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HfO2 functional stack films based resistive switching memories with controlled SET and RESET voltages
SOSI-13446; No of Pages 4 Solid State Ionics xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Al2O3/HfO2 functional stack films based resistive switching memories with controlled SET and RESET voltages Lin Chen ⁎, Ya-Wei Dai, Qing-Qing Sun ⁎, Jiao-Jiao Guo, Peng Zhou, David Wei Zhang State Key Laboratory of ASIC and System, School of Microelectronics, Department of Materials Science, Fudan University, Shanghai, China
a r t i c l e
i n f o
Article history: Received 8 April 2014 Received in revised form 29 August 2014 Accepted 29 August 2014 Available online xxxx Keywords: Resistive switching Random circuit breaker network Atomic layer deposition
a b s t r a c t Two Resistive Random Access Memory (RRAM) devices were designed with the thin Al2O3/thick HfO2 stack as a functional layer. These devices used different thickness of Al2O3 layer and same thickness of HfO2 layer. The fluctuation of the SET and the RESET voltages, the main obstacle which blocks the application of RRAMs based on transition metal oxide, leads to the instability of the RRAM. The random circuit breaker network (RCB) model points out that the fluctuation of the voltages is the universal problem of RRAM devices and it originated from the working principles of devices. With this structure, the random formation and rupture of conducting filaments are limited within the Al2O3 film, which own lower k value, near the anode region instead of random formation and rupture in the whole functional layer. And experiments find that for the device with 5 nm Al2O3 film, the distributions of the 80% VSET and VRESET are limited within 0.2 V (from 0.6 V to 0.8 V) and 0.25 V (from −0.5 V to − 0.25 V), respectively. For comparison, distributions for the device with 10 nm Al2O3 film are within 1.1 V (from 0.6 V to 1.7 V) and 1 V (from −1.3 V to 0.3 V), respectively. © 2014 Elsevier B.V. All rights reserved.
The scaling of conventional nonvolatile memory is going to encounter the technical and physical limits in the near future [1,2]. RRAM based on binary transition metal oxides (TMO) such as NiO, TiO2, ZrO2, Nb2O5, and HfO2 have advantages in higher density, simpler structure [3], more robust endurance, better retention, faster write/erase speed and lower write/erase voltages compared to the conventional flash memories based on charges storage, as well as lower power consumption than alternative candidates such as phase-change memory (PCM) and magneto-resistance random access memory (MRAM) [4,5]. Among the many types of transition metal oxide (TMO) materials, HfO2 has lots of properties which are suitable for electrical devices, such as thermo dynamic stability, low hysteresis in I–V and C–V measurement, simple constituent and high break-down fields [6–8]. Moreover, HfO2based materials are industry friendly since they are already used in complementary metal-oxide-semiconductor (CMOS) devices at 22 nm technology node. It is a highly promising candidate for future generations of non-volatile memories. On the other hand, the instability of switching voltages is currently one of the most serious obstacles to the RRAM applications. Previous studies about RRAMs based on the TMO mainly supported the filament mechanism. It was confirmed that the resistance switching behaviors were due to the formation and rupture of the conducting filament (CF) within the oxide films [9]. Recently, an outstanding work dedicated to the mechanism of the instability of switching voltages, the random circuit breaker (RCB) network model [10], explained that the wide
distributions of switching voltages was due to the randomly turning on or off a circuit breaker in the network which corresponded to the formation and rupture of a small segment of the conductive filaments. Based on this theory, we designed a RRAM device with the thin low-k/thick high-k stack as functional layer. According to the Gauss's law, the electric field distributed in the thin low k layer is much stronger than that distributed in the high k layer, so, the rupture and formation of the filaments would likely to happen within the region of low-k layer near the anode region.
⁎ Corresponding authors. E-mail addresses: [email protected] (L. Chen), [email protected] (Q.-Q. Sun).
Fig. 1. (a) The TEM image of the RRAM for Al2O3/HfO2 = 5 nm/10 nm; (b) the TEM image and EDX spectrometry of the RRAM for Al2O3/HfO2 = 10 nm/10 nm.
http://dx.doi.org/10.1016/j.ssi.2014.08.014 0167-2738/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: L. Chen, et al., Al2O3/HfO2 functional stack films based resistive switching memories with controlled SET and RESET voltages..., Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.08.014
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L. Chen et al. / Solid State Ionics xxx (2014) xxx–xxx
Fig. 2. XPS depth profiles of the two RRAM device cells with Al2O3/HfO2 (5 nm/10 nm) functional stack (a), and Al2O3/HfO2 = 10 nm/10 nm functional stack (b).
This work proposed two RRAMs using Al2O3/ HfO2 stack as resistive switching layers ruled by the principle of electric field modulation in the switching region. These two devices were designed by two different thickness of the Al2O3 film with the same thickness of HfO2 film beneath the alumina. One structure is 5 nm Al2O3 film on the 10 nm HfO2 film, and the other one is 10 nm Al2O3 film on the 10 nm HfO2 film. The results show that the device with 5 nm Al2O3 film has much better performance. The distribution of VSET and VRESET is much smaller for the device with 5 nm Al2O3 film, compared to the device with 10 nm
Al2O3 film. RRAM elements with 10 nm HfO2 were also fabricated as control sample. A thermally oxidized (300 nm SiO2) silicon wafer was used as a substrate. Subsequently, a 20 nm thick titanium (Ti) adhesion layer and 80 nm thick platinum (Pt) layer were deposited on the SiO2 /Si substrate by electron beam evaporation. The platinum layer acts as the bottom electrode. After that, 100 cycles HfO2 film was prepared at 300 °C by atomic layer deposition (ALD) using Tetrakis (EthylMethylAmide) Hafnium (TEMAH) and H2O as precursors on the bottom electrode. Subsequently, 41 cycle and 82 cycle Al2O3 films were deposited by ALD at 200 °C on the previously deposited HfO2 film using TriMethylAluminum (TMA) and H2O as precursors to form two different RRAM devices, respectively. 80 nm thick TiN was deposited by PVD as top electrode (TE). The TE had a disk shape with a diameter of 150 μm. Then metal-insulator-metal (MIM) structure was formed for memory device characterization. The film thickness was measured by SOPRA GES5E Spectroscopic Ellipsometer (SE) and transmission electron microscopy (TEM). The current–voltage characteristics of TiN/Al2O3/HfO2/Pt structure were measured by the Agilent B1500A Semiconductor Device analyzer at room temperature with biased top electrode and grounded bottom electrode. Besides, we conducted the X-ray photoelectron spectroscopy (XPS) analysis with a depth profiling function and X-ray diffraction (XRD) by ESCALAB250 THERMO SCIENTIFIC and Bruker D8. Fig. 1(a) shows the TEM image of the RRAM with 41 cycle Al2O3 film on the 100 cycle HfO2 film. As shown in Fig. 1(a), the dark color part is the Pt bottom electrode. The shallow black part next to Pt is the HfO2 film, and the gray layer is the Al2O3 film. The thickness of the Al2O3 and HfO2 layers was estimated to be 5 nm and 10 nm, respectively. Fig. 1(b) shows the TEM picture of the sample with 82 cycles Al2O3 film on the same 100 cycles HfO2 film and the thickness of the Al2O3
Fig. 3. (a) The schematic structure of TiN/Al2O3/HfO2/Pt devices and TiN/ HfO2/Pt devices; (b) cycling bipolar resistive switching of TiN/ HfO2 (10 nm)/Pt devices shows poor SET and RESET stabilities; (c) bipolar resistive switching traces for TiN/Al2O3 (5 nm)/HfO2 (10 nm)/Pt devices; (d) bipolar resistive switching traces for TiN/Al2O3 (10 nm)/HfO2 (10 nm)/Pt devices.
Please cite this article as: L. Chen, et al., Al2O3/HfO2 functional stack films based resistive switching memories with controlled SET and RESET voltages..., Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.08.014
L. Chen et al. / Solid State Ionics xxx (2014) xxx–xxx
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Fig. 4. (a) The histograms of the of the VSET and VRESET of 500 switching cycles of the RRAM for Al2O3/HfO2 (5 nm/10 nm); (b) the histograms of the of the VSET and VRESET of 500 switching cycles of the RRAM for Al2O3/HfO2 (10 nm/10 nm); (c) the 500 times endurance test of the two devices; (d) the schematic energy band diagrams of the device of 5 nm Al2O3 and 10 nm HfO2.
layer was estimated to be 10 nm when the HfO2 layers was estimated to be 10 nm. According to Fig. 1(a) and (b), both the HfO2 film and the Al2O3 film are amorphous in the two devices. Fig. 2 is the XPS depth profiles of the elements of the two samples exposed by Ar+ bombarding with different exposing times. There is no carbon residue existing in the both bulk films indicating the complete reaction during the ALD process. The alumina layer in the Fig. 2(b) is found to be thicker (almost two times) than that in the Fig. 2(a) which is consisted to the TEM analysis. The XRD spectra (figures not shown) of the two devices do not show any peak from HfO2 and Al2O3, which are consisted with the previously TEM analysis. The schematic structure of TiN/Al2O3/HfO2/Pt devices and TiN/ HfO2/ Pt devices were shown in Fig. 3(a), and during the test, the voltage was applied to the top electrode of the TiN film while the Pt bottom electrode was grounded. Pt bottom electrode was grounded. Fig. 3(b) shows the typical bipolar switching current–voltage (I–V) characteristics of the TiN/HfO2/Pt devices. Large distribution of SET and RESET voltages can be observed in cycling operation due to the random formation and rupture of conductive filaments in HfO2 film. Fig. 3(c) and (d) shows the reproducible bipolar switching I–V characteristics of the TiN/Al2O3/HfO2/Pt devices with different Al2O3 thicknesses. When the voltage sweeping from 0 V to 3 V, a compliance current of 1 mA was applied to avoid the hard breakdown of the device and limited the variation of the ON-state resistance which will further control the variation of the Voff when the device is switched back to OFF-state. During the positive sweeping, an abrupt increase of current is observed where the device is switched to ON-state and reaches a
low resistance state (LRS) from the high resistance state (HRS). The voltage where the current increases abruptly is defined as VSET or Von. Subsequently, sweeping the voltage from zero to a certain negative voltage, the devices are switched from the LRS to the HRS. The voltage where the curve has the inflection point is the VRESET or VOFF. It is apparent that the electrical characteristics have been improved when the Al2O3 film was reduced from 10 nm to 5 nm with the same TiN/ Al2O3/HfO2/Pt stack structure. Panels (a) and (b) in Fig. 4 are the histograms of the VSET and VRESET of the TiN/Al2O3/HfO2/Pt devices with different Al2O3 thicknesses after 500 switching cycles, showing the statistical electrical performance. Compared to the device with 82 cycles Al2O3 film, the distribution of SET and RESET voltages of the device with 41 cycles Al2O3 film are obviously much narrower. For the device with 41 cycles Al2O3 film, more than 80% of the SET voltages are distributed between 0.6 V and 0.8 V, and on the other hand, more than 80% of the RESET voltages are distributed between −0.5 V and −0.25 V. But for the RRAM with 82 cycles Al2O3 film, the SET voltages vary at the region from 0.6 V to 1.7 V, while the RESET voltages are located at the region of − 1.3 V to − 0.3 V. Fig. 4(c) is the HRS and LRS resistance distributions during the endurance cycling test for the two devices with the readout voltage of the HRS and LRS are both 0.05 V. It shows that, compared with the RRAM of 82 cycles Al2O3 film, the device with 41 cycles Al2O3 film has much more stable low resistance (LR) and high resistance (HR) values. Moreover, the values of the HR and LR of the device with 82 cycles Al2O3 film are somewhat fluctuation, and both the HR and LR values show some degradation during the test.
Please cite this article as: L. Chen, et al., Al2O3/HfO2 functional stack films based resistive switching memories with controlled SET and RESET voltages..., Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.08.014
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It was reported that the switching behavior of HfO2 is related to the formation and rupture of the conductive filament (CF) [3,9,11,12]. Since the dielectric constant of Al2O3 is about 9 and the HfO2 is 25 [7], the electric field distributed in Al2O3 is far greater than that in the HfO2 film when a voltage is applied on the device. As a result, the formation and rupture of the filament are limited in the lower dielectric Al2O3 film. Fig. 4(d) is the schematic energy band diagram of one device. In the diagram, the film thickness of the Al2O3 is 5 nm while HfO2 is 10 nm. In this case, if we apply a 2.0 V voltage on the anode, the uniform electrical field distributed in the Al2O3 film is 2.32 × 108 V/m while that distributed in the HfO2 film is about 8 × 107 V/m. The electric field distributed in the Al2O3 film is about 3 times of that in the HfO2 film. So the formation and rupture of the filament are more likely to happen in the Al2O3 film. By RCB network model, the formation and rupture of the filament correspond to the turning on or off the breakers in the Al2O3 resign. When the Al2O3 is thinner, the breakers in the Al2O3 film is less and the randomness of the switching on or off of the breakers is better controlled, the device is more stable. In general, by introducing thin Al2O3 film/ thick HfO2 film stack and limiting the formation and rupture of the filaments in the thin Al2O3 (lower k value) layer ruled by the principle of electric field modulation, we found that compared with the device with/without Al2O3 film, the stability of the device with 5 nm Al2O3 film is significantly improved. The distribution of the SET and RESET voltages is more narrow and the resistance ration is also improved. This structure provides a possible
solution to the promising application of the RRAMs based on transition metal oxides. This work was supported by the National Natural Science Foundation of China (61376093, 61106108, 61376094), Shanghai Municipal Science and Technology Commission (13QA1400400), National Science and Technology Major Project (2011ZX02707) and Innovation Program of Shanghai Municipal Education Commission (12ZZ010). References [1] A. Sawa, Mater. Today 11 (2008) 28. [2] C.Y. Lin, C.Y. Wu, C.Y. Wu, C.C. Lin, T.Y. Tseng, Thin Solid Films 516 (2–4) (2007) 444. [3] L. Chen, Y. Xu, Q.Q. Sun, P. Zhou, P.F. Wang, S.J. Ding, W. Zhang, IEEE Electron Device Lett. 31 (2010) 1296. [4] A. Pirovano, A.L. Lacaita, A. Benvenuti, F. Pellizzer, R. Bez, IEEE Trans. Electron Devices 51 (3) (2004) 452–459. [5] R. Bruchhaus, R. Muenstermann, T. Menke, C. Hermes, F. Lentz, R. Weng, R. Dittmann, R. Waser, Curr. Appl. Phys. 11 (2011) e75–e78. [6] M.Y. Chan, T. Zhang, V. Ho, P.S. Lee, Microelectron. Eng. 85 (2008) 2420. [7] M.F. Li, Modern CMOS device, Fudan Lecture Notes2010. [8] J.M. Khoshman, M.E. Kordesch, Surf. Coat. Technol. 201 (2006) 3530. [9] G. Bersuker, D.C. Gilmer, D. Veksler, J. Yum, H. Park, S. Lian, L. Vandelli, A. Padovani, L. Larcher, K. McKenna, A. Shluger, V. Iglesias, M. Porti, M. Nafria, W. Taylor, P.D. Kirsch, R. Jammy, International Electron Devices Meeting, 2010, pp. 19.6.1–19.6.4. [10] S.H. Chang, J.S. Lee, S.C. Chae, S.B. Lee, C. Liu, B. Kahng, D.W. Kim, T.W. Noh, Phys. Rev. Lett. 20 (2009) 1154. [11] L. Goux, P. Czarnecki, Y.Y. Chen, L. Pantisano, X.P. Wang, R. Degraeve, B. Govoreanu, M. Jurczak, D.J. Wouters, L. Altimime, Appl. Phys. Lett. 97 (2010) 243509. [12] P. Calka, E. Martinez, D. Lafond, H. Dansas, S. Tirano, V. Jousseaume, F. Bertin, C. Guedj, Microelectron. Eng. 88 (2011) 1140.
Please cite this article as: L. Chen, et al., Al2O3/HfO2 functional stack films based resistive switching memories with controlled SET and RESET voltages..., Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.08.014
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Al2O3/HfO2 functional stack films based resistive switching memories with controlled SET and RESET voltages Lin Chen ⁎, Ya-Wei Dai, Qing-Qing Sun ⁎, Jiao-Jiao Guo, Peng Zhou, David Wei Zhang State Key Laboratory of ASIC and System, School of Microelectronics, Department of Materials Science, Fudan University, Shanghai, China
a r t i c l e
i n f o
Article history: Received 8 April 2014 Received in revised form 29 August 2014 Accepted 29 August 2014 Available online xxxx Keywords: Resistive switching Random circuit breaker network Atomic layer deposition
a b s t r a c t Two Resistive Random Access Memory (RRAM) devices were designed with the thin Al2O3/thick HfO2 stack as a functional layer. These devices used different thickness of Al2O3 layer and same thickness of HfO2 layer. The fluctuation of the SET and the RESET voltages, the main obstacle which blocks the application of RRAMs based on transition metal oxide, leads to the instability of the RRAM. The random circuit breaker network (RCB) model points out that the fluctuation of the voltages is the universal problem of RRAM devices and it originated from the working principles of devices. With this structure, the random formation and rupture of conducting filaments are limited within the Al2O3 film, which own lower k value, near the anode region instead of random formation and rupture in the whole functional layer. And experiments find that for the device with 5 nm Al2O3 film, the distributions of the 80% VSET and VRESET are limited within 0.2 V (from 0.6 V to 0.8 V) and 0.25 V (from −0.5 V to − 0.25 V), respectively. For comparison, distributions for the device with 10 nm Al2O3 film are within 1.1 V (from 0.6 V to 1.7 V) and 1 V (from −1.3 V to 0.3 V), respectively. © 2014 Elsevier B.V. All rights reserved.
The scaling of conventional nonvolatile memory is going to encounter the technical and physical limits in the near future [1,2]. RRAM based on binary transition metal oxides (TMO) such as NiO, TiO2, ZrO2, Nb2O5, and HfO2 have advantages in higher density, simpler structure [3], more robust endurance, better retention, faster write/erase speed and lower write/erase voltages compared to the conventional flash memories based on charges storage, as well as lower power consumption than alternative candidates such as phase-change memory (PCM) and magneto-resistance random access memory (MRAM) [4,5]. Among the many types of transition metal oxide (TMO) materials, HfO2 has lots of properties which are suitable for electrical devices, such as thermo dynamic stability, low hysteresis in I–V and C–V measurement, simple constituent and high break-down fields [6–8]. Moreover, HfO2based materials are industry friendly since they are already used in complementary metal-oxide-semiconductor (CMOS) devices at 22 nm technology node. It is a highly promising candidate for future generations of non-volatile memories. On the other hand, the instability of switching voltages is currently one of the most serious obstacles to the RRAM applications. Previous studies about RRAMs based on the TMO mainly supported the filament mechanism. It was confirmed that the resistance switching behaviors were due to the formation and rupture of the conducting filament (CF) within the oxide films [9]. Recently, an outstanding work dedicated to the mechanism of the instability of switching voltages, the random circuit breaker (RCB) network model [10], explained that the wide
distributions of switching voltages was due to the randomly turning on or off a circuit breaker in the network which corresponded to the formation and rupture of a small segment of the conductive filaments. Based on this theory, we designed a RRAM device with the thin low-k/thick high-k stack as functional layer. According to the Gauss's law, the electric field distributed in the thin low k layer is much stronger than that distributed in the high k layer, so, the rupture and formation of the filaments would likely to happen within the region of low-k layer near the anode region.
⁎ Corresponding authors. E-mail addresses: [email protected] (L. Chen), [email protected] (Q.-Q. Sun).
Fig. 1. (a) The TEM image of the RRAM for Al2O3/HfO2 = 5 nm/10 nm; (b) the TEM image and EDX spectrometry of the RRAM for Al2O3/HfO2 = 10 nm/10 nm.
http://dx.doi.org/10.1016/j.ssi.2014.08.014 0167-2738/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: L. Chen, et al., Al2O3/HfO2 functional stack films based resistive switching memories with controlled SET and RESET voltages..., Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.08.014
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L. Chen et al. / Solid State Ionics xxx (2014) xxx–xxx
Fig. 2. XPS depth profiles of the two RRAM device cells with Al2O3/HfO2 (5 nm/10 nm) functional stack (a), and Al2O3/HfO2 = 10 nm/10 nm functional stack (b).
This work proposed two RRAMs using Al2O3/ HfO2 stack as resistive switching layers ruled by the principle of electric field modulation in the switching region. These two devices were designed by two different thickness of the Al2O3 film with the same thickness of HfO2 film beneath the alumina. One structure is 5 nm Al2O3 film on the 10 nm HfO2 film, and the other one is 10 nm Al2O3 film on the 10 nm HfO2 film. The results show that the device with 5 nm Al2O3 film has much better performance. The distribution of VSET and VRESET is much smaller for the device with 5 nm Al2O3 film, compared to the device with 10 nm
Al2O3 film. RRAM elements with 10 nm HfO2 were also fabricated as control sample. A thermally oxidized (300 nm SiO2) silicon wafer was used as a substrate. Subsequently, a 20 nm thick titanium (Ti) adhesion layer and 80 nm thick platinum (Pt) layer were deposited on the SiO2 /Si substrate by electron beam evaporation. The platinum layer acts as the bottom electrode. After that, 100 cycles HfO2 film was prepared at 300 °C by atomic layer deposition (ALD) using Tetrakis (EthylMethylAmide) Hafnium (TEMAH) and H2O as precursors on the bottom electrode. Subsequently, 41 cycle and 82 cycle Al2O3 films were deposited by ALD at 200 °C on the previously deposited HfO2 film using TriMethylAluminum (TMA) and H2O as precursors to form two different RRAM devices, respectively. 80 nm thick TiN was deposited by PVD as top electrode (TE). The TE had a disk shape with a diameter of 150 μm. Then metal-insulator-metal (MIM) structure was formed for memory device characterization. The film thickness was measured by SOPRA GES5E Spectroscopic Ellipsometer (SE) and transmission electron microscopy (TEM). The current–voltage characteristics of TiN/Al2O3/HfO2/Pt structure were measured by the Agilent B1500A Semiconductor Device analyzer at room temperature with biased top electrode and grounded bottom electrode. Besides, we conducted the X-ray photoelectron spectroscopy (XPS) analysis with a depth profiling function and X-ray diffraction (XRD) by ESCALAB250 THERMO SCIENTIFIC and Bruker D8. Fig. 1(a) shows the TEM image of the RRAM with 41 cycle Al2O3 film on the 100 cycle HfO2 film. As shown in Fig. 1(a), the dark color part is the Pt bottom electrode. The shallow black part next to Pt is the HfO2 film, and the gray layer is the Al2O3 film. The thickness of the Al2O3 and HfO2 layers was estimated to be 5 nm and 10 nm, respectively. Fig. 1(b) shows the TEM picture of the sample with 82 cycles Al2O3 film on the same 100 cycles HfO2 film and the thickness of the Al2O3
Fig. 3. (a) The schematic structure of TiN/Al2O3/HfO2/Pt devices and TiN/ HfO2/Pt devices; (b) cycling bipolar resistive switching of TiN/ HfO2 (10 nm)/Pt devices shows poor SET and RESET stabilities; (c) bipolar resistive switching traces for TiN/Al2O3 (5 nm)/HfO2 (10 nm)/Pt devices; (d) bipolar resistive switching traces for TiN/Al2O3 (10 nm)/HfO2 (10 nm)/Pt devices.
Please cite this article as: L. Chen, et al., Al2O3/HfO2 functional stack films based resistive switching memories with controlled SET and RESET voltages..., Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.08.014
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Fig. 4. (a) The histograms of the of the VSET and VRESET of 500 switching cycles of the RRAM for Al2O3/HfO2 (5 nm/10 nm); (b) the histograms of the of the VSET and VRESET of 500 switching cycles of the RRAM for Al2O3/HfO2 (10 nm/10 nm); (c) the 500 times endurance test of the two devices; (d) the schematic energy band diagrams of the device of 5 nm Al2O3 and 10 nm HfO2.
layer was estimated to be 10 nm when the HfO2 layers was estimated to be 10 nm. According to Fig. 1(a) and (b), both the HfO2 film and the Al2O3 film are amorphous in the two devices. Fig. 2 is the XPS depth profiles of the elements of the two samples exposed by Ar+ bombarding with different exposing times. There is no carbon residue existing in the both bulk films indicating the complete reaction during the ALD process. The alumina layer in the Fig. 2(b) is found to be thicker (almost two times) than that in the Fig. 2(a) which is consisted to the TEM analysis. The XRD spectra (figures not shown) of the two devices do not show any peak from HfO2 and Al2O3, which are consisted with the previously TEM analysis. The schematic structure of TiN/Al2O3/HfO2/Pt devices and TiN/ HfO2/ Pt devices were shown in Fig. 3(a), and during the test, the voltage was applied to the top electrode of the TiN film while the Pt bottom electrode was grounded. Pt bottom electrode was grounded. Fig. 3(b) shows the typical bipolar switching current–voltage (I–V) characteristics of the TiN/HfO2/Pt devices. Large distribution of SET and RESET voltages can be observed in cycling operation due to the random formation and rupture of conductive filaments in HfO2 film. Fig. 3(c) and (d) shows the reproducible bipolar switching I–V characteristics of the TiN/Al2O3/HfO2/Pt devices with different Al2O3 thicknesses. When the voltage sweeping from 0 V to 3 V, a compliance current of 1 mA was applied to avoid the hard breakdown of the device and limited the variation of the ON-state resistance which will further control the variation of the Voff when the device is switched back to OFF-state. During the positive sweeping, an abrupt increase of current is observed where the device is switched to ON-state and reaches a
low resistance state (LRS) from the high resistance state (HRS). The voltage where the current increases abruptly is defined as VSET or Von. Subsequently, sweeping the voltage from zero to a certain negative voltage, the devices are switched from the LRS to the HRS. The voltage where the curve has the inflection point is the VRESET or VOFF. It is apparent that the electrical characteristics have been improved when the Al2O3 film was reduced from 10 nm to 5 nm with the same TiN/ Al2O3/HfO2/Pt stack structure. Panels (a) and (b) in Fig. 4 are the histograms of the VSET and VRESET of the TiN/Al2O3/HfO2/Pt devices with different Al2O3 thicknesses after 500 switching cycles, showing the statistical electrical performance. Compared to the device with 82 cycles Al2O3 film, the distribution of SET and RESET voltages of the device with 41 cycles Al2O3 film are obviously much narrower. For the device with 41 cycles Al2O3 film, more than 80% of the SET voltages are distributed between 0.6 V and 0.8 V, and on the other hand, more than 80% of the RESET voltages are distributed between −0.5 V and −0.25 V. But for the RRAM with 82 cycles Al2O3 film, the SET voltages vary at the region from 0.6 V to 1.7 V, while the RESET voltages are located at the region of − 1.3 V to − 0.3 V. Fig. 4(c) is the HRS and LRS resistance distributions during the endurance cycling test for the two devices with the readout voltage of the HRS and LRS are both 0.05 V. It shows that, compared with the RRAM of 82 cycles Al2O3 film, the device with 41 cycles Al2O3 film has much more stable low resistance (LR) and high resistance (HR) values. Moreover, the values of the HR and LR of the device with 82 cycles Al2O3 film are somewhat fluctuation, and both the HR and LR values show some degradation during the test.
Please cite this article as: L. Chen, et al., Al2O3/HfO2 functional stack films based resistive switching memories with controlled SET and RESET voltages..., Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.08.014
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It was reported that the switching behavior of HfO2 is related to the formation and rupture of the conductive filament (CF) [3,9,11,12]. Since the dielectric constant of Al2O3 is about 9 and the HfO2 is 25 [7], the electric field distributed in Al2O3 is far greater than that in the HfO2 film when a voltage is applied on the device. As a result, the formation and rupture of the filament are limited in the lower dielectric Al2O3 film. Fig. 4(d) is the schematic energy band diagram of one device. In the diagram, the film thickness of the Al2O3 is 5 nm while HfO2 is 10 nm. In this case, if we apply a 2.0 V voltage on the anode, the uniform electrical field distributed in the Al2O3 film is 2.32 × 108 V/m while that distributed in the HfO2 film is about 8 × 107 V/m. The electric field distributed in the Al2O3 film is about 3 times of that in the HfO2 film. So the formation and rupture of the filament are more likely to happen in the Al2O3 film. By RCB network model, the formation and rupture of the filament correspond to the turning on or off the breakers in the Al2O3 resign. When the Al2O3 is thinner, the breakers in the Al2O3 film is less and the randomness of the switching on or off of the breakers is better controlled, the device is more stable. In general, by introducing thin Al2O3 film/ thick HfO2 film stack and limiting the formation and rupture of the filaments in the thin Al2O3 (lower k value) layer ruled by the principle of electric field modulation, we found that compared with the device with/without Al2O3 film, the stability of the device with 5 nm Al2O3 film is significantly improved. The distribution of the SET and RESET voltages is more narrow and the resistance ration is also improved. This structure provides a possible
solution to the promising application of the RRAMs based on transition metal oxides. This work was supported by the National Natural Science Foundation of China (61376093, 61106108, 61376094), Shanghai Municipal Science and Technology Commission (13QA1400400), National Science and Technology Major Project (2011ZX02707) and Innovation Program of Shanghai Municipal Education Commission (12ZZ010). References [1] A. Sawa, Mater. Today 11 (2008) 28. [2] C.Y. Lin, C.Y. Wu, C.Y. Wu, C.C. Lin, T.Y. Tseng, Thin Solid Films 516 (2–4) (2007) 444. [3] L. Chen, Y. Xu, Q.Q. Sun, P. Zhou, P.F. Wang, S.J. Ding, W. Zhang, IEEE Electron Device Lett. 31 (2010) 1296. [4] A. Pirovano, A.L. Lacaita, A. Benvenuti, F. Pellizzer, R. Bez, IEEE Trans. Electron Devices 51 (3) (2004) 452–459. [5] R. Bruchhaus, R. Muenstermann, T. Menke, C. Hermes, F. Lentz, R. Weng, R. Dittmann, R. Waser, Curr. Appl. Phys. 11 (2011) e75–e78. [6] M.Y. Chan, T. Zhang, V. Ho, P.S. Lee, Microelectron. Eng. 85 (2008) 2420. [7] M.F. Li, Modern CMOS device, Fudan Lecture Notes2010. [8] J.M. Khoshman, M.E. Kordesch, Surf. Coat. Technol. 201 (2006) 3530. [9] G. Bersuker, D.C. Gilmer, D. Veksler, J. Yum, H. Park, S. Lian, L. Vandelli, A. Padovani, L. Larcher, K. McKenna, A. Shluger, V. Iglesias, M. Porti, M. Nafria, W. Taylor, P.D. Kirsch, R. Jammy, International Electron Devices Meeting, 2010, pp. 19.6.1–19.6.4. [10] S.H. Chang, J.S. Lee, S.C. Chae, S.B. Lee, C. Liu, B. Kahng, D.W. Kim, T.W. Noh, Phys. Rev. Lett. 20 (2009) 1154. [11] L. Goux, P. Czarnecki, Y.Y. Chen, L. Pantisano, X.P. Wang, R. Degraeve, B. Govoreanu, M. Jurczak, D.J. Wouters, L. Altimime, Appl. Phys. Lett. 97 (2010) 243509. [12] P. Calka, E. Martinez, D. Lafond, H. Dansas, S. Tirano, V. Jousseaume, F. Bertin, C. Guedj, Microelectron. Eng. 88 (2011) 1140.
Please cite this article as: L. Chen, et al., Al2O3/HfO2 functional stack films based resistive switching memories with controlled SET and RESET voltages..., Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.08.014