Metal-insulator-metal (MIM) structure was fabricated by partially anodizing aluminum film followed by deposition of another aluminum film. Unipolar resistive switching between a high-resistance state and a low-resistance state with a high resistance ratio (> ∼ 10 4) was observed from the structure. The switching occurred without the requirement of a forming process, which was attributed to the pre-existing conductive filaments in the Al-rich Al x O y layer formed by the anodization. Each resistance state exhibited Ohmic behavior which could be explained by the metallic conduction and electron hopping from one isolated state to the next in the Al-rich Al x O y layer. The MIM structure showed good memory characteristics. © 2009 American Institute of Physics.published_or_final_versio

Chen, TP

Fung, S

Liu, Y

Liu, Z

Yang, M

Zhu, W

Journal of Applied Physics

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Title Resistive switching in aluminum/anodized aluminum filmstructure without forming processAuthor(s) Zhu, W; Chen, TP; Liu, Z; Yang, M; Liu, Y; Fung, SCitation Journal Of Applied Physics, 2009, v. 106 n. 9Issued Date 2009URL http://hdl.handle.net/10722/80535Rights Journal of Applied Physics. Copyright © American Institute ofPhysics.Resistive switching in aluminum/anodized aluminum film structure withoutforming processW. Zhu,1 T. P. Chen,1,a Z. Liu,1 M. Yang,1 Y. Liu,2 and S. Fung31School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798,Singapore2State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Scienceand Technology of China, Chengdu, Sichuan 610054, People’s Republic of China3Department of Physics, The University of Hong Kong, Hong KongReceived 1 September 2009; accepted 23 September 2009; published online 10 November 2009Metal-insulator-metal MIM structure was fabricated by partially anodizing aluminum filmfollowed by deposition of another aluminum film. Unipolar resistive switching between ahigh-resistance state and a low-resistance state with a high resistance ratio 104 was observedfrom the structure. The switching occurred without the requirement of a forming process, which wasattributed to the pre-existing conductive filaments in the Al-rich AlxOy layer formed by theanodization. Each resistance state exhibited Ohmic behavior which could be explained by themetallic conduction and electron hopping from one isolated state to the next in the Al-rich AlxOylayer. The MIM structure showed good memory characteristics. © 2009 American Institute ofPhysics. doi:10.1063/1.3253722I. INTRODUCTIONResistive switching phenomenon in metal oxides such asNiO,1–4 TiO2,5,6 ZrO2,7 CuxO,8 and AlxOy5,9 attracts greatinterest for a possible application in nonvolatile memory de-vices. Some complicated materials such as Cr-dopedSrZrO310 and V-doped SrZrO311 also show the switchingphenomenon, but their switching mechanisms could be dif-ferent from each other. The resistance switching behaviorcould be explained by various mechanisms including chargetrapping in dielectric layer, space-charge-limited conductionprocesses, ion conduction and electrodeposition, Mott transi-tion, and conductive filaments CFs.4 However, it is stilldifficult to use a complete physical model to describe switch-ing phenomenon, as switching behavior may be defined bythe dielectric materials, electrode materials, fabrication pro-cedure, or even experiment setup. For most of the materials,a forming process, which is the first switching from initialstate to low-resistance state LRS, is usually required. Theforming process in metal oxides could be a kind of softbreakdown in the oxide film. It is not desirable for practicalapplications as the “forming” voltage is usually much higherthan the set voltage. In this work, unipolar resistive switch-ing in a partially anodized aluminum film was observed. Noforming process was required, which is attributed to the pre-existing CFs formed by the excess aluminum in the Al-richAlxOy layer of the partially anodized aluminum film.II. EXPERIMENTA 10 nm thick silicon dioxide layer was first thermallygrown on a p-type silicon substrate in order to protect thesilicon substrate during the anodization. Then a 500 nm thickAl film was deposited on the silicon dioxide layer by dcsputtering of an Al target with purity of 99.99%. Anodizationof the Al film was undertaken in a 0.15 M /L solution ofoxalic acid at 25 °C, by applying a constant voltage of 40 Vusing a Keithley 2400 source meter. A 22 cm2 platinummesh was used as the cathode. The anodizing time was about120 min. The chemical states of the film were analyzed byx-ray photoelectron spectroscopy XPS Kratos AXIS Ultrawith monochromatic Al K 1486.71 eV x-ray radiation.Figure 1 shows the XPS peak of Al 2p core level taken fromthe film. The Al 2p peak of the film can be decomposed intotwo components, i.e., metallic Al and Al2O3. The Al compo-nent indicates the existence of Al-rich AlxOy layer. A trans-mission electron microscope TEM image of the anodizedaluminum film is shown in the inset of Fig. 1. The thicknessof the AlxOy film is about 30–50 nm, as can be seen in theTEM image. A 200 nm thick aluminum film was then depos-ited on the anodized film as top electrode by electron beamaElectronic mail: echentp@ntu.edu.sg.70 71 72 73 74 75 76 77 78 79 80 81 82020040060080010001200Intensity(Arb.Unit)Binding Energy (eV)Al Al2O3AlAlxOy 50nmFIG. 1. XPS peak of Al 2p core level taken from the anodized aluminumfilm. The inset shows the cross-sectional TEM image of the partially anod-ized Al film.JOURNAL OF APPLIED PHYSICS 106, 093706 20090021-8979/2009/1069/093706/4/$25.00 © 2009 American Institute of Physics106, 093706-1Downloaded 07 Nov 2010 to 147.8.21.59. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsevaporation to form the metal-insulator i.e., the AlxOylayer-metal i.e., the bottom unreacted Al layer MIMstructure. The diameter of the electrode is 200 m. Current-voltage I-V measurements were carried out with a Keithley4200 semiconductor characterization system at room tem-perature.III. RESULTS AND DISCUSSIONFigure 2a shows the typical unipolar switching charac-teristics of the MIM structure. By sweeping the voltage ap-plied to the top electrode from 0 to +1.1 V with step of1 mV, a bistable resistive switching was observed. As thebiasing voltage was increased to a certain voltage Vresetwith a current compliance of 0.1 A, a sudden drop of currentappeared the current dropped from 10−2 to 10−8 A,i.e., the so-called reset process occurred, without any priorforming process. In the reset process the resistance wasswitched from a LRS to a high-resistance state HRS, andthen the sample maintained at the HRS. In the second volt-age sweeping starting from 0 V with a smaller current com-pliance of 0.01 A, the HRS was maintained until the voltagewas increased to certain value Vset at which an abrupt in-crease in current appeared. This process is known as the“set” process and it is defined as the resistive transition fromthe HRS to the LRS. A smaller compliance current was usedin the set process to avoid the hard breakdown in the oxidelayer. The reset and set processes were highly reproducible.On the other hand, as shown in the inset of Fig. 2a, similarreset and set processes were also observed in the negative-voltage sweeping. The independence of the reset and set pro-cesses on voltage polarity suggests that the interfaces of Alelectrode/AlxOy and AlxOy/unreacted Al did not play an im-portant role and the switching was mainly determined by theAlxOy layer. Figure 2b shows the comparison of the virginswitching cycle with the tenth switching cycle. As can beseen in the figure, the switching behavior is basically thesame in the two cases. The result further confirms that theforming process is not necessary for switching to occur.The switching behaviors observed in this work can beexplained with the CF model. As revealed by the XPS analy-sis, there was a high concentration of metallic aluminumexisting in the Al-rich AlxOy layer. The metallic aluminumpreformed the CFs, yielding the initial LRS. This explainswhy no prior forming process was required for the reset/setprocesses. The pre-existing CFs, which were formed by theexcess aluminum in the AlxOy layer, were alternatively con-nected and disconnected during the set/reset operation, andthe situation could be somewhat different from that of othermetal oxides. A high current was produced at a small voltagedue to a large number of the pre-existing CFs. The suddenincrease in resistance at the reset transition is attributed to thedissolution of some of the CFs as a consequence of Jouleheating caused by the high current. In the set process, someof the broken CFs could be reconnected and or new CFscould be formed as a result of electrochemical reactions suchas reduction reactions occurring in the Al-rich AlxOy layerunder the influence of both the electric field and the electroninjection from the cathode.Figure 3 shows the I-V characteristics in log scale of0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.110-910-810-710-610-510-410-310-210-1First sweepSecond sweepSetI(A)V (V)Reset0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.110-910-810-710-610-510-410-310-210-1I(A)|V| (V)Negative voltage0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.810-910-810-710-610-510-410-310-210-1Virgin switching cycle10th switching cycleI(A)V (V)(b)(a)FIG. 2. a Resistive switching in the Al /AlxOy /Al structure under positivebias. The inset shows the switching under negative bias. b Comparison ofthe virgin switching cycle with the tenth switching cycle.10-210-110-1310-1210-1110-1010-910-810-710-610-510-410-310-210-1Power-law fitting I = AVnn =1.01I(A)V (V)n =0.99Symbols: measurementLRSHRSFIG. 3. Color online Plots of log I vs log V at low voltage region VVreset for the LRS and HRS.093706-2 Zhu et al. J. Appl. Phys. 106, 093706 2009Downloaded 07 Nov 2010 to 147.8.21.59. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsboth the LRS and HRS for VVreset. In this low field region,both the LRS and HRS follow a power law, i.e., I=AVn,where n is an exponent factor n=1 for perfect Ohmic con-duction and A is a coefficient which depends on resistancestate. As shown in Fig. 3, n is very close to 1 for both theLRS and HRS, indicating that the CFs for both the LRS andHRS had an Ohmic behavior. A similar Ohmic conductionwas reported in stoichiometric ZrO2 films.7 The Ohmic be-havior could be explained by the following two mechanisms.Conduction in the form of nanoscale metal conductors wasformed by a high concentration of excess metallic aluminumin the Al-rich AlxOy layer. On the other hand, at low fields,current could be carried by thermally excited electrons hop-ping from one isolated state to the next in the Al-rich AlxOylayer, which would exhibit an Ohmic behavior also. In otherwords, the two mechanisms were responsible for the currentconduction. Both the LRS and HRS had the same conductionmechanisms, but they differed in the number and or thephysical dimensions of the CFs. This argument is supportedby the result shown in Fig. 3. As can be seen in the figure,both the LRS and HRS had about the same n value, but theyhave a large difference in A value the A value of the LRS isabout six orders higher than that of the HRS. Therefore, theswitching between the LRS and the HRS basically reflectsthe changes in the number and or the physical dimensions ofthe CFs.Figure 4 shows the variation in resistance for both theLRS and HRS with the number of switching set/resetcycles. To minimize the influence of the electrical field dur-ing the resistance measurement, the resistance was measuredat as low as 0.1 V Vreset. The resistance ratio of the HRSto the LRS is larger than 104. The large memory windoweasily satisfies the requirement for distinguishing the twostates in memory application. The inset of Fig. 4 shows thedistributions of both Vreset and Vset. The distribution of Vresetwas quite narrow around 0.4–0.5 V, the distribution of Vsetwas relatively larger around 1.0–1.4 V, but a window of0.5 V between the Vreset and the Vset was still achieved. Ascan be seen in Fig. 4, the resistance of the HRS was not asstable as that of the LRS, showing a larger fluctuation in thereset transition. However, both the HRS and the LRS had agood stability before switching occurring, as can be seen inFig. 5 which shows the reading endurance for both the HRSand LRS. The results shown in Figs. 4 and 5 indicate that theMIM structure is promising in the resistive random accessmemory application.IV. CONCLUSIONIn summary, the Al-rich AlxOy layer formed by partiallyanodizing Al thin film showed unipolar and repeatableswitching between the LRS and the HRS with a high resis-tance ratio 104. The switching occurred without therequirement of a forming process, which is attributed to thepre-existing CFs in the Al-rich AlxOy layer. Both the LRSand HRS in low voltage region exhibited Ohmic behaviorwhich can be explained by the metallic conduction and elec-tron hopping from one isolated state to the next in the Al-richAlxOy layer. The MIM structure consisting of the Al-richAlxOy layer showed good memory characteristics.ACKNOWLEDGMENTSThis work has been financially supported by NationalResearch Foundation of Singapore NRF-G-CRP 2007-01.Y. Liu acknowledges National Natural Science Foundation ofChina NSFC under Project No. 60806040.1D. Ielmini, C. Cagli, and F. Nardi, Appl. Phys. Lett. 94, 063511 2009.2D. C. Kim, S. Seo, S. E. Ahn, D.-S. Suh, M. J. Lee, B.-H. Park, and I. K.Yoo, Appl. Phys. Lett. 88, 202102 2006.3S. Seo, M. J. Lee, D. H. Seo, E. J. Jeoung, D.-S. Suh, Y. S. Joung, and I.K. Yoo, Appl. Phys. Lett. 85, 5655 2004.4U. Russo, D. Ielmini, C. Cagli, and A. L. Lacaita, IEEE Trans. ElectronDevices 56, 186 2009.5K. M. Kim, B. J. Choi, B. W. Koo, S. Choi, D. S. Jeong, and C. S. Hwang,Electrochem. Solid-State Lett. 9, G343 2006.0 20 40 60 80 100101102103104105106107108109LRSHRSResistance(ohm)Number of set/reset cycles0.00.20.40.60.81.01.21.41.61.82.0V(V)VresetVsetFIG. 4. Color online Variation in resistance for both the LRS and HRSwith number of reset/set cycles. The resistance was measured at 0.1 V. Theinset shows the distributions of both Vreset and Vset.0 100 200 300 400 50010-1110-1010-910-810-710-610-510-410-310-210-1SetSetResetI(A)Number of readingsResetFIG. 5. Reading stability of both the LRS and HRS. The current was mea-sured at 0.1 V, and the time period corresponding to 100 readings is about200 s.093706-3 Zhu et al. J. Appl. Phys. 106, 093706 2009Downloaded 07 Nov 2010 to 147.8.21.59. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions6C. Rohde, B. J. Choi, D. S. Jeong, S. Choi, J.-S. Zhao, and C. S. Hwang,Appl. Phys. Lett. 86, 262907 2005.7X. Wu, P. Zhou, J. Li, L. Y. Chen, H. B. Lv, Y. Y. Lin, and T. A. Tang,Appl. Phys. Lett. 90, 183507 2007.8H. B. Lv, M. Yin, X. F. Fu, Y. L. Song, L. Tang, P. Zhou, C. H. Zhao, T.A. Tang, B. A. Chen, and Y. Y. Lin, IEEE Electron Device Lett. 29, 3092008.9S. Kim and Y. K. Choi, Appl. Phys. Lett. 92, 223508 2008.10C. Y. Liu, P. H. Wu, A. Wang, W. Y. Jang, J. C. Young, K. Y. Chiu, and T.Y. Tseng, IEEE Electron Device Lett. 26, 351 2005.11C.-C. Lin, B.-C. Tu, C.-C. Lin, and C.-H. Lin, IEEE Electron Device Lett.27, 725 2006.093706-4 Zhu et al. J. Appl. Phys. 106, 093706 2009Downloaded 07 Nov 2010 to 147.8.21.59. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

Resistive switching in aluminum/anodized aluminum film structure without forming process

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Resistive switching in aluminum/anodized aluminum film structure without forming process

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