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Kubota, Masato, Nigo, Seisuke, [Kitazawa, Hideaki](https://orcid.org/0000-0002-9756-2311), [Harada, Yoshitomo](https://orcid.org/0000-0001-9380-2106), Kido, Giyuu, Hirayama, Taisei, [Kato, Seiichi](https://orcid.org/0000-0002-6427-5463)

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[Conduction band caused by oxygen vacancies in aluminum oxide for resistance random access memory](https://mdr.nims.go.jp/datasets/a026c3b5-4b50-4a23-9163-a2ff7badf0ce)

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untitledJ. Appl. Phys. 112, 033711 (2012); https://doi.org/10.1063/1.4745048 112, 033711© 2012 American Institute of Physics.Conduction band caused by oxygenvacancies in aluminum oxide for resistancerandom access memoryCite as: J. Appl. Phys. 112, 033711 (2012); https://doi.org/10.1063/1.4745048Submitted: 05 March 2012 . Accepted: 17 July 2012 . Published Online: 07 August 2012Seisuke Nigo, Masato Kubota, Yoshitomo Harada, Taisei Hirayama, Seiichi Kato, Hideaki Kitazawa, andGiyuu KidoARTICLES YOU MAY BE INTERESTED INEffect of vacancy-type oxygen deficiency on electronic structure in amorphous aluminaApplied Physics Letters 98, 042102 (2011); https://doi.org/10.1063/1.3548549Oxygen vacancy levels and electron transport in Applied Physics Letters 96, 032905 (2010); https://doi.org/10.1063/1.3293440Conduction mechanism of TiN/HfOx/Pt resistive switching memory: A trap-assisted-tunneling modelApplied Physics Letters 99, 063507 (2011); https://doi.org/10.1063/1.3624472https://images.scitation.org/redirect.spark?MID=176720&plid=1087013&setID=379065&channelID=0&CID=358625&banID=519992917&PID=0&textadID=0&tc=1&type=tclick&mt=1&hc=4b0cee398e0882d8e6fbc34bd2c841e21a6383ff&location=https://doi.org/10.1063/1.4745048https://doi.org/10.1063/1.4745048https://aip.scitation.org/author/Nigo%2C+Seisukehttps://aip.scitation.org/author/Kubota%2C+Masatohttps://aip.scitation.org/author/Harada%2C+Yoshitomohttps://aip.scitation.org/author/Hirayama%2C+Taiseihttps://aip.scitation.org/author/Kato%2C+Seiichihttps://aip.scitation.org/author/Kitazawa%2C+Hideakihttps://aip.scitation.org/author/Kido%2C+Giyuuhttps://doi.org/10.1063/1.4745048https://aip.scitation.org/action/showCitFormats?type=show&doi=10.1063/1.4745048https://aip.scitation.org/doi/10.1063/1.3548549https://doi.org/10.1063/1.3548549https://aip.scitation.org/doi/10.1063/1.3293440https://doi.org/10.1063/1.3293440https://aip.scitation.org/doi/10.1063/1.3624472https://aip.scitation.org/doi/10.1063/1.3624472https://doi.org/10.1063/1.3624472Conduction band caused by oxygen vacancies in aluminum oxidefor resistance random access memorySeisuke Nigo,1,a) Masato Kubota,2 Yoshitomo Harada,1 Taisei Hirayama,3 Seiichi Kato,1Hideaki Kitazawa,1 and Giyuu Kido11National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan2Japan Atomic Energy Agency, 2-4 Shirane Shirakata, Tokai-mura, Ibaraki 319-1195, Japan3Rigaku Corporation, 3-9-12 Matsubara-cho, Akishima, Tokyo 196-8666, Japan(Received 5 March 2012; accepted 17 July 2012; published online 7 August 2012)As a next-generation memory, we have developed a rare-metal-free memory using Al oxide witha high-density of oxygen vacancies (Vos). The electronic structure has been simulated usingfirst-principles calculations. In this paper, we report the electronic structure of the band gap,analyzed using thermally stimulated current measurements, to evaluate the simulated results. Weobserved electronic states corresponding to resistance changes for the first time. These results showthat Voþ2 (electron empty Vo) changes to Voþ1 by electron injection; the overlapped Voþ1 electronchanges into a “Vo conduction band” (VoCB), and the changed structure is stabilized by structuralrelaxation of Al ions around Vo. VoCB is considered as a kind of mid-gap impurity band. Theorigin of the on/off switching is considered to be generation/degeneration of the VoCB caused byincreasing/decreasing numbers of Vo electrons. Based on knowledge of the electronic mechanism,we have changed metal/insulator/metal structure to a metal/insulator/semiconductor structure anddecreased the reset-current to 7 lA. The Vos of Al oxide are considered to be useful for electronicmemory storage. VC 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4745048]I. INTRODUCTIONResistance random access memory (ReRAM), as thenext-generation memory to succeed flash memory, is beingdeveloped using many kinds of transition-metal oxides.1However, the switching mechanisms are not yet fully clear.The switching mechanism of a transition-metal-oxide ReRAMis believed to be caused by O ion migration.2 Recently, it wasclarified by x-ray photoelectron spectroscopy (XPS) that asmall XPS energy shift occurs by redox reactions, such as2TaO2 þ O2� $ Ta2O5 þ 2e.3,4 However, the electronic stateitself has not yet been observed. We have developed ReRAMusing Al anodized oxide (AlOx) instead of a transition-metaloxide, focusing on oxygen vacancies (Vos) localized in theAlOx cell boundaries.5Pioneering researches on the electronic properties ofanodized AlOx have been continued systematically since1964 by Hickmott. He proposed energy-band diagrams ofimpurity bands with double Schottky barriers of structure Al/AlOx/Au and reported the existence of defects at a density of1019�1020 cm�3 with a broad energy distribution among theband gaps of AlOx.6 His detailed researches showed thatdefects are introduced at the time of anodization, and thatelectrons were injected from the Al electrode into AlOx byFowler�Nordheim (FN) tunneling.7 The negative resistancephenomenon of AlOx discovered by Hickmott8 is consideredto lead directly to resistive switching of AlOx-ReRAM.However, since he used an anodized AlOx film as thin as28 nm, a cell boundary did not exist, and therefore no atten-tion was paid to Vos localized at the cell boundary. As aresult, the switching mechanism caused by a high-density ofVos, such as 1021 cm�3, was overlooked until our research.Theoretically supporting our research, Momida et al. simu-lated the electronic structure of amorphous alumina contain-ing a high-density of Vos using first-principles calculationsfor the first time and the simulation results have beenpublished.9In this paper, we report an experimental study of theelectronic states, analyzed using the thermally stimulatedcurrent (TSC), to evaluate the Vo conduction band (VoCB)model, based on the simulation results. Next, we show ametal/insulator/semiconductor (MIS) AlOx-ReRAM struc-ture developed with the aim of significantly reducing thereset-current. In addition, to understand the resistive switch-ing mechanism driven by injection/extraction of electrons,we show schematically the electronic state changes amongthe band gaps using TSC spectra and a band model. Finally,we propose using a dry-type AlOx film, similar to the cellboundary, instead of anodized AlOx, which is not suitablefor semiconductor manufacturing processes. Generally, aswitching mechanism driven by electron injection/extractionis advantageous compared with the ion-migration mecha-nism of other types of ReRAM with respect to cycle endur-ance and power saving.II. EXPERIMENTAn Al(top)/AlOx/Al(bottom) memory cell was fabri-cated. First, a 0.5-mm-thick Al sheet (99.99%) was treatedby electrochemical polishing in a mixed solution ofperchloric acid and ethanol. The Al sheet was anodized usinga two-step anodization process.10 The Al sheet was anodizedat a voltage of 40 V in oxalic acid (0.3 M) at 293 K for 3 h.a)Author to whom correspondence should be addressed. Electronic mail:NIGO.Seisuke@nims.go.jp.0021-8979/2012/112(3)/033711/6/$30.00 VC 2012 American Institute of Physics112, 033711-1JOURNAL OF APPLIED PHYSICS 112, 033711 (2012)http://dx.doi.org/10.1063/1.4745048http://dx.doi.org/10.1063/1.4745048http://dx.doi.org/10.1063/1.4745048Then, the anodic oxide layer was removed in a mixture ofphosphoric acid (6 wt. %) and chromic acid (1.8 wt. %) at333 K for 1 h. The Al sheet was then anodized again for 64 sunder the same conditions as in the first step. After drying,an Al electrode of thickness 80 nm with a diameter of300 lm was formed using a heat evaporation system, and Cuwire was bonded with Ag paste. An Al (top)/AlOx/pþ-Si(bottom) memory cell was fabricated as a MIS-type ReRAM.The native oxide on the surface of the pþ-Si wafer (boronhigh-doped Si, 0.001–0.005 X cm) was removed using a so-lution of 1% HF (10 min), and Ar-ion sputtering (DC 100 W,1 min). An Al thin film of thickness 200 nm sputtered on thepþ-Si wafer was anodized using the same conditions as inthe second step of production of the Al/AlOx/Al memorycell. The subsequent processes were the same as the proc-esses in production of the Al/AlOx/Al memory cell. An ano-dized Al sheet was vertically sliced to a thickness of 100 nmusing a focused ion-beam (Hitachi FB-2000 A). The slicedsamples were observed using transmission electron micros-copy (TEM) at 200 kV (JEOL JEM-2100 F). A planar sectionof the AlOx film was observed using high-resolution TEM(Hitachi H9500). The electronic structures of specimens ofthickness 70 nm were determined using electron-energy-lossspectroscopy (EELS; Gatan GIF/Model 863), with a beamspot diameter of 1 nm. The localized existence of AlOx oxy-gen vacancies was determined, as shown in Fig. 1. The I–Vcharacteristics were measured using a source-meter unit(Keithley 2400). A current-regulating diode (CRD) of 28 lAwas inserted in series to the measuring circuit. Thesweep range, the sweep speed, and the step size were 63 V,0.5 V s�1, and 0.5 V, respectively. The electronic state of theoff-state sample of the Al/AlOx/Al memory cell was meas-ured using a TSC measurement system (Rigaku TSC-FETT2000). The sweep range, the rate of temperature increase,and the applied voltage were 80–550 K, 0.15 K s�1, and 0 V,respectively.III. RESULTS AND DISCUSSIONSA. Oxygen vacancies and electronic state analysisFirst of all, in the case of AlOx-ReRAMs, unlike transition-metal oxide ReRAMs, such as Pt/Ta2O5�x/TaO2�x/Pt, we con-sidered that the origin of the resistive switching is an increase/decrease in Vo electrons rather than O ion movement. Nano-holes and cell boundaries are generated by self-formation inthe Al anodization process. The existence of localized Vos inthe cell boundary of AlOx was revealed by the O K-edgespectra satellite peak in EELS as follows. The cell boundaryis visible as light gray lines of width approximately 20 nm inthe TEM image shown in Figs. 1(b) and 1(c). A satellitepeak at 534 eV is observed at the cell boundary at point Bin Fig. 1(d), although it is not observed at points A and C.Furthermore, as shown in Fig. 1(e), the EELS depth profileindicates that satellite peaks exist continuously in regionE but not in regions D and F of the vertical cell boundary.Generally, the satellite peak at 534 eV is considered to becaused by Vos of the metal oxide. We focused on the Vos asuseful electronic storage sites.The above-mentioned simulation results (Ref. 9) suggestthat Voþ2 (electron empty Vo) changes to Voþ1 (electronhalf-filled Vo) as a result of electron injection in the case ofVoþ1 densities of 1021 cm�3 or more; overlapped Voþ1 elec-trons generate a VoCB among the band gaps of AlOx. Thechanged electronic state is stabilized by local atomic relaxa-tions around the Vos, as shown in Fig. 2(a). A model of the“Vo pseudo-cluster” is Al3O12(Vo)1, in which three Al atomsare located around one Vo and each Al atom is surroundedby four O atoms, and is part of a super-cell of 120 atoms; themodel was simulated using an Al48O72 crystal structure as atypical semi-stable amorphous structure, using molecular dy-namics (see Ref. 9 for details). An electron trapped at a Vosite is localized in this state and oozes out to 12 O atomsaround the Voþ1, as shown in yellow in Fig. 2(a). Therefore,FIG. 1. Oxygen vacancies localized at thecell boundary of AlOx. (a) Schematiccross-section of AlOx. (b) AlOx planeTEM image. (c) AlOx vertical TEMimage. (d) and (e) EELS spectra of OK-edge measured at A, B, C and every redpoint. D: Surface of AlOx, E: inner do-main, F: interface of Al substrate. Arrowindicates satellite peaks caused by oxygenvacancies of AlOx.033711-2 Nigo et al. J. Appl. Phys. 112, 033711 (2012)the bound energy of the inner-shell electron of the O atom isconsidered to be decreased by the charge of the Voþ1 elec-tron. As a result, the O K-edge of the EELS spectrum shiftstowards a lower energy. This energy shift is considered to bepart of the probable origin of the satellite peak of EELSshown in Fig. 1. Although further research is required forconfirmation, the Voþ1 electron shown in yellow in Fig. 2(a)seems to be an electron with a broad distribution in a clusterconsisting of three Al atoms, 12 O atoms, and one Vo. Voþ1electrons are considered to be delocalized by increasing innumber and overlapping spatially, generating VoCB.TSC measurements are used to detect impurity levels insemiconductors or insulators. Trapped electrons at impuritylevels at a low temperature, such as 80 K, are excited up tothe conduction band by constant heating and emitted to theelectrode. The current emitted to the measurement circuit isrecorded against temperature, providing information on theenergy of trapped electrons and holes. A diagram of a TSCmeasurement system is shown in Fig. 2(c). Generally, delo-calized electrons cannot be measured by TSC, althoughlocalized electrons and holes can be measured precisely byTSC. As a trial, we measured an on-state sample of an AlOx-ReRAM of approximately 10 X; the measurement wasimpossible because of the noise current of 0.1 mA inducedby the femto-ammeter. The horizontal axis of Fig. 2(b) is theactivation energy of the trapped electron (Et), calculatedusing the following equation:11Et ¼ kTmlnðTm4=bÞ; (1)where k is the Boltzmann constant, Tm is the TSC peak tem-perature, and b is a heating rate of 0.15 K s�1. We show,here, three typical electronic states, measured using the TSCmethod, as electronic states of different resistances in theoff-state sample of AlOx-ReRAM, as shown in Fig. 2(b). Indetail, it became clear that electronic states 0.15–0.41 eVbelow the framework conduction band (FCB) are generated,corresponding to decreases in resistance. Electronic states inthis energy range do not exist in sample C but exist in thecases of samples A and B. If the number of Voþ1 electronsincreases more than that of sample A, VoCB will be gener-ated by delocalization of Voþ1 electrons, and simultaneously,the electronic state will change to metallic conduction; i.e.,the electronic state 0.15–0.41 eV below the FCB is consid-ered as a bud of the VoCB or remains of the VoCB. It istherefore presumed that these electronic states change intothe VoCB by increasing the number of Voþ1 electrons. Weshow these electronic state changes later, in Fig. 5.The origin of the switching mechanism of a transition-metal oxide ReRAM was revealed by XPS, as mentionedabove. Although we tried using XPS measurements to detectthe binding-energy shift of AlOx, we failed. This is becausethe change occurs not in bulk but in the nanodomain of thecell boundary of width �20 nm, as shown in Fig. 1(b), so theXPS signal is so small that it is undetectable. On the otherhand, we directly observed a clear change in the electronicstate using the TSC method for the first time, as mentionedabove. Since the Voþ1 electron exists at a level 0.15–0.41 eVbelow the FCB, the electron is expected to provide long-termelectronic storage. However, it becomes difficult to extractthe Voþ1 electron by electronic excitation up to the FCB. Inother words, a feature of AlOx-ReRAM is an easy set-operation and a difficult reset-operation, which is similar toother types of ReRAM.B. Resistive properties and I–V characteristics ofmetal/insulator/metal (MIM) type AlOx-ReRAMA typical unipolar-type I–V curve of a memory cell isshown in Fig. 3(c); it was measured by changing the voltageat room temperature as follows: 0! þ3! 0! þ2! 0 V.The high-resistance state (HRS) becomes a low-resistancestate (LRS) at 2.5 V in the process of which it increases to3 V, and the current is limited to 28 lA by a CRD, as shownFIG. 2. Structural change in Vo cluster andTSC measurements. (a) Structural relaxationof Al ions around the Vo by one electroncharge/discharge. Blue numerical valuesshow Al-Vo distance changes (Vo0 base)by first-principle calculation. (b) TSC ofoff-state of 100 KX, 200 KX, and 5 MX.(c) Illustration of TSC method.033711-3 Nigo et al. J. Appl. Phys. 112, 033711 (2012)in Fig. 3(c). Subsequently, by increasing the applied voltageto 2 V without the CRD, a larger reset-current than the set-current changes LRS to HRS at 1.2 V.Based on the temperature dependence of resistanceshown in Fig. 3(b), it was clear that the conduction mecha-nism in the HRS is a state of hopping conduction, in whichthe resistance increases exponentially with decreasing temper-ature, and the conduction mechanism in the LRS is a metallicconduction in which the resistance decreases linearly withdecreasing temperature. Since the LRS is reset to HRS if thecurrent increases to the reset-current as a result of a decreasein temperature, the increase in current accompanying adecrease in temperature must be controlled to be smaller thanthe reset-current. We therefore measured the temperature de-pendence of the resistance (Fig. 3(b)) with a voltage of only0.1 mV. In the case of unipolar drive, a larger current than theset-current causes a reset mechanism, as shown in Fig. 3(c).The restriction that the reset-current must be larger than theset-current is a fatal fault of low-power-consumption memory.C. MIS-type AlOx-ReRAM and mechanism studyTo reduce the reset-current significantly, we developeda MIS-type AlOx-ReRAM with a bipolar drive, as shown inFig. 4(a). Since the origin of the on/off switching is consid-ered to be generation/degeneration of VoCB by increasing/decreasing the numbers of Vo electrons, blocking the inflowof current at the time of reset is considered to be effective inreducing the reset-current. We therefore changed the struc-ture from a MIM-type Al/AlOx/Al to a MIS-type Al/AlOx/pþ-Si and were able to decrease the reset-current to 7 lAusing a p–n junction, as shown in Fig. 4(b).In addition, we show in Fig. 4(c) a schematic illustrationof the on/off sequential mechanism based on the Vo bandmodels to help understand the MIS-type AlOx-ReRAM.By increasing the applied voltage to the threshold valueVon, the electron that passed the Schottky barrier by FNtunneling is trapped at Voþ2. Voþ2 changes into Voþ1, andthe energy level of Voþ1 falls to the band gap from the FCBbottom simultaneously with electron trapping, as shown inFig. 4(c-1). The changed electronic structure is stabilized byshrinking of the Al ions around the Vo. As a result of increas-ing the number of Voþ1 electrons, the Voþ1 electrons overlapand delocalize, and the conductive electronic state of VoCBis generated, as shown in Fig. 4(c-2). This state is the on-state of metallic conduction, caused by VoCB. The set-mechanism is considered to be as shown in Eq. (2). Byincreasing the reverse voltage, since the reverse current isprevented by the p–n junction, Vo electrons are instantlyextracted by a reverse electric field. The reset-mechanism isconsidered to be as shown in Eq. (3); the state is stabilizedby the spreading of Al ions, and the energy level of Voþ2increases to near the FCB bottom, as shown in Fig. 4(c-3).The Voþ1 electrons are localized as a result of their decreas-ing number. Simultaneously, the metallic conductionreturns to band-insulator hopping conduction, as shown inFig. 4(c-4). The spreading and shrinking of Al ions aroundthe Vo correspond to changes in the Al-Vo distance shown inFig. 2(a). The aforementioned mechanisms can be expressedsimply as follows:Voþ2 þ e� ! Voþ1 : reduction of Vo by electron injection! on-state by delocalized Vo electrons (2)Voþ2  Voþ1 � e� : oxidation of Vo by electron extraction! off-state by localized Vo electrons (3)These reactions are accompanied by a sub-reaction inwhich the electronic structure of Vo is stabilized by structuralrelaxation of Al ions around the Vo. The aforementioned sim-ulation results show that structural relaxation of amorphousAlOx is larger and more stable than that of crystalline Al2O3,FIG. 3. Resistive properties of AlOx-ReRAM memory cell. (a) Basic structure of MIM-type of AlOx-ReRAM, and measurement circuit with a CRD. (b) Resistancechange with temperature in HRS and LRS. (c) Typical unipolar-type I-V characteristics. Set current limited to 28lA by CRD.033711-4 Nigo et al. J. Appl. Phys. 112, 033711 (2012)i.e., a bistable electronic structural change in crystalline Al2O3does not occur, even if it contains a high-density of Vos (seeRef. 9 for details).Two reactions, Voþ2 þ e� ! Voþ1 and Voþ2 þ 2 e�! Vo0, progress simultaneously as a result of electron injec-tion. Vo0 is non-conductive, because of full electronic occupa-tion, and does not participate in resistance changes. Thegeneration of a VoCB as a result of an increase in the numberof Voþ1s decreases the effective voltage, and electron injectionby FN tunneling stops because the Schottky barrier is revivedby a decrease in the effective voltage, i.e., the reaction Voþ2þ 2 e� ! Vo0 does not contribute to the resistance change.In 2002, Hickmott reported 10 metal oxides, namely,Ta2O5, PrOx, TiO2, Nb2O5, ZrO2, SiO, CeO2, MgO, Y2O3,and anodized Al2O3, that show negative resistance.12 Inrecent years, all the metal oxides listed by Hickmott havebeen verified by many researchers to be candidate materialsfor ReRAM. In general, defects have adverse effects onsemiconductors, and therefore the removal of defects, suchas Vo is important. However, in the case of AlOx-ReRAM,high-density Vo in anodized Al2O3 is considered to serveas useful electronic storage sites for electron injection/extraction.D. Electronic structure change studyTo help further understanding of the electronic struc-tures of AlOx-ReRAM, Fig. 5 shows a schematic diagram ofthe electronic structures of the on/off state, based on TSCanalyses. The electronic states of typical off-states, based onFig. 2(b), are shown in the orange area in Fig. 5. TSC mea-surement of the on-state is impossible because of delocalizedelectrons. The electronic state of the on-state was thereforepresumed, based on clear tendencies of changes in the off-state, to be as follows. In the case of a high resistance of5 MX, electrons do not exist in the energy range 0.15–0.41 eV below the FCB, as shown in Fig. 5(a-1). However,when the resistance decreases from 5 MX to 200 KX, elec-trons appear in the energy range 0.15–0.41 eV below theFCB, as shown in Fig. 5(a-2). Furthermore, when the resist-ance decreases from 200 kX to 100 kX, the TSC increasessharply in a narrower part of the same energy range, asshown in Fig. 5(a-3). If the resistance decreases to 100 X orless, TSC measurements become impossible because of delo-calized electrons. Such a state is a delocalized electronicstate, namely, the on-state. Since TSC is measured in thetemperature range above 80 K, the electronic state in therange 0–0.15 eV below the FCB cannot be revealed usingTSC. The number of electrons near the measurement limit of0.15 eV below the FCB increases sharply, corresponding to adecrease in the resistance, as shown in Fig. 2(b). The delo-calized electrons caused by an increase in the number ofelectrons are therefore also considered to exist above thelevel of 0.15 eV below the FCB. Since VoCB is generatedwith the delocalized electrons, the Fermi level (Ef) of the on-state is considered to exist at a level above 0.15 eV below theFCB, as shown in Fig. 5(b).Although the electronic states in the band gap, except at0.15–1.28 eV below FCB, are not clarified, a band gap of6.5 eV was clarified by EELS measurements of the AlOx cellFIG. 4. MIS-type AlOx-ReRAM and a mechanism image. (a) I-V characteristic measurement circuit with CRD. (b) Typical bipolar-type I-V characteristic ofthree cycles. Set current is limited to 28 lA by CRD. Reset current is decreased to 7 lA by p-n junction effect. (c) An image of on/off mechanism based on theband model. Sequence numbers 1-4 in Fig. 4(c) are corresponded to the numbers 1-4 in Fig. 4(b).033711-5 Nigo et al. J. Appl. Phys. 112, 033711 (2012)boundary. The valence band top level of AlOx, estimatedusing UV photoelectron spectroscopy (UPS), exists at 7.8 eVbelow the vacuum level, and the shape of the valence bandnear the top of AlOx is similar to the valence band of crystal-line Al2O3. To summarize these measurement results forelectronic states, a schematic diagram of the on/off-stateelectronic structures of AlOx-ReRAM is shown in Fig. 5.Finally, since a dry process is more suitable in semicon-ductor manufacturing process than a wet-type anodizationprocess, we are extending our research to a sputtered AlOxfilm, based on knowledge of the cell boundary of anodizedAlOx. Although the production process of AlOx does notneed to be anodization, the electronic structure must be simi-lar to the cell boundary of anodized AlOx, containing a high-density of Vos, i.e., 1021 cm�3 or more. A remarkable featureof Vos is the ability to receive and release electrons withstructural relaxation, without any chemical reaction.IV. CONCLUSIONSWe showed a resistive switching mechanism of anAlOx-ReRAM based on the electronic states revealed byTSC. This system is expected to have high endurance, equiv-alent to that of dynamic random access memory (DRAM),because it is driven by increasing/decreasing numbers ofelectrons, similar to DRAM. To verify the mechanism, anddecrease the reset-current, we changed a MIM-type AlOx-ReRAM to a MIS type, and decreased the reset-current to7 lA, as expected. Although the properties of materials withhigh-density Vos are still under study, in the future, Vo hostmaterials may be not limited to AlOx; well-designed Vomaterials are expected to provide excellent and rare-metal-free electronic materials.ACKNOWLEDGMENTSThis work was performed under a subsidy from the Ele-ments Science and Technology Project of MEXT, Japan.The authors are grateful to Kobelco Research Inc. and theFukuryo Semicon Engineering Co. for TEM and EELSmeasurements.1A. Sawa, Mater. Today 11, 28 (2008).2M.-J. Lee, C. B. 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G. Milnes, Mater. Sci. Eng. B5, 397 (1990).12T. W. Hickmott, J. Appl. Phys. 88, 2805 (2000).FIG. 5. Schematic illustrations of electronicstructures of on/off-state based on TSC. (a)Electronic states 0.15-1.28 eV below theconduction band measured by TSC areshown in the orange area as typical off-states. DOS patterns are TSC curves rotatedto the right through 90�. (b) Electronic stateof on-state is estimated from tendencies inchanges in the off-state. FCB and VoCBdenote the framework conduction band andthe Vo conduction band, respectively. A-Aand B-B arrows indicate the cross-sectionline of the schematic energy-band diagramof Al/AlOx/Al structure corresponding tothe on/off-state.033711-6 Nigo et al. J. Appl. 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