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Oxygen vacancies control transition of resistive switching mode in single-crystal TiO2 memory device Jun Ge, and Mohamed Chaker ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 28 Apr 2017 Downloaded from http://pubs.acs.org on April 30, 2017

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ACS Applied Materials & Interfaces

Oxygen vacancies control transition of resistive switching mode in single-crystal TiO2 memory device Jun Ge*, Mohamed Chaker* Institut National de Recherche Scientifique, Centre Énergie Matériaux Télécommunications 1650, boulevard Lionel-Boulet, Varennes, Québec, J3X 1S2, Canada KEYWORDS: resistive memory, titanium dioxide, oxygen vacancies, resistive switching mode, single-crystal ABSTRACT: Epitaxial TiO2 thin films were grown by radio-frequency magnetron sputtering on conductive Nb-SrTiO3 substrates. X-ray photoelectron spectroscopy reveals that the oxygen vacancies inside the TiO2 films can be dramatically reduced by post-annealing treatment in an oxygen atmosphere. The decreasing concentration of oxygen vacancies modifies the resistive switching mechanism from a valence change mode to a electrochemical metallization mode, resulting in a high switching ratio ( ͘≥105), a small electronic leakage current in the high-resistance (≥109 Ω) state and a highly controlled quantized conductance in the low-resistance state. These results allow understanding the relation between different resistive switching mechanisms as well as the quantized conductance for multi-level data storage application.

INTRODUCTION Redox-based resistive random access memory (ReRAM) has been intensively studied as one of the promising candidates for next-generation nonvolatile devices due to its simple structure, excellent scalability, fast switching speed, etc. 1-2 These devices reversibly and reproducibly switch between a conducting state (ON state) and an insulating state (OFF state). It is commonly accepted and experimentally proved that the resistance state switching in ReRAM is based on the formation and disruption of the nanoscale conductive filament (CF). 3-4 As the CF size decreases to the atomic size, scattering might be absent, resulting in ballistic electron transport and quantized conductance (QC), 5 which could be utilized for multi-level storage memory 6 if the QC behavior can be well modulated. Depending on the charged species that form CFs, two types of ReRAM emerge: the electrochemical metallization memories (ECM) with metal cations and the valence change memories (VCM) with oxygen anions (or oxygen vacancies). As a prototype material for ReRAM, TiO2 is well known as a VCM material due to its rich phase structure and easy migration of oxygen ions. 7-9 Therefore, extensive results 9-15 have been demonstrated for TiO2 films with an amorphous or polycrystalline structure, in which the amount of oxygen vacancies is significant. In contrast, up to now the papers dealing with resistive switching (RS) in single-crystal TiO2 are scarce, possibly owing to their unfavorable low defect concentration for VCM switching. 16-20 Recently, an anomalous ECM-type RS behavior was found in single-crystal TiO2 films, which shows a very high switching ratio (~107) and tightly controlled QC, revealing the promising prospect of single-crystal TiO2 films for RS devices. 19-20 The authors attributed the high ON/OFF ratio and QC in RS of the TiO2 device to an oxygen vacancy-assisted mechanism. On the other hand, Ti cations movement has been observed in TiO2 film14 and recent studies on Ti/TiOx structures show that

the ECM can be achieved from VCM by suppressing the mobility of oxygen ions while the motion of metal cations dominates.21 These findings necessitate reconsidering the ECM-type behavior observed in the single-crystal TiO2 films and deeply understanding the competition between two mechanisms, namely, the ECM and VCM modes. However, although technically feasible, the ECM-type switching and transition of RS mode in TiO2 films remains scarcely reported. Hence, exploring alternative ways to control the RS mode in ReRAM is of highly scientific curiosity and technological significance. In this work, single-crystal TiO2 films were grown on Nb-SrTiO3 substrates by radio-frequency magnetron sputtering. Post-annealing was shown to be a powerful method to significantly modify the phase structure and chemical state of films, hence the functional properties. 22 Regoutz et al.23 have recently reported an optimization method using the post-annealing treatment for TiO2-based VCM. The change of phase structure and oxygen content after the annealing leads to higher switching ratio and lower power consumption. Meanwhile, oxygen vacancies are well-known for controlling and optimizing the material performance in oxides.24-25 Therefore, our experiments are designed to investigate the effect of post-annealing with focus on the influence of oxygen vacancies on the RS mode of TiO2 and the corresponding difference in device performance. This would enable to identify the mechanism behind and to develop fabrication strategies for memory devices with high ON/OFF ratio, small electronic leakage current in the high-resistance state and highly controlled QC in the low-resistance state. EXPERIMENTAL SECTION 2.1 Deposition of Epitaxial Anatase TiO2 Thin Films. Epitaxial TiO2 films were grown by radio-frequency magnetron sputtering (Kurt J. Lesker CMS-18) using a

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TiO2 target (Kurt J. Lesker, 99.99%) on conductive (100) Nb-doped SrTiO3 (0.7% wt Nb, MTI Corp). Prior to deposition, the substrates were etched by buffered HF and heated to 850oC in an oxygen atmosphere for 30 min to create an atomic flat surface. After introducing the substrates, the chamber was pumped down at a base pressure of 6.0×10-7 torr. The target-to-substrate distance is 16.5 cm. During deposition, the temperature of each substrate was maintained at 700 oC and the pressure at 5 mTorr using a 50/50 mixture of argon and oxygen. After deposition, some samples were post-annealed under an oxygen atmosphere at 700oC for 30 minutes by rapid thermal annealing. For the electrical measurement, a series of square top electrodes, Au (160 nm) /Ti (10 nm), were deposited by electron evaporation (Kurt J. Lesker AXXIS) at room temperature, with the side length from 10 to 100 µm and patterned with photolithography and lift-off. The back surface of the Nb-SrTiO3 substrate was coated with 10 nm Ti/100 nm Au as a bottom electrode. 2.2 Characterization

(101) reflection of TiO2 films and SrTiO3. The film is aligned according to (001)anatase/(001)STO and (100)anatase/(100)STO. To further determine the degree of out-of-plane orientation, a rocking curve was performed around the (004) reflection, which provides a full-width half maximum of 0.7° (Fig. 1c). In addition, atomic force microscopy shows that the resulting films are smooth with a root-mean-square surface roughness of ∼0.3 nm (Figure 1d).

X-ray diffraction was employed to investigate the crystallinity of the deposited TiO2 thin film using a high-resolution X-ray diffraction (Panalytical X-Pert PRO MRD) with a Cu Kα radiation (λ = 1.5404 Å). The epitaxial relation between film and substrates was further determined by rocking curve and phi-scan measurements. The film thicknesses were measured using SEM and X-ray specular reflectivity where the log intensity versus 2θ angles from 0° to 6° shows oscillations at a frequency directly related to the thickness of the deposited film. The chemical valence state of the TiO2 films was analyzed by X-ray photoelectron spectroscopy (XPS), using a VG Escalab 220i XL electron analyzer with a monochromated Al Kα X-ray source. The film composition was determined using CasaXPS (ver.2.3.16) peak fitting with a Shirley background subtraction. The electrical measurements were performed on a microprobe station with a temperature-controlled chuck. The RS behavior of the films with a thickness of about 15 nm was measured in air by Keithley 2400 with a two-probe configuration. A sweeping voltage V was applied to the top electrode with the bottom electrode grounded. The HRS and LRS resistances (RHRS and RLRS) were measured at a read voltage of +0.3 V.

Figure 1. (a) X-ray diffraction θ-2θ spectrum, (b) φ scan and (c) rocking curve of the as-deposited TiO2/Nb-SrTiO3 sample. (d) AFM image of the as-deposited film. To analyze the chemical valence states of the constituent elements, XPS investigation was performed. Figure 2a and 2b shows the typical Ti 2p XPS spectrum of the TiO2 films. The Ti 2p3/2 and 2p1/2 peak are located at about 458.2 eV and 464.0 eV, respectively. These values correspond to those of the Ti4+ ion, suggesting that these ions dominate in all the samples. 17 However, depending on if the sample is annealed or not, the O 1s XPS spectrum of the TiO2 films shows quite different chemical states of oxygen ions. Figure 2c presents the O 1s peak of a fresh TiO2 film, which can be fitted by two nearly Gaussian components, centered at 528.7 eV and 531.5 eV, respectively. The peak at 531.5 eV can be associated with oxygen vacancies in the TiO2 matrix. 27 These results indicate that numerous oxygen vacancies exist in the as-deposited TiO2 thin film. In contrast, the O 1s peak of the post-annealed sample can still be fitted by two single Gaussian functions, as demonstrated in Figure 2d, but with a much smaller area contribution of the peak at 531.5 eV, which is a strong indication that the oxygen vacancies have decreased due to the annealing under an oxygen atmosphere.

RESULTS AND DISCUSSION Figure 1a presents the θ-2θ X-ray diffraction (XRD) patterns of a ~30 nm-thick TiO2 film before and after post-annealing. The patterns only show the (004) peak of anatase TiO2, which indicates that the films do not contain any impurity phase. Moreover, the TiO2 (004) peak shifts toward higher 2θ angle after the post-annealing treatment, indicating a smaller lattice parameter compared with that of the fresh sample. The bulk lattice constant a and c of anatase TiO2 are 3.79 Å and 9.51 Å, respectively, while the lattice constant of cubic SrTiO3 is 3.91 Å. The out-of-plane lattice constant in the post-annealed sample (9.47 Å) is smaller than the bulk constant. By contrast, the out-of-plane lattice constant in fresh samples (9.54 Å) is larger than the bulk constant, although these films experience in-plane tensile strain. The crystal volume increase of the fresh sample can be attributed to oxygen vacancies in the oxides,26 considering the relatively low partial pressure of oxygen during deposition. The in-plane texturing of TiO2 thin films was confirmed by XRD φ scan (Fig. 1b) on the

Figure 2. X-ray photoelectron spectrum of the TiO2 films for Ti 2p (a), (b) and O 1s (c), (d).


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Figure 3. Typical I–V characteristics of the TiO2/Nb-SrTiO3 devices on linear (a), (c) and semi-log (b), (d) scales. The gray loop is depicted from the first sweep and the blue loop presents a regular RS switching after 10 switching cycles. Figure 3 presents the RS characteristics of the devices and a reading voltage of 0.3 V. For the fresh samples, the determined by applying a sweeping voltage, where the HRS varies between about 200 to 104 Ω (Fig.S5(a)). In positive direction corresponds to the current flowing from contrast, the HRS varies between about 200 to 104 Ω TiO2 to Nb-SrTiO3 substrate. SET is defined as switching (Fig.S5(a)). In contrast, the HRS of post-annealed sample presents very high resistances all well-above 108 Ω, which from a high-resistance state (HRS) towards a low-resistance state (LRS), whereas RESET represents the is the detection limit of our equipment (Fig.S5(b)). reverse process. The electroforming sweep (gray) and the Furthermore, the retention characteristics of the device at RS switching loop after 10 switching cycles (blue) for fresh room temperature are shown in Fig. S6. For the fresh TiO2 are shown in Figure 3a and 3b on linear and semi-log sample (Fig.S6(a)), the two resistance states of the device are maintained up to 104 s without apparent degradation. scale, respectively. The compliance current here is 5 mA. First, the initial resistance, i.e. the resistance measured in However, the LRS of post-annealed sample reveals an the high-resistance state during the first switching is weak, increasing resistance with the time (Fig.S6(b)). A similar about 500 Ω, which could be related to the high phenomenon was also reported recently for a Cu/SiO2/W concentration of oxygen vacancies inside the TiO2 film. 8,9 ECM device and is attributed to spontaneous filament decay with quantized conductance levels.29 Second, the electroforming process is not necessary for the film, as we can see that the first and the 10th switching The redox-based RS in various materials can be explained loop have the similar SET and RESET voltages, which are by various mechanisms: electrostatic/electronic, 30-31 about 2.2V and -0.5V, respectively. Such characteristics valence change, 10, 32 thermochemical, 33-34 electrochemical were reported for different kinds of RS materials, where the metallization, 29, 35 and phase change mechanisms. 36-37 In “route” of the conductive filament is readily formed due to addition, conduction charge effects such as carrier the high amount of charged oxygen anions. 9, 28 Third, the screening effect 38 and carrier concentration effect 39 might ON/OFF ratio is around 10 and the RESET process reveals also influence the RS behavior. For a transition metal oxide a slow descending tendency. By contrast, the sample after like TiO2, the phase change mechanism is generally ruled post-annealing in oxygen shows a very different RS out. The thermochemical mechanism usually generates behavior. Figures 3c presents the results on a linear scale unipolar RS, which is not the case in our TiO2 films. The and Fig. 3d on a logarithmic scale for a clearer focus on the electrostatic/electronic mechanism has been reported as a switching properties. First, the post- annealed film requires possible bipolar RS mechanism for TiO2 films. 16-18 much fewer compliance currents (Icomp) to stabilize the However, in this case, the RESET process is always LRS, a minimum of 50 µA, with the current of 400 µA gradual and a nonlinear (square-law) I−V characteristic is shown here, which is indicated as the maximum current commonly observed in the LRS, while our films exhibit a during the SET process. Second, the initial resistance of the clearly linear I-V relation. Also, this mechanism typically sample is very high (≥109 Ω) as compared with that of the shows electrode area-dependent Resistance of LRS (RLRS). fresh sample, attributing to the much lower concentration Figure 4 compares RLRS for top electrodes of different sizes of oxygen vacancies. Third, the resistance totally recovers (10, 25, 50, and 100 µm for side length, respectively). to its initial state after RESET, with a very high ON/OFF Clearly RLRS is almost constant for different device sizes, ratio of ≥105, which is at least three orders of magnitude suggesting the filamentary nature of the LRS. 3,5 Therefore, over that of the fresh sample. Fourth, as compared to the the remaining possible mechanisms would be valence fresh sample, the post-annealed sample has a very clean change and electrochemical metallization mechanism. and abrupt RESET process (Figure 3c), which is consistent We will first focus on the RS mechanism in the fresh with its high ON/OFF ratio. To further compare their RS samples. As we have shown above, the fresh samples properties, the write endurance of the fresh and demonstrate a typical VCM-type RS behavior with the post-annealed devices was measured, as shown in Fig S4. moderate ON/OFF ratio and gradual RESET processes. To The I-V curves obtained over 150 cycles are shown in further confirm the filamentary characteristics, we analyzed semilogarithmic scale in Fig. S4(a) (fresh sample) and Fig. the charge transport properties of the fresh sample. The S4(b) (post-annealed sample). Their reset/set switching logarithmic plots and linear fits of the I-V curves for the characteristics were measured using the DC sweep mode positive voltage sweep and negative voltage sweep are



shown in Fig.S1(a) and S1(b). The linear I-V relationship with the slope of about 1 indicates the formation of CFs in the device after the SET process.38 To examine the type of conductivity of the filament, we performed temperature-dependent (293-393 K) measurements of the LRS (Fig. S2), showing the metallic behavior. The metallic I-V behavior in TiO2 VCM devices has also been demonstrated in previous studies,6, 40 in which metallic oxygen deficient TinO2n-1 nanophases are believed to form the CF. However, previous research has demonstrated evidences that the metallic CF could also be formed by Ti cation metallization.14, 21, 41-42 Ti cation mobility in TiOx is supported by studies on the passivation of metals in liquid electrolytes, where cation diffusion coefficients (in dense oxide films) comparable to those of oxygen vacancies have been reported.41-42 Moreover, some very recent studies14, 21 have provided clear evidence that the cations in thin films of TiOx are mobile under the influence of an electric field and can actively participate in the resistive switching process in competition with the oxygen vacancies. In our RS structure, an Au/Ti top electrode was used and thus provides the source of Ti ions under a positive electric field. Therefore, in view of identifying the drifting cations, we investigated the RS behavior of Au/TiO2/Nb-SrTiO3 structure. By studying the RS behavior without Ti layers as top electrodes, we can then rule out the Ti cation contribution during the RS process. The RS behavior of the fresh sample, shown in Fig. S3(a), demonstrates a typical bipolar RS character. Compared with the devices using Au/Ti electrodes, the device shows higher RHRS, which could be ascribed to the Schottky barrier formed at the Au/TiO2 interface. In this case, the switching mechanism can only be associated with the migration, accumulation and rearrangement of oxygen vacancies, which strongly hints that the pristine TiO2 films are VCM, creating a possible new phase with a different valence state of cations. In contrast, post-annealed TiO2 structures with only Au top electrode were proved to be not switchable after several electroforming attempts (Fig. S3(b)). This result indicates that the oxygen vacancies in post-annealed TiO2 should not account for the formation of CF and that the Ti cations play a key role in the RS behavior of post-annealed TiO2 devices. Together with the very high ON/OFF ratio, the abrupt SET/RESET process and the quantum conductance that will be discussed later, it is plausible that the post-annealed TiO2 devices are ECM. Determining the RS mechanisms of our TiO2 devices would require examining the nature of filaments by transmission electron microscopy, which is beyond the scope of the present article.

post-annealed sample and Icomp=5 mA for the fresh sample, showing no dependence of RLRS on the device electrode area; Based on the discussion above, we can explain the difference between the fresh and post-annealed TiO2 films under the assumption that different species form the CF and a change from VCM-to ECM-type switching due to the competition between the movement of Ti cations and that of oxygen anions, which was already reported in Ta/TaOx and Ti/TiOx by using an inserting a graphene layer. 21, 35 Figure 5 illustrates a schematic diagram of the switching mechanism: Fig. 5a and Fig. 5c show the initial state of two kinds of RS structure, among which the as-deposited sample contains more oxygen vacancies. When a positive voltage is applied to the Au/Ti top electrodes, oxidation occurs in this electrochemically active material. Therefore Ti4+ cations are generated, which could be described as Ti=Ti4++4eMeanwhile, oxygen vacancies are also generated and move under the electric field (Fig. 5b and Fig. 5f). As shown in Fig. 5c and Fig. 5g, the successive precipitations of Ti metal atoms, being reduced from Ti4+ cations at the cathode, compete with the filament made of oxygen vacancies. In the devices treated by post-annealing in an oxygen atmosphere, the Ti filament finally reaches the top electrode and forms a highly conductive path in the on state, further suppressing the formation of oxygen vacancies filament and enable an ECM-type switching.

Figure 5. (a–d) Schematic VCM-dominated switching kinetics in a TiO2/Nb-SrTiO3 device (a) The initial state with randomly distributed mobile oxygen ions and TiOx at the interface. (b) The nucleation and subsequent growth of CFs composed of oxygen vacancies during the forming process. (c) The LRS with a full CF d) The HRS with a partially ruptured CF at its thinnest region. (e–h) Schematic ECM-dominated switching kinetics (e) The initial state with randomly distributed mobile oxygen ions and TiOx at the interface. (f) The nucleation and subsequent growth of CFs composed of metal Ti atom during the forming process. (g) The LRS with a full CF (h) The HRS with a partially ruptured CF at its thinnest region. Scaling of filamentary-type ReRAM cells to nanoscale dimensions will eventually lead to a QC of the low resistance state. Here, we first investigated the conductance changes during the resistive switching processes for both fresh and annealed samples. Representative plots of the conductance changes are shown in Figure 6a and 6b during

Figure 4. RLRS versus device area after SET processes (15 SET processes for each device size) under Icomp=400 µA for


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the RESET process. The electrical conductance G was calculated as G = I/V. G was recorded in units of the quantum conductance G0=2e2/h, where e is electron charge and h is Planck’s constant. For the fresh sample, the current decreases continuously with no visible conductance plateau, indicating the absence of QC. In contrast, multiple current jumps were clearly detected in the post-annealed sample, each followed by a stable conductance plateau. This quantized switching phenomenon was consistently observed for several post annealed devices. To better reveal the effective control over stabilized QC by electrical means, we measured each RLRS at +0.3 V right after a SET process under the corresponding Icomp and a histogram of the LRS conductance GLRS for the 150 successive cycles as shown in Figure 6c. Histogram peaks corresponding to quantized conductance values for each Icomp can be seen, demonstrating that the GLRS can be monitored within quantum of conductance by imposing different Icomp during the SET process. The highly-controlled QC behaviors observed here, which has the potential to reach multi-level data storage, can be attributed to the creation and annihilation of Ti conducting filaments at the atomic scale inside the insulating TiO2. On the other hand, the large LRS conductance of the fresh sample implies a large area of conductive filament contact with the electrode, which leads to difficulties in observation of QC.

CONCLUSION In conclusion, epitaxial TiO2 thin films grown on Nb-doped SrTiO3 by radio-frequency magnetron sputtering provide evidence that the concentration of oxygen vacancies plays a significant role in the RS mode. The low concentration of oxygen vacancies would suppress their formation of conductive filament thus making an unfavorable condition to achieve a VCM-type RS switching. Fortunately, using a Ti top electrode would provide an additional way, i.e. ECM-type switching, which was rarely observed in TiO2-based RS devices up to now. Such change to ECM mode enables a large improvement of RS properties such as a higher ON/OFF ratio, smaller electronic leakage current in the HRS, lower SET and RESET current and highly controlled QC in the low-resistance state. Our work has added a strong proof that suggests VCM and ECM are intrinsically linked,21, 35 and that the characteristics of the ReRAM cells are tunable by proper defect engineering. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx xxx xxx Charge transport characterstics, RLRS dependence with temperature, RS behaviors of Au/TiO2/Nb-SrTiO3 structure, Cycling evolution of devices, Write endurance, Retention properties. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. ACKNOWLEDGEMENT The authors would like to thank Christophe Chabanier for his support for HRXRD and XPS measurements. The authors would also like to thank Catalin Harnagea for his help during AFM measurement. The authors would like to thank the Canada Research Chair Program and the “Fonds de Recherche du Québec – Nature et Technologies (FRQNT)” for the financial support. REFERENCE 1. Waser, R.; Aono, M., Nanoionics-based Resistive Switching Memories. Nat. Mater. 2007, 6 (11), 833-840. 2. Waser, R.; Dittmann, R.; Staikov, G.; Szot, K., Redox-Based Resistive Switching Memories – Nanoionic Mechanisms, Prospects, and Challenges. Adv. Mater. 2009, 21 (25-26), 2632-2663. 3. Chen, J. Y.; Huang, C. W.; Chiu, C. H.; Huang, Y. T.; Wu, W. W., Switching Kinetic of VCM-Based Memristor: Evolution and Positioning of Nanofilament. Adv. Mater. 2015, 27 (34), 5028-5033. 4. Jang, M. H.; Agarwal, R.; Nukala, P.; Choi, D.; Johnson, A. T. C.; Chen, I. W.; Agarwal, R., Observing Oxygen Vacancy Driven Electroforming in Pt–TiO2–Pt

Figure 6. Measured conductance of (a) fresh sample and (b) post-annealed sample as a function of the bias voltage during the RESET process. (c) Histogram of GLRS in units of G0= 2e2/h over 150 dc sweep cycles under Icomp ranging from 50 µA to 800 µA.



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