure S8 in Supporting Information). It indicates that the symmetric switching behavior is not associated with the top metal electrodes or its deposition process, but the water-driven passive layer. These PFM results are actually surprising and interesting, because the formation of passive layer on the top of ferroelectric thin films has usually been considered as a detrimental 9 ACS Paragon Plus Environment

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phenomenon so far. However, our PFM results clearly demonstrate that the chemicallyinduced passive layer can make the hysteresis loops more symmetric and improve the bistability of polarization states in BTO FTJs. Lastly, we address a couple of issues related to our studies. In many previous works regarding the water adsorption, only hydroxyls have been considered important as effective surface-adsorbates on thin films. However, we argue that protons may also play a crucial role in stabilizing the internal atomic structure and polarization configuration. The chargescreening mechanism by such electropositive defects in a thin film has already been reported in several works27, 35. In BiFeO3 films, for example, oxygen vacancies segregate at the nearsurface layer to screen the negative polarization charges27. Similarly, in our BTO system, the protons are the most plausible candidate for the electropositive elements. It is well known that water can provide protons and hydroxyls to the BTO surface by the dissociative adsorption4,

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. Given that protons easily diffuse inside perovskite oxides37-42, the

electropositive protons will be able to migrate from the top surface to the lower region to screen polarization charges. We believe that the effect of oxygen vacancies, which might be another plausible candidate, is minimal here. If oxygen vacancies were actually accumulated at the top few unit cells, this could be seen as a comparatively larger lattice spacing in COBRA results. On the contrary, the COBRA data showed that the lattice spacing of the top 2 unit cells clearly decreased after a water treatment as shown in Figure 3(c). This indicates that the oxygen vacancies were filled up by hydroxyls dissociated from water. On the other hand, the interstitial protons do not make any severe change in the lattice spacing of perovskite oxides38. Another approach to prove the formation of the passive layer is by electrical capacitance measurements in FTJ structures42. The result is shown in Supporting Information Figure S9. The increased y-axis intercept of the water-treated junction indicates that the water-treatment 10 ACS Paragon Plus Environment

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produced a passive layer with a dielectric constant lower than normal BTO. The water-driven passive layer was reproducibly observed in many samples and not spontaneously removed after aging in ambient air for 3 days, as shown in Supporting Information Figure S10. We also confirmed that the water-driven passive layer was not destroyed even after the repetitive hysteresis cycles as shown in Supporting Information S11. “Additionally, to conclusively demonstrate that this structural change is directly associated with the polar hydroxyls and protons, we have performed a similar experiment using a non-polar aprotic solvent, anhydrous hexane (Sigma-Aldrich) and a polar aprotic solvent, dichloromethane (SigmaAldrich), instead of using water. As shown in Supporting Information Figure S12, there was no peak shift in the out-of-plane θ-2θ XRD pattern after the hexane- and the dichloromethane-treatment for one hour. This result indicates that the structural change and the resulting polarization pinning can be achieved only by the surface hydroxylation and the proton migration from water.” As a final note, we confirmed that the passive layer thickness could be precisely controlled by changing the water-treatment time duration (Figure S13 in Supporting Information). This result is helpful for finding the optimum thickness of the water-induced passive layer for the best performance of FTJs. In conclusion, we demonstrated the control of polarization imprint of BTO thin films by chemically-induced surface polarization pinning. When a BTO thin film was exposed to water, the 2~3 top unit cells of the BTO formed a passive layer with polarization pinned at an upward orientation. We have investigated the chemically-induced structural change by XRD and COBRA studies. As a proof of concept, we fabricated FTJ structures using the watertreated BTO thin film. The water-treated samples exhibited symmetric hysteresis behavior, a smaller polarization imprint, and bistable equivalent polarization states, all of which are advantageous for device applications. This result can pave the way to a chemical approach for controlling the properties of ferroelectric thin films, enabling practical applications. 11 ACS Paragon Plus Environment

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Figure 1. (a) Schematics depicting the spontaneous polarization in an as-grown BTO thin film. Positive (negative) polarization charges can be screened by the electrons of SRO (surface oxygen vacancies) effectively. Thus, the spontaneous downward polarization can be stabilized. (b) Energy diagram showing the asymmetric polarization states due to built-in potential in the BTO thin film. (c) Schematics depicting the chemically-induced polarization pinning effect in a BTO thin film. In the top passive layer, the positive (negative) polarization charges can be screened by hydroxyls (positively-charged elements or defects). In the lower BTO region, the positive (negative) polarization charges can be screened by the electrons of SRO (positively-charged elements or defects). (d) Energy diagram for the water-treated BTO thin film. Polarization states in the lower BTO thin film is symmetric, while that in the top passive layer is strongly asymmetric. (e) Expected shape of hysteresis loops of BTO thin film before and after the water treatment.


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Figure 2. AFM topography image of an (a) as-grown and (b) the water-treated BTO thin film. (c) X-ray photoelectron spectroscopy (XPS), O 1s core-level spectrum for the as-grown BTO thin film. The largest green peak centered on ~529.1 eV corresponds to the lattice oxygen in BTO. The red (centered on ~530.3 eV) and blue (centered on ~531.4 eV) peaks correspond to hydroxyls that filled oxygen vacancy sites and adsorbed on metal sites, respectively. The experimental and the fitted spectra are given by the black squares and the pink curves, respectively. (d) XPS spectrum for the water-treated film. The increased hydroxyl peaks clearly show that the BTO surface has been hydroxylated after the water treatment. (e) Reciprocal space map (RSM) of the as-grown BTO thin film grown on SRO/STO. (f) RSM of the water-treated BTO thin film. The XRD data indicate that the water treatment does not change the in-plane lattice parameters of the BTO film, and that only the c-lattice parameter decreases slightly after the water treatment.


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Figure 3. Schematic diagrams depicting the atomic structures of BTO thin films (a) before and (b) after the water-treatment. (c) Lattice spacing of each unit cell along the (001) direction, measured by B-B spacing in as-grown (black circles) and water-treated (red squares) BTO films. The blue-colored region shows the chemically-induced passive layer. (d) B-site off-center displacement determined by COBRA in the as-grown (black circles) and the water-treated (red squares) BTO thin film. Note that the B-site atoms obviously moved upward in the passive layer (blue-colored region).


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Figure 4. (a) Schematics showing the FTJ structure based on an as-grown BTO thin film. Au/Co was used as the top metal electrodes. (b) PFM amplitude and (c) phase hysteresis loops measured in the FTJ structure. (d) Schematics showing the FTJ structure based on a water-treated BTO thin film. The chemically-induced surface polarization in the top passive layer is depicted by blue arrows. (e) and (f) shows its PFM amplitude and phase signals, respectively.


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ASSOCIATED CONTENT Supporting Information. Expected built-in field in BTO thin film, RHEED monitoring results, additional XRD spectra, stability of the passive layer, the effect of non-polar solvent, COBRA data, capacitance measurement result, additional PFM data and the time response of water treatment. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] (C. B. Eom) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The work at University of Wisconsin-Madison was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DE-FG02-06ER46327 (conceiving the project, fabrication and structural characterization of thin films). The research at University of Nebraska–Lincoln (PFM measurements) was supported by the National Science Foundation (NSF) through Materials Research Science and Engineering Center (MRSEC) under Grant DMR-1420645. The authors declare no competing financial interest.


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