- Email: [email protected]
TiO2 for resistive random access memory
Journal of Alloys and Compounds 722 (2017) 753e759
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Mechanism for an enhanced resistive switching effect of bilayer NiOx/TiO2 for resistive random access memory Guangdong Zhou a, b, c, *, Lihua Xiao a, Shuangju Zhang a, Bo Wu d, Xiaoqin Liu a, Ankun Zhou e a
Guizhou Institute of Technology, Guizhou 550003, China Institute for Clean Energy & Advanced Materials (ICEAM), Southwest University, Chongqing 400715, China Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy, Chongqing 400715, China d Institute of Theoretical Physics, Zunyi Normal College, Zunyi 563002, China e China Kunming Institute of Botany, Chinese Academy Science, Kunming 650201, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 March 2017 Received in revised form 9 June 2017 Accepted 16 June 2017 Available online 19 June 2017
Bilayer of NiOx/TiO2 thin film spin-coated and sputtering-deposited on the fluorine doped tin oxide (FTO) substrate is employed to develop a resistive random access memory device. An enhanced resistive switching (RS) behavior, which with appropriate resistance ratio of ~103, switching cycle endurance for 102 and long retention time for 104 s, is observed in the bilayer NiOx/TiO2 based device. Construction of contact-potential barrier, formation and rupture of a localized conduction filaments and migration of oxygen vacancy existed in the interface near electrodes co-contribute to the enhanced RS memory effects, but the migration of Agþ, Ni2xþ and diffusion of oxygen vacancies are the dominated ones. This work might give an insight into the mechanism of RS memory behaviors of an oxide-stacked structure device. © 2017 Published by Elsevier B.V.
Keywords: Bilay NiOx/TiO2 films Resistive switching memory Migration of oxygen vacancy Ag conduction filamens
1. Introduction According to the report from the International Technology Roadmap for Semiconductor Industry (ITRS), the memory technologies based on electrically operated metal-oxide-semiconductor devices, for example, the flash memory and dynamic random access memory (DRAM), are currently reaching their physical limits in scalability [1]. To solve the problem, tremendous efforts in discovering of new materials and designing of structures have been carried out for decades, but the ineffective storage density and high leakage current value are become major hurdles [2e4]. New type nonvolatile memory devices, for instance, the ferroelectric random access memory (FeRAM) [5], Magnetic Random Access Memory (MRAM) [6], and resistive random access memory (ReRAM) [7e12] have been extensively focused recent years. Especially, the ReRAM with a high/low resistance state after operating forward and reversed sweep voltage is believed to be one of the most promising
* Corresponding author. Guizhou Institute of Technology, Guizhou 550003, China. E-mail addresses: [email protected], [email protected] (G. Zhou). http://dx.doi.org/10.1016/j.jallcom.2017.06.178 0925-8388/© 2017 Published by Elsevier B.V.
candidates to substitute the conventional flash memory or DRAM in the near future [13]. Up today, the ReRAM behaviors have been observed in different material systems: i) the transition metal oxides (TiO2, NiOx, HfO2) [14e16]; ii) multiferroic materials (BiMnO3, BiFeO3, FeWO4) [17e19]; iii) organic and organic-doped materials [20e22]; iv) inorganic composites [23,24]. In addition, the application of ReRAM in biosensors, neurotransmitter detectors and transistors are become very attractive in recent years [25,26]. In the year of 2017, the Semiconductor Manufacturing International Corporation (SMIC) and Crossbar Corporation co-develop a 40-nm ReRAM chip, which meaning the ReRAM will enter a stage of industrialization production [27]. Although great progress, breakthrough and advance have made just for several years, but there are some important issues for the ReRAM application and integrated with other electronic device, such as the stability of dynamic switching cycles, retention time and mechanism of the resistive switching (RS) are still unclear. Particularly, the mechanism of ReRAM is still unclear because there are large numbers of physical and chemical models are proposed to explain the RS behaviors. For instance, in transition metal oxide system, the injected electrons trapping or detrapping from trapsites are believed to result in the RS memory effects [28,29],
754
G. Zhou et al. / Journal of Alloys and Compounds 722 (2017) 753e759
while, in multiferroic material systems, the synergistic effects of magnetism and conductive ion are responsible for observed RS memory behaviors [17,30]. Furthermore, chemically active metal electrodes (i.e. Ag, Cu) easily ionized under an external electric field and migration in organic-based memory cells are also believed to contribute to the RS memory behaviors [20,31e33]. In our previous works, we have founded that the formation and rapture of Ag filaments are responsible for the RS memory behaviors observed in albumen-doped eggshell [23] and the metallic conduction paths and migration of oxygen vacancy co-contribute to the charge storage phenomenon [34e36]. Li et al. has been reported that the moisture come from the air easily reacted with the oxygen vacancy on the surface of TiO2 layers and Messerschmitt et al. has found that the redox-based RS memory was very sensitive to the moisture [37,38]. Avoiding or decreasing the influence of moisture on TiO2ebased RS memory behavior and making its corresponding RS mechanism clear is an important issue. Hwang et al. had been reported that bilayer Pt/n-Type TiO2/p-Type NiO/Pt structure device show an enhanced RS behavior results from the rupture and formation of multi-filaments, and the H2O related reaction with TiO2 has not taken into account [39]. However, if the Ni oxides show non-stoichiometry and the reaction with H2O coming from environment can be suppressed, whether the enhanced RS memory can be obtained? In this paper, therefore, post-anneal process and sputterdeposition are employed to develop the bilayer NiOx/TiO2 ReRAM device. The redox-based Ag and Ni metallic conductive paths and migration of oxide vacancies are discussed. 2. Experimental details i) Fabrication of ReRAM device. The precursor solution fabricated using the titanium isopropoxide (C12H28O4Ti) and isopropanol (C3H8O) with volume ratio of 1:3 is spin-coated on FTO substrates with 3000 rpm for 30 s. The substrates coated with precursor solution are annealed at 550 C in air for 6 h. The NiOx films are deposited the annealed substrates using radio frequency magnetron sputtering at the power of 100 W for 10 min. The mixture (Ar: O2 ¼ 3:1) with 2 Pa are employed to be reaction gases. The thickness of 120 nm Ag, Au and Pt top electrodes with diameter of 200 mm are deposited on the surface of NiOx/TiO2 films using steel mask. Therefore, the sandwich structure of Ag/NiOx/TiO2/FTO is developed. ii) Current-voltage hysteresis measurement. The voltage-current hysteresis curves are measured using the probe station (Lakeshore TTPX-78495) and electrochemical workstation (CHI, RST5202F) at room temperature. The cross section images, surface morphology features and chemical components are characterized by the field emission scanning electron microscopy (FE-SEM, JSM6510), atomic force microscopy (AFM, SPI-3800N, Seiko Co) and X-ray photoelectron spectroscopy (XPS, 250Xi). 3. Results and discussion The cross section structure FE-SEM image of a fabricated film is shown in Fig. 1 (a), which indicating that the TiO2 and NiOx layers have a homogeneous distribution and uniformly physical thickness. By roughly estimating, the thickness of the bilayer NiOx/TiO2 is ~100 nm. It is worth noting that the surface of the bilayer films show the island-like shape. Therefore, the AFM technology is employed to investigation the surface of active films. One can see that the surface area with 1 1 mm scale of AFM image shows an uniform and smooth characteristics, as shown in Fig. 1 (b). The chemical component of the bilayer of NiOx/TiO2 is studied by XPS technology. Fig. 1 (c) is the XPS spectrum of survey binding energy
for the fabricated films indicating that the core shell of 1s of C, 2p of Ti, 1s of O and 2p of Ni can be detected. The XPS elaborate binding energy peaks for the Ti 2p and Ni 2p are shown in Fig. 1 (d) and (e). The areas of Ti 2p1/2 peaks are approximately half of the corresponding Ti 2p3/2. The binding energy of Ti 2p3/2 locates at 457.5 eV, and Ti 2p1/2 locates at 463.2 eV, and the spin-orbital splitting is 5.7 eV, as shown in Fig. 1 (d), demonstrating the main oxidation state of Ti is þ4 [40]. Similarly, Fig. 1 (e) highlights that the corelevels of Ni 2p3/2 and Ni 2p1/2 locate at the binding energy peaks of 854.3 and 872.1 eV, respectively, and the spin-orbital splitting is 17.8 eV, which illustrating the main oxidation state of Ni is þ2,but there is non-negligible contribution from the Niþ3 state [41]. All of XPS spectrum presented in this works are calibrated using the bond of carbon-carbon surface bond binding energies of 284.8 eV. Based the analysis from Fig. 1(a~e), the bilayer active NiOx/TiO2 films with a physical thickness of 100 nm, well distribution and uniform surface states are obtained using the spin-coating and sputtering method. The typical current-voltage (I-V) measurements are operated on the fabricated device at room temperature, and the voltage scan rate is 1 V/s. The device with single NiOx active layer is fabricated. The I-V curves of the device with the structure of Ag/NiOx/FTO show an obvious hysteresis phenomenon both at the negative and positive voltage range, although the I-V hysteresis of negative voltage region is smaller and weaker than the positive voltage region, as shown in Fig. 2 (a). The asymmetric I-V hysteresis obtained after stressing forward and reversed voltage sweep demonstrates that the single layer NiOx based device shows an electronic resistive switching memory phenomenon. Similarly, the single TiO2 device with Ag/TiO2/FTO shows the asymmetric I-V hysteresis phenomenon at both negative and positive voltage range, as shown in Fig. 2 (b). However, the asymmetric I-V curves exhibit a stronger rectification effect at positive voltage region and more stable RS memory effects at negative voltage region than the NiOx layer based device. The bilayer NiOx/TiO2 based devices with the structure of Ag/NiOx/ TiO2/FTO are developed at room temperature. Different with single layer of NiOx or TiO2 based device, a symmetrical I-V hysteresis observed in the bilayer of NiOx/TiO2 device indicates that the RS memory behaviors can be obtained, as shown in Fig. 2 (c). By comparison, an enhanced RS memory behavior is observed in the Ag/NiOx/TiO2/FTO device. The enhanced bipolar RS memory contains six stages, as demonstrated in Fig. 2 (c). In the stage 1, the current density of bilayer device shows a very little change when operating the sweep voltage from 0 to 2 V, which illustrate the device maintains a high resistance state (HRS or OFF). The voltage of 2 V is called as threshold value or SET voltage for the ReRAM. In the stage 2, when the operating external voltage reaches or exceeds the SET value, the current density of the bilayer of NiOx/TiO2 device steeply and sharply increase to a high current value, which indicates the resistance of device switches from the HRS to low resistance state (LRS or ON). In electronics, the logical “1” defines high voltage level, while low voltage level defines as “0”. After stressing a continuous bias voltage from a low to high value, the memory cell appears two different current states. In the stage 3, the LRS is well maintained when stressing a reversed voltage sweep from 2 to 0 V. In the stage 4, the LRS can be well maintained as well, even the stressing reversed voltage operated from 0 to 2 V. It is worth noting that the operating voltage sweeps from 2 to 1.8 V, the device is maintained the LRS, which might imply the charges stored in and captured by deep traps are extracted to contribute the RS memory effects. The voltage of 1.8 V is called as the RESET voltage, when the operating external voltage exceeds this value, the resistance of device will switches from LRS to back the HRS. Therefore, in the stage of 5, when operating voltage from 2 to 1.8 V, the current density of
G. Zhou et al. / Journal of Alloys and Compounds 722 (2017) 753e759
755
Fig. 1. (a) Cross-section FE-SEM image of the NiOx/TiO2 film coated onto FTO substrates at room temperature. (b) AFM image of the surface of NiOx/TiO2 sample. (c) XPS spectra of survey for the fabricated films, the C 1s, Ti 2p, O1s and Ni 2p can be detected. The binding energy peaks of (d) Ti 2p and (e) Ni 2p are demonstrated for the NiOx/TiO2 sample.
the bilayer device sharply decreases to a low value. In the stage of 6, the HRS can be well maintained when stressing the bias voltage form 1.8 to 0 V. Therefore, the bilayer of NiOx/TiO2 device shows two states (HRS and LRS), at the voltage ranges (0 / 2 V,2 / 0 V), the device keeps the “1” state, while at the voltage ranges (2 / 0 / 2/1.8 V), the device maintains the “0” state. As one of nonvolatile ReRAM device, the smaller of SET or RESET voltage, the more stability of LRS and HRS, and the higher ratio of HRS/LRS, the better of the device is, because these properties mean high storage density, low power consumption and high sensitivity. In this experiment, the bare NiOx/TiO2/FTO samples are operated I-V measurement, which the W tip with diameter of 2 mm substituted to Ag top electrodes partially penetrate into the active layer, as shown in Fig. 2 (d). By compared with the Ag/NiOx/TiO2/FTO device, not only the I-V curve shows a very little sloop and small saturation current in both the positive and negative voltage range, but also exhibits an unstable I-V hysteresis as the increasing of cycles. Despite that, the symmetric I-V loops implying the RS memory behavior is only contributed by NiOx/TiO2 bilayer. The I-V curves hysteresis observed in all of fabricated devices (Ag/NiOx/FTO, Ag/TiO2/FTO, Ag/NiOx/TiO2/FTO, NiOx/TiO2/FTO) indicating the RS memory are obtained, particularly, the bilayer Ag/ NiOx/TiO2/FTO device presents an enhanced bipolar RS memory effects. The I-V hysteresis of Fig. 2 (a~c) presents different RS memory behaviors for different fabricated device. The NiOx active layer shows typical electronic resistive switching memory behaviors, while TiO2 layer shows an obvious rectification effect at positive voltage region, interestingly, an enhanced RS memory phenomenon
is observed for the bilayer NiOx/TiO2. The electron transport and conductive ions migration in these fabricated devices are needed to further verify. To verify the conduction mechanism for the RS memory, the loglog scale I-V curves are conducted using Fig. 2 (a~d) and their corresponding fitting for HRS and LRS at the positive voltage region are plotted. To the Ag/NiOx/FTO device, the I-V curve in the HRS is comprised of the low Ohmic-like conduction (I ~ V0.89) and Child,s law region (I ~ V2.19), and in the LRS shows Ohmic-like conduction (I ~ V1.15), as shown in Fig. 3 (a). For the Ag/TiO2/FTO device, the HRS region contains three regions, Ohmic conduction region (I ~ V1.06), Child,s law conductive region (I-V2.12) and current steeply increase region (I ~ V3.72), similarly, the LRS region is dominated by the Ohmic-like conduction (I ~ V1.16), as shown in Fig. 3 (b). Similarly, the bilayer Ag/NiOx/TiO2/FTO based device in HRS region are dominated by Ohmic-like conduction (I-V1.15) and Child,s law conduction (I-V2.07), and the Ohimc-like conduction (I ~ V0.93) plays the main role, as shown in Fig. 3 (c). By comparison, the bare films of NiOx/TiO2 shows other type conduction mechanism in both the HRS and LRS region, as shown in Fig. 3 (d), which indicating the Ag electrodes play an important role in the enhanced RS memory behaviors. According to the space charge limited current (SCLC) model, the current density can be described as:
Vmþ1 Jf 2mþ1 d
(1)
where the J, V, d, m are the current density, operating bias voltage, the thickness of active layer and fitting index, respectively. The
756
G. Zhou et al. / Journal of Alloys and Compounds 722 (2017) 753e759
Fig. 2. Typical current-voltage hysteresis of device with the structure of (a) Ag/NiOx/FTO, (b) Ag/TiO2/FTO, (c) Ag/NiOx/TiO2/FTO and (d) NiOx/TiO2/FTO. The current-voltage loops of the Ag/NiOx/TiO2/FTO devices imply that an enhanced bipolar resistive switching memory behavior can be obtained.
Fig. 3. The double-logarithm scale current-voltage curves and their corresponding fitting results for the fabricated devices of (a) Ag/NiOx/FTO, (b) Ag/TiO2/FTO, (c) Ag/NiOx/TiO2/FTO and (d) NiOx/TiO2/FTO, respectively. The ohmic-like and traps-based space charge limited conduction mechanisms dominate the low resistance state and high resistance state, respectively.
G. Zhou et al. / Journal of Alloys and Compounds 722 (2017) 753e759
m 0, when the m ¼ 0, equation (1) becomes:
Jf
1 V d
(2)
The I ~ V indicates the Ohimc conduction. When the m ¼ 1, equation (1) can be written as [42].
J¼
9 g ε0 mεr 2 V 8 gþ1 d3
(3)
The g is the ratio of free electrons/trapped electrons, εr is the relative permittivity, ε0 is the vacuum permittivity, m is the carrier mobility. The relation of current-voltage of equation (3) means the Child,s law. While the index larger than the 1, the current steeply increasing as the bias voltage. The HRS region of the fabricated devices seemed to be dominated by the SCLC model, but the Ohmic-like conduction is the main domination one in the LRS region. Thus, it can be deduced that the charges trapping and de-trapping from trap-sites, such as the oxygen vacancies, metallic ion defects or the interface defects, results in the RS memory behavior. It should be noted that the I-V fitting results of the Ag/NiOx/TiO2/FTO are more in line with the SCLC models than others, which indicating the NiOx added plays a critical role in the enhanced RS memory. To further make the conduction clear, the Agþ conduction filaments, Ni2xþ based
757
conductive filaments, migration of oxygen vacancies and the charges trapping and de-trapping from the oxygen vacancy sites are proposed to interpret the RS memory behaviors. There are five possible conductions should be taken into account: i) migration of oxygen vacancies; ii) charges trapping and de-trapping from oxygen vacancies existed in interface; iii) redoxbased Agþ conduction filaments; iv) cation ion of Ni2xþ based localized paths and v) potential barrier of interface. Before the SET process, the devices are in HRS state, as shown in Fig. 4 (a). When applied low positive bias voltage, the injected charges have enough kinetic energy to overcome the potential barrier to enter the interface, in this case, oxygen vacancies in the interfaces near the FTO electrodes are gradually filled by the injected charges, and oxygen vacancies coming from the interfaces near the Ag electrodes and body of films are gradually driven to migrate to bottom electrodes of FTO. In addition, an oxidation reaction occurs for the Ag electrodes (Ag / Agþ þ e), and then the Agþ is reduced near the bottom electrodes (Agþ þ e / Ag) [43]. The composite NiOx containing the Ni2þ and Ni3þ is verified by the XPS spectrum, by comparison, the Ni2þ easily migrate to form a localized conduction filaments under external bias voltage because the Ni3þ with higher valence has difficulty in moving and the free of Ni3þ might be reduced by injected charges (Ni3þ þ e / Ni2þ) [44]. If the positive bias voltage successively increases to reach or exceed the SET voltage value, all of trap sites are filled by injected charges, the migration of oxygen vacancies, Ni (Ni2xþ þ 2x e / Ni) localized
Fig. 4. (a) After stressing a positive low external electric field, the Agþionized from Ag electrode migrates into the NiOx/TiO2 active layer. The oxygen vacancies existed in the interface or near the Ag electrodes are driven to migrate to FTO electrode or filled by injected charges. (b) The oxygen vacancy-based conduction filaments, Ag redox-based conduction paths and Ni2xþ of metallic-based conduction filaments are forming under a high successive electric field resulting in the device switching from high resistance state to low resistance state, which corresponding to the SET process. (c) Low resistance sates are maintained when stressing a reversed electric field, but all of forming conduction filaments become weak or even rupture at some localized region. (d) All of conduction filaments are broken resulting in the resistance switching from high resistance state to low resistance state, which corresponding to the RESET process.
758
G. Zhou et al. / Journal of Alloys and Compounds 722 (2017) 753e759
Fig. 5. (a) Resistive switching memory effects for the NiOx/TiO2 active layer but with different electrodes of Ag, Au and Pt, respectively. (b) Operating different compliance currents of 100 mA, 1 mA and 0.1 mA, respectively. (c) Switching dynamically cycles properties for the developed device of Ag/NiOx/TiO2/FTO. (d) Retention time test for 104 s, and resistance ratio large than 103.
conduction paths and metallic Ag filaments are formed resulting in the devices switching from HRS to LRS, which corresponding the SET process, as shown in Fig. 4 (b). When the SET process is finished, the LRS will be well maintained until the operating negative voltage reaches or exceeds the RESET voltage value. When stressing an negative bias voltage, the Ag accumulated near the bottom electrodes is gradually oxidized as Agþ (Ag/Agþ þ e), which will re-migrates to top electrodes, where the Agþ is reduced as Ag again. Under the negative bias voltage, the charges captured by traps are gradually trapping from these trap sites and the oxygen vacancies are gradually driven back to the near interface region, as shown in Fig. 4 (c). Once the operating negative voltage reaches or exceeds the RESET voltage (1.8 V), all of conduction filaments are ruptured resulting in the devices switching from LRS to HRS, which corresponding the RESET process, as shown in Fig. 4 (d). The formation and rupture of the redox-based Agþ conductive filaments ascribed the chemically active properties [45,46]. Therefore, to further study the effects of Agþ, the inactive electrodes (Pt and Au) are employed. The obvious RS memory behaviors can be detected for the Au-, Pt- and Ag-based devices, but the higher resistance ratio is obtained for the Ag-based device, as shown in Fig. 5 (a). The Agþ based conductive paths, therefore, play an important role for the enhanced RS memory effects. I-V characteristics of Ag/NiOx/TiO2/FTO memory cells under different compliance currents are shown in Fig. 5 (b). The compliance currents were lowered from the 100 to 1 mA, the reversible resistive switching was feasible, although the resistance ON/OFF ratio decreases. However, the resistive switching memory in HRS shown much more noisy points, and if the compliance currents was smaller than 1 mA, the endurance of the resistance states became very poor, which data not presented here. In order to investigate the endurance of dynamic
switching cycles, the compliance current of 100 mA was employed. The SET and RESET process and resistance ratio of >103 were available after stressing for 100 cycles, as shown in Fig. 5 (c). The retention time of resistive switching memory presented the resistance ratio of >103 at reading voltage of 200 mV for 104 s, as shown in Fig. 5 (d). Therefore, the developed devices fabricated using bilayer NiOx/TiO2 as active films show an enhanced RS memory effects, favorable endurance and retention characteristics.
4. Conclusion An enhanced RS memory behavior with favorable endurance, retention and resistance ON/OFF ratio was observed for the bilayer structure of Ag/NiOx/TiO2/FTO. The formation and rapture of the redox-based Ag conduction filaments, construction and breaking of Ni paths, charge trapping and de-trapping from the trap sites of interface near electrodes and the migration of oxygen vacancies leading a conductive path co-contributed the enhanced RS memory phenomenon. Among these mechanisms, the Agþ filaments and migration of oxygen vacancies may mainly dominate one. The distribution, concentration and migration of the oxygen vacancies, the in-situ observation of nanoscale Agþ and Ni filaments are needed further studied.
Acknowledgements This work was partly supported by the National Natural Science Foundation of China (11304410), Youth Science Foundation of Education Ministry (QJHKZ [2012] 084) of China.
G. Zhou et al. / Journal of Alloys and Compounds 722 (2017) 753e759
References [1] L. Wilson, International Technology Roadmap for Semiconductors (ITRS), Semiconductor Industry Association, 2013. [2] Q.A. Vu, Y.S. Shin, Y.R. Kim, V.L. Nguyen, W.T. Kang, H. Kim, D.H. Luong, M. Lee, K. Lee, D.S. Ko, J. Heo, S. Park, Y.H. Lee, W.J. Yu, Nat. Commun. 7 (2016) 12725. [3] M.B. Sarkar, A. Mondal, B. Choudhuri, J. Alloys Compd. 654 (2016) 529e533. [4] C.U. Kshirsagar, W. Xu, Y. Su, M.C. Robbins, C.H. Kim, S.J. Koester, ACS Nano 10 (2016) 8457e8464. [5] C.H. Chiu, C.W. Huang, Y.H. Hsieh, J.Y. Chen, C.F. Chang, Y.H. Chu, W.W. Wu, Nano Energy 34 (2017) 103e110. [6] E. Liu, J. Swerts, S. Couet, S. Mertens, Y. Tomczak, T. Lin, V. Spampinato, A. Franquet, S.V. Elshocht, G. Kar, A. Furnemont, J.D. Boeck, Appl. Phys. Lett. 108 (2016) 132405. [7] B. Sun, X. Zhang, G. Zhou, C. Zhang, P. Li, Y. Xia, Y. Zhao, J. Alloys Compd. 694 (2017) 464e470. [8] N. Onofrio, D. Guzman, A. Strachan, Nat. Mater. 14 (2015) 440e446. [9] H. Wang, Y. Zhu, D. Fu, J. Alloys Compd. 695 (2017) 2669e2671. [10] P. Kumar, S. Maikap, S. Ginnaram, J. Qiu, D. Jana, S. Chakrabarti, S. Samanta, K. Singh, A. Roy, S. Jana, M. Dutta, Y. Chang, H. Cheng, R. Mahapatra, H. Chiu, J. Yang, J. Electrochem. Soc. 164 (2017) B127eB135. [11] D. Jana, S. Samanta, S. Maikap, H. Cheng, Appl. Phys. Lett. 108 (2016) 011605. [12] S. Roy, S. Maikap, G. Sreekanth, M. Dutta, D. Jana, Y.Y. Chen, J.R. Yang, J. Alloys Compd. 637 (2015) 517e523. € a €slompolo, N. Tuomisto, L. Yao, S. Majumdar, J. Vijayakumar, [13] Q.H. Qin, L. Ak A. Casiraghi, S. Inkinen, B. Chen, A. Zugarramurdi, M. Puska, S. Dijken, Adv. Mater. 28 (2016) 6852e6859. [14] L. Zou, W. Hu, W. Xie, D. Bao, J. Alloys Compd. 693 (2017) 1180e1184. [15] X. Chen, W. Hu, Y. Li, S. Wu, D. Bao, Appl. Phys. Lett. 108 (2016) 053504. [16] K. Lee, Y. Tchoe, H. Yoon, H. Baek, K. Chung, S. Lee, C. Yoon, B.H. Park, G.C. Yi, Sci. Rep. 6 (2016) 27451. [17] B. Sun, C.M. Li, Phys. Chem. Chem. Phys. 17 (2015) 6718e6721. [18] A.Q. Jiang, C. Wang, K.J. Jin, X.B. Liu, J.F. Scott, C.S. Hwang, T.A. Tang, H.B. Lu, G.Z. Yang, Adv. Mater. 23 (2011) 1277e1281. € [19] M.Y. Han, B. Ozyilmaz, Y. Zhang, P. Kim, Phys. Rev. Lett. 98 (2007) 206805. [20] B. Zhang, G. Liu, Y. Chen, C. Wang, K.G. Neoh, T. Bai, E.T. Kang, Chem. Plus. Chem. 77 (2012) 74. [21] H. Wang, B. Zhu, X. Ma, Y. Hao, X. Chen, Small 12 (2016) 2715. [22] H. Wang, F. Meng, B. Zhu, W.R. Leow, Y. Liu, X. Chen, Adv. Mater. 27 (2015) 7670. [23] G. Zhou, B. Sun, A. Zhou, B. Wu, Curr. Appl. Phys. 17 (2017) 235e239.
759
[24] G. Zhou, B. Sun, B. Wu, X. Liu, S. Zhang, A. Zhou, Compos. Struct. 166 (2017) 177e183. [25] D.H. Kim, Y.S. Kim, J. Amsden, B. Panilaitis, D.L. Kaplan, F.G. Omenetto, M.R. Zakin, J.A. Rogers, Appl. Phys. Lett. 95 (2009) 133701. [26] B. Zhu, H. Wang, W.R. Leow, Y. Cai, X.J. Loh, M.Y. Han, X. Chen, Adv. Mater. 28 (2016) 4250e4265. [27] http://www.ednchina.com/news/article/20170118smic. [28] J.J. Yang, M.D. Pickett, X. Li, D.A. Ohlberg, D.R. Stewart, R.S. Williams, Nat. Nanotechnol. 3 (2008) 429e433. [29] J. Jin, J. Zhang, R.E. Kemal, Y. Luo, P. Bao, M. Althobaiti, D. Hesp, V.R. Dhanak, Z. Zheng, I.Z. Mitrovic, S. Hall, A. Song, J. Alloys Compd. 673 (2016) 54e59. [30] D. Li, W. Zheng, D. Zheng, J. Gong, L. Wang, C. Jin, P. Li, H. Bai, ACS Appl. Mater. Interfaces 8 (2016) 3977e3984. [31] A.K. Mahapatro, R. Agrawal, S. Ghosh, J. Appl. Phys. 96 (2004) 3583. [32] S. Nau, S. Sax, E.J.W.L. Kratochvil, Adv. Mater. 26 (2014) 2508e2513. [33] B. Sun, X. Zhang, G.D. Zhou, P. Li, Y. Zhang, H. Wang, Y. Xia, Y. Zhao, Org. Electron 42 (2017) 181e186. [34] G. Zhou, B. Wu, Z. Li, Z. Xiao, S. Li, P. Li, Curr. Appl. Phys. 15 (2015) 279e284. [35] G. Zhou, B. Wu, X. Liu, P. Li, S. Zhang, B. Sun, A. Zhou, Phys. Chem. Chem. Phys. 18 (2016) 6509e6514. [36] G. Zhou, B. Wu, X. Liu, Z. Li, S. Zhang, A. Zhou, X. Yang, J. Alloys Compds. 678 (2016) 31e35. [37] Y. Li, Y. Gao, Phys. Rev. Lett. 112 (2014) 206101. [38] F. Messerschmitt, M. Kubicek, J.L.M. Rupp, Adv. Funct. Mater. 25 (2015) 5117e5125. [39] K.M. Kim, S.J. Song, G.H. Kim, J.Y. Seok, M.H. Lee, J.H. Yoon, J. Park, C.S. Hwang, Adv. Funct. Mater. 21 (2011) 1587. [40] X.L. Shao, K.M. Kim, K.J. Yoon, S.J. Song, J.H. Yoon, H.J. Kim, T.H. Park, D.E. Kwon, Y.J. Kwon, Y.M. Kim, X.W. Hu, J.S. Zhao, C.S. Hwang, Nanoscale 8 (2016) 16455e16466. [41] S. Oswald, W. Bruckner, Surf. Interface Anal. 36 (2004) 17e22. € Metin, S¸. Aydog lu, O. an, K. Meral, J. Alloys Compds. [42] Z. Çaldıran, M. S¸inoforog 631 (2015) 261e265. [43] Z. Xu, Y. Bando, W. Wang, X. Bai, D. Golberg, ACS Nano 4 (2010) 2515e2522. [44] K. Oka, T. Yanagida, K. Nagashim, H. Tanaka, T. Kawai, J. Am. Chem. Soc. 131 (2009) 3434e3435. [45] P. Han, B. Sun, S. Cheng, F. Yua, B. Jiao, Q. Wu, J. Alloys Compds 681 (2016) 216e521. [46] G. Zhou, Z. Lu, Y. Yao, G. Wang, X. Yang, A. Zhou, P. Li, B.F. Ding, Q. Song, J. Appl. Phys. 121 (2017) 155302.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Mechanism for an enhanced resistive switching effect of bilayer NiOx/TiO2 for resistive random access memory Guangdong Zhou a, b, c, *, Lihua Xiao a, Shuangju Zhang a, Bo Wu d, Xiaoqin Liu a, Ankun Zhou e a
Guizhou Institute of Technology, Guizhou 550003, China Institute for Clean Energy & Advanced Materials (ICEAM), Southwest University, Chongqing 400715, China Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy, Chongqing 400715, China d Institute of Theoretical Physics, Zunyi Normal College, Zunyi 563002, China e China Kunming Institute of Botany, Chinese Academy Science, Kunming 650201, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 March 2017 Received in revised form 9 June 2017 Accepted 16 June 2017 Available online 19 June 2017
Bilayer of NiOx/TiO2 thin film spin-coated and sputtering-deposited on the fluorine doped tin oxide (FTO) substrate is employed to develop a resistive random access memory device. An enhanced resistive switching (RS) behavior, which with appropriate resistance ratio of ~103, switching cycle endurance for 102 and long retention time for 104 s, is observed in the bilayer NiOx/TiO2 based device. Construction of contact-potential barrier, formation and rupture of a localized conduction filaments and migration of oxygen vacancy existed in the interface near electrodes co-contribute to the enhanced RS memory effects, but the migration of Agþ, Ni2xþ and diffusion of oxygen vacancies are the dominated ones. This work might give an insight into the mechanism of RS memory behaviors of an oxide-stacked structure device. © 2017 Published by Elsevier B.V.
Keywords: Bilay NiOx/TiO2 films Resistive switching memory Migration of oxygen vacancy Ag conduction filamens
1. Introduction According to the report from the International Technology Roadmap for Semiconductor Industry (ITRS), the memory technologies based on electrically operated metal-oxide-semiconductor devices, for example, the flash memory and dynamic random access memory (DRAM), are currently reaching their physical limits in scalability [1]. To solve the problem, tremendous efforts in discovering of new materials and designing of structures have been carried out for decades, but the ineffective storage density and high leakage current value are become major hurdles [2e4]. New type nonvolatile memory devices, for instance, the ferroelectric random access memory (FeRAM) [5], Magnetic Random Access Memory (MRAM) [6], and resistive random access memory (ReRAM) [7e12] have been extensively focused recent years. Especially, the ReRAM with a high/low resistance state after operating forward and reversed sweep voltage is believed to be one of the most promising
* Corresponding author. Guizhou Institute of Technology, Guizhou 550003, China. E-mail addresses: [email protected], [email protected] (G. Zhou). http://dx.doi.org/10.1016/j.jallcom.2017.06.178 0925-8388/© 2017 Published by Elsevier B.V.
candidates to substitute the conventional flash memory or DRAM in the near future [13]. Up today, the ReRAM behaviors have been observed in different material systems: i) the transition metal oxides (TiO2, NiOx, HfO2) [14e16]; ii) multiferroic materials (BiMnO3, BiFeO3, FeWO4) [17e19]; iii) organic and organic-doped materials [20e22]; iv) inorganic composites [23,24]. In addition, the application of ReRAM in biosensors, neurotransmitter detectors and transistors are become very attractive in recent years [25,26]. In the year of 2017, the Semiconductor Manufacturing International Corporation (SMIC) and Crossbar Corporation co-develop a 40-nm ReRAM chip, which meaning the ReRAM will enter a stage of industrialization production [27]. Although great progress, breakthrough and advance have made just for several years, but there are some important issues for the ReRAM application and integrated with other electronic device, such as the stability of dynamic switching cycles, retention time and mechanism of the resistive switching (RS) are still unclear. Particularly, the mechanism of ReRAM is still unclear because there are large numbers of physical and chemical models are proposed to explain the RS behaviors. For instance, in transition metal oxide system, the injected electrons trapping or detrapping from trapsites are believed to result in the RS memory effects [28,29],
754
G. Zhou et al. / Journal of Alloys and Compounds 722 (2017) 753e759
while, in multiferroic material systems, the synergistic effects of magnetism and conductive ion are responsible for observed RS memory behaviors [17,30]. Furthermore, chemically active metal electrodes (i.e. Ag, Cu) easily ionized under an external electric field and migration in organic-based memory cells are also believed to contribute to the RS memory behaviors [20,31e33]. In our previous works, we have founded that the formation and rapture of Ag filaments are responsible for the RS memory behaviors observed in albumen-doped eggshell [23] and the metallic conduction paths and migration of oxygen vacancy co-contribute to the charge storage phenomenon [34e36]. Li et al. has been reported that the moisture come from the air easily reacted with the oxygen vacancy on the surface of TiO2 layers and Messerschmitt et al. has found that the redox-based RS memory was very sensitive to the moisture [37,38]. Avoiding or decreasing the influence of moisture on TiO2ebased RS memory behavior and making its corresponding RS mechanism clear is an important issue. Hwang et al. had been reported that bilayer Pt/n-Type TiO2/p-Type NiO/Pt structure device show an enhanced RS behavior results from the rupture and formation of multi-filaments, and the H2O related reaction with TiO2 has not taken into account [39]. However, if the Ni oxides show non-stoichiometry and the reaction with H2O coming from environment can be suppressed, whether the enhanced RS memory can be obtained? In this paper, therefore, post-anneal process and sputterdeposition are employed to develop the bilayer NiOx/TiO2 ReRAM device. The redox-based Ag and Ni metallic conductive paths and migration of oxide vacancies are discussed. 2. Experimental details i) Fabrication of ReRAM device. The precursor solution fabricated using the titanium isopropoxide (C12H28O4Ti) and isopropanol (C3H8O) with volume ratio of 1:3 is spin-coated on FTO substrates with 3000 rpm for 30 s. The substrates coated with precursor solution are annealed at 550 C in air for 6 h. The NiOx films are deposited the annealed substrates using radio frequency magnetron sputtering at the power of 100 W for 10 min. The mixture (Ar: O2 ¼ 3:1) with 2 Pa are employed to be reaction gases. The thickness of 120 nm Ag, Au and Pt top electrodes with diameter of 200 mm are deposited on the surface of NiOx/TiO2 films using steel mask. Therefore, the sandwich structure of Ag/NiOx/TiO2/FTO is developed. ii) Current-voltage hysteresis measurement. The voltage-current hysteresis curves are measured using the probe station (Lakeshore TTPX-78495) and electrochemical workstation (CHI, RST5202F) at room temperature. The cross section images, surface morphology features and chemical components are characterized by the field emission scanning electron microscopy (FE-SEM, JSM6510), atomic force microscopy (AFM, SPI-3800N, Seiko Co) and X-ray photoelectron spectroscopy (XPS, 250Xi). 3. Results and discussion The cross section structure FE-SEM image of a fabricated film is shown in Fig. 1 (a), which indicating that the TiO2 and NiOx layers have a homogeneous distribution and uniformly physical thickness. By roughly estimating, the thickness of the bilayer NiOx/TiO2 is ~100 nm. It is worth noting that the surface of the bilayer films show the island-like shape. Therefore, the AFM technology is employed to investigation the surface of active films. One can see that the surface area with 1 1 mm scale of AFM image shows an uniform and smooth characteristics, as shown in Fig. 1 (b). The chemical component of the bilayer of NiOx/TiO2 is studied by XPS technology. Fig. 1 (c) is the XPS spectrum of survey binding energy
for the fabricated films indicating that the core shell of 1s of C, 2p of Ti, 1s of O and 2p of Ni can be detected. The XPS elaborate binding energy peaks for the Ti 2p and Ni 2p are shown in Fig. 1 (d) and (e). The areas of Ti 2p1/2 peaks are approximately half of the corresponding Ti 2p3/2. The binding energy of Ti 2p3/2 locates at 457.5 eV, and Ti 2p1/2 locates at 463.2 eV, and the spin-orbital splitting is 5.7 eV, as shown in Fig. 1 (d), demonstrating the main oxidation state of Ti is þ4 [40]. Similarly, Fig. 1 (e) highlights that the corelevels of Ni 2p3/2 and Ni 2p1/2 locate at the binding energy peaks of 854.3 and 872.1 eV, respectively, and the spin-orbital splitting is 17.8 eV, which illustrating the main oxidation state of Ni is þ2,but there is non-negligible contribution from the Niþ3 state [41]. All of XPS spectrum presented in this works are calibrated using the bond of carbon-carbon surface bond binding energies of 284.8 eV. Based the analysis from Fig. 1(a~e), the bilayer active NiOx/TiO2 films with a physical thickness of 100 nm, well distribution and uniform surface states are obtained using the spin-coating and sputtering method. The typical current-voltage (I-V) measurements are operated on the fabricated device at room temperature, and the voltage scan rate is 1 V/s. The device with single NiOx active layer is fabricated. The I-V curves of the device with the structure of Ag/NiOx/FTO show an obvious hysteresis phenomenon both at the negative and positive voltage range, although the I-V hysteresis of negative voltage region is smaller and weaker than the positive voltage region, as shown in Fig. 2 (a). The asymmetric I-V hysteresis obtained after stressing forward and reversed voltage sweep demonstrates that the single layer NiOx based device shows an electronic resistive switching memory phenomenon. Similarly, the single TiO2 device with Ag/TiO2/FTO shows the asymmetric I-V hysteresis phenomenon at both negative and positive voltage range, as shown in Fig. 2 (b). However, the asymmetric I-V curves exhibit a stronger rectification effect at positive voltage region and more stable RS memory effects at negative voltage region than the NiOx layer based device. The bilayer NiOx/TiO2 based devices with the structure of Ag/NiOx/ TiO2/FTO are developed at room temperature. Different with single layer of NiOx or TiO2 based device, a symmetrical I-V hysteresis observed in the bilayer of NiOx/TiO2 device indicates that the RS memory behaviors can be obtained, as shown in Fig. 2 (c). By comparison, an enhanced RS memory behavior is observed in the Ag/NiOx/TiO2/FTO device. The enhanced bipolar RS memory contains six stages, as demonstrated in Fig. 2 (c). In the stage 1, the current density of bilayer device shows a very little change when operating the sweep voltage from 0 to 2 V, which illustrate the device maintains a high resistance state (HRS or OFF). The voltage of 2 V is called as threshold value or SET voltage for the ReRAM. In the stage 2, when the operating external voltage reaches or exceeds the SET value, the current density of the bilayer of NiOx/TiO2 device steeply and sharply increase to a high current value, which indicates the resistance of device switches from the HRS to low resistance state (LRS or ON). In electronics, the logical “1” defines high voltage level, while low voltage level defines as “0”. After stressing a continuous bias voltage from a low to high value, the memory cell appears two different current states. In the stage 3, the LRS is well maintained when stressing a reversed voltage sweep from 2 to 0 V. In the stage 4, the LRS can be well maintained as well, even the stressing reversed voltage operated from 0 to 2 V. It is worth noting that the operating voltage sweeps from 2 to 1.8 V, the device is maintained the LRS, which might imply the charges stored in and captured by deep traps are extracted to contribute the RS memory effects. The voltage of 1.8 V is called as the RESET voltage, when the operating external voltage exceeds this value, the resistance of device will switches from LRS to back the HRS. Therefore, in the stage of 5, when operating voltage from 2 to 1.8 V, the current density of
G. Zhou et al. / Journal of Alloys and Compounds 722 (2017) 753e759
755
Fig. 1. (a) Cross-section FE-SEM image of the NiOx/TiO2 film coated onto FTO substrates at room temperature. (b) AFM image of the surface of NiOx/TiO2 sample. (c) XPS spectra of survey for the fabricated films, the C 1s, Ti 2p, O1s and Ni 2p can be detected. The binding energy peaks of (d) Ti 2p and (e) Ni 2p are demonstrated for the NiOx/TiO2 sample.
the bilayer device sharply decreases to a low value. In the stage of 6, the HRS can be well maintained when stressing the bias voltage form 1.8 to 0 V. Therefore, the bilayer of NiOx/TiO2 device shows two states (HRS and LRS), at the voltage ranges (0 / 2 V,2 / 0 V), the device keeps the “1” state, while at the voltage ranges (2 / 0 / 2/1.8 V), the device maintains the “0” state. As one of nonvolatile ReRAM device, the smaller of SET or RESET voltage, the more stability of LRS and HRS, and the higher ratio of HRS/LRS, the better of the device is, because these properties mean high storage density, low power consumption and high sensitivity. In this experiment, the bare NiOx/TiO2/FTO samples are operated I-V measurement, which the W tip with diameter of 2 mm substituted to Ag top electrodes partially penetrate into the active layer, as shown in Fig. 2 (d). By compared with the Ag/NiOx/TiO2/FTO device, not only the I-V curve shows a very little sloop and small saturation current in both the positive and negative voltage range, but also exhibits an unstable I-V hysteresis as the increasing of cycles. Despite that, the symmetric I-V loops implying the RS memory behavior is only contributed by NiOx/TiO2 bilayer. The I-V curves hysteresis observed in all of fabricated devices (Ag/NiOx/FTO, Ag/TiO2/FTO, Ag/NiOx/TiO2/FTO, NiOx/TiO2/FTO) indicating the RS memory are obtained, particularly, the bilayer Ag/ NiOx/TiO2/FTO device presents an enhanced bipolar RS memory effects. The I-V hysteresis of Fig. 2 (a~c) presents different RS memory behaviors for different fabricated device. The NiOx active layer shows typical electronic resistive switching memory behaviors, while TiO2 layer shows an obvious rectification effect at positive voltage region, interestingly, an enhanced RS memory phenomenon
is observed for the bilayer NiOx/TiO2. The electron transport and conductive ions migration in these fabricated devices are needed to further verify. To verify the conduction mechanism for the RS memory, the loglog scale I-V curves are conducted using Fig. 2 (a~d) and their corresponding fitting for HRS and LRS at the positive voltage region are plotted. To the Ag/NiOx/FTO device, the I-V curve in the HRS is comprised of the low Ohmic-like conduction (I ~ V0.89) and Child,s law region (I ~ V2.19), and in the LRS shows Ohmic-like conduction (I ~ V1.15), as shown in Fig. 3 (a). For the Ag/TiO2/FTO device, the HRS region contains three regions, Ohmic conduction region (I ~ V1.06), Child,s law conductive region (I-V2.12) and current steeply increase region (I ~ V3.72), similarly, the LRS region is dominated by the Ohmic-like conduction (I ~ V1.16), as shown in Fig. 3 (b). Similarly, the bilayer Ag/NiOx/TiO2/FTO based device in HRS region are dominated by Ohmic-like conduction (I-V1.15) and Child,s law conduction (I-V2.07), and the Ohimc-like conduction (I ~ V0.93) plays the main role, as shown in Fig. 3 (c). By comparison, the bare films of NiOx/TiO2 shows other type conduction mechanism in both the HRS and LRS region, as shown in Fig. 3 (d), which indicating the Ag electrodes play an important role in the enhanced RS memory behaviors. According to the space charge limited current (SCLC) model, the current density can be described as:
Vmþ1 Jf 2mþ1 d
(1)
where the J, V, d, m are the current density, operating bias voltage, the thickness of active layer and fitting index, respectively. The
756
G. Zhou et al. / Journal of Alloys and Compounds 722 (2017) 753e759
Fig. 2. Typical current-voltage hysteresis of device with the structure of (a) Ag/NiOx/FTO, (b) Ag/TiO2/FTO, (c) Ag/NiOx/TiO2/FTO and (d) NiOx/TiO2/FTO. The current-voltage loops of the Ag/NiOx/TiO2/FTO devices imply that an enhanced bipolar resistive switching memory behavior can be obtained.
Fig. 3. The double-logarithm scale current-voltage curves and their corresponding fitting results for the fabricated devices of (a) Ag/NiOx/FTO, (b) Ag/TiO2/FTO, (c) Ag/NiOx/TiO2/FTO and (d) NiOx/TiO2/FTO, respectively. The ohmic-like and traps-based space charge limited conduction mechanisms dominate the low resistance state and high resistance state, respectively.
G. Zhou et al. / Journal of Alloys and Compounds 722 (2017) 753e759
m 0, when the m ¼ 0, equation (1) becomes:
Jf
1 V d
(2)
The I ~ V indicates the Ohimc conduction. When the m ¼ 1, equation (1) can be written as [42].
J¼
9 g ε0 mεr 2 V 8 gþ1 d3
(3)
The g is the ratio of free electrons/trapped electrons, εr is the relative permittivity, ε0 is the vacuum permittivity, m is the carrier mobility. The relation of current-voltage of equation (3) means the Child,s law. While the index larger than the 1, the current steeply increasing as the bias voltage. The HRS region of the fabricated devices seemed to be dominated by the SCLC model, but the Ohmic-like conduction is the main domination one in the LRS region. Thus, it can be deduced that the charges trapping and de-trapping from trap-sites, such as the oxygen vacancies, metallic ion defects or the interface defects, results in the RS memory behavior. It should be noted that the I-V fitting results of the Ag/NiOx/TiO2/FTO are more in line with the SCLC models than others, which indicating the NiOx added plays a critical role in the enhanced RS memory. To further make the conduction clear, the Agþ conduction filaments, Ni2xþ based
757
conductive filaments, migration of oxygen vacancies and the charges trapping and de-trapping from the oxygen vacancy sites are proposed to interpret the RS memory behaviors. There are five possible conductions should be taken into account: i) migration of oxygen vacancies; ii) charges trapping and de-trapping from oxygen vacancies existed in interface; iii) redoxbased Agþ conduction filaments; iv) cation ion of Ni2xþ based localized paths and v) potential barrier of interface. Before the SET process, the devices are in HRS state, as shown in Fig. 4 (a). When applied low positive bias voltage, the injected charges have enough kinetic energy to overcome the potential barrier to enter the interface, in this case, oxygen vacancies in the interfaces near the FTO electrodes are gradually filled by the injected charges, and oxygen vacancies coming from the interfaces near the Ag electrodes and body of films are gradually driven to migrate to bottom electrodes of FTO. In addition, an oxidation reaction occurs for the Ag electrodes (Ag / Agþ þ e), and then the Agþ is reduced near the bottom electrodes (Agþ þ e / Ag) [43]. The composite NiOx containing the Ni2þ and Ni3þ is verified by the XPS spectrum, by comparison, the Ni2þ easily migrate to form a localized conduction filaments under external bias voltage because the Ni3þ with higher valence has difficulty in moving and the free of Ni3þ might be reduced by injected charges (Ni3þ þ e / Ni2þ) [44]. If the positive bias voltage successively increases to reach or exceed the SET voltage value, all of trap sites are filled by injected charges, the migration of oxygen vacancies, Ni (Ni2xþ þ 2x e / Ni) localized
Fig. 4. (a) After stressing a positive low external electric field, the Agþionized from Ag electrode migrates into the NiOx/TiO2 active layer. The oxygen vacancies existed in the interface or near the Ag electrodes are driven to migrate to FTO electrode or filled by injected charges. (b) The oxygen vacancy-based conduction filaments, Ag redox-based conduction paths and Ni2xþ of metallic-based conduction filaments are forming under a high successive electric field resulting in the device switching from high resistance state to low resistance state, which corresponding to the SET process. (c) Low resistance sates are maintained when stressing a reversed electric field, but all of forming conduction filaments become weak or even rupture at some localized region. (d) All of conduction filaments are broken resulting in the resistance switching from high resistance state to low resistance state, which corresponding to the RESET process.
758
G. Zhou et al. / Journal of Alloys and Compounds 722 (2017) 753e759
Fig. 5. (a) Resistive switching memory effects for the NiOx/TiO2 active layer but with different electrodes of Ag, Au and Pt, respectively. (b) Operating different compliance currents of 100 mA, 1 mA and 0.1 mA, respectively. (c) Switching dynamically cycles properties for the developed device of Ag/NiOx/TiO2/FTO. (d) Retention time test for 104 s, and resistance ratio large than 103.
conduction paths and metallic Ag filaments are formed resulting in the devices switching from HRS to LRS, which corresponding the SET process, as shown in Fig. 4 (b). When the SET process is finished, the LRS will be well maintained until the operating negative voltage reaches or exceeds the RESET voltage value. When stressing an negative bias voltage, the Ag accumulated near the bottom electrodes is gradually oxidized as Agþ (Ag/Agþ þ e), which will re-migrates to top electrodes, where the Agþ is reduced as Ag again. Under the negative bias voltage, the charges captured by traps are gradually trapping from these trap sites and the oxygen vacancies are gradually driven back to the near interface region, as shown in Fig. 4 (c). Once the operating negative voltage reaches or exceeds the RESET voltage (1.8 V), all of conduction filaments are ruptured resulting in the devices switching from LRS to HRS, which corresponding the RESET process, as shown in Fig. 4 (d). The formation and rupture of the redox-based Agþ conductive filaments ascribed the chemically active properties [45,46]. Therefore, to further study the effects of Agþ, the inactive electrodes (Pt and Au) are employed. The obvious RS memory behaviors can be detected for the Au-, Pt- and Ag-based devices, but the higher resistance ratio is obtained for the Ag-based device, as shown in Fig. 5 (a). The Agþ based conductive paths, therefore, play an important role for the enhanced RS memory effects. I-V characteristics of Ag/NiOx/TiO2/FTO memory cells under different compliance currents are shown in Fig. 5 (b). The compliance currents were lowered from the 100 to 1 mA, the reversible resistive switching was feasible, although the resistance ON/OFF ratio decreases. However, the resistive switching memory in HRS shown much more noisy points, and if the compliance currents was smaller than 1 mA, the endurance of the resistance states became very poor, which data not presented here. In order to investigate the endurance of dynamic
switching cycles, the compliance current of 100 mA was employed. The SET and RESET process and resistance ratio of >103 were available after stressing for 100 cycles, as shown in Fig. 5 (c). The retention time of resistive switching memory presented the resistance ratio of >103 at reading voltage of 200 mV for 104 s, as shown in Fig. 5 (d). Therefore, the developed devices fabricated using bilayer NiOx/TiO2 as active films show an enhanced RS memory effects, favorable endurance and retention characteristics.
4. Conclusion An enhanced RS memory behavior with favorable endurance, retention and resistance ON/OFF ratio was observed for the bilayer structure of Ag/NiOx/TiO2/FTO. The formation and rapture of the redox-based Ag conduction filaments, construction and breaking of Ni paths, charge trapping and de-trapping from the trap sites of interface near electrodes and the migration of oxygen vacancies leading a conductive path co-contributed the enhanced RS memory phenomenon. Among these mechanisms, the Agþ filaments and migration of oxygen vacancies may mainly dominate one. The distribution, concentration and migration of the oxygen vacancies, the in-situ observation of nanoscale Agþ and Ni filaments are needed further studied.
Acknowledgements This work was partly supported by the National Natural Science Foundation of China (11304410), Youth Science Foundation of Education Ministry (QJHKZ [2012] 084) of China.
G. Zhou et al. / Journal of Alloys and Compounds 722 (2017) 753e759
References [1] L. Wilson, International Technology Roadmap for Semiconductors (ITRS), Semiconductor Industry Association, 2013. [2] Q.A. Vu, Y.S. Shin, Y.R. Kim, V.L. Nguyen, W.T. Kang, H. Kim, D.H. Luong, M. Lee, K. Lee, D.S. Ko, J. Heo, S. Park, Y.H. Lee, W.J. Yu, Nat. Commun. 7 (2016) 12725. [3] M.B. Sarkar, A. Mondal, B. Choudhuri, J. Alloys Compd. 654 (2016) 529e533. [4] C.U. Kshirsagar, W. Xu, Y. Su, M.C. Robbins, C.H. Kim, S.J. Koester, ACS Nano 10 (2016) 8457e8464. [5] C.H. Chiu, C.W. Huang, Y.H. Hsieh, J.Y. Chen, C.F. Chang, Y.H. Chu, W.W. Wu, Nano Energy 34 (2017) 103e110. [6] E. Liu, J. Swerts, S. Couet, S. Mertens, Y. Tomczak, T. Lin, V. Spampinato, A. Franquet, S.V. Elshocht, G. Kar, A. Furnemont, J.D. Boeck, Appl. Phys. Lett. 108 (2016) 132405. [7] B. Sun, X. Zhang, G. Zhou, C. Zhang, P. Li, Y. Xia, Y. Zhao, J. Alloys Compd. 694 (2017) 464e470. [8] N. Onofrio, D. Guzman, A. Strachan, Nat. Mater. 14 (2015) 440e446. [9] H. Wang, Y. Zhu, D. Fu, J. Alloys Compd. 695 (2017) 2669e2671. [10] P. Kumar, S. Maikap, S. Ginnaram, J. Qiu, D. Jana, S. Chakrabarti, S. Samanta, K. Singh, A. Roy, S. Jana, M. Dutta, Y. Chang, H. Cheng, R. Mahapatra, H. Chiu, J. Yang, J. Electrochem. Soc. 164 (2017) B127eB135. [11] D. Jana, S. Samanta, S. Maikap, H. Cheng, Appl. Phys. Lett. 108 (2016) 011605. [12] S. Roy, S. Maikap, G. Sreekanth, M. Dutta, D. Jana, Y.Y. Chen, J.R. Yang, J. Alloys Compd. 637 (2015) 517e523. € a €slompolo, N. Tuomisto, L. Yao, S. Majumdar, J. Vijayakumar, [13] Q.H. Qin, L. Ak A. Casiraghi, S. Inkinen, B. Chen, A. Zugarramurdi, M. Puska, S. Dijken, Adv. Mater. 28 (2016) 6852e6859. [14] L. Zou, W. Hu, W. Xie, D. Bao, J. Alloys Compd. 693 (2017) 1180e1184. [15] X. Chen, W. Hu, Y. Li, S. Wu, D. Bao, Appl. Phys. Lett. 108 (2016) 053504. [16] K. Lee, Y. Tchoe, H. Yoon, H. Baek, K. Chung, S. Lee, C. Yoon, B.H. Park, G.C. Yi, Sci. Rep. 6 (2016) 27451. [17] B. Sun, C.M. Li, Phys. Chem. Chem. Phys. 17 (2015) 6718e6721. [18] A.Q. Jiang, C. Wang, K.J. Jin, X.B. Liu, J.F. Scott, C.S. Hwang, T.A. Tang, H.B. Lu, G.Z. Yang, Adv. Mater. 23 (2011) 1277e1281. € [19] M.Y. Han, B. Ozyilmaz, Y. Zhang, P. Kim, Phys. Rev. Lett. 98 (2007) 206805. [20] B. Zhang, G. Liu, Y. Chen, C. Wang, K.G. Neoh, T. Bai, E.T. Kang, Chem. Plus. Chem. 77 (2012) 74. [21] H. Wang, B. Zhu, X. Ma, Y. Hao, X. Chen, Small 12 (2016) 2715. [22] H. Wang, F. Meng, B. Zhu, W.R. Leow, Y. Liu, X. Chen, Adv. Mater. 27 (2015) 7670. [23] G. Zhou, B. Sun, A. Zhou, B. Wu, Curr. Appl. Phys. 17 (2017) 235e239.
759
[24] G. Zhou, B. Sun, B. Wu, X. Liu, S. Zhang, A. Zhou, Compos. Struct. 166 (2017) 177e183. [25] D.H. Kim, Y.S. Kim, J. Amsden, B. Panilaitis, D.L. Kaplan, F.G. Omenetto, M.R. Zakin, J.A. Rogers, Appl. Phys. Lett. 95 (2009) 133701. [26] B. Zhu, H. Wang, W.R. Leow, Y. Cai, X.J. Loh, M.Y. Han, X. Chen, Adv. Mater. 28 (2016) 4250e4265. [27] http://www.ednchina.com/news/article/20170118smic. [28] J.J. Yang, M.D. Pickett, X. Li, D.A. Ohlberg, D.R. Stewart, R.S. Williams, Nat. Nanotechnol. 3 (2008) 429e433. [29] J. Jin, J. Zhang, R.E. Kemal, Y. Luo, P. Bao, M. Althobaiti, D. Hesp, V.R. Dhanak, Z. Zheng, I.Z. Mitrovic, S. Hall, A. Song, J. Alloys Compd. 673 (2016) 54e59. [30] D. Li, W. Zheng, D. Zheng, J. Gong, L. Wang, C. Jin, P. Li, H. Bai, ACS Appl. Mater. Interfaces 8 (2016) 3977e3984. [31] A.K. Mahapatro, R. Agrawal, S. Ghosh, J. Appl. Phys. 96 (2004) 3583. [32] S. Nau, S. Sax, E.J.W.L. Kratochvil, Adv. Mater. 26 (2014) 2508e2513. [33] B. Sun, X. Zhang, G.D. Zhou, P. Li, Y. Zhang, H. Wang, Y. Xia, Y. Zhao, Org. Electron 42 (2017) 181e186. [34] G. Zhou, B. Wu, Z. Li, Z. Xiao, S. Li, P. Li, Curr. Appl. Phys. 15 (2015) 279e284. [35] G. Zhou, B. Wu, X. Liu, P. Li, S. Zhang, B. Sun, A. Zhou, Phys. Chem. Chem. Phys. 18 (2016) 6509e6514. [36] G. Zhou, B. Wu, X. Liu, Z. Li, S. Zhang, A. Zhou, X. Yang, J. Alloys Compds. 678 (2016) 31e35. [37] Y. Li, Y. Gao, Phys. Rev. Lett. 112 (2014) 206101. [38] F. Messerschmitt, M. Kubicek, J.L.M. Rupp, Adv. Funct. Mater. 25 (2015) 5117e5125. [39] K.M. Kim, S.J. Song, G.H. Kim, J.Y. Seok, M.H. Lee, J.H. Yoon, J. Park, C.S. Hwang, Adv. Funct. Mater. 21 (2011) 1587. [40] X.L. Shao, K.M. Kim, K.J. Yoon, S.J. Song, J.H. Yoon, H.J. Kim, T.H. Park, D.E. Kwon, Y.J. Kwon, Y.M. Kim, X.W. Hu, J.S. Zhao, C.S. Hwang, Nanoscale 8 (2016) 16455e16466. [41] S. Oswald, W. Bruckner, Surf. Interface Anal. 36 (2004) 17e22. € Metin, S¸. Aydog lu, O. an, K. Meral, J. Alloys Compds. [42] Z. Çaldıran, M. S¸inoforog 631 (2015) 261e265. [43] Z. Xu, Y. Bando, W. Wang, X. Bai, D. Golberg, ACS Nano 4 (2010) 2515e2522. [44] K. Oka, T. Yanagida, K. Nagashim, H. Tanaka, T. Kawai, J. Am. Chem. Soc. 131 (2009) 3434e3435. [45] P. Han, B. Sun, S. Cheng, F. Yua, B. Jiao, Q. Wu, J. Alloys Compds 681 (2016) 216e521. [46] G. Zhou, Z. Lu, Y. Yao, G. Wang, X. Yang, A. Zhou, P. Li, B.F. Ding, Q. Song, J. Appl. Phys. 121 (2017) 155302.