ITO for invisible electronics application

Journal of Non-Crystalline Solids 406 (2014) 102–106

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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol

Transparent amorphous memory cell: A bipolar resistive switching in ZnO/Pr0.7Ca0.3MnO3/ITO for invisible electronics application R. Zhang a, J. Miao a,b,⁎, F. Shao a, W.T. Huang a, C. Dong a, X.G. Xu a, Y. Jiang a a b

School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Institut für Physik, Johannes Gutenberg-Universität Mainz, 55099 Mainz, Germany

a r t i c l e

i n f o

Article history: Received 21 May 2014 Received in revised form 29 September 2014 Accepted 30 September 2014 Available online xxxx Keywords: ZnO; PCMO; Amorphous; Resistive switching; Optical transparent

a b s t r a c t ZnO/Pr0.7Ca0.3MnO3 (ZnO/PCMO) amorphous thin films were grown on an indium-tin-oxide (ITO)/glass by pulsed laser deposition at room temperature. Interestingly, a stable bipolar resistive switching behavior of the ITO/ZnO/PCMO/ITO cell can be longer than 2.5 × 103 cycles. The on/off ratio of switching behaviors is as high as 104. The structure of ITO/ZnO/PCMO/ITO exhibits a high average transparency of 79.6% in the visible range with a maximum transparency of 84.6% at 590 nm wavelength. The conductive mechanism during switching cycling in our structure can be described by a trapped-control space charge limited current behavior. The ZnO/ PCMO/ITO/glass structure shows a potential of the transparent memory for future invisible electronics devices. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Recently, a resistive switching (RS) behavior was extensively investigated for resistive random access memory (RRAM) due to its simple structure, nonvolatile memory, and low power consumption [1,2]. The RS behavior has been observed in many transitional metal oxides, such as ZnO [3,4], TaOx [5], TiO2 [6], and InGaZnO [7]. Moreover, some perovskite oxides, such as BiFeO3 [8], Pr0.7Ca0.3MnO3 [9,10], and SrZrO3 [11] also exhibit the resistive switching behavior. Although various models have been proposed, such as rupture filaments [12,13], Schottky barrier [14], and trap-controlled space charge limited current [15], the conductive mechanism of RS behavior is still not well understood. Some transitional metal oxides (such as Gd2O3 [16], ZnO [17], and Pr0.7Ca0.3MnO3 [2]) are also highly transparent in the visible region with a large optical bandgap (N3 eV). Specifically, the ZnO and Pr0.7Ca0.3MnO3 (PCMO) films exhibit a good transparent and electrical performance. Therefore, implemented with a transparent electrode (ITO), a transparent cell composed of PCMO, ZnO, and ITO films may be potentially applied as a transparent RRAM. However, the resistive switching performance of amorphous ZnO/PCMO bilayer has not been investigated until now. In this work, the bipolar resistive switching of ITO/ZnO/PCMO/ITO transparent sandwich was investigated. The device shows a stable ⁎ Corresponding author at: School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. E-mail address: [email protected] (J. Miao).

http://dx.doi.org/10.1016/j.jnoncrysol.2014.09.055 0022-3093/© 2014 Elsevier B.V. All rights reserved.

resistive switching up to 2.5 × 103 cycles and a ratio of on/off as high as 104. An average transparency of 79.6% in the visible range was observed in the ITO/ZnO/PCMO/ITO/glass. Furthermore, all deposition steps were done at room temperature (RT) without any additional heating, which is beneficial to the industrial mass production. Our ZnO/PCMO/ITO/glass cell shows a potential application on transparent RRAM for invisible electronics. 2. Experimental details The stoichiometrically ceramic targets of ZnO, Pr0.7Ca0.3MnO3 (PCMO) were synthesized via the traditional solid state reactions. A PCMO (~100 nm) layer was deposited on ITO/glass substrate by pulse laser deposition (PLD) under an oxygen pressure of 10 Pa at RT. Then, a ZnO film (~60 nm) was grown on the top of PCMO by PLD under an oxygen pressure of 15 Pa at RT. The energy and repetition frequency of the laser pulses were 300 mJ and 5 Hz, respectively. Finally, to form a metal/insulator/metal cell, top ITO electrodes were sputtered on the as-deposited ZnO/PCMO films through a metal shadow mask at RT. No any additional heat treatments were conducted after the device's fabrication. The phases structure was identified by X-ray diffraction (XRD, MAC Science, M21X, CuKa1 0.15406 Å). The surface morphology was characterized by a scanning electron microscope (Zeiss, SUPRA 55). The stoichiometry of the film was analyzed by energy dispersive X-ray spectroscopy (EDS). The resistive switching and cycling performance of current–voltage properties was measured by a Keithley 2400c semiconductor device analyzer with a sweeping speed of 5 mV s−1. The

R. Zhang et al. / Journal of Non-Crystalline Solids 406 (2014) 102–106

103 +

optical transmittance spectrum of the films was recorded with an UV– visible-near infrared spectrophotometer (Varian, Cary-5000).

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2400c

ITO

ZnO PCMO ITO

3. Results and discussions

Glass

Fig. 1 shows the XRD patterns of ZnO/Pr0.7Ca0.3MnO3 films on ITO/ glass substrate deposited at room temperature. Except for ITO no sharp peaks can be found. This indicates an amorphous structure of the ZnO/PCMO films. The inset of Fig. 1 shows an elemental analysis of the Ta/ZnO/PCMO/ITO/glass by EDS. As evidence, Pr, Ca, Mn, and Zn elements were found in the spectrum with proper concentration. Fig. 2(a) shows the surface morphology of ZnO/Pr0.7Ca0.3MnO3 films on the ITO/glass by a FE-SEM. It can be seen that the surface of the heterostructure was rather smooth and composed of small grains with an average diameter of 20 nm. The inset of Fig. 2(a) schematizes the heterostructure of the ZnO/PCMO/ITO/glass. Fig. 2(b) shows the crosssectional views of the ZnO/PCMO/ITO/glass. The thicknesses of ZnO and PCMO layers are about 60 nm and 100 nm, respectively. Moreover, it can be seen as a sharp interface between ZnO and PCMO layers, which is beneficial to its electrical performance. Fig. 3(a) shows the resistive switching behaviors of ITO/ZnO/ Pr0.7Ca0.3MnO3/ITO transparent stacks. The arrows indicate the sweeping direction along the current during the voltage sweeping. A typical bipolar switching behavior can be observed in the ZnO/PCMO film. Moreover, the forming process to active the resistive switching was unnecessary in the ITO/ZnO/PCMO/ITO structure. The characteristics of forming-free behavior may be attributed to its proper chemical stoichiometry in the oxide [18]. Furthermore, the switching from the low resistance state (LRS) to high-resistance state (HRS) happened at an applied positive voltage of 2.0 V–2.3 V, whereas the switching from HRS to LRS occurred at a negative bias of − 2.0 V–− 2.6 V. The reason for the switching of resistance may be explained as the change of the Schottky-like barrier at the interface [19]; it's well known that if the surface state density is large enough, the Schottky-like barrier width or height is determined by the net charge in the interface states [20,21]; with the increasing voltage, the degree of band bending is changed and in turn caused the changed contact resistance. It should be noted that there's an abrupt change in current at positive bias of +1 V. During this process, some movable traps (oxygen vacancy) were filled by the injected carriers and form a conducting path, which is called a conducting filament [22]. However, when the voltage reached 1 V, the weakest point in the conducting filament is destroyed, resulting in a

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Intensity (a.u.)

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●

ZnO/PCMO/ITO/glass

●

20

ITO 30

ITO/glass 40

50

60

70

80

2θ (degree) Fig. 1. XRD patterns of amorphous ZnO/PCMO/ITO/glass and ITO/glass, respectively. (Inset) EDS elemental analysis of ZnO/PCMO/ITO/glass.

Fig. 2. FE-SEM images of the ZnO/PCMO/ITO/glass. (a) Planar-view; (b) cross-sectional view.

change of electronic properties [23] and a rupture of filaments in HRS [24]. Thus, at 1 V the current suddenly decreases from 6 mA to 3 mA. A similar phenomenon has been reported in Pt/HfO2/TiN metal/insulator/metal structures [25]. As shown in Fig. 3(b), stable and reproducible symmetric switching characteristics can be maintained longer than 2.5 × 103 sweep cycles. This feature reveals its promising reliability of the ITO/ZnO/PCMO/ITO structure for nonvolatile memory device. With increasing number of switching, the forming process of the conducting path may be different, as well as the potential energy of the weakest point in the conducting filament [22]. Moreover, a competition between Joule heating and thermal dissipation exists [26]. It is noteworthy that there are two kinds of resistive switching mechanisms that exist in the ZnO/Pr0.7Ca0.3MnO3 film during a switching process. As shown in Fig. 3, the resistance switching of the PCMO/ZnO films can be changed abruptly from HRS to LRS (defined as a set process), while the resistance switching was changed gradually from LRS to HRS (defined as a reset process). This kind of set process could be affected by the quality in the PCMO/ZnO film, such as crystallinity and density of grain boundary and defects [27]. Meanwhile, the filamentary conducting paths are sensitive to the measuring condition [28,29]. Furthermore, different band structures at the PCMO/ZnO interfaces may lead to a different kind of switching process in the ITO/ZnO/PCMO/ITO structure. To investigate the conducting mechanism during the switching process, the conductive behaviors of the ITO/ZnO/PCMO/ITO device were investigated. As shown in Fig. 4, the I–V curves in the LRS and HRS regions were replotted using a double-logarithmic scale at positive and

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R. Zhang et al. / Journal of Non-Crystalline Solids 406 (2014) 102–106

12

(a)

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50th 100th 250th

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I V slope=1.16

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I V slope=1.94

reset

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Fig. 3. Typical I–V characteristic of the ITO/ZnO/PCMO/ITO stacks: (a) 1st circle, (b) 50th, 150th, and 250th circles.

negative fields. It can be seen that the observed I–V curves are in agreement with a trap-controlled space charge limited current (SCLC) behavior [30], 9 V2 εr εo μS 3 ; 8 d

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Voltage(V)

-3

1

-8

I¼

0.1

-1

(b)

4

-12

I V slope=1.12

-7

ð1Þ

where ε0 is the dielectric constant of vacuum; εr is the dielectric constant of the film; d is the thickness of the film; μ is the electron mobility; and S is the area of the electrode. As shown in Fig. 4, the total I–V characteristics of the ITO/ZnO/PCMO/ITO device were divided into three parts: the ohmic region (I–V), the Child's square law region (I–V2), and the steep increasing in the current region. Fig. 5 shows the electrode area dependence of HRS and LRS resistance for the ITO/ZnO/PCMO/ITO device. The area of electrodes was dependent on the resistance of HRS, whereas simultaneously correlated with the resistance of LRS. This indicated that a conductive filament mechanism may dominate the conduction of LRS [31]. It is well known that the internal charge defects (oxygen vacancies, metallic defects, grain boundaries, etc.) result in a formation of the conductive filaments [31]. Based on the above analysis, the scenario of bipolar resistive switching in the ITO/ZnO/PCMO/ITO structure can be summarized. Under an external bias, the defect in ZnO/PCMO/ITO films may act as a trapped center and led to SCLC conducting behavior [32,33]. Moreover, the oxygen vacancy annihilation at the ITO/PCMO interface may be also responsible for a conducting rupture of filaments during the switching process.

0.1

Voltage(V)

1

Fig. 4. Double-logarithmic scale of I–V characteristics of the ITO/ZnO/PCMO/ITO structure for both negative and positive bias regions.

Fig. 6 shows the fatigue performance of resistance switching in an ITO/ZnO/PCMO/ITO structure between the two defined states up to 2.5 × 103 cycles. The resistance state of LRS is relatively stable while a slight fluctuation was found in the resistance state of HRS. The ratio of RHRS/RLRS of the ITO/ZnO/PCMO/ITO structures is higher than 104. Compared to the reported values [2,17,34], the ZnO/PCMO/ITO structure exhibits a higher switching ratio and better resistance stability. Thus, the

108 107

Resistance(Ω)

Current (mA)

8

-5

set

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I V slope=1.77

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I V slope=1.07

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st

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(a)

106

HRS

105 104 103

LRS

102 101

1.5x104 3.0x104 4.5x104 6.0x104 7.5x104

TE area (μm2) Fig. 5. Top electrode area dependence of resistance in the ITO/ZnO/PCMO/ITO structure for the LRS and HRS regions.

R. Zhang et al. / Journal of Non-Crystalline Solids 406 (2014) 102–106 9

10

transparency of 84.6% at 590 nm. This ZnO/PCMO/ITO/glass structure holds promise for potential applications in transparent RRAM invisible electronics.

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Resistance (Ω)

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Acknowledgments

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HRS

This work was supported by the National Basic Research Program of China (No. 2012CB932702), National Science Foundation of China (Nos. 11174031, 51371024, 51325101, 51271020), NCET, PCSIRT, Beijing Municipal Natural Science Foundation (No. 2122037), Beijing Nova program (No. 2011031), and Fundamental Research Funds for the Central Universities.

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Ratioon/off > 10

LRS

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100.0

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3

3

3

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5.0x10 1.0x10 1.5x10 2.0x10 2.5x10 3.0x10

Switching cycles 3

Fig. 6. Endurance switching in the ITO/ZnO/PCMO/ITO device up to 2.5 × 10 times.

ITO/ZnO/PCMO/ITO structures with a lower operating voltage, higher operating speed, and better resistance stability have a good potential in the memory applications. Fig. 7 shows the optical transmittance spectrum of the ITO/ZnO/ Pr0.7Ca0.3MnO3/ITO/glass cell. As shown in the figure, the structure exhibits an average transparency of 79.6% in the visible range (450– 750 nm). Moreover, a maximum transparency of 84.6% was observed at the wavelength of 590 nm. Compared to the reported values, the ITO/ZnO/PCMO/ITO/glass structure exhibits better optical transparent properties (79.6%) than the single film of PCMO [2] (77%) and ZnO [35] (75%). More interestingly, as shown in the inset of Fig. 7, a clear photograph without any distortion or refraction of the university's logo (USTB) can be seen. Those optical transparent results strongly support the possible application of the ITO/PCMO/ZnO/ITO structures in the transparent RRAM devices. 4. Conclusions The amorphous ZnO/PCMO/ITO/glass cell exhibited both a bipolar resistive switching character and a good transparency for the visible range. The device shows a stable resistive switching up to 2.5 × 103 cycles and the ratio of on/off is as high as 104. The conductive switching behavior in the ZnO/PCMO films is related to the trap-controlled SCLC mechanism [36]. An average transparency of 79.6% in the visible range was observed in the ITO/ZnO/PCMO/ITO/glass with a maximum

100

ITO/ZnO/PCMO/ITO/glass 80

Transmittance (%)

105

ITO/glass 60 40 20 0 300

400

500

600

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800

Wavelength (nm) Fig. 7. Optical transmittance properties of the ITO/ZnO/PCMO/ITO/glass cell. (Inset) A clear photography of the underlying logo of USTB without any distortion or refraction.

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