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Nonvolatile unipolar resistive switching behavior of amorphous BiFeO3 films
Accepted Manuscript Nonvolatile unipolar resistive switching behavior of amorphous BiFeO3 films Haoliang Deng, Ming Zhang, Tong Li, Jizhou Wei, Shangjie Chu, Minyong Du, Hui Yan PII: DOI: Reference:
S0925-8388(15)00823-3 http://dx.doi.org/10.1016/j.jallcom.2015.03.110 JALCOM 33721
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
15 February 2015 13 March 2015 14 March 2015
Please cite this article as: H. Deng, M. Zhang, T. Li, J. Wei, S. Chu, M. Du, H. Yan, Nonvolatile unipolar resistive switching behavior of amorphous BiFeO3 films, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/ 10.1016/j.jallcom.2015.03.110
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nonvolatile unipolar resistive switching behavior of amorphous BiFeO3 films
Haoliang Deng
a, b
, Ming Zhang a*, Tong Li c , Jizhou Wei a, Shangjie Chu a, Minyong Du a, Hui
Yan a
a
College of Materials Science and Engineering, Beijing University of Technology, Beijing, 100124, China
b
School of Science, Tianjin University of Technology and Education, Tianjin, 300222, China
c
School of Electronic Engineering, Tianjin University of Technology and Education, Tianjin, 300222,
China
ABSTRACT
Amorphous BiFeO3 thin films were grown on an indium tin oxide glass substrate using RF magnetron sputtering at room temperature. The resistive switching behavior of amorphous BiFeO3
thin films was investigated. The Au/amorphous BiFeO3/indium tin oxide device exhibited the stable unipolar resistive switching behavior with a substantial resistance ratio (larger than 102) between low and high resistance states and the excellent retention performance. The low resistance state exhibited a linear Ohmic behavior, while the high resistance state conduction could be ascribed to the trap controlled space-charge limited conduction mechanism. Based on the X-ray photoelectron spectroscopy, the observed resistive switching behavior was mainly attributed to the electric field
induced migration of oxygen vacancies.
Keywords: Amorphous BiFeO3; Thin films; Resistive switching; Oxygen vacancies
_________ * Corresponding author. E-mail address: [email protected] (M. Zhang)
1. Introduction With the advantages of high operation speed, high storage density, and low power consumption, the resistive random access memory (RRAM) based on the resistive switching (RS) effect has been regarded as one of the most promising candidates for next-generation nonvolatile memory [1, 2]. The resistive switching effect has been observed in many binary metal oxides [3, 4], and perovskite type oxides [5, 6]. BiFeO3 (BFO) is currently the most investigated multiferroic material, which shows distinctive properties such as large spontaneous polarization, high Curie temperature (TC ~ 1103 K), and high Neel temperature (TN ~ 643 K) [7, 8]. Moreover, the resistive switching effect was also found in BFO films. The resistive switching in polycrystalline BFO films was reported and ascribed to the redistribution of oxygen vacancies under electric field [9]. The polarization modulated switchable diode effect and the ferroelectric resistive switching behavior were observed in epitaxial BFO films [10]. Shuai et al. [11] showed that the control of oxygen vacancies concentration by varying oxygen partial pressure during film deposition plays an important role to achieve large switching effect. Recently, there have been more investigations on RRAM devices fabricated at low temperatures (≤ 200 ℃ ) due to their potential applications in the transparent flexible electronics [12, 13]. Furthermore, films grown at low temperatures are preferred for integrating the oxide materials with existing semiconductor device technology. Amorphous Pr0.7Ca0.3MnO3 films were grown at room temperature (RT), and their resistive switching properties were reported [14]. However, the resistive switching properties of amorphous BFO (ABFO) films have not been reported yet. In the present work, the ABFO films were prepared by using RF magnetron sputtering at RT and their structural and the resistive switching behavior were investigated.
2. Experimental details The amorphous BFO thin films were deposited on an indium tin oxide (ITO)/glass substrate by using the RF magnetron sputtering system. The deposition was performed at room temperature using a Bi1.1FeO3 ceramic target in the pure argon gas with a flow rate of 10 sccm. The working pressure and the power to sputter the BFO films were controlled at 1 Pa and 100W, respectively. X-ray diffraction (XRD) was used for structural and phase characterization. Thickness ~ 250nm for ABFO thin film was determined by scanning electron microscopy (SEM). X-ray photoelectron spectroscopy was applied to evaluate chemical bonding states of the elements in the films. In order to measure the electrical
properties of ABFO thin film, the Au top electrodes with 1mm diameter were deposited on the as-deposited ABFO thin film through a shadow mask using DC sputtering at room temperature. The current-voltage (I-V) and retention characteristics were measured using a Keithley 2636B source meter. The direction for current flowing from Au top electrode to ITO bottom electrode was defined as a positive direction. The compliance current for measuring RS characteristics was 10 mA to protect the device from permanent hard breakdown. All measurements were carried out at room temperature.
3. Results and discussion Fig. 1(a) shows the XRD patterns of as-deposited ABFO film grown at RT and ITO/glass substrate. Absence of any characteristic XRD peak of BFO can be observed indicating that the amorphous phase of BFO was developed in the film grown on the ITO/glass substrate. Fig. 1(b) shows a cross-sectional SEM image of the ABFO film grown on the ITO/glass substrate at RT. This image demonstrates that the 280 nm-thick ABFO film was well formed on the ITO electrode. Fig. 2 shows the typical current-voltage (I-V) characteristics of the Au/ABFO/ITO device obtained by applying unipolar voltage sweeps. Initially, the pristine film was found to be high resistance state (HRS). Under positive DC bias sweeping voltage, the current flowing through the device was found to be increase abruptly at around 14.4 V, as shown in the inset of Fig. 2. This is known as initial forming or electroforming process, in relevant to the defect induced soft dielectric breakdown. A compliance current of 1 mA was kept fixed during forming process to prevent the device from being damaged by the leakage current. After this step, the device turned to low resistance state (LRS). Now as the voltage was again swept from 0 to 1 V, a rapid decrease in current was observed at a voltage of ~ 0.4 V indicating the device was switched from LRS to HRS, which is called the “reset” process. The HRS was found to remain conserved even when the applied bias voltage was removed. In HRS, as the voltage was swept again from 0 to 2 V, a sudden increase in current was observed at ~ 1.12 V and the device was switched back into LRS, which is called the “set” progress. Thus, steady and reproducible unipolar RS behavior was demonstrated in the Au/ABFO/ITO device. To investigate the non-volatility of the unipolar RS effect, pulsed voltage measurements were carried out. Fig. 3 presents the retention performance of the Au/ABFO/ITO device under 0.1 V reading voltage with a delay time of 1 s for reading a current value at room temperature. The small reading pulse voltage was repeated one thousand times to read the resistance of both HRS and LRS,
respectively. Over a time period of ~ 103 s, the resistance values of HRS and LRS were found to be almost unchanged, which is an indicator of excellent device stability. The resistances ratio of HRS to LRS appears to be greater than 2 orders of magnitude at room temperature, which meets the requirement for distinguishing the two states in nonvolatile memory applications. To identify the conduction mechanism of the Au/ABFO/ITO device, the I-V curves are replotted in log-log scale in Fig. 4. Curve fitting is performed for both the LRS and HRS curves in various bias regimes. As can be seen in Fig. 4(a), the I-V curve for LRS exhibits a linear Ohmic behavior with a slope of 1.03, which is considered to be the result of the conductive filament during the forming process [15-17]. Fig. 4(b) presents the I-V curve for HRS and the slope of the curve fitting appears to be 1.07 at low fields. This phenomenon is brought about by ohmic conduction, and is similar to those in other amorphous metal oxides [16, 17]. The slope rises continuously until the device is switched to LRS, reaching 2.01 at higher voltages. This kind of I-V behavior could be explained based on the trap-controlled space-charge-limited current conduction (SCLC) mechanism [18]. According to the trap-controlled SCLC mechanism, the low voltage regime corresponds to Ohmic conduction (I-V), while in the higher voltage regime the conduction is dominated by the Child’s square law (I - V2). In the Child’s law region, the concentration of free electrons due to carrier injection greatly exceeds the equilibrium concentration in the BBFO thin film, and contributes to an increase in leakage current [19, 20]. Until now, several models have been developed to explain the RS behaviors of RRAM devices, such as charge trapping and detrapping, metal-insulator transition, and conductive filaments (CFs) [21-23]. In this study, the observed unipolar RS behavior in the Au/ABFO/ITO device could be interpreted by the CFs model [21, 22]. According to this model, the switching of resistance states of the corresponding device occurs through the creation and rupture of CFs created by the high density of defects, i.e., oxygen vacancies and/or metal interstitials in the film. For the BiFeO3 films, the chemical valance fluctuation of Fe is very sensitive to the oxygen vacancies because of the fact that the presence of Fe2+ ion is a prerequisite for charge compensation of oxygen vacancies [24]. Therefore, it is necessary to detect the ionic valences of the Fe in the films. The XPS analysis of as grown films was carried out. Fig. 5(a) shows the XPS spectra of Fe 2p of the ABFO film. Two main peaks for Fe 2p3/2 and Fe 2p1/2 can be seen in the inset of Fig. 5(a). Moreover, satellite peak is also identified, which is considered to be characteristic of the oxidation state of Fe. Fig. 5(a) illustrates the analysis of Fe 2p3/2
peak by Gaussian fitting. The Fe 2p3/2 peak position was determined to be at around 709.9 eV for Fe2+ ions and 7ll.2 eV for Fe3+ ions. In other words, it reveals the coexistence of Fe2+ ions and Fe3+ ions in the ABFO films. It was accepted that the electron hops between Fe 3+ ions and Fe2+ ions with
charged compensated oxygen vacancies. More amount of Fe2+ ions mean more amount of the oxygen vacancies [25]. Fig. 5(b) shows the XPS spectra of O 1s for the ABFO film. The O 1s peak can be fitted by two Gaussian curves. The peak centered at 529.3 eV was ascribed to oxygen ions bonded with metal ions (Bi and Fe) to form the metal oxides (labeled as OM). The other peak of higher binding energy centered at 530.6 eV was attributed to oxygen ions near oxygen vacancies (labeled as O V). It has been pointed out that in perovskite oxide thin films traps are mainly formed due to the oxygen vacancies [26]. These trapping centers and their physical separation were found to be responsible for transition between different resistance states due to the trapping and detrapping of charge carriers [27]. Hence, oxygen vacancies could be the key factor to provoke the switching of resistance states of the Au/ABFO/ITO device. During the initial forming process, the charged defects associated with oxygen vacancies in the ABFO film could be percolated in a network to form CFs under a high electric field, which serves as the conduction channels between the top and bottom electrodes. At the reset process, a large current flowing through the conductive filaments generates local Joule heating. When the heat accumulates to a certain level in the ABFO film, the out-diffusion of the oxygen vacancies causes the destruction of CFs, prompting the device to switch back to HRS. During the set process, the conduction channels through the oxygen vacancies can be re-established in the high applied electric field and the device is switched back to LRS again.
4. Conclusions In summary, ABFO films were grown on ITO/glass substrate using RF magnetron sputtering at room temperature. The resistive switching behavior of ABFO film was investigated. The Au/ABFO/ITO device showed nonvolatile unipolar resistive switching behavior with a resistance ratio larger than 102 as well as the excellent retention performance. The LRS exhibited a linear Ohmic behavior, while the conduction in the HRS could be attributed to the SCLC mechanism. The RS mechanism based on the conducting filaments model was suggested to explain the switching behavior of the ABFO film. The conductive filaments formation was mainly ascribed to the electric field
induced migration of oxygen vacancies. These results provide a possible application of ABFO film in
nonvolatile memory applications.
Acknowledgements This research was supported by the National Natural Science Foundation of China (Grant No. 11174021), and the Beijing Natural Science Foundation (Grant No. 2122007). References [1] A. Sawa, Mater. Today 11 (2008) 28-36. [2] R. Waser, R. Dittmann, G. Staikov, K. Szot, Adv. Mater. 21 (2009) 2632-2663. [3] K. M. Kim, B. J. Choi, Y. C. Shin, S. Choi, C. S. Huang, Appl. Phys. Lett. 91 (2007) 012907. [4] H. Zhang, L. Liu, B. Gao, Y. Qui, X. Liu, J. Lu, R. Han, J. Kang, B. Yu, Appl. Phys. Lett. 98 (2011) 042105. [5] Q. Liu, N. J. Wu, A. Ignatiev, Appl. Phys. Lett. 76 (2000) 2749. [6] A. Sawa, T. Fujii, M. Kawasaki, Y. Tokura, Appl. Phys. Lett. 85 (2004) 4073. [7] J. V. Rivera, H. Schmid, Ferroelectrics 204 (1997) 23-33. [8] J. Wang, J. B. Neaton, H. Zheng, V. Nagarajan, S. B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D. G. Schlom, U. V. Waghmare, N. A. Spaldin, K. M. Rabe, M. Wuttig, R. Ramesh, Science 299 (2003) 1719-1722. [9] K. Yin, M. Li, Y. Liu, C. He, F. Zhuge, B. Chen, W. Lu, X. Pan, R. W. Li, Appl. Phys. Lett. 97 (2010) 042101. [10] C. Wang, K. J. Jin, Z. Xu, L. Wang, C. Ge, H. Lu, H. Guo, M. He, G.Yang, Appl. Phys. Lett. 98 (2011) 192901. [11] Y. Shuai, S. Q. Zhou, D. Burger, M. Helm, H. Schmidt, J. Appl. Phys. 109 (2011) 124117. [12] J. W. Seo, J. W. Park, K. S. Lim, S. J. Kang, Y. H. Hong, J. H. Yang, L. Fang, G. Y. Sung, H. K. Kim, Appl. Phys. Lett. 95 (2009) 133508. [13] K. Nagashima, T. Yanagida, K. Oka, T. Kawai, Appl. Phys. Lett. 94 (2009) 242902. [14] Tae-Geun Seong, Kyu Bum Choi, In-Tae Seo, Joon-Ho Oh, Ji Won Moon, Kwon Hong, Sahn Nahm, Applied Physics Letters 100 (2012)212111. [15] H. Peng, T. Wu, Appl. Phys. Lett. 95 (2009) 152106. [16] J. S. Rajachidambaram, S. Murali, J. F. Conley, Jr., S. L. Golledge, G. S. Herman, J. Vac. Sci. Technol. B 31(2013) 01A104.
[17] B. Tulu, W. Z. Chang, J. P. Chu, S. F. Wang, Appl. Phys. Lett. 103 (2013) 252904. [18] Q. Liu, W. Guan, S. Long, R. Jia, M. Liu, J. Chen, Appl. Phys. Lett. 92 (2008) 012117. [19] H. Peng, T. Wu, Appl. Phys. Lett. 95 (2009) 152106. [20] C. H. Jung, S. I. Woo, Y. S. Kim, K. S. No, Thin Solid Films 519 (2011) 3291. [21] K. Szot, W. Speier, G. Bihlmayer, R. Waser, Nat. Mater. 5 (2006) 312. [22] J. P. Strachan, M. D. Pickett, J. J. Yang, S. Aloni, A. L. D. Kilcoyne, G. Medeiros-Ribeiro, R. S. Williams, Adv. Mater. 22 (2010) 3573. [23] J. W. Zhao, J. Sun, H. Q. Huang, F. J. Liu, Z. F. Hu, X. Q. Zhang, Appl. Surf. Sci. 258 (2012) 4588. [24] Y. Shuai, S. Zhou, D. Burger, M. Helm, H. Schmidt, J. Appl. Phys. 109 (2011) 124117. [25] H. Yang, Y.Q. Wang, H. Wang, Q.X. Jia, Appl. Phys. Lett. 96 (2010) 012909. [26] Y. Xia, W. He, L. Chen, X. Meng, Z. Liu, Appl. Phys. Lett. 90 (2007) 022907. [27] W. Z. Chang, J. P. Chu, S. F. Wang, Appl. Phys. Lett. 101 (2012) 012102.
Figure captions Fig. 1. (a) XRD patterns and (b) SEM cross-section image of the ABFO thin film grown on a ITO/Glass substrate at RT. Fig. 2. The I-V characteristic of the Au/ABFO/ITO device. Inset shows electroforming process. Fig. 3. The retention characteristic of the Au/ABFO/ITO device at room temperature. Fig. 4. I-V curve fitting for (a) LRS and (b) HRS of the Au/ABFO/ITO device in log-log scale. Fig. 5. The XPS spectra of (a) Fe 2p and (b) O 1s of as grown ABFO film, respectively.
Highlights
Amorphous BiFeO3 (ABFO) films was grown by magnetron sputtering at RT.
Nonvolatile unipolar resistive switching behavior in an Au/ABFO/ITO device.
High resistance ratio of over 102 and excellent retention performance.
oxygen vacancies
S0925-8388(15)00823-3 http://dx.doi.org/10.1016/j.jallcom.2015.03.110 JALCOM 33721
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
15 February 2015 13 March 2015 14 March 2015
Please cite this article as: H. Deng, M. Zhang, T. Li, J. Wei, S. Chu, M. Du, H. Yan, Nonvolatile unipolar resistive switching behavior of amorphous BiFeO3 films, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/ 10.1016/j.jallcom.2015.03.110
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nonvolatile unipolar resistive switching behavior of amorphous BiFeO3 films
Haoliang Deng
a, b
, Ming Zhang a*, Tong Li c , Jizhou Wei a, Shangjie Chu a, Minyong Du a, Hui
Yan a
a
College of Materials Science and Engineering, Beijing University of Technology, Beijing, 100124, China
b
School of Science, Tianjin University of Technology and Education, Tianjin, 300222, China
c
School of Electronic Engineering, Tianjin University of Technology and Education, Tianjin, 300222,
China
ABSTRACT
Amorphous BiFeO3 thin films were grown on an indium tin oxide glass substrate using RF magnetron sputtering at room temperature. The resistive switching behavior of amorphous BiFeO3
thin films was investigated. The Au/amorphous BiFeO3/indium tin oxide device exhibited the stable unipolar resistive switching behavior with a substantial resistance ratio (larger than 102) between low and high resistance states and the excellent retention performance. The low resistance state exhibited a linear Ohmic behavior, while the high resistance state conduction could be ascribed to the trap controlled space-charge limited conduction mechanism. Based on the X-ray photoelectron spectroscopy, the observed resistive switching behavior was mainly attributed to the electric field
induced migration of oxygen vacancies.
Keywords: Amorphous BiFeO3; Thin films; Resistive switching; Oxygen vacancies
_________ * Corresponding author. E-mail address: [email protected] (M. Zhang)
1. Introduction With the advantages of high operation speed, high storage density, and low power consumption, the resistive random access memory (RRAM) based on the resistive switching (RS) effect has been regarded as one of the most promising candidates for next-generation nonvolatile memory [1, 2]. The resistive switching effect has been observed in many binary metal oxides [3, 4], and perovskite type oxides [5, 6]. BiFeO3 (BFO) is currently the most investigated multiferroic material, which shows distinctive properties such as large spontaneous polarization, high Curie temperature (TC ~ 1103 K), and high Neel temperature (TN ~ 643 K) [7, 8]. Moreover, the resistive switching effect was also found in BFO films. The resistive switching in polycrystalline BFO films was reported and ascribed to the redistribution of oxygen vacancies under electric field [9]. The polarization modulated switchable diode effect and the ferroelectric resistive switching behavior were observed in epitaxial BFO films [10]. Shuai et al. [11] showed that the control of oxygen vacancies concentration by varying oxygen partial pressure during film deposition plays an important role to achieve large switching effect. Recently, there have been more investigations on RRAM devices fabricated at low temperatures (≤ 200 ℃ ) due to their potential applications in the transparent flexible electronics [12, 13]. Furthermore, films grown at low temperatures are preferred for integrating the oxide materials with existing semiconductor device technology. Amorphous Pr0.7Ca0.3MnO3 films were grown at room temperature (RT), and their resistive switching properties were reported [14]. However, the resistive switching properties of amorphous BFO (ABFO) films have not been reported yet. In the present work, the ABFO films were prepared by using RF magnetron sputtering at RT and their structural and the resistive switching behavior were investigated.
2. Experimental details The amorphous BFO thin films were deposited on an indium tin oxide (ITO)/glass substrate by using the RF magnetron sputtering system. The deposition was performed at room temperature using a Bi1.1FeO3 ceramic target in the pure argon gas with a flow rate of 10 sccm. The working pressure and the power to sputter the BFO films were controlled at 1 Pa and 100W, respectively. X-ray diffraction (XRD) was used for structural and phase characterization. Thickness ~ 250nm for ABFO thin film was determined by scanning electron microscopy (SEM). X-ray photoelectron spectroscopy was applied to evaluate chemical bonding states of the elements in the films. In order to measure the electrical
properties of ABFO thin film, the Au top electrodes with 1mm diameter were deposited on the as-deposited ABFO thin film through a shadow mask using DC sputtering at room temperature. The current-voltage (I-V) and retention characteristics were measured using a Keithley 2636B source meter. The direction for current flowing from Au top electrode to ITO bottom electrode was defined as a positive direction. The compliance current for measuring RS characteristics was 10 mA to protect the device from permanent hard breakdown. All measurements were carried out at room temperature.
3. Results and discussion Fig. 1(a) shows the XRD patterns of as-deposited ABFO film grown at RT and ITO/glass substrate. Absence of any characteristic XRD peak of BFO can be observed indicating that the amorphous phase of BFO was developed in the film grown on the ITO/glass substrate. Fig. 1(b) shows a cross-sectional SEM image of the ABFO film grown on the ITO/glass substrate at RT. This image demonstrates that the 280 nm-thick ABFO film was well formed on the ITO electrode. Fig. 2 shows the typical current-voltage (I-V) characteristics of the Au/ABFO/ITO device obtained by applying unipolar voltage sweeps. Initially, the pristine film was found to be high resistance state (HRS). Under positive DC bias sweeping voltage, the current flowing through the device was found to be increase abruptly at around 14.4 V, as shown in the inset of Fig. 2. This is known as initial forming or electroforming process, in relevant to the defect induced soft dielectric breakdown. A compliance current of 1 mA was kept fixed during forming process to prevent the device from being damaged by the leakage current. After this step, the device turned to low resistance state (LRS). Now as the voltage was again swept from 0 to 1 V, a rapid decrease in current was observed at a voltage of ~ 0.4 V indicating the device was switched from LRS to HRS, which is called the “reset” process. The HRS was found to remain conserved even when the applied bias voltage was removed. In HRS, as the voltage was swept again from 0 to 2 V, a sudden increase in current was observed at ~ 1.12 V and the device was switched back into LRS, which is called the “set” progress. Thus, steady and reproducible unipolar RS behavior was demonstrated in the Au/ABFO/ITO device. To investigate the non-volatility of the unipolar RS effect, pulsed voltage measurements were carried out. Fig. 3 presents the retention performance of the Au/ABFO/ITO device under 0.1 V reading voltage with a delay time of 1 s for reading a current value at room temperature. The small reading pulse voltage was repeated one thousand times to read the resistance of both HRS and LRS,
respectively. Over a time period of ~ 103 s, the resistance values of HRS and LRS were found to be almost unchanged, which is an indicator of excellent device stability. The resistances ratio of HRS to LRS appears to be greater than 2 orders of magnitude at room temperature, which meets the requirement for distinguishing the two states in nonvolatile memory applications. To identify the conduction mechanism of the Au/ABFO/ITO device, the I-V curves are replotted in log-log scale in Fig. 4. Curve fitting is performed for both the LRS and HRS curves in various bias regimes. As can be seen in Fig. 4(a), the I-V curve for LRS exhibits a linear Ohmic behavior with a slope of 1.03, which is considered to be the result of the conductive filament during the forming process [15-17]. Fig. 4(b) presents the I-V curve for HRS and the slope of the curve fitting appears to be 1.07 at low fields. This phenomenon is brought about by ohmic conduction, and is similar to those in other amorphous metal oxides [16, 17]. The slope rises continuously until the device is switched to LRS, reaching 2.01 at higher voltages. This kind of I-V behavior could be explained based on the trap-controlled space-charge-limited current conduction (SCLC) mechanism [18]. According to the trap-controlled SCLC mechanism, the low voltage regime corresponds to Ohmic conduction (I-V), while in the higher voltage regime the conduction is dominated by the Child’s square law (I - V2). In the Child’s law region, the concentration of free electrons due to carrier injection greatly exceeds the equilibrium concentration in the BBFO thin film, and contributes to an increase in leakage current [19, 20]. Until now, several models have been developed to explain the RS behaviors of RRAM devices, such as charge trapping and detrapping, metal-insulator transition, and conductive filaments (CFs) [21-23]. In this study, the observed unipolar RS behavior in the Au/ABFO/ITO device could be interpreted by the CFs model [21, 22]. According to this model, the switching of resistance states of the corresponding device occurs through the creation and rupture of CFs created by the high density of defects, i.e., oxygen vacancies and/or metal interstitials in the film. For the BiFeO3 films, the chemical valance fluctuation of Fe is very sensitive to the oxygen vacancies because of the fact that the presence of Fe2+ ion is a prerequisite for charge compensation of oxygen vacancies [24]. Therefore, it is necessary to detect the ionic valences of the Fe in the films. The XPS analysis of as grown films was carried out. Fig. 5(a) shows the XPS spectra of Fe 2p of the ABFO film. Two main peaks for Fe 2p3/2 and Fe 2p1/2 can be seen in the inset of Fig. 5(a). Moreover, satellite peak is also identified, which is considered to be characteristic of the oxidation state of Fe. Fig. 5(a) illustrates the analysis of Fe 2p3/2
peak by Gaussian fitting. The Fe 2p3/2 peak position was determined to be at around 709.9 eV for Fe2+ ions and 7ll.2 eV for Fe3+ ions. In other words, it reveals the coexistence of Fe2+ ions and Fe3+ ions in the ABFO films. It was accepted that the electron hops between Fe 3+ ions and Fe2+ ions with
charged compensated oxygen vacancies. More amount of Fe2+ ions mean more amount of the oxygen vacancies [25]. Fig. 5(b) shows the XPS spectra of O 1s for the ABFO film. The O 1s peak can be fitted by two Gaussian curves. The peak centered at 529.3 eV was ascribed to oxygen ions bonded with metal ions (Bi and Fe) to form the metal oxides (labeled as OM). The other peak of higher binding energy centered at 530.6 eV was attributed to oxygen ions near oxygen vacancies (labeled as O V). It has been pointed out that in perovskite oxide thin films traps are mainly formed due to the oxygen vacancies [26]. These trapping centers and their physical separation were found to be responsible for transition between different resistance states due to the trapping and detrapping of charge carriers [27]. Hence, oxygen vacancies could be the key factor to provoke the switching of resistance states of the Au/ABFO/ITO device. During the initial forming process, the charged defects associated with oxygen vacancies in the ABFO film could be percolated in a network to form CFs under a high electric field, which serves as the conduction channels between the top and bottom electrodes. At the reset process, a large current flowing through the conductive filaments generates local Joule heating. When the heat accumulates to a certain level in the ABFO film, the out-diffusion of the oxygen vacancies causes the destruction of CFs, prompting the device to switch back to HRS. During the set process, the conduction channels through the oxygen vacancies can be re-established in the high applied electric field and the device is switched back to LRS again.
4. Conclusions In summary, ABFO films were grown on ITO/glass substrate using RF magnetron sputtering at room temperature. The resistive switching behavior of ABFO film was investigated. The Au/ABFO/ITO device showed nonvolatile unipolar resistive switching behavior with a resistance ratio larger than 102 as well as the excellent retention performance. The LRS exhibited a linear Ohmic behavior, while the conduction in the HRS could be attributed to the SCLC mechanism. The RS mechanism based on the conducting filaments model was suggested to explain the switching behavior of the ABFO film. The conductive filaments formation was mainly ascribed to the electric field
induced migration of oxygen vacancies. These results provide a possible application of ABFO film in
nonvolatile memory applications.
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Figure captions Fig. 1. (a) XRD patterns and (b) SEM cross-section image of the ABFO thin film grown on a ITO/Glass substrate at RT. Fig. 2. The I-V characteristic of the Au/ABFO/ITO device. Inset shows electroforming process. Fig. 3. The retention characteristic of the Au/ABFO/ITO device at room temperature. Fig. 4. I-V curve fitting for (a) LRS and (b) HRS of the Au/ABFO/ITO device in log-log scale. Fig. 5. The XPS spectra of (a) Fe 2p and (b) O 1s of as grown ABFO film, respectively.
Highlights
Amorphous BiFeO3 (ABFO) films was grown by magnetron sputtering at RT.
Nonvolatile unipolar resistive switching behavior in an Au/ABFO/ITO device.
High resistance ratio of over 102 and excellent retention performance.
oxygen vacancies