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ITO devices on glass
Microelectronic Engineering 88 (2011) 2608–2610
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Characteristics of multilevel bipolar resistive switching in Au/ZnO/ITO devices on glass Yong Han a, Kyoungah Cho b, Sangsig Kim a,b,⇑ a b
Department of Nano Semiconductor Engineering, Korea University, 5-1, Anam-dong, Sungbuk-gu, Seoul 137-701, Republic of Korea Department of Electrical Engineering, Korea University, 5-1, Anam-dong, Sungbuk-gu, Seoul 137-701, Republic of Korea
a r t i c l e
i n f o
Article history: Available online 12 February 2011 Keywords: ReRAM Resistive switching ZnO Multilevel
a b s t r a c t In this study, we characterize multilevel bipolar switching in devices composed of Au/ZnO/ITO constructed on glass. The ratio of the measured resistance in the low-resistance state (LRS) to that in the high-resistance state (HRS) depends on the starting sweep voltage. The HRS/LRS ratio increases from 10 to 104 as the starting sweep voltage changes from 1 to 3.5 V. Moreover, the Au/ZnO/ITO devices can complete more than 102 cycles and maintain their characteristics for up to 10 years. A more detailed description on the multilevel bipolar switching of our devices will be given in this paper. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental procedures
Resistive switching random access memory (ReRAM) is a promising candidate for next-generation nonvolatile memory devices owing to its high speed, high density, and simple structures [1–9]. Recently, multilevel resistive switching in ReRAM has attracted considerable attention because of its extremely high storage density [10–12]. To date, multilevel switching has been observed by controlling the compliance current, but this causes distortion in the electrical characteristics of ReRAM devices [12–14]. Hence, in this study we propose control of starting sweep voltages as a new method of obtaining multilevel switching. We fabricate ReRAM consisting of Au/ZnO/ITO structures on glass substrates in order to examine the application in transparent electronic systems. ZnO is an n-type semiconductor with wide band gap (3.37 eV), so this semiconducting memory material is transparent in the visible range. In recent years, ZnO ReRAM devices based on ZnO films have been actively developed, since ZnO’s transparency is useful in transparent electronics [4–7]. The studies on ZnO ReRAM devices have been primarily focused on single switching characteristics of ReRAM devices fabricated on glass. Therefore, in this study, we investigate the multilevel switching behaviors of ZnO ReRAM devices fabricated on glass by control of starting sweep voltages.
ZnO thin films approximately 20 nm thick were deposited on glass substrates coated with indium tin oxide (ITO) at room temperature by RF magnetron sputtering in a mixture of Ar and O2 at a pressure of 6 10 3 Torr. Then 100-nm-thick Au electrodes with a diameter of 300 lm were deposited on the ZnO films by thermal evaporation. The electrical characteristics of a Au/ZnO/ ITO structure fabricated in this way were examined with an HP4155C semiconductor parameter analyzer at room temperature in air.
⇑ Corresponding author at: Department of Nano Semiconductor Engineering, Korea University, 5-1, Anam-dong, Sungbuk-gu, Seoul 137-701, Republic of Korea. Tel.: +82 2 3290 3245; fax: +82 2 3290 3894. E-mail address: [email protected] (S. Kim). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.02.058
3. Results and discussion Fig. 1 shows a schematic of a ReRAM device with a Au/ZnO/ITO structure on a glass substrate. Fig. 2(a) shows a representative I–V curve of the device with bipolar resistive switching behavior when the dc voltage is swept in the sequence 2 ? 0 ? 4 ? 0 ? 2 V. When the dc applied voltage on the Au electrode is swept from 2 to 4 V, the resistance increases dramatically at about 2 V, and the HRS is maintained while the dc applied voltage is swept from 4 to 0 V. When the dc voltage on the Au electrode is swept from 0 to 2 V, the HRS switches to the LRS. The formations of LRS and HRS are ascribed to the formation and annihilation of conducting filaments constructed with oxygen vacancies existing in the ZnO film, respectively [15]. The conduction behavior in the LRS is considered to be ohmic, since the linear slope is close to 1 in the log I log V curve, as shown in Fig. 2(b). In contrast, two types of conduction behavior are observed in the HRS. Ohmic conduction is observed at low voltage
Y. Han et al. / Microelectronic Engineering 88 (2011) 2608–2610
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Fig. 1. Schematic of ZnO ReRAM structure used in this study. Fig. 3. Endurance characteristics of the Au/ZnO/ITO device.
Fig. 4. Retention characteristics of the Au/ZnO/ITO device.
Fig. 2. (a) Representative I–V curve of the Au/ZnO/ITO device when dc voltage is swept in the sequence 2 V ? 0 ? 4 ? 0 ? 2 V, and (b) log I and log V plot of the LRS and HRS.
(<0.9 V), and trap-related space-charge-limited conduction appears at high voltage (>1.0 V). These results agree with those of previous studies of ZnO-based ReRAM devices [16–19]. The endurance properties of the Au/ZnO/ITO devices are shown in Fig. 3. The resistance values read at 0.55 V are used as the HRS and LRS. The switching characteristics of the Au/ZnO/ITO device persist over 102 cycles, which is comparable to the results of previous studies [5,6,12,13]. Fig. 4 shows the device’s retention characteristics at room temperature. The LRS and HRS are maintained
after 104 s without any significant change, and the characteristics are maintainable for up to 10 years. Multilevel switching of the Au/ZnO/ITO device is obtained by changing start voltage for sweeping, as illustrated in Fig. 5. The I–V characteristics measured at different starting sweep voltages are shown in Fig. 5(a), indicating that the LRS/HRS ratio depends on the starting sweep voltage. In addition, as the starting sweep voltage is reduced from 1 to 3.5 V, the switching point between the HRS and LRS shifts from <2 V to >3 V as shown in Fig. 5(a). The shifts of the switching point could be explained as follows. The sweeping started at 3.5 V generates more conducting filaments driven by oxygen vacancies during set process and the rupture of the conducting filaments occurs at higher reset voltage than that started at 1 V. To investigate multilevel switching in the Au/ ZnO/ITO device in detail, its resistance in the LRS and HRS is shown in Fig. 5(b). The values of the HRS and LRS correspond to the HRS and LRS read at 0.55 V in Fig. 5(a), respectively. Regardless of the order in the successive cycles of sweeping, the ratio of HRS to LRS is proportional to the absolute value of starting sweep voltage, although some fluctuations occur. The HRS/LRS ratio is approximately 10 at 1 V and increases to 104 at 3.5 V. As the sweeping starts at more negative voltage, the reset voltage becomes more positive and consequently, the HRS value increases owing to more significant annihilation of conducting filaments constructed with oxygen vacancies occurring at the voltage. Thus, multilevel switching in ReRAM devices with bipolar behavior could be realized by control of starting sweep voltages.
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Y. Han et al. / Microelectronic Engineering 88 (2011) 2608–2610
changing the start voltage for sweeping. The HRS/LRS ratio is approximately 10 at 1 V and increases to 104 at 3.5 V. The multilevel switching method developed in this study is applicable to next-generation nonvolatile memory devices with high storage density. Acknowledgements This work was supported by the IT R&D program of MKE/KEIT (10030559, Development of next-generation high-performance organic/nano materials and printing process technology), the Nano R&D Program (M10703000980-08M0300-98010), the World Class University Project (WCU, R32-2008-000-10082-0) of the Ministry of Education, Science, and Technology (Korea Science and Engineering Foundation), and the Hynix-Korea University NanoSemiconductor Program. References
Fig. 5. (a) Typical I–V characteristics of the Au/ZnO/ITO device when the sweeping is started at 1 V, 2 V, and 3.5 V, and (b) resistance states of the Au/ZnO/ITO when the sweeping is started at 1 V, 1.5 V, 2 V, 3 V, and 3.5 V.
4. Conclusion We investigated the resistive switching characteristics of Au/ZnO/ITO devices fabricated on glass substrates. The fabricated ReRAM devices exhibit bipolar switching behaviors and retention characteristics maintainable for up to 10 years. In addition, multilevel resistive switching in the ReRAM device is realized by
[1] S.H. Lee, H.J. Kim, D.J. Yun, S.W. Rhee, K.J. Yong, Appl. Phys. Lett. 95 (2009) 262113. [2] L.M. Kukreja, A.K. Das, P. Misra, Bull. Mater. Sci. 32 (2009) 247. [3] K. Nagashima, T. Yanagida, K. Oka, T. Kawai, Appl. Phys. Lett. 94 (2009) 242902. [4] P. Misra, A.K. Das, L.M. Kukreja, Phys. Status Solidi (c) 7 (2010) 1718. [5] J.W. Seo, J.W. Park, K.S. Lim, J.H. Yang, S.J. Kang, Appl. Phys. Lett. 93 (2008) 223505. [6] J.W. Seo, J.W. Park, K.S. Lim, S.J. Kang, Y.H. Hong, J.H. Yang, L. Fang, G.Y. Sung, H.K. Kim, Appl. Phys. Lett. 95 (2009) 133508. [7] L. Shi, D. Shang, J. Sun, B. Shen, Appl. Phys. Express. 2 (2009) 101602. [8] N. Xu, L.F. Liu, X. Sun, C. Chen, Y. Wang, D.D. Han, X.Y. Liu, R.Q. Han, J.F. Kang, B. Yu, Semicond. Sci. Technol. 23 (2008) 075019. [9] D.S. Jeong, H. Schroeder, R. Waser, Appl. Phys. Lett. 89 (2006) 082909. [10] M. Lauters, B. McCarthy, G.E. Jabbour, D. Sarid, Appl. Phys. Lett. 89 (2006) 013507. [11] C. Schindler, S.C.P. Thermadam, R. Waser, M.N. Kozicki, IEEE Trans. Electron. Dev. 54 (2007) 2762. [12] Y. Wang, Q. Liu, S. Long, W. Wang, Q. Wang, M. Zhang, S. Zhang, Y. Li, Q. Zuo, J. Yang, M. Liu, Nanotechnology 21 (2010) 045202. [13] U. Russo, D. Kamalanathan, D. Ielmini, A.L. Lacaita, M.N. Kozicki, IEEE Trans. Electron. Dev. 56 (2009) 1040. [14] Y.S. Lai, C.H. Tu, D.L. Kwong, Appl. Phys. Lett. 87 (2005) 122101. [15] N. Xu, L. Liu, X. Sun, Z. Lin, D. Han, Y. Wang, R. Han, J. Kang, B. Yu, Appl. Phys. Lett. 92 (2008) 232112. [16] R. Dong, D.S. Lee, W.F. Xiang, S.J. Oh, D.J. Seong, S.H. Heo, H.J. Choi, M.J. Kwon, S.N. Seo, M.B. Pyun, M. Hasan, H. Hwanga, Appl. Phys. Lett. 90 (2007) 042107. [17] K.M. Kim, B.J. Choi, Y.C. Shin, S. Choi, C.S. Hwang, Appl. Phys. Lett. 91 (2007) 012907. [18] K.M. Kim, B.J. Choi, D.S. Jeong, C.S. Hwang, S.W. Han, Appl. Phys. Lett. 89 (2006) 162912. [19] S. Tsui, A. Baikalov, J. Cmaidalka, Y.Y. Sun, Y.Q. Wang, Y.Y. Xue, C.W. Chu, L. Chen, A.J. Jacobson, Appl. Phys. Lett. 85 (2004) 317.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Characteristics of multilevel bipolar resistive switching in Au/ZnO/ITO devices on glass Yong Han a, Kyoungah Cho b, Sangsig Kim a,b,⇑ a b
Department of Nano Semiconductor Engineering, Korea University, 5-1, Anam-dong, Sungbuk-gu, Seoul 137-701, Republic of Korea Department of Electrical Engineering, Korea University, 5-1, Anam-dong, Sungbuk-gu, Seoul 137-701, Republic of Korea
a r t i c l e
i n f o
Article history: Available online 12 February 2011 Keywords: ReRAM Resistive switching ZnO Multilevel
a b s t r a c t In this study, we characterize multilevel bipolar switching in devices composed of Au/ZnO/ITO constructed on glass. The ratio of the measured resistance in the low-resistance state (LRS) to that in the high-resistance state (HRS) depends on the starting sweep voltage. The HRS/LRS ratio increases from 10 to 104 as the starting sweep voltage changes from 1 to 3.5 V. Moreover, the Au/ZnO/ITO devices can complete more than 102 cycles and maintain their characteristics for up to 10 years. A more detailed description on the multilevel bipolar switching of our devices will be given in this paper. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental procedures
Resistive switching random access memory (ReRAM) is a promising candidate for next-generation nonvolatile memory devices owing to its high speed, high density, and simple structures [1–9]. Recently, multilevel resistive switching in ReRAM has attracted considerable attention because of its extremely high storage density [10–12]. To date, multilevel switching has been observed by controlling the compliance current, but this causes distortion in the electrical characteristics of ReRAM devices [12–14]. Hence, in this study we propose control of starting sweep voltages as a new method of obtaining multilevel switching. We fabricate ReRAM consisting of Au/ZnO/ITO structures on glass substrates in order to examine the application in transparent electronic systems. ZnO is an n-type semiconductor with wide band gap (3.37 eV), so this semiconducting memory material is transparent in the visible range. In recent years, ZnO ReRAM devices based on ZnO films have been actively developed, since ZnO’s transparency is useful in transparent electronics [4–7]. The studies on ZnO ReRAM devices have been primarily focused on single switching characteristics of ReRAM devices fabricated on glass. Therefore, in this study, we investigate the multilevel switching behaviors of ZnO ReRAM devices fabricated on glass by control of starting sweep voltages.
ZnO thin films approximately 20 nm thick were deposited on glass substrates coated with indium tin oxide (ITO) at room temperature by RF magnetron sputtering in a mixture of Ar and O2 at a pressure of 6 10 3 Torr. Then 100-nm-thick Au electrodes with a diameter of 300 lm were deposited on the ZnO films by thermal evaporation. The electrical characteristics of a Au/ZnO/ ITO structure fabricated in this way were examined with an HP4155C semiconductor parameter analyzer at room temperature in air.
⇑ Corresponding author at: Department of Nano Semiconductor Engineering, Korea University, 5-1, Anam-dong, Sungbuk-gu, Seoul 137-701, Republic of Korea. Tel.: +82 2 3290 3245; fax: +82 2 3290 3894. E-mail address: [email protected] (S. Kim). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.02.058
3. Results and discussion Fig. 1 shows a schematic of a ReRAM device with a Au/ZnO/ITO structure on a glass substrate. Fig. 2(a) shows a representative I–V curve of the device with bipolar resistive switching behavior when the dc voltage is swept in the sequence 2 ? 0 ? 4 ? 0 ? 2 V. When the dc applied voltage on the Au electrode is swept from 2 to 4 V, the resistance increases dramatically at about 2 V, and the HRS is maintained while the dc applied voltage is swept from 4 to 0 V. When the dc voltage on the Au electrode is swept from 0 to 2 V, the HRS switches to the LRS. The formations of LRS and HRS are ascribed to the formation and annihilation of conducting filaments constructed with oxygen vacancies existing in the ZnO film, respectively [15]. The conduction behavior in the LRS is considered to be ohmic, since the linear slope is close to 1 in the log I log V curve, as shown in Fig. 2(b). In contrast, two types of conduction behavior are observed in the HRS. Ohmic conduction is observed at low voltage
Y. Han et al. / Microelectronic Engineering 88 (2011) 2608–2610
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Fig. 1. Schematic of ZnO ReRAM structure used in this study. Fig. 3. Endurance characteristics of the Au/ZnO/ITO device.
Fig. 4. Retention characteristics of the Au/ZnO/ITO device.
Fig. 2. (a) Representative I–V curve of the Au/ZnO/ITO device when dc voltage is swept in the sequence 2 V ? 0 ? 4 ? 0 ? 2 V, and (b) log I and log V plot of the LRS and HRS.
(<0.9 V), and trap-related space-charge-limited conduction appears at high voltage (>1.0 V). These results agree with those of previous studies of ZnO-based ReRAM devices [16–19]. The endurance properties of the Au/ZnO/ITO devices are shown in Fig. 3. The resistance values read at 0.55 V are used as the HRS and LRS. The switching characteristics of the Au/ZnO/ITO device persist over 102 cycles, which is comparable to the results of previous studies [5,6,12,13]. Fig. 4 shows the device’s retention characteristics at room temperature. The LRS and HRS are maintained
after 104 s without any significant change, and the characteristics are maintainable for up to 10 years. Multilevel switching of the Au/ZnO/ITO device is obtained by changing start voltage for sweeping, as illustrated in Fig. 5. The I–V characteristics measured at different starting sweep voltages are shown in Fig. 5(a), indicating that the LRS/HRS ratio depends on the starting sweep voltage. In addition, as the starting sweep voltage is reduced from 1 to 3.5 V, the switching point between the HRS and LRS shifts from <2 V to >3 V as shown in Fig. 5(a). The shifts of the switching point could be explained as follows. The sweeping started at 3.5 V generates more conducting filaments driven by oxygen vacancies during set process and the rupture of the conducting filaments occurs at higher reset voltage than that started at 1 V. To investigate multilevel switching in the Au/ ZnO/ITO device in detail, its resistance in the LRS and HRS is shown in Fig. 5(b). The values of the HRS and LRS correspond to the HRS and LRS read at 0.55 V in Fig. 5(a), respectively. Regardless of the order in the successive cycles of sweeping, the ratio of HRS to LRS is proportional to the absolute value of starting sweep voltage, although some fluctuations occur. The HRS/LRS ratio is approximately 10 at 1 V and increases to 104 at 3.5 V. As the sweeping starts at more negative voltage, the reset voltage becomes more positive and consequently, the HRS value increases owing to more significant annihilation of conducting filaments constructed with oxygen vacancies occurring at the voltage. Thus, multilevel switching in ReRAM devices with bipolar behavior could be realized by control of starting sweep voltages.
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Y. Han et al. / Microelectronic Engineering 88 (2011) 2608–2610
changing the start voltage for sweeping. The HRS/LRS ratio is approximately 10 at 1 V and increases to 104 at 3.5 V. The multilevel switching method developed in this study is applicable to next-generation nonvolatile memory devices with high storage density. Acknowledgements This work was supported by the IT R&D program of MKE/KEIT (10030559, Development of next-generation high-performance organic/nano materials and printing process technology), the Nano R&D Program (M10703000980-08M0300-98010), the World Class University Project (WCU, R32-2008-000-10082-0) of the Ministry of Education, Science, and Technology (Korea Science and Engineering Foundation), and the Hynix-Korea University NanoSemiconductor Program. References
Fig. 5. (a) Typical I–V characteristics of the Au/ZnO/ITO device when the sweeping is started at 1 V, 2 V, and 3.5 V, and (b) resistance states of the Au/ZnO/ITO when the sweeping is started at 1 V, 1.5 V, 2 V, 3 V, and 3.5 V.
4. Conclusion We investigated the resistive switching characteristics of Au/ZnO/ITO devices fabricated on glass substrates. The fabricated ReRAM devices exhibit bipolar switching behaviors and retention characteristics maintainable for up to 10 years. In addition, multilevel resistive switching in the ReRAM device is realized by
[1] S.H. Lee, H.J. Kim, D.J. Yun, S.W. Rhee, K.J. Yong, Appl. Phys. Lett. 95 (2009) 262113. [2] L.M. Kukreja, A.K. Das, P. Misra, Bull. Mater. Sci. 32 (2009) 247. [3] K. Nagashima, T. Yanagida, K. Oka, T. Kawai, Appl. Phys. Lett. 94 (2009) 242902. [4] P. Misra, A.K. Das, L.M. Kukreja, Phys. Status Solidi (c) 7 (2010) 1718. [5] J.W. Seo, J.W. Park, K.S. Lim, J.H. Yang, S.J. Kang, Appl. Phys. Lett. 93 (2008) 223505. [6] J.W. Seo, J.W. Park, K.S. Lim, S.J. Kang, Y.H. Hong, J.H. Yang, L. Fang, G.Y. Sung, H.K. Kim, Appl. Phys. Lett. 95 (2009) 133508. [7] L. Shi, D. Shang, J. Sun, B. Shen, Appl. Phys. Express. 2 (2009) 101602. [8] N. Xu, L.F. Liu, X. Sun, C. Chen, Y. Wang, D.D. Han, X.Y. Liu, R.Q. Han, J.F. Kang, B. Yu, Semicond. Sci. Technol. 23 (2008) 075019. [9] D.S. Jeong, H. Schroeder, R. Waser, Appl. Phys. Lett. 89 (2006) 082909. [10] M. Lauters, B. McCarthy, G.E. Jabbour, D. Sarid, Appl. Phys. Lett. 89 (2006) 013507. [11] C. Schindler, S.C.P. Thermadam, R. Waser, M.N. Kozicki, IEEE Trans. Electron. Dev. 54 (2007) 2762. [12] Y. Wang, Q. Liu, S. Long, W. Wang, Q. Wang, M. Zhang, S. Zhang, Y. Li, Q. Zuo, J. Yang, M. Liu, Nanotechnology 21 (2010) 045202. [13] U. Russo, D. Kamalanathan, D. Ielmini, A.L. Lacaita, M.N. Kozicki, IEEE Trans. Electron. Dev. 56 (2009) 1040. [14] Y.S. Lai, C.H. Tu, D.L. Kwong, Appl. Phys. Lett. 87 (2005) 122101. [15] N. Xu, L. Liu, X. Sun, Z. Lin, D. Han, Y. Wang, R. Han, J. Kang, B. Yu, Appl. Phys. Lett. 92 (2008) 232112. [16] R. Dong, D.S. Lee, W.F. Xiang, S.J. Oh, D.J. Seong, S.H. Heo, H.J. Choi, M.J. Kwon, S.N. Seo, M.B. Pyun, M. Hasan, H. Hwanga, Appl. Phys. Lett. 90 (2007) 042107. [17] K.M. Kim, B.J. Choi, Y.C. Shin, S. Choi, C.S. Hwang, Appl. Phys. Lett. 91 (2007) 012907. [18] K.M. Kim, B.J. Choi, D.S. Jeong, C.S. Hwang, S.W. Han, Appl. Phys. Lett. 89 (2006) 162912. [19] S. Tsui, A. Baikalov, J. Cmaidalka, Y.Y. Sun, Y.Q. Wang, Y.Y. Xue, C.W. Chu, L. Chen, A.J. Jacobson, Appl. Phys. Lett. 85 (2004) 317.