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Pt memory device
Applied Surface Science 360 (2016) 338–341
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Coexistence of bipolar and unipolar resistive switching behaviors in the double-layer Ag/ZnS-Ag/CuAlO2 /Pt memory device Lei Zhang, Haiyang Xu ∗ , Zhongqiang Wang ∗ , Hao Yu, Jiangang Ma, Yichun Liu Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory for UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun, China
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Article history: Received 7 September 2015 Accepted 2 November 2015 Available online 6 November 2015 Keywords: Resistive random access memory (RRAM) Bipolar and unipolar resistive switching (BRS and URS) Composition of conducting filaments
a b s t r a c t The coexistence of uniform bipolar and unipolar resistive-switching (RS) characteristics was demonstrated in a double-layer Ag/ZnS-Ag/CuAlO2 /Pt memory device. By changing the compliance current (CC) from 1 mA to 10 mA, the RS behavior can be converted from the bipolar mode (BRS) to the unipolar mode (URS). The temperature dependence of low resistance states further indicates that the CFs are composed of the Ag atoms and Cu vacancies for the BRS mode and URS mode, respectively. For this double-layer structure device, the thicker conducting filaments (CFs) will be formed in the ZnS-Ag layer, and it can act as tip electrodes. Thus, the formation and rupture of these two different CFs are located in the CuAlO2 layer, realizing the uniform and stable BRS and URS. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Resistive random access memory (RRAM) has attracted much research interest, since it is regarded as an alternative to the flash memories [1]. Based on whether the resistive switching (RS) behavior depends on electric polarity or not, RRAMs can be classified into two types: bipolar and unipolar RS (BRS and URS) [2,3]. The BRS mode is usually based on the electric-field driven migration effect of active metal ions (e.g., Ag or Cu) to realize the formation/rupture of conducting filaments (CFs) [4,5]. In contrast, the rupture of CFs in URS mode is usually attributed to the power-induced Joule heating effect [6–8]. Thus, the BRS and URS are generally demonstrated in different systems corresponding with these above two RS models. Recently, the coexistence of BRS and URS characteristics was also obtained in several single RRAM devices, and the transition between BRS and URS can be obtained by changing operation parameters such as compliance current (CC) or operation voltage [9–11]. The coexistence of two RS modes is benefit to expand its application scopes in multilevel nonvolatile memory [12,13]. Though the RS behavior has been observed in various materials, especially in many transition metal oxides such as HfO2 , Ta2 O5 , TiO2 , ZnO, less attention was paid to p-type oxides probably due to the limited availability of p-type materials. In our previous stud-
∗ Corresponding authors. E-mail addresses: [email protected] (H. Xu), [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.apsusc.2015.11.022 0169-4332/© 2015 Elsevier B.V. All rights reserved.
ies, we have demonstrated a RRAM device based on p-type CuAlO2 [14]. The device exhibited URS behavior, and its CF was observed to consist of Cu-vacancy acceptor defects, which is quite different from the prevailing oxygen-vacancy based model in n-type oxides. Further, if the metal Ag active electrode is introduced into the pCuAlO2 RRAM devices (where the p-CuAlO2 not only serves as a switching layer for the formation of Cu-vacancy-CFs, but also as a electrolyte layer for the formation of Ag conductive bridge), the BRS and URS behaviors will be expected to co-exist in the Ag/pCuAlO2 /Pt memory cell due to the migration of both Cu-vacancies and Ag ions to form different CFs. However, when a planar electrode is used, the RRAM devices usually suffer from a large dispersion in the RS parameters due to the high degree of randomness in the formation/rupture process of the CFs. Our previous studies have indicated that the insertion of a ZnS interlayer with embedded Ag nanoclusters (ZnS-Ag) between the Ag active electrode and the electrolyte layer can improve RS uniformity, because nanoscale Ag-CFs preferentially form in the ZnS-Ag interlayer, which acts as tip electrodes for the RS. Based on the above consideration, we designed and fabricated the Ag/ZnSAg/CuAlO2 /Pt double-layer memory device in this work, achieving the uniform BRS and URS behaviors by changing the CCs. The composition of CFs in the BRS mode and URS mode was also investigated by studying the temperature dependence of their low resistance states. Furthermore, the RS model, involving the migration of both Ag ions and Cu vacancies with the different threshold voltages, is proposed to explain the transformation mechanism from BRS to URS.
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Fig. 1. Typical BRS characteristic of the double-layer Ag/ZnS-Ag/CuAlO2 /Pt memory device, the inset shows the device structure and the “forming” process.
2. Experimental conduction A structural diagram of the Ag/ZnS-Ag/CuAlO2 /Pt memory device is shown in the inset of Fig. 1 (a). Firstly, a 40 nm-thick CuAlO2 layer was deposited on Pt/Ti/SiO2 /Si substrates by magnetron sputtering in 2 Pa O2 atmosphere. Then, a 40 nm-thick ZnS-Ag layer was sequentially deposited by co-sputtering ZnS ceramic target and Ag slices in 2 Pa Ar atmosphere. Our previous works indicate that most of sputtered Ag species can aggregate into metallic Ag nanoclusters embedded in ZnS host matrix [5]. Finally, the Ag active electrode was thermally evaporated on the top to complete the fabrication of the double-layer device. We define that the positive current flows from the Ag to Pt electrode in the following electrical measurements. 3. Experimental results A forming process is required to activate the RS behavior by applying a positive voltage (∼1.1 V) on the Ag electrode (see the inset of Fig. 1) under the CC of 1 mA. After the electroforming process, the typical BRS behavior was obtained in this device (see
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Fig. 1). A positive voltage (∼0.2 V) triggers the abrupt increase of current to the CC (1 mA), and the resistance switches from the high resistance state (HRS) to the low resistance state (LRSB ). Afterwards, a negative voltage sweep (∼−0.1 V) is necessary to recover the resistance back to the HRS. Interestingly, once the CC increases to 10 mA, the RS mode can be converted from the BRS to the URS. It is noted that a following transformation (“forming”) process (see Fig. 2 (a)) is necessary to realize the URS behavior. As can be observed in Fig. 2(a), when the positive voltage is swept with the 10 mA CC, two set processes (SET I and SET II) can be found, and the current abruptly increases at the threshold voltage around 0.2 V and 1.4 V, respectively. Three distinct states are demonstrated: HRS (∼106 ), LRSB (∼200 ) and a much lower resistance state (LRST ∼ 20 ). Accordingly, two reset processes (RESET I and RESET II) can be observed when increasing the negative voltage sweep, which makes the device switch back from LRST to the HRS (see Fig. 2(a)). It is important to note that the value of the intermediate state in the reset process is similar to that of LRSB in the set process, indicating that the two intermediate states should have the same conductive mechanism. After this transformation process, the device shows the typical URS under 10 mA CC. The reset processes has no dependence on the voltage polarity in the URS mode, which can be either positive or negative, as shown in Fig. 2(b). A set voltage (∼0.7 V) and a reset voltage (∼0.15 V) can switch the resistance state between high and low resistance values (HRS and LRSU , herein, LRSU ∼ 20 ) in URS mode. These above results indicate that both the BRS and URS modes can be achieved in the single Ag/ZnS-Ag/CuAlO2 /Pt memory device. To better understand the BRS and URS mechanism, their RS parameters were summarized and compared, including set/reset voltage (VSET /VRESET ) and high/low resistance. As shown in Fig. 2(c), the similar absolute value of VRESET (∼0.15 V) and HRS (∼106 ) can be seen in both of these two modes for this double-layer device, which means the HRS may have the same physical conduction nature. Further, we consider that the RS region might have been localized in CuAlO2 sub-layer. Considering our previous work, the electric-field can be locally enhanced and become more concentrated around the embedded Ag nanoclusters. Thus, the Ag-CFs will form preferentially in the ZnS-Ag sub-layer and their size can
Fig. 2. (a) The current–voltage curve of BRS-to-URS transformation process with the 10 mA CC. (b) The typical URS behavior after undergoing the transformation process. (c) The statistical data of VSET /VRESET for the BRS and URS behaviors. (d) The variation of LRSB resistance with changing CC from 0.8 to 2 mA.
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Fig. 3. (a and b) The resistance distribution of HRS, LRSB and LRSU in BRS and URS modes obtained from typical 100 switching cycles. (c and d) The temperature dependence of the LRSB and LRSU , respectively (read-out voltages: 0.05 V).
increase with the help of embedded Ag nanoclusters [5]. These incomplete Ag-CFs can act as tip electrodes, resulting in the RS localization in the CuAlO2 sub-layer in both BRS and URS modes. Though the RS does not occur in the ZnS-Ag sub-layer, its insertion can effectively improved RS uniformity and stability. Fig. 3 (a) and (b) illustrates the evolution of RS characteristics with 100 cycles in BRS and URS modes, respectively. The relative fluctuations (standard deviation divided by mean value) of HRS, LRSB and LRSU are 10.5%, 18.5% and 8.6%, which are obviously reduced in contrast to the single-layer CuAlO2 memory device [14] or some reported devices with coexisted BRS and URS behaviors [9,15,16]. On the other hand, the VSET and low resistance values are quite different between BRS and URS behaviors, suggesting that the CFs of BRS and URS modes should have different composition and switching mechanism. Our thought is supported by evidence as follows: (1) a self-limitation of LRSB can be observed in the BRS mode. That is, when changing the CCs from 0.8 to 2 mA, the LRSB keeps almost invariable in the measurements (see Fig. 2(d)). Such a selflimitation behavior may be due to the limited quantity of Ag atoms which migrates into the CuAlO2 layer. Thus, the decreased resistance of LRSU , compared with LRSB , cannot attribute to the increase of Ag CFs’ size. That indirectly indicates the different composition
of the CFs in the BRS and URS modes. (2) Importantly, we found that the temperature coefficient of LRSB and LRSU are different, which illustrate the intrinsic difference of CFs in the two modes, as discussed below. To investigate the composition of CFs in the BRS and URS modes, the temperature dependence of LRSB and LRSU were carried out in Fig. 3(c) and (d). Both the low resistance values increase linearly with the temperature, showing the typical metallic conduction. The dependences can be well fitted by R(T) = R0 [1 + ˛(T − T0 )], where R0 is the resistance at temperature T0 , and ˛ is a temperature coefficient of resistance. By choosing T0 as 298 K, the ˛ is calculated to be 1.13 × 10−3 K−1 and 4.19 × 10−4 K−1 , respectively, for the LRSB and LRSU . The ˛ of LRSB is close to that of Ag nano-wires [17], which confirms the formation of Ag-CFs. Thus, the BRS mechanism can be interpreted as follows: (I) the anodic dissolution of Ag atoms; (II) the transport of Ag+ cations through the CuAlO2 film; (III) the reduction of Ag+ cations and the crystallization of Ag-CFs (Fig. 4 (a)); (IV) the rupture of Ag-CFs by the electromigration and redox processes. In addition, the Ag-CFs will be preferentially ruptured in the CuAlO2 layer in the reset process due to the difference in CFs’ size between the two layers, as shown in Fig. 4(b). Thus, the formation and rupture of Ag-CFs in the CuAlO2 layer should be accepted as the BRS
Fig. 4. The schematic diagrams of the RS mechanism and the CFs’ configuration for the BRS behavior (a and b), for the BRS-to-URS transformation (c and d), and the URS behavior (e and f).
L. Zhang et al. / Applied Surface Science 360 (2016) 338–341
behavior under the low CC of 1 mA. We can see that the ˛ of LRSU is one order of magnitude smaller than that of LRSB , as shown in Fig. 3(c) and (d). Furthermore, the ˛ value of LRSU is similar to that of Cu-vacancy CFs in single-layer p-type CuAlO2 memory device [14]. Thus, the CFs of LRSU may be composed of Cu-vacancy-CFs rather than Ag-CFs. The formation and rupture of Cu-vacancy-CFs are responsible for the URS behavior in the case of high CC (10 mA). Why can the different CFs, based on Cu-vacancies and Ag ions, coexist in the present device? How do they form? The transformation process, as illustrated in Fig. 2(a), seems to answer these questions. The Ag-CFs first form with a positive voltage since Ag+ cations are easier mobile than oxygen ions (the BRS mode shows the smaller VSET than the URS mode.) Once the larger positive voltage is applied to the LRSB with 10 mA CC, the Cu-vacancies (in other words, oxygen ions) will further migrate, forming Cu-vacancy-CFs in the CuAlO2 film, and the device converts from LRSB to LRST (see Fig. 2(a)). Thus, as indicated in Fig. 4(c), the LRST shows a parallel resistance of Cu-vacancy-CFs and Ag-CFs in the CuAlO2 layer (herein, the resistance of Ag-CFs in the ZnS-Ag sub-layer can be ignored due to their relatively large size). It is noted that this parallel resistance (LRST ) is almost equal to the resistance of Cu-vacancy-CFs (namely, LRSU ) because there is an order of magnitude resistance difference in between Cu-vacancy-CFs and Ag-CFs. In fact, similar RS model, including the sequential migration of metal ions (e.g., Ag or Cu) and oxygen ions at different electricfield strength, has been reported in several single memory devices [9,12]. When a negative voltage is applied to the LRST , the Cuvacancy-CFs will rupture first through the combined effect of the Joule heating and electric field. However, the Ag-CFs still exist in the CuAlO2 layer, which is consistent with the experimental observation that the two intermediate states in the set and reset processes have almost the same resistance. When a larger negative voltage is applied to the LRSB , the Ag-CFs will rupture based on the electricfield driven migration effect, as shown in Fig. 4(d). Afterward, the device shows the URS behavior, and the RS mechanism can attribute to the formation and rupture of Cu-vacancy-CFs in the CuAlO2 layer, as shown in Fig. 4(e) and (f). Herein, we think that only the Cu-vacancies participate in the following RS after this transformation process, realizing the URS behavior for the device. Why do the Ag ions not migrate in the following RS process? The reason may attribute to the different rupture degree of Cu-vacancy-CFs and Ag-CFs in the transformation process. It is noted that about −0.4 V reset voltage is needed to rupture the Ag-CFs in this transformation process, which is four times bigger than the VRESET in the BRS mode. Previous reports indicate that the rupture degree is related with the VRESET [18,19]. Thus, the rupture degree of the Ag-CFs will increase in the transformation process due to the increased VRESET . In addition, some studies have indicated that the switching thickness is only a few nanometers for the CFs consisting of oxygen-related defects, while it will increase to dozens of nanometers for the active metal CFs [18–20]. Thus, compared with Ag-CFs, the smaller switching thickness of Cu-vacancy-CFs is expected. Once another voltage is applied to the device, it results in a stronger electric field applied on the Cu-vacancy-CFs’ gap, which favors the formation and rupture of Cu-vacancy-CFs in the CuAlO2 layer, as shown in Fig. 4(e). However, the further studies are necessary to clearly understand the RS process of BRS and URS behaviors by some more direct characterization methods. 4. Conclusion In conclusion, the double-layer Ag/ZnS-Ag/CuAlO2 /Pt memory device was demonstrated, and the introduction of the ZnS-Ag layer makes the RS region localized in CuAlO2 layer, improving the RS
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uniformity. The devices show interesting coexistence of URS and BRS modes, which can be well explained in terms of the sequential migration model of Ag ions and Cu-vacancies to form different CFs. The present work, targeting the implementation of URS and BRS switching in a single device, is helpful to expand RRAM application on the multilevel memory. Acknowledgements This work is supported by the NSFC for Excellent Young Scholars (No. 51422201), General Program of NSFC (No. 51172041, 51372035, and 61404026), 973 Program (No. 2012CB933703), “111” project (No. B13013), Higher Education Doctoral Program (No. 20130043110004), the Fund from Jilin Province (Nos. 20121802, 201201061 and 20140520106JH). References [1] H.Y. Lee, Y.S. Chen, P.S. Chen, T.Y. Wu, F. Chen, C.C. Wang, P.J. Tzeng, M.J. Tsai, C. Lien, Low-power and nanosecond switching in robust hafnium oxide resistive memory with a thin Ti cap, IEEE Electron Device Lett. 31 (2010) 44–46. [2] D.H. Kwon, K.M. Kim, J.H. Jang, J.M. Jeon, M.H. Lee, G.H. Kim, X.S. Li, G.S. Park, B. Lee, S. Han, M. Kim, C.S. Hwang, Atomic structure of conducting nanofilaments in TiO2 resistive switching memory, Nat. Nanotechnol. 5 (2010) 148–153. [3] X.W. Sun, G.Q. Li, X.A. Zhang, L.H. Ding, W.F. Zhang, Coexistence of the bipolar and unipolar resistive switching behaviors in Au/SrTiO3 /Pt cells, J. Phys. D: Appl. Phys. 44 (2011) 5404–5409. [4] L. Zhang, H.Y. Xu, Z.Q. Wang, X.N. Zhao, J.G. Ma, Y.C. Liu, Improved resistive switching characteristics by introducing Ag-nanoclusters in amorphous-carbon memory, Mater. Lett. 154 (2015) 98–102. [5] L. Zhang, H.Y. Xu, Z.Q. Wang, X.N. Zhao, J.G. Ma, Y.C. Liu, Localized resistive switching in a ZnS-Ag/ZnS double-layer memory, J. Phys. D: Appl. Phys. 47 (2014) 5101–5106. [6] M. Terai, Y. Sakotsubo, S. Kotsuji, H. Hada, Resistance controllability of Ta2 O5 /TiO2 stack ReRAM for low-voltage and multilevel operation, IEEE Electron Device Lett. 31 (2010) 204–206. [7] L.L. Zou, W. Hu, W. Xie, R.Q. Chen, N. Qin, B.J. Li, D.H. Bao, Impacts of Au-doping on the performance of Cu/HfO2 /Pt RRAM devices, Appl. Surf. Sci. 311 (2014) 697–702. [8] W.H. Guan, S.B. Long, Q. Liu, M. Liu, W. Wang, Nonpolar nonvolatile resistive switching in Cu doped ZrO2 , IEEE Electron Device Lett. 29 (2008) 434–437. [9] W. Hu, X.M. Chen, G.H. Wu, Y.T. Lin, N. Qin, D.H. Bao, Bipolar and tri-state unipolar resistive switching behaviors in Ag/ZnFe2 O4 /Pt memory devices, Appl. Phys. Lett. 101 (2012) 3501–3504. [10] L. Goux, J.G. Lisoni, M. Jurczak, D.J. Wouters, L. Courtade, Ch. Muller, Coexistence of the bipolar and unipolar resistive-switching modes in NiO cells made by thermal oxidation of Ni layers, J. Appl. Phys. 107 (2010) 4512–4514. [11] D.S. Jeong, H. Schroeder, R. Waser, Coexistence of bipolar and unipolar resistive switching behaviors in a Pt/TiO2 /Pt stack, Electrochem. Solid State Lett. 10 (2007) 51–53. [12] T. Liu, M. Verma, Y. Kang, M.K. Orlowski, Coexistence of bipolar and unipolar switching of Cu and oxygen vacancy nanofilaments in Cu/TaOx /Pt resistive devices, ECS Solid State Lett. 1 (2012) 11–13. [13] D.L. Xu, Y. Xiong, M.H. Tang, B.W. Zeng, Y.G. Xiao, Bipolar and unipolar resistive switching modes in Pt/Zn0.99 Zr0.01 O/Pt structure for multi-bit resistance random access memory, Appl. Phys. Lett. 104 (2014) 3501–3504. [14] L. Zhang, H.Y. Xu, Z.Q. Wang, H. Yu, X.N. Zhao, J.G. Ma, Y.C. Liu, Oxygen-concentration effect on p-type CuAlOx resistive switching behaviors and the nature of conducting filaments, Appl. Phys. Lett. 104 (2014) 3512–3515. [15] S. Lee, H. Kim, J. Park, K. Yong, Coexistence of unipolar and bipolar resistive switching characteristics in ZnO thin films, J. Appl. Phys. 108 (2010) 6101–6103. [16] V.S. Yalishev, Y.S. Kim, B.H. Park, S.U. Yuldashev, Resistance states dependence of photoluminescence in Ag/ZnO/Pt structures, Appl. Phys. Lett. 99 (2011) 2101–2103. [17] A. Bid, A. Bora, A.K. Raychaudhuri, Temperature dependence of the resistance of metallic nanowires (diameter ≥15 nm): applicability of Bloch–Grüneisen theorem, Phys. Rev. B 74 (2006) 5426–5434. [18] Y.E. Syu, T.C. Chang, J.H. Lou, T.M. Tsai, K.C. Chang, M.J. Tsai, Y.L. Wang, M. Liu, S.M. Sze, Atomic-level quantized reaction of HfOx memristor, Appl. Phys. Lett. 102 (2013) 2903–2906. [19] S. Kim, S. Choi, J. Lee, W. Lu, Tuning resistive switching characteristics of tantalum oxide memristors through Si doping, ACS Nano 8 (2014) 10262–10269. [20] Y.C. Yang, P. Gao, S. Gaba, T. Chang, X.Q. Pan, W. Lu, Observation of conducting filament growth in nanoscale resistive memories, Nat. Commun. 3 (2012) 732–739.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Coexistence of bipolar and unipolar resistive switching behaviors in the double-layer Ag/ZnS-Ag/CuAlO2 /Pt memory device Lei Zhang, Haiyang Xu ∗ , Zhongqiang Wang ∗ , Hao Yu, Jiangang Ma, Yichun Liu Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory for UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun, China
a r t i c l e
i n f o
Article history: Received 7 September 2015 Accepted 2 November 2015 Available online 6 November 2015 Keywords: Resistive random access memory (RRAM) Bipolar and unipolar resistive switching (BRS and URS) Composition of conducting filaments
a b s t r a c t The coexistence of uniform bipolar and unipolar resistive-switching (RS) characteristics was demonstrated in a double-layer Ag/ZnS-Ag/CuAlO2 /Pt memory device. By changing the compliance current (CC) from 1 mA to 10 mA, the RS behavior can be converted from the bipolar mode (BRS) to the unipolar mode (URS). The temperature dependence of low resistance states further indicates that the CFs are composed of the Ag atoms and Cu vacancies for the BRS mode and URS mode, respectively. For this double-layer structure device, the thicker conducting filaments (CFs) will be formed in the ZnS-Ag layer, and it can act as tip electrodes. Thus, the formation and rupture of these two different CFs are located in the CuAlO2 layer, realizing the uniform and stable BRS and URS. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Resistive random access memory (RRAM) has attracted much research interest, since it is regarded as an alternative to the flash memories [1]. Based on whether the resistive switching (RS) behavior depends on electric polarity or not, RRAMs can be classified into two types: bipolar and unipolar RS (BRS and URS) [2,3]. The BRS mode is usually based on the electric-field driven migration effect of active metal ions (e.g., Ag or Cu) to realize the formation/rupture of conducting filaments (CFs) [4,5]. In contrast, the rupture of CFs in URS mode is usually attributed to the power-induced Joule heating effect [6–8]. Thus, the BRS and URS are generally demonstrated in different systems corresponding with these above two RS models. Recently, the coexistence of BRS and URS characteristics was also obtained in several single RRAM devices, and the transition between BRS and URS can be obtained by changing operation parameters such as compliance current (CC) or operation voltage [9–11]. The coexistence of two RS modes is benefit to expand its application scopes in multilevel nonvolatile memory [12,13]. Though the RS behavior has been observed in various materials, especially in many transition metal oxides such as HfO2 , Ta2 O5 , TiO2 , ZnO, less attention was paid to p-type oxides probably due to the limited availability of p-type materials. In our previous stud-
∗ Corresponding authors. E-mail addresses: [email protected] (H. Xu), [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.apsusc.2015.11.022 0169-4332/© 2015 Elsevier B.V. All rights reserved.
ies, we have demonstrated a RRAM device based on p-type CuAlO2 [14]. The device exhibited URS behavior, and its CF was observed to consist of Cu-vacancy acceptor defects, which is quite different from the prevailing oxygen-vacancy based model in n-type oxides. Further, if the metal Ag active electrode is introduced into the pCuAlO2 RRAM devices (where the p-CuAlO2 not only serves as a switching layer for the formation of Cu-vacancy-CFs, but also as a electrolyte layer for the formation of Ag conductive bridge), the BRS and URS behaviors will be expected to co-exist in the Ag/pCuAlO2 /Pt memory cell due to the migration of both Cu-vacancies and Ag ions to form different CFs. However, when a planar electrode is used, the RRAM devices usually suffer from a large dispersion in the RS parameters due to the high degree of randomness in the formation/rupture process of the CFs. Our previous studies have indicated that the insertion of a ZnS interlayer with embedded Ag nanoclusters (ZnS-Ag) between the Ag active electrode and the electrolyte layer can improve RS uniformity, because nanoscale Ag-CFs preferentially form in the ZnS-Ag interlayer, which acts as tip electrodes for the RS. Based on the above consideration, we designed and fabricated the Ag/ZnSAg/CuAlO2 /Pt double-layer memory device in this work, achieving the uniform BRS and URS behaviors by changing the CCs. The composition of CFs in the BRS mode and URS mode was also investigated by studying the temperature dependence of their low resistance states. Furthermore, the RS model, involving the migration of both Ag ions and Cu vacancies with the different threshold voltages, is proposed to explain the transformation mechanism from BRS to URS.
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Fig. 1. Typical BRS characteristic of the double-layer Ag/ZnS-Ag/CuAlO2 /Pt memory device, the inset shows the device structure and the “forming” process.
2. Experimental conduction A structural diagram of the Ag/ZnS-Ag/CuAlO2 /Pt memory device is shown in the inset of Fig. 1 (a). Firstly, a 40 nm-thick CuAlO2 layer was deposited on Pt/Ti/SiO2 /Si substrates by magnetron sputtering in 2 Pa O2 atmosphere. Then, a 40 nm-thick ZnS-Ag layer was sequentially deposited by co-sputtering ZnS ceramic target and Ag slices in 2 Pa Ar atmosphere. Our previous works indicate that most of sputtered Ag species can aggregate into metallic Ag nanoclusters embedded in ZnS host matrix [5]. Finally, the Ag active electrode was thermally evaporated on the top to complete the fabrication of the double-layer device. We define that the positive current flows from the Ag to Pt electrode in the following electrical measurements. 3. Experimental results A forming process is required to activate the RS behavior by applying a positive voltage (∼1.1 V) on the Ag electrode (see the inset of Fig. 1) under the CC of 1 mA. After the electroforming process, the typical BRS behavior was obtained in this device (see
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Fig. 1). A positive voltage (∼0.2 V) triggers the abrupt increase of current to the CC (1 mA), and the resistance switches from the high resistance state (HRS) to the low resistance state (LRSB ). Afterwards, a negative voltage sweep (∼−0.1 V) is necessary to recover the resistance back to the HRS. Interestingly, once the CC increases to 10 mA, the RS mode can be converted from the BRS to the URS. It is noted that a following transformation (“forming”) process (see Fig. 2 (a)) is necessary to realize the URS behavior. As can be observed in Fig. 2(a), when the positive voltage is swept with the 10 mA CC, two set processes (SET I and SET II) can be found, and the current abruptly increases at the threshold voltage around 0.2 V and 1.4 V, respectively. Three distinct states are demonstrated: HRS (∼106 ), LRSB (∼200 ) and a much lower resistance state (LRST ∼ 20 ). Accordingly, two reset processes (RESET I and RESET II) can be observed when increasing the negative voltage sweep, which makes the device switch back from LRST to the HRS (see Fig. 2(a)). It is important to note that the value of the intermediate state in the reset process is similar to that of LRSB in the set process, indicating that the two intermediate states should have the same conductive mechanism. After this transformation process, the device shows the typical URS under 10 mA CC. The reset processes has no dependence on the voltage polarity in the URS mode, which can be either positive or negative, as shown in Fig. 2(b). A set voltage (∼0.7 V) and a reset voltage (∼0.15 V) can switch the resistance state between high and low resistance values (HRS and LRSU , herein, LRSU ∼ 20 ) in URS mode. These above results indicate that both the BRS and URS modes can be achieved in the single Ag/ZnS-Ag/CuAlO2 /Pt memory device. To better understand the BRS and URS mechanism, their RS parameters were summarized and compared, including set/reset voltage (VSET /VRESET ) and high/low resistance. As shown in Fig. 2(c), the similar absolute value of VRESET (∼0.15 V) and HRS (∼106 ) can be seen in both of these two modes for this double-layer device, which means the HRS may have the same physical conduction nature. Further, we consider that the RS region might have been localized in CuAlO2 sub-layer. Considering our previous work, the electric-field can be locally enhanced and become more concentrated around the embedded Ag nanoclusters. Thus, the Ag-CFs will form preferentially in the ZnS-Ag sub-layer and their size can
Fig. 2. (a) The current–voltage curve of BRS-to-URS transformation process with the 10 mA CC. (b) The typical URS behavior after undergoing the transformation process. (c) The statistical data of VSET /VRESET for the BRS and URS behaviors. (d) The variation of LRSB resistance with changing CC from 0.8 to 2 mA.
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Fig. 3. (a and b) The resistance distribution of HRS, LRSB and LRSU in BRS and URS modes obtained from typical 100 switching cycles. (c and d) The temperature dependence of the LRSB and LRSU , respectively (read-out voltages: 0.05 V).
increase with the help of embedded Ag nanoclusters [5]. These incomplete Ag-CFs can act as tip electrodes, resulting in the RS localization in the CuAlO2 sub-layer in both BRS and URS modes. Though the RS does not occur in the ZnS-Ag sub-layer, its insertion can effectively improved RS uniformity and stability. Fig. 3 (a) and (b) illustrates the evolution of RS characteristics with 100 cycles in BRS and URS modes, respectively. The relative fluctuations (standard deviation divided by mean value) of HRS, LRSB and LRSU are 10.5%, 18.5% and 8.6%, which are obviously reduced in contrast to the single-layer CuAlO2 memory device [14] or some reported devices with coexisted BRS and URS behaviors [9,15,16]. On the other hand, the VSET and low resistance values are quite different between BRS and URS behaviors, suggesting that the CFs of BRS and URS modes should have different composition and switching mechanism. Our thought is supported by evidence as follows: (1) a self-limitation of LRSB can be observed in the BRS mode. That is, when changing the CCs from 0.8 to 2 mA, the LRSB keeps almost invariable in the measurements (see Fig. 2(d)). Such a selflimitation behavior may be due to the limited quantity of Ag atoms which migrates into the CuAlO2 layer. Thus, the decreased resistance of LRSU , compared with LRSB , cannot attribute to the increase of Ag CFs’ size. That indirectly indicates the different composition
of the CFs in the BRS and URS modes. (2) Importantly, we found that the temperature coefficient of LRSB and LRSU are different, which illustrate the intrinsic difference of CFs in the two modes, as discussed below. To investigate the composition of CFs in the BRS and URS modes, the temperature dependence of LRSB and LRSU were carried out in Fig. 3(c) and (d). Both the low resistance values increase linearly with the temperature, showing the typical metallic conduction. The dependences can be well fitted by R(T) = R0 [1 + ˛(T − T0 )], where R0 is the resistance at temperature T0 , and ˛ is a temperature coefficient of resistance. By choosing T0 as 298 K, the ˛ is calculated to be 1.13 × 10−3 K−1 and 4.19 × 10−4 K−1 , respectively, for the LRSB and LRSU . The ˛ of LRSB is close to that of Ag nano-wires [17], which confirms the formation of Ag-CFs. Thus, the BRS mechanism can be interpreted as follows: (I) the anodic dissolution of Ag atoms; (II) the transport of Ag+ cations through the CuAlO2 film; (III) the reduction of Ag+ cations and the crystallization of Ag-CFs (Fig. 4 (a)); (IV) the rupture of Ag-CFs by the electromigration and redox processes. In addition, the Ag-CFs will be preferentially ruptured in the CuAlO2 layer in the reset process due to the difference in CFs’ size between the two layers, as shown in Fig. 4(b). Thus, the formation and rupture of Ag-CFs in the CuAlO2 layer should be accepted as the BRS
Fig. 4. The schematic diagrams of the RS mechanism and the CFs’ configuration for the BRS behavior (a and b), for the BRS-to-URS transformation (c and d), and the URS behavior (e and f).
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behavior under the low CC of 1 mA. We can see that the ˛ of LRSU is one order of magnitude smaller than that of LRSB , as shown in Fig. 3(c) and (d). Furthermore, the ˛ value of LRSU is similar to that of Cu-vacancy CFs in single-layer p-type CuAlO2 memory device [14]. Thus, the CFs of LRSU may be composed of Cu-vacancy-CFs rather than Ag-CFs. The formation and rupture of Cu-vacancy-CFs are responsible for the URS behavior in the case of high CC (10 mA). Why can the different CFs, based on Cu-vacancies and Ag ions, coexist in the present device? How do they form? The transformation process, as illustrated in Fig. 2(a), seems to answer these questions. The Ag-CFs first form with a positive voltage since Ag+ cations are easier mobile than oxygen ions (the BRS mode shows the smaller VSET than the URS mode.) Once the larger positive voltage is applied to the LRSB with 10 mA CC, the Cu-vacancies (in other words, oxygen ions) will further migrate, forming Cu-vacancy-CFs in the CuAlO2 film, and the device converts from LRSB to LRST (see Fig. 2(a)). Thus, as indicated in Fig. 4(c), the LRST shows a parallel resistance of Cu-vacancy-CFs and Ag-CFs in the CuAlO2 layer (herein, the resistance of Ag-CFs in the ZnS-Ag sub-layer can be ignored due to their relatively large size). It is noted that this parallel resistance (LRST ) is almost equal to the resistance of Cu-vacancy-CFs (namely, LRSU ) because there is an order of magnitude resistance difference in between Cu-vacancy-CFs and Ag-CFs. In fact, similar RS model, including the sequential migration of metal ions (e.g., Ag or Cu) and oxygen ions at different electricfield strength, has been reported in several single memory devices [9,12]. When a negative voltage is applied to the LRST , the Cuvacancy-CFs will rupture first through the combined effect of the Joule heating and electric field. However, the Ag-CFs still exist in the CuAlO2 layer, which is consistent with the experimental observation that the two intermediate states in the set and reset processes have almost the same resistance. When a larger negative voltage is applied to the LRSB , the Ag-CFs will rupture based on the electricfield driven migration effect, as shown in Fig. 4(d). Afterward, the device shows the URS behavior, and the RS mechanism can attribute to the formation and rupture of Cu-vacancy-CFs in the CuAlO2 layer, as shown in Fig. 4(e) and (f). Herein, we think that only the Cu-vacancies participate in the following RS after this transformation process, realizing the URS behavior for the device. Why do the Ag ions not migrate in the following RS process? The reason may attribute to the different rupture degree of Cu-vacancy-CFs and Ag-CFs in the transformation process. It is noted that about −0.4 V reset voltage is needed to rupture the Ag-CFs in this transformation process, which is four times bigger than the VRESET in the BRS mode. Previous reports indicate that the rupture degree is related with the VRESET [18,19]. Thus, the rupture degree of the Ag-CFs will increase in the transformation process due to the increased VRESET . In addition, some studies have indicated that the switching thickness is only a few nanometers for the CFs consisting of oxygen-related defects, while it will increase to dozens of nanometers for the active metal CFs [18–20]. Thus, compared with Ag-CFs, the smaller switching thickness of Cu-vacancy-CFs is expected. Once another voltage is applied to the device, it results in a stronger electric field applied on the Cu-vacancy-CFs’ gap, which favors the formation and rupture of Cu-vacancy-CFs in the CuAlO2 layer, as shown in Fig. 4(e). However, the further studies are necessary to clearly understand the RS process of BRS and URS behaviors by some more direct characterization methods. 4. Conclusion In conclusion, the double-layer Ag/ZnS-Ag/CuAlO2 /Pt memory device was demonstrated, and the introduction of the ZnS-Ag layer makes the RS region localized in CuAlO2 layer, improving the RS
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uniformity. The devices show interesting coexistence of URS and BRS modes, which can be well explained in terms of the sequential migration model of Ag ions and Cu-vacancies to form different CFs. The present work, targeting the implementation of URS and BRS switching in a single device, is helpful to expand RRAM application on the multilevel memory. Acknowledgements This work is supported by the NSFC for Excellent Young Scholars (No. 51422201), General Program of NSFC (No. 51172041, 51372035, and 61404026), 973 Program (No. 2012CB933703), “111” project (No. B13013), Higher Education Doctoral Program (No. 20130043110004), the Fund from Jilin Province (Nos. 20121802, 201201061 and 20140520106JH). References [1] H.Y. Lee, Y.S. Chen, P.S. Chen, T.Y. Wu, F. Chen, C.C. Wang, P.J. Tzeng, M.J. Tsai, C. Lien, Low-power and nanosecond switching in robust hafnium oxide resistive memory with a thin Ti cap, IEEE Electron Device Lett. 31 (2010) 44–46. [2] D.H. Kwon, K.M. Kim, J.H. Jang, J.M. Jeon, M.H. Lee, G.H. Kim, X.S. Li, G.S. Park, B. Lee, S. Han, M. Kim, C.S. Hwang, Atomic structure of conducting nanofilaments in TiO2 resistive switching memory, Nat. Nanotechnol. 5 (2010) 148–153. [3] X.W. Sun, G.Q. Li, X.A. Zhang, L.H. Ding, W.F. Zhang, Coexistence of the bipolar and unipolar resistive switching behaviors in Au/SrTiO3 /Pt cells, J. Phys. D: Appl. Phys. 44 (2011) 5404–5409. [4] L. Zhang, H.Y. Xu, Z.Q. Wang, X.N. Zhao, J.G. Ma, Y.C. Liu, Improved resistive switching characteristics by introducing Ag-nanoclusters in amorphous-carbon memory, Mater. Lett. 154 (2015) 98–102. [5] L. Zhang, H.Y. Xu, Z.Q. Wang, X.N. Zhao, J.G. Ma, Y.C. Liu, Localized resistive switching in a ZnS-Ag/ZnS double-layer memory, J. Phys. D: Appl. Phys. 47 (2014) 5101–5106. [6] M. Terai, Y. Sakotsubo, S. Kotsuji, H. Hada, Resistance controllability of Ta2 O5 /TiO2 stack ReRAM for low-voltage and multilevel operation, IEEE Electron Device Lett. 31 (2010) 204–206. [7] L.L. Zou, W. Hu, W. Xie, R.Q. Chen, N. Qin, B.J. Li, D.H. Bao, Impacts of Au-doping on the performance of Cu/HfO2 /Pt RRAM devices, Appl. Surf. Sci. 311 (2014) 697–702. [8] W.H. Guan, S.B. Long, Q. Liu, M. Liu, W. Wang, Nonpolar nonvolatile resistive switching in Cu doped ZrO2 , IEEE Electron Device Lett. 29 (2008) 434–437. [9] W. Hu, X.M. Chen, G.H. Wu, Y.T. Lin, N. Qin, D.H. Bao, Bipolar and tri-state unipolar resistive switching behaviors in Ag/ZnFe2 O4 /Pt memory devices, Appl. Phys. Lett. 101 (2012) 3501–3504. [10] L. Goux, J.G. Lisoni, M. Jurczak, D.J. Wouters, L. Courtade, Ch. Muller, Coexistence of the bipolar and unipolar resistive-switching modes in NiO cells made by thermal oxidation of Ni layers, J. Appl. Phys. 107 (2010) 4512–4514. [11] D.S. Jeong, H. Schroeder, R. Waser, Coexistence of bipolar and unipolar resistive switching behaviors in a Pt/TiO2 /Pt stack, Electrochem. Solid State Lett. 10 (2007) 51–53. [12] T. Liu, M. Verma, Y. Kang, M.K. Orlowski, Coexistence of bipolar and unipolar switching of Cu and oxygen vacancy nanofilaments in Cu/TaOx /Pt resistive devices, ECS Solid State Lett. 1 (2012) 11–13. [13] D.L. Xu, Y. Xiong, M.H. Tang, B.W. Zeng, Y.G. Xiao, Bipolar and unipolar resistive switching modes in Pt/Zn0.99 Zr0.01 O/Pt structure for multi-bit resistance random access memory, Appl. Phys. Lett. 104 (2014) 3501–3504. [14] L. Zhang, H.Y. Xu, Z.Q. Wang, H. Yu, X.N. Zhao, J.G. Ma, Y.C. Liu, Oxygen-concentration effect on p-type CuAlOx resistive switching behaviors and the nature of conducting filaments, Appl. Phys. Lett. 104 (2014) 3512–3515. [15] S. Lee, H. Kim, J. Park, K. Yong, Coexistence of unipolar and bipolar resistive switching characteristics in ZnO thin films, J. Appl. Phys. 108 (2010) 6101–6103. [16] V.S. Yalishev, Y.S. Kim, B.H. Park, S.U. Yuldashev, Resistance states dependence of photoluminescence in Ag/ZnO/Pt structures, Appl. Phys. Lett. 99 (2011) 2101–2103. [17] A. Bid, A. Bora, A.K. Raychaudhuri, Temperature dependence of the resistance of metallic nanowires (diameter ≥15 nm): applicability of Bloch–Grüneisen theorem, Phys. Rev. B 74 (2006) 5426–5434. [18] Y.E. Syu, T.C. Chang, J.H. Lou, T.M. Tsai, K.C. Chang, M.J. Tsai, Y.L. Wang, M. Liu, S.M. Sze, Atomic-level quantized reaction of HfOx memristor, Appl. Phys. Lett. 102 (2013) 2903–2906. [19] S. Kim, S. Choi, J. Lee, W. Lu, Tuning resistive switching characteristics of tantalum oxide memristors through Si doping, ACS Nano 8 (2014) 10262–10269. [20] Y.C. Yang, P. Gao, S. Gaba, T. Chang, X.Q. Pan, W. Lu, Observation of conducting filament growth in nanoscale resistive memories, Nat. Commun. 3 (2012) 732–739.