Solid-State Electronics 76 (2012) 40–43

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Solid-State Electronics

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Effect of annealing temperature on resistance switching behaviorof Mg0.2Zn0.8O thin films deposited on ITO glass

Shu-ming Gao, Hua Wang ⇑, Ji-wen Xu, Chang-lai Yuan, Xiao-wen ZhangSchool of Materials Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China

a r t i c l e i n f o

Article history:Received 5 December 2011Received in revised form 8 May 2012Accepted 15 May 2012Available online 2 July 2012

The review of this paper was arranged by Y.Kuk

Keywords:Mg0.2Zn0.8OResistance switchingAnnealing temperatureSol–gel

0038-1101/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.sse.2012.05.009

⇑ Corresponding author. Tel.: +86 773 2192509; faxE-mail address: wh65@tom.com (H. Wang).

a b s t r a c t

Mg0.2Zn0.8O thin films for resistance random access memory application were prepared on ITO glass sub-strates by sol–gel method, and the effect of annealing temperature on resistance switching behavior hasbeen studied. With the increase of annealing temperature, the crystallinity of Mg0.2Zn0.8O thin films canbe improved, the turn on voltage, the turn off voltage, the resistance of low resistance state and the resis-tance ratio of high resistance state to low resistance state showed an increasing, but the endure of switch-ing cycles decreased and the resistance of high resistance state exhibited a degradation.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

In the past decade, resistive random access memory (RRAM) hasbeen widely investigated as next-generation nonvolatile memorydevices due to its simple structure and compatibility with comple-mentary metal oxide semiconductor technology. The resistiveswitching behavior has been discovered in various materialsincluding binary oxide thin films such as TiO2 [1], NiO [2], ZrO2

[3]. Recently, S and Co doped ZnO thin films have also been foundto exhibit resistive switching characteristic [4], MgxZn1�xO thinfilms and RRAM devices based on ZnO films have been activelydeveloped for transparent electronics [5–12]. It is well known thatMg doped ZnO thin films are important wide bandgap semiconduc-tor materials for various optoelectronic device applications due tothe realization of bandgap engineering. Chen et al. reported thefabrication of RRAM device with the structure of Pt/Mg0.2Zn0.8O/Pt [13,14], which exhibited good resistive switching properties.However, this device is not suitable for transparent electronicsbecause both the electrodes and the substrates are not transparent.Besides, the synthesis temperature of 650 �C is too high for thegrowth of MgxZn1�xO:Mg films on glass substrate. Cao et al. re-ported that the AZO/MZO/AZO structure RRAM devices have beenprepared by pulsed laser deposition (PLD) at room temperature [9].

In this study, the transparent RRAM devices with a structure ofAg/Mg0.2Zn0.8O/ITO were prepared on indium tin oxide (ITO) glass

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substrate by sol–gel method. The effects of annealing temperatureon the resistance switching behavior were studied.

2. Experimental procedures

Mg0.2Zn0.8O thin films were deposited by sol–gel spin coating meth-od onto ITO glass substrates. Zn(CH3COO)2�2H2O, Mg(CH3COO)2�4H2O(purity 99.9%, Xilong chemical Co., Ltd.), 2-methoxyethanol and dieth-anolamine were used as starting materials, solvent and stabilizer,respectively. After stirring continuously at room temperature, thenthe stable and homogeneous solutions of 0.3 mol/L were prepared.The thin films were prepared by spin coating at 3000 rpm for 30 s. Eachlayer was heated at 250 �C to remove organic residuals before spinningthe next layer. To prevent the cracking and disruption to crystallization,each spin layer must be thin enough to ensure the simultaneous evap-oration of all solvent. The above spin coating and preheating procedurewas repeated until the desired film thickness was achieved. Finally, thesamples of Mg0.2Zn0.8O thin films were annealed at 350 �C, 400 �C and440 �C for 1 h in air ambient respectively.

For resistance switching measurement, the top Ag electrodeswith a diameter of 200 lm were deposited by RF magnetron sputter-ing with a metal shadow mask to form the Ag/Mg0.2Zn0.8O/ITO struc-ture device cell. The crystalline phase was determined by X-raydiffraction (XRD) with Cu Ka radiation (AXS D8-ADVANCE, Bruker).The surfaces of the annealed samples were observed by a scanningelectron microscope (JSM-5610LV, JEOL). The current–voltage(I–V) characteristics of the resistance switching behavior were ob-tained using digital source meter (2636A, Keithley). The thickness

S.-m. Gao et al. / Solid-State Electronics 76 (2012) 40–43 41

of thin films is about 350 nm measured by profilometer (XP-100,Ambios).

Fig. 3. I–V curves of Ag/Mg0.2Zn0.8O/ITO devices at different annealing tempera-tures, the inset is the log(I)–V curves.

3. Results and discussion

Fig. 1 shows the XRD patterns of Mg0.2Zn0.8O thin filmsannealed at different temperatures. As observed from the XRDpatterns, the peaks of ITO (222), (400) and (240) were very dis-tinct, which is come from the ITO glass substrate. The other peaksbelong to zinc oxide hexagonal wurtzite structure. The full width athalf maximum of ZnO peak decreases slightly, which indicates thecrystallinity of Mg0.2Zn0.8O thin films can be improved by theannealing process.

Fig. 2 shows the Ag/Mg0.2Zn0.8O/ITO RRAM cell structure. TheI–V characteristics of bipolar resistive switching behavior at differ-ent annealing temperatures were measured by direct current (DC)voltage sweeping in the sequence of 0 ? 3 ? 0 ? �3 ? 0 V, the re-sults were shown in Fig. 3 and the inset represents the log(I)–Vcurves for Fig. 3. Two distinct resistance states for different anneal-ing temperature are observed. The ‘‘RESET’’ implies resistanceswitching from low resistance state (LRS) to high resistance state(HRS), while ‘‘SET’’ is resistance switching from HRS to LRS. Theturn off voltage (VOFF) is defined as the sweeping voltage whenthe ‘‘RESET’’ occurs, and the turn on voltage (VON) is the sweepingvoltage when the‘‘SET’’ takes place. When the DC voltage appliedon the Ag electrode is swept from 0 to 3 V, the ‘‘SET’’ process occursat about 1.0 V for the sample annealed at 350 �C, 1.8 V for the

Fig. 1. XRD patterns of Mg0.2Zn0.8O thin films at different annealing temperatures.

Fig. 2. The schematic structure of the RRAM device.

sample annealed at 400 �C, and 2.4 V for the sample annealed at440 �C. The LRS is maintained while the DC voltage is swept from3 to 0 V. When the DC voltage applied on the Ag electrode is sweptfrom 0 to �3 V, the ‘‘RESET’’ process occurs and current abruptlydecreased at �1.8 V for the sample annealed at 350 �C, �2.0 V forthe sample annealed at 400 �C and �2.5 V for the sample annealedat 440 �C. In general, the VON and VOFF increased with increasing inannealing temperature.

To understand the conduction mechanisms of Ag/Mg0.2Zn0.8O/ITO devices, the typical I–V curve has been transformed into thelog(I)–log(V) plot as shown in Fig. 4 and linear fitting results ofthe log(I)–log(V) plot of the samples annealed at 350 �C in the po-sitive voltage regions. The log(I)–log(V) plot in the LRS was com-pletely linear with a slope of 1, indicating the ohmic conductionmechanism. The electrical transport behavior in the HRS at lowvoltage section also follows the ohmic conduction mechanism(the slope is about 1 in the log(I)–log(V) plot). Whereas a nonlineareffect in the log(I)–log(V) plot (the slope is about 2) was observedat high voltage section, this is the typical behavior of trap-con-trolled space–charge–limited conduction (SCLC) [15]. Fig. 5 pre-sents the log(I)–log(V) plots of the positive branch for differentannealing temperature samples. In Fig. 5, it can be seen that the

Fig. 4. log(I)–log(V) plot of the Mg0.2Zn0.8O film annealed at 350 �C in the positivevoltage regions, the lines show the fitting results in both HRS and LRS.

Fig. 5. log(I)–log(V) plots of the Mg0.2Zn0.8O films at different annealing temper-atures in the positive voltage regions.

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samples annealed at different temperature are similar in conduc-tion mechanisms. However, the ohmic conduction mechanism ex-tends to a larger voltage range in the HRS while SCLC conductionmechanism has also been weakened as the annealing temperatureincreases.

To study the device endurance, the resistance of HRS (RHRS) andLRS (RLRS) of Mg0.2Zn0.8O devices were measured at a bias voltage of0.2 V as a function of switching cycles, as shown in Fig. 6. It can beseen that the initial resistance ratio of RHRS to RLRS (RHRS/RLRS) andthe RLRS increased as annealing temperature increases from350 �C to 440 �C. However, the RHRS declined sharply as annealingtemperature increased after more switching cycles. In other words,RHRS/RLRS decreased slowly with the increasing of switching cyclesin 350 �C annealing, indicating that the Mg0.2Zn0.8O thin film an-nealed at 350 �C has the better endurance characteristics. Besides,the fluctuation of RHRS can be observed in the initial testing cyclesand become relatively stable after more switching cycles. Thescanty tiny filaments existed in the Mg0.2Zn0.8O thin film afterthe ‘‘RESET’’ process, which is the root cause of the resistance fluc-tuation in the initial testing cycles. However, the number of tinyfilament increases with the increasing of switching cycles, a massof tiny filaments makes the fluctuation in resistance values become

Fig. 6. Endurance characteristics of the Ag/Mg0.2Zn0.8O/ITO device.

relatively stable and the RHRS decrease after more switching cycles.Fluctuation of RHRS is also observed by Kukreja et al. [16]. Althoughthe endurance properties of Ag/Mg0.2Zn0.8O/ITO prepared at roomtemperature by sol–gel method is not as good as that of AZO/MZO/AZO prepared by PLD at room temperature [9], Ag/Mg0.2Zn0.8O/ITO prepared by optimum sol–gel process would besuitable for nonvolatile memory applications.

In this study, the generation and recovery of oxygen vacancieswith non-lattice oxygen ions and SCLC conduction mechanismcan be used to explain the bipolar resistive switching phenome-non in the Ag/Mg0.2Zn0.8O/ITO devices. It is inevitable that somedefects such as oxygen vacancies, gain boundaries, and interstitialatoms exist in polycrystalline Mg0.2Zn0.8O thin films, act as trapcenters for charge carriers and cause the trap-controlled SCLCtransport behavior. When a voltage is applied, grain boundariesand dislocations make it easy for the point defects, probably oxy-gen vacancy, to diffuse at a high electric field, resulting in perco-lating conduction filaments, thus the LSR transition. Xu et al. [12]also suggested that the generation and recovery of oxygen vacan-cies with non-lattice oxygen ions are considered to be the mech-anism of resistance switching in TiN/ZnO/Pt devices. The carrierstransport or hop through the filament path, exhibiting Ohmic I–Vbehavior and holding onto the LRS until the tiny filaments areruptured, then, the device switches to HRS. A possible reasonfor the rupture of the conduction filaments in the present deviceis the vanishing of the oxygen vacancies near the Mg0.2Zn0.8O/ITOinterface by the oxygen diffusion from ITO that works as an oxy-gen reservoir [8]. In addition to oxygen diffusion, the transport ofindium ion and the restoring of oxygen ions by the indium ionunder the bias voltage with different polarity is also possible tocause the resistance switching [8,17]. The metallic filamentsmay not be destroyed completely in the HRS ? LRS process andthe remaining fragments cause the inhomogeneous conductionin the Mg0.2Zn0.8O film. Defect-healing may occur during anneal-ing, and the larger grain size for the high temperature annealedsamples suggests fewer defects within the conducting paths.Therefore, the fewer defects in the high temperature annealedMg0.2Zn0.8O films, either result from defect-healing or less grainboundaries, are the root cause of the increase in VON, VOFF, andRLRS [12,18].

4. Conclusion

Mg0.2Zn0.8O thin films with good resistive switching propertiescan be prepared on ITO glass substrates by sol–gel method. Withthe increase of annealing temperature, the grain size and the crys-tallinity of Mg0.2Zn0.8O films can be improved. The VON, VOFF, RLRS

and RHRS/RLRS increase as the annealing temperature increases,which is due to the defect-healing effect of Mg0.2Zn0.8O films. TheRHRS declined sharply and the RLRS increased slightly as annealingtemperature increases, indicating that the Mg0.2Zn0.8O thin filmsannealed at 350 �C has the better endurance characteristics. Thegeneration and recovery of oxygen vacancies with non-lattice oxy-gen ions and SCLC conduction mechanism considered being themechanism of resistance switching in the Ag/Mg0.2Zn0.8O/ITO de-vices. The scanty tiny filaments existed in HRS after the ‘‘RESET’’process was the root cause of the resistance of Mg0.2Zn0.8O filmfluctuation in the initial testing cycles and a mass of tiny filamentsmake the resistance fluctuation become relatively stable and theRHRS decreased after more switching cycles.

Acknowledgement

This work is supported by the Natural Science Foundation ofChina (Grant No. 61066001).

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