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High-resolution transmission electron microscopy study on bipolar resistive switching behavior in TiO2 thin films
Materials Science in Semiconductor Processing 15 (2012) 37–40
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
High-resolution transmission electron microscopy study on bipolar resistive switching behavior in TiO2 thin films Ying Li a,n, Gaoyang Zhao b, Xiaofei Zhou b, Lining Pan b, Yang Ren b a b
Advanced Material Analysis Center, Xi’an University of Technology, Box 759#, No. 5, Jinhua South Road, Xi’an, Shaanxi 710048, China Material Science and Engineering School, Xi’an University of Technology, Xi’an, Shaanxi 710048, China
a r t i c l e i n f o
abstract
Available online 23 July 2011
We fabricated TiO2 thin films the by sol–gel process. Successful I–V curves can be obtained in the Cu/TiO2/ATO structure device in which TiO2 thin film was calcined at 300 1C. The bipolar resistive switching behavior was observed and the ratio of Roff/Ron can be increased to 104. The switching voltage changes from 4.8 to 3.5 V when the current compliance drops from 10 to 0.1 mA. We also investigated the microstructure by HRTEM technology. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Resistive switching HRTEM TiO2 Sol–gel
1. Introduction Recently, the resistive switching behaviors of thin films, which are used in high density resistance random access memories (RRAMs) have shown great promise for the next generation nonvolatile memory [1–3]. The intensive research and development of RRAM were triggered by the nonvolatile memory. As for the resistive switching mechanism, there are several kinds of switching phenomena like redox effects in the resistance changes, filament formation and the percolation of the defects in the oxide [4,5]. Researchers used all kinds of techniques to proof the mechanism such as conductive atomic force microscopy (C-AFM), high resolution transmission electrical microscopy (HRTEM) and scanning electrical microscopy (SEM) [6,7]. In a recent study, we studied the resistive switching behaviors of the sol–gel processed TiO2 thin film in the metal–oxide–semiconductor structure. The bipolar resistive switching behavior can be observed successfully in this structure. 2. Experimental The whole process of fabrication is shown in Fig. 1. The substrate is self-made antimony tin oxide (ATO) glass n
Corresponding author. E-mail address: [email protected] (Y. Li).
1369-8001/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2011.07.001
substrate and ATO film is transparent and conductive with transmission 92.0% and resistivity 3.5 10 3 O cm. [8]. The ATO glass substrates were cleaned with propanol and acetone step by step. We used tetraethylorthosilicate (TBT) and ethanol (EtOH) as starting materials while using acetylacetone (AcAc) as a chemical modifier. The components TBT, EtOH and AcAc were mixed in terms of molar ratio 1:20:3. TiO2 solution could be obtained after stirring for 8 h. Then a dip-coating method was used to make a TiO2 gel film on the ATO glass substrate. After heat treating the gel films for 15 min, organic compounds were evaporated and 20 nm thick TiO2 thin films were obtained. Then the TiO2 thin film was covered by a mask and copper (99.99%) was evaporated by high vacuum thermal evaporation at a speed of 20 nm/min at 2.0 10 4 Pa for 1 min. Finally, the metal–oxide–semiconductor structure Cu 20 nm/TiO2 20 nm/ATO was achieved (see Ref. [9]). Finally, the metal–oxide–semiconductor structure Cu/ TiO2/ATO was achieved. All the electrical measurements were performed by Keithley 2400 analyzer at room temperature. The micro structural characterize was carried out by HRTEM (JEOL 3010, Japan). 3. Results and discussions For I–V measurements, probes contact top electrode (Cu) and bottom electrode (ATO), respectively. The bottom electrode was grounded. A Schottky barrier contact
38
Y. Li et al. / Materials Science in Semiconductor Processing 15 (2012) 37–40
Fig. 1. A schematic of the whole fabrication process.
Fig. 2. Typical I–V characteristics of the Cu/TiO2/ATO device.
exists between Cu and TiO2 films. A forming process is necessary to activate the Cu/TiO2/ATO structure at the fresh state. After that, stable and repeatable I–V curves can be observed. The TiO2 thin film in the structure was calcined at 300, 400 and 500 1C. All samples can show bipolar resistive switching [9]. However, the bipolar resistive switching characterization was the most stable when TiO2 thin film in the structure was calcined at 300 1C. Fig. 2 shows the I–V characteristics of the
Cu/TiO2/ATO structure in which the TiO2 thin film was calcined at 300 1C. The current compliance (CC) is applied to this sample at (a) 10, (b) 1 and (c) 0.1 mA. The bipolar resistive switching behavior was observed and the ratio of Roff/Ron can be increased to 104. For (a) I–V curve, when the voltage reaches 4.4 V, the current increases rapidly, which shows the switching of the OFF state to the ON state. During the second I–V sweep at the negative side, when the voltage reaches 4.2 V, the current decreases
Y. Li et al. / Materials Science in Semiconductor Processing 15 (2012) 37–40
rapidly, which shows the switching of the ON state to the OFF state. This behavior can be repeated more than several times. For (b) I–V curve, the current compliance is applied at 1 mA. When the voltage reaches 4.2 V, the current increases rapidly, which shows the switching of the OFF state to the ON state. During the second I–V sweep at the negative side, when the voltage reaches 2.2 V, the current decreases rapidly, which shows the switching of the ON state to the OFF state. This behavior can also be repeated. For (c) I–V curve, the current compliance is applied at 0.1 mA. When the voltage reaches 2.5 V, the current increases rapidly, which shows the switching of the OFF state to the ON state. During the second I–V sweep at the negative side, when the voltage reaches 0.3 V, the current decreases rapidly, which shows the switching of the ON state to the OFF state. This behavior can also be repeated. These three I–V curves all show bipolar resistive switching with different current compliances. The switching voltage changes from 4.4 to 2.5 V when the current compliance dropped from 10 to 0.1 mA. There are two main mechanisms concluded in literatures. One is the filamentary mechanism where the conducting path is formed by the oxygen vacancy or the top electrode metal ions. The other one is the effect of the interface in the metal–insulator–metal structure including the potential barrier height from the metal work function and the Fermi level of the oxide films. An explanation of this resistive switching behavior filamentary mechanism including metal filament and non-metal filament has been reported [10–12]. When TiO2 thin film contacts with top electrode Cu, the work function of Cu is 4.65 eV [13]. A low Schottky barrier contact makes the electron transfer between the metal and the oxide films easy. It is possible to break the Schottky barrier and to form conduction Cu-rich filament. According to Fig. 2, all I–V curves show typical bipolar resistive switching characteristics. The Cu ions go up and down through TiO2 film
39
Fig. 4. HRTEM images of Cu/TiO2/ATO structure device with the selected area diffraction (SAD) pattern.
Fig. 5. HRTEM images of Cu/TiO2/ATO structure device.
Fig. 3. Interface image of Cu/TiO2/ATO structure device.
when we applied positive and negative voltage, respectively. Therefore it shows bipolar resistive switch. For further investigation of microstructure, we took a single unit of the sample for HRTEM observations. A crosssection sample of Cu/TiO2/ATO structure device in which TiO2 thin film was calcined at 300 1C was successfully made by the low angel Argon milling machine (Gatan, USA). The milling angle was below 81. Micro structural analysis has taken on the foundation of HRTEM observations. Fig. 3 shows the interface image of Cu/TiO2/ATO sandwiched structure. This TEM image confirmed the formation of distinct interface layer between Cu top electrode and TiO2
40
Y. Li et al. / Materials Science in Semiconductor Processing 15 (2012) 37–40
thin film. The bottom electrode (BE) is the ATO film and the thickness is about 400 nm. The TiO2 work layer is about 50 nm and is amorphous according to the selected area diffraction pattern. The Cu top electrode (TE) is about 50 nm. This information agreed with the thickness parameters. We make a high resolution image of the interface in order to investigate the structure of the interlayer between TiO2 thin film and ATO bottom electrode. In Fig. 4, there are several SnO2 grains in the ATO thin film (BT). The inset shows selected area diffraction (SAD) pattern of SnO2 grain. It shows tetragonal phase and was in close agreement with the JCPDS cards No. 770452. [8]. The interlayer is highlighted by the shadow lines. The inset shows the SAD pattern image of the amorphous TiO2 thin film. Fig. 5 is also a high resolution image of the interface between amorphous TiO2 thin film and ATO bottom electrode. The image of the SnO2 crystal lattice can be also seen. 4. Conclusions In summary, we fabricated TiO2 thin films by sol–gel deposition. Successful Cu/TiO2/ATO structure device in which TiO2 thin film was calcined at 300 1C can be obtained. The bipolar resistive switching behavior was observed and the ratio of Roff/Ron can be increased to 104. The switching voltage changes from 4.8 to 3.5 V when the current compliance drops from 10 to 0.1 mA. We also investigate the switching device by HRTEM. TiO2 thin film is amorphous and the interfaces between the top electrode and TiO2 thin film were observed.
Acknowledgements This study was partly supported by the National Natural Science Foundation of China (No. 50772088). We also thank the Foundation of Excellent Doctoral Dissertation of Xian University of Technology and the Scientific Research Projects of Shaanxi Education Department (No. 11JK0806). References [1] Lin Chih-Y, Lee Dai-Y, Wang Sheng-Y, Lin Chun-C, Tseng Tseung-Y, Surface Coat Technology 203 (2008) 480–483. [2] Shima Hisashi, Tamai Yukio, Microelectronics Journal 40 (2009) 628–632. [3] M.J. Rozenberg, I.H. Inoue, M.J. Sanchez, Physical Review Letters 92 (17) (2004) 178302–178306. [4] B.J. Choi, D.S. Jeong, S.K. Kim, et al., Applied Physics Letters 98 (2005) 037715–037725. [5] R. Waser, Microelectronics Engineering 86 (2009) 1925–1928. [6] X. Guo, C. Schindler, M. Stephan, R. Waser, Applied Physics Letters 91 (13) (2007) 133513–133516. [7] C. Gopalan, M.N. Kozicki, S. Bhagat, et al., Journal of Non-Crystal Solids 353 (2007) 1844–1848. [8] X. Zhi, G. Zhao, T. Zhu, Y. Li, Surface and Interface Analysis 40 (2008) 67–70. [9] Y. Li, G. Zhao, X. Zhou, L. Pan, Y. Ren, Journal of Sol–Gel Science and Technology 56 (2010) 61–65. [10] J. Park, S. Jung, Microelectronic Engineering doi:10.1016/ j.mee.2011.03.050. [11] J.Y. Kim, H.Y. Jeong, Current Applied Physics doi:10.1016/ j.cap.2010.12.038. [12] Y.H. Do, J.S. Kwak, J.P. Hong, Thin Solids Films 518 (2010) 4408–4411. [13] W.G. Kim, S.W. Rhee, Microelectronic Engineering 87 (2010) 98–103.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
High-resolution transmission electron microscopy study on bipolar resistive switching behavior in TiO2 thin films Ying Li a,n, Gaoyang Zhao b, Xiaofei Zhou b, Lining Pan b, Yang Ren b a b
Advanced Material Analysis Center, Xi’an University of Technology, Box 759#, No. 5, Jinhua South Road, Xi’an, Shaanxi 710048, China Material Science and Engineering School, Xi’an University of Technology, Xi’an, Shaanxi 710048, China
a r t i c l e i n f o
abstract
Available online 23 July 2011
We fabricated TiO2 thin films the by sol–gel process. Successful I–V curves can be obtained in the Cu/TiO2/ATO structure device in which TiO2 thin film was calcined at 300 1C. The bipolar resistive switching behavior was observed and the ratio of Roff/Ron can be increased to 104. The switching voltage changes from 4.8 to 3.5 V when the current compliance drops from 10 to 0.1 mA. We also investigated the microstructure by HRTEM technology. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Resistive switching HRTEM TiO2 Sol–gel
1. Introduction Recently, the resistive switching behaviors of thin films, which are used in high density resistance random access memories (RRAMs) have shown great promise for the next generation nonvolatile memory [1–3]. The intensive research and development of RRAM were triggered by the nonvolatile memory. As for the resistive switching mechanism, there are several kinds of switching phenomena like redox effects in the resistance changes, filament formation and the percolation of the defects in the oxide [4,5]. Researchers used all kinds of techniques to proof the mechanism such as conductive atomic force microscopy (C-AFM), high resolution transmission electrical microscopy (HRTEM) and scanning electrical microscopy (SEM) [6,7]. In a recent study, we studied the resistive switching behaviors of the sol–gel processed TiO2 thin film in the metal–oxide–semiconductor structure. The bipolar resistive switching behavior can be observed successfully in this structure. 2. Experimental The whole process of fabrication is shown in Fig. 1. The substrate is self-made antimony tin oxide (ATO) glass n
Corresponding author. E-mail address: [email protected] (Y. Li).
1369-8001/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2011.07.001
substrate and ATO film is transparent and conductive with transmission 92.0% and resistivity 3.5 10 3 O cm. [8]. The ATO glass substrates were cleaned with propanol and acetone step by step. We used tetraethylorthosilicate (TBT) and ethanol (EtOH) as starting materials while using acetylacetone (AcAc) as a chemical modifier. The components TBT, EtOH and AcAc were mixed in terms of molar ratio 1:20:3. TiO2 solution could be obtained after stirring for 8 h. Then a dip-coating method was used to make a TiO2 gel film on the ATO glass substrate. After heat treating the gel films for 15 min, organic compounds were evaporated and 20 nm thick TiO2 thin films were obtained. Then the TiO2 thin film was covered by a mask and copper (99.99%) was evaporated by high vacuum thermal evaporation at a speed of 20 nm/min at 2.0 10 4 Pa for 1 min. Finally, the metal–oxide–semiconductor structure Cu 20 nm/TiO2 20 nm/ATO was achieved (see Ref. [9]). Finally, the metal–oxide–semiconductor structure Cu/ TiO2/ATO was achieved. All the electrical measurements were performed by Keithley 2400 analyzer at room temperature. The micro structural characterize was carried out by HRTEM (JEOL 3010, Japan). 3. Results and discussions For I–V measurements, probes contact top electrode (Cu) and bottom electrode (ATO), respectively. The bottom electrode was grounded. A Schottky barrier contact
38
Y. Li et al. / Materials Science in Semiconductor Processing 15 (2012) 37–40
Fig. 1. A schematic of the whole fabrication process.
Fig. 2. Typical I–V characteristics of the Cu/TiO2/ATO device.
exists between Cu and TiO2 films. A forming process is necessary to activate the Cu/TiO2/ATO structure at the fresh state. After that, stable and repeatable I–V curves can be observed. The TiO2 thin film in the structure was calcined at 300, 400 and 500 1C. All samples can show bipolar resistive switching [9]. However, the bipolar resistive switching characterization was the most stable when TiO2 thin film in the structure was calcined at 300 1C. Fig. 2 shows the I–V characteristics of the
Cu/TiO2/ATO structure in which the TiO2 thin film was calcined at 300 1C. The current compliance (CC) is applied to this sample at (a) 10, (b) 1 and (c) 0.1 mA. The bipolar resistive switching behavior was observed and the ratio of Roff/Ron can be increased to 104. For (a) I–V curve, when the voltage reaches 4.4 V, the current increases rapidly, which shows the switching of the OFF state to the ON state. During the second I–V sweep at the negative side, when the voltage reaches 4.2 V, the current decreases
Y. Li et al. / Materials Science in Semiconductor Processing 15 (2012) 37–40
rapidly, which shows the switching of the ON state to the OFF state. This behavior can be repeated more than several times. For (b) I–V curve, the current compliance is applied at 1 mA. When the voltage reaches 4.2 V, the current increases rapidly, which shows the switching of the OFF state to the ON state. During the second I–V sweep at the negative side, when the voltage reaches 2.2 V, the current decreases rapidly, which shows the switching of the ON state to the OFF state. This behavior can also be repeated. For (c) I–V curve, the current compliance is applied at 0.1 mA. When the voltage reaches 2.5 V, the current increases rapidly, which shows the switching of the OFF state to the ON state. During the second I–V sweep at the negative side, when the voltage reaches 0.3 V, the current decreases rapidly, which shows the switching of the ON state to the OFF state. This behavior can also be repeated. These three I–V curves all show bipolar resistive switching with different current compliances. The switching voltage changes from 4.4 to 2.5 V when the current compliance dropped from 10 to 0.1 mA. There are two main mechanisms concluded in literatures. One is the filamentary mechanism where the conducting path is formed by the oxygen vacancy or the top electrode metal ions. The other one is the effect of the interface in the metal–insulator–metal structure including the potential barrier height from the metal work function and the Fermi level of the oxide films. An explanation of this resistive switching behavior filamentary mechanism including metal filament and non-metal filament has been reported [10–12]. When TiO2 thin film contacts with top electrode Cu, the work function of Cu is 4.65 eV [13]. A low Schottky barrier contact makes the electron transfer between the metal and the oxide films easy. It is possible to break the Schottky barrier and to form conduction Cu-rich filament. According to Fig. 2, all I–V curves show typical bipolar resistive switching characteristics. The Cu ions go up and down through TiO2 film
39
Fig. 4. HRTEM images of Cu/TiO2/ATO structure device with the selected area diffraction (SAD) pattern.
Fig. 5. HRTEM images of Cu/TiO2/ATO structure device.
Fig. 3. Interface image of Cu/TiO2/ATO structure device.
when we applied positive and negative voltage, respectively. Therefore it shows bipolar resistive switch. For further investigation of microstructure, we took a single unit of the sample for HRTEM observations. A crosssection sample of Cu/TiO2/ATO structure device in which TiO2 thin film was calcined at 300 1C was successfully made by the low angel Argon milling machine (Gatan, USA). The milling angle was below 81. Micro structural analysis has taken on the foundation of HRTEM observations. Fig. 3 shows the interface image of Cu/TiO2/ATO sandwiched structure. This TEM image confirmed the formation of distinct interface layer between Cu top electrode and TiO2
40
Y. Li et al. / Materials Science in Semiconductor Processing 15 (2012) 37–40
thin film. The bottom electrode (BE) is the ATO film and the thickness is about 400 nm. The TiO2 work layer is about 50 nm and is amorphous according to the selected area diffraction pattern. The Cu top electrode (TE) is about 50 nm. This information agreed with the thickness parameters. We make a high resolution image of the interface in order to investigate the structure of the interlayer between TiO2 thin film and ATO bottom electrode. In Fig. 4, there are several SnO2 grains in the ATO thin film (BT). The inset shows selected area diffraction (SAD) pattern of SnO2 grain. It shows tetragonal phase and was in close agreement with the JCPDS cards No. 770452. [8]. The interlayer is highlighted by the shadow lines. The inset shows the SAD pattern image of the amorphous TiO2 thin film. Fig. 5 is also a high resolution image of the interface between amorphous TiO2 thin film and ATO bottom electrode. The image of the SnO2 crystal lattice can be also seen. 4. Conclusions In summary, we fabricated TiO2 thin films by sol–gel deposition. Successful Cu/TiO2/ATO structure device in which TiO2 thin film was calcined at 300 1C can be obtained. The bipolar resistive switching behavior was observed and the ratio of Roff/Ron can be increased to 104. The switching voltage changes from 4.8 to 3.5 V when the current compliance drops from 10 to 0.1 mA. We also investigate the switching device by HRTEM. TiO2 thin film is amorphous and the interfaces between the top electrode and TiO2 thin film were observed.
Acknowledgements This study was partly supported by the National Natural Science Foundation of China (No. 50772088). We also thank the Foundation of Excellent Doctoral Dissertation of Xian University of Technology and the Scientific Research Projects of Shaanxi Education Department (No. 11JK0806). References [1] Lin Chih-Y, Lee Dai-Y, Wang Sheng-Y, Lin Chun-C, Tseng Tseung-Y, Surface Coat Technology 203 (2008) 480–483. [2] Shima Hisashi, Tamai Yukio, Microelectronics Journal 40 (2009) 628–632. [3] M.J. Rozenberg, I.H. Inoue, M.J. Sanchez, Physical Review Letters 92 (17) (2004) 178302–178306. [4] B.J. Choi, D.S. Jeong, S.K. Kim, et al., Applied Physics Letters 98 (2005) 037715–037725. [5] R. Waser, Microelectronics Engineering 86 (2009) 1925–1928. [6] X. Guo, C. Schindler, M. Stephan, R. Waser, Applied Physics Letters 91 (13) (2007) 133513–133516. [7] C. Gopalan, M.N. Kozicki, S. Bhagat, et al., Journal of Non-Crystal Solids 353 (2007) 1844–1848. [8] X. Zhi, G. Zhao, T. Zhu, Y. Li, Surface and Interface Analysis 40 (2008) 67–70. [9] Y. Li, G. Zhao, X. Zhou, L. Pan, Y. Ren, Journal of Sol–Gel Science and Technology 56 (2010) 61–65. [10] J. Park, S. Jung, Microelectronic Engineering doi:10.1016/ j.mee.2011.03.050. [11] J.Y. Kim, H.Y. Jeong, Current Applied Physics doi:10.1016/ j.cap.2010.12.038. [12] Y.H. Do, J.S. Kwak, J.P. Hong, Thin Solids Films 518 (2010) 4408–4411. [13] W.G. Kim, S.W. Rhee, Microelectronic Engineering 87 (2010) 98–103.