First Observation of Ferroelectricity in â¼1 nm Ultrathin

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First observation of ferroelectricity in #1 nm ultra-thin semiconducting BaTiO films 3

Seung Ran Lee, Lkhagvasuren Baasandorj, Jung Won Chang, In Woong Hwang, Jung Rae Kim, Jeong-Gyu Kim, Kyung-Tae Ko, Seung Bo Shim, Min Woo Choi, Mujin You, Chan-Ho Yang, Jinhee Kim, and Jonghyun Song Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04326 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

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First observation of ferroelectricity in ∼1 nm ultrathin semiconducting BaTiO3 films Seung Ran Lee,†,‡, Lkhagvasuren Baasandorj,¶, Jung Won Chang,§ In Woong Hwang, Jung Rae Kim,‡ Jeong-Gyu Kim,,# Kyung-Tae Ko,,# Seung Bo Shim,† Min Woo Choi, Mujin You, ǂ Chan-Ho Yang, ǂ Jinhee Kim,† and Jonghyun Song ,* † Korea Research Institute of Standards and Science, Daejeon 34113, Republic of Korea, ‡ Center for Correlated Electron Systems, Institute for Basic Science (IBS) & Department of Physics and Astronomy, Seoul National University (SNU), Seoul 08826, Republic of Korea,  Max Planck POSTECH Center for Complex Phase Materials, Pohang University of Science and Technology, Pohang 37673, Republic of Korea, ¶ University of Science and Technology, Daejeon 34113, Republic of Korea, § Department of Display and Semiconductor Physics, Korea University, Sejong 30019, Republic of Korea, # Department of Physics, Pohang University of Science and Technology, Pohang 37673, Republic of Korea, ǂ Department of Physics, KAIST, Daejeon 34141, Republic of Korea,  Department of Physics, Chungnam National University, Daejeon 34134, Republic of Korea

KEYWORDS : ultra-thin BaTiO3, ferroelectricity, semiconducting ferroelectric, LaAlO3/SrTiO3, 2-dimensional electron gas.

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ABSTRACT

The requirements of multi-functionality in thin-film systems have led to the discovery of unique physical properties and degrees of freedom, which exist only in film forms. With progress in growth techniques, one can decrease the film thickness to the scale of a few nanometers (~nm), where its unique physical properties are still pronounced. Among advanced ultra-thin film systems, ferroelectrics have generated tremendous interest. As a prototype ferroelectric, the electrical properties of BaTiO3 (BTO) films have been extensively studied and it has been theoretically predicted that ferroelectricity sustains down to ~nm thick films. However, efforts towards determining the minimum thickness for ferroelectric films have been hindered by practical issues surrounding large leakage currents. In this study, we used a few nm-thick BTO films, exhibiting semiconducting characteristics, grown on a LaAlO3(LAO)/SrTiO3(STO) heterostructure. In particular, we utilized two-dimensional (2D) electron gas at the LAO/STO heterointerface as the bottom electrode in these capacitor junctions. We demonstrate that the BTO film exhibits ferroelectricity at room temperature even when it is only ~2 unit-cells thick and the total thickness of the capacitor junction can be reduced to less than ~4 nm. Observation of ferroelectricity in ultra-thin semiconducting films and the resulting shrunken capacitor thickness will expand the applicability of ferroelectrics in the next generation of functional devices.


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The fundamental thickness limitation of Si-based nano-electronic devices has triggered extensive investigations for alternative materials that are compatible with conventional Si-based devices with new functionality and/or multi-functionality to maximize the integrity of next generation nano-electronic devices. Among the many candidate systems, perovskites have attracted a lot of attention owing to its versatility in physical phenomena appearing even in a single material. In particular, ferroelectricity in perovskites has been highlighted due to its switchable character by an electric field. The structural properties of most perovskite ferroelectrics are compatible with other perovskite crystalline materials,[1-4] enabling high-quality integration of multifunctional perovskite-based electronic devices. However, similar questions before asked to Si-based nano-electronic devices regarding the fundamental thickness limitation without losing their inherent properties also have recently been raised in the ferroelectric research. Pioneering theoretical predictions had led earlier research on the determination of the film thickness below which ferroelectricity disappears in insulating BaTiO3 (BTO) films. In the theoretical studies, depolarization field of the ultra-thin BTO films and screening length of their electrodes were considered as the most important factors determining the thickness for sustained ferroelectricity. They predicted the critical thickness dC to be 0.08 uc ≤ dC ≤ 6 uc (here, uc denotes unit cell(s)).[5-10] On the other hand, earlier polarization-electric field (P-E) hysteresis loop experiments conducted with a capacitor structure (top-electrode(TE)/BTO/bottomelectrode(BE)) in SrRuO3/BTO/SrRuO3 (SRO/BTO/SRO) showed the sustainment of ferroelectric character down to 5 nm. The 3.2 nm thick BTO film, however, showed no ferroelectric hysteresis loop due to large leakage current;[11] therefore, the insulating BTO film in Ref. [11] had dC > 3.2 nm (7–8 uc), which is higher compared to the theoretical prediction. Later,


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by introducing piezoresponse force microscopy (PFM), an electrical contact area between the PFM tip and the BTO film could be considerably reduced to smaller than 100 nm2, which reduced the leakage current. The method using the conducting PFM tip as a TE yielded a 3 uc thick insulating BTO film that showed ferroelectric characteristics.[12] Note that in their study, the

authors

induced

large

out-of-plane

tensile

strain

by

depositing

BTO

on

La0.67Sr0.33MnO3(LSMO)/NdGaO3(NGO) substrates to enhance ferroelectricity. In this study, we obtained semiconducting ferroelectric BTO film as thin as 1–2 nm. Instead of conventional metallic electrodes in oxide electronics such as LSMO and SRO, we used the quasi 2D electron gas (or liquid) system (q2-DES) formed at a LaAlO3/SrTiO3 (LAO/STO) heterointerface as the BE. Because we used 6 uc thick LAO layers, the entire height of our integrated device (BTO/LAO/STO) can be extremely thinned less than 4 nm for the ~2–3 uc thick BTO layer. This indicates the advantage of using q2-DES at LAO/STO hetero-interface as a BE. The BTO films exhibited metallic-like behavior near room temperature (RT) when the thickness is between 3 uc and 15 uc. PFM measurements found ferroelectric signals in all BTO films at RT, including even the 2 uc thick film. The local electronic structure of the BTO films was examined by X-ray absorption spectroscopy (XAS), which verified the presence of ferroelectricity down to 2 uc in our films. To date, our report is the first observation of a semiconducting ferroelectric BTO film, which has only been reported in the reduced bulk BTO.[13-16] Furthermore, to the best of our knowledge, direct experimental measurements using PFM for extremely thin BTO have been rare. The crystalline quality of our films was examined by X-ray diffraction (XRD) spectra (see Figure S1 in Supporting Information). The atomic-scale flatness of each layer was confirmed by atomic force microscopy, shown in Figures S2a and S2b. Further details regarding the structural


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properties and growth modes are provided in Supporting Information. The out-of-plane lattice constants of LAO and BTO layers were calculated as 3.825 Å and 4.279 Å, respectively, from Figure S1a. The X-ray reciprocal space mapping (XRSM) in Figure S1b shows that all peaks from the LAO and BTO films and the STO substrate lie on the same vertical line, indicating that their respective in-plane lattice constants are identical to that of STO.[17] Comparing the experimentally obtained lattice constant with the bulk lattice constants of LAO and BTO,[1] we determined the tetragonality (the ratio between the out-of-plane and in-plane lattice constants, c/a) of our BTO film to be ~1.096, which is larger than that of a previously reported 1 nm insulating ferroelectric BTO film (c/a ∼ 1.051).[12] Electrical properties of the BTO films were measured in a capacitor structure, as drawn in Figure 1a. Electric current flows from an Al TE to a q2-DES LAO/STO interfacial BE. Here, the capacitor structure can be considered as a series resistance of all layers and interfacial contacts formed between each layer. To distinguish the electric characteristics of the BTO layer from other contributors, we varied the thickness of the BTO films (dBTO) from 2 uc to 17 uc, keeping the thickness of the LAO layers and all growth conditions invariant. Thickness-dependent resistance of the BTO capacitors is presented in Figure 1b. The resistance of the stacked junctions reduce as temperature (T) decreases, within the dBTO range of 3 10 uc, indicating metallic-like behavior. Our previous study[18] unambiguously proves similar resistancetemperature (R-T) relation for the junction without a BTO layer, denoted 0 uc in Figure 1b. Because we only changed dBTO in the current study, we can estimate resistivity of the BTO film by plotting thickness-inverse current values in Figure 1c. The resistivity values are obtained from the I-V characteristics as displayed in Figure S3a–S3f (see Supporting Information). The simple Ohm’s law was introduced to fit the data in Figure 1c, indicated by the red solid line,


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which gives a value of

BTO

~0.6 Ω·cm at RT. The evaluated

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BTO

value is in the regime of

semiconductors. In earlier works, semiconducting BTO films were obtained by introducing oxygen defects or dopants,[19-23] which exhibit both metallic-like and insulating electric character. It may mean possible existence of O deficiency in our BTO films. Although the amount of O defect sites is less than what can be detected by XAS (as shown in Figure 4), we cannot completely exclude out the possible oxygen deficiency enough to induce the conducting electrical character as in SrTiO3-δ.[24] On the other hand, other sources such as electrons in the q2-DES at the LAO/STO hetero-interface can also cause conductivity by tunneling through the few nm thick insulating LAO. It has been long questioned whether conductive (non-insulating) materials can also exhibit insulating bulk characteristics such as ferroelectricity. The ferroelectricity in semiconducting materials have been studied experimentally[14-16] as well as theoretically,[25,26] which has been empirically probed with indirect methods. Moreover, most studies have been performed in bulk BTO by using reduced single crystal or sintered ceramics, but never in thin film form. If ferroelectricity sustains in semiconducting ultra-thin BTO films, the next question would be regarding the minimum thickness required to maintain the ferroelectric properties. With our BTO ultra-thin films, we directly measured ferroelectric hysteresis using PFM at RT, and then obtained d33 loops. The measurements on the 2, 3, and 5 uc thick BTO films on LAO/STO (2-, 3-, 5- BTO/LAO/STO) are presented in Figures 2d–2i, while Figures 2b–2c show the corresponding measurements on LAO/STO without the BTO layer (0-BTO/LAO/STO) for comparison. Note that we used the q2-DES at the LAO/STO hetero-interface as an electrode. We measured the bare BTO surfaces directly at RT (Cypher Highest Resolution Fast Scanning AFM, Oxford Instruments). The amplitudes of the 2-, 3-, 5-BTO/LAO/STO in Figures 2d, 2f, and 2h,


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respectively, show clear butterfly shapes that are all successively switchable. Note that the imprint (off-set of the butterfly shape from 0 V) is found in all measured samples which is usually attributed to interfacial effects with electrodes.[27] The phases of the d33 loops in Figures 2e, 2g, and 2i corresponding to the amplitude signals of each junction also exhibit switchable behavior, indicating the existence of ferroelectric character in the stacked junction. In addition, domain switching voltages between the amplitude and the phase data are identical, also indicating the sustainment of ferroelectricity in the stacked junctions. One may infer other possible origins of the observation of ferroelectric signal in our stacked junctions such as a strained LAO layer,[28] oxygen vacancy migration in the LAO layer,[29,30] and also from the STO substrates. However, note that LAO is not even piezoelectric in any case[31] and our electrical measurement scheme confines applied electrical fields within BTO and LAO layers; STO substrates are not exposed to external bias voltage. For the observation of PFM piezoresponse from the oxygen vacancy migration in LAO/STO, a thick LAO layer having more than 10 uc is required,[29,30] while we used 6 uc thick LAO layer for all samples in this study. Decisively, as shown for the 0-BTO/LAO/STO in Figures 2b–2c, no signs of ferroelectricity were observed in samples without a BTO layer. These results confirm that the observed ferroelectricity comes only from the BTO layers for 2-, 3-, 5-BTO/LAO/STO. To further examine the role of possible migration of oxygen vacancies in the junctions in the observed PFM response, we successively increased the bias voltage sweeps up to voltages of 1 V or more to reach current densities high enough for ionic migration in electric devices.[32,33] Figures S4a–S4d show the I-V sweep data of 3−15 uc-thick BTO films on LAO/STO (see Supporting Information). Near RT, all devices still have their own linear I-V characteristics, with the slope identical to those in Figure S3 possessing no hysteretic behavior. In general, ionic


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migration produces variation of resistance in electronics such as degradation[32] which is not observed in our samples. Therefore, we can rule out possible ionic migration in our junctions during electrical measurements. On the other hand, the BTO/LAO/STO samples have shown little change in conductance for repeated measurements over several months. These facts indicate that BTO/LAO/STO is very stable at an ambient environment presumably due to the chemically stable BTO as the top protective layer. Note that the LAO/STO interface is easily affected by ambient environment.[34] Some insights regarding the ferroelectricity in the semiconducting BTO film are found in the self-polarization images displayed in Figure 3. As shown in Figure 3a, no change was observed for the entire area by writing with ±8 V on 0-BTO/LAO/STO, as expected from Figures 2b and 2c, reconfirming the absence of ferroelectricity without BTO layer. In Figures 3b and 3d, the PFM responses in the outer-most as-grown areas with 0 V are same (2 uc thick, Figure 3b) or opposite (5 uc thick, Figure 3d) to that of the inner areas written with +8 V. Such area without application of electrical writing process for either up or down polarization clearly have different polarization direction, the down- and upward for the 2 and 5 uc thick BTO films, respectively. Note that the 3 uc thick BTO has both down- and upward self-polarization, while only downward one is displayed in Figure 3c. In general, the self-polarization is usually affected by the depolarization field, which plays a significant role in the ferroelectric properties. Besides surface charges, BTO itself used in this study has mobile carriers which may also be able to screen either external electrical field or depolarization field, resulting in changes in the self-polarization direction as dBTO changes. To check the stability of ferroelectricity in our samples, we measured self-polarization images at specific intervals of time for 5-BTO/LAO/STO. Our observations


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indicate that the ferroelectricity of BTO/LAO/STO junction is more stable compared to that of BTO(12 uc)/SRO/STO (see Figure S6 in Supporting Information). The electronic evidence to verify ferroelectricity in the ~1 nm thick BTO film was examined by analyzing the local electronic structure of BTO. The soft X-ray absorption spectra measured at Ti L-edge and O K-edge are displayed in Figure 4. Red and blue lines indicate the spectra measured by using E//c and E⊥c linear photon polarizations, respectively. Figure 4a displays Ti L-edge spectra. For clarity, the top pane displays full range of Ti L2,3-edge spectra of a 2 uc thin film, which is that of a typical Ti4+ ion in octahedral symmetry. The spectra are separated into L3(2p3/2) and L2(2p1/2)-edge because of the spin-orbit splitting of 2p core hole final states. Each edge shows two white lines which reflect the t2g (yz, zx, xy) and eg (x2–y2, 3z2–r2) states of crystal field splitting. The crystal field levels are split further under an elongated tetragonal distortion. When the Ti4+ ion sits in the center, the energy of yz and zx (3z2–r2) orbitals is lower than that of xy (x2–y2), and the absorption white lines corresponding to E//c spectrum(red) locates at a lower photon energy compared to E⊥c. On the other hand, the E//c spectrum of t2g peak is observed at a slightly higher photon energy as the Ti4+ ion does not sit at the center of the elongated tetragonal cage.[35] The ferroelectricity of BTO is triggered by the off-centering of the Ti4+ ion accompanying the dynamical change of hybridization.[36] In the ferroelectric BTO, the Ti 3d – O 2p hybridization between Ti ion and apical oxygen strengthens and it results in the higher ligand field energy of yz and zx orbitals. Because the pre-edge peak of O K-edge spectra represent the ligand hole states (Lyz Lzx Lxy) hybridizing with Ti 3d t2g orbitals, the energy separation of the pre-edge represents the energetic position of anti-bonding states, i.e. the energy levels of ligand field splitting. As can be seen in Figure 4b, the energy position of the E//c spectrum is higher than that


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of the E⊥c spectrum. Because the Ti4+ ion is located at the off-center of the elongated tetragonal TiO6 cage, the hybridization strengths of the yz and zx orbitals become stronger than that of the xy orbital. It results in the splitting of t2g levels where the yz and zx ligand field levels are observed at a higher photon energy than the xy level, as shown in Figure 4b. As an another origin for the observed XAS, we can consider an electronic reconstruction at the TiO2 termination layer in BTO film. Its electronic structure can be very similar to that of 2q-DES at the LAO/STO hetero-interface. [37,38] In this case, the xy band is also located lower than the yz and zx bands, as in an n-type LAO/STO interface. However, the energy splitting in O K-edge spectra of the 2 uc film is larger than that of the LAO/STO system.[39] Taking these observations together with the PFM results discussed in Figures 2 and 3, we can conclude that the lower energy of the xy orbital can only be attributed to the off-centering of the Ti4+ ion. Although the splitting of the 2 uc film is relatively smaller than that of the 5 uc film, the sign of linear dichroism remains identical. This observation is sufficient to conclude that the local symmetry is off-centered tetragonal. Because BTO on STO is under a huge compressive strain, the elongated tetragonal coordination and Coulomb repulsion of planar oxygens push the Ti4+ ion along the c-axis. Consequently, the ferroelectricity of compressively strained BTO exhibits huge robustness even in an ultra-thin 2 uc film. In summary, we have observed the existence of ~1 nm thick semiconducting ferroelectric BTO films at RT for the first time. Various experimental evidence from transport measurements, PFM measurements, and soft X-ray linear dichroism clearly support the semiconducting ferroelectric character of our BTO samples. This observation can provide critical insights for the development of both fundamental understanding and applications of ferroelectrics. Furthermore, the hybridized system of ~nm thick ferroelectric and q2-DES electrodes shows that the thickness of


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next-generation devices can be reduced using innovative methods, which may make it possible to have adaptable two-dimensional devices in the future.

Experimental Section PFM Measurements: To obtain the d33 loops, we swept the bias voltage in the range of –6 V to 6 V for all BTO layers with different thickness of 2, 3, 5 uc while it was -8V to 8 V for the sample without BTO layer (0 uc BTO). For complete ferroelectric domain switching, sufficient electric field (E) is required to pole dipoles along up- or downward direction. Therefore, as the thickness of the films increase, the applied bias voltage must be sufficiently higher to completely switch the ferroelectric domain along up-/downward direction. Therefore, such phase differences of thicker (3, 5 uc) BTO films in the d33 loops may be attributed to insufficient external electric field. Note that the smaller phase differences in the thicker BTO films does not mean that there may exist polarization switching with that small angle, i.e. 120° domain switching in the 3 uc thick BTO. Rather, formation and propagation of domain wall in such ultra-thin ferroelectric films under insufficient electric field may be critical. [40] X-ray absorption Spectroscopy: The XAS measurements at Ti L-edge and O K-edge were performed at 2A beamline at the Pohang Light Source. Total electron yield method was used to acquire the spectra, and the drain currents of samples and a gold mesh were measured simultaneously in order to normalize beam current. Photon incidence angle was fixed to be 70° to the normal vector (c-axis) of the films, and the linear photon polarizations of E⊥c and E//c were switched by using the elliptical polarized undulator. Hence, it enables us to obtain the linear dichroism spectra without changing the measurement geometry.


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Figure 1. The BTO thickness dependent transport properties. a) Three-terminal electrical measurement schematic of the BTO/LAO/STO stacked junction and b) measured resistance values (R) vs temperature (T) with varying the BTO thickness (dBTO). Al TEs were deposited in a square form (500

500 µm2) with the thickness of 50 nm. c) Inverse current vs dBTO for different

values of dBTO; the red solid line is plotted with the Ohm’s law. The 0 uc in 1b denotes the R vs T of the LAO/STO bilayer without the BTO layer obtained in the device shown in schematic 1a. In addition, the inset in 1b indicates the R vs T with the 2 uc BTO layer on the LAO/STO.


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Figure 2. The d33 measurements of piezoresponse force microscope (PFM) for 0 − 5 uc BTO films measured on the bare BTO surfaces. a) Schematic diagram of PFM measurement. b), d), f), h) Amplitudes and c), e), g), i) phases of d33 hysteresis loops from PFM of the 0, 2, 3, and 5 uc thick BTO layers are presented, respectively. In all the measurement processes, the conducting LAO/STO interface was grounded.


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b

a 0V

0V

Deg

+8V

-8V

-8V

+8V

2 uc

0 uc

c

d 0V

0V

+8V

+8V V -8V

-8V

3 uc

5 uc

𝟏𝟎𝝁𝒎 × 𝟏𝟎𝝁𝒎 Figure 3. PFM electrical writing and directional changes of polarization for different thicknesses of BTO: a) 0 uc, b) 2 uc, c) 3 uc, and d) 5 uc. +8 V and –8 V is applied separately in the regions, distinguished by solid lines. The voltage was applied to the sample relative to the tip. The outermost areas are in their pristine states without application of electrical fields. The vertical dark lines at the edges of the different polarity of the bias voltages are artificially drawn due to unknown mechanism of the voltage polarity switching of our PFM equipment.


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Figure 4. Soft X-ray absorption linear dichroism spectra of BTO thin films. a) Ti L2,3-edge spectra of a 2 uc thin film (top) and magnified L3-edge spectra of 2 uc and 5 uc films (bottom). b) O K-edge spectra of a 2 uc thin film (top) and magnified pre-edge spectra of 2 uc and 5 uc films (bottom). Pre-edge reflects oxygen 2p holes hybridized with Ti 3d t2g orbitals. Red and blue lines indicate that the spectrum is measured by using E//c and E ⊥ c linear photon polarizations, respectively. Red and blue bars indicate the energy positions of white lines.


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ASSOCIATED CONTENT Supporting Information. Structural characterization by XRD measurements, growth of BTO/LAO on STO substrates and surface morphology of these BTO/LAO/STO trilayers, I-V characteristics of BTO/LAO/STO trilayers depending on the thickness of BTO layers, I-V characteristics of BTO/LAO/STO trilayers with higher bias voltages, electrical break down with much higher bias voltage during IV measurements, relaxation behavior of PFM piezoresponse over time (PDF)

AUTHOR INFORMATION Corresponding Author **E-mail: [email protected] Author Contributions S.R. Lee and L. Bassanforj deposited all oxide films and S. R. Lee and J. H. Song characterized their structural properties. L. Bassanforj, J. W. Chang, and I. W. Hwang fabricated capacitor structures and measured electrical properties of them. J. R. Kim, M. W. Choi, M. You, and C.-H. Yang examined PFM hysteresis loops and signals and S. R. Lee, J. R. Kim, and J. Kim analyzed ferroelectricity from PFM data. J.-G. Kim and K.-T. Ko performed XAS experiments. S. R. Lee wrote the main manuscript text; K.-T. Ko analyzed and described XAS data while the details of structural and electrical properties of devices was reviewed and revised by S. B. Shim, J. H. Song, and J. Kim. S. R. Lee prepared figures 1 and 2, J. R. Kim figures 3, J.-G. Kim and K.-T. Ko figure 4.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Research Center Program of IBS (Institute for Basic Science) in Korea (IBS-R009-D1) and the National Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2016K1A4A4A01922028) and Basic Science Research Program (2016R1A2B4008706). This work was also supported by the NRF Grant funded by the Korean Government via the Creative Research Center for Lattice Defectronics (Grant No. NRF2017R1A3B1023686). ABBREVIATIONS nm, nano-meter; BTO, BaTiO3; LAO, LaAlO3; STO, SrTiO3; uc, unit cell; dC, critical thickness ; TE, top-electrode; BE, bottom-electrode; SRO, SrRuO3; P-E, polarization-electric field; PFM, piezoresponse force microscopy; q2-DES, quasi 2-Dimensional electron gas; RT, room temperature; XAS, X-ray absorption spectroscope; XRD, X-ray diffraction; XRSM, X-ray reciprocal space mapping; I-V, current-voltage.


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REFERENCES (1)

Schlom, D. G.; Chen, L.-Q.; Fennie, C. J.; Gopalan, V.; Muller, D. A.; Pan, X.; Ramesh, R.; Uecker, R.: Elastic strain engineering of ferroic oxides. MRS Bulletin 2014, 39, 118130.

(2)

Garcia, V.; Bibes, M.: Ferroelectric tunnel junctions for information storage and processing. Nat. Commun. 2014, 5.

(3)

Gruverman, A.: Recent advances in functional testing of ferroelectric nanostructures. Ferroelectrics 2012, 433, 88-106.

(4)

Ishiwara, H.: Ferroelectric Random Access Memories. J. Nanosci. Nanotechnol. 2012, 12, 7619-7627.

(5)

Junquera, J.; Ghosez, P.: Critical thickness for ferroelectricity in perovskite ultrathin films. Nature 2003, 422, 506-509.

(6)

Gerra, G.; Tagantsev, A. K.; Setter, N.; Parlinski, K.: Ionic polarizability of conductive metal oxides and critical thickness for ferroelectricity in BaTiO3. Phys. Rev. Lett. 2006, 96, 107603.

(7)

Aguado-Puente, P.; Junquera, J.: Ferromagneticlike closure domains in ferroelectric ultrathin films: First-principles simulations. Phys. Rev. Lett. 2008, 100, 177601.

(8)

Stengel, M.; Vanderbilt, D.; Spaldin, N. A.: Enhancement of ferroelectricity at metal-oxide interfaces. Nat. Mater. 2009, 8, 392-397.

(9)

Liu, Y.; Lou, X.; Bibes, M.; Dkhil, B.: Effect of a built-in electric field in asymmetric ferroelectric tunnel junctions. Phys. Rev. B 2013, 88, 024106.


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Page 20 of 24

(10) Zheng, Y.; Cai, M. Q.; Woo, C. H.: Critical properties of symmetric nanoscale metal– ferroelectric–metal capacitors. Acta. Materialia. 2010, 58, 3050-3058. (11) Kim, Y. S.; Kim, D. H.; Kim, J. D.; Chang, Y. J.; Noh, T. W.; Kong, J. H.; Char, Park, K.Y. D.; Bu, S. D.; Yoon, J.-G.; Chung, J.-S.: Critical thickness of ultrathin ferroelectric BaTiO3 films. Appl. Phys. Lett. 2005, 86, 102907. (12) Garcia, V.; Fusil, S.; Bouzehouane, K.; Enouz-Vedrenne, S.; Mathur, N. D.; Barthelemy, A.; Bibes, M.: Giant tunnel electroresistance for non-destructive readout of ferroelectric states. Nature 2009, 460, 81-84. (13) Jeong, I. K.; Lee, S.; Jeong, S.-Y.; Won, C. J.; Hur, N.; Llobet, A.: Structural evolution across the insulator-metal transition in oxygen-deficient BaTiO3- studied using neutron total scattering and Rietveld analysis. Phys. Rev. B 2011, 84, 064125. (14) Kolodiazhnyi, T.; Tachibana, M.; Kawaji, H.; Hwang, J.; Takayama-Muromachi, E.: Persistence of ferroelectricity in BaTiO3 through the insulator-metal transition. Phys. Rev. Lett. 2010, 104, 147602. (15) Hwang, J.; Kolodiazhnyi, T.; Yang, J.; Couillard, M.: Doping and temperature-dependent optical properties of oxygen-reduced BaTiO3-. Phys. Rev. B 2010, 82, 214109. (16) Fujioka, J.; Doi, A.; Okuyama, D.; Morikawa, D.; Arima, T.; Okada, K. N.; Kaneko, Y.; Fukuda, T.; Uchiyama, H.; Ishikawa, D.; Baron, A. Q. R.; Kato, K.; Takata, M.; Tokura, Y.: Ferroelectric-like metallic state in electron doped BaTiO3. Sci. Rep. 2015, 5.


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Nano Letters

(17) Eom, C. B.; Cava, R. J.; Fleming, R. M.; Phillips, J. M.; vanDover, R. B.; Marshall, J. H.; Hsu, J. W. P.; Krajewski, J. J.; Peck, W. F.: Single-crystal epitaxial thin films of the isotropic metallic oxides Sr1–xCaxRuO3 (0 ≤ x ≤ 1). Science 1992, 258, 1766-1769. (18) Lee, J. S.; Seung, S. K.; Lee, S. R.; Chang, J.-W.; Noh, H.; Baasandorj, L.; Shin, H. S.; Shim, S.-B.; Song, J.; Kim, J.: Gate-controlled metal–insulator transition in the LaAlO3/SrTiO3 system with sub-critical LaAlO3 thickness. Phys. Status Solidi RRL 2012, 6, 472-474. (19) Jonker, G. H.: Some aspects of semiconducting barium titanate. Solid-State Electron. 1964, 7, 895-903. (20) Zhao, T.; Chen, Z.-H.; Chen, F.; Lu, H.-B.; Yang, G.-Z.; Cheng, H.-S.: Electrical and optical properties of strongly reduced epitaxial BaTiO3−x thin films. Appl. Phys. Lett. 2000, 77, 4338-4340. (21) Heywang, W.: Semiconducting barium titanate. J. Mater. Sci. 1971, 6, 1214-1224. (22) Ihrig, H.; Hennings, D.: Electrical transport properties of n-type BaTiO3. Phys. Rev. B 1978, 17, 4593-4599. (23) Gillot, C.; Michenaud, J. P.; Maglione, M.; Jannot, B.: DC electrical resistivity of Nbdoped BaTiO3 and EPR measurements. Solid State Commun. 1992, 84, 1033-1038. (24) Herranz, G.; Basletic ´, M.; Bibes, M.; Carrétéro, C.; Tafra, E.; Jacquet, E.; Bouzehouane, K.; Deranlot, C.; Hamzić, A.; Broto, J.-M.; Barthélémy, A.; Fert, A.: High Mobility in LaAlO3/SrTiO3 Heterostructures: Origin, Dimensionality, and Perspectives. Phys. Rev. Lett. 2007, 98, 216803.


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Page 22 of 24

(25) Anderson, P. W.; Blount, E. I.: Symmetry considerations on martensitic transformations: "Ferroelectric" metals? Phys. Rev. Lett. 1965, 14, 217-219. (26) Wang, Y.; Liu, X.; Burton, J. D.; Jaswal, S. S.; Tsymbal, E. Y.: Ferroelectric instability under screened Coulomb interactions. Phys. Rev. Lett. 2012, 109, 247601. (27) Grossmann, M.; Lohse, O.; Bolten, D.; Boettger, U.; Schneller, T.; Waser, R.: The interface screening model as origin of imprint in PbZrxTi1

− xO3

thin films. I. Dopant,

illumination, and bias dependence. J. Appl. Phys. 2002, 92, 2680-2687. (28) Haeni, J. H.; Irvin, P.; Chang, W.; Uecker, R.; Reiche, P.; Choudhury, Y. L. Li, S.; Tian, W.; Hawley, M. E.; Craigo, B.; Tagantsev, A. K.; Pan, X. Q.; Streiffer, S. K.; Chen, L. Q.; Kirchoefer, S. W.; Levy, J.; Schlom, D. G.: Room-temperature ferroelectricity in strained SrTiO3. Nature 2004, 430, 758-761. (29) Bark, C. W.; Sharma, P.; Wang, Y.; Baek, S. H.; Lee, S.; Ryu, S.; Folkman, C. M.; Paudel, T. R.; Kumar, A.; Kalinin, S. V.; Sokolov, A.; Tsymbal, E. Y.; Rzchowski, M. S.; Gruverman, A.; Eom, C. B.: Switchable Induced polarization in LaAlO3/SrTiO3 heterostructures. Nano Letters 2012, 12, 1765-1771. (30) Li, C.; Cao, Y.; Bai, Y.; Li, A.; Zhang, S.; Wu, D.: Electromechanical Response from LaAlO3/SrTiO3 Heterostructures. ACS Appl. Mater. Interfaces 2015, 7, 10146-10151. (31) Cancellieri, C.; Fontaine, D.; Gariglio, S.; Reyren, N.; Caviglia, A. D.; Fête, A.; Leake, S. J.; Pauli, S. A.; Willmott, P. R.; Stengel, M.; Ghosez, P.; Triscone, J. M.: Electrostriction at the LaAlO3/SrTiO3 interface. Phys. Rev. Lett. 2011, 107, 056102.


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(32) Nian, Y. B.; Strozier, J.; Wu, N. J.; Chen, X.; Ignatiev, A.: Evidence for an oxygen diffusion model for the electric pulse induced resistance change effect in transition-metal oxides. Phys. Rev. Lett. 2007, 98, 146403. (33) Strukov, D. B.; Williams, R. S.: Exponential ionic drift: fast switching and low volatility of thin-film memristors. Appl. Phys. A 2009, 94, 515-519. (34) Bi, F.; Bogorin, D. F.; Cen, C.; Bark, C. W.; Park, J.-W.; Eom, C.-B.; Levy, J.: “Watercycle” mechanism for writing and erasing nanostructures at the LaAlO3/SrTiO3 interface. Appl. Phys. Lett. 2010, 97, 173110. (35) Schmitz-Antoniak, C.; Schmitz, D.; Borisov, P.; de Groot, F. M. F.; Stienen, S.; Warland, A.; Krumme, B.; Feyerherm, R.; Dudzik, E.; Kleemann, W.; Wende, H.: Electric in-plane polarization in multiferroic CoFe2O4/BaTiO3 nanocomposite tuned by magnetic fields. Nature Commun. 2013, 4, 2051. (36) Ghosez, P.; Michenaud, J. P.; Gonze, X.: Dynamical atomic charges: The case of ABO3 compounds. Phys. Rev. B 1998, 58, 6224-6240. (37) Chang, Y. J.; Bostwick, A.; Kim, Y. S,; Horn, K.; Rotenberg, E.: Structure and correlation effects in semiconducting SrTiO3. Phys. Rev. B 2010, 81, 235109. (38) Delugas, P.; Filippetti, A.; Fiorentini, V.; Bilc, D. I.; Fontaine, D.; Ghosez, P.: Spontaneous 2-Dimensional Carrier Confinement at the n-Type SrTiO3/LaAlO3 Interface. Phys. Rev. Lett. 2011, 106, 166807. (39) Lee, J.-S.; Xie, Y. W.; Sato, H. K.; Bell, C.; Hikita, Y.; Hwang, H. Y.; Kao, C.-C.: Titanium dxy ferromagnetism at the LaAlO3/SrTiO3 interface. Nature Mater. 2013, 12, 703-706.


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(40) Jo, J. Y.; Kim, Y. S.; Noh, T. W.; Yoon, J.-G.; Song, T. K.: Coercive fields in ultrathin BaTiO3 capacitors. Appl. Phys.Lett. 2006, 89, 232909.


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