Japanese Journal of Applied Physics Vol. 47, No. 3, 2008, pp. 1635–1638 #2008 The Japan Society of Applied Physics
Structural Properties and Resistance-Switching Behavior of Thermally Grown NiO Thin Films Dong-Wook K IM, Ranju J UNG1 , Bae Ho PARK2 , Xiang-Shu L I1 , Chanwoo P ARK, Seongmo S HIN, Dong-Chirl K IM1 , Chang Won LEE1 , and Sunae S EO1 Department of Applied Physics, Hanyang University, Ansan, Kyeonggi 426-791, Korea 1 Samsung Advanced Institute of Technology, Suwon, Kyeonggi 440-600, Korea 2 Department of Physics, Konkuk University, Seoul 143-701, Korea (Received September 7, 2007; revised November 23, 2007; accepted November 27, 2007; published online March 14, 2008)
We investigated the structural and electrical properties of polycrystalline NiO thin films on Pt electrodes formed by thermal oxidation. A Ni–Pt alloy phase was found at the interface, which could be explained by the oxidation kinetics and reactions of Ni, NiO, and Pt. An increase in the oxidation temperature decreased the volume of the alloy layer and improved the crystalline quality of the NiO thin films. Pt/NiO/Pt structures were fabricated, and they showed reversible resistance switching from a high-resistance state (HRS) to a low-resistance state (LRS) and vice versa during unipolar current–voltage measurements. The oxidation temperature affected (did not affect) the HRS (LRS) resistance of the Pt/NiO/Pt structures. This indicated that the transport characteristics of HRS and LRS should be different. [DOI: 10.1143/JJAP.47.1635] KEYWORDS: NiO, thermal oxidation, resistance switching
1.
Introduction
The resistance-switching behavior of metal/oxide/metal sandwich structures has attracted considerable attention due to its potential for nonvolatile random-access memory applications.1–11) Among numerous resistance-switching oxides, NiO is one of the most intensively investigated materials owing to its good retention properties.1–7,10,11) However, a clear understanding of the switching mechanism is still lacking. Recently, the roles of metal/oxide interfaces have gained increasing interest regarding improvement of the device performance as well as revealing the switching mechanism.2–9) Very thin interfacial layers, such as IrO2 and Ni–Pt alloy layers, can decrease the switching-voltage fluctuation of metal/NiO/metal structures.3,4) It is very likely that the resistance switching is based on the reduction and oxidation of the oxide layers.3–10) Such chemical reactions seem to be assisted by heat.9) This indicates that determining the reaction at the metal/NiO interface at elevated temperatures is very important in understanding the resistance-switching mechanism of NiO thin films. Careful investigations of the interface microstructures and the related transport properties can allow us to understand the nature and origin of the resistance-switching. In this report we present the results of structural and electrical investigations of thermally grown NiO thin films in the temperature range of 350 – 550 C. At the NiO/Pt interface, a Ni–Pt alloy layer was found. Such alloy formation can be understood on the basis of the oxidation kinetics and reactions of Ni, NiO, and Pt. The transport characteristics and resistance-switching behaviors of the Pt/ NiO/Pt structures are then discussed. 2.
Experiment
The NiO thin films were prepared by evaporating Ni films on Pt electrodes and subsequent thermal oxidation. First, 40nm-thick Ni films were deposited on commercial platinized Si substrates (150-nm-thick Pt/10 nm TiOx /300 nm SiO2 /Si,
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purchased from Inostek) using an e-beam evaporator, with a base pressure of less than 1 10 7 Torr. The TiOx layer was used as an adhesion layer for the Pt electrode to improve thermal stability.12) Then, the films were annealed in a tube furnace, which was evacuated using a mechanical pump and then filled with oxygen at 400 Torr.11) The oxidation time was 1 h, which was sufficient to oxidize the 40-nm-thick Ni films completely at annealing temperatures 350 C.13,14) The crystalline quality and phase composition of the films were analyzed by grazing-incidence X-ray diffraction (XRD). At low angles of incidence, the X-rays penetrate only the uppermost layers of a sample, enabling surface-sensitive characterization. Microstructural and compositional analyses were performed using a field-emission transmission electron microscope (TEM) equipped with an energy-dispersive spectrometer (EDS). The sample used for the TEM measurement was made by ion milling and the thickness was around 60 – 70 nm. For the EDS measurements, the incident probe size was 1 nm and the spatial resolution was 3 nm. The obtained EDS spectra were quantified using the ES Vision software. The absolute quantification by EDS is difficult, but the relative comparison is reasonable with experimental error within 5 atomic %. For the current–voltage (I–V) measurements, top electrodes of 30 nm Au/10 nm Pt were evaporated using a shadow mask with an electrode area of 100 100 mm2 . The metal layers were deposited on the NiO thin films using the e-beam evaporator. The resistance-switching characteristics were investigated using an Agilent 4155C semiconductor parameter analyzer at room temperature. 3.
Results and Discussion
Figure 1 shows grazing-incidence XRD –2 scan results for NiO thin films prepared at 350, 450, and 550 C. The XRD patterns show that the NiO thin films have single-phase and polycrystalline structures. No remaining Ni peak can be observed, as anticipated. The NiO phase peaks are stronger and narrower for the films oxidized at higher temperatures, indicating better crystalline quality. For Ni oxidation, the growth is primarily via cation (Ni ion) migration; Ni–Ni bonds are broken in the Ni layer, and Ni ions can diffuse to
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Fig. 2. Cross-sectional TEM images of 70-nm-thick NiO films grown at (a) 350 C and (b) 550 C on a 150 nm Pt/10 nm TiOx /300 nm SiO2 /Si substrate. A bump is seen in (a), marked by white arrows. White dashed lines denote the NiO/Pt interface. The scale bar is 50 nm.
550 °C 30
40
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2θ (Deg.) Fig. 1. Grazing incidence XRD patterns of NiO films formed by the oxidation of Ni films on platinized Si substrates at temperatures from 350 to 550 C: NiO ( ), Pt ( ), and Ni–Pt ( ).
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Ni O Pt
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the surface to form the oxide layer.13–16) The cations can be transported through lattice defects, such as vacancies and interstitials, or via grain boundaries through solid-state diffusion.15) This implies that either low a oxidation temperature or insufficient annealing time can increase the defect concentration of the oxidized layer. Since the oxidation time was fixed to be 1 h in this study, the oxidation temperature is main parameter affecting the structural properties of the NiO thin films. In Fig. 1, a broad peak at around 70 is observed, and the peak intensity is higher for the films oxidized at lower temperatures. A similar broad peak was observed for NiO thin films sputtered on Pt electrodes, which was identified as the Ni–Pt phase.4) The broad peak may indicate the compositional variation of the alloy layer. The central peak position corresponds to the (220) plane of fcc-structured ˚. Ni–Pt alloy with a lattice constant of 3.81 A The peak intensity of the alloy phase decreases with increasing oxidation temperature. Our evaporated Ni films might have a Ni–Pt layer at the Ni/Pt interface even prior to the heat treatment. Gambardella and Kern found that a large fraction of Ni atoms could be exchanged with Pt atoms at relatively low temperatures to form an alloy (150 – 300 K).17) While evaporating the Ni thin films on the Pt electrodes, the samples may be heated via radiation from the melted source material. This may promote the alloy formation. During the oxidation, Ni ions from both Ni and Ni–Pt layers diffuse to the surface forming oxide layers. Thus, Ni ions from the alloy may be gradually depleted, resulting in a decrease in the alloy layer volume. This scenario explains the XRD results well and will be discussed in detail later. Figures 2(a) and 2(b) show cross-sectional TEM images of NiO films grown at 350 and 550 C, respectively. The NiO layer is 60 nm thick, which agrees well with the expected value considering the difference in the molar volumes of Ni and NiO.14) Both the NiO films and the Pt electrodes have granular structures. Note that there is a bump at the interface for the 350 C-grown sample, marked by white arrows in Fig. 2(a). Dashed lines, denoting the NiO/Pt interface, clearly reveal the existence of the bump. In the
60 40 20 0 1
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Position Fig. 3. EDS spectra at different points on the NiO/Pt film: (a) TEM image showing the acquisition points for analysis and (b) the elemental composition distributions. The dashed line denotes the NiO/Pt interface.
case of the 550 C-grown sample, such bumps are hard to find. Figure 3(a) shows a cross-sectional TEM image of a 350 C-grown sample, which show bumps at the interface. The interface bumps have different atomic number contrast compared with neighboring regions, indicating compositional variation. TEM–EDS was used for the chemical analyses of the bumps. Figure 3(b) shows that the bump [region of Fig. 3(a)] is Pt-rich. The dark region at the interface (`) has a similar composition to the film interior (´), namely NiO. These results clearly reveal that the bump has a Ni–Pt alloy phase. A similar inhomogeneously formed Ni–Pt layer was reported for sputtered samples.4) Figures 4(a) and 4(b) show high-resolution TEM images of NiO thin films prepared at 350 and 550 C, respectively.
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Fig. 4. High-resolution TEM images of the NiO/Pt interface region of NiO films formed at (a) 350 and (b) 550 C. 1st SET
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Voltage (V) Fig. 6. Typical I–V curves of a Pt/NiO/Pt structure obtained during successive resistance switching measurements with a current compliance of 10 mA (ISET ). VSET indicates the high-to-low resistance switching (SET) voltage. VRESET and IRESET denote the voltage and current at which the low-to-high resistance switching (RESET) occurs, respectively. The line of solid circles corresponds to the first SET process.
Fig. 5. Schematic diagrams illustrating cross-sectional view of the samples during the oxidation process (a) platinized Si substrate with a Ni film, (b) intermediate stage of the oxidation: an oxidized layer in the top region, a Ni layer remaining in the middle, and an intermixed Ni–Pt layer at the bottom, (c) NiO film with an interfacial alloy layer, and (d) sufficiently oxidized layer without Ni–Pt alloy layer at the interface.
For the 350 C-grown sample, the NiO film consists of nanocrystallites with poorly aligned crystalline axes. In addition, clear contrast cannot be observed at the interface in Fig. 4(a), indicating the aforementioned Ni–Pt alloy formation. Increasing the oxidation temperature to 550 C significantly improves the crystalline quality of the NiO and Pt thin films, as shown in Fig. 4(b). Also, note that the NiO/Pt interface is well-defined. The grain size of the NiO thin films is increased at the higher temperature, which is similar to the result given in a previous report.14) Both NiO and Pt grains are preferentially aligned along their (111) planes, indicating highly textured film growth. One the basis of the above results, a possible scenario involved in the interfacial reaction and oxidation during our growth process can be suggested, as illustrated in Fig. 5. As discussed above, the existence of a Ni–Pt layer can be assumed for the evaporated Ni/Pt films [Fig. 5(a)]. During the heat treatment, elemental Ni from the upper region of the Ni film migrates outward and forms an oxide. Meanwhile, some of the elemental Ni close to the Pt electrodes might be intermixed into the alloy. The interdiffusion rate of the Ni and Pt layers may be inhomogeneous across the interface, since the diffusion is enhanced around defects such as grain boundaries. Thus, the alloy layer may have a nonuniform
thickness [Fig. 5(b)]. The alloy layer, as well as the Ni layers, is expected to be oxidized. The oxidation rate of the alloy will be limited by the outward diffusion rate of Ni in the alloy since Pt is a noble metal. Thus, the alloy layer may remain at the interface, surrounded by the oxidized layer [Fig. 5(c)].16) This reasonably explains how the Ni–Pt alloy layer can form bumps at the interface, as shown in Fig. 3. The schematic diagram of Fig. 5(c) is thought to correspond to the cross-sectional view of the sample formed at 350 C. As oxidation proceeds, the consumption of Ni in the Ni–Pt layer will decrease the volume of the alloy layer. Therefore, sufficient oxidation will remove the Ni ions from the Ni–Pt layer and result in a well-defined NiO/Pt interface separate from the alloy layer [Fig. 5(d)]. Since the films grown at 550 C have a negligible Ni–Pt layer at the interface from the XRD and TEM results, their cross-section is thought to be similar to that shown in Fig. 5(d). Figure 6 shows several exemplary I–V curves of a sample grown at 350 C, obtained during resistance-switching measurements. The bias voltage was swept from 0 to 3 V for the high-to-low resistance switching (‘‘SET’’ process) and from 0 to 1 V for the low-to-high resistance switching (‘‘RESET’’ process). None of the samples used in this study require a forming process; the initial voltage required to induce the low-resistance state was similar to the SET voltage. [Note: The solid line of filled circles corresponds to the first SET process.] During the SET process, the applied current was limited to 10 mA to avoid permanent damage to the samples [Hereafter, this compliance current is called the ‘‘SET’’ current (ISET ) to clarify its role in the switching]. Figure 6 clearly shows the variation in the switching current and voltage.1–7) VSET , VRESET , and IRESET denote the voltages and current at which the resistance switching occurs, respectively. The variation of VRESET is much smaller than that of VSET , as reported elsewhere.1–7) Figure 7 shows the switching characteristics of the Pt/ NiO/Pt structures with NiO thin films oxidized at 350 and
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microstructural influences on the resistance-switching behaviors of NiO thin films has not yet been achieved and more systematic investigations are required.
-2
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10-3 10-4 Set
10-5 350 °C 550 °C
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Voltage (V) Fig. 7. Typical resistance-switching characteristics of Pt/NiO/Pt structures, in which the NiO thin films are oxidized at different temperatures.
550 C. The average values of the SET/RESET voltages (VSET =VRESET ) do not show significant dependence on the oxidation temperature.11) The fluctuation of VSET for the 550 C grown sample is larger than that for the 350 C grown sample. For sputtered NiO thin films, it was reported that a local Ni–Pt phase at the interface decreased the switchingvoltage fluctuation.4) Thermally grown NiO thin films seem to have similar behaviors, since the lower-temperaturegrown samples have a larger volume of the alloy layer, as revealed in the XRD and TEM results. Although there is fluctuation of the measured data, the high-resistance-state (HRS) resistance is larger for the sample grown at the higher temperature. The XRD and TEM data show that a high oxidation temperature improves the microstructural property of the thermally grown NiO thin films. This indicates that the defect concentration significantly affects the HRS conduction of our NiO thin films. The HRS transport mechanism of Pt/NiO/Pt structures has been attributed to the correlated barrier polaronhopping model; Ni vacancies and defects can enhance the electrical conduction.1,10) Our results also support such an explanation. In contrast, low-resistance-state (LRS) resistance does not show strong dependence on the oxidation temperature, as shown in Fig. 7. This highlights the clear distinction between HRS and LRS transport. It is known that the LRS conduction has an inhomogeneous current distribution.6,7) Although the detailed switching process is unclear, the following scenario has been proposed: the SET process increases the defect density and disorder, resulting in the formation of a percolative network of metallic paths, socalled filaments.1–7) When an electric field is applied to a disordered material, the resulting current and power densities may have strong spatial variation.7) The initiation of the breakdown process has been attributed to a region with high electric field strength (high-field spots) or to the generation of a large quantity of Joule heat (hot spots).18) This implies that the location and density of the filaments may depend on the microstructures and the defect concentration of the NiO films. However, Fig. 7 shows that the LRS resistance of the NiO thin films is not markedly affected by the oxidation temperature. A complete understanding of the
Conclusions
The microstructures and resistance-switching characteristics were investigated in detail for Pt/NiO/Pt structures, in which the NiO thin films were prepared by thermal oxidation. The interface microstructures as well as the crystalline quality of the NiO thin films showed considerable variations depending on the oxidation temperature. These structures exhibited reproducible resistance switching during unipolar I–V measurements. The samples prepared at higher temperatures had larger HRS resistance, indicating the importance of defect-mediated conduction. Conversely, the LRS conduction, which is dominated by local filamentary paths, did not depend noticeably on the oxidation temperature. Further investigations are expected to reveal the effects of the microstructures on the resistance-switching behavior. Acknowledgments This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2007-331-C00083). One of the authors (B.H.P.) was supported by the Basic Research Program of the Korea Science and Engineering Foundation, Grant No. R01-2006000-10883-0. 1) 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. 85 (2004) 5655. 2) S. Seo, M. J. Lee, D. C. Kim, S. E. Ahn, B.-H. Park, I. K. Yoo, I. S. Byun, I. R. Hwang, S. H. Kim, J.-S. Kim, J. S. Choi, J. H. Lee, S. H. Jeon, S. H. Hong, and B. H. Park: Appl. Phys. Lett. 87 (2005) 263507. 3) D. C. Kim, M. J. Lee, S. E. Ahn, S. Seo, J. C. Park, I. K. Yoo, I. G. Baek, H. J. Kim, E. K. Yim, J. E. Lee, S. O. Park, H. S. Kim, U.-I. Chung, J. T. Moon, and B. I. Ryu: Appl. Phys. Lett. 88 (2006) 232106. 4) R. Jung, M. Lee, S. Seo, D. C. Kim, G.-S. Park, K. Kim, S. Ahn, Y.-S. Park, I.-K. Yoo, J.-S. Kim, and B. H. Park: Appl. Phys. Lett. 91 (2007) 022112. 5) K. Kinoshita, T. Tamura, M. Aoki, Y. Sugiyama, and H. Tanaka: Appl. Phys. Lett. 89 (2006) 103509. 6) Y. Sato, K. Kinoshita, M. Aoki, and Y. Sugiyama: Appl. Phys. Lett. 90 (2007) 033503. 7) I. H. Inoue, S. Yasuda, H. Akinaga, and H. Takagi: Phys. Rev. B 77 (2008) 035105. 8) M. Fujimoto, H. Koyama, Y. Hosoi, K. Ishihara, and S. Kobayashi: Jpn. J. Appl. Phys. 45 (2006) L310. 9) K. M. Kim, B. J. Choi, and C. S. Hwang: Appl. Phys. Lett. 90 (2007) 242906. 10) K. Jung, H. Seo, Y. Kim, H. Im, J. Hong, J.-W. Park, and J.-K. Lee: Appl. Phys. Lett. 90 (2007) 052104. 11) D.-W. Kim, B. H. Park, R. Jung, and S. Seo: Jpn. J. Appl. Phys. 46 (2007) 5205. 12) J.-L. Cao, A. Solbach, U. Klemradt, T. Weirich, J. Mayer, H. Horm-Solle, U. Bo¨ttger, P. J. Schorn, T. Schneller, and R. Waser: J. Appl. Phys. 99 (2006) 114107. 13) B. C. Sales, M. B. Maple, and F. L. Veron III: Phys. Rev. B 18 (1978) 486. 14) A. M. Lo´pez-Beltra´n and A. Mendoza-Galva´n: Thin Solid Films 503 (2006) 40. 15) G. R. Wallwork: Rep. Prog. Phys. 39 (1976) 401. 16) A. Schro¨der, W. Sitte, I. Rom, G. Kothleitner, and F. Hofer: Solid State Ionics 141–142 (2001) 177. 17) P. Gambardella and K. Kern: Surf. Sci. 475 (2001) L229. 18) M. So¨derberg: Phys. Rev. B 35 (1987) 352.
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