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Pt/ZrO /p+-Si sandwich structure fabricated by reactive sput- tering shows two stable resistance states. By applying proper bias, resistance switching from one to ...
IEEE ELECTRON DEVICE LETTERS, VOL. 26, NO. 9, OCTOBER 2005

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Resistance Switching of the Nonstoichiometric Zirconium Oxide for Nonvolatile Memory Applications Dongsoo Lee, Hyejung Choi, Hyunjun Sim, Dooho Choi, Hyunsang Hwang, Myoung-Jae Lee, Sun-Ae Seo, and I. K. Yoo

Abstract—The resistance switching behavior and switching mechanism of nonstoichiometric zirconium oxide thin films were investigated for nonvolatile memory application. The Pt/ZrO /p+ -Si sandwich structure fabricated by reactive sputtering shows two stable resistance states. By applying proper bias, resistance switching from one to another state can be obtained. The composition in ZrO thin films were confirmed from X-ray photoelectron spectroscope (XPS) analysis, which showed three layers such as top stoichiometric ZrO2 layer with high resistance, transition region with medium resistance, and conducting ZrO bulk layer. The resistance switching can be explained by electron trapping and detrapping of excess Zr+ ions in transition layer which control the distribution of electric field inside the oxide, and, hence the current flow. Index Terms—n-MOSFET, nonstoichiometric zirconium oxide, resistance random access memory (RAM), resistive switching, switching mechanism, tri-layer structure.

I. INTRODUCTION

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ECENTLY, various new memory devices such as polymer random access memory (PoRAM) [1], magnetic random access memory (MRAM) [2], Flash memory [3], and resistance random access memory (RRAM) have been proposed for future nonvolatile memory device applications. Among theses new memory devices, resistance RAM (RRAM) shows various advantages such as simple device structure, low power operation, high density integration, high-speed operations, and compatibility of current CMOS process [4]–[6]. Since the early 1960s, the resistance switching behaviors of various oxides such as Nb O [7], [8], TiO [9], NiO [10], Al O [11] and ferroelectrics oxide [12] have been investigated. To explain resistance switching behavior, various models by Hickmott, Simmoms-Verderber, Barriac and Dearnaley were proposed [13]. However, clear switching mechanisms have not yet been reported in detail [14]–[21].

Manuscript received May 10, 2005; revised June 9, 2005. This work was supported in part by the Samsung Advanced Institute of Technology (SAIT) and in part by the Ministry of Science and Technology (MOST) under the National Research and Development Project. The review of this paper was arranged by Editor C.-P. Chang. D. Lee, H. Choi, H. Sim, D. Choi, and H. Hwang are with the Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea (e-mail: [email protected]). M.-J. Lee, S.-A. Seo, and I. K. Yoo are with the Samsung Advanced Institute of Technology (SAIT), Suwon 440-600, Korea. Digital Object Identifier 10.1109/LED.2005.854397

Fig. 1. (a) HR-TEM image of the ZrO film grown at 400 C. (b) Inset is an XRD spectrum SEM photograph showing a cross-sectional view of nonstoichiometric zirconium oxide film.

In this letter, we have investigated the resistance switching characteristics of a nonstoichiometric ZrO prepared by reactive sputtering for nonvolatile memory application. II. EXPERIMENT After the standard cleaning of a p -type silicon wafer, the nonstoichimetric ZrO films were deposited at 400 C by the radio frequency (RF) reactive magnetron sputtering from a Zr target. The flow rate of the oxygen to argon was 1 : 30. To grow a thin stoichiometric oxide layer, post deposition annealing was performed in oxygen ambient at 250 C for 10 min. The Pt/ZrO /p -Si device with 100-nm-thick platinum (Pt) gate electrode was fabricated by photolithography. The n-MOSFET device with ZrO resistor layer in source region was also fabricated by the conventional MOSFET process. A high resolution transmission electron microscope (HR-TEM) was used to observe a microstructure. The stoichiometry of the deposited sample was measured using 2.236 MeV He by Rutherford backscattering spectrometry (RBS) analysis. The chemical binding energy analysis was performed using X-ray photoelectron spectroscope (XPS). The current–voltage (I–V) characteristics of samples were measured by an HP4155 semiconductor parameter analyzer. III. RESULTS AND DISCUSSION The stoichiometry of the deposited film was analyzed by RBS. Using the RBS data (not shown), we confirmed that the average oxygen to zirconium ratio is 1.65. Fig. 1(a) shows the

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IEEE ELECTRON DEVICE LETTERS, VOL. 26, NO. 9, OCTOBER 2005

Fig. 2. XPS spectra of Zr 3d (a) top oxide: from top to 20 nm in depth, (b) transition region: from 20 and to 42 nm, and (c) bulk region. (d) Schematic representation of the structure of the ZrO film in depth.

high resolution TEM image which indicates a good crystalline structure. Based on an X-ray diffraction (XRD) pattern as shown in the inset of Fig. 1, the ZrO film shows the mixture structure with tetragonal (101) and monoclinic (020) planes. The monoclinic (020) plane shows a higher XRD intensity than that of the tetragonal (101) plane. From the cross-sectional scanning electron microscope (SEM) photograph shown in Fig. 1(b), we can see two layers of ZrO on the substrate (referred to here as “top layer and bulk layer”). To confirm composition and binding energy, XPS depth profiling was performed as shown in Fig. 2. From XPS depth profiling, ZrO film was composed of three layers (referred to here as “top layer, transition layer, and conducting bulk layer”). The XPS for metallic Zr and ZrO is 179.04 peak position of Zr and 183.2 eV, respectively. The additional peak is related to Zr for metallic Zr and ZrO ( eV). The XPS spectra of the surface of the films from Fig. 2(a) shows that Zr 3d peak is associated with ZrO layer indicating the stoichiometric oxide layer. In contrast, the bulk ZrO layer and transition layer show both peaks from metallic Zr and ZrO which indicate metal excess nonstoichiometric ZrO layer. Fig. 3(a) shows typical I–V characteristics of the MIS device with ZrO as reversible switched resistive layer. The negative differential resistance (NDR) behavior was clearly observed. Fig. 3(b) shows typical switching behavior of set and reset current. To implement resistor memory with n-MOSFET, Pt/ZrO /n -Si structure was used at the source side of nMOSFET. Fig. 3(c) depicts – characteristics of n-MOSFET with resistor memory in source region. By adjusting resistance state of ZrO with pulse stress, we can control transistor current. The pulse stress time was 100 ns and pulse amplitude for on and off condition was 6 and 2 V, respectively. If there is voltage drop in source region, the effective gate bias can be reduced significantly – ). Fig. 3(d) shows retention (

Fig. 3. (a) I–V characteristics of high resistance state and low resistance state for Pt/ZrO /Si device (b) Reset/set current characteristics as a function of curves for a long channel enhancement switching cycles. (c) I versus V mode n-MOSFET. (d) Retention characteristics of n-MOSFET .

characteristics of n-MOSFET. The resistor memory shows good retention characteristics for both high and low resistance states. Although various models were proposed to explain resistance switching, the clear physical model is not yet available, especially for binary metal oxide. Based on experimental data, we propose switching model of ZrO as shown in Fig. 4. From our XPS data as shown in Fig. 2, the ZrO films have metallic zirconium defects in the stoichiometrc zirconium oxide matrix and it is composed of three layers such as top stoichiometric ZrO layer with high resistance, transition region with medium resistance and conducting ZrO bulk layer. To activate the RRAM device, high forming voltage stress (or forming process) is necessary. The forming voltage depends on film property, and it is in the range between 5 and 7 V. Considering top insulating ZrO layer, soft-breakdown of top oxide layer is necessary to activate resistance switching as shown in Fig. 4(a). The bulk ZrO shows very high conductivity. The sheet resistance of as deposited ZrO measured by both four-point probe and Hall . The thickness of ZrO and demeasurement is about 12.4 cm , respectively. The vice area is about 120 nm, and 9 calculated bulk resistance of ZrO is about 2 . However, based on the – curve, the device resistance is about . The significant difference of resistance value can 1 to 10 be explained by the resistance of transition layer and top oxide layer. Therefore, during the initial forming stress condition, high voltage is only applied on top insulating layer and it causes soft-breakdown of top insulating layer. Considering the transition layer with Zr excess, high voltage stress causes ionization of metallic Zr and generates positive Zr ions. The positive charges in ZrO cause band bending which in turn enhances current flow through the transition layer as shown in Fig. 4(c). As shown in Fig. 3(a), the critical bias

LEE et al.: RESISTANCE SWITCHING OF NONSTOICHIOMETRIC ZIRCONIUM OXIDE

Fig. 4. Proposed resistive switching mechanism of the ZrO film: (a) band diagram after forming, (b) high-resistance state without positive charge, and (c) low resistance state with positive charge.

(or set voltage, over 1.6 V) enables the ZrO film to the low resistance state. If we apply bias above the critical value, we expect electron accumulation at transition layer due to the limited current flow at top oxide layer. It might causes recombination of electron with positive Zr ion which in turn reduces electric field and current flow as shown in Fig. 4(b). This model is similar to that of Simmons and Verderber model to explain voltage-controlled negative resistance and reversible memory effects. [20] IV. CONCLUSION The resistance switching behavior of ZrO has been investigated for nonvolatile memory device application. Both MIS capacitor and MISFET with resistance memory at source region clearly show two distinct memory states. By applying proper bias, resistance switching from one state to another can be obtained. The resistance switching and NDR behavior can be explained by electron trapping and detrapping at Zr -trap in ZrO matrix. Considering simple device structure and compatibility with current CMOS technology, RRAM devices shows a good promise for future nonvolatile memory device applications. REFERENCES [1] L. Ma, S. Pyo, J. Ouyang, Q. Xu, and Y. Yang, “Nonvolatile electrical bistability of organic/metal cluster/organic system,” Appl. Phys. Lett., vol. 82, pp. 1419–1421, 2003. [2] D. C. Worledge and D. W. Abraham, “Conducting atomic-force-microscope electrical characterization of submicron magnetic tunnel junctions,” Appl. Phys. Lett., vol. 82, pp. 4522–4524, 2003.

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