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The enhancement of unipolar resistive switching behavior via an amorphous TiOx layer formation in Dy2O3-based forming-free RRAM
Solid-State Electronics 89 (2013) 12–16
Contents lists available at SciVerse ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
The enhancement of unipolar resistive switching behavior via an amorphous TiOx layer formation in Dy2O3-based forming-free RRAM Hongbin Zhao a, Hailing Tu a,b,⇑, Feng Wei a, Xinqiang Zhang a, Yuhua Xiong a, Jun Du a,b a b
Advanced Electronic Materials Institute, General Research Institute for Nonferrous Metals, Beijing 100088, People’s Republic of China National Engineering Research Center for Semiconductor Materials, General Research Institute for Nonferrous Metals, Beijing 100088, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 25 December 2012 Received in revised form 14 June 2013 Accepted 21 June 2013
Keywords: Dysprosium oxide Titanium oxide Resistive switching
a b s t r a c t We present effects of an amorphous TiOx layer formation on the behavior of a unipolar resistive switching memory device, which consists of Pt/Ti embedding layer (Ti-EL)/Dy2O3/Pt structure. The better properties have been obtained from the Pt/Ti-EL/Dy2O3/Pt system, including lower switching voltage, higher switching uniformity, and better endurance, besides, a reversed set/reset process is observed, in comparison with Pt/Dy2O3/Pt device. It is considered that the spontaneous formation of an amorphous TiOx layer and Ti–Pt–O nano-crystal clusters from Ti layer between Pt top electrode and Dy2O3 film is the main factor for the improvement of unipolar resistive switching behavior. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Over last decades, there has been increasing interest in the resistive switching random access memory (RRAM), due to its low power consumption, high switching speed, high data storage density, and high CMOS compatibility for the application of next generation nonvolatile memories [1–5]. In particular, the unipolar resistive switching (RS) characteristics of many oxides, such as TiO2 [6], NiO [7], and Gd2O3 [8], which depend on the amplitude of the applied voltage but not on the polarity, have attracted much attention due to the potential application on multi-stacked, high density, and simplified selector circuit structure [9,10]. Although many unipolar RS systems have shown promising aspects to be future nonvolatile memory [11,12], some certain important issues concerning unipolar RS, including the need of large forming bias for device initialization, the high program/erase voltage, and the non-uniformity in the distribution of switching parameters, are some of the critical unsolved problems [5]. Hence, improving performance of the unipolar devices is urgently desired. In recent years, rare-earth oxide materials have been studied for RRAM application because these materials exhibit unipolar ‘‘forming-free’’ behavior which can simplify the circuit design [13,14]. Among these rare-earth oxides, Dy2O3 is particular attractive for its advantages including large band gap, high resistivity, and dielec⇑ Corresponding author at: Advanced Electronic Materials Institute, General Research Institute for Nonferrous Metals, Beijing 100088, People’s Republic of China. Tel.: +86 10 82241242; fax: +86 10 82241246. E-mail address: [email protected] (H. Tu). 0038-1101/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.sse.2013.06.011
tric constant for future CMOS technology application [15]. In addition, Dy2O3 was reported to have a great potential for the application in RRAM recently [14]. It is reported that the introduction of active metal oxide interlayer in resistive switching devices has significant influence on the RS behaviors [16]. Up to date, the effects of amorphous metal oxide layer formation on bipolar RS behavior have been intensively studied [17,18], but these effects on rare-earth based RRAM materials with unipolar RS behavior have rarely been reported. Since Ti is well known as one of the most readily oxidizable metal, a thin Ti embedding layer (Ti-EL) is expected to modify the RS behavior with a typical Dy2O3-based RRAM through affecting oxygen-related defects distribution in rare-earth oxides film. In this paper, by embedding a Ti-EL to fabricate Pt/TiEL/Dy2O3/Pt structure device, we found that the Pt/Ti-EL/Dy2O3/Pt device performs a reversed set/reset process in comparison with Pt/Dy2O3/Pt structure device. Besides, excellent unipolar RS behavior including forming-free, low switching voltage, narrow switching voltage distribution, high uniformity of high/low resistance state, and good cycling endurance has also been obtained. The formation of TiOx layer, which plays a crucial role in improving the RS characteristics, is fully explored to be the key for RRAM design.
2. Experimental procedure The schematic illustrations of two-terminal devices studied in this work are depicted in Fig. 1a and b. A 120-nm-thick Pt bottom electrode (BE) was deposited by DC magnetron sputtering method. Between the Pt BE and SiO2/Si substrate, a thin Ti layer was in-
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Fig. 1. The cross-sectional schematic illustrations of the (a) Pt/Dy2O3/Pt and (b) Pt/Ti-EL/Dy2O3/Pt device structures. Unipolar RS of (c) Pt/Dy2O3/Pt and (d) Pt/Ti-EL/Dy2O3/Pt device. The inset shows distributions of operation voltage. A and C represent Vset and Vreset of Pt/Dy2O3/Pt devices; B and D represent Vset and Vreset of Pt/Ti-EL/Dy2O3/Pt devices, respectively. The data are from switching 200 cycles.
serted to enhance the adhesion of Pt BE on SiO2/Si substrate. Then, a 37-nm-thick Dy2O3 layer was deposited on Pt BE by radio frequency (RF) magnetron sputtering technique using Dy2O3 ceramic as sputtering target. During the deposition process, temperature and Ar + O2 working pressure (the oxygen partial pressure was 3%) were kept at room temperature and 2.5 Pa, respectively. A 5nm-thick Ti-EL was in-situ deposited on Dy2O3 film. Finally, a 15nm-thick Pt top electrode (TE) was deposited using DC magnetron sputtering method. For comparison, the Pt/Dy2O3/Pt device was prepared under the same process. The conventional photolithography method was used to define cell sizes with 100 lm 100 lm as mask. The deposited films were examined by a field emission highresolution transmission electron microscopy (HRTEM) (Tecnai F30). The X-ray photoelectron spectroscopy (XPS) (ESCALAB 250) was employed to determine the chemical bonding state of the Dy2O3 thin films. The RS behaviors of the device were characterized at room temperature using a Keithley-4200 semiconductor parameter analyzer in the I–V sweep mode. The switching behaviors of the samples were measured by applying a negative bias voltage to the top electrode with the bottom electrode grounded. 3. Results and discussion Fig. 1c and d shows typical I–V characteristics of the Pt/Dy2O3/Pt and Pt/Ti-EL/Dy2O3/Pt devices, respectively. They indicate that the resistance switches abruptly from high resistivity state (HRS) to low resistivity state (LRS) or LRS to HRS, which does not depend on the polarity, showing a typical unipolar RS characteristic. It is well known that most pristine oxides require an additional ‘‘electro-forming’’ process prior to exhibiting successful RS [19,20]. However, in our samples, the forming process was required neither in the Pt/Dy2O3/Pt devices nor in the Pt/Ti-EL/Dy2O3/Pt devices. It is worth noting that, although both devices exhibit ‘‘forming-free’’ behavior, the first RS cycle of the two devices is significantly differ-
ent. For the Pt/Dy2O3/Pt devices, the fresh memory devices are always at HRS, and the devices are switching from HRS to LRS by a relatively smaller bias sweep compared with ‘‘electro-forming’’ device. While for the Pt/TiOx/Dy2O3/Pt devices, the fresh memory devices are always at LRS and switch from LRS to HRS by a bias sweep, which is the same as the reported switching behavior in ‘‘forming-free’’ rare-earth oxides [13,14]. The operation voltage and switching uniformity of the Pt/Ti-EL/ Dy2O3/Pt devices are significantly different from those of the Pt/ Dy2O3/Pt devices, as shown in Fig. 1c and d. Firstly, the operation voltage of both set (Vset) and reset (Vreset) is visibly reduced. Secondly, the switching uniformity of the Pt/Ti-EL/Dy2O3/Pt RRAM devices becomes more stable. As is shown in the inset of Fig. 1d, the average values (standard deviations) of Vset and Vreset for the Pt/Ti/ Dy2O3/Pt devices are 0.54 (0.13) and 0.2 (0.01) V which are lower than 0.87 (0.26) V and 0.53 (0.04) V of the Pt/Dy2O3/Pt devices. In order to clarify the physical explanation of the improved switching properties of Pt/Ti-EL/Dy2O3/Pt devices, the chemical composition of Dy2O3 films with/without Ti-EL was analyzed by XPS. Fig. 2a shows the chemical states of Ti 2p3/2 region in the Pt/ Ti-EL/Dy2O3/Pt devices after Ar+ ion etching off Pt top electrode. The peak at 458.7 eV is assigned to the Ti4+ state, which indicates that because of oxygen migration from the Dy2O3 film to the Ti film, resulting in TiOx layer formation. Fig. 2b shows the chemical states of Dy 4d5/2 region in the Dy2O3 layer between Pt TE and Pt BE. The peaks at 153.2 eV and 156.5 eV are assigned to the metallic Dy0 state and the Dy3+ state, respectively. The content of metallic Dy in the oxide films can result in high concentration of oxygen vacancies, which contribute to the formation of the conductive filaments. Fig. 2c plots Dy 4d5/2 XPS spectra of Pt/Dy2O3/Pt (red line)1 and Pt/Ti-EL/Dy2O3/Pt (black line) structures obtained after Ar+ ion sput1 For interpretation of color in Figs. 2 and 3, the reader is referred to the web version of this article.
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Fig. 2. XPS spectra of (a) Ti 2p3/2 in Pt/Ti-EL/Dy2O3/Pt sample and (b) Dy 4d5/2 in Pt/Dy2O3/Pt sample. Comparison of the XPS spectra of (c) Dy 4d5/2 in Pt/Dy2O3/Pt and Pt/TiEL/Dy2O3/Pt samples and (d) Pt 4f in Pt and Ti-Pt-O of Pt/Ti-EL/Dy2O3/Pt sample.
tering, respectively. We can obviously observe a lower shift in the binding energy of the Dy 4d5/2 feature in Pt/Ti-EL/Dy2O3/Pt device. This is a clear hint on the existence of a less oxide component, which suggests a reduction in oxygen component in the Dy2O3 bulk layer of Pt/Ti-EL/Dy2O3/Pt device. We have further analyzed the Ti-Dy2O3 interface by HRTEM, and the result is shown in Fig. 3. A clear thin amorphous interface layer about 7.5 nm is found between Pt TE and Dy2O3 Layer and believed to be TiOx Combined with the XPS analysis in Fig. 2a. Furthermore, in the image shown in Fig. 3b, contrast regions marked with a red dotted line with the Pt layer and TiOx layer are remarkably observed and identified as Ti–Pt–O nano-crystal clusters. The HRTEM image shows that the contrast of Ti–Pt–O nano-crystal clusters is darker than the Ti4O7 layer and lighter than the Pt layer, which is consistent with the report about the formation of an amorphous Ti–Pt–O layer between a Ti4O7 layer and a Pt layer [21]. It was reported that a Pt layer deposited onto a clean TiO2 surface could form island, and it was also reported that metals with low formation
energy of oxides could be turn into sub-oxide upon contact with TiO2 [22,23]. The chemical state of Pt has been investigated by XPS to explore the formation of Ti–Pt–O nano-crystal clusters, as shown in Fig. 2d. The peaks of Pt 4f shift toward high binding energy with etching along depth profiles, which indicates the formation of Ti–Pt–O nano-crystal clusters. The data strongly support the formation of Ti–Pt–O nano-crystal clusters. For these reasons, we presume that the nano-crystal clusters marked with a red dotted line in Fig. 3b are Ti–Pt–O. And the formation of Ti–Pt–O nano-crystal clusters also proved that the TiOx layer formed before the Pt layer deposited. It is reported by Lin [24] et al that Ti as top electrode contacting with ZrO2 will cause the formation of a-TiOx and a thin ZrOx layer. In our HRTEM observation of Pt/Ti-EL/Dy2O3/Pt samples, however, the contrast difference between Dy2O3 and DyOx is indistinguishable. Thus, based on the results of XPS analysis and HRTEM image, we believe that the oxygen vacancies of the Dy2O3 film modified by
Fig. 3. (a) A low-magnification cross-section TEM image, and (b) HRTEM image for the Pt/Ti-EL/Dy2O3/Pt sample.
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Fig. 4. Illustration of switching mechanism for (a) Pt/Dy2O3/Pt and (b) Pt/Ti-EL/Dy2O3/Pt samples.
Ti-EL in Pt/Ti-EL/Dy2O3/Pt sample are distributed in the whole bulk region instead of the interface. The conductive filaments formation and rupture with local oxygen vacancies accumulation and depletion model are generally in agreement with the mechanism in the metal–insulator–metal structure RRAM, and the device with this mechanism usually shows unipolar behavior [25]. It has been reported that the formation of conductive filaments should be attributed to the localized agglomeration of the oxygen vacancies (VO ) in Dy2O3 film [14]. The formation of the oxygen vacancies filament is attributed to the migration of oxygen induced by the electric field and joule heat of the electric current. In Fig. 4a and b, we describe microscopic schematic diagrams of conductive filaments formation and rupture in the resistive switching process of both Pt/Dy2O3/Pt and Pt/Ti-EL/ Dy2O3/Pt devices. As shown in Fig. 4a, there are oxygen vacancies in the fresh Pt/Dy2O3/Pt devices, which were formed during the deposition process of Dy2O3 film. The oxygen vacancies are discretely distributed in the whole bulk region of the Dy2O3 film without any metallic conductive filaments formation, which results in the initial HRS. In the first set process, when voltage is applied to the device, the oxygen vacancies tend to drift toward the cathode. Owing to the drift in a short distance is out of order, metallic conductive filament can be randomly formed for the existing of large number of oxygen vacancies. Once the conductive filament formed, electric potential will mainly load on the conductive filament because the current flow mostly through the conductive filament,
which lead to the oxygen vacancies stop drifting. In the reset process, the rupture of the filament is occurring due to the heat generated by the large current flow. As opposed to the Pt/Dy2O3/Pt devices, the initial resistance state of the Pt/Ti-EL/Dy2O3/Pt devices is LRS, because of the metallic conductive filaments preexisting in the fresh device, as shown in Fig. 4b. We can draw a scenario of the resistive switching behavior as follows. After deposition of Ti film on the Dy2O3 film, Ti atoms attach to the surface of the Dy2O3. Owing to the strong oxygen affinity of Ti metal, oxygen ions from the Dy2O3 layer tends to migrate toward the interface between Ti film and Dy2O3, and resulting in forming amorphous TiOx interlayer. As the interface keeps extracting of the oxygen ions, the domains consisting of oxygen vacancies are generated continuously in the bulk region of the Dy2O3 film. With this creation of oxygen deficient domains, the formation process of conductive filaments that connect domains throughout the TE and BE region is achieved. Therefore, the initial resistance of the Pt/Ti-EL/Dy2O3/Pt devices reveals in LRS. The latter RS behavior of the set and reset process is same as Pt/Dy2O3/Pt devices. Based on the above analysis, the properties improvement of Pt/ Ti-EL/Dy2O3/Pt devices can be derived from the formation of the TiOx layer and the Ti–Pt–O nano-crystal clusters separately. (i) The TiOx layer formation strongly affects the distribution and movement of oxygen vacancies in the Dy2O3 film and results in the conductive filaments formation, which is the reason for the reverse in the set/reset process between Pt/Ti-EL/Dy2O3/Pt devices
Fig. 5. (a) Endurance characteristics of the Pt/Ti-EL/Dy2O3/Pt device. (b) Retention property of the HRS and LRS at room temperature.
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and Pt/Dy2O3/Pt devices. (ii) Ti–Pt–O nano-crystal clusters, formed from an amorphous TiOx layer and Pt electrode, act as a protrusion to reduce the effective voltage required for switching [26]. and (iii) Usually, the randomly formed conductive filaments result in the fluctuation of switching voltages. While for our Pt/Ti-EL/Dy2O3/Pt device, the formation of Ti–Pt–O nano-crystal clusters can draw strong electric field and lead to the formation and rupture of the conductive filaments reproducible at a fixed point [27], revealing an obvious improvement of switching uniformity. Furthermore, with the formation of TiOx interlayer as a diffusion barrier which blocks the oxygen inside the Dy2O3 layer infusion to the atmosphere, the endurance becomes better for the Pt/Ti-EL/ Dy2O3/Pt devices [28]. Endurance characteristic of the Pt/Ti-EL/ Dy2O3/Pt devices is given in Fig. 5a. Noting that, after 1000 successive switching cycles, the resistance ratio between HRS and LRS is still more than 106 without any degradation. Retention properties of Pt/Ti-EL/Dy2O3/Pt devices at room temperature are also investigated, which demonstrate the ability of nonvolatile memory devices to retain the data after writing the stored information. As seen from Fig. 5b, the resistance values of both states exhibit little change over 106 s. The good endurance and retention properties provide a perspective that the Pt/Ti-EL/Dy2O3/Pt memory devices are potentially suitable for nonvolatile memory applications. 4. Conclusions We have investigated the unipolar RS behavior of Dy2O3-based RRAM device by embedding a thin Ti layer between Pt TE and Dy2O3. A reversed set/reset process is observed, and the RS characteristics of the Pt/Ti-EL/Dy2O3/Pt device are greatly improved comparison with Pt/Dy2O3/Pt device. The improvement of the RS characteristics can be attributed to the formation of the amorphous TiOx layer which modifies the oxygen vacancies in the Dy2O3 RS layer, as well as Ti–Pt–O nano-crystal, as a protrusion, which reduces the effective electric field required for switching and draws strong electric field enhancement to reduce switching fluctuation. The Pt/Ti-EL/Dy2O3/Pt device with characteristics of forming-free property, low switching voltage, high switching uniformity, good endurance, and long retention time have the potential for future memory application.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 50932001, 51202013, 51102020), Important National Science & Technology Specific Projects (Grant No. 2009ZX02039-005). References [1] Beck A, Bednorz JG, Gerber C, Rosseel C, Widmer D. Appl Phys Lett 2000;77:139. [2] Liu SQ, Wu NJ, Ignatiev A. Appl Phys Lett 2000;76:2749. [3] Waser R, Aono M. Nat Mater 2007;6:833. [4] Burr GW, Kurdi BN, Scott JC, Lam CH. IBM J Res Dev 2008;52:449. [5] Waser R, Dittmann R, Staikov G, Szot K. Adv Mater 2009;21:2632. [6] Rohde C, Choi BJ, Jeong DS, Choi S, Zhao JS, Hwangb CS. Appl Phys Lett 2005;86:262907. [7] Lee MJ, Kim SI, Lee CB, Yin H, Ahn SE, Kang BS, et al. Adv Funct Mater 2009;19:1587. [8] Szot K, Speier W, Bihlmayer G, Waser R. Nat Mater 2006;5:312. [9] Chae SC, Lee JS, Kim S, Lee SB, Chang SH, Liu C, et al. Noh T W. Adv Mater 2008;20:6. [10] Kim KM, Jeong DS, Hwang CS. Nanotechnology 2011;22:254002. [11] Lee MJ, Han S, Jeon SH, Park BH, Kang BS, Ahn SE, et al. Nano Lett 2009;9:1476. [12] Kwon DH, Kim KM, Jang JH, Jeon JM, Lee MH, Kim GH, et al. Nat Nanotech 2010;5:148. [13] Cao X, Li XM, Gao XD, Yu WD, Liu XJ, Zhang YW, et al. J Appl Phys 2009;106:073723. [14] Pan TM, Lu CH. Appl Phys Lett 2011;99:113509. [15] Chang SC, Deng SY, Lee JYM. Appl Phys Lett 2006;89:053504. [16] Yu Y, Lee B, Wong HSP. IEEE Electron Dev Lett 2010;31:1449. [17] Liao ZL, Wang ZZ, Meng Y, Liu ZY, Gao P, Gang JL, et al. Appl Phys Lett 2009;94:253503. [18] Liu Z, Zhang P, Meng Y, Tian H, Li J, Pan X, et al. Appl Phys Lett 2012;100:143506. [19] Huang SY, Chang TC, Chen MC, Chen SC, Lo HP, Huang HC, et al. Solid State Electron 2011;63:189. [20] Jeong DS, Schroeder H, Waser R. Nanotechnology 2009;20:375201. [21] Ko DS, Kim SI, Ahn TY, Kim SD, Oh YH, Kima YW. Appl Phys Lett 2012;101:053502. [22] Diebold U. Surf Sci Rep 2003;48:53. [23] Yang JJ, Strachan JP, Mial F, Ahang MX, Pickett MD, Douglas WY, et al. Appl Phys A 2011;102:785. [24] Lin CY, Wu CY, Wu CY, Tsenga TY, Hu C. J Appl Phys 2007;102:094101. [25] Sawa A. Mater Today 2008;11:28. [26] Fujiwara K, Yajima T, Nakamura Y, Rozenberg MJ, Takagi H. Appl Phys Express 2009;2:081401. [27] Jung R, Lee MJ, Seo S, Kim DC, Park GS, Kim K, et al. Appl Phys Lett 2007;91:022112. [28] Shen W, Dittmann R, Breuer U, Waser R. Appl Phys Lett 2008;93:222102.
Contents lists available at SciVerse ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
The enhancement of unipolar resistive switching behavior via an amorphous TiOx layer formation in Dy2O3-based forming-free RRAM Hongbin Zhao a, Hailing Tu a,b,⇑, Feng Wei a, Xinqiang Zhang a, Yuhua Xiong a, Jun Du a,b a b
Advanced Electronic Materials Institute, General Research Institute for Nonferrous Metals, Beijing 100088, People’s Republic of China National Engineering Research Center for Semiconductor Materials, General Research Institute for Nonferrous Metals, Beijing 100088, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 25 December 2012 Received in revised form 14 June 2013 Accepted 21 June 2013
Keywords: Dysprosium oxide Titanium oxide Resistive switching
a b s t r a c t We present effects of an amorphous TiOx layer formation on the behavior of a unipolar resistive switching memory device, which consists of Pt/Ti embedding layer (Ti-EL)/Dy2O3/Pt structure. The better properties have been obtained from the Pt/Ti-EL/Dy2O3/Pt system, including lower switching voltage, higher switching uniformity, and better endurance, besides, a reversed set/reset process is observed, in comparison with Pt/Dy2O3/Pt device. It is considered that the spontaneous formation of an amorphous TiOx layer and Ti–Pt–O nano-crystal clusters from Ti layer between Pt top electrode and Dy2O3 film is the main factor for the improvement of unipolar resistive switching behavior. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Over last decades, there has been increasing interest in the resistive switching random access memory (RRAM), due to its low power consumption, high switching speed, high data storage density, and high CMOS compatibility for the application of next generation nonvolatile memories [1–5]. In particular, the unipolar resistive switching (RS) characteristics of many oxides, such as TiO2 [6], NiO [7], and Gd2O3 [8], which depend on the amplitude of the applied voltage but not on the polarity, have attracted much attention due to the potential application on multi-stacked, high density, and simplified selector circuit structure [9,10]. Although many unipolar RS systems have shown promising aspects to be future nonvolatile memory [11,12], some certain important issues concerning unipolar RS, including the need of large forming bias for device initialization, the high program/erase voltage, and the non-uniformity in the distribution of switching parameters, are some of the critical unsolved problems [5]. Hence, improving performance of the unipolar devices is urgently desired. In recent years, rare-earth oxide materials have been studied for RRAM application because these materials exhibit unipolar ‘‘forming-free’’ behavior which can simplify the circuit design [13,14]. Among these rare-earth oxides, Dy2O3 is particular attractive for its advantages including large band gap, high resistivity, and dielec⇑ Corresponding author at: Advanced Electronic Materials Institute, General Research Institute for Nonferrous Metals, Beijing 100088, People’s Republic of China. Tel.: +86 10 82241242; fax: +86 10 82241246. E-mail address: [email protected] (H. Tu). 0038-1101/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.sse.2013.06.011
tric constant for future CMOS technology application [15]. In addition, Dy2O3 was reported to have a great potential for the application in RRAM recently [14]. It is reported that the introduction of active metal oxide interlayer in resistive switching devices has significant influence on the RS behaviors [16]. Up to date, the effects of amorphous metal oxide layer formation on bipolar RS behavior have been intensively studied [17,18], but these effects on rare-earth based RRAM materials with unipolar RS behavior have rarely been reported. Since Ti is well known as one of the most readily oxidizable metal, a thin Ti embedding layer (Ti-EL) is expected to modify the RS behavior with a typical Dy2O3-based RRAM through affecting oxygen-related defects distribution in rare-earth oxides film. In this paper, by embedding a Ti-EL to fabricate Pt/TiEL/Dy2O3/Pt structure device, we found that the Pt/Ti-EL/Dy2O3/Pt device performs a reversed set/reset process in comparison with Pt/Dy2O3/Pt structure device. Besides, excellent unipolar RS behavior including forming-free, low switching voltage, narrow switching voltage distribution, high uniformity of high/low resistance state, and good cycling endurance has also been obtained. The formation of TiOx layer, which plays a crucial role in improving the RS characteristics, is fully explored to be the key for RRAM design.
2. Experimental procedure The schematic illustrations of two-terminal devices studied in this work are depicted in Fig. 1a and b. A 120-nm-thick Pt bottom electrode (BE) was deposited by DC magnetron sputtering method. Between the Pt BE and SiO2/Si substrate, a thin Ti layer was in-
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Fig. 1. The cross-sectional schematic illustrations of the (a) Pt/Dy2O3/Pt and (b) Pt/Ti-EL/Dy2O3/Pt device structures. Unipolar RS of (c) Pt/Dy2O3/Pt and (d) Pt/Ti-EL/Dy2O3/Pt device. The inset shows distributions of operation voltage. A and C represent Vset and Vreset of Pt/Dy2O3/Pt devices; B and D represent Vset and Vreset of Pt/Ti-EL/Dy2O3/Pt devices, respectively. The data are from switching 200 cycles.
serted to enhance the adhesion of Pt BE on SiO2/Si substrate. Then, a 37-nm-thick Dy2O3 layer was deposited on Pt BE by radio frequency (RF) magnetron sputtering technique using Dy2O3 ceramic as sputtering target. During the deposition process, temperature and Ar + O2 working pressure (the oxygen partial pressure was 3%) were kept at room temperature and 2.5 Pa, respectively. A 5nm-thick Ti-EL was in-situ deposited on Dy2O3 film. Finally, a 15nm-thick Pt top electrode (TE) was deposited using DC magnetron sputtering method. For comparison, the Pt/Dy2O3/Pt device was prepared under the same process. The conventional photolithography method was used to define cell sizes with 100 lm 100 lm as mask. The deposited films were examined by a field emission highresolution transmission electron microscopy (HRTEM) (Tecnai F30). The X-ray photoelectron spectroscopy (XPS) (ESCALAB 250) was employed to determine the chemical bonding state of the Dy2O3 thin films. The RS behaviors of the device were characterized at room temperature using a Keithley-4200 semiconductor parameter analyzer in the I–V sweep mode. The switching behaviors of the samples were measured by applying a negative bias voltage to the top electrode with the bottom electrode grounded. 3. Results and discussion Fig. 1c and d shows typical I–V characteristics of the Pt/Dy2O3/Pt and Pt/Ti-EL/Dy2O3/Pt devices, respectively. They indicate that the resistance switches abruptly from high resistivity state (HRS) to low resistivity state (LRS) or LRS to HRS, which does not depend on the polarity, showing a typical unipolar RS characteristic. It is well known that most pristine oxides require an additional ‘‘electro-forming’’ process prior to exhibiting successful RS [19,20]. However, in our samples, the forming process was required neither in the Pt/Dy2O3/Pt devices nor in the Pt/Ti-EL/Dy2O3/Pt devices. It is worth noting that, although both devices exhibit ‘‘forming-free’’ behavior, the first RS cycle of the two devices is significantly differ-
ent. For the Pt/Dy2O3/Pt devices, the fresh memory devices are always at HRS, and the devices are switching from HRS to LRS by a relatively smaller bias sweep compared with ‘‘electro-forming’’ device. While for the Pt/TiOx/Dy2O3/Pt devices, the fresh memory devices are always at LRS and switch from LRS to HRS by a bias sweep, which is the same as the reported switching behavior in ‘‘forming-free’’ rare-earth oxides [13,14]. The operation voltage and switching uniformity of the Pt/Ti-EL/ Dy2O3/Pt devices are significantly different from those of the Pt/ Dy2O3/Pt devices, as shown in Fig. 1c and d. Firstly, the operation voltage of both set (Vset) and reset (Vreset) is visibly reduced. Secondly, the switching uniformity of the Pt/Ti-EL/Dy2O3/Pt RRAM devices becomes more stable. As is shown in the inset of Fig. 1d, the average values (standard deviations) of Vset and Vreset for the Pt/Ti/ Dy2O3/Pt devices are 0.54 (0.13) and 0.2 (0.01) V which are lower than 0.87 (0.26) V and 0.53 (0.04) V of the Pt/Dy2O3/Pt devices. In order to clarify the physical explanation of the improved switching properties of Pt/Ti-EL/Dy2O3/Pt devices, the chemical composition of Dy2O3 films with/without Ti-EL was analyzed by XPS. Fig. 2a shows the chemical states of Ti 2p3/2 region in the Pt/ Ti-EL/Dy2O3/Pt devices after Ar+ ion etching off Pt top electrode. The peak at 458.7 eV is assigned to the Ti4+ state, which indicates that because of oxygen migration from the Dy2O3 film to the Ti film, resulting in TiOx layer formation. Fig. 2b shows the chemical states of Dy 4d5/2 region in the Dy2O3 layer between Pt TE and Pt BE. The peaks at 153.2 eV and 156.5 eV are assigned to the metallic Dy0 state and the Dy3+ state, respectively. The content of metallic Dy in the oxide films can result in high concentration of oxygen vacancies, which contribute to the formation of the conductive filaments. Fig. 2c plots Dy 4d5/2 XPS spectra of Pt/Dy2O3/Pt (red line)1 and Pt/Ti-EL/Dy2O3/Pt (black line) structures obtained after Ar+ ion sput1 For interpretation of color in Figs. 2 and 3, the reader is referred to the web version of this article.
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Fig. 2. XPS spectra of (a) Ti 2p3/2 in Pt/Ti-EL/Dy2O3/Pt sample and (b) Dy 4d5/2 in Pt/Dy2O3/Pt sample. Comparison of the XPS spectra of (c) Dy 4d5/2 in Pt/Dy2O3/Pt and Pt/TiEL/Dy2O3/Pt samples and (d) Pt 4f in Pt and Ti-Pt-O of Pt/Ti-EL/Dy2O3/Pt sample.
tering, respectively. We can obviously observe a lower shift in the binding energy of the Dy 4d5/2 feature in Pt/Ti-EL/Dy2O3/Pt device. This is a clear hint on the existence of a less oxide component, which suggests a reduction in oxygen component in the Dy2O3 bulk layer of Pt/Ti-EL/Dy2O3/Pt device. We have further analyzed the Ti-Dy2O3 interface by HRTEM, and the result is shown in Fig. 3. A clear thin amorphous interface layer about 7.5 nm is found between Pt TE and Dy2O3 Layer and believed to be TiOx Combined with the XPS analysis in Fig. 2a. Furthermore, in the image shown in Fig. 3b, contrast regions marked with a red dotted line with the Pt layer and TiOx layer are remarkably observed and identified as Ti–Pt–O nano-crystal clusters. The HRTEM image shows that the contrast of Ti–Pt–O nano-crystal clusters is darker than the Ti4O7 layer and lighter than the Pt layer, which is consistent with the report about the formation of an amorphous Ti–Pt–O layer between a Ti4O7 layer and a Pt layer [21]. It was reported that a Pt layer deposited onto a clean TiO2 surface could form island, and it was also reported that metals with low formation
energy of oxides could be turn into sub-oxide upon contact with TiO2 [22,23]. The chemical state of Pt has been investigated by XPS to explore the formation of Ti–Pt–O nano-crystal clusters, as shown in Fig. 2d. The peaks of Pt 4f shift toward high binding energy with etching along depth profiles, which indicates the formation of Ti–Pt–O nano-crystal clusters. The data strongly support the formation of Ti–Pt–O nano-crystal clusters. For these reasons, we presume that the nano-crystal clusters marked with a red dotted line in Fig. 3b are Ti–Pt–O. And the formation of Ti–Pt–O nano-crystal clusters also proved that the TiOx layer formed before the Pt layer deposited. It is reported by Lin [24] et al that Ti as top electrode contacting with ZrO2 will cause the formation of a-TiOx and a thin ZrOx layer. In our HRTEM observation of Pt/Ti-EL/Dy2O3/Pt samples, however, the contrast difference between Dy2O3 and DyOx is indistinguishable. Thus, based on the results of XPS analysis and HRTEM image, we believe that the oxygen vacancies of the Dy2O3 film modified by
Fig. 3. (a) A low-magnification cross-section TEM image, and (b) HRTEM image for the Pt/Ti-EL/Dy2O3/Pt sample.
H. Zhao et al. / Solid-State Electronics 89 (2013) 12–16
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Fig. 4. Illustration of switching mechanism for (a) Pt/Dy2O3/Pt and (b) Pt/Ti-EL/Dy2O3/Pt samples.
Ti-EL in Pt/Ti-EL/Dy2O3/Pt sample are distributed in the whole bulk region instead of the interface. The conductive filaments formation and rupture with local oxygen vacancies accumulation and depletion model are generally in agreement with the mechanism in the metal–insulator–metal structure RRAM, and the device with this mechanism usually shows unipolar behavior [25]. It has been reported that the formation of conductive filaments should be attributed to the localized agglomeration of the oxygen vacancies (VO ) in Dy2O3 film [14]. The formation of the oxygen vacancies filament is attributed to the migration of oxygen induced by the electric field and joule heat of the electric current. In Fig. 4a and b, we describe microscopic schematic diagrams of conductive filaments formation and rupture in the resistive switching process of both Pt/Dy2O3/Pt and Pt/Ti-EL/ Dy2O3/Pt devices. As shown in Fig. 4a, there are oxygen vacancies in the fresh Pt/Dy2O3/Pt devices, which were formed during the deposition process of Dy2O3 film. The oxygen vacancies are discretely distributed in the whole bulk region of the Dy2O3 film without any metallic conductive filaments formation, which results in the initial HRS. In the first set process, when voltage is applied to the device, the oxygen vacancies tend to drift toward the cathode. Owing to the drift in a short distance is out of order, metallic conductive filament can be randomly formed for the existing of large number of oxygen vacancies. Once the conductive filament formed, electric potential will mainly load on the conductive filament because the current flow mostly through the conductive filament,
which lead to the oxygen vacancies stop drifting. In the reset process, the rupture of the filament is occurring due to the heat generated by the large current flow. As opposed to the Pt/Dy2O3/Pt devices, the initial resistance state of the Pt/Ti-EL/Dy2O3/Pt devices is LRS, because of the metallic conductive filaments preexisting in the fresh device, as shown in Fig. 4b. We can draw a scenario of the resistive switching behavior as follows. After deposition of Ti film on the Dy2O3 film, Ti atoms attach to the surface of the Dy2O3. Owing to the strong oxygen affinity of Ti metal, oxygen ions from the Dy2O3 layer tends to migrate toward the interface between Ti film and Dy2O3, and resulting in forming amorphous TiOx interlayer. As the interface keeps extracting of the oxygen ions, the domains consisting of oxygen vacancies are generated continuously in the bulk region of the Dy2O3 film. With this creation of oxygen deficient domains, the formation process of conductive filaments that connect domains throughout the TE and BE region is achieved. Therefore, the initial resistance of the Pt/Ti-EL/Dy2O3/Pt devices reveals in LRS. The latter RS behavior of the set and reset process is same as Pt/Dy2O3/Pt devices. Based on the above analysis, the properties improvement of Pt/ Ti-EL/Dy2O3/Pt devices can be derived from the formation of the TiOx layer and the Ti–Pt–O nano-crystal clusters separately. (i) The TiOx layer formation strongly affects the distribution and movement of oxygen vacancies in the Dy2O3 film and results in the conductive filaments formation, which is the reason for the reverse in the set/reset process between Pt/Ti-EL/Dy2O3/Pt devices
Fig. 5. (a) Endurance characteristics of the Pt/Ti-EL/Dy2O3/Pt device. (b) Retention property of the HRS and LRS at room temperature.
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and Pt/Dy2O3/Pt devices. (ii) Ti–Pt–O nano-crystal clusters, formed from an amorphous TiOx layer and Pt electrode, act as a protrusion to reduce the effective voltage required for switching [26]. and (iii) Usually, the randomly formed conductive filaments result in the fluctuation of switching voltages. While for our Pt/Ti-EL/Dy2O3/Pt device, the formation of Ti–Pt–O nano-crystal clusters can draw strong electric field and lead to the formation and rupture of the conductive filaments reproducible at a fixed point [27], revealing an obvious improvement of switching uniformity. Furthermore, with the formation of TiOx interlayer as a diffusion barrier which blocks the oxygen inside the Dy2O3 layer infusion to the atmosphere, the endurance becomes better for the Pt/Ti-EL/ Dy2O3/Pt devices [28]. Endurance characteristic of the Pt/Ti-EL/ Dy2O3/Pt devices is given in Fig. 5a. Noting that, after 1000 successive switching cycles, the resistance ratio between HRS and LRS is still more than 106 without any degradation. Retention properties of Pt/Ti-EL/Dy2O3/Pt devices at room temperature are also investigated, which demonstrate the ability of nonvolatile memory devices to retain the data after writing the stored information. As seen from Fig. 5b, the resistance values of both states exhibit little change over 106 s. The good endurance and retention properties provide a perspective that the Pt/Ti-EL/Dy2O3/Pt memory devices are potentially suitable for nonvolatile memory applications. 4. Conclusions We have investigated the unipolar RS behavior of Dy2O3-based RRAM device by embedding a thin Ti layer between Pt TE and Dy2O3. A reversed set/reset process is observed, and the RS characteristics of the Pt/Ti-EL/Dy2O3/Pt device are greatly improved comparison with Pt/Dy2O3/Pt device. The improvement of the RS characteristics can be attributed to the formation of the amorphous TiOx layer which modifies the oxygen vacancies in the Dy2O3 RS layer, as well as Ti–Pt–O nano-crystal, as a protrusion, which reduces the effective electric field required for switching and draws strong electric field enhancement to reduce switching fluctuation. The Pt/Ti-EL/Dy2O3/Pt device with characteristics of forming-free property, low switching voltage, high switching uniformity, good endurance, and long retention time have the potential for future memory application.
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