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Resistance-Switching Characteristics of Polycrystalline Nb2O5 for Nonvolatile Memory Application Hyunjun Sim, Dooho Choi, Dongsoo Lee, Sunae Seo, Myong-Jae Lee, In-Kyeong Yoo, and Hyunsang Hwang
Abstract—The resistance switching characteristics of polycrystalline Nb2 O5 film prepared by pulsed-laser deposition (PLD) were investigated for nonvolatile memory application. Reversible resistance-switching behavior from a high resistance state to a lower state was observed by voltage stress with current compliance. The reproducible resistance-switching cycles were observed and the resistance ratio was as high as 50–100 times. The resistance switching was observed under voltage pulse as short as 10 ns. The estimated retention lifetime at 85 C was sufficiently longer than ten years. Considering its excellent electrical and reliability characteristics, Nb2 O5 shows strong promise for future nonvolatile memory applications. Index Terms—Nb2 O5 , pulsed laser deposition (PLD), resistance switching, switching retention, switching time.
I. INTRODUCTION
R
ECENTLY, new nonvolatile memory devices such as polymer random access memory (RAM) [1], magnetic RAM [2], Flash memory [3], and resistance RAM [4] have been extensively investigated for future memory device applications. Among these new memory devices, resistance RAM (ReRAM) shows excellent memory characteristics such as low power operation, high density integration, and high-speed write and erase operations [4]. Since the early 1960s, the resistance switching behaviors of various oxides such as Nb O [5], TiO [6], NiO [7], Al O [8], and ferroelectrics oxide [9] have been investigated. To explain the resistance-switching behavior, various models by Hickmott, Simmoms–Verderber, Barriac, and Dearnaley were proposed [10]. However, a clear switching mechanism and memory switching characteristics under short (nanosecond) pulse stress have not yet been reported in detail [11], [12]. In this letter, we have investigated the resistance-switching characteristics of polycrystalline Nb O thin films prepared by pulsed laser deposition (PLD) under pulse stress for nonvolatile memory application.
Manuscript received February 18, 2005. This work was supported by the Samsung Advanced Institute of Technology and the National Research Program for the 0.1 Terabit Non-Volatile Memory Development sponsored by the Korea Ministry of Science and Technology. The review of this letter was arranged by Editor A. Chatterjee. H. Sim, D. Choi, D. Lee, 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]). S. Seo, M.-J. Lee, and I.-K. Yoo are with the Samsung Advanced Institute of Technology, Suwon 440-600, Korea. Digital Object Identifier 10.1109/LED.2005.846592
Fig. 1. 2.236-MeV 4He RBS spectra and XRD spectrum (inset) of a 58-nm-thick Nb O film annealed at 800 C deposited on Si substrate.
II. EXPERIMENTAL After standard wafer cleaning followed by HF-last cleaning to maintain a hydrogen-passivated silicon surface, a 58-nm-thick Nb O film was directly deposited on p-type Si (100) substrate /cm ) by a PLD with boron doping concentrations ( process in oxygen ambient using a KrF excimer laser (248 nm in wavelength, 30 ns in pulse width, and operated at 1 Hz). The substrate temperature was 700 C, and the oxygen partial pressure was 30 mtorr. After depositing the Nb O film, postannealing was performed at 800 C for 5 min in nitrogen ambient at atmospheric pressure in a tube furnace for crystallization. An approximately 150-nm-thick platinum top electrode was deposited by RF magnetron sputtering through a shadow cm electrode area. Reproducible curmask with rent–voltage ( – ) switching measurements were performed using a parameter analyzer (HP 4155A) controlled by a software program. The structural investigation of Nb O film was carried out using X-ray diffraction (XRD) with a Cu–K X-ray source and stoichiometry analyses of film were conducted by Rutherford backscattering spectroscopy (RBS) III. RESULTS AND DISCUSSION The composition analyses of polycrystalline Nb O annealed at 800 C conducted by RBS are shown in Fig. 1. The simulated composition ratio of niobium (Nb) to oxygen (O) atoms
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SIM et al.: RESISTANCE-SWITCHING CHARACTERISTICS OF POLYCRYSTALLINE
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Fig. 3. Resistance ratio as a function of switching time. I –V characteristics of two conduction states after pulse stress (inset).
Fig. 2. (a) Current–voltage characteristics of the high-resistance and low-resistance states for Pt/Nb O /Si device. (b) Reproducible resistance switching was shown.
in Nb O film was approximately 0.4, which yields stoichiometric Nb O film. The inset of Fig. 1 shows the XRD spectra of 58-nm-thick Nb O film annealed at 800 C. Various peaks in the XRD pattern indicate that the film was polycrystalline by annealing at 800 C. In contrast, the as-deposited sample at 700 C shows no clear XRD peaks (not shown). We observed two distinct conduction states of Pt–Nb O –Si devices, as shown in Fig. 2(a). Gate bias was directly applied to Pt-gate with substrate at ground. The high-resistance state was switched to a low-resistance state by voltage stress with current compliance [13]. The transition from high resistance to low resistance was defined as “set.” Subsequently, the transition from the lower resistance state to the higher resistance state was occurred by voltage stress with current compliance, and it was defined as “reset.” The reset and set voltages were approximately 2 and 3.5 V, respectively. The reproducible resistance-switching cycles were observed, as shown in Fig. 2(b). The current compliance of the reset and set state was approximately 500 and 10 A, respectively. The resisk ) to the high-retance ratios of the low-resistance state ( sistance state ( 1 M ) ranged from 50 to 100 times. Considering real memory device operation, memory switching under pulse stress is important. We have investigated the resistance-switching time by means of a short voltage pulse. Fig. 3 shows the resistance ratio as a function of switching time. The resistance ratio between the high conductance state and low conductance state by voltage pulse stress also ranged from 50 to 100 times. The inset of Fig. 3 shows the – characteristics of two conduction states after pulse stress.
Fig. 4. Degradation of resistance ratio measured at 85 C. The estimated degradation of resistance ratio at 85 C for 10 years is approximately 7%.
We observed clear resistance switching at pulsewidth as short as 10 ns. We investigated the degradation of resistance ratio at 85 C, as shown in Fig. 4. The estimated degradation of the resistance ratio at 85 C for 10 years is approximately 7%, indicating the potential for nonvolatile memory applications. Although various models have been proposed to explain the switching mechanism of binary oxides, a mechanism that can consistently explain experimental results is not available [14]–[17]. IV. CONCLUSION The resistance switching characteristics of polycrystalline Nb O films by PLD process were investigated for nonvolatile memory application. The reproducible switching cycles by voltage stress with current compliance were observed. The k ) resistance ratio between the high conductance state ( and low conductance state ( M ) ranged from 50 to 100 times. Under voltage pulse amplitude of 4.5 V and a pulsewidth of 10 ns, clear resistance switching was observed. We also observed a sufficiently stable memory state at 85 C, and the memory lifetime was longer than ten years. Although the
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switching mechanism is not yet clearly understood, considering the excellent memory switching behavior and reliability of Nb O , ReRAM based on Nb O shows strong promise for future nonvolatile memory applications. REFERENCES [1] Li. 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. [3] S. Choi, M. Cho, H. Hwang, and J. Kim, “Improved metal-oxide-nitride-oxide-silicon-type flash device with high- dielectrics for blocking layer,” J. Appl. Phys., vol. 94, pp. 5408–5410, 2003. [4] W. W. Zhuang, W. Pan, B. D. Ulrich, J. J. Lee, L. Stecker, A. Burmaster, D. R. Evans, S. T. Hsu, M. Tajiri, A. Shimaoka, K. Inoue, T. Naka, N. Awaya, K. Sakiyama, Y. Wang, S. Q. Liu, N. J. Wu, and A. Ignatiev, “Novell colossal magnetroresistive thin film nonvolatile resistance random access memory (RRAM),” in IEDM Tech. Dig., 2002, pp. 193–196. [5] W. R. Hiatt and T. W. Hickmott, “Bistable switching in niobium oxide diode,” Appl. Phys. Lett., vol. 6, pp. 106–108, 1965. [6] F. Argall, “Switching phenomena in Titanium oxide thin films,” SolidState Electron., vol. 11, pp. 535–541, 1968. [7] J. F. Gibbons and W. E. Beadle, “Switching properties of thin NiO films,” Solid-State Electron., vol. 7, pp. 785–797, 1964.
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