Dynamic Processes of Resistive Switching in Metallic Filament-Based

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Dynamic Processes of Resistive Switching in Metallic Filament-Based Organic Memory Devices Shuang Gao, Cheng Song,* Chao Chen, Fei Zeng, and Feng Pan* Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: Dynamic formation/rupture processes of metallic filament have been clarified in solid electrolyte- and oxidebased resistive memory devices, whereas they remain exclusive in organic ones. Here we report these dynamic processes in Cu/poly (3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester/indium−tin oxide (ITO) structure, which exhibits a typical bipolar resistive switching effect. Under illumination, an open circuit voltage of −0.15 V exists in highresistance state, yet it vanishes in low-resistance state owing to the emergence of Cu filament. By combining the symmetry of current−voltage curves with corresponding energy band diagrams in different resistance states, it is demonstrated that the Cu filament grows from Cu/organics interface, ends at organics/ITO interface, and ruptures near organics/ITO interface. This work might advance the insight into resistive switching mechanisms in organic-based resistive memories.

1. INTRODUCTION As a potential candidate for conventional charge storage based nonvolatile Flash memory, resistive random access memory (RRAM) has received much attention and been realized by use of a variety of materials, including organics,1−3 oxides,4−7 solid electrolytes,8,9 amorphous silicon, and so on.10 Related resistive switching mechanisms are generally classified into metallic filament,1,2,4−6 charge trapping/detrapping,3 redox reaction,7 and interfacial barrier regulation.11 In particular, metallic filament-based RRAM possesses unique advantages in aspects of thermal stability,2 write/erase speed,4 and retention time,12 making them attractive for practical memories. Exploring the dynamic formation/rupture processes of metallic filament is a long-term goal due to its crucial importance in performance optimization of RRAM, such as accurate manipulation of switching behaviors with desired values. Recently, in situ transmission electron microscope (TEM) observations of these dynamic processes have been conducted in solid electrolyte-8,9 and oxide-6,13 based memory devices. However, the opposite conclusion was obtained for the rupture site of metallic filament, which locates near electrochemically active (usually Cu or Ag) and inert (e.g., W and Pt) electrodes in solid electrolyte- and oxide-based devices, respectively. For organicbased RRAM, the metallic filament in low resistance state (LRS) has been reported,1,2 whereas its dynamic processes are still under exploring. Here we investigate this as-yetundetermined issue using exemplary top electrode (TE)/ storage medium/bottom electrode (BE) sandwich structure in an all-electrical fashion. A mixture of poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester, P3HT:PCBM in short, has been selected as the storage medium in this work for three reasons. First, the P3HT:PCBM is one typical type of bulk heterojunction composite and has been widely studied as the active layer in photovoltaic devices.14 Second, only the © 2012 American Chemical Society

write-once-read-many-times memory effect of this composite has been researched to date.15,16 Finally, the presence of strongly coordinating hetero-sulfur atoms in P3HT is beneficial to the formation of metallic filament.17 Studies of the open circuit voltages (VOC) and the symmetry of current−voltage (I−V) curves in different resistance states give rise to the clear dynamic picture of the formation/rupture of Cu filament.

2. EXPERIMENTAL METHODS Indium−tin oxide (ITO, 160 nm in thickness)-coated glass substrates were obtained from CSG Holding. Both P3HT and PCBM were purchased from Synwit Technology. ITO-coated glass substrates were first cleaned with detergent. Subsequently, they were ultrasonic cleaned in acetone, isopropyl alcohol, and deionized water with a period of 15 min, respectively. The P3HT:PCBM, mass ratio of 1:0.8, was dissolved in dichlorobenzene with a total concentration of 27 mg/mL. The solution was spin-coated on ITO substrates at 1000 rpm for 60 s. As for TE, Cu film with a thickness of ∼200 nm was deposited by direct current magnetron sputtering at room temperature. The device area was defined by use of a shadow mask with diameters ranging from 300 to 1000 μm. Crosssectional sample of the sandwich structure was fabricated by focused ion beam system and examined by TEM (Philips EM430). Electrical properties were totally characterized on an Agilent B1500A semiconductor device analyzer under ambient conditions. During test, the devices were fixed on a CASCADE M150 measurement platform, which can be heated to 300 °C and has a light source. Voltages were applied to TE with ITO Received: June 4, 2012 Revised: July 26, 2012 Published: August 6, 2012 17955

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(BE) grounded the entire time. All I−V curves were obtained under dark conditions if there is no special mark. PPMS was employed to obtain the dependence between resistance and temperature (between 200 and 300 K) in different resistance states. To avoid oxidation, the devices were stored in a glovebox filled with nitrogen apart from test time.

the trilayer structure, which will be further discussed in Section 3.3. Successive 100 switching cycles were performed in a single device, and statistical analysis was made to get further information on switching stability (Figure 2a,b). Note that

3. RESULTS AND DISCUSSION 3.1. Switching Property of Cu/P3HT:PCBM/ITO. Figure 1a shows a schematic of the sample layout and our

Figure 2. Cumulative probability of (a) resistance and (b) threshold voltage. (c) Retention property in both LRS and HRS. (d) Switching character under pulsed voltages. Figure 1. (a) Schematic of the sample layout and measurement configuration. (b) Cross-sectional TEM image of the sandwich structure. (c) Typical I−V curve of bipolar resistive switching behavior. (d) I−V curve of LRS in a log−log scale and fitted slope. The insert shows log(I)−V1/2 curve of HRS.

the resistances of both HRS and LRS scatter in a certain extent during cycling, especially for HRS. Also, the threshold voltage of reset process (Vreset) spreads between −2.1 and −0.3 V, exhibiting a narrower distribution than that of set process (Vset, 0.5 to 4.7 V). These phenomena are commonly seen in metallic filament based RRAM.2,4 The retention performance displayed in Figure 2c shows little degradation of resistance in two states even after 106 s, revealing the reliability of the present bipolar resistive switching effect. Furthermore, to meet practical application, switching property under pulsed voltages is also characterized and shown in Figure 2d. Short pulses with amplitudes of 5, −3, and 0.1 V are adopted to write, erase, and read the device, respectively. According to the current (blue line) under each read pulse, we demonstrate the feasibility of switching the present organic RRAM by 300 ns pulses, which is much faster than conventional flash memory. 3.2. Existence of Cu Filament. In general, VOC will arise under light when bulk heterojunction composite is sandwiched by two electrodes with different work functions. In this section, we focus on the difference of VOC in LRS and HRS to see the effects of resistive switching on VOC. The minima of absolute current (Imin) in I−V curves for both HRS and LRS are observed at V = 0 V in dark, as seen in Figure 3a,b. This situation changes dramatically when the devices are under light. VOC = −0.15 V is observed only in HRS, and the I−V curves of LRS are almost identical with that in dark. Herein, the drift of Imin from V = 0 V in HRS was previously observed likely due to the capacitance of the sandwich structure, which relates to voltage sweep directions.10 To exclude this effect, we adopt both forward (−0.5 → 0.5 V) and reverse (0.5 → −0.5 V) voltage sweep directions. As clearly displayed in the Figure, there is no difference in these two inverse voltage sweep directions. Consequently, it is beyond doubt that the drift of Imin under light originates from photovoltaic effect. Free

measurement configuration. A cross-sectional TEM image of the sandwich structure in Figure 1b reveals that the laminated structure has two clear interfaces, and the thickness of organic layer is ∼85 nm. Figure 1c depicts the typical I−V curve of resistive switching cycle on a semilogarithmic scale, which belongs to bipolar resistive switching behavior. There is no apparent electro-forming operation needed to induce resistive switching in our devices, which is advantageous to practical application. Under positive voltage sweep, the current increases gradually due to the intrinsic conductivity of organic layer (step 1). Remarkably, a set process occurs at a voltage of ∼4.5 V (step 2), which triggers an abrupt change from a high resistance state (HRS) of ∼106 Ω to a low resistance state of ∼103 Ω, producing a memory window of ∼103. The LRS can hold even if the voltage is removed, revealing the nature of nonvolatility. When an opposite voltage is applied (step 4), the device transforms back to HRS at about −2 V (step 5), that is, the socalled reset process. In step 6, the device remains in HRS when the voltage is scanned back from −4 to 0 V. To clarify the conduction and switching mechanisms of the memory device, we replotted the I−V curve in LRS on a log− log scale, as shown in Figure 1d. Obviously, the current is proportional to the applied voltage with a slope of ∼1.1, obeying Ohm’s law, which implies the formation of Cu metallic filament during the set process. For HRS, log(I)−V1/2 curve is depicted in the insert of Figure 1d and exhibits good linear relation, which agrees well with the thermionic emission model,18 indicating the existence of a large Schottky barrier in 17956

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nanowire into account.22,23 It is noteworthy that α decreases as the cross-sectional area of the nanowire is reduced owing to surface diffuse scattering.23 Hence, thinner Cu nanowire in the present case exhibits somehow smaller α compared with α = 1.45 × 10−3 (ref 22) and 2.5 × 10−3 K−1 (ref 23). The relations between resistance and cell area in both resistance states are depicted in Figure 3f. For these experiments, every data point is the average of 10 different cells with the same TE area. During all set processes, a current compliance of 2 mA is adopted to avoid permanent breakdown. In HRS, resistance decreases with increasing TE area with a slope of approximate −1.05, obeying R ∝ S−1 (S represents cell area), which is attributed to homogeneous current through total cell area. Nonetheless, resistance almost remains unchanged with increasing TE area in LRS, which is also a typical characteristic of memory cells with filament mechanism.2 In addition, we substituted Cu TE with electrochemically inert Pt TE, and no resistive switching behavior was observed (data not shown), which gives further support for metallic filament mechanism. 3.3. Dynamic Processes of Resistive Switching. We now turn toward the dynamic formation/rupture processes of Cu filament during switching cycles in an all-electrical fashion. Figure 4a displays the first switching cycle of a virgin memory

Figure 3. I−V curves measured in dark and under light of (a) HRS and (b) LRS. Schematics of (c) the existence of VOC in HRS and (d) the elimination of VOC in LRS. Dependence of resistance on (e) temperature and (f) TE area in both HRS and LRS. The insert of panel e shows the temperature coefficient α in LRS at 300 K.

electrons and holes are generated around TE under light in both states (Figure 3c,d). On the basis of the theory of photovoltaic cells, electrons and holes tend to move toward electrodes with lower and higher work function, respectively.14 Because the work function of Cu (∼4.65 eV)19 is smaller than that of ITO (∼4.8 eV),20 electrons and holes accumulate, respectively, at Cu TE and ITO BE, resulting in the existence of VOC at HRS, schematically shown in Figure 3c. As for LRS (Figure 3d), however, a Cu filament formed during the set process neutralizes these free carriers, leading to the elimination of VOC. We should point out here that VOC in our measurement conditions is qualitative owing to the limit of light source and the shadow effect of Cu TE, which is not comparable to that in standard photovoltaic devices. Besides, two more experiments have been taken to testify the existence of Cu filament in LRS. Figure 3e shows the dependence of resistance on temperature from 200 to 300 K in both resistance states. The resistance in HRS decreases with increasing temperature, exhibiting a typical semiconductive property due to the nature of the organic layer. In contrast, it is apparent in LRS that the resistance decreases as decreasing temperature, exhibiting the feature of metallic conduction mechanism. In general, the dependence of metallic resistance on temperature can be expressed as R(T) = R0[1 + α(T − T0)], in which R0 represents the resistance at T0 and α is the temperature coefficient. As demonstrated in the previous work, the value of α is also a powerful way to clarify the switching mechanism.21 According to these data in Figure 3e, we obtain the temperature coefficient α = 1.1 × 10−3 K−1 at 300 K, indicating the formation of the Cu filament with a diameter of ∼10 nm by taking the resistivity of 5.67 × 10−8 Ω m for Cu

Figure 4. (a) I−V curves of the first switching circle in a virgin memory device. I−V curves from −0.7 to 0.7 V and corresponding energy band diagrams in (b) IRS, (c) LRS, and (d) HRS.

device. Before set process, the I−V curve of the initial resistance state (IRS) from −0.7 to 0.7 V was measured and shown in Figure 4b. Afterward, a positive voltage sweep (0 → 5 → 0 V) was adopted to switch the cell into LRS. I−V curve at this state was subsequently measured from −0.7 to 0.7 V (Figure 4c). Finally, a negative voltage sweep (0→ −4 → 0 V) was applied to reset the cell into HRS, and its I−V curve from −0.7 to 0.7 V was also recorded and depicted in Figure 4d. Evidently, I−V curves in IRS (Figure 4b) and HRS (Figure 4d) are asymmetric, whereas that in LRS (Figure 4c) is highly symmetric. These phenomena could be explained by energy band diagrams relevant to each resistance state. Because of the difference of work functions between Cu (∼4.65 eV)19 and ITO (∼4.8 eV),20 schematically depicted in the insert of Figure 4b, Schottky barrier of Cu/organics is lower than that of organics/ITO by 0.15 eV. According to ref 24, the current is 17957

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2) ions, that is, Cu → Cun+ + ne−. Under the positive electric field, Cu cations move toward the ITO BE. In contrast with solid electrolytes, which are fast ionic conductors, the organic layer has a lower Cu cation mobility.25,26 Accordingly, Cu cations would be reduced by the injected electrons inside the organic layer without reaching the ITO cathode. Once the number of the reduced Cu atoms exceeds its solubility in the organic layer, Cu atoms would precipitate and form protrusions from the Cu TE (Figure 5b). Subsequently, the protrusions prolong and finally reach the BE, switching the device into LRS, as depicted in Figure 5c. Instead of multiple filaments, only a dominant filament probably corresponds to LRS in our Cu/ P3HT:PCBM/ITO devices for the following reasons: (i) with voltage sweep mode of 1 mV/step, only abrupt changes appear in I−V curves during set process (similar to Figure 1c), even with larger current compliance of 10 mA, which is different from the multiple discrete current steps originating from the multiple filaments27 and (ii) no obvious and regular discrete resistance steps are obtained in reset process. Just as the above discussion, the weakest part of Cu filament is located near Cu/ organics interface, consistent with the observation in oxidebased RRAM.6,13 When the applied voltage is reversed, the dissolution of Cu filament primarily takes place at the weakest part due to the electrochemical effect assisted by Joule heating. Consequently, the device transforms to HRS (Figure 5d). This finding provides a reasonable interpretation for the observed partial metallic filaments in refs 1. and 2. In the following voltage sweeping cycles, organic layer between the residual filament and the Cu TE would act as the effective medium, producing the alternation of set and reset process.

commonly dominated by the reverse-biased Schottky barrier. On the basis of our scenario, Schottky barriers of organics/ITO and Cu/organics in IRS play a dominant role under positive and negative bias, respectively. Hence, the absolute value of current in negative bias would be larger than its positive counterpart, leading to the asymmetric I−V curve. The symmetric I−V characteristic in Figure 4c is caused by the emergence of a metallic Cu filament connecting TE and BE formed during the set process. The corresponding sketch is presented in the insert, based on the previous observation of only one dominant filament in LRS.6 We now address the question where the rupture of filament occurs. This issue unavoidably refers to two established points: (i) the metallic filament always has a conical shape and (ii) the rupture of filament occurs at its weakest part.5,6,9,13 On the basis of these two points and the fact that the HRS after reset process is achieved owing to partial rupture of the filament,1,6,13 it is quite natural to expect that Cu filament ruptures near Cu/organics or organics/ITO interfaces. In addition, I−V characteristics in HRS are dominated by the insulating gap sandwiched between the electrode and the residual filament.5 If it ruptured near the Cu/organics interface, then the Cu TE and residual Cu filament would stay fact to face, and correspondingly two similar Schottky barriers in series-opposing connection would result in symmetric I−V curve, which is contradictory with the present I−V curve in HRS. As a result, during the reset process, Cu filament ruptures at its weakest part locating near organics/ ITO, and two different Schottky barriers (Cu/organics and organics/ITO) reappear (the inset of Figure 4d), leading to asymmetric I−V curve qualitatively and quantitatively similar to that in IRS, as displayed Figure 4d. On the basis of the discussion above, we propose a dynamic picture for the formation/rupture of Cu filament in our memory devices. The virgin device in Figure 5a possesses two clear interfaces in view of the TEM image in Figure 1b. When the TE is positive-biased, Cu atoms are ionized to Cun+ (n = 1,

4. CONCLUSIONS In summary, the bipolar resistive switching effect has been observed in Cu/P3HT:PCBM/ITO structure with good retention property longer than 106 s and fast switching speed of 300 ns. Conduction mechanisms in LRS and HRS obey Ohm’s law and thermionic emission model, respectively. Under light, VOC = −0.15 V exists in HRS and vanishes in LRS due to the emergence of Cu metallic filament. By analyzing the symmetry of I−V curves and corresponding energy band diagrams in different resistance states, the dynamic processes of resistive switching in metallic filament-based organic RRAM are built. Under positive voltage, the Cu filament grows from Cu/ organics interface and ends at organics/ITO interface. Subsequently, with reverse voltage, the Cu filament ruptures at its weakest part locating near organics/ITO interface, switching the device back to HRS.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.S). panf@mail. tsinghua.edu.cn (F.P). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Chuang Yao for his help on device fabrication and valuable discussions. This work was supported by National Hitech (R&D) Project of China (grant no. 2009AA034001), National Natural Science Foundation of China (grant no. 50871060), and National Basic Research Program of China (grant no. 2010CB832905).

Figure 5. Schematics of dynamic resistive switching processes. (a) Virgin device structure with two clear interfaces. (b) Growing of Cu filament due to oxidation and reduction of Cu atoms under positive voltage. (c) Cu filament finally reaches BE, leading to the appearance of LRS. (d) The weakest part of Cu filament near organics/ITO interface ruptures under negative voltage, switching the device back to HRS. 17958

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