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Dependence of nonvolatile memory characteristics on curing temperature for polymer memory-cell embedded with Au nanocrystals in poly(N-vinylcarbazole)
Current Applied Physics 11 (2011) e25ee29
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Dependence of nonvolatile memory characteristics on curing temperature for polymer memory-cell embedded with Au nanocrystals in poly(N-vinylcarbazole) Jong-Dae Lee, Hyun-Min Seung, Kyoung-Cheol Kwon, Jea-gun Park* National Program Center for Tera-bit-level Nonvolatile Memory Development, HIT 101 Hanyang University, Seoul 133-791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 October 2010 Accepted 1 December 2010 Available online 1 February 2011
We investigated the dependence of nonvolatile memory characteristics on curing temperature for a polymer nonvolatile 4F2 memory-cell embedded with Au nanocrystals in poly(N-vinylcarbazole) (PVK). The curing temperature is an important factor for determining the size, shape, and isolation presence of Au nanocrystals embedded in the PVK layer. When the curing temperature rises above 300 C, w8-nm spherical Au nanocrystals are uniformly distributed and well isolated. Thus, the nonvolatile memory-cell demonstrated a memory margin of (Ion/Ioff) of 9.8 101, retention time of 1 105 s and more than 100 program/erase endurance cycles. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: Nonvolatile memory Nanocrystals Au PVK
1. Introduction
2. Experiments
Demand for very high-density integrated, low-cost, flexible devices has been increasing. Organic memory-cell is promising candidates for tera-bit level memory-cell because organic memorycell has shown a minimum device feature size of 4F2, the possibility of a flexible device, and fast access and storage of several tens of nanoseconds. Thus, many groups have reported organic memory-cell using small-molecule memory-cell [1e5] or polymer memory-cell [5e8]. In particular, it has been reported that small-molecule Alq3: Aluminun trise(8hydroxyquinoline) 4F2 memory-cell demonstrated nonvolatile multi-bit and multi-level cell operation [1]. For smallmolecule memory-cell, however, the temperature of post-processes, such as passivation and inter-metal connection, should be lower than approximately 50 C to prevent degradation of the smallmolecule layer. We developed a polymer (PVK) nonvolatile 4F2 memory-cell embedded with Au nanocrystals because PVK can be sustained above the process temperature of 350 C [9,10]. This memory-cell was fabricated with the PVK layer embedded with Au nanocrystals between the cross-bar top and bottom Al electrodes. In addition, the dependence of curing temperature on nonvolatile memory characteristics for memory-cell was investigated using high resolution cross-sectional transmission electron microscopy (x-TEM) and electrical measurements.
The polymer nonvolatile 4F2 memory-cell consisted of a device structure of a PVK layer embedded with Au nanocrystals between cross-bar top and bottom Al electrodes patterned by width of 1 mm, as shown in Fig. 1. First, the 80-nm bottom Al electrode on the silicon dioxide layer grown on the silicon substrate was thermally evaporated at an evaporation rate of 5.0 Å/s in a vacuum chamber. Then, the bottom PVK layer, dissolved using chloroform with 0.8 wt%, was spin-coated with a rotation velocity of 2000 rpm for 99 s and subsequently baked at 120 C for 2 min. Afterward, the 5-nm-thick Au film was thermally evaporated at an evaporation rate of 0.1 Å/s. The upper PVK layer was spin-coated with the same process as with the bottom PVK layer and was followed by curing at 200, 250 or 300 C for 2 h to produce Au nanocrystals embedded in the PVK layer. Finally, the top Al electrode was thermally evaporated using cross-patterned mask against the bottom Al electrode. The PVK (molecular weight: 700,000e1,100,000) was purchased from TCI (Tokyo Chemical Industry). The physical device structure was characterized using x-TEM for the cross-sectional TEM samples made using a focus-ion beam. The nonvolatile memory characteristics were estimated by measuring dc and ac current vs. voltage (IeV) using a computer-interfaced Agilent source-measure unit, model 4155C. 3. Results and discussion
* Corresponding author. Fax: þ82 2 2295 6868. E-mail address: [email protected] (J.-g. Park). 1567-1739/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.12.037
Fig. 2 shows the physical structures of the polymer memory-cell embedded with Au nanocrystals produced by various curing
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J.-D. Lee et al. / Current Applied Physics 11 (2011) e25ee29
Fig. 1. Nonvolatile 4F2 polymer (PVK) memory-cell embedded with Au nanocrystals: (a) perspective view of polymer memory-cell, (b) chemical structure of PVK.
temperatures. Fig. 2(a) presents an x-TEM image for a polymer memory-cell embedded with Au nanocrystals produced by curing at 200 C for 2 h. The uniform 5-nm Au film began to be agglomerated by curing at 200 C for 2 h. As a result, the agglomerated Au film thickness was w9.2 nm. Fig. 2(b) presents an x-TEM image of a polymer memory-cell embedded with Au nanocrystals produced by curing at 250 C for 2 h. The uniform 5-nm Au film by curing at 200 C for 2 h was transformed into dumbbell-shaped Au nanocrystals. The Au nanocrystals were not completely isolated from one another and their size was approximately w8.4 nm. Fig. 2(c) presents an x-TEM image of a polymer memory-cell embedded
with Au nanocrystals produced by curing at 300 C for 2 h. The uniform 5-nm Au film by curing at 300 C for 2 h was transformed into Au spherical nanocrystals. The Au nanocrystals were well isolated from one another and their size was approximately w7.8 nm. By comparing Fig. 2(a)e(c), the Au nanocrystal size decreased from w9.2 nm to w7.8 nm, and the Au crystal morphology changed from a thin film to spherical nanocrystals when the curing temperature increased from 200 to 300 C. The mechanism of curing process of the thin Au film embedded in the PVK layer producing spherical Au nanocrystals can be understood by considering the thermal expansion coefficient and material
Fig. 2. x-TEM images of polymer (PVK) memory-cell embedded with Au nanocrystals evaporated with w5-nm Au film followed by curing at (a) 200, (b) 250, and (c) 300 C.
J.-D. Lee et al. / Current Applied Physics 11 (2011) e25ee29
phase difference between PVK and Au films. The thermal expansion coefficient for the PVK film (5 105 K1) is approximately 3.5 times larger than that for Au film (1.4 105 K1), and the PVK film is in an amorphous phase while the Au film is in a poly-crystal phase, as shown in Fig. 2 [19]. The thermal expansion coefficient difference between the PVK and Au films during the curing process above 200 C would generate a severe strain at the interface between these films, thereby increasing the interfacial energy between the PVK and Au films. To release this increased interfacial energy, Au atoms migrate to the interface and then the uniform Au film becomes a coalescence film and finally is transformed into spherical Au nanocrystals to minimize the total surface energy at the interface between the PVK and Au films or Au nanocrystals. Note that these morphology changing processes could only be achieved based on the phase separation between the amorphous (PVK film) and poly-crystal (Au film) phases. Fig. 3 shows a magnified x-TEM image and chemical composition analysis using electron dispersion spectroscopy of a polymer nonvolatile 4F2 memory-cell fabricated with 5-nm Au film embedded in the PVK layer, followed by curing at 300 C for 2 h. The upper PVK layer, Au nanocrystals, and bottom PVK layer were 160, 8 and 127 nm thick, respectively. The Au nanocrystals located at the center of the PVK layer were uniformly distributed and well isolated from one another. The crystal structure of the Au nanocrystals was a face-centered cubic consisting of mainly (111), (200) and (311), see the m-diffraction pattern in the insert in Fig. 3. In addition, the black solid, red solid, blue open, and green open circles represent carbon, oxygen, Al, and Au, respectively. It is obvious that Au nanocrystals were not oxidized. The nonvolatile memory characteristics for the polymer memory-cell embedded with Au nanocrystals (w7.7 nm in diameter) cured at 300 C for 2 h are shown in Fig. 4. The dc IeV curve of the nonvolatile polymer memory-cell is shown in Fig. 4(a). First, the current slightly increased with the applied voltage, called the first low-current state (Ioff), when the applied bias was swept from 0 to the threshold voltage (Vth: 2.3 V). Then, the current rapidly increased with the applied bias when the applied bias was swept from Vth to the program voltage (Vp: 3.6 V). Afterward, the current decreased with increasing applied bias, called a negative
Fig. 3. Cross-sectional TEM image and energy dispersive X-ray spectroscopy (EDS) spectra of polymer nonvolatile memory-cell.
e27
differential resistance (NDR) region, when the applied bias was swept from Vp to the erase voltage (Ve: 6.2 V). Finally, the current slightly increased with the applied bias, called the second lowcurrent state, when the applied bias was swept above Ve. The current then followed the path of the second low-current state, NDR, and high-current state (called Ion) when the applied bias was swept from 8.0 V to 0 V. The sweeping of the applied bias from 0, 8.0, and 0 V evidently demonstrates nonvolatile memory characteristics. The memory margin (Ion/Ioff) of this memory-cell was w9.8 101. This memory-cell showed a symmetrical dc IeV curve when the applied bias was swept form 0, 8.0, and 0 V. However, this symmetrical dc IeV curve for the polymer memory-cell is different from that for a resistive random-access-memory cell
Fig. 4. Electrical characteristics of polymer (PVK) memory-cell embedded with Au nanocrystals: (a) dc IeV characteristic, (b) retention-time, and (c) program/erase endurance cycles.
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J.-D. Lee et al. / Current Applied Physics 11 (2011) e25ee29
[10e15]. For nonvolatile memory operation, the high-current state (Ion) could be obtained by reading at 1.0 V after programming by applying a bias of Vp (3.6 V). Otherwise, the low-current state (Ioff) could be obtained by reading at 1.0 V after erasing by applying a bias of above Ve (8.0 V). Fig. 4(b) shows the retention-time characteristic for the memory-cell. The Ion for reading at 1.0 V after programming by applying a bias of Vp (3.6 V) was w1.2 102 A, and the Ioff for reading at 1.0 V after erasing by applying a bias of above Ve (8.0 V) was w5.5 104 A. The retention-time with a memory margin (Ion/Ioff) of w2.1 101 was w1.0 105 s, which could be expanded to 10 years. Fig. 4(c) presents the endurance characteristics of the program and erases cycles obtained from another cell. The memory margin (Ion/Ioff) initially decreased from w6.9 101 and then saturated at w8.8 when the number of program and erase cycles increased to above 100. This initial decrease in the memory margin (Ion/Ioff) at early program/erase
cycles is probably associated with the unstable interface between Au nanocrystals and surrounding PVK. More detailed investigation is necessary to figure out this trend of program and erase cycles in Fig. 4(c). Fig. 5 shows the current conduction mechanism of the nonvolatile PVK layer embedded with Au nanocrystals. The low-current state (Ioff) from 0 V to Vth correlated well with the space-chargelimited-current (SCLC), which is proportional to Vn, where n is an index and V is the applied voltage, as shown in Fig. 5(a) [16]. In addition, both current conduction at Vth w Vp and NDR region correlated well with FowlereNordheim (FeN) tunneling, which is proportional to V2exp(c/V), where c is a constant corresponding to the tunneling barrier, as shown in Fig. 5(c) and (d) [17]. Furthermore, the second low-current conduction above Ve correlated with SCLC, as shown in Fig. 5(d). Finally, the high-current state (Ion) correlated with SCLC. The indexes (n) of the low-current state (Ioff)
a
b
c
d
e
Fig. 5. Current conduction mechanism for nonvolatile polymer (PVK) memory operation: (a) 0 w Vth (Ioff state), (b) Vth w Vp (electron charge on Au nanocrystals), (c) NDR region (electron discharge on Au nanocrystals), (d) above Ve , (e) 0 w Vp (Ion state).
J.-D. Lee et al. / Current Applied Physics 11 (2011) e25ee29
(1.53), second low-current state (1.56), and high-current state (Ion) were slightly different from one another since electron charge (Vth w Vp) and discharge (NDR region) states were slightly different from each other. In summary, the current conduction mechanism of the polymer (PVK) memory-cell embedded with Au nanocrystals is similar to that of the small-molecular memory-cell embedded with Ni nanocrystals surrounded with NiO [1]. 4. Conclusion The nonvolatile 4F2 polymer (PVK) memory-cell embedded with Au nanocrystals showed typical nonvolatile memory characteristics such as memory margin (Ion/Ioff) of 9.8 101, retention time of 1 105 s, and more than 100 program/erase endurance cycles. The structure of polymer memory-cell is similar to that of a smallmolecule memory-cell except the conductive organic material (i.e., polymer or small-molecule) and with or without a surrounding tunneling barrier (i.e., Au or Ni surrounded with a NiO tunneling barrier). However, nonvolatile memory characteristics for polymer memory-cell seem to be worse in memory margin and program/ erase endurance cycles than those of small-molecular memory-cell. However, polymer memory-cell would be good candidates for nonvolatile 4F2 cross-bar memory-cell with terra-bit level integration since the process temperature could be sustained up to 350 C. Therefore, further studies, such as on a stable hydrophilic interface between the polymer layer and Au nanocrystals and the implementation of a tunneling barrier surrounding Au nanocrystals, are necessary to improve nonvolatile memory characteristics such as memory margin (Ion/Ioff), retention-time, and endurance of program/erase cycles for polymer memory-cell [1,18].
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Acknowledgements This research was supported by “The National Research Program for Terabit Nonvolatile Memory Development” sponsored by the Korean Ministry of Knowledge Economy.
References [1] J.G. Park, W.S. Nam, S.H. Seo, Y.G. Kim, Y.H. Oh, G.S. Lee, U.G. Paik, Nano Lett. 9 (4) (2009) 1713. [2] L.P. Ma, J. Liu, Y. Yang, Appl. Phys. Lett. 80 (2002) 2997. [3] J.G. Park, G.S. Lee, K.S. Chae, J. Korean Phys. Soc. 48 (2006) 1505. [4] R.B. Kaner, J. Ouyang, Y. Yang, Appl. Phys. Lett. 90 (2007) 053101. [5] L.D. Bozano, B.W. Kean, M. Beinhoff, K.R. Carter, P.M. Rice, J.C. Scott, Adv. Funct. Mater. 15 (2005) 1933. [6] J.Y. Ouyang, C.W. Chu, C.R. Szmanda, L.P. Ma, Y. Yang, Nat. Mater. 3 (2004) 918. [7] M. Lauters, B. McCarthy, D. Sarid, Appl. Phys. Lett. 89 (2006) 013507. [8] C.W. Chu, J. Ouyang, J.H. Tseng, Y. Yang, Adv. Mater. 17 (2005) 1440. [9] J. Pielichowsjki, J. Therm. Anal. Calorim. 4 (1972) 339. [10] Data Sheet No. 263, Poly (N-vinylcarbazole). Polysciences, Inc, Warrington, Penn, 1990. [11] A. Sawa, T. Fujii, M. Kawasaki, Y. Tokura, Appl. Phys. Lett. 85 (2004) 4073. [12] Y.S. Lai, C.H. Tu, D.L. Kwong, Appl. Phys. Lett. 87 (2005) 122101. [13] S.S. Baek, D.J. Lee, S.H. Hong, O.Y. Kim, M.H. Ree, Adv. Func. Mater. 17 (2007) 2637. [14] Q.D. Ling, Y. Song, S.J. Ding, C. Zhu, D.S.H. Chan, D.L. Kwong, E.T. Kang, K.G. Neoh, Adv. Mater. 17 (2005) 455. [15] Q.D. Ling, D.J. Liaw, E.Y.H. Teo, C. Zhu, D.S.H. Chan, E.T. Kang, K.G. Neoh, Polymer 48 (2007) 5182. [16] J.G. Simmons, R.R. Verderber, Proc. R. Soc. Lond. A 301 (1967) 77. [17] S.M. Sze, K.K. Ng, Physic of Semiconductor Devices, third ed.. John Wiley & Sons Inc., New Jersey, 2007. [18] S.H. Seo, W.S. Nam, J.S. Kim, S.Y. Lee, T.H. Shim, J.G. Park, Curr. Appl. Phys. 10 (2010) e32. [19] Polymer Data Handbook, vol. 925, Oxford University Press, Inc., 1999.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Dependence of nonvolatile memory characteristics on curing temperature for polymer memory-cell embedded with Au nanocrystals in poly(N-vinylcarbazole) Jong-Dae Lee, Hyun-Min Seung, Kyoung-Cheol Kwon, Jea-gun Park* National Program Center for Tera-bit-level Nonvolatile Memory Development, HIT 101 Hanyang University, Seoul 133-791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 October 2010 Accepted 1 December 2010 Available online 1 February 2011
We investigated the dependence of nonvolatile memory characteristics on curing temperature for a polymer nonvolatile 4F2 memory-cell embedded with Au nanocrystals in poly(N-vinylcarbazole) (PVK). The curing temperature is an important factor for determining the size, shape, and isolation presence of Au nanocrystals embedded in the PVK layer. When the curing temperature rises above 300 C, w8-nm spherical Au nanocrystals are uniformly distributed and well isolated. Thus, the nonvolatile memory-cell demonstrated a memory margin of (Ion/Ioff) of 9.8 101, retention time of 1 105 s and more than 100 program/erase endurance cycles. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: Nonvolatile memory Nanocrystals Au PVK
1. Introduction
2. Experiments
Demand for very high-density integrated, low-cost, flexible devices has been increasing. Organic memory-cell is promising candidates for tera-bit level memory-cell because organic memorycell has shown a minimum device feature size of 4F2, the possibility of a flexible device, and fast access and storage of several tens of nanoseconds. Thus, many groups have reported organic memory-cell using small-molecule memory-cell [1e5] or polymer memory-cell [5e8]. In particular, it has been reported that small-molecule Alq3: Aluminun trise(8hydroxyquinoline) 4F2 memory-cell demonstrated nonvolatile multi-bit and multi-level cell operation [1]. For smallmolecule memory-cell, however, the temperature of post-processes, such as passivation and inter-metal connection, should be lower than approximately 50 C to prevent degradation of the smallmolecule layer. We developed a polymer (PVK) nonvolatile 4F2 memory-cell embedded with Au nanocrystals because PVK can be sustained above the process temperature of 350 C [9,10]. This memory-cell was fabricated with the PVK layer embedded with Au nanocrystals between the cross-bar top and bottom Al electrodes. In addition, the dependence of curing temperature on nonvolatile memory characteristics for memory-cell was investigated using high resolution cross-sectional transmission electron microscopy (x-TEM) and electrical measurements.
The polymer nonvolatile 4F2 memory-cell consisted of a device structure of a PVK layer embedded with Au nanocrystals between cross-bar top and bottom Al electrodes patterned by width of 1 mm, as shown in Fig. 1. First, the 80-nm bottom Al electrode on the silicon dioxide layer grown on the silicon substrate was thermally evaporated at an evaporation rate of 5.0 Å/s in a vacuum chamber. Then, the bottom PVK layer, dissolved using chloroform with 0.8 wt%, was spin-coated with a rotation velocity of 2000 rpm for 99 s and subsequently baked at 120 C for 2 min. Afterward, the 5-nm-thick Au film was thermally evaporated at an evaporation rate of 0.1 Å/s. The upper PVK layer was spin-coated with the same process as with the bottom PVK layer and was followed by curing at 200, 250 or 300 C for 2 h to produce Au nanocrystals embedded in the PVK layer. Finally, the top Al electrode was thermally evaporated using cross-patterned mask against the bottom Al electrode. The PVK (molecular weight: 700,000e1,100,000) was purchased from TCI (Tokyo Chemical Industry). The physical device structure was characterized using x-TEM for the cross-sectional TEM samples made using a focus-ion beam. The nonvolatile memory characteristics were estimated by measuring dc and ac current vs. voltage (IeV) using a computer-interfaced Agilent source-measure unit, model 4155C. 3. Results and discussion
* Corresponding author. Fax: þ82 2 2295 6868. E-mail address: [email protected] (J.-g. Park). 1567-1739/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.12.037
Fig. 2 shows the physical structures of the polymer memory-cell embedded with Au nanocrystals produced by various curing
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J.-D. Lee et al. / Current Applied Physics 11 (2011) e25ee29
Fig. 1. Nonvolatile 4F2 polymer (PVK) memory-cell embedded with Au nanocrystals: (a) perspective view of polymer memory-cell, (b) chemical structure of PVK.
temperatures. Fig. 2(a) presents an x-TEM image for a polymer memory-cell embedded with Au nanocrystals produced by curing at 200 C for 2 h. The uniform 5-nm Au film began to be agglomerated by curing at 200 C for 2 h. As a result, the agglomerated Au film thickness was w9.2 nm. Fig. 2(b) presents an x-TEM image of a polymer memory-cell embedded with Au nanocrystals produced by curing at 250 C for 2 h. The uniform 5-nm Au film by curing at 200 C for 2 h was transformed into dumbbell-shaped Au nanocrystals. The Au nanocrystals were not completely isolated from one another and their size was approximately w8.4 nm. Fig. 2(c) presents an x-TEM image of a polymer memory-cell embedded
with Au nanocrystals produced by curing at 300 C for 2 h. The uniform 5-nm Au film by curing at 300 C for 2 h was transformed into Au spherical nanocrystals. The Au nanocrystals were well isolated from one another and their size was approximately w7.8 nm. By comparing Fig. 2(a)e(c), the Au nanocrystal size decreased from w9.2 nm to w7.8 nm, and the Au crystal morphology changed from a thin film to spherical nanocrystals when the curing temperature increased from 200 to 300 C. The mechanism of curing process of the thin Au film embedded in the PVK layer producing spherical Au nanocrystals can be understood by considering the thermal expansion coefficient and material
Fig. 2. x-TEM images of polymer (PVK) memory-cell embedded with Au nanocrystals evaporated with w5-nm Au film followed by curing at (a) 200, (b) 250, and (c) 300 C.
J.-D. Lee et al. / Current Applied Physics 11 (2011) e25ee29
phase difference between PVK and Au films. The thermal expansion coefficient for the PVK film (5 105 K1) is approximately 3.5 times larger than that for Au film (1.4 105 K1), and the PVK film is in an amorphous phase while the Au film is in a poly-crystal phase, as shown in Fig. 2 [19]. The thermal expansion coefficient difference between the PVK and Au films during the curing process above 200 C would generate a severe strain at the interface between these films, thereby increasing the interfacial energy between the PVK and Au films. To release this increased interfacial energy, Au atoms migrate to the interface and then the uniform Au film becomes a coalescence film and finally is transformed into spherical Au nanocrystals to minimize the total surface energy at the interface between the PVK and Au films or Au nanocrystals. Note that these morphology changing processes could only be achieved based on the phase separation between the amorphous (PVK film) and poly-crystal (Au film) phases. Fig. 3 shows a magnified x-TEM image and chemical composition analysis using electron dispersion spectroscopy of a polymer nonvolatile 4F2 memory-cell fabricated with 5-nm Au film embedded in the PVK layer, followed by curing at 300 C for 2 h. The upper PVK layer, Au nanocrystals, and bottom PVK layer were 160, 8 and 127 nm thick, respectively. The Au nanocrystals located at the center of the PVK layer were uniformly distributed and well isolated from one another. The crystal structure of the Au nanocrystals was a face-centered cubic consisting of mainly (111), (200) and (311), see the m-diffraction pattern in the insert in Fig. 3. In addition, the black solid, red solid, blue open, and green open circles represent carbon, oxygen, Al, and Au, respectively. It is obvious that Au nanocrystals were not oxidized. The nonvolatile memory characteristics for the polymer memory-cell embedded with Au nanocrystals (w7.7 nm in diameter) cured at 300 C for 2 h are shown in Fig. 4. The dc IeV curve of the nonvolatile polymer memory-cell is shown in Fig. 4(a). First, the current slightly increased with the applied voltage, called the first low-current state (Ioff), when the applied bias was swept from 0 to the threshold voltage (Vth: 2.3 V). Then, the current rapidly increased with the applied bias when the applied bias was swept from Vth to the program voltage (Vp: 3.6 V). Afterward, the current decreased with increasing applied bias, called a negative
Fig. 3. Cross-sectional TEM image and energy dispersive X-ray spectroscopy (EDS) spectra of polymer nonvolatile memory-cell.
e27
differential resistance (NDR) region, when the applied bias was swept from Vp to the erase voltage (Ve: 6.2 V). Finally, the current slightly increased with the applied bias, called the second lowcurrent state, when the applied bias was swept above Ve. The current then followed the path of the second low-current state, NDR, and high-current state (called Ion) when the applied bias was swept from 8.0 V to 0 V. The sweeping of the applied bias from 0, 8.0, and 0 V evidently demonstrates nonvolatile memory characteristics. The memory margin (Ion/Ioff) of this memory-cell was w9.8 101. This memory-cell showed a symmetrical dc IeV curve when the applied bias was swept form 0, 8.0, and 0 V. However, this symmetrical dc IeV curve for the polymer memory-cell is different from that for a resistive random-access-memory cell
Fig. 4. Electrical characteristics of polymer (PVK) memory-cell embedded with Au nanocrystals: (a) dc IeV characteristic, (b) retention-time, and (c) program/erase endurance cycles.
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J.-D. Lee et al. / Current Applied Physics 11 (2011) e25ee29
[10e15]. For nonvolatile memory operation, the high-current state (Ion) could be obtained by reading at 1.0 V after programming by applying a bias of Vp (3.6 V). Otherwise, the low-current state (Ioff) could be obtained by reading at 1.0 V after erasing by applying a bias of above Ve (8.0 V). Fig. 4(b) shows the retention-time characteristic for the memory-cell. The Ion for reading at 1.0 V after programming by applying a bias of Vp (3.6 V) was w1.2 102 A, and the Ioff for reading at 1.0 V after erasing by applying a bias of above Ve (8.0 V) was w5.5 104 A. The retention-time with a memory margin (Ion/Ioff) of w2.1 101 was w1.0 105 s, which could be expanded to 10 years. Fig. 4(c) presents the endurance characteristics of the program and erases cycles obtained from another cell. The memory margin (Ion/Ioff) initially decreased from w6.9 101 and then saturated at w8.8 when the number of program and erase cycles increased to above 100. This initial decrease in the memory margin (Ion/Ioff) at early program/erase
cycles is probably associated with the unstable interface between Au nanocrystals and surrounding PVK. More detailed investigation is necessary to figure out this trend of program and erase cycles in Fig. 4(c). Fig. 5 shows the current conduction mechanism of the nonvolatile PVK layer embedded with Au nanocrystals. The low-current state (Ioff) from 0 V to Vth correlated well with the space-chargelimited-current (SCLC), which is proportional to Vn, where n is an index and V is the applied voltage, as shown in Fig. 5(a) [16]. In addition, both current conduction at Vth w Vp and NDR region correlated well with FowlereNordheim (FeN) tunneling, which is proportional to V2exp(c/V), where c is a constant corresponding to the tunneling barrier, as shown in Fig. 5(c) and (d) [17]. Furthermore, the second low-current conduction above Ve correlated with SCLC, as shown in Fig. 5(d). Finally, the high-current state (Ion) correlated with SCLC. The indexes (n) of the low-current state (Ioff)
a
b
c
d
e
Fig. 5. Current conduction mechanism for nonvolatile polymer (PVK) memory operation: (a) 0 w Vth (Ioff state), (b) Vth w Vp (electron charge on Au nanocrystals), (c) NDR region (electron discharge on Au nanocrystals), (d) above Ve , (e) 0 w Vp (Ion state).
J.-D. Lee et al. / Current Applied Physics 11 (2011) e25ee29
(1.53), second low-current state (1.56), and high-current state (Ion) were slightly different from one another since electron charge (Vth w Vp) and discharge (NDR region) states were slightly different from each other. In summary, the current conduction mechanism of the polymer (PVK) memory-cell embedded with Au nanocrystals is similar to that of the small-molecular memory-cell embedded with Ni nanocrystals surrounded with NiO [1]. 4. Conclusion The nonvolatile 4F2 polymer (PVK) memory-cell embedded with Au nanocrystals showed typical nonvolatile memory characteristics such as memory margin (Ion/Ioff) of 9.8 101, retention time of 1 105 s, and more than 100 program/erase endurance cycles. The structure of polymer memory-cell is similar to that of a smallmolecule memory-cell except the conductive organic material (i.e., polymer or small-molecule) and with or without a surrounding tunneling barrier (i.e., Au or Ni surrounded with a NiO tunneling barrier). However, nonvolatile memory characteristics for polymer memory-cell seem to be worse in memory margin and program/ erase endurance cycles than those of small-molecular memory-cell. However, polymer memory-cell would be good candidates for nonvolatile 4F2 cross-bar memory-cell with terra-bit level integration since the process temperature could be sustained up to 350 C. Therefore, further studies, such as on a stable hydrophilic interface between the polymer layer and Au nanocrystals and the implementation of a tunneling barrier surrounding Au nanocrystals, are necessary to improve nonvolatile memory characteristics such as memory margin (Ion/Ioff), retention-time, and endurance of program/erase cycles for polymer memory-cell [1,18].
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Acknowledgements This research was supported by “The National Research Program for Terabit Nonvolatile Memory Development” sponsored by the Korean Ministry of Knowledge Economy.
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