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Pt memory devices for RRAM application
Microelectronic Engineering 88 (2011) 1628–1632
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Improved resistive switching properties of Ti/ZrO2/Pt memory devices for RRAM application Sheng-Yu Wang a, Chen-Han Tsai a, Dai-Ying Lee a, Chih-Yang Lin b, Chun-Chieh Lin c, Tseung-Yuen Tseng a,⇑ a
Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, Hsinchu 300, Taiwan Global Mixed-Mode Technology Inc., Hsinchu 300, Taiwan c Department of Electrical Engineering, National Dong Hwa University, Hualien 974, Taiwan b
a r t i c l e
i n f o
Article history: Received 2 September 2009 Received in revised form 26 July 2010 Accepted 28 November 2010 Available online 1 April 2011 Keywords: Nonvolatile memory RRAM ZrO2 Resistive switching
a b s t r a c t In this study, we have investigated the effect of current compliance during forming process on resistive switching (RS) characteristics of the Ti/ZrO2/Pt device. The higher the current compliance is, the larger RS operation voltage is needed. The Ti/ZrO2/Pt device with high device yield can be operated over 10,000 RS cycles by sweeping dc voltage, and the on and off two memory states exhibit good stability under 0.3 V stress voltage. Moreover, the data retention of both memory states is over 105 s. As applying +6-V 10-ns and 3-V 10-ns voltage pulses on the device, there are operation errors observed during continuous write-read-erase-read cycles until increasing the pulse width to 50 ns. Nondestructive readout tests are also performed on the Ti/ZrO2/Pt device before and after 103 pulse cycles without any obvious degradation observed. Compared with reported ZrO2-based memory devices, our Ti/ZrO2/Pt device exhibits better RS properties and has a high potential for memory application. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The next-generation nonvolatile memory (NVM) has attracted extensive attention due to the conventional memories approaching their scaling limits. Recently, various new memory devices, especially resistance random access memory (RRAM), have been proposed for future nonvolatile memory device applications [1–17]. RRAM composed of a simple metal–insulator–metal (M–I–M) structure has the advantages of low-power consumption, highspeed operation, good scalability, and high density integration. Due to these excellent characteristics, the resistive switching (RS) characteristics of various metal oxides, such as ZrO2, TiO2, NiO, Al2O3, SrZrO3, SrTiO3, and Pr1 xCaxMnO3, have been extensively studied. ZrO2 has been investigated for high-k gate dielectric applications owing to its high dielectric constant, simple composition, and easy fabrication. Therefore, the physical, chemical, and electrical properties of ZrO2 can be referenced in many detailed studies. As for ZrO2 in RRAM application, Lee et al. have recently reported the resistive switching behavior of nonstoichiometric zirconium oxide [11]. However, the device yield of the nonstoichiometric zirconium oxide is very low; therefore, many methods are proposed to improve the device yield such as using stoichiometric ZrO2, embedding nanocrystals, doping Cu impurities, and implanting Zr+ ion and Au [10–17].
⇑ Corresponding author. Tel.: +886 3 5731879; fax: +886 3 5724361. E-mail address: [email protected] (T.-Y. Tseng). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2010.11.058
In this study, we report a bipolar reproducible RS behavior in the Ti/ZrO2/Pt memory device. From the statistical measurements on 50 devices at random location on wafers, near 100% device yield of the ZrO2-based device is successfully achieved by stacking Ti top electrode. The effects of the current compliance during forming process on the RS properties of the device are studied. The reproducibility, retention, nondestructive readout, and electrical pulseinduced resistance (EPIR) changes properties of the memory device are also investigated.
2. Experimental Thirty to 75-nm-thick ZrO2 films were deposited on the Pt/Ti/ SiO2/Si substrates at 200 °C by a radio-frequency (rf) magnetron sputter employing a ceramic ZrO2 target. The working pressure to sputter ZrO2 was 10 mTorr, which was maintained by a gas mixture of oxygen and argon at a mixing ratio of 1:2 with a total flow rate 18 sccm. According to the X-ray photoelectron spectroscopy result (not shown here), the composition of ZrO2 film is estimated to be approximately ZrO1.7, indicating about 15% oxygen vacancies in oxygen sublattice. To achieve the M–I–M structure, a 150-nmthick Ti top electrode with a diameter of 250 lm was deposited by electron beam evaporation. Electrical characteristics were measured by using Agilent 4155C and Agilent 81110A at room temperature. Current–voltage (I–V) and current–time (I–t) curves were measured by Agilent 4155C. While performing the writeread-erase-read pulse cycles, Agilent 81110A was adopted to
S.-Y. Wang et al. / Microelectronic Engineering 88 (2011) 1628–1632
generate voltage pulses to switch the resistance states of the device and then the resistance states were measured by the Agilent 4155C. The voltage bias was applied on the Ti top electrode with the Pt bottom electrode common. 3. Results and discussion Before initiating a RS behavior, a forming process is necessary to activate the memory devices into the high conductive state (ONstate), which is achieved by sweeping voltage bias over a forming voltage with a current compliance (not shown here). Fig. 1(a) shows that the forming voltage depends on the ZrO2 thickness in the Ti/ZrO2/Pt device, where each result was estimated from the measured values for 20 devices. Kinoshita et al. have reported that forming process is equivalent to dielectric breakdown by performing the time dependent dielectric breakdown (TDDB) experiment,
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and after forming process, the conducting filaments are determined within the RS films [3]. Therefore, as the ZrO2 thickness increases, it can sustain higher breakdown voltage, and thus, the higher forming voltage is observed as shown in Fig. 1(a). Fig. 1(b) depicts the typical bipolar RS behaviors of the Ti/ZrO2/Pt device with various current compliances imposed on the device during the forming process. After forming process, a negative voltage bias exceeding the switch-off voltage (VOFF) is required to switch the device into low conductive state (OFF-state). Then, by sweeping the positive bias over a certain voltage (VON), the Ti/ZrO2/Pt device is switched back into ON-state, where an abrupt increase in current is observed; as a result, a RS cycle is completed. It has been reported that the current compliance influences the formation of percolating filaments [16,18]. In this study, as increasing the current compliance during forming process, the ON-state current increases (Fig. 1(c)), showing that the more or stronger
Fig. 1. (a) Statistic diagram of forming voltages with various ZrO2 thicknesses, (b) I–V characteristics of the Ti/ZrO2/Pt device correlated with the various current compliances in the forming process, and (c) corresponding resistance ratio and current values of the two memory states.
Fig. 2. Endurance performance of the Ti/ZrO2/Pt devices by sweeping dc voltage bias. The inset shows the currents of ON-state and OFF-state traced during 10,000 times.
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conducting filaments are formed within ZrO2 film. In Fig. 1(b), higher VOFF, consequently, is required to switch the device off. Due to the higher current compliance causing the more or stronger conducting filaments formed and higher VOFF required, it leads to increasing the current of OFF-state. Hence, the resistance ratio between the two memory states maintains near constant in Fig. 1(c). After the forming process with current compliance of 5 mA, the cycling endurance characteristics of the Ti/ZrO2/Pt device measured in dc sweep mode are shown in Fig. 2, demonstrating the 10th, 100th, 1000th and 10,000th RS cycles. There is only a little distortion among above I–V curves, and the I–V hysteresis is still apparent with the switching cycles up to 10,000 times. The inset
in Fig. 2 depicts the evolution of RS characteristics by tracing the currents of both ON-state and OFF-state at 0.3 V versus RS cycles. In spite of a little variation of RS characteristics, the memory device can be switched continuously between the ON-state and the OFFstate without exhibiting any operation failure. Interestingly, the effect of current compliance of the forming process on the endurance cycle has been investigated in this work. It is well known that imposing higher current compliance would make conducting filaments permanent with an absence of RS properties, which could be understood in the similar context of the irreversible oxide breakdown [18]. Accordingly, the higher the forming compliance (>10 mA) performed, the less endurance cycles are observed
Fig. 3. (a) Non-destructive readout test result of both ON- and OFF-states over 104 s, and the resistance ratio between them closed to two orders. (b) Data retention of both ON- and OFF-states of the device over 105 s.
Fig. 4. (a) First five write-read-erase-read cycles performed by applying voltage pulses of +6 V and 3 V to switch the device on and off, alternately. (b) As increasing pulse cycles, evolution of both ON- and OFF-states with pulse width of 10 ns, (c) 20 ns, and (d) 50 ns, respectively.
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Fig. 5. Nondestructive readout properties of the Ti/ZrO2/Pt device. After 103 write/erase cycles, both ON-state and OFF-state are still kept stable over 104 s.
Table 1 Device structures, resistance values of two memory states, RS voltages, pulse operation parameters, and device yield of the current ZrO2-based memory devices reported by several research groups. Device structure
a
Ti/ZrO2/Pt Pt/ZrOX/p+-Si TiN/ZrO2/Pt Au/nc-Au ZrO2/n+-Sib Cr/Au-implanted ZrO2/n+-Si Cu/Cu-doped ZrO2/Pt Cr/Zr+-implanted ZrO2/n+-Si a b
RON
ROFF
|VON| (V)
|VOFF|
RS cycles
|PulseON|
|PulseOFF|
Device yield
Reference
150–2 kX 3 kX 300–800 X 5 k–166 kX 70–30 kX 100 X 4 kX
10 k–100 kX 43 kX 4 k–80 kX 9 M–83 MX 2 M–300 MX 100 MX 200 MX
0.9 1.6 0.7–1.1 2.6–3.5 8 2.1–3.6 3.2
1.4 1.1 0.5–0.8 1–3 2.2 0.8–1.5 3.1
10,000 80 480 100 200 N.A. N.A.
6 V/50 ns 6 V/100 ns 1.5 V/1 ls N.A. 12 V/50 ns N.A. N.A.
3 V/50 ns 2 V/100 ns 2 V/1 ls N.A. 6 V/100 ns N.A. N.A.
100% 43% N.A. 73% 100% Higher than that undoped Higher than that unimplanted
This work [11,12] [13] [14] [15] [16] [17]
TE/resistive layer/BE. nc: nanocrystals.
(<10,000 cycles). However, as imposing the lower forming compliance (<1 mA) on our memory device, the number of endurance cycles is found to dramatically decrease (<100 cycles). This might be attributed to the less or weaker conducting filaments formed within ZrO2 film, leading to the unstable RS properties such as the ‘‘set fail’’ phenomena observed [19]. Fig. 3(a) shows the capability of the Ti/ZrO2/Pt device to retain both memory states under 0.3 V. After applying the dc voltage to switch the fresh device into ONstate or OFF-state, the currents of both memory states are kept stable over 104 s with the resistance ratio more than 100 times, confirming the nonvolatile nature and the nondestructive readout property. Moreover, the data retention as shown in Fig. 3(b) demonstrates both ON-state and OFF-state maintain their own current values over 105 s at room temperature. To be easily compatible with the modern integrated circuit operation, we further investigate the possibility of electrical pulse-induced resistance (EPIR) changes in the Ti/ZrO2/Pt device. Fig. 4(a) depicts the measured result of the first five write-readerase-read cycles. After applying a 3-V 10-ns voltage pulse to switch the device off, the OFF-state is read at 0.3 V with a duration time of 3 s. Subsequently, the ON-state is achieved by +6-V 10-ns voltage pulse and also read at 0.3 V. However, as increasing the write-read-erase-read cycles, there are a lot of intermediate memory states observed as shown in Fig. 4(b), leading to write/erase operation errors. Moreover, when increasing the pulse width to 20 ns, the frequency of the appeared soft errors is obviously lowered as shown in Fig. 4(c). In Fig. 4(d), these operation errors disap-
pear without changing the pulse height, until the pulse width is increased to 50 ns. Under above pulse conditions, we demonstrate that the write-read-erase-read cycle in the Ti/ZrO2/Pt device is over 103 times. Fig. 5 depicts the nondestructive readout properties of the Ti/ ZrO2/Pt device under a stress voltage of 0.3 V. Even after 103 write/erase cycles, both the ON-state and OFF-state are distinguishable during successive reading over 104 s and the corresponding resistance ratio maintains closed to 100 times. Therefore, the important properties of nondestructive readout and good reliability are demonstrated in this device. The Ti/ZrO2/Pt device shows a high possibility to be integrated into CMOS circuit with a high speed operation. Table 1 lists device structures, resistance values of two memory states, RS voltages, pulse operation parameters, and device yield of our and the reported ZrO2-based memory devices. According to Table 1, our Ti/ZrO2/Pt device shows better RS characteristics among the reported ZrO2-based devices even without any additional process, such as embedding nanocrystals, doping impurities, and implanting ions. Therefore, our Ti/ZrO2/Pt device has the relatively higher potential in the commercial memory applications.
4. Conclusions In summary, the effects of current compliance during forming process are investigated in the Ti/ZrO2/Pt device. As increasing
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the current compliance, both VON and VOFF are raised with a near constant resistance ratio. By choosing a proper current compliance, the device is demonstrated to be operated over 10,000 RS cycles by sweeping dc voltage, and two memory states possess good stability. The write/erase operation errors are observed with applying +6-V 10-ns and 3-V 10-ns voltage pulses, even when increasing the pulse width to 20 ns. However, by applying +6-V 10-ns and 3-V 10-ns voltage pulses, there are no such errors during 1000 successive write-read-erase-read cycles. The Ti/ZrO2/Pt device also exhibits nondestructive readout properties before and after 103 pulse cycles. Compared with other currently reported ZrO2-based memory devices, our Ti/ZrO2/Pt device with near 100% device yield can be easily realized for practical memory application due to the simple structure and fewer fabrication processes. Acknowledgment This work was supported by the National Science Council, Taiwan, under Project NSC 96-2628-E009-166-MY3. References [1] I.G. Baek, M.S. Lee, S. Seo, M.J. Lee, D.H. Seo, D.S. Suh, J.C. Park, S.O. Park, H.S. Kim, I.K. Yoo, U.-In Chung, J.T. Moon, Int. Electron Devices Meet., Tech. Dig. (2004) 587. [2] T.N. Fang, S. Kaza, S. Haddad, A. Chen, Y.C. Wu, Z. Lan, S. Avanzino, D. Liao, C. Gopalan, S. Choi, S. Mahdavi, M. Buynoski, Y. Lin, C. Marrian, C. Bill, M. VanBuskirk, M. Taguchi, Int. Electron Devices Meet., Tech. Dig. (2006) 789.
[3] K. Kinoshita, T. Tamura, M. Aoki, Y. Sugiyama, H. Tanaka, Appl. Phys. Lett. 89 (2006) 103509. [4] A. Sawa, T. Fujii, M. Kawasaki, Y. Tokura, Appl. Phys. Lett. 85 (2004) 4073. [5] C.Y. Lin, C.Y. Wu, C.Y. Wu, T.C. Lee, F.L. Yang, C. Hu, T.Y. Tseng, IEEE Electron Device Lett. 28 (2007) 366. [6] C.Y. Lin, C.Y. Wu, C.Y. Wu, T.Y. Tseng, C. Hu, J. Appl. Phys. 102 (2007) 094101. [7] C.Y. Lin, S.Y. Wang, D.Y. Lee, T.Y. Tseng, J. Electrochem. Soc. 155 (2008) H615. [8] C.Y. Lin, C.Y. Wu, C.Y. Wu, C. Hu, T.Y. Tseng, J. Electrochem. Soc. 154 (2007) G189. [9] C.C. Lin, C.Y. Lin, M.H. Lin, C.H. Lin, T.Y. Tseng, IEEE Trans. Electron Devices 54 (2007) 3146. [10] X. Wu, P. Zhou, J. Li, L.Y. Chen, H.B. Lv, Y.Y. Lin, T.A. Tang, Appl. Phys. Lett. 90 (2007) 183507. [11] D. Lee, H. Choi, H. Sim, D. Choi, H. Hwang, M. Lee, S. Seo, I.K. Yoo, IEEE Electron Device Lett. 26 (2005) 719. [12] D. Lee, D. Seong, H.J. Choi, I. Jo, R. Dong, W. Xiang, S. Oh, M. Pyun, S. Seo, S. Heo, M. Jo, D. Hwang, H.K. Park, M. Chang, M. Hasan, H. Hwang, Int. Electron Devices Meet., Tech. Dig. (2006) 346733. [13] B. Sun, Y.X. Liu, L.F. Liu, N. Xu, Y. Wang, X.Y. Liu, R.Q. Han, J.F. Kang, J. Appl. Phys. 105 (2009) 061630. [14] W. Guan, S. Long, R. Jia, M. Liu, Appl. Phys. Lett. 91 (2007) 062111. [15] Q. Liu, W. Guan, S. Long, M. Liu, S. Zhang, Q. Wang, J. Chen, J. Appl. Phys. 104 (2008) 114514. [16] W. Guan, S. Long, Q. Liu, M. Liu, W. Wang, IEEE Electron Device Lett. 29 (2008) 434. [17] Q. Liu, W. Guan, S. Long, R. Jia, M. Liu, J. Chen, Appl. Phys. Lett. 92 (2008) 012117. [18] D.C. Kim, S. Seo, S.E. Ahn, D.-S. Suh, M.J. Lee, B.-H. Park, I.K. Yoo, I.G. Baek, H.-J. Kim, E.K. Yim, J.E. Lee, S.O. Park, H.S. Kim, U.-In Chung, J.T. Moon, B.I. Ryu, Appl. Phys. Lett. 88 (2006) 202102. [19] D.C. Kim, M.J. Lee, S.E. Ahn, S. Seo, J.C. Park, I.K. Yoo, I.G. Baek, H.J. Kim, E.K. Yim, J.E. Lee, S.O. Park, H.S. Kim, U.-In Chung, J.T. Moon, B.I. Ryu, Appl. Phys. Lett. 88 (2006) 232106.
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Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Improved resistive switching properties of Ti/ZrO2/Pt memory devices for RRAM application Sheng-Yu Wang a, Chen-Han Tsai a, Dai-Ying Lee a, Chih-Yang Lin b, Chun-Chieh Lin c, Tseung-Yuen Tseng a,⇑ a
Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, Hsinchu 300, Taiwan Global Mixed-Mode Technology Inc., Hsinchu 300, Taiwan c Department of Electrical Engineering, National Dong Hwa University, Hualien 974, Taiwan b
a r t i c l e
i n f o
Article history: Received 2 September 2009 Received in revised form 26 July 2010 Accepted 28 November 2010 Available online 1 April 2011 Keywords: Nonvolatile memory RRAM ZrO2 Resistive switching
a b s t r a c t In this study, we have investigated the effect of current compliance during forming process on resistive switching (RS) characteristics of the Ti/ZrO2/Pt device. The higher the current compliance is, the larger RS operation voltage is needed. The Ti/ZrO2/Pt device with high device yield can be operated over 10,000 RS cycles by sweeping dc voltage, and the on and off two memory states exhibit good stability under 0.3 V stress voltage. Moreover, the data retention of both memory states is over 105 s. As applying +6-V 10-ns and 3-V 10-ns voltage pulses on the device, there are operation errors observed during continuous write-read-erase-read cycles until increasing the pulse width to 50 ns. Nondestructive readout tests are also performed on the Ti/ZrO2/Pt device before and after 103 pulse cycles without any obvious degradation observed. Compared with reported ZrO2-based memory devices, our Ti/ZrO2/Pt device exhibits better RS properties and has a high potential for memory application. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The next-generation nonvolatile memory (NVM) has attracted extensive attention due to the conventional memories approaching their scaling limits. Recently, various new memory devices, especially resistance random access memory (RRAM), have been proposed for future nonvolatile memory device applications [1–17]. RRAM composed of a simple metal–insulator–metal (M–I–M) structure has the advantages of low-power consumption, highspeed operation, good scalability, and high density integration. Due to these excellent characteristics, the resistive switching (RS) characteristics of various metal oxides, such as ZrO2, TiO2, NiO, Al2O3, SrZrO3, SrTiO3, and Pr1 xCaxMnO3, have been extensively studied. ZrO2 has been investigated for high-k gate dielectric applications owing to its high dielectric constant, simple composition, and easy fabrication. Therefore, the physical, chemical, and electrical properties of ZrO2 can be referenced in many detailed studies. As for ZrO2 in RRAM application, Lee et al. have recently reported the resistive switching behavior of nonstoichiometric zirconium oxide [11]. However, the device yield of the nonstoichiometric zirconium oxide is very low; therefore, many methods are proposed to improve the device yield such as using stoichiometric ZrO2, embedding nanocrystals, doping Cu impurities, and implanting Zr+ ion and Au [10–17].
⇑ Corresponding author. Tel.: +886 3 5731879; fax: +886 3 5724361. E-mail address: [email protected] (T.-Y. Tseng). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2010.11.058
In this study, we report a bipolar reproducible RS behavior in the Ti/ZrO2/Pt memory device. From the statistical measurements on 50 devices at random location on wafers, near 100% device yield of the ZrO2-based device is successfully achieved by stacking Ti top electrode. The effects of the current compliance during forming process on the RS properties of the device are studied. The reproducibility, retention, nondestructive readout, and electrical pulseinduced resistance (EPIR) changes properties of the memory device are also investigated.
2. Experimental Thirty to 75-nm-thick ZrO2 films were deposited on the Pt/Ti/ SiO2/Si substrates at 200 °C by a radio-frequency (rf) magnetron sputter employing a ceramic ZrO2 target. The working pressure to sputter ZrO2 was 10 mTorr, which was maintained by a gas mixture of oxygen and argon at a mixing ratio of 1:2 with a total flow rate 18 sccm. According to the X-ray photoelectron spectroscopy result (not shown here), the composition of ZrO2 film is estimated to be approximately ZrO1.7, indicating about 15% oxygen vacancies in oxygen sublattice. To achieve the M–I–M structure, a 150-nmthick Ti top electrode with a diameter of 250 lm was deposited by electron beam evaporation. Electrical characteristics were measured by using Agilent 4155C and Agilent 81110A at room temperature. Current–voltage (I–V) and current–time (I–t) curves were measured by Agilent 4155C. While performing the writeread-erase-read pulse cycles, Agilent 81110A was adopted to
S.-Y. Wang et al. / Microelectronic Engineering 88 (2011) 1628–1632
generate voltage pulses to switch the resistance states of the device and then the resistance states were measured by the Agilent 4155C. The voltage bias was applied on the Ti top electrode with the Pt bottom electrode common. 3. Results and discussion Before initiating a RS behavior, a forming process is necessary to activate the memory devices into the high conductive state (ONstate), which is achieved by sweeping voltage bias over a forming voltage with a current compliance (not shown here). Fig. 1(a) shows that the forming voltage depends on the ZrO2 thickness in the Ti/ZrO2/Pt device, where each result was estimated from the measured values for 20 devices. Kinoshita et al. have reported that forming process is equivalent to dielectric breakdown by performing the time dependent dielectric breakdown (TDDB) experiment,
1629
and after forming process, the conducting filaments are determined within the RS films [3]. Therefore, as the ZrO2 thickness increases, it can sustain higher breakdown voltage, and thus, the higher forming voltage is observed as shown in Fig. 1(a). Fig. 1(b) depicts the typical bipolar RS behaviors of the Ti/ZrO2/Pt device with various current compliances imposed on the device during the forming process. After forming process, a negative voltage bias exceeding the switch-off voltage (VOFF) is required to switch the device into low conductive state (OFF-state). Then, by sweeping the positive bias over a certain voltage (VON), the Ti/ZrO2/Pt device is switched back into ON-state, where an abrupt increase in current is observed; as a result, a RS cycle is completed. It has been reported that the current compliance influences the formation of percolating filaments [16,18]. In this study, as increasing the current compliance during forming process, the ON-state current increases (Fig. 1(c)), showing that the more or stronger
Fig. 1. (a) Statistic diagram of forming voltages with various ZrO2 thicknesses, (b) I–V characteristics of the Ti/ZrO2/Pt device correlated with the various current compliances in the forming process, and (c) corresponding resistance ratio and current values of the two memory states.
Fig. 2. Endurance performance of the Ti/ZrO2/Pt devices by sweeping dc voltage bias. The inset shows the currents of ON-state and OFF-state traced during 10,000 times.
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conducting filaments are formed within ZrO2 film. In Fig. 1(b), higher VOFF, consequently, is required to switch the device off. Due to the higher current compliance causing the more or stronger conducting filaments formed and higher VOFF required, it leads to increasing the current of OFF-state. Hence, the resistance ratio between the two memory states maintains near constant in Fig. 1(c). After the forming process with current compliance of 5 mA, the cycling endurance characteristics of the Ti/ZrO2/Pt device measured in dc sweep mode are shown in Fig. 2, demonstrating the 10th, 100th, 1000th and 10,000th RS cycles. There is only a little distortion among above I–V curves, and the I–V hysteresis is still apparent with the switching cycles up to 10,000 times. The inset
in Fig. 2 depicts the evolution of RS characteristics by tracing the currents of both ON-state and OFF-state at 0.3 V versus RS cycles. In spite of a little variation of RS characteristics, the memory device can be switched continuously between the ON-state and the OFFstate without exhibiting any operation failure. Interestingly, the effect of current compliance of the forming process on the endurance cycle has been investigated in this work. It is well known that imposing higher current compliance would make conducting filaments permanent with an absence of RS properties, which could be understood in the similar context of the irreversible oxide breakdown [18]. Accordingly, the higher the forming compliance (>10 mA) performed, the less endurance cycles are observed
Fig. 3. (a) Non-destructive readout test result of both ON- and OFF-states over 104 s, and the resistance ratio between them closed to two orders. (b) Data retention of both ON- and OFF-states of the device over 105 s.
Fig. 4. (a) First five write-read-erase-read cycles performed by applying voltage pulses of +6 V and 3 V to switch the device on and off, alternately. (b) As increasing pulse cycles, evolution of both ON- and OFF-states with pulse width of 10 ns, (c) 20 ns, and (d) 50 ns, respectively.
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Fig. 5. Nondestructive readout properties of the Ti/ZrO2/Pt device. After 103 write/erase cycles, both ON-state and OFF-state are still kept stable over 104 s.
Table 1 Device structures, resistance values of two memory states, RS voltages, pulse operation parameters, and device yield of the current ZrO2-based memory devices reported by several research groups. Device structure
a
Ti/ZrO2/Pt Pt/ZrOX/p+-Si TiN/ZrO2/Pt Au/nc-Au ZrO2/n+-Sib Cr/Au-implanted ZrO2/n+-Si Cu/Cu-doped ZrO2/Pt Cr/Zr+-implanted ZrO2/n+-Si a b
RON
ROFF
|VON| (V)
|VOFF|
RS cycles
|PulseON|
|PulseOFF|
Device yield
Reference
150–2 kX 3 kX 300–800 X 5 k–166 kX 70–30 kX 100 X 4 kX
10 k–100 kX 43 kX 4 k–80 kX 9 M–83 MX 2 M–300 MX 100 MX 200 MX
0.9 1.6 0.7–1.1 2.6–3.5 8 2.1–3.6 3.2
1.4 1.1 0.5–0.8 1–3 2.2 0.8–1.5 3.1
10,000 80 480 100 200 N.A. N.A.
6 V/50 ns 6 V/100 ns 1.5 V/1 ls N.A. 12 V/50 ns N.A. N.A.
3 V/50 ns 2 V/100 ns 2 V/1 ls N.A. 6 V/100 ns N.A. N.A.
100% 43% N.A. 73% 100% Higher than that undoped Higher than that unimplanted
This work [11,12] [13] [14] [15] [16] [17]
TE/resistive layer/BE. nc: nanocrystals.
(<10,000 cycles). However, as imposing the lower forming compliance (<1 mA) on our memory device, the number of endurance cycles is found to dramatically decrease (<100 cycles). This might be attributed to the less or weaker conducting filaments formed within ZrO2 film, leading to the unstable RS properties such as the ‘‘set fail’’ phenomena observed [19]. Fig. 3(a) shows the capability of the Ti/ZrO2/Pt device to retain both memory states under 0.3 V. After applying the dc voltage to switch the fresh device into ONstate or OFF-state, the currents of both memory states are kept stable over 104 s with the resistance ratio more than 100 times, confirming the nonvolatile nature and the nondestructive readout property. Moreover, the data retention as shown in Fig. 3(b) demonstrates both ON-state and OFF-state maintain their own current values over 105 s at room temperature. To be easily compatible with the modern integrated circuit operation, we further investigate the possibility of electrical pulse-induced resistance (EPIR) changes in the Ti/ZrO2/Pt device. Fig. 4(a) depicts the measured result of the first five write-readerase-read cycles. After applying a 3-V 10-ns voltage pulse to switch the device off, the OFF-state is read at 0.3 V with a duration time of 3 s. Subsequently, the ON-state is achieved by +6-V 10-ns voltage pulse and also read at 0.3 V. However, as increasing the write-read-erase-read cycles, there are a lot of intermediate memory states observed as shown in Fig. 4(b), leading to write/erase operation errors. Moreover, when increasing the pulse width to 20 ns, the frequency of the appeared soft errors is obviously lowered as shown in Fig. 4(c). In Fig. 4(d), these operation errors disap-
pear without changing the pulse height, until the pulse width is increased to 50 ns. Under above pulse conditions, we demonstrate that the write-read-erase-read cycle in the Ti/ZrO2/Pt device is over 103 times. Fig. 5 depicts the nondestructive readout properties of the Ti/ ZrO2/Pt device under a stress voltage of 0.3 V. Even after 103 write/erase cycles, both the ON-state and OFF-state are distinguishable during successive reading over 104 s and the corresponding resistance ratio maintains closed to 100 times. Therefore, the important properties of nondestructive readout and good reliability are demonstrated in this device. The Ti/ZrO2/Pt device shows a high possibility to be integrated into CMOS circuit with a high speed operation. Table 1 lists device structures, resistance values of two memory states, RS voltages, pulse operation parameters, and device yield of our and the reported ZrO2-based memory devices. According to Table 1, our Ti/ZrO2/Pt device shows better RS characteristics among the reported ZrO2-based devices even without any additional process, such as embedding nanocrystals, doping impurities, and implanting ions. Therefore, our Ti/ZrO2/Pt device has the relatively higher potential in the commercial memory applications.
4. Conclusions In summary, the effects of current compliance during forming process are investigated in the Ti/ZrO2/Pt device. As increasing
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the current compliance, both VON and VOFF are raised with a near constant resistance ratio. By choosing a proper current compliance, the device is demonstrated to be operated over 10,000 RS cycles by sweeping dc voltage, and two memory states possess good stability. The write/erase operation errors are observed with applying +6-V 10-ns and 3-V 10-ns voltage pulses, even when increasing the pulse width to 20 ns. However, by applying +6-V 10-ns and 3-V 10-ns voltage pulses, there are no such errors during 1000 successive write-read-erase-read cycles. The Ti/ZrO2/Pt device also exhibits nondestructive readout properties before and after 103 pulse cycles. Compared with other currently reported ZrO2-based memory devices, our Ti/ZrO2/Pt device with near 100% device yield can be easily realized for practical memory application due to the simple structure and fewer fabrication processes. Acknowledgment This work was supported by the National Science Council, Taiwan, under Project NSC 96-2628-E009-166-MY3. References [1] I.G. Baek, M.S. Lee, S. Seo, M.J. Lee, D.H. Seo, D.S. Suh, J.C. Park, S.O. Park, H.S. Kim, I.K. Yoo, U.-In Chung, J.T. Moon, Int. Electron Devices Meet., Tech. Dig. (2004) 587. [2] T.N. Fang, S. Kaza, S. Haddad, A. Chen, Y.C. Wu, Z. Lan, S. Avanzino, D. Liao, C. Gopalan, S. Choi, S. Mahdavi, M. Buynoski, Y. Lin, C. Marrian, C. Bill, M. VanBuskirk, M. Taguchi, Int. Electron Devices Meet., Tech. Dig. (2006) 789.
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