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Spectroscopic Proof of the Correlation between Redox-State and Charge-Carrier Transport at the Interface of Resistively Switching Ti/PCMO Devices Anja Herpers, Christian Lenser, Chanwoo Park, Francesco Offi, Francesco Borgatti, Giancarlo Panaccione, Stephan Menzel, Rainer Waser, and Regina Dittmann* Resistance random access memory (RRAM), which utilizes two or more resistive states of a material system for data storage, has attracted considerable attention as a future non-volatile memory concept.[1,2] Besides a large variety of binary oxides, complex transition metal oxides, e.g., manganites,[3–7] titanates, and zirconates,[8,9] usually exhibit different resistance states at opposite polarities of electrical stimulation and could therefore be employed as RRAM. It has become widely accepted that this so-called bipolar resistive switching in oxides is in most cases connected with a voltage-driven oxygen vacancy movement and a resulting redox-process.[10] However, the current knowledge of the microscopic details of the redox-processes is very limited. For most n-conducting oxides, e.g., TiO2, TaOx, and SrTiO3, it has been demonstrated that resistive switching and the related redox-processes do not take place homogeneously beneath the whole electrode area but are restricted to a spatially defined filament region that evolves during the so-called electroforming process.[11–14] Interestingly enough, there is another class of bipolar resistive switching oxide systems, such as several manganites, for which it was demonstrated that the high and low resistive state currents scale with the electrode area,[15–17] implying that forming and switching take place beneath the whole electrode. Manganite thin films in general form an ohmic contact with noble metals, such as Pt and Au,[18] whereas oxidizable electrodes, such as Al, Ti, or Ta, exhibit non-linear current-voltage relations, which is a prerequisite for the observation of resistive Dr. A. Herpers,[+] Dr. C. Lenser,[+] C. Park, Dr. S. Menzel, Prof. R. Waser, Prof. R. Dittmann Peter Grünberg Institute Research Center Jülich 52425, Germany E-mail: [email protected] Dr. F. Offi CNISM and Dipartimento di Scienze Universita Roma Tre Rome, Italy Dr. F. Borgatti CNR – Istituto per lo Studio dei Materiali Nanostrutturati (ISMN) via P. Gobetti 101 I-40129 Bologna, Italy Dr. G. Panaccione CNR – Istituto Officina dei Materiali (IOM) Laboratorio TASC, S.S.14, Km 163.5 I-34149 Trieste, Italy [+]A.H. and C.L. contributed equally to this work.
DOI: 10.1002/adma.201304054
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switching. As a result of the use of non-noble metals, an interface oxide layer is formed,[19–21] which in principle could also be directly involved in the switching effect. If cation intermixing at the interfaces is also taken into account,[22] the situation becomes even more complicated. Recently, we have shown that after electroforming of Ti/ Pr0.48Ca0.52MnO3/SrRuO3 (Ti/PCMO/SRO) thin film devices the chemical changes at the interface between the Ti electrode and the PCMO layer dominate the resistance of the multilayer stack.[23] The reactive metal electrode forms an oxide layer at the Ti/PCMO interface during its deposition[19–21] prior to device operation, and applying a voltage to the stack was shown to increase the thickness of the TiO2 layer at the interface, and to deplete the underlying PCMO of oxygen further. While electroforming in resistive switching systems is often accompanied by significant changes in the oxygenation states,[14,24] the chemical changes upon switching are expected to be much less pronounced, occur on a much smaller length scale and have therefore not been reported for a similar system so far. However, in this work we were able to detect small but significant chemical changes between the two reversible resistance states. Based on the knowledge of the electroforming mechanism, we present an encompassing view of the field-induced valence change during resistive switching at the Ti/PCMO interface. The chemical changes at the Ti/PCMO interface in four different resistive states are investigated by Hard X-ray Photoelectron Spectroscopy (HAXPES), and correlated to the transport-mechanism of charge-carriers across the multilayer stack for each state. A notable difference between the high and low resistive states can be explained through a convolution of several conduction mechanisms, which is in agreement with the observed chemical changes. Figure 1a shows an exemplary current–voltage relation of the Pt (7 nm)/Ti (4 nm)/PCMO (20 nm)/SRO (30 nm)/SrTiO3 stack, with an inset depicting the sample geometry. In the following, we distinguish between four different resistive states on the bipolar switching hysteresis. The initial resistance state after deposition of the top electrode is referred to as “pristine state” (PS), and exhibits the lowest resistance. Electroforming is achieved by the application of a positive voltage sweep of up to +6.5 V, and is called “formed state” (FS). After forming, a negative voltage sweep to –6 V sets the system into the “low resistance state” (LRS), and a subsequent sweep to a positive voltage of +6.5 V resets the system into the “high resistance state” (HRS). The four states can be ordered by their resistance according to PS < LRS < HRS < FS for the samples discussed
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in this work. It should be noted that the switching follows a counter-eightwise polarity,[13] in the same way as the filamentary switching of titania-based systems reported before. Due to the constraints imposed on the sample geometry by the limited photoelectron mean-free-path in the HAXPES experiment, the top electrode thickness was reduced to Pt (3 nm)/Ti (7 nm), without observing a major change of switching properties. Large electrode arrays were fabricated to fill the field-of-view of the photoelectron-analyzer with thin film devices completely, as schematically depicted in Figure 1b. In addition to the patterned arrays, a free PCMO reference thin film was measured (Figure 2c). As shown in our previous work,[23] the shift in the Mn spectra is induced by the redoxreaction during the Ti deposition according to: 3+ 2+ 3+ 4+ (Pr0.48 Ca0.52 )(Mn0.48 Mn0.52 )O32− + Ti 3+ 2+ 3+ 4+ 2− ⇔ (Pr0.48Ca0.52 )(Mn 0.48+2 x Mn 0.52 − 2 x )O3− x + TiOx
(1)
The influence of the electrical treatment on the oxidation state of the top electrode is shown in Figure 2a. The Ti 2p spectrum recorded in the pristine state exhibits two distinct features at a binding energy of 454.1 eV and 458.5 eV, which can be identified as the Ti 2p3/2 emission line of metallic Ti0 and fully oxidized Ti4+, respectively. For a good reproduction of the spectral envelope, a least-squares fitting analysis with one spin-orbit split doublet pair for each possible oxidation state of Ti was performed. To minimize the free parameters of the fit and adhere to physical principles, the area ratio of the doublet peaks was set to 2:1, the doublet separation was set to 6.17 eV for Ti4+ and 5.54 eV for metallic Ti0, with decreasing intermediate values for the decreasing oxidation state of Tin+. A minor contribution from the underlying SrRuO3 bottom
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Figure 1. (a) Quasistatic I(V) curve of the Pt/Ti/PCMO/SRO/STO stack, investigated with HAXPES. The voltage is applied to the Pt top electrode and the SRO is set on ground potential. (b) Geometry of the HAXPES sample.
electrode, arising from the area between the electrodes, was accounted for by a single peak, corresponding to the Ru 3p3/2 line at 463.4 eV. With increasing resistance state, a spectral weight transfer from the low binding energy components to the high binding energy components of the complex Ti 2p envelope is observed. The area percentages of all Ti oxidation states are shown in Figure 2d. While the Ti4+ component is the dominant line in all four spectra, a monotonous decrease can be observed in the metallic Ti0 line with increasing resistance, while the Ti4+ component increases with increasing resistance. In a similar manner, Figure 2b and 2e depict the O 1s spectra and the area percentage of the three constituent peaks, respectively. Two main components can be observed in each spectrum at 529.5 eV and 530.5 eV binding energy, which can be assigned to PCMO and TiOx, respectively, while the third component can be assigned to surface contaminants (mainly residual photoresist). It should be noted that the electronic configuration of the oxygen ion ([He]2s22p6), unlike that of Ti, does not change appreciably in different Ti-suboxides, and therefore a single O 1s line is sufficient for a good reproduction of the spectral envelope. In analogy to the trend observed in the Ti spectra, a spectral weight transfer from PCMO to TiOx is observed, increasing with increasing resistance state. In accordance with these observations, the Mn 2p emission doublet (Figure 2f) shows a low binding energy shoulder at the Mn 2p3/2 line for all states. This shoulder indicates a shift of the average Mn valence toward Mn3+, in accordance with oxygen removal from the PCMO (Equation (1)), and is slightly enhanced by the electrical treatment. As the effect upon switching between LRS and HRS is rather small and the Mn signal is detected from deeper lying regions of the stack than the Ti, the signal-to-noise level of the data is insufficient to detect differences between theses electrical states in the Mn spectrum. The photoemission spectra of the Ti 2p, O 1s and Mn 2p lines clearly show that a redox-reaction is induced through electrical biasing, influencing the resistance of both the Ti electrode and the PCMO layer. The monotonous increase of electrode oxidation with stack resistance implies a direct role of the interfacial oxide layer in the resistance change. To investigate if the electrode oxidation alone determines the resistance of the multilayer stack, we have performed transport measurements at low voltages for each resistive state, corresponding to a non-destructive read-out. The low voltage characteristics of four representative electrodes – measured after the HAXPES experiment – are shown in Figure 3. The PS (Figure 3a) and LRS (Figure 3c) exhibit almost symmetric, non-linear current responses, whereas a notable asymmetry of the current response can be observed for resistive states written by a positive voltage, i.e., FS (Figure 3b) and HRS (Figure 3d). Taking into account the complex multilayered structure of the system, we have performed a qualitative analysis of the conduction mechanisms that dominate each resistive state. Since the same oxide stack with a high work function metal as top electrode shows a linear and low-resistive I(V) relation,[15,23] the increased resistance of the pristine
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Figure 2. (a–c) Ti 2p/O 1s/Mn 2p XPS spectra collected on the four regions, sorted from low resistance (pristine) to high resistance (formed). The spectrum named PCMO refers to the unpatterned reference sample. (d,e) Area percentages of the constituent components of the Ti 2p and O 1s envelope, respectively. (f) Close-up of the Mn 2p3/2 edge.
Ti/PCMO stack can be attributed to the redox-reaction at the interface. A non-linear circuit element, describing the field-acceleration of the polaron transport in the oxygendepleted PCMO3-x layer,[25] is used to model the measured I(V)-behavior of the PS (see Figure 3a). An accurate fit can only be obtained by extending the circuit with a serial ohmic resistor, which describes the linear contributions of the metals Pt, Ti and SRO, the Ti suboxides TiOx (0 < x < 2)[26] and the completely oxidized PCMO, which possesses a linear I(V) behavior due to its small barrier for polaron hopping. In a serial connection of two circuit elements, the voltages are added while the current is constant. For the model describing the PS, we inverted the function of Ihop(Vhop) and added the voltage drop upon the ohmic resistance R:
Vhop+R (I ) = Vhop (I ) + VR (I ) =
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ΔE ⎞ ⎛ π 2kBTr arsinh ⎜ e kBT ⋅ I ⎟ + R ⋅ I ea ⎝ neaω A ⎠
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(2)
where T is the temperature, r the thickness of the oxygen-depleted region, a the hopping distance, n the polaron density, ω the hopping frequency, A the pad area and ΔE the activation energy for hopping; kB is the Boltzmann constant and e the electron charge. Interestingly, the same description can be extended to the LRS (see Figure 3c) with reasonable parameter modifications (Table 1), but fails to describe either FS or HRS. For the latter two, it is necessary to include an additional, asymmetric circuit element into the model. As the HAXPES data shows a larger amount of Ti4+ for these states, it is conceivable that a closed interfacial TiO2 layer acts as a barrier for charge carrier transport.[27] An asymmetric tunnel junction, as described by Simmons,[28] located in the TiOx/TiO2/PCMO interface region yielded a very good fit when added to the serial circuit used to describe PS and LRS. For a trapezoidal tunnel barrier with two different barrier heights ϕ1 < ϕ2 and thickness d, asymmetry can only be found for voltages higher than the specific barrier height. For lower voltages, a symmetric function has to be applied:
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Figure 3. Electrical read-out data for the different resistance states: (a) In the pristine state the Ti oxide layer is formed homogenously, (b) after electroforming a TiO2 tunnel barrier is formed, (c) after a negative voltage sweep the tunnel barrier is bridged, (d) for the HRS the tunnel barrier is reformed. For each data set the fit including the applied model consisting of two or three serial circuit elements is depicted: Polaron hopping Δ, ohmic resistance Ω and tunnel barrier Φ.
PS
FS
LRS
HRS
r [nm]
0.65
1.6
0.79
0.95
R [Ω]
450
11000
2370
700
d [nm]
2.94
ϕ1 [eV]
0.08
0.10
ϕ2 [eV]
0.915
0.85
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2.84
hopping region and the ohmic resistance, and add the voltages to obtain the overall voltage drop. Using this calculation we could find suitable parameter sets for both FS and HRS (Table 1), resulting in good fits as shown in Figure 3b and 3d, respectively. For details regarding the fitting parameters, refer to the experimental section. From these two different serial circuits necessary to reproduce the experimental data, we conclude that charge-carrier transport across the multilayer stack is not determined by a single conduction mechanism, but rather by a combination of several mechanisms and their respective contribution to the total resistance. The transition between PCMO and TiO2 as the dominating contribution to the stack resistance can now be understood as follows: Prior to the deposition of the Ti electrode, the PCMO is a highly doped oxide that behaves as a p-type semiconductor.[29,30] The deposition of Ti reduces the underlying PCMO before device operation, which causes the existence of a thin interfacial PCMO3−x layer where the hopping conductivity is significantly reduced through the presence of oxygen vacancies interrupting the double-exchange chains along Mn–O–Mn bonds.[31] According to the redox-reaction between Ti and Mn (Equation (1)), the Ti layer is simultaneously oxidized to a
2 eV ⎧ ⎡ ⎤ 23π mdϕ 23/2 1+ ϕ2 ⎪ 1.1eA(eV +ϕ − ϕ )2 ⎢ − 23π mdϕ23/2 ⎛ − ⎥ 2eV ⎞ 2 1 6 h ( eV +ϕ 2 −ϕ1 ) 6 h ( eV +ϕ 2 −ϕ1 ) ⎪ − ⎜1+ e ⎢e ⎥ 2 ⎟ ⎝ ϕ2 ⎠ 4π hϕ 2d ⎪ ⎢ ⎥ ⎪ ⎢⎣ ⎥⎦ ⎪ 4π d m 4π d m ⎡ ϕ ϕ eV ϕ ϕ − + − − + + eV ⎤ 1 2 1 2 ⎪ eA I(Vsim ) = ⎨ (ϕ 1 +ϕ 2 − eV )e h − (ϕ 1 +ϕ 2 +eV )e h ⎥ 2 ⎢ ⎦ ⎪ 4π hd ⎣ ⎪ 2 eV ⎡ 23π mdϕ 3/2 ⎤ 23π mdϕ13/2 1+ ϕ1 ⎪ 1 2 ⎢ − − ⎥ ⎪ 1.1eA(eV − ϕ 2 +ϕ 1 ) ⎢e 6 h ( eV −ϕ2 +ϕ1 ) − ⎛ 1 + 2eV ⎞ e 6 h ( eV −ϕ2 +ϕ1 ) ⎥ 2 ⎜ ⎟ ⎪ ⎝ ⎠ ϕ 4π hϕ 1d 1 ⎢ ⎥ ⎪ ⎢⎣ ⎥⎦ ⎩
where m is the effective mass and h is the Planck constant. Since the hopping region, the tunnel barrier and the ohmic resistance are in series, the voltage Vsim(I) is added to the voltage drop caused by hopping and ohmic resistance: Vhop+sim+R(I) = Vhop(I) + Vsim(I) + VR(I). An inversion of Equation (3) is analytically not possible. To circumvent this problem, we varied the voltage Vsim in small increments and calculated the current flowing through the tunnel barrier. As the current in a serial circuit is conserved, we can calculate the resulting voltage drop at the polaron
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Table 1. Fitting parameters for different resistance states.
⎫ ⎪ ⎪ eV < −ϕ 1 ⎪ ⎪ ⎪ ⎪ ⎬ e |V | ≤ ϕ ⎪ ⎪ ⎪ ⎪ eV > ϕ 2 ⎪ ⎪ ⎭
(3)
heterogeneous TiOx layer, which shows significant conductivity,[32] as sketched in Figure 3a. During electroforming, oxygen ions migrate - due to the gradient in the electrical potential - toward the positively biased top electrode, where the Ti is further oxidized and a closed TiO2 layer is formed. The TiO2 (χTiO2 = 4.0 eV)[33] acts as an asymmetric tunnel barrier, because it is sandwiched between two electrodes with different work functions (ΦTi = 4.3 eV, χPCMO = 4.9 eV).[34] Despite the fact that this oxidation is
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homogeneous across the whole electrode, it is conceivable that the oxidation front shows in-plane inhomogeneities on the nanometer scale, as sketched in the inset of Figure 3b. Such inhomogeneities might be a consequence of the polycrystalline structure of the electrode, or due to the nanoscale roughness of the oxygen diffusion front formed during electroforming, or both.[32] The suggested in-plane inhomogeneities of the Ti oxidation state at the PCMO/Ti interface would also be consistent with the experimentally confirmed spatial inhomogeneities induced by similar redox-processes at the SrTiO3/Ti interface which take place during the deposition of the Ti layer.[35] When a negative bias is applied to the formed device, this nanoscale roughness leads to the local reduction of the TiO2 layer. As sketched in the inset of Figure 3c, the tunnel barrier is shorted by reduced, conductive TiOx channels, and the device returns to a symmetric I(V)-characteristics in the low resistance state. Returning to the high resistive state through another positive voltage, the tunnel barrier is restored, along with the asymmetric character of the device (see Figure 3d). It might also be possible to describe each individual curve using different conduction mechanisms (supplementary information). However, the crucial advantage of our proposed model is that it is in agreement with the movement of oxygen vacancies expected from the direction of the applied field and we can fit all four resistive states in a consistent way with a limited number of physically reasonable parameters. In summary, we have experimentally proven that resistive switching in Pt/Ti/PCMO/SRO/STO devices is based on a redox-process, which mainly happens on the Ti -side. The different resistance states are determined by the amount of fully oxidized Ti-ions in the stack, implying a reversible redox-reaction at the interface that governs the formation and shortening of an insulating tunnel barrier. In particular, we have shown that the limiting factor for charge-carrier transport through the PCMO multilayer stack can be changed from tunneling through a thin insulator at high resistance to field-enhanced polaron hopping at low resistance.
0.8 mm wide, where every electrode in the stripe has been subjected to an identical electrical treatment. The spacing between the pads was about 10 µm. The photoelectron emission from the buried regions of interest is ensured by the large electron inelastic mean free path (IMFP) typical of the hard X-ray excitation regime, of the order of 8 nm. The fixed parameters for the circuit modeling were chosen as follows: For the polaron hopping mechanism, room temperature and a polaron hopping distance of a = 4 Å according to the Mn-Mn distance was assumed. The pad size is A = (50 µm)2, and the polaron density was set to n = 1 nm-3 according to Hall measurements. An attempt rate for hopping of ω = 1013 Hz corresponding to the phonon frequency was assumed. From the fits of the PS and LRS, a hopping barrier of about ΔE = 0.4 eV was determined for the highly reduced PCMO layer, which was then fixed for the fitting of the data sets for FS and HRS. This value exceeds the value for completely oxidized PCMO ΔE ≈ 0.1 eV,[25] because the conduction via double exchange got interrupted by oxygen vacancies. For the Simmons tunnel barrier, an effective mass m of 1.19·me was used. A hopping transport mechanism is also conceivable for the Ti suboxide region, but the alternation in hopping layer thickness is inconsistent with the direction of oxygen vacancy movement.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was funded by the European Union Council under the 7th Framework Program (FP7) grant no. 246102 IFOX and has been in part supported by the Deutsche Forschungsgemeinschaft (SFB 917). The synchrotron radiation experiments were performed at the P09 beamline of Petra-III under proposal I-20110545 EC. Thanks are due to the beamline staff Dr. Andrei Gloskovskii for technical support. The HAXPES instrument at beamline P09 is jointly operated by the University of Würzburg (R. Claessen), the University of Mainz (C. Felser) and DESY. Funding by the Federal Ministry of Education and Research (BMBF) under contracts 05KS7UM1, 05K10UMA, 05KS7WW3, and 05K10WW1 is gratefully acknowledged. Received: August 12, 2013 Revised: February 12, 2014 Published online: April 6, 2014
Experimental Details For the samples intended for switching devices, 30 nm SrRuO3 (SRO) (650 °C, 0.133 mbar) as bottom electrode (BE) and 20 nm Pr0.48Ca0.52MnO3 (PCMO) (700 °C, 0.133 mbar) thin films were epitaxially grown in–situ on TiO2-terminated SrTiO3 (STO) (001) single crystals by pulsed laser deposition. 7 nm Ti and a 3 nm Pt capping layer were subsequently deposited by sequential DC sputtering without breaking the vacuum. The top electrode (TE) stack Ti/Pt was patterned using photolithography. As PCMO reference, a 40 nm thick PCMO film was grown under the same conditions directly on STO. HAXPES measurements were performed at beamline P09, PETRA III, HASYLAB. The experimental geometry was chosen to maximize bulk sensitivity, with the angle between the incident X-ray beam and the electron analyzer set to 90°, and the X-ray beam impinging on the sample at a grazing angle of 2°. The photon energy was set at 6 keV and was periodically verified by measuring the Fermi edge spectrum of a reference Au strip in electrical contact to the sample. The overall resolution was 0.35 eV, as determined by the width of the Au Fermi-edge. All the measurements were performed at room temperature. Due to the large footprint of the X-ray beam, single devices were not suitable for HAXPES measurements. Therefore device arrays were fabricated, consisting of 50 × 50 µm² electrodes arranged in stripes about 7 mm long and
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[1] R. Waser, M. Aono, Nat. Mater. 2007, 6, 833. [2] R. Waser, R. Dittmann, G. Staikov, K. Szot, Adv. Mater. 2009, 21, 2632. [3] A. Asamitsu, Y. Tomioka, H. Kuwahara, Y. Tokura, Nature 1997, 388, 50. [4] H. Oshima, K. Miyano, Y. Konishi, M. Kawasaki, Y. Tokura, Appl. Phys. Lett. 1999, 75, 1473. [5] Y. B. Nian, J. Strozier, N. J. Wu, X. Chen, A. Ignatiev, Phys. Rev. Lett. 2007, 98, 146403/1. [6] M. Quintero, P. Levy, A. G. Leyva, M. J. Rozenberg, Phys. Rev. Lett. 2007, 98, 116601/1. [7] A. Sawa, T. Fujii, M. Kawasaki, Y. Tokura, Appl. Phys. Lett. 2004, 85, 4073. [8] A. Beck, J. G. Bednorz, C. Gerber, C. Rossel, D. Widmer, Appl. Phys. Lett. 2000, 77, 139. [9] Y. Watanabe, J. G. Bednorz, A. Bietsch, Gerber-Ch, D. Widmer, A. Beck, S. J. Wind, Appl. Phys. Lett. 2001, 78, 3738.
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[22] T. Yamamoto, R. Yasuhara, I. Ohkubo, H. Kumigashira, M. Oshima, J. Appl. Phys. 2011, 110, 53707/1. [23] F. Borgatti, C. Park, A. Herpers, F. Offi, R. Egoavil, Y. Yamashita, A. Yang, M. Kobata, K. Kobayashi, J. Verbeeck, G. Panaccione, R. Dittmann, Nanoscale 2013, 5, 3954. [24] R. Dittmann, R. Muenstermann, I. Krug, D. Park, T. Menke, J. Mayer, A. Besmehn, F. Kronast, C. M. Schneider, R. Waser, Proc. IEEE 2012, 100, 1979. [25] S. Schramm, J. Hoffmann, C. Jooss, J. Phys. - Condensed Matter 2008, 20, 395231. [26] S. Harada, K. Tanaka, H. Inui, J. Appl. Phys. 2010, 108, 83703/1. [27] M. D. Pickett, D. B. Strukov, J. L. Borghetti, J. J. Yang, G. S. Snider, D. R. Stewart, R. S. Williams, J. Appl. Phys. 2009, 106, 074508. [28] J. G. Simmons, J. Appl. Phys. 1963, 34, 2581. [29] L. P. Gor’kov, V. Z. Kresin, Phys. Rep. 2004, 400, 149. [30] A. Sawa, T. Fujii, M. Kawasaki, Y. Tokura, Appl. Phys. Lett. 2005, 86, 112508. [31] J. M. D. Coey, M. Viret, S. von Molnar, Adv. Phys. 1999, 48, 167. [32] J. H. Kuo, H. U. Anderson, D. M. Sparlin, J. Solid State Chem. 1990, 87, 55. [33] Y. Nosaka, K. Norimatsu, H. Miyama, Chem. Phys. Lett. 1984, 106, 128. [34] D. Reagor, S. Lee, Y. Li, Q. Jia, J. Appl. Phys. 2004, 95, 7971. [35] S. Stille, C. Lenser, R. Dittmann, A. Koehl, I. Krug, R. Muenstermann, J. Perlich, C. M. Schneider, U. Klemradt, R. Waser, Appl. Phys. Lett. 2012, 100, 223503/1.
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[10] M. J. Rozenberg, M. J. Sanchez, R. Weht, C. Acha, F. Gomez-Marlasca, P. Levy, Phys. Rev. B 2010, 81, 115101. [11] J. P. Strachan, J. J. Yang, R. Muenstermann, A. Scholl, G. Medeiros-Ribeiro, D. R. Stewart, R. S. Williams, Nanotechnology 2009, 20, 485701. [12] F. Miao, J. P. Strachan, J. J. Yang, M.-X. Zhang, I. Goldfarb, A. C. Torrezan, P. Eschbach, R. D. Kelly, G. Medeiros-Ribeiro, R. S. Williams, Adv. Mater. 2011, 23, 5633. [13] R. Muenstermann, T. Menke, R. Dittmann, R. Waser, Adv. Mater. 2010, 22, 4819. [14] Ch. Lenser, A. Kuzmin, J. Purans, A. Kalinko, R. Waser, R. Dittmann, J. Appl. Phys. 2012, 111, 76101/1. [15] A. Sawa, Mater. Today 2008, 11, 28. [16] M. Hasan, R. Dong, H. J. Choe, D. S. Lee, D. J. Seong, M. B. Pyun, H. Hwang, Appl. Phys. Lett. 2008, 92, 202102. [17] R. Meyer, L. Schloss, J. Brewer, R. Lambertson, W. Kinney, J. Sanchez, D. Rinerson, Proc. NVMTS 2008, 54. [18] K. Tsubouchi, I. Ohkubo, H. Kumigashira, M. Oshima, Y. Matsumoto, K. Itaka, T. Ohnishi, M. Lippmaa, H. Koinuma, Adv. Mater. 2007, 19, 1711. [19] K. Shono, H. Kawano, T. Yokota, M. Gomi, Appl. Phys. Express 2008, 1, 55002/1. [20] S. Asanuma, H. Akoh, H. Yamada, A. Sawa, Phys. Rev. B 2009, 80, 235113/1. [21] H. Kawano, K. Shono, T. Yokota, M. Gomi, Appl. Phys. Express 2008, 1, 101901/1.
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Spectroscopic Proof of the Correlation between Redox-State and Charge-Carrier Transport at the Interface of Resistively Switching Ti/PCMO Devices Anja Herpers, Christian Lenser, Chanwoo Park, Francesco Offi, Francesco Borgatti, Giancarlo Panaccione, Stephan Menzel, Rainer Waser, and Regina Dittmann* Resistance random access memory (RRAM), which utilizes two or more resistive states of a material system for data storage, has attracted considerable attention as a future non-volatile memory concept.[1,2] Besides a large variety of binary oxides, complex transition metal oxides, e.g., manganites,[3–7] titanates, and zirconates,[8,9] usually exhibit different resistance states at opposite polarities of electrical stimulation and could therefore be employed as RRAM. It has become widely accepted that this so-called bipolar resistive switching in oxides is in most cases connected with a voltage-driven oxygen vacancy movement and a resulting redox-process.[10] However, the current knowledge of the microscopic details of the redox-processes is very limited. For most n-conducting oxides, e.g., TiO2, TaOx, and SrTiO3, it has been demonstrated that resistive switching and the related redox-processes do not take place homogeneously beneath the whole electrode area but are restricted to a spatially defined filament region that evolves during the so-called electroforming process.[11–14] Interestingly enough, there is another class of bipolar resistive switching oxide systems, such as several manganites, for which it was demonstrated that the high and low resistive state currents scale with the electrode area,[15–17] implying that forming and switching take place beneath the whole electrode. Manganite thin films in general form an ohmic contact with noble metals, such as Pt and Au,[18] whereas oxidizable electrodes, such as Al, Ti, or Ta, exhibit non-linear current-voltage relations, which is a prerequisite for the observation of resistive Dr. A. Herpers,[+] Dr. C. Lenser,[+] C. Park, Dr. S. Menzel, Prof. R. Waser, Prof. R. Dittmann Peter Grünberg Institute Research Center Jülich 52425, Germany E-mail: [email protected] Dr. F. Offi CNISM and Dipartimento di Scienze Universita Roma Tre Rome, Italy Dr. F. Borgatti CNR – Istituto per lo Studio dei Materiali Nanostrutturati (ISMN) via P. Gobetti 101 I-40129 Bologna, Italy Dr. G. Panaccione CNR – Istituto Officina dei Materiali (IOM) Laboratorio TASC, S.S.14, Km 163.5 I-34149 Trieste, Italy [+]A.H. and C.L. contributed equally to this work.
DOI: 10.1002/adma.201304054
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switching. As a result of the use of non-noble metals, an interface oxide layer is formed,[19–21] which in principle could also be directly involved in the switching effect. If cation intermixing at the interfaces is also taken into account,[22] the situation becomes even more complicated. Recently, we have shown that after electroforming of Ti/ Pr0.48Ca0.52MnO3/SrRuO3 (Ti/PCMO/SRO) thin film devices the chemical changes at the interface between the Ti electrode and the PCMO layer dominate the resistance of the multilayer stack.[23] The reactive metal electrode forms an oxide layer at the Ti/PCMO interface during its deposition[19–21] prior to device operation, and applying a voltage to the stack was shown to increase the thickness of the TiO2 layer at the interface, and to deplete the underlying PCMO of oxygen further. While electroforming in resistive switching systems is often accompanied by significant changes in the oxygenation states,[14,24] the chemical changes upon switching are expected to be much less pronounced, occur on a much smaller length scale and have therefore not been reported for a similar system so far. However, in this work we were able to detect small but significant chemical changes between the two reversible resistance states. Based on the knowledge of the electroforming mechanism, we present an encompassing view of the field-induced valence change during resistive switching at the Ti/PCMO interface. The chemical changes at the Ti/PCMO interface in four different resistive states are investigated by Hard X-ray Photoelectron Spectroscopy (HAXPES), and correlated to the transport-mechanism of charge-carriers across the multilayer stack for each state. A notable difference between the high and low resistive states can be explained through a convolution of several conduction mechanisms, which is in agreement with the observed chemical changes. Figure 1a shows an exemplary current–voltage relation of the Pt (7 nm)/Ti (4 nm)/PCMO (20 nm)/SRO (30 nm)/SrTiO3 stack, with an inset depicting the sample geometry. In the following, we distinguish between four different resistive states on the bipolar switching hysteresis. The initial resistance state after deposition of the top electrode is referred to as “pristine state” (PS), and exhibits the lowest resistance. Electroforming is achieved by the application of a positive voltage sweep of up to +6.5 V, and is called “formed state” (FS). After forming, a negative voltage sweep to –6 V sets the system into the “low resistance state” (LRS), and a subsequent sweep to a positive voltage of +6.5 V resets the system into the “high resistance state” (HRS). The four states can be ordered by their resistance according to PS < LRS < HRS < FS for the samples discussed
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in this work. It should be noted that the switching follows a counter-eightwise polarity,[13] in the same way as the filamentary switching of titania-based systems reported before. Due to the constraints imposed on the sample geometry by the limited photoelectron mean-free-path in the HAXPES experiment, the top electrode thickness was reduced to Pt (3 nm)/Ti (7 nm), without observing a major change of switching properties. Large electrode arrays were fabricated to fill the field-of-view of the photoelectron-analyzer with thin film devices completely, as schematically depicted in Figure 1b. In addition to the patterned arrays, a free PCMO reference thin film was measured (Figure 2c). As shown in our previous work,[23] the shift in the Mn spectra is induced by the redoxreaction during the Ti deposition according to: 3+ 2+ 3+ 4+ (Pr0.48 Ca0.52 )(Mn0.48 Mn0.52 )O32− + Ti 3+ 2+ 3+ 4+ 2− ⇔ (Pr0.48Ca0.52 )(Mn 0.48+2 x Mn 0.52 − 2 x )O3− x + TiOx
(1)
The influence of the electrical treatment on the oxidation state of the top electrode is shown in Figure 2a. The Ti 2p spectrum recorded in the pristine state exhibits two distinct features at a binding energy of 454.1 eV and 458.5 eV, which can be identified as the Ti 2p3/2 emission line of metallic Ti0 and fully oxidized Ti4+, respectively. For a good reproduction of the spectral envelope, a least-squares fitting analysis with one spin-orbit split doublet pair for each possible oxidation state of Ti was performed. To minimize the free parameters of the fit and adhere to physical principles, the area ratio of the doublet peaks was set to 2:1, the doublet separation was set to 6.17 eV for Ti4+ and 5.54 eV for metallic Ti0, with decreasing intermediate values for the decreasing oxidation state of Tin+. A minor contribution from the underlying SrRuO3 bottom
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Figure 1. (a) Quasistatic I(V) curve of the Pt/Ti/PCMO/SRO/STO stack, investigated with HAXPES. The voltage is applied to the Pt top electrode and the SRO is set on ground potential. (b) Geometry of the HAXPES sample.
electrode, arising from the area between the electrodes, was accounted for by a single peak, corresponding to the Ru 3p3/2 line at 463.4 eV. With increasing resistance state, a spectral weight transfer from the low binding energy components to the high binding energy components of the complex Ti 2p envelope is observed. The area percentages of all Ti oxidation states are shown in Figure 2d. While the Ti4+ component is the dominant line in all four spectra, a monotonous decrease can be observed in the metallic Ti0 line with increasing resistance, while the Ti4+ component increases with increasing resistance. In a similar manner, Figure 2b and 2e depict the O 1s spectra and the area percentage of the three constituent peaks, respectively. Two main components can be observed in each spectrum at 529.5 eV and 530.5 eV binding energy, which can be assigned to PCMO and TiOx, respectively, while the third component can be assigned to surface contaminants (mainly residual photoresist). It should be noted that the electronic configuration of the oxygen ion ([He]2s22p6), unlike that of Ti, does not change appreciably in different Ti-suboxides, and therefore a single O 1s line is sufficient for a good reproduction of the spectral envelope. In analogy to the trend observed in the Ti spectra, a spectral weight transfer from PCMO to TiOx is observed, increasing with increasing resistance state. In accordance with these observations, the Mn 2p emission doublet (Figure 2f) shows a low binding energy shoulder at the Mn 2p3/2 line for all states. This shoulder indicates a shift of the average Mn valence toward Mn3+, in accordance with oxygen removal from the PCMO (Equation (1)), and is slightly enhanced by the electrical treatment. As the effect upon switching between LRS and HRS is rather small and the Mn signal is detected from deeper lying regions of the stack than the Ti, the signal-to-noise level of the data is insufficient to detect differences between theses electrical states in the Mn spectrum. The photoemission spectra of the Ti 2p, O 1s and Mn 2p lines clearly show that a redox-reaction is induced through electrical biasing, influencing the resistance of both the Ti electrode and the PCMO layer. The monotonous increase of electrode oxidation with stack resistance implies a direct role of the interfacial oxide layer in the resistance change. To investigate if the electrode oxidation alone determines the resistance of the multilayer stack, we have performed transport measurements at low voltages for each resistive state, corresponding to a non-destructive read-out. The low voltage characteristics of four representative electrodes – measured after the HAXPES experiment – are shown in Figure 3. The PS (Figure 3a) and LRS (Figure 3c) exhibit almost symmetric, non-linear current responses, whereas a notable asymmetry of the current response can be observed for resistive states written by a positive voltage, i.e., FS (Figure 3b) and HRS (Figure 3d). Taking into account the complex multilayered structure of the system, we have performed a qualitative analysis of the conduction mechanisms that dominate each resistive state. Since the same oxide stack with a high work function metal as top electrode shows a linear and low-resistive I(V) relation,[15,23] the increased resistance of the pristine
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Figure 2. (a–c) Ti 2p/O 1s/Mn 2p XPS spectra collected on the four regions, sorted from low resistance (pristine) to high resistance (formed). The spectrum named PCMO refers to the unpatterned reference sample. (d,e) Area percentages of the constituent components of the Ti 2p and O 1s envelope, respectively. (f) Close-up of the Mn 2p3/2 edge.
Ti/PCMO stack can be attributed to the redox-reaction at the interface. A non-linear circuit element, describing the field-acceleration of the polaron transport in the oxygendepleted PCMO3-x layer,[25] is used to model the measured I(V)-behavior of the PS (see Figure 3a). An accurate fit can only be obtained by extending the circuit with a serial ohmic resistor, which describes the linear contributions of the metals Pt, Ti and SRO, the Ti suboxides TiOx (0 < x < 2)[26] and the completely oxidized PCMO, which possesses a linear I(V) behavior due to its small barrier for polaron hopping. In a serial connection of two circuit elements, the voltages are added while the current is constant. For the model describing the PS, we inverted the function of Ihop(Vhop) and added the voltage drop upon the ohmic resistance R:
Vhop+R (I ) = Vhop (I ) + VR (I ) =
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ΔE ⎞ ⎛ π 2kBTr arsinh ⎜ e kBT ⋅ I ⎟ + R ⋅ I ea ⎝ neaω A ⎠
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(2)
where T is the temperature, r the thickness of the oxygen-depleted region, a the hopping distance, n the polaron density, ω the hopping frequency, A the pad area and ΔE the activation energy for hopping; kB is the Boltzmann constant and e the electron charge. Interestingly, the same description can be extended to the LRS (see Figure 3c) with reasonable parameter modifications (Table 1), but fails to describe either FS or HRS. For the latter two, it is necessary to include an additional, asymmetric circuit element into the model. As the HAXPES data shows a larger amount of Ti4+ for these states, it is conceivable that a closed interfacial TiO2 layer acts as a barrier for charge carrier transport.[27] An asymmetric tunnel junction, as described by Simmons,[28] located in the TiOx/TiO2/PCMO interface region yielded a very good fit when added to the serial circuit used to describe PS and LRS. For a trapezoidal tunnel barrier with two different barrier heights ϕ1 < ϕ2 and thickness d, asymmetry can only be found for voltages higher than the specific barrier height. For lower voltages, a symmetric function has to be applied:
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Figure 3. Electrical read-out data for the different resistance states: (a) In the pristine state the Ti oxide layer is formed homogenously, (b) after electroforming a TiO2 tunnel barrier is formed, (c) after a negative voltage sweep the tunnel barrier is bridged, (d) for the HRS the tunnel barrier is reformed. For each data set the fit including the applied model consisting of two or three serial circuit elements is depicted: Polaron hopping Δ, ohmic resistance Ω and tunnel barrier Φ.
PS
FS
LRS
HRS
r [nm]
0.65
1.6
0.79
0.95
R [Ω]
450
11000
2370
700
d [nm]
2.94
ϕ1 [eV]
0.08
0.10
ϕ2 [eV]
0.915
0.85
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2.84
hopping region and the ohmic resistance, and add the voltages to obtain the overall voltage drop. Using this calculation we could find suitable parameter sets for both FS and HRS (Table 1), resulting in good fits as shown in Figure 3b and 3d, respectively. For details regarding the fitting parameters, refer to the experimental section. From these two different serial circuits necessary to reproduce the experimental data, we conclude that charge-carrier transport across the multilayer stack is not determined by a single conduction mechanism, but rather by a combination of several mechanisms and their respective contribution to the total resistance. The transition between PCMO and TiO2 as the dominating contribution to the stack resistance can now be understood as follows: Prior to the deposition of the Ti electrode, the PCMO is a highly doped oxide that behaves as a p-type semiconductor.[29,30] The deposition of Ti reduces the underlying PCMO before device operation, which causes the existence of a thin interfacial PCMO3−x layer where the hopping conductivity is significantly reduced through the presence of oxygen vacancies interrupting the double-exchange chains along Mn–O–Mn bonds.[31] According to the redox-reaction between Ti and Mn (Equation (1)), the Ti layer is simultaneously oxidized to a
2 eV ⎧ ⎡ ⎤ 23π mdϕ 23/2 1+ ϕ2 ⎪ 1.1eA(eV +ϕ − ϕ )2 ⎢ − 23π mdϕ23/2 ⎛ − ⎥ 2eV ⎞ 2 1 6 h ( eV +ϕ 2 −ϕ1 ) 6 h ( eV +ϕ 2 −ϕ1 ) ⎪ − ⎜1+ e ⎢e ⎥ 2 ⎟ ⎝ ϕ2 ⎠ 4π hϕ 2d ⎪ ⎢ ⎥ ⎪ ⎢⎣ ⎥⎦ ⎪ 4π d m 4π d m ⎡ ϕ ϕ eV ϕ ϕ − + − − + + eV ⎤ 1 2 1 2 ⎪ eA I(Vsim ) = ⎨ (ϕ 1 +ϕ 2 − eV )e h − (ϕ 1 +ϕ 2 +eV )e h ⎥ 2 ⎢ ⎦ ⎪ 4π hd ⎣ ⎪ 2 eV ⎡ 23π mdϕ 3/2 ⎤ 23π mdϕ13/2 1+ ϕ1 ⎪ 1 2 ⎢ − − ⎥ ⎪ 1.1eA(eV − ϕ 2 +ϕ 1 ) ⎢e 6 h ( eV −ϕ2 +ϕ1 ) − ⎛ 1 + 2eV ⎞ e 6 h ( eV −ϕ2 +ϕ1 ) ⎥ 2 ⎜ ⎟ ⎪ ⎝ ⎠ ϕ 4π hϕ 1d 1 ⎢ ⎥ ⎪ ⎢⎣ ⎥⎦ ⎩
where m is the effective mass and h is the Planck constant. Since the hopping region, the tunnel barrier and the ohmic resistance are in series, the voltage Vsim(I) is added to the voltage drop caused by hopping and ohmic resistance: Vhop+sim+R(I) = Vhop(I) + Vsim(I) + VR(I). An inversion of Equation (3) is analytically not possible. To circumvent this problem, we varied the voltage Vsim in small increments and calculated the current flowing through the tunnel barrier. As the current in a serial circuit is conserved, we can calculate the resulting voltage drop at the polaron
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Table 1. Fitting parameters for different resistance states.
⎫ ⎪ ⎪ eV < −ϕ 1 ⎪ ⎪ ⎪ ⎪ ⎬ e |V | ≤ ϕ ⎪ ⎪ ⎪ ⎪ eV > ϕ 2 ⎪ ⎪ ⎭
(3)
heterogeneous TiOx layer, which shows significant conductivity,[32] as sketched in Figure 3a. During electroforming, oxygen ions migrate - due to the gradient in the electrical potential - toward the positively biased top electrode, where the Ti is further oxidized and a closed TiO2 layer is formed. The TiO2 (χTiO2 = 4.0 eV)[33] acts as an asymmetric tunnel barrier, because it is sandwiched between two electrodes with different work functions (ΦTi = 4.3 eV, χPCMO = 4.9 eV).[34] Despite the fact that this oxidation is
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homogeneous across the whole electrode, it is conceivable that the oxidation front shows in-plane inhomogeneities on the nanometer scale, as sketched in the inset of Figure 3b. Such inhomogeneities might be a consequence of the polycrystalline structure of the electrode, or due to the nanoscale roughness of the oxygen diffusion front formed during electroforming, or both.[32] The suggested in-plane inhomogeneities of the Ti oxidation state at the PCMO/Ti interface would also be consistent with the experimentally confirmed spatial inhomogeneities induced by similar redox-processes at the SrTiO3/Ti interface which take place during the deposition of the Ti layer.[35] When a negative bias is applied to the formed device, this nanoscale roughness leads to the local reduction of the TiO2 layer. As sketched in the inset of Figure 3c, the tunnel barrier is shorted by reduced, conductive TiOx channels, and the device returns to a symmetric I(V)-characteristics in the low resistance state. Returning to the high resistive state through another positive voltage, the tunnel barrier is restored, along with the asymmetric character of the device (see Figure 3d). It might also be possible to describe each individual curve using different conduction mechanisms (supplementary information). However, the crucial advantage of our proposed model is that it is in agreement with the movement of oxygen vacancies expected from the direction of the applied field and we can fit all four resistive states in a consistent way with a limited number of physically reasonable parameters. In summary, we have experimentally proven that resistive switching in Pt/Ti/PCMO/SRO/STO devices is based on a redox-process, which mainly happens on the Ti -side. The different resistance states are determined by the amount of fully oxidized Ti-ions in the stack, implying a reversible redox-reaction at the interface that governs the formation and shortening of an insulating tunnel barrier. In particular, we have shown that the limiting factor for charge-carrier transport through the PCMO multilayer stack can be changed from tunneling through a thin insulator at high resistance to field-enhanced polaron hopping at low resistance.
0.8 mm wide, where every electrode in the stripe has been subjected to an identical electrical treatment. The spacing between the pads was about 10 µm. The photoelectron emission from the buried regions of interest is ensured by the large electron inelastic mean free path (IMFP) typical of the hard X-ray excitation regime, of the order of 8 nm. The fixed parameters for the circuit modeling were chosen as follows: For the polaron hopping mechanism, room temperature and a polaron hopping distance of a = 4 Å according to the Mn-Mn distance was assumed. The pad size is A = (50 µm)2, and the polaron density was set to n = 1 nm-3 according to Hall measurements. An attempt rate for hopping of ω = 1013 Hz corresponding to the phonon frequency was assumed. From the fits of the PS and LRS, a hopping barrier of about ΔE = 0.4 eV was determined for the highly reduced PCMO layer, which was then fixed for the fitting of the data sets for FS and HRS. This value exceeds the value for completely oxidized PCMO ΔE ≈ 0.1 eV,[25] because the conduction via double exchange got interrupted by oxygen vacancies. For the Simmons tunnel barrier, an effective mass m of 1.19·me was used. A hopping transport mechanism is also conceivable for the Ti suboxide region, but the alternation in hopping layer thickness is inconsistent with the direction of oxygen vacancy movement.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was funded by the European Union Council under the 7th Framework Program (FP7) grant no. 246102 IFOX and has been in part supported by the Deutsche Forschungsgemeinschaft (SFB 917). The synchrotron radiation experiments were performed at the P09 beamline of Petra-III under proposal I-20110545 EC. Thanks are due to the beamline staff Dr. Andrei Gloskovskii for technical support. The HAXPES instrument at beamline P09 is jointly operated by the University of Würzburg (R. Claessen), the University of Mainz (C. Felser) and DESY. Funding by the Federal Ministry of Education and Research (BMBF) under contracts 05KS7UM1, 05K10UMA, 05KS7WW3, and 05K10WW1 is gratefully acknowledged. Received: August 12, 2013 Revised: February 12, 2014 Published online: April 6, 2014
Experimental Details For the samples intended for switching devices, 30 nm SrRuO3 (SRO) (650 °C, 0.133 mbar) as bottom electrode (BE) and 20 nm Pr0.48Ca0.52MnO3 (PCMO) (700 °C, 0.133 mbar) thin films were epitaxially grown in–situ on TiO2-terminated SrTiO3 (STO) (001) single crystals by pulsed laser deposition. 7 nm Ti and a 3 nm Pt capping layer were subsequently deposited by sequential DC sputtering without breaking the vacuum. The top electrode (TE) stack Ti/Pt was patterned using photolithography. As PCMO reference, a 40 nm thick PCMO film was grown under the same conditions directly on STO. HAXPES measurements were performed at beamline P09, PETRA III, HASYLAB. The experimental geometry was chosen to maximize bulk sensitivity, with the angle between the incident X-ray beam and the electron analyzer set to 90°, and the X-ray beam impinging on the sample at a grazing angle of 2°. The photon energy was set at 6 keV and was periodically verified by measuring the Fermi edge spectrum of a reference Au strip in electrical contact to the sample. The overall resolution was 0.35 eV, as determined by the width of the Au Fermi-edge. All the measurements were performed at room temperature. Due to the large footprint of the X-ray beam, single devices were not suitable for HAXPES measurements. Therefore device arrays were fabricated, consisting of 50 × 50 µm² electrodes arranged in stripes about 7 mm long and
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[1] R. Waser, M. Aono, Nat. Mater. 2007, 6, 833. [2] R. Waser, R. Dittmann, G. Staikov, K. Szot, Adv. Mater. 2009, 21, 2632. [3] A. Asamitsu, Y. Tomioka, H. Kuwahara, Y. Tokura, Nature 1997, 388, 50. [4] H. Oshima, K. Miyano, Y. Konishi, M. Kawasaki, Y. Tokura, Appl. Phys. Lett. 1999, 75, 1473. [5] Y. B. Nian, J. Strozier, N. J. Wu, X. Chen, A. Ignatiev, Phys. Rev. Lett. 2007, 98, 146403/1. [6] M. Quintero, P. Levy, A. G. Leyva, M. J. Rozenberg, Phys. Rev. Lett. 2007, 98, 116601/1. [7] A. Sawa, T. Fujii, M. Kawasaki, Y. Tokura, Appl. Phys. Lett. 2004, 85, 4073. [8] A. Beck, J. G. Bednorz, C. Gerber, C. Rossel, D. Widmer, Appl. Phys. Lett. 2000, 77, 139. [9] Y. Watanabe, J. G. Bednorz, A. Bietsch, Gerber-Ch, D. Widmer, A. Beck, S. J. Wind, Appl. Phys. Lett. 2001, 78, 3738.
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[22] T. Yamamoto, R. Yasuhara, I. Ohkubo, H. Kumigashira, M. Oshima, J. Appl. Phys. 2011, 110, 53707/1. [23] F. Borgatti, C. Park, A. Herpers, F. Offi, R. Egoavil, Y. Yamashita, A. Yang, M. Kobata, K. Kobayashi, J. Verbeeck, G. Panaccione, R. Dittmann, Nanoscale 2013, 5, 3954. [24] R. Dittmann, R. Muenstermann, I. Krug, D. Park, T. Menke, J. Mayer, A. Besmehn, F. Kronast, C. M. Schneider, R. Waser, Proc. IEEE 2012, 100, 1979. [25] S. Schramm, J. Hoffmann, C. Jooss, J. Phys. - Condensed Matter 2008, 20, 395231. [26] S. Harada, K. Tanaka, H. Inui, J. Appl. Phys. 2010, 108, 83703/1. [27] M. D. Pickett, D. B. Strukov, J. L. Borghetti, J. J. Yang, G. S. Snider, D. R. Stewart, R. S. Williams, J. Appl. Phys. 2009, 106, 074508. [28] J. G. Simmons, J. Appl. Phys. 1963, 34, 2581. [29] L. P. Gor’kov, V. Z. Kresin, Phys. Rep. 2004, 400, 149. [30] A. Sawa, T. Fujii, M. Kawasaki, Y. Tokura, Appl. Phys. Lett. 2005, 86, 112508. [31] J. M. D. Coey, M. Viret, S. von Molnar, Adv. Phys. 1999, 48, 167. [32] J. H. Kuo, H. U. Anderson, D. M. Sparlin, J. Solid State Chem. 1990, 87, 55. [33] Y. Nosaka, K. Norimatsu, H. Miyama, Chem. Phys. Lett. 1984, 106, 128. [34] D. Reagor, S. Lee, Y. Li, Q. Jia, J. Appl. Phys. 2004, 95, 7971. [35] S. Stille, C. Lenser, R. Dittmann, A. Koehl, I. Krug, R. Muenstermann, J. Perlich, C. M. Schneider, U. Klemradt, R. Waser, Appl. Phys. Lett. 2012, 100, 223503/1.
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COMMUNICATION
[10] M. J. Rozenberg, M. J. Sanchez, R. Weht, C. Acha, F. Gomez-Marlasca, P. Levy, Phys. Rev. B 2010, 81, 115101. [11] J. P. Strachan, J. J. Yang, R. Muenstermann, A. Scholl, G. Medeiros-Ribeiro, D. R. Stewart, R. S. Williams, Nanotechnology 2009, 20, 485701. [12] F. Miao, J. P. Strachan, J. J. Yang, M.-X. Zhang, I. Goldfarb, A. C. Torrezan, P. Eschbach, R. D. Kelly, G. Medeiros-Ribeiro, R. S. Williams, Adv. Mater. 2011, 23, 5633. [13] R. Muenstermann, T. Menke, R. Dittmann, R. Waser, Adv. Mater. 2010, 22, 4819. [14] Ch. Lenser, A. Kuzmin, J. Purans, A. Kalinko, R. Waser, R. Dittmann, J. Appl. Phys. 2012, 111, 76101/1. [15] A. Sawa, Mater. Today 2008, 11, 28. [16] M. Hasan, R. Dong, H. J. Choe, D. S. Lee, D. J. Seong, M. B. Pyun, H. Hwang, Appl. Phys. Lett. 2008, 92, 202102. [17] R. Meyer, L. Schloss, J. Brewer, R. Lambertson, W. Kinney, J. Sanchez, D. Rinerson, Proc. NVMTS 2008, 54. [18] K. Tsubouchi, I. Ohkubo, H. Kumigashira, M. Oshima, Y. Matsumoto, K. Itaka, T. Ohnishi, M. Lippmaa, H. Koinuma, Adv. Mater. 2007, 19, 1711. [19] K. Shono, H. Kawano, T. Yokota, M. Gomi, Appl. Phys. Express 2008, 1, 55002/1. [20] S. Asanuma, H. Akoh, H. Yamada, A. Sawa, Phys. Rev. B 2009, 80, 235113/1. [21] H. Kawano, K. Shono, T. Yokota, M. Gomi, Appl. Phys. Express 2008, 1, 101901/1.
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