Nonvolatile resistance memory switching in polycrystalline ZnO thin films grown by ISERP ISERP

Amit Kumar et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 1, Issue No. 2, 118 - 122

Nonvolatile resistance memory switching in polycrystalline ZnO thin films grown by RF magnetron sputtering Manoj Kumar

Department of Electrical, Electronic & Information Engineering, Toyohashi University of Technology, Tempakucho, Hibariga-oka1-1, Toyohashi-4418580, Japan Panwarm72@yahoo.com

(poly-N-vinylcarbazole), [3]. However, bulk and thin films of transition-metal oxides (TMO) materials, such as TiO2 [4], NiO [5], ZrO [6] Nb2O5 [7] and ZnO [8] are also known to show a resistive switching behavior. In comparison with ternary or quaternary oxide semiconductor films such as doped (SrZrO3) or (Pr,Ca)MnO3, binary metal oxides have the advantage of a simple fabrication process and are more compatible with complementary metal-oxide semiconductor processing [9]. Note that TMO materials have gained considerable interest because of its excellent material properties. The wide bandgap (3.3 eV) and highly transparent in visible region make TMO materials a potential candidate to realize a fully transparent RRAM (TRRAM) device which could be implemented in various electronic system. The TRRAM has great feasibility for novel electronic applications in the near future. Among the known TMO materials, which have been currently investigated for the development of RRAM, more recently ZnO has emerged showing great potential due to its abundant availability in nature, rugged structural and chemical behaviour, highly evolved growth technologies, nontoxic, inexpensive and compatibility with complementary metal oxide semiconductor technology. ZnO is II– VI n-type semiconductor with direct and wide band gap of about 3.3 eV at room temperature. ZnO films show good optical transmittance in the visible and near-infrared region. Its electrical properties can be tailored in a broad range from metal like to insulator like by suitable impurity doping. Thus, it is expected to achieve a fully transparent RRAM based on ZnO [10].

Abstract In this paper, the resistance switching characteristics of polycrystalline ZnO thin film were investigated for non-volatile memory. ZnO thin films grown by RF magnetron sputtering on Ti/SiO2/Si substrate were highly resistive and caxis oriented. The ZnO thin film showed reliable and repeated switching behavior under small voltage. Resistance ratios of the high resistance state to low resistance state were obtained to be several orders of magnitude up to 20 cycles. It is believed that this resistance change is due to the difference in point defect density. Both the low and high resistance states showed linear log current versus log voltage characteristics in the low voltage. The observed results were examined on the basis of filamentary conductions with the contribution of local Joule heating effect. The achieved characteristics of the resistive switching in ZnO thin films provide a possible solution for nonvolatile memory applications. Keywords: ZnO thin films; RF sputtering; Nonvolatile resistive memory switching

Beer Pal Singh

Department of Physics, Ch. Charan Singh University, Meerut – 250 004, U. P. India drbeerpal@gmail.com

Amit Kumar

Institute of Engineering & Technology, North Extn., M.I.A. Alwar-301030, Rajasthan, India amitpanwar889@gmail.com

1. Introduction

Resistance switching random access memory (RRAM) has drawn considerable attention for the application in non-volatile memory elements in semiconductor memory devices. The resistive switching phenomenon, which signifies a drastic change in resistance in some materials, has reportedly been found in ferromagnetic oxide materials (Pr1−xCaxMnO3), [1] doped perovskite oxide materials (SrZrO3), [2] and organic materials

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Amit Kumar et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 1, Issue No. 2, 118 - 122

(110) ZnO

(002) Ti (102) Ti

(002) ZnO

2Deg.)

ZnO thin films on Ti/SiO2/n-Si were grown by RF magnetron sputtering technique. The deposition was carried out in a vacuum chamber evacuated to a base pressure of 2 × 10-6 Torr. ZnO thin films were grown at rf power of 100 W. During the film deposition substrate temperature of 300 oC and the deposition time of 5 min was kept constant. High purity O2 (99.999%) was used as the sputtering gas to achieve highly resistive ZnO thin films. The partial pressure was adjusted by the use of electronic mass flow meters and total working pressure was fixed at 10 mTorr. The thickness of the resulting ZnO thin film was estimated to be around 150 nm. The ZnO powder was mixed for 24 h by using ball milling. The mixture was grinded to a fine powder and pressed into a pellet by cold isostatic press at 800 Kg-cm-2, and the pellet was sintered at 1000 oC for 4 h. The pellet obtained was used as the target for preparation of ZnO thin films. A Riggaku (Japan, D/max-II with CuK radiation) x-ray diffraction (XRD) system was employed to study crystallographic orientation of the films. Both top and bottom contacts were deposited using RF sputtering technique. The direction of current flow was kept from top to bottom electrode and the measurements were carried out in voltage sweeping mode.

2. Experimental Details

1(b). The grain size determined from SEM was in the range of 25–30 nm.

Intensity (a. u.)

In this article, a report on the resistance switching characteristics of highly resistive ZnO thin films grown by RF magnetron sputtering is presented. It is demonstrated that the ZnO thin film shows reliable and reproducible switching phenomenon.

3. Results and Discussions

Figure 1 (a) shows XRD pattern of the ZnO thin film grown on Ti/SiO2/Si substrate. In the present study, the ZnO thin film revealed polycrystalline structure. The XRD spectrum exhibited (002) diffraction peak of ZnO film, indicating preferred highly c-axis orientation. In addition, Si and Ti peaks also emerged in the spectrum. The full widths at half maximum of the (002) peak of ZnO thin film was found to be 0.25o. SEM image showed low density packing of grains with a uniform dimension, which is illustrated in Fig.

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Figure 1. (a) XRD spectrum (b) SEM image of the ZnO thin film grown on Ti/SiO2/Si substrate

During the I-V measurements of resistive switching, Ti bottom electrode was grounded and positive bias voltage was applied to the top electrode. The resistance of the ZnO thin film was measured through room temperature I–V measurements. The resistive switching characteristic of the devices are investigated and is demonstrated in Fig. 2.

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3.0x10

-2

2.5x10

-2

2.0x10

-2

1.5x10

-2

1.0x10

-2

5.0x10

-3

Reset Set

0.0 -5.0x10

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Voltage (V) Figure 2. Typical I–V characteristics of ZnO thin film showing resistive switching

LRS

-2

Current (A)

The activation of the devices as a nonvolatile memory requires a forming process with current compliance of 7 mA in the present work, wherein the ZnO thin film is transformed from its initial high resistance state (HRS) to low resistance state (LRS) on application of an external bias voltage (3.0 ~3.5 V) known as the initial forming voltage. The initial forming voltage of 3.0 ~ 3 5 V in the current study is favourably comparable with the recently reported forming voltages in ZnO films and much smaller than those reported for other metal oxides [11, 12]. After the initial forming process, the voltage swept from 0 to 2.0 V, A sudden drop in current was observed at a voltage of ~ 1.6 V indicating abrupt increase in the resistance of ZnO thin film and switching from LRS (OFF state) reached into HRS (ON state). When the voltage reswept from zero to positive voltage, an increase in current was observed at ~ 3 V and ZnO thin film switched back to the LRS state and nonvolatile switching was achieved. The plot of the log I-log V of the LRS in the device is shown in Fig. 3 (a). It is seen from figure that the log I-log V characteristic is linear with a slope of approximately 0.9, indicating conduction behavior Ohmic transport. However, at higher bias voltages (> 0 9 V), the I–V curve showed deviation from the linearity, which may be due to the heating induced increase in the resistance conducting filaments before the onset of rupture process due to excess heating, which leads the film to HRS. Kim et al. [13] have also observed such deviation from linear

I–V characteristics at high voltages. The log I-log V characteristic of ZnO film in the HRS is shown in Fig. 3 (b). As is seen from figure that the I-V characteristic in HRS is also reveal linear behavior at very low voltage which is governed by Ohm’s law. However at slight high voltages, the I-V characteristic showed nonlinear behaviour. This nonlinear I-V characteristic in the high voltage region can be explained by the Poole–Frenkel (PF) emission theory. Since the ZnO thin film is grown at low-temperature generally contain large number of point and structural defects which may act as carrier traps during the conduction process. These results are consistent with those of a previous report that investigated the characteristics of RRAM with ZnO thin film [14].

Current (mA)

Amit Kumar et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 1, Issue No. 2, 118 - 122

-3

0.13534

Voltage (V)

HRS -2

Current (mA)

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0.36788

-3

Voltage (V)

Figure 3. Log–log plot of I–V characteristics of ZnO film in (a) LRS and (b) HRS

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Amit Kumar et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 1, Issue No. 2, 118 - 122

HRS

4. Conclusion

LRS 2

Resistance (ohm)

flow through the filaments is large enough so that the most of the conductive filaments are disrupted via the Joule heating effect and bringing the material in HRS. However, some conductive filaments still remain, and they can contribute to the conduction, the Ohmic conduction, in the HRS at a low voltage region. Though, at high voltage region, the conduction behavior is dominated by Pooleâ&#x20AC;&#x201C;Frenkel emission, which is closely related to the defects in the ZnO thin films. It is well recognized that the extended defects play an important role in the formation of the conductive filament and stable resistive switching. ZnO thin film has a polycrystalline structure in this study as is shown in Fig. 1 (a), structural defects specially grain boundaries may provide a considerable contribution to the formation of the conductive filament and the stable resistive switching.

Figure 4 shows the resistances of the ZnO thin film in the two well determined states of LRS and HRS were measured for different switching cycles to study the device survival. It is noticed from the figure that the resistance in ZnO thin film in LRS was observed a stable switching property while, there is a slight fluctuation of resistance in the HRS during 20 cycles. Other group has also been observed large fluctuations in the resistance of ZnO thin film in the HRS with switching cycles [11]. It is believed that the results show a significant reliable performance of the device for nonvolatile memory application.

Switching Cycle

Figure 4 switching cycling characteristics for the HRS and LRS

Resistance switching mechanism in ZnO thin film is not yet clearly understood. One of the feasible models for the formation of linear conducting paths in various metal oxide films is the formation of conducting filaments. Conducting filaments are a variety of extended defects which can be created by the alignment of the pre-existing structural defects such as oxygen vacancies, Zn interstitials and dislocations etc along the grain boundaries of the polycrystalline films due to the presence of a strong electric field during the initial part of the formation process. In the initial forming process, tiny and less conductive filaments are formed through the defects of the ZnO thin film. The filaments become stronger and more conductive with further increase in voltage and current flow suddenly increases manifolds resulting in a transition to the LRS. Hence, the conduction in the LRS can be expressed by Ohmic behavior. In the reset process case, it is believed that the current

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Highly resistive and c-axis oriented polycrystalline ZnO thin films were grown on Ti/SiO2/Si substrate by RF magnetron sputtering technique for studied their resistive switching behaviour. The ZnO thin film based device exhibits reliable resistive switching properties such as low switching voltage and switching cycling. The I-V measurements are closely related to conductive filament formation during voltage application. Hence, the observed results offer the possibility of realizing transparent nonvolatile memory devices. Acknowledgement The authors wish to express their thanks to Dr. J. P. Kar for valuable discussion and comments. Reference [1] S. Q. Liu, N. J. Wu, and A. Ignatiev, Appl. Phys. Lett. vol. 76, pp. 2749-2751, March 2000. [2] Y. Watanabe, J. G. Bednorz, A. Bietsch, Ch. Gerber, D. Widmer, A. Beck, and S. J. Wind, Appl. Phys. Lett. vol. 78, pp. 3738-3740, April 2001. [3] Y. S. Lai, C. H. Tu, D. L. Kwong, and J. S. Chen, Appl. Phys. Lett. vol. 87, pp. 122101-122101-3, September 2005. [4] B. J. Choi, D. S. Jeong, S. K. Kim, S. Choi, J. H. Oh, C. Rohde, H. J. Kim, C. S. Hwang, K. Szot, R. Waser, B. Reichenberg, and S. Tiedke, J. Appl. Phys. vol. 98 pp. 033715-033715-10 August 2005.

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[5] S. Seo, M. J. Lee, D. H. Seo, E. J. Jeoung, D. S. Suh, Y. S. Joung, I. K. Yoo, I. R. Hwang, S. H. Kim, I. S. Byun, J. S. Kim, J. S. Choi, and B. H. Park, Appl. Phys. Lett. vol. 85, pp. 5655-5657, December 2004. [6] S. Kim, I. Byun, I. Hwang, J. Kim, J. Choi, B. H. Park, S. Seo, M. J. Lee, D. H. Seo, D. S. Suh, Y. S. Joung, and I. K. Yoo, Jpn. J. Appl. Phys., Part 2 vol. 44, pp. L345-L347, February 2005. [7] H. J. Sim, D. H. Choi, D. S. Lee, S. A. Seo, M. J. Lee, I. K. Yoo, and H. S. Hwang, IEEE Electron Device Lett. vol. 26, pp. 292-294, May 2005. [8] M. Villafuerte, S. P. Heluani, G. Juarez, G. Simonelli, G. Braunstein, and S. Duhalde, Appl. Phys. Lett. vol. 90, pp. 052105-052105-3, January 2007. [9] X. Wu, P. Zhou, J. Li, L. Y. Chen, H. B. Lv, Y. Y. Lin, and T. A. Tang, Appl. Phys. Lett. vol. 90, pp. 183507183507-3, May 2007. [10] L. M. Kukreja, A. K. Das and P. Miisra, Bull. Mater. Sci., vol. 32, pp. 247-252, June 2009. [11] W -Y Chang, Y -C Lai, T -B Wu, S -F Wang, F. Chen and M –J Tsai, Appl. Phys. Lett. vol. 92, pp. 022110022110-3, January 2008. [12] H -Y Lee, P -S Chen, C -C Wang, S. Maikap, P -J Tzeng, C -H Lin, L -S Lee and M –J Tsai, Jap. J. Appl. Phys. vol. 46, pp. 2157-2163, April 2007. [13] K-M Kim and C-S Hwang, Appl. Phys. Lett. vol. 94 pp. 122109-122109-3 March 2009.

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