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Annealing effect on the bipolar resistive switching memory of NiZn ferrite films
Journal of Alloys and Compounds 779 (2019) 794e799
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Annealing effect on the bipolar resistive switching memory of NiZn ferrite films Lei Wu a, b, c, *, Chunhui Dong d, **, Xiaoqiang Wang b, c, Jinsheng Li b, c, Mingya Li b, c a
School of Materials Science and Engineering, Northeastern University, Shenyang 110189, People's Republic of China School of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao 066004, People's Republic of China c Key Laboratory of Dielectric and Electrolyte Functional Material Hebei Province, Qinhuangdao 066004, People's Republic of China d China Electronics Technology Group Corporation No. 13 Research Institute, Shijiazhuang 050051, People's Republic of China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 October 2018 Received in revised form 24 November 2018 Accepted 26 November 2018 Available online 27 November 2018
A detailed understanding of the resistive switching behaviors and relevant physical mechanism is key to controlling nonvolatile memory devices. Pt/Ni0.5Zn0.5Fe2O4/Pt was synthesized by radio frequency magnetron sputtering method at room temperature. The typical bipolar resistive switching effects were detected in the annealed Ni0.5Zn0.5Fe2O4 thin films. Annealing effect on the bipolar resistive switching memory have been investigated. Good stability, identifiability, and excellent retention were obtained at the same time. Conductive filament mechanism, consisting of oxygen vacancies and reduced cations, was used to explain the physical mechanism in Pt/NZFO/Pt memory devices. The present results further enhance the applicability of spinel ferrite oxides in nonvolatile memory devices. © 2018 Elsevier B.V. All rights reserved.
Keywords: Spinel ferrite Ni0.5Zn0.5Fe2O4 thin film Resistive switching effect Conductive filament
1. Introduction Since the physical limitations of typical silicon-based flash memory devices, the high-performance nonvolatile memory is greatly demanded in modern information technology [1e3]. Resistive random access memory (RRAM), as one of the most promising candidates among the emerging nonvolatile memory technology, depending on its nonvolatility, ultrafast operating speeds, high scalability, and low power consumption, has attracted many attentions of researchers [4e6]. Based on resistive switching (RS) effect in metal/oxide/metal (MIM) sandwich configuration, the MIM memory cell can be switched reversibly between at least two different resistance states after electroforming, including highresistance state (HRS) and low-resistance state (LRS) [7]. According to the operating electrical polarity, RRAM can be classified into two types of RS effects. One type is an electrical polarity dependent “bipolar-switching” and another is a polarity independent ‘‘unipolar-switching’’. The physical mechanism of bipolar type is usually regarded as electrochemical migration or a redox reaction, and the
* Corresponding author. School of Materials Science and Engineering, Northeastern University, Shenyang 110189, People's Republic of China. ** Corresponding author. E-mail addresses: [email protected] (L. Wu), [email protected] (C. Dong). https://doi.org/10.1016/j.jallcom.2018.11.345 0925-8388/© 2018 Elsevier B.V. All rights reserved.
unipolar type is usually interpreted as Joule heating effect. At first, RS effect was investigated in several binary transition metal oxides such as ZnO, TiO2, and NiO [8e10]. Recently, some ferrites (NiFe2O4, CoFe2O4) and ferroelectrics materials (BaTiO3, BiFeO3, SrRuO3) were both found to exhibit the RS behavior [11e15]. Therein, the RS effects of ferrites were particularly attractive, which broaden its potential use in novel, multifunctional device applications. Hu et al. [16] investigated the unipolar RS properties of CoFe2O4 thin films and reported that the RS mechanism might be the formation and rupture of oxygen vacancies related filaments. Ismail et al. [17] studied the nonpolar switching characteristics of ZnFe2O4 thin film, where SET and RESET processes do not depend upon the polarity. The coexistence of unipolar and bipolar RS behaviors in NiFe2O4 thin film by doping Ag nanoparticles have been reported by Hao et al. [18], they suggest that the Joule heating effect and electrochemical redox reaction play a key role in the unipolar RS and bipolar RS modes. As one of the most versatile and technologically important ferrite materials, spinel ferrites have been intensively investigated for its rich electronic and magnetic properties. The emerging RS effects, combined with its electronic and magnetic properties, further enhance the possibility of ferrites-based nonvolatile multi-level memory devices. Thus, it is of significant importance to research spinel ferrites-based RS structures and elucidate the RS physical mechanism. In this study,
L. Wu et al. / Journal of Alloys and Compounds 779 (2019) 794e799
in order to further understand the underlying physical origin of RS effects, the annealing effects on bipolar RS memory devices have been investigated. The typical bipolar RS effects were detected in annealed Ni0.5Zn0.5Fe2O4 thin films. Good stability, identifiability, and excellent retention were obtained at the same time. Conductive filament mechanism, consisting of oxygen vacancies and reduced cations, was used to explain the physical mechanism in Pt/NZFO/Pt memory devices. The results will further broaden the development space of spinel ferrites materials.
2. Experimental details The NZFO films were deposited on Pt/Ti/SiO2/Si substrates under room temperature by radio frequency magnetron sputtering with a base pressure below 5 105 Pa. High-purity Ar (99.999%) and O2 (99.999%) with an Ar/O2 ratio of 1:4 were introduced into the chamber during deposition process, and th e sputtering pressure was 2.5 Pa. Thickness of the obtained films was approximately 200 nm. The deposited films were annealed at different temperatures for 1 h in ambient air. Circular Pt top electrodes with diameters of 200 mm were deposited on top of the as-prepared NZFO thin films using a shadow mask for electrical measurements. The as-synthesized samples were characterized by an X-ray diffractometer (XRD, X0 Pert PRO PHILIPS with Cu Ka radiation, l ¼ 1.54056 Å). The morphologies of the samples were characterized using scanning electron microscopy (SEM, Hitachi S4800). The static magnetic properties were measured by vibrating sample magnetometer (VSM EV9). The X-ray photoelectron spectroscopy (XPS) study was carried out using a Kratos Axis Ultra DLD photoelectron spectrometer with a monochromatic Al Ka source. A
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Keithley 2400 source measurement unit was employed to test the electrical properties of NZFO films. All tests were performed at room temperature.
3. Results and discussions The XRD pattern of unannealed NZFO thin film is exhibited in Fig. 1(a), pure spinel NZFO (JCPDS No. 86-0234) is detected, polycrystalline structures are obtained, and no other impurities peaks are found. Fig. 1(b) displays the surface SEM image of NZFO thin film, the NZFO thin film is consists of nanoparticles, with a relatively uniform and flat surface, the average grain size of NZFO thin film is around 20 nm. Fig. 1(d) shows the cross-sectional SEM images of NZFO/Pt heterostructure, the compact degree of cross section is fairly high. Both NZFO and Pt thin films are well grown on Si substrate, with thicknesses of approximately 200 nm and 100 nm, respectively. Magnetic property was measured using a vibrating sample magnetometer at room temperature (RT), and the M-H curve for NZFO thin film are presented in Fig. 1(c), the inset shows the scaling-up curves. The saturation magnetization was about 233 emu/cc and the coercivity was about 200 Oe. According to previous reports [19,20], the magnetic mechanism of ferrites can be explained on the basis of super exchange model. Tetrahedral sites are occupied by Zn2þ, while Fe3þ and Ni2þ ions occupy both tetrahedral and octahedral sites. The difference between the A-B sites gives the net magnetic moment to the material. Typical IeV characteristics of Pt/NZFO/Pt planar structure is shown in Fig. 2(a), here, the NZFO thin film was unannealed. The inset image of Fig. 2(a) depicts the schematic diagram of Pt/NZFO/Pt structure, Pt top electrodes with the diameter of 200 mm were
Fig. 1. (a) XRD pattern of NZFO thin film. (b) Surface SEM image of NZFO thin film. (c) The MeH curves of NZFO thin film measured at room temperature, the inset images show enlarged views in small fields. (d) The cross-sectional SEM image of NZFO/Pt/Si structures.
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Fig. 2. (a) The I-V curve of the unannealed NZFO thin film, the left inset image shows the schematic diagram of experiment, the right inset image is the I-V curve in semi-log scale. (b) The I-V characteristic of the NZFO thin film annealed at 600 C, the inset image is the I-V curve in semi-log scale and the measurement schematic diagram.
deposited on top of as-prepared NZFO thin films for electrical measurements. A continuous voltage sweep with a sequence 0 V/þ6 V/0 V/-6 V/0 V was applied. The IeV curve shows a symmetric feature rather than a rectifying one, the current hysteresis is owing to the leakage current [21]. In order to achieve the resistive switching behavior of Pt/NZFO/Pt structure, the NZFO thin films were annealed at different temperature. The filament-type resistive switching takes place in our devices after annealing process, the current suddenly increases at the forming voltage and the current compliance is essential for preventing the hard breakdown. As shown in Fig. 2(b), the typical bipolar resistive switching effect was detected after the electroforming process, the NZFO thin film was annealed at 600 C. Before applying voltage the films were insulating with a high resistance, during sweeping the bias voltage
from 0 to 5 V, the current suddenly increased to 10 mA when the applied voltage was 2.5 V. This “set” action turned the film to a low resistive state (LRS or ON state). The film held on the LRS until applied voltage reached 1.1 V, when a “reset” action occurred which turned the film to a high resistive state (HRS or OFF state). The OFF state of the sandwiched structures can be read and reprogrammed to the ON state again in the subsequent positive sweeps, thus completing the bipolar “write-read-erase-readrewrite” cycle for a non-volatile random access memory device. The bipolar resistive switching behaviors have been obtained in the annealed NZFO thin films (500 C, 600 C, 700 C, and 800 C), and the effect of annealed temperature would be illustrated in the following section. To investigate the time-retention characteristics of RS behaviors,
Fig. 3. Retention characteristics of the NZFO thin film switched in ON and OFF states. The NZFO thin film annealed at (a) 500 C, (b) 600 C, (c) 700 C, (d) 800 C.
L. Wu et al. / Journal of Alloys and Compounds 779 (2019) 794e799
the variations of the HRS and LRS read at 0.01 V as a function of time are displayed in Fig. 3. Fig. 3(a)-(d) display the HRS and LRS of the NZFO thin film annealed at 500 C, 600 C, 700 C and 800 C for 1 h, respectively. With the increasing of annealed temperature, the value of resistance remained relatively stable no matter in the HRS or LRS. However, the distributions of resistance were more concentrated at 600 C and 700 C, which indicated the repeatability of resistance was affected by the annealed temperature. When the annealed temperature is too high or too low, the dispersion of both the HRS and LRS will become wider. This result indicated the uniformity and stability of ions or defects distribution were decreased in the NZFO thin film annealed at 500 C and 800 C. Additional researches were needed to explore the reason for these changes. The repeatability of resistance is one of the most important parameters reflecting the performance of future nonvolatile memory devices. The ROFF/RON ratio of about 200 remains discernible until the end of the test, making the periphery circuit easy to distinguish the storage information. To further understand the underlying RS mechanism of the Pt/ NZFO/Pt structure, the conduction mechanisms in the LRS and HRS have been investigated in detail, from which transport properties of carriers can be understood deeply. Fig. 4(a) shows the typical IeV characteristics in the positive bias plotted in double-logarithmic scale. Evidently, the IeV curve presents a linear dependence on voltage with a slope of ~0.99 at the LRS, the Ohmic behavior is
Fig. 4. (a) I-V characteristics in the positive bias plotted in double-log scale, inset: the plot of log(I/V) vs. V1/2 at HRS, the red line is a linear fit, (b) shows the negative bias region. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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obeyed, consistent with the formation of conductive filaments during the set process. Whereas, the IeV curves at the HRS are complicated and can be divided into two regions. In the low voltage range of 0e0.6 V, the IeV relationship shows a linear dependence on the voltage, corresponding to the Ohmic region with a slope of ~1.08. In contrast, the IeV characteristics exhibits a nonlinear behavior with a slope of ~1.44 when V > 0.6 V. The plot of log(I/V) vs V1/2 at the HRS illustrated in the inset of Fig. 4(a) a shows a linear characteristic, according to previous reports [8,16,22], the conduction of the HRS can be described by the Poole-Frenkel (PeF) type hopping conduction probably caused by defects such as oxygen vacancy (VO). XPS results in Fig. 6 (c) and (d) confirmed the existence of VO in the annealed NZFO thin film. For the negative bias region, the IeV curve in both the LRS and HRS is similar to that of the positive bias region except for the transformational part of the LRS to HRS. XPS was used to further confirm the elemental composition and analyze the valence state of NZFO thin film. As shown in Fig. 5(a), the signals of Ni, Zn, Fe and O can be identified in the product, consistent with the formation of NZFO, wherein C 1s (284.6 eV) is used for calibration. Fig. 5(b) shows the high resolution core spectrum for Ni 2p, Ni 2p1/2 peak locates at 872.1 eV, and Ni 2p3/2 peak locates at 854.7 eV, indicating that the Ni ions are divalent [23]. Form Fig. 5(c), the signals at 1044.1 eV and 1020.9 eV can be attributed to Zn 2p1/2 and Zn 2p3/2 of Zn2þ [24,25]. Fig. 5(d) shows the high-resolution scan of the Fe 2p region, Fe 2p3/2 peak can be fitting into two peaks, which are 711.6 eV and 709.7 eV correspond to Fe3þ and Fe2þ states, respectively [26,27]. With the increasing of annealing temperature, the proportion of Fe2þ is enhanced, which is corresponding to the variation trend of the oxygen vacancies. Previous reports suggest that the valence change between Fe3þ and Fe2þ is responsible for the RS behaviors [12,28], and the cations with changed valence were the important component of conducting filaments in our thin films. Fig. 6(a) displays the characteristic spectrum for O 1s of unannealed and annealed (600 C, 800 C) NZFO thin film, the shape of O 1s spectrum was distinctly different before and after annealing process. By fitting with the Lorentzian-Gaussian function, Fig. 6(b)(d) shows the fitting results of O1s. For unannealed NZFO thin film, O 1s curve was isolated with three symmetric peaks centered at 529.6 eV, 531.3 eV and 532.7 eV, which were assigned to ordered lattice oxygen ions, oxygen vacancies, and adsorbed oxygen, respectively [26,29]. However, for annealed NZFO thin film, O 1s curve can be fitted by two symmetric peaks centered at 529.6 eV and 531.3 eV, the adsorbed oxygen was almost nonexistent, and the proportion of oxygen vacancies increased correspondingly. This result indicated that the amount of oxygen vacancies play a critical role in terms of resistive switching behavior, which is consistent with previous reports [11,30]. The observation of oxygen vacancies as well as the reduction of cations could cause the increase in electrical conductivity, the formation and rupture of oxygen vacancies and reduced cations related filament model could explain the physical mechanism in Pt/ NZFO/Pt memory devices. In the set process, as shown in Fig. 7(a), the oxygen vacancies along with the reduction of cations gave rise to excess electrons that get redistributed. Thus, the samples with oxygen vacancies exhibited increased conductivity, the conduction mechanism is dominated by Ohmic conduction in the LRS. In the reset process, as displayed in Fig. 7(b), the oxygen vacancies would annihilate because of the thermal effect and the changes in the valences of cations because of the redox effect, eventually leading to the rupture of the filaments, the device switched from the LRS to HRS. The Poole-Frenkel emission mechanism is dominant in the HRS, indicating the existence of partial oxygen defects in the thin film.
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Fig. 5. (a) XPS survey spectrum of unannealed and annealed NZFO thin film. Characteristic peaks of (b) Ni 2p. (c) Zn 2p. (d) Fe 2p.
Fig. 6. (a) Characteristic peaks of O1s. The fitting results of (b) unannealed, (c) annealed at 600 C, (d) annealed at 800 C NZFO thin film.
L. Wu et al. / Journal of Alloys and Compounds 779 (2019) 794e799
Fig. 7. Diagrams showing formation and rupture of conducting filaments in the (a) LRS and (b) HRS, respectively.
4. Conclusions In conclusion, Pt/Ni0.5Zn0.5Fe2O4/Pt was synthesized by radio frequency magnetron sputtering method at room temperature. The typical bipolar resistive switching effects were detected in the annealed Ni0.5Zn0.5Fe2O4 thin films. Annealing effect on the bipolar resistive switching memory device have been investigated. Good stability, identifiability, and excellent retention were obtained at the same time. Conductive filament mechanism, consisting of oxygen vacancies and reduced cations, was used to explain the physical mechanism in the Pt/NZFO/Pt memory devices. The present results further enhance the applicability of spinel ferrite oxides in nonvolatile memory devices. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant Nos. 51602045 and 51602042), the Fundamental Research Funds for the Central Universities (Grant Nos. N162304013), and the Natural Science Foundation of Hebei Province (Grant Nos. E2017501082 and E2018501042), and the Scientific Research Foundation of Northeastern University at Qinhuangdao (Grant No. XNB201716). References [1] A. Wedig, M. Luebben, D.Y. Cho, M. Moors, K. Skaja, V. Rana, T. Hasegawa, K.K. Adepalli, B. Yildiz, R. Waser, I. Valov, Nanoscale cation motion in TaOx, HfOx and TiOx memristive systems, Nat. Nanotechnol. 11 (2016) 67e74. [2] E.J. Yoo, M. Lyu, J.H. Yun, C.J. Kang, Y.J. Choi, L. Wang, Resistive switching behavior in organic-inorganic hybrid CH3NH3PbI3-xClx perovskite for resistive random access memory devices, Adv. Mater. 27 (2015) 6170e6175. [3] K. Cai, M. Yang, H. Ju, S. Wang, Y. Ji, B. Li, K.W. Edmonds, Y. Sheng, B. Zhang, N. Zhang, S. Liu, H. Zheng, K. Wang, Electric field control of deterministic current-induced magnetization switching in a hybrid ferromagnetic/ferroelectric structure, Nat. Mater. 16 (2017) 712e716. [4] P. Zhang, C. Gao, B. Xu, L. Qi, C. Jiang, M. Gao, D. Xue, Structural phase transition effect on resistive switching behavior of MoS2-polyvinylpyrrolidone nanocomposites films for flexible memory devices, Small 12 (2016) 2077e2084. [5] T. Yanagida, K. Nagashima, K. Oka, M. Kanai, A. Klamchuen, B.H. Park, T. Kawai, Scaling effect on unipolar and bipolar resistive switching of metal oxides, Sci. Rep. 3 (2013) 1657. [6] T.C. Chang, K.C. Chang, T.M. Tsai, T.J. Chu, S.M. Sze, Resistance random access memory, Mater. Today 19 (2016) 254e264.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Annealing effect on the bipolar resistive switching memory of NiZn ferrite films Lei Wu a, b, c, *, Chunhui Dong d, **, Xiaoqiang Wang b, c, Jinsheng Li b, c, Mingya Li b, c a
School of Materials Science and Engineering, Northeastern University, Shenyang 110189, People's Republic of China School of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao 066004, People's Republic of China c Key Laboratory of Dielectric and Electrolyte Functional Material Hebei Province, Qinhuangdao 066004, People's Republic of China d China Electronics Technology Group Corporation No. 13 Research Institute, Shijiazhuang 050051, People's Republic of China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 October 2018 Received in revised form 24 November 2018 Accepted 26 November 2018 Available online 27 November 2018
A detailed understanding of the resistive switching behaviors and relevant physical mechanism is key to controlling nonvolatile memory devices. Pt/Ni0.5Zn0.5Fe2O4/Pt was synthesized by radio frequency magnetron sputtering method at room temperature. The typical bipolar resistive switching effects were detected in the annealed Ni0.5Zn0.5Fe2O4 thin films. Annealing effect on the bipolar resistive switching memory have been investigated. Good stability, identifiability, and excellent retention were obtained at the same time. Conductive filament mechanism, consisting of oxygen vacancies and reduced cations, was used to explain the physical mechanism in Pt/NZFO/Pt memory devices. The present results further enhance the applicability of spinel ferrite oxides in nonvolatile memory devices. © 2018 Elsevier B.V. All rights reserved.
Keywords: Spinel ferrite Ni0.5Zn0.5Fe2O4 thin film Resistive switching effect Conductive filament
1. Introduction Since the physical limitations of typical silicon-based flash memory devices, the high-performance nonvolatile memory is greatly demanded in modern information technology [1e3]. Resistive random access memory (RRAM), as one of the most promising candidates among the emerging nonvolatile memory technology, depending on its nonvolatility, ultrafast operating speeds, high scalability, and low power consumption, has attracted many attentions of researchers [4e6]. Based on resistive switching (RS) effect in metal/oxide/metal (MIM) sandwich configuration, the MIM memory cell can be switched reversibly between at least two different resistance states after electroforming, including highresistance state (HRS) and low-resistance state (LRS) [7]. According to the operating electrical polarity, RRAM can be classified into two types of RS effects. One type is an electrical polarity dependent “bipolar-switching” and another is a polarity independent ‘‘unipolar-switching’’. The physical mechanism of bipolar type is usually regarded as electrochemical migration or a redox reaction, and the
* Corresponding author. School of Materials Science and Engineering, Northeastern University, Shenyang 110189, People's Republic of China. ** Corresponding author. E-mail addresses: [email protected] (L. Wu), [email protected] (C. Dong). https://doi.org/10.1016/j.jallcom.2018.11.345 0925-8388/© 2018 Elsevier B.V. All rights reserved.
unipolar type is usually interpreted as Joule heating effect. At first, RS effect was investigated in several binary transition metal oxides such as ZnO, TiO2, and NiO [8e10]. Recently, some ferrites (NiFe2O4, CoFe2O4) and ferroelectrics materials (BaTiO3, BiFeO3, SrRuO3) were both found to exhibit the RS behavior [11e15]. Therein, the RS effects of ferrites were particularly attractive, which broaden its potential use in novel, multifunctional device applications. Hu et al. [16] investigated the unipolar RS properties of CoFe2O4 thin films and reported that the RS mechanism might be the formation and rupture of oxygen vacancies related filaments. Ismail et al. [17] studied the nonpolar switching characteristics of ZnFe2O4 thin film, where SET and RESET processes do not depend upon the polarity. The coexistence of unipolar and bipolar RS behaviors in NiFe2O4 thin film by doping Ag nanoparticles have been reported by Hao et al. [18], they suggest that the Joule heating effect and electrochemical redox reaction play a key role in the unipolar RS and bipolar RS modes. As one of the most versatile and technologically important ferrite materials, spinel ferrites have been intensively investigated for its rich electronic and magnetic properties. The emerging RS effects, combined with its electronic and magnetic properties, further enhance the possibility of ferrites-based nonvolatile multi-level memory devices. Thus, it is of significant importance to research spinel ferrites-based RS structures and elucidate the RS physical mechanism. In this study,
L. Wu et al. / Journal of Alloys and Compounds 779 (2019) 794e799
in order to further understand the underlying physical origin of RS effects, the annealing effects on bipolar RS memory devices have been investigated. The typical bipolar RS effects were detected in annealed Ni0.5Zn0.5Fe2O4 thin films. Good stability, identifiability, and excellent retention were obtained at the same time. Conductive filament mechanism, consisting of oxygen vacancies and reduced cations, was used to explain the physical mechanism in Pt/NZFO/Pt memory devices. The results will further broaden the development space of spinel ferrites materials.
2. Experimental details The NZFO films were deposited on Pt/Ti/SiO2/Si substrates under room temperature by radio frequency magnetron sputtering with a base pressure below 5 105 Pa. High-purity Ar (99.999%) and O2 (99.999%) with an Ar/O2 ratio of 1:4 were introduced into the chamber during deposition process, and th e sputtering pressure was 2.5 Pa. Thickness of the obtained films was approximately 200 nm. The deposited films were annealed at different temperatures for 1 h in ambient air. Circular Pt top electrodes with diameters of 200 mm were deposited on top of the as-prepared NZFO thin films using a shadow mask for electrical measurements. The as-synthesized samples were characterized by an X-ray diffractometer (XRD, X0 Pert PRO PHILIPS with Cu Ka radiation, l ¼ 1.54056 Å). The morphologies of the samples were characterized using scanning electron microscopy (SEM, Hitachi S4800). The static magnetic properties were measured by vibrating sample magnetometer (VSM EV9). The X-ray photoelectron spectroscopy (XPS) study was carried out using a Kratos Axis Ultra DLD photoelectron spectrometer with a monochromatic Al Ka source. A
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Keithley 2400 source measurement unit was employed to test the electrical properties of NZFO films. All tests were performed at room temperature.
3. Results and discussions The XRD pattern of unannealed NZFO thin film is exhibited in Fig. 1(a), pure spinel NZFO (JCPDS No. 86-0234) is detected, polycrystalline structures are obtained, and no other impurities peaks are found. Fig. 1(b) displays the surface SEM image of NZFO thin film, the NZFO thin film is consists of nanoparticles, with a relatively uniform and flat surface, the average grain size of NZFO thin film is around 20 nm. Fig. 1(d) shows the cross-sectional SEM images of NZFO/Pt heterostructure, the compact degree of cross section is fairly high. Both NZFO and Pt thin films are well grown on Si substrate, with thicknesses of approximately 200 nm and 100 nm, respectively. Magnetic property was measured using a vibrating sample magnetometer at room temperature (RT), and the M-H curve for NZFO thin film are presented in Fig. 1(c), the inset shows the scaling-up curves. The saturation magnetization was about 233 emu/cc and the coercivity was about 200 Oe. According to previous reports [19,20], the magnetic mechanism of ferrites can be explained on the basis of super exchange model. Tetrahedral sites are occupied by Zn2þ, while Fe3þ and Ni2þ ions occupy both tetrahedral and octahedral sites. The difference between the A-B sites gives the net magnetic moment to the material. Typical IeV characteristics of Pt/NZFO/Pt planar structure is shown in Fig. 2(a), here, the NZFO thin film was unannealed. The inset image of Fig. 2(a) depicts the schematic diagram of Pt/NZFO/Pt structure, Pt top electrodes with the diameter of 200 mm were
Fig. 1. (a) XRD pattern of NZFO thin film. (b) Surface SEM image of NZFO thin film. (c) The MeH curves of NZFO thin film measured at room temperature, the inset images show enlarged views in small fields. (d) The cross-sectional SEM image of NZFO/Pt/Si structures.
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Fig. 2. (a) The I-V curve of the unannealed NZFO thin film, the left inset image shows the schematic diagram of experiment, the right inset image is the I-V curve in semi-log scale. (b) The I-V characteristic of the NZFO thin film annealed at 600 C, the inset image is the I-V curve in semi-log scale and the measurement schematic diagram.
deposited on top of as-prepared NZFO thin films for electrical measurements. A continuous voltage sweep with a sequence 0 V/þ6 V/0 V/-6 V/0 V was applied. The IeV curve shows a symmetric feature rather than a rectifying one, the current hysteresis is owing to the leakage current [21]. In order to achieve the resistive switching behavior of Pt/NZFO/Pt structure, the NZFO thin films were annealed at different temperature. The filament-type resistive switching takes place in our devices after annealing process, the current suddenly increases at the forming voltage and the current compliance is essential for preventing the hard breakdown. As shown in Fig. 2(b), the typical bipolar resistive switching effect was detected after the electroforming process, the NZFO thin film was annealed at 600 C. Before applying voltage the films were insulating with a high resistance, during sweeping the bias voltage
from 0 to 5 V, the current suddenly increased to 10 mA when the applied voltage was 2.5 V. This “set” action turned the film to a low resistive state (LRS or ON state). The film held on the LRS until applied voltage reached 1.1 V, when a “reset” action occurred which turned the film to a high resistive state (HRS or OFF state). The OFF state of the sandwiched structures can be read and reprogrammed to the ON state again in the subsequent positive sweeps, thus completing the bipolar “write-read-erase-readrewrite” cycle for a non-volatile random access memory device. The bipolar resistive switching behaviors have been obtained in the annealed NZFO thin films (500 C, 600 C, 700 C, and 800 C), and the effect of annealed temperature would be illustrated in the following section. To investigate the time-retention characteristics of RS behaviors,
Fig. 3. Retention characteristics of the NZFO thin film switched in ON and OFF states. The NZFO thin film annealed at (a) 500 C, (b) 600 C, (c) 700 C, (d) 800 C.
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the variations of the HRS and LRS read at 0.01 V as a function of time are displayed in Fig. 3. Fig. 3(a)-(d) display the HRS and LRS of the NZFO thin film annealed at 500 C, 600 C, 700 C and 800 C for 1 h, respectively. With the increasing of annealed temperature, the value of resistance remained relatively stable no matter in the HRS or LRS. However, the distributions of resistance were more concentrated at 600 C and 700 C, which indicated the repeatability of resistance was affected by the annealed temperature. When the annealed temperature is too high or too low, the dispersion of both the HRS and LRS will become wider. This result indicated the uniformity and stability of ions or defects distribution were decreased in the NZFO thin film annealed at 500 C and 800 C. Additional researches were needed to explore the reason for these changes. The repeatability of resistance is one of the most important parameters reflecting the performance of future nonvolatile memory devices. The ROFF/RON ratio of about 200 remains discernible until the end of the test, making the periphery circuit easy to distinguish the storage information. To further understand the underlying RS mechanism of the Pt/ NZFO/Pt structure, the conduction mechanisms in the LRS and HRS have been investigated in detail, from which transport properties of carriers can be understood deeply. Fig. 4(a) shows the typical IeV characteristics in the positive bias plotted in double-logarithmic scale. Evidently, the IeV curve presents a linear dependence on voltage with a slope of ~0.99 at the LRS, the Ohmic behavior is
Fig. 4. (a) I-V characteristics in the positive bias plotted in double-log scale, inset: the plot of log(I/V) vs. V1/2 at HRS, the red line is a linear fit, (b) shows the negative bias region. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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obeyed, consistent with the formation of conductive filaments during the set process. Whereas, the IeV curves at the HRS are complicated and can be divided into two regions. In the low voltage range of 0e0.6 V, the IeV relationship shows a linear dependence on the voltage, corresponding to the Ohmic region with a slope of ~1.08. In contrast, the IeV characteristics exhibits a nonlinear behavior with a slope of ~1.44 when V > 0.6 V. The plot of log(I/V) vs V1/2 at the HRS illustrated in the inset of Fig. 4(a) a shows a linear characteristic, according to previous reports [8,16,22], the conduction of the HRS can be described by the Poole-Frenkel (PeF) type hopping conduction probably caused by defects such as oxygen vacancy (VO). XPS results in Fig. 6 (c) and (d) confirmed the existence of VO in the annealed NZFO thin film. For the negative bias region, the IeV curve in both the LRS and HRS is similar to that of the positive bias region except for the transformational part of the LRS to HRS. XPS was used to further confirm the elemental composition and analyze the valence state of NZFO thin film. As shown in Fig. 5(a), the signals of Ni, Zn, Fe and O can be identified in the product, consistent with the formation of NZFO, wherein C 1s (284.6 eV) is used for calibration. Fig. 5(b) shows the high resolution core spectrum for Ni 2p, Ni 2p1/2 peak locates at 872.1 eV, and Ni 2p3/2 peak locates at 854.7 eV, indicating that the Ni ions are divalent [23]. Form Fig. 5(c), the signals at 1044.1 eV and 1020.9 eV can be attributed to Zn 2p1/2 and Zn 2p3/2 of Zn2þ [24,25]. Fig. 5(d) shows the high-resolution scan of the Fe 2p region, Fe 2p3/2 peak can be fitting into two peaks, which are 711.6 eV and 709.7 eV correspond to Fe3þ and Fe2þ states, respectively [26,27]. With the increasing of annealing temperature, the proportion of Fe2þ is enhanced, which is corresponding to the variation trend of the oxygen vacancies. Previous reports suggest that the valence change between Fe3þ and Fe2þ is responsible for the RS behaviors [12,28], and the cations with changed valence were the important component of conducting filaments in our thin films. Fig. 6(a) displays the characteristic spectrum for O 1s of unannealed and annealed (600 C, 800 C) NZFO thin film, the shape of O 1s spectrum was distinctly different before and after annealing process. By fitting with the Lorentzian-Gaussian function, Fig. 6(b)(d) shows the fitting results of O1s. For unannealed NZFO thin film, O 1s curve was isolated with three symmetric peaks centered at 529.6 eV, 531.3 eV and 532.7 eV, which were assigned to ordered lattice oxygen ions, oxygen vacancies, and adsorbed oxygen, respectively [26,29]. However, for annealed NZFO thin film, O 1s curve can be fitted by two symmetric peaks centered at 529.6 eV and 531.3 eV, the adsorbed oxygen was almost nonexistent, and the proportion of oxygen vacancies increased correspondingly. This result indicated that the amount of oxygen vacancies play a critical role in terms of resistive switching behavior, which is consistent with previous reports [11,30]. The observation of oxygen vacancies as well as the reduction of cations could cause the increase in electrical conductivity, the formation and rupture of oxygen vacancies and reduced cations related filament model could explain the physical mechanism in Pt/ NZFO/Pt memory devices. In the set process, as shown in Fig. 7(a), the oxygen vacancies along with the reduction of cations gave rise to excess electrons that get redistributed. Thus, the samples with oxygen vacancies exhibited increased conductivity, the conduction mechanism is dominated by Ohmic conduction in the LRS. In the reset process, as displayed in Fig. 7(b), the oxygen vacancies would annihilate because of the thermal effect and the changes in the valences of cations because of the redox effect, eventually leading to the rupture of the filaments, the device switched from the LRS to HRS. The Poole-Frenkel emission mechanism is dominant in the HRS, indicating the existence of partial oxygen defects in the thin film.
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Fig. 5. (a) XPS survey spectrum of unannealed and annealed NZFO thin film. Characteristic peaks of (b) Ni 2p. (c) Zn 2p. (d) Fe 2p.
Fig. 6. (a) Characteristic peaks of O1s. The fitting results of (b) unannealed, (c) annealed at 600 C, (d) annealed at 800 C NZFO thin film.
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Fig. 7. Diagrams showing formation and rupture of conducting filaments in the (a) LRS and (b) HRS, respectively.
4. Conclusions In conclusion, Pt/Ni0.5Zn0.5Fe2O4/Pt was synthesized by radio frequency magnetron sputtering method at room temperature. The typical bipolar resistive switching effects were detected in the annealed Ni0.5Zn0.5Fe2O4 thin films. Annealing effect on the bipolar resistive switching memory device have been investigated. Good stability, identifiability, and excellent retention were obtained at the same time. Conductive filament mechanism, consisting of oxygen vacancies and reduced cations, was used to explain the physical mechanism in the Pt/NZFO/Pt memory devices. The present results further enhance the applicability of spinel ferrite oxides in nonvolatile memory devices. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant Nos. 51602045 and 51602042), the Fundamental Research Funds for the Central Universities (Grant Nos. N162304013), and the Natural Science Foundation of Hebei Province (Grant Nos. E2017501082 and E2018501042), and the Scientific Research Foundation of Northeastern University at Qinhuangdao (Grant No. XNB201716). References [1] A. Wedig, M. Luebben, D.Y. Cho, M. Moors, K. Skaja, V. Rana, T. Hasegawa, K.K. Adepalli, B. Yildiz, R. Waser, I. Valov, Nanoscale cation motion in TaOx, HfOx and TiOx memristive systems, Nat. Nanotechnol. 11 (2016) 67e74. [2] E.J. Yoo, M. Lyu, J.H. Yun, C.J. Kang, Y.J. Choi, L. Wang, Resistive switching behavior in organic-inorganic hybrid CH3NH3PbI3-xClx perovskite for resistive random access memory devices, Adv. Mater. 27 (2015) 6170e6175. [3] K. Cai, M. Yang, H. Ju, S. Wang, Y. Ji, B. Li, K.W. Edmonds, Y. Sheng, B. Zhang, N. Zhang, S. Liu, H. Zheng, K. Wang, Electric field control of deterministic current-induced magnetization switching in a hybrid ferromagnetic/ferroelectric structure, Nat. Mater. 16 (2017) 712e716. [4] P. Zhang, C. Gao, B. Xu, L. Qi, C. Jiang, M. Gao, D. Xue, Structural phase transition effect on resistive switching behavior of MoS2-polyvinylpyrrolidone nanocomposites films for flexible memory devices, Small 12 (2016) 2077e2084. [5] T. Yanagida, K. Nagashima, K. Oka, M. Kanai, A. Klamchuen, B.H. Park, T. Kawai, Scaling effect on unipolar and bipolar resistive switching of metal oxides, Sci. Rep. 3 (2013) 1657. [6] T.C. Chang, K.C. Chang, T.M. Tsai, T.J. Chu, S.M. Sze, Resistance random access memory, Mater. Today 19 (2016) 254e264.
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