Open Access

Self-rectifying performance in the sandwiched structure of Ag/In-Ga-Zn-O/Pt bipolar resistive switching memory

  • Xiaobing Yan1, 2Email author,
  • Hua Hao1,
  • Yingfang Chen1,
  • Shoushan Shi1,
  • Erpeng Zhang1,
  • Jianzhong Lou1 and
  • Baoting Liu3
Nanoscale Research Letters20149:548

https://doi.org/10.1186/1556-276X-9-548

Received: 16 July 2014

Accepted: 26 September 2014

Published: 2 October 2014

Abstract

We reported that the resistive switching of Ag/In-Ga-Zn-O/Pt cells exhibited self-rectifying performance at low-resistance state (LRS). The self-rectifying behavior with reliability was dynamic at elevated temperature from 303 to 393 K. The Schottky barrier originated from the interface between Ag electrode and In-Ga-Zn-O films, identified by replacing Ag electrode with Cu and Ti metals. The reverse current at 1.2 V of LRS is strongly suppressed and more than three orders of magnitude lower than the forward current. The Schottky barrier height was calculated as approximately 0.32 eV, and the electron injection process and resistive switching mechanism were discussed.

Keywords

Resistive switchingSelf-rectifyingSchottky barrier

Background

Resistive switching random access memory (RRAM) has attracted considerable interests for its potential in the next-generation high-density nonvolatile memories due to its low power operation, long date retention, and fast write/read speed[1]. In order to realize high-density memory, the cross-bar architecture, in which memory cells are integrated between the word and bit lines, has been presented for achieving a cell size of 4 F 2 of RRAM application[2]. However, there is a serious problem for this specific application because of the sneaking current issue due to cross-talk effect in cross-bar structure. For example, in a simple 2 × 2 array, a cell at high-resistance state (HRS) will be misread when three surrounding cells at low-resistance state (LRS) form an unexpected sneak current path as shown in Figure 1a[3]. While in a large memory array, e.g., m × n (m, n > 2), the number of the sneak current path is huge, which makes such leakage current unreasonable. Some solutions have been reported in the form of devices such as transistors, selection devices, and complementary resistive switch (CRS) where the logic bit is coded in two different reset (high-resistance) states for RRAM cross-bar integration[36]. Especially, a rectifying diode (1D) or transistor (1 T) integrated with RRAM cell is one significant method to block sneak current path[7]. Unfortunately, it will increase fabrication cost and power dissipation for integrating a diode and transistor, which also influences memory performance. Recently, the self-rectifying resistive memory has been actively demonstrated[8, 9], which exhibits the obvious rectification effect in LRS, so this manifestation can alleviate the cross-talk effect without serially connecting a diode. Therefore, much work has being done to investigate self-rectifying effect in RRAM. However, it is quite difficult to obtain the self-rectifying characteristics for most the binary metal oxide material, which may be resulted from the fact that conductive filaments (CFs) with metallic property are generated and it leads to an ohmic contact at the interface between the metal electrode and oxide films, so no rectifying behavior was observed. Sawa and Yang et al. reported that complex crystalline oxides Pr0.7Ca0.3MnO3 and Cr-doped SrZrO3 can realize the resistive switching with self-rectifying effect[10, 11]. In this study, we report another complex oxide amorphous In-Ga-Zn-O as the resistive switching storage films, which were widely demonstrated in TFTs and so on, due to its low processing temperature, good transparency, and high mobility[12]. The metals Ag and Pt were used as top and bottom electrodes. The resistive switching device with good endurance has a remarkable self-rectifying effect at LRS. The Schottky barrier behavior between metal Ag and In-Ga-Zn-O films was investigated in detail, and the electron injection process and resistive switching mechanism in the Ag/In-Ga-Zn-O/Pt device structure were also discussed.
Figure 1

Schematic of a sneak current path and I-V characteristics of Ag/In-Ga-Zn-O/Pt resistive memory. (a) The sneak current path as a dotted line in cross-bar array. When HRS cell is read, the sneak current path is formed due to the surrounding LRS cells. (b)-(d) The I-V characteristics of Ag/In-Ga-Zn-O/Pt resistive memory on a semilogarithmic scale with different range of voltage sweep, and the voltage is swept in the direction as follows: 1 → 2 → 3 → 4 process. The inset of (d) shows the LRS of the resistive memory (upper) and the resistance of the device scaling with the area size of the top electrode (lower).

Methods

The 80-nm-thick In-Ga-Zn-O film was under the ambient pressure of 5 × 10-4 Pa. The In-Ga-Zn-O films was prepared on a Pt substrate by magnetron sputtering technique at a power of 100 W in 0.5 Pa atmosphere of Ar + O2 mixture (Ar/O2 flow rate ratio = 50:25) at 450°C. Then postdeposition annealing process was carried out at 450°C in O2 ambiance for 30 min. Then, Ag, Ti, Cu, and Pt were deposited as top electrodes by direct current (DC) magnetron sputtering using a metal shadow mask. The top electrode with 70 nm in thickness was deposited at room temperature. The base pressure of the sputtering chamber was below 2 × 10-4 Pa, and the working pressure was 3 Pa maintained by a gas mixture of argon. The diameter of the top electrodes was 0.1 mm, and the DC power was 10 W. Keithley 2400 source-measure unit (Keithley Instruments Inc., Cleveland, OH, USA) and probe station were employed to measure the electrical characteristics and switching properties. The bottom electrode Pt was grounded, and voltage sweeps were always applied to the Ag top electrode.

Results and discussion

Figure 1 demonstrates the representative I-V characteristic with 0.18, 0.5, and 1.3 V max voltage sweep range; the rectification effects were observed Figure 1c,d. The direction of the voltage sweep 0 → 1.3 → 0 → -1.3 → 0 V is denoted by the numbered arrows in Figure 1d. A remarkable resistive switching was obtained, and the memory cell can be switched between HRS and LRS reversibly. Moreover, the resistive switching was of bipolar type because a reversal polarity of voltage was applied to the cell for transforming the resistance state. It is worth noting that there is no current jump in HRS or LRS as in Ag/STO and Ag/electrolyte/Pt structure resistive switching devices, in which formation and dissolution of Ag filaments are ascribed for the resistive switching mechanism[13, 14]. In addition, the filaments show localization feature; however, we can find obvious size dependence of current of HRS and LRS. The Ag electrodes with two different areas have 0.1 and 0.3 mm diameters. The resistance of the device is scaling with the area size of the top electrode as shown in the inset of Figure 1d. So, no filaments were formed in our device, and the switching mechanism should be different with the phenomenon of electrolytes, and we would discuss the mechanism in following content. The I-V curve of the voltage sweep -1.6 → 1.6 V was measured after the cell was switched to LRS in the inset of Figure 1d. We can observe the Schottky-diode-like behavior in Ag/In-Ga-Zn-O/Pt memory cell, and the reverse current at -1.2 V of LRS is about more than three orders of magnitude lower than forward current at 1.2 V due to the remarkable suppression by the barrier as shown in Figure 1d.

The temperature dependence of the I-V characteristics was also investigated for current transport mechanism as shown in Figure 2. The I-V curves of the LRS in Ag/In-Ga-Zn-O/Pt were tested from 303 to 393 K. It can be found that the rectifying effects exist at all testing temperature ranges. Besides, the absolute value of the current increases gradually as the temperature increases, implying that the resistance value of LRS decreases as the temperature increases as shown in inset of Figure 2. It could be ascribed to a typical semiconducting behavior, which is similar to the result reported by Chiu et al.[13]. This conducting characteristic in the LRS is notably different from the metallic-filament-based systems[14, 15], in which the current in the LRS shows a increasing trend when the ambient temperature increases. Moreover, the rectifying effect is still dynamic at 393 K as shown in Figure 2. In-Ga-Zn-O is an n-type semiconductor, but the devices have larger current in positive segment. The current in the negative part was suppressed. The reverse current is the negative segment; the forward current is the positive segment. Therefore, the self-rectifying effect is due to a potential barrier formation at the Ag/In-Ga-Zn-O interface. To confirm this thought, Cu and Ti are also fabricated as the top electrodes by DC magnetron sputtering. We found that both currents of the negative part of the cell with Cu and Ti electrodes are smaller than the current of the positive part. Compared to the top Ag electrode, a more remarkable rectifying characteristic is observed with top Cu or Ti electrode as shown in Figure 3. This phenomenon indicates that the top electrode dominates the rectifying effect at LRS. It is also natural to suggest that the Ag/In-Ga-Zn-O junction has a Schottky characteristic. Therefore, the barrier formed between the top electrode and In-Ga-Zn-O film is employed to explain the self-rectifying effect. Furthermore, the work functions of Cu, Ti, and Ag are 4.65, 4.33, and 4.26 eV, respectively. Therefore, the higher barriers would be formed at the interface between various top electrode metals and In-Ga-Zn-O films due to the different metal work functions, which are responsible for the different I-V characteristics. When negative voltage sweep is applied to the top electrode Ag, the barrier at the top Ag electrode/In-Ga-Zn-O interface blocks the electrons flowing from the top Ag electrode to bottom Pt electrode, which makes the negative current much smaller than the positive value. Although the self-rectifying behaviors appear as a dependence on electrode materials with different work functions, it is necessary to investigate the detailed current transport mechanism due to the complexity of the conduction process in the resistive switching device.
Figure 2

I-V characteristics of Ag/In-Ga-Zn-O/Pt in the LRS were tested from 303 to 393 K. The inset is the temperature dependence of the resistance in LRS.

Figure 3

I-V characteristics of resistive memory with top Cu, Ti, and Ag electrodes. The rectifying effect at LRS is shown.

To further investigate the resistive switching and current transport mechanisms of the Ag/In-Ga-Zn-O/Pt cell, the I-V curve was fitted by the space-charge-limited conduction (SCLC) mechanisms, which is similar to the results reported in [Ref.[16, 17]. Figure 4a shows the linear fitting under the relation of ln(I) vs. ln(V) of the previous I-V curve of Figure 1d at the positive voltage sweep region at HRS, respectively. It can be seen from the figure that there are three different slope regions. In region I, the slope is about 1.1, implying the ohmic conduction at the low-voltage region. When the applied voltage is low, there are more thermally generated free electrons than injected electrons from the electrode, thus, the current depends on the applied field and the conductivity of the films. It is followed by a region characterized by a large slope of 1.6, region II, corresponding to Child's law. With the increase of applied voltage, the injected carriers become predominant and a trap filled-limited current is observed, where the I-V characteristics follow Child's law. In region III, the slope of 3.2 corresponds the accomplishment of trap-filling process[18, 19]. The sharp increase of the leakage current occurs at the so-called trap-filled limit voltage (VTFL) 0.18 V. The injected electrons are transported inside the bulk films (In-Ga-Zn-O) and are partly captured by the traps in the In-Ga-Zn-O films, while others contribute to the total current. This transport process is recognized as trap-controlled SCLC. When the bias voltage is smaller than the trap-filled limit voltage VTFL in I-V curves, the electron injected from the Pt electrode into the insulator In-Ga-Zn-O will distribute in the insulator and cannot transform the insulator to affect the current in In-Ga-Zn-O thin film. So, we also cannot obtain the resistive switching effect as shown in Figure 1b, in which the max voltage sweep is 0.18 V. When the bias voltage exceeds the VTFL, the electron injected from the bottom electrode Pt can affect the insulator. This can be explained that the deep traps in the In-Ga-Zn-O thin film were completely filled, then the injected electron from the bottom electrode Pt can pass through the thin film, resulting into the increase of the leakage current. In general, it would be caused by the existence of the current jump due to resistance changed by bulk films in set process of their device. As for the I-V curve of the LRS of Figure 4b, the relation ln(I) vs. V1/2 can be fitted linearly in LRS as shown in Figure 4b, indicating Schottky emission mechanism due to the existence of Schottky barrier at Ag/In-Ga-Zn-O interface. It is also in accordance with above analysis as shown in Figure 3.
Figure 4

The ln( I )-ln( V ) curves. (a) The ln(I)-ln(V) curves of HRS in the high-field region of Figure 1. (b) The ln(I) as a function of V1/2 at the LRS.

In order to confirm the Schottky-type electron injection at the Ag/In-Ga-Zn-O junction of the Schottky barrier, the reverse current of the I-V curve was measured at temperatures ranging from 303 to 393 K in Figure 5a[2023]. Figure 5b demonstrates the variation of the Ln(J/T2) at applied voltages ranging from -0.02 to -0.2 V as ranged in shadow area of Figure 5a with the inverse of the temperature according to the Schottky emission in Equation 1:
Ln J / T 2 = - q Φ b + q qV / 4 π d ε 0 ε r / kT + Ln A *
(1)
where Φb, d, ε0, εr, and A* are the interface potential barrier height, depletion layer thickness, dielectric permittivity of the vacuum, dynamic dielectric constant, and effective Richardson's constant, respectively. The activation energies at each voltage - q Φ b + q qV / 4 π d ε 0 ε r were obtained from the slopes. The activation energy decreases linearly with which is in agreement with the Schottky-type thermionic emission theory. The interfacial potential barriers Φb at interface were obtained by extrapolating the plots to V = 0. By extrapolating to V = 0, the Schottky barrier height at 0 V of the Ag/In-Ga-Zn-O interface was determined to be 0.32 eV. The dynamic dielectric constant εr is extracted to be 7.9, according to the fitting results of a Schottky conduction model.
Figure 5

The ln( I )-ln( V ) curves and conventional Richardson plot of LRS. (a) The ln(I)-ln(V) curves of LRS for Ag/In-Ga-Zn-O/Pt resistive memory measured from 303 to 393 K. (b) The conventional Richardson plot for the LRS of resistive memory.

Based on the aforementioned results, the resistive switching device of Ag/In-Ga-Zn-O/Pt with self-rectifying effects shows a potential to be used in cross-bar structure arrays. Therefore, the endurance and retention properties of the cell could be tested as shown in Figure 6a,b. Figure 6a demonstrates the endurance performance of the resistive memory at the LRS. After the resistive memory device was operated for changing to LRS, the current was recorded at -0.5 and 0.5 V as readout voltages, respectively. After 240 cycles of set and reset operation by DC sweeping, the forward/reverse current ratio keeps almost unchanged. Figure 6b shows that the currents of HRS and LRS have no obvious fluctuations after 4.3 × 104 s, implying good retention property. A voltage of 0.25 V was employed as read voltage for HRS for LRS. The reproducible and reliable rectifying effect of Ag/In-Ga-Zn-O/Pt device can effectively reduce the misreading in cross-bar architectures.
Figure 6

Endurance performance of resistive memory at LRS (a) and retention property of the device (b).

Conclusions

In conclusion, we fabricated the Ag/In-Ga-Zn-O/Pt structure device by RF magnetron sputtering method in this study. The device exhibited good bipolar resistive switching and superior self-rectifying effect. Schottky diode model was employed to explain the mechanism of the self-rectifying characteristics, and the Schottky barrier height is calculated by measuring the I-V curves and fitting the data at different temperatures. The experimental results confirm that the resistive switching of Ag/In-Ga-Zn-O/Pt structure can become a promising candidate as non-volatile memory devices using in cross-bar structure.

Declarations

Acknowledgements

This work was financially supported by National Natural Science Foundation of China under Grant Nos. 61306098, 61475041, and 11374086, the Natural Science Foundation of Hebei Province (E2012201088, E2013201176), and the Science Research Program of University in Hebei Province (ZH2012019).

Authors’ Affiliations

(1)
College of Electron and Information Engineering, Hebei University
(2)
The Laboratory of Nano-Fabrication and Novel Devices Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences
(3)
Department of Physics, Hebei University

References

  1. Sawa A: Resistive switching in transition metal oxides. Mater Today 2008, 11: 28–36.View ArticleGoogle Scholar
  2. Kang BS, Ahn SE, Lee MJ, Stefanovich G, Kim KH, Xianyu WX, Lee CB, Park Y, Baek IG, Park BH: High-current-density CuOx/InZnOx thin-film diodes for cross-point memory applications. Adv Mater 2008, 20: 3066–3069. 10.1002/adma.200702932View ArticleGoogle Scholar
  3. Linn E, Rosezin R, Kuegeler C, Waser R: Complementary resistive switches for passive nanocrossbar memories. Nat Mater 2010, 9: 403–406. 10.1038/nmat2748View ArticleGoogle Scholar
  4. Nardi F, Balatti S, Larentis S, Ielmini D: Complementary switching in metal oxides: toward diode-less crossbar RRAMs. International Electron Devices Meeting: In Technical Digest; 2011:31.1.1–31.1.4.Google Scholar
  5. Yang YC, Sheridan P, Lu W: Complementary resistive switching in tantalum oxide-based resistive memory devices. Appl Phys Lett 2012, 100: 203112. 10.1063/1.4719198View ArticleGoogle Scholar
  6. Tang GS, Zeng F, Chen C, Liu HY, Gao S, Song C, Lin Y, Chen G, Pan F: Programmable complementary resistive switching behaviours of a plasma-oxidised titanium oxide nanolayer. Nanoscale Res Lett 2013, 5: 422–428.View ArticleGoogle Scholar
  7. Cho B, Kim TW, Song S, Ji Y, Jo M, Hwang H, Jung GY, Lee T: Rewritable switching of one diode-one resistor nonvolatile organic memory devices. Adv Mater 2010, 22: 1228–1232. 10.1002/adma.200903203View ArticleGoogle Scholar
  8. Lv HB, Li YT, Liu Q, Long S: Self-rectifying resistive-switching device with α-Si/WO3 bilayer. IEEE Electron Device Lett 2013, 34: 229–231.View ArticleGoogle Scholar
  9. Park S, Jung S, Siddik M, Jo M, Park J, Kim S, Lee W, Shin J, Lee D, Choi G, Woo J, Cha E, Lee BH, Hwang H: Self-formed Schottky barrier induced selector-less RRAM for cross-point memory applications. Phys Status Solidi (RRL) 2012, 6: 454–456. 10.1002/pssr.201206382View ArticleGoogle Scholar
  10. Sawa A, Fujii T, Kawasaki M, Tokura Y: Hysteretic current-voltage characteristics and resistance switching at a rectifying Ti/Pr0.7Ca0.3MnO3 interface. Appl Phys Lett 2004, 85: 4073–4075. 10.1063/1.1812580View ArticleGoogle Scholar
  11. Yang MK, Kim DY, Park JW, Lee JK: Resistive switching behavior of Cr-doped SrZrO3 perovskite thin films for random access memory applications. J Korean Phys Soc 2005, 47: 313–316. 10.3938/jkps.47.313View ArticleGoogle Scholar
  12. Nomura K, Ohta H, Takagi A, Kamiya T, Hirano M, Hosono H: Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors. Nature 2004, 432: 488–492. 10.1038/nature03090View ArticleGoogle Scholar
  13. Chiu F-C, Li P-W, Chang W-Y: Reliability characteristics and conduction mechanisms in resistive switching memory devices using ZnO thin films. Nanoscale Res Lett 2012, 7: 178. 10.1186/1556-276X-7-178View ArticleGoogle Scholar
  14. Yan XB, Li K, Yin J, Xia YD, Guo HX, Chen L, Liu ZG: The resistive switching mechanism of Ag/SrTiO3/Pt memory cells. Electrochemical Solid-State Lett 2010, 13: H87-H89. 10.1149/1.3279689View ArticleGoogle Scholar
  15. Yan XB, Yin J, Guo HX, Su Y, Xu B, Li HT, Yan DW, Yan DW, Liu ZG: Bipolar resistive switching performance of the nonvolatile memory cells based on (AgI)0.2(Ag2MoO4 )0.8 solid electrolyte films. J Appl Phys 2009, 106: 054501. 10.1063/1.3211293View ArticleGoogle Scholar
  16. Chen MC, Chang TC, Huang SY, Chen SC, Hu CW, Tsai CT, Sze SM: Bipolar resistive switching characteristics of transparent indium gallium zinc oxide resistive random access memory. Electrochemical Solid-State Lett 2010, 13: H191-H193. 10.1149/1.3360181View ArticleGoogle Scholar
  17. Kim CH, Jang YH, Hwang HJ, Song CH, Yang YS, Cho JH: Bistable resistance memory switching effect in amorphous InGaZnO thin films. Appl Phys Lett 2010, 97: 062109. 10.1063/1.3479527View ArticleGoogle Scholar
  18. Wang SY, Cheng BL, Wang C, Dai SY, Lu HB, Zhou YL, Chen ZH, Yang GZ: Reduction of leakage current by Co doping in Pt/Ba0.5Sr0.5TiO3/Nb-SrTiO3 capacitor. Appl Phys Lett 2004, 84: 4116–4118. 10.1063/1.1755421View ArticleGoogle Scholar
  19. Chiu FC: A review on conduction mechanisms in dielectric films. Adv Mater Sci Eng 2014, 2014: 578168.Google Scholar
  20. Shin YC, Song J, Kim KM, Choi BJ, Choi S, Lee HJ, Kim GH, Eom T, Hwang CS: (In, Sn)2O3/TiO2/Pt Schottky-type diode switch for the TiO2 resistive switching memory array. Appl Phys Lett 2008, 92: 162904. 10.1063/1.2912531View ArticleGoogle Scholar
  21. Huang JJ, Kuo CW, Chang WC, Hou TH: Transition of stable rectification to resistive-switching in Ti/TiO2/Pt oxide diode. Appl Phys Lett 2010, 96: 262901. 10.1063/1.3457866View ArticleGoogle Scholar
  22. Hwang CS, Lee BT, Kang CS, Kim JW, Lee KH, Cho HJ, Horii H, Kim WD, Lee SI, Roh YB, Lee MY: A comparative study on the electrical conduction mechanisms of (Ba0.5Sr0.5)TiO3 thin films on Pt and IrO2 electrodes. J Appl Phys 1998, 83: 3703. 10.1063/1.366595View ArticleGoogle Scholar
  23. Zuo Q, Long S, Liu Q, Zhang S, Wang Q, Li Y, Wang Y, Liu M: Self-rectifying effect in gold nanocrystal-embedded zirconium oxide resistive memory. J Appl Phys 2009, 106: 073724. 10.1063/1.3236632View ArticleGoogle Scholar

Copyright

© Yan et al.; licensee Springer. 2014

This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.

Advertisement