Metal Oxide Resistive Switching Memory

Here, wereview the research progress of metal oxide memory including thephysical mechanism of switching, state of the art deviceperformances, as well as device cell structure for integra

Shimeng Yu, Byoungil Lee, H.-S Philip Wong Center for Integrated Systems and Department of Electrical Engineering, Stanford University

13.1 Introduction

Information storage device is a key component of nanoelectronicsystems Conventional memories such as SRAM, DRAM, andFLASH are facing formidable device scaling challenges Emergingmemory device concepts have been actively pursued both in industryand academia in hopes of finding solutions for future informationstorage needs [1-2] The ideal characteristics for a memory deviceinclude fast programming speed (~ns), long retention time (>10years), low power consumption, good reliability, high integrateddensity, and continued scalability Several candidates have beenproposed to achieve the above goals, such as magnetoresistiverandom access memory (MRAM) [3], ferroelectric random accessmemory (FeRAM) [4] In recent years, memory devices based onthe electrically switchable resistance phenomenon have beenextensively studied The basic principle of this kind of memory is to

use a high resistance state (HRS) or low resistance state (LRS) tostore the information data “0” or “1”, and the transition between thetwo states can be triggered by electrical inputs Roughly speaking,resistive switching memories can be classified into three groups [5]:(i) phase-change memory (PCM) based on chalcogenides [6-9],which relies on the temperature induced change between thecrystalline phase (corresponding to LRS) and the amorphous phase(corresponding to HRS) (ii) programmable-metallization-cell (PMC)memory based on solid electrolytes or polymers [10-13], whichrelies on the formation (corresponding to LRS) or the rupture(corresponding to HRS) of a metallic conducting bridge betweentwo electrodes, and (iii) resistance random access memory (RRAM)based on metal oxides Among these resistive switching memories,metal oxide memory is promising for practical applications due to itscompatibility with silicon CMOS fabrication technology Here, wereview the research progress of metal oxide memory including thephysical mechanism of switching, state of the art deviceperformances, as well as device cell structure for integration into amemory array.

The negative differential resistances phenomenon in oxides wasfirstly reported in the 1960s [14-15], then it was reviewed by G.Dearnaley et al [16] in 1970 Recent work on the resistive switchingmetal oxide memory can be traced back to the discovery ofhysteresis I-V characteristics in perovskite oxides [17-19] such as

Pr0.7Ca0.3MnO3, SrTiO3, SrZrO3, etc in the late 1990s and the early2000s Since 2004, the research activities have focused on binary

metal oxides [20-26] such as NiO, TiO2, ZrO2, ZnO, Cu2O, Al2O3,HfO2, etc because of the simplicity of the material and goodcompatibility with silicon CMOS fabrication process Recently, theperformance of perovskite oxide memories and related physicalmechanism of resistive switching were reviewed by R Waser et al.[27-28] and A Sawa [29] Therefore, here we focus this review onbinary metal oxide memory.

13.1.1 Device Operation

It is necessary to first introduce some basic concepts andterminologies about metal oxide memory The typical metal oxidememory cell is a simple metal-insulator-metal (MIM) structure, asshown in Fig 1 (a) The switching event from high resistance state(HRS) to low resistance state (LRS) is referred to as the “set”process Conversely, the switching event from LRS to HRS isreferred to as the “reset” process In some cases, for the freshsamples in its initial resistance state (IRS), a larger voltage is needed

to trigger on the resistive switching behaviors for the subsequentcycles This is called the “electroforming” or “forming” process Theswitching modes of metal oxide memory can be broadly classifiedinto two switching modes: unipolar and bipolar Unipolar switchingmeans the switching direction depends on the amplitude of theapplied voltage but not on the polarity, thus set/reset can occur at thesame polarity, and usually it can symmetrically occur at bothforward and reversed voltages, as illustrated in Fig 1(b) Bipolarswitching means the switching direction depends on the polarity of

the applied voltage, thus set can only occur at one polarity and resetcan only occur at the reverse polarity, as illustrated in Fig 1 (c) Toavoid a hard dielectric breakdown in the set process, it isrecommended to enforce a set compliance current, which is usuallyprovided by the semiconductor parameter analyzer, or morepractically, by a memory cell selection (or access) transistor/diode or

a series resistor To read the data from the cell, a small voltage isapplied that does not affect the state of the memory cell For non-volatile application, the cell should retain its state at standby modeand free from disturb from reading/writing of neighboring cells

Fig 1 (a) Schematic of metal-insulator-metal (MIM) structure for metal oxide

memory cell, and schematic of metal oxide memory’s I-V curves, showing two modes of operation: (b) unipolar and (c) bipolar

13.1.2 Device Characteristics

To further understand how the metal oxide memory works, thedevice characteristics of HfO2 based memory cell [30-31] fromIndustrial Technology Research Institute (ITRI) are presented as anexample Fig 2 (a) shows the transmission electron microscopy(TEM) image of the concave structure device of TiN/Ti/HfOx/TiNstack with 30 nm cell size; here 5 nm HfOx layer is used for the mainswitching layer, TiN is used for the electrode material, and a thin Tibuffer layer is used for improving the switching stability Fig 2 (b)shows the typical I-V curve of such memory cell A 200 µA setcompliance current is enforced, and the device exhibits bipolarswitching Fig 2 (c) shows the switching endurance testing result

The set/reset programming condition is +1.5 V/-1.4 V pulse with

500 µs width, and it is seen that after 106 switching cycles, theHRS/LRS resistance ratio is still larger than 100, although there issome degradation Fig 2 (d) shows the data retention testing result,the measurement is performed at 150 °C, and it is seen that 10 yearslifetime is expected using a simple linear extrapolation Besides, fastswitching speed (~5 ns), and multi-bit storage potential is alsodemonstrated in the HfO2 memory

Besides HfO2, other binary metal oxides such as NiO, TiO2, ZrO2,ZnO, Cu2O, Al2O3 have been extensively explored Up to now,dozens of binary metal oxides have been found to exhibits electric-field-induced bistable resistance switching behavior Most of themetals are transition metals, and some are lanthanide series metals.The materials for switching layer and electrode layer aresummarized in Table 13.1 Note that some nitrides, e.g TiN are alsoused for the electrode layer

Fig 2 (a) Transmission electron microscopy (TEM) image of the concave

structure device of TiN/Ti/HfO x /TiN stack; (b) Typical I-V curve of the device with the cell size of 30 nm; (c) Endurance testing by 500 µs pulse: successive 10 6

switching cycles is achieved; (d) Retention testing at 150 °C: 10 years lifetime is expected Reprinted from Ref [30-31]

Table 13.1 Summary of the materials that has been used for binary metal oxide

memory Yellow metals corresponding binary oxides are used for switching layer; blue metals are used for electrode layer.

13.2 Possible Physical Mechanism for Resistive

Switching in Metal Oxides

The physical mechanism for resistive switching in metal oxidememory is still under a heated debate According to ITRS 2007 [32],resistive switching mechanism can be roughly classified into threegroups: thermal effect, electronic effect, or ionic effect Thermaleffect refers to the thermal dissolution of conductive filamentstriggered by local Joule-heating and works like a traditionalhousehold fuse albeit at the nanoscale [33-34] It is also referred to

as fuse/anti-fuse switching type The electronic effect mechanismconjectures that injected charges are trapped by interface defects.These trapped charges modify the Schottky barrier height betweenelectrodes and oxides and thereby changing the conductance throughthe MIM structure [35-36] Another case of electronic effect is Mottmetal-insulator-transition due to the strong electron-electroncorrelation in some transition metal oxides [37-38] Ionic effectrefers to the migration of ions with related electrochemical reactions

to form a conducting bridge between electrodes [39-40] It is alsoreferred to as reduction/oxidation type Actually, this ionic effect isjust the operating principle of programmable-metallization-cellmemory in solid electrolytes or polymers as mentioned before.Metal oxides can also serve as fast ions conductors for someparticular metal ions like Ag+ and Cu2+ [41-42] Here we plan tofocus on the intrinsic properties of metal oxides, thus we would notinclude those metal oxides memory with Ag or Cu electrodes in the

following discussions

So far, no single model mentioned above can explain all theexperimental results obtained in various materials It seems that thethermal dissolution model can address parts of the unipolarswitching characteristics, while the ionic migration model canexplain most of the bipolar switching characteristics However, thedistinction between the two switching behaviors is ambiguous Here

we discuss several key issues such as the conducting mechanism, thenature of electroforming/set/reset process, the effect of electrodematerials on switching modes, with the aim of seeking a rationalmechanism for both unipolar and bipolar switching characteristics

13.2.1 Conduction Mechanism

A lot of conductive atomic force microscopy (C-AFM)measurements [43-47] have been reported in the literature, revealingthat conductive filaments (CFs) with nanoscale diameters are formed

in the oxide layer during the set process, and large conductingcurrent passes through these filaments in LRS Fig 3 shows one ofthese C-AFM measurements results [45] Typically, CFs are sparselyand non-homogeneously distributed under the electrodes Thismeans that the conducting area in LRS is an extremely small portion

of the entire electrode area, which is typically quite large in mostexperiments Therefore, the conductance in LRS would not decrease

as much as the conductance in HRS does when the electrode area isdecreased, resulting a larger HRS/LRS resistance ratio, which is abenefit of scaling down memory cell size It has been experimentally

demonstrated that these CFs can be formed by C-AFM tipindividually [48], but in most cases multiple CFs exist in the oxidelayer, which is believed to be responsible for the multi-levelswitching behaviors exhibited in some metal oxide memory devices[49-50] Although the filamentary conduction mechanism is widelyrecognized, some researchers argued that a bulk conductionmechanism dominates the LRS resistance in their TiO2 devices [51].They proposed that charged dopants (mainly oxygen vacancies)migrate under a voltage bias to the TiO2/electrode interface Theinterfacial changes modify the Schottky barrier height at theinterface and lead to the resistive switching in their devices Inessence, such arguments do not contradict with the filamentaryconduction mechanism, provided that the so-called bulk conduction

is regarded as many parallel CFs It should be noted that if CFs aredense enough in ultra-scaled devices, the bulk conduction behaviormay arise A usual experiment that aims at distinguishingfilamentary or bulk conduction is to measure the trend of theHRS/LRS resistance ratio versus the cell area If the ratio goes upwhen the cell area is scaled down, filamentary conduction prevails

If the ratio remains almost constant when the cell is scaled down,bulk conduction prevails However, so far, these experiments havenot been performed for devices with truly nanometer scale size (<

100 nm) memory cells and therefore the results are not yetconclusive

Fig 3 Conductive atomic force microscopy (C-AFM) of LRS (a) and HRS (b)

conductance in Al 2 O 3 memory, showing filamentary conduction dominates in LRS (scanned area: 2 µm×2 µm; maximum y scale: (a) 105 nA, (b) 5 nA) Reprinted from [45]

The CFs are conjectured to be made up of oxygen vacancies (Vo)

in most metal oxide memory devices, and this assumption wasconfirmed by micro X-ray fluorescence (XRF) and X-ray absorptionnear-edge spectroscopy (XANES) [52] It is well known that Vo canact as an effective donor in n-type metal oxides Ab-initiocalculation of the rutile TiO2 electronic structure [53] reveals that Vocan produce a defect state within the band gap This state is occupied

by two electrons that are localized on Ti 3d orbital of the nearest Tiatoms to Vo If a chain of such Vo forms, it is expected that electronsdelocalization occurs and the hopping probability can increaseremarkably along these CFs and ultimately lead to the LRS.Although Vo plays an important role in the switching behaviors ofmost metal oxide memory devices, the composition of CFs is notlimited to Vo There are quite a few reports that the CFs in NiOmemory are composed of excess metallic Ni in NiO This suggestion

is confirmed by measurement of the dependence of the LRSresistance on temperature [54] and XPS composition analysis [55] Itwas further verified by electron energy loss spectroscopy (EELS)that there exists a Ni-rich phase at the grain boundaries, whichimplies that the CFs of Ni are formed as precipitates at the grainboundaries [56] A simple way to distinguish whether the CFs aremetallic or semiconducting is to measure the LRS resistancetemperature dependency If the LRS resistance goes up with the

increase of temperature, the CFs are metallic and may consist ofmetal precipitates On the contrary, if the LRS resistance drops withthe increase of temperature, CFs are semiconducting and mayconsist of Vo.

There are many efforts to fit the I-V characteristics of HRS andLRS to current conduction models in the literature Most of thereports show an Ohmic relationship in the LRS And in thesemiconducting CFs, Mott variable hopping conduction [57] isproposed to dominate in LRS, and this assumption was supported bysome temperature varying measurements [58-59] and ACconductance measurement [60] But, the conduction fitting results inHRS are quite diverse, Poole-Frenkel emission (I~V*exp(V1/2)) [61-62], Schottky emission (I~exp(V1/2)) [63-64], the space chargelimited current (SCLC) characteristic (the Ohmic region I~V,followed by the child’s square law region I~V2) [65-66], wereobserved in various metal oxide memories The diverse I-Vcharacteristics in HRS involving different leakage mechanism may

be associated with the different dielectric properties or differentfabrication processes conditions, e.g annealing temperature,annealing ambient, and the properties of the interface between theoxides and the electrodes Nevertheless, the key issue of metaloxides memory modeling is not the leakage current mechanism inHRS, but to investigate the mechanism that triggers the resistiveswitching So in the next section, we will discuss the forming/set andreset process, respectively

13.2.2 Electroforming/Set/Reset Process with Oxygen Migration

Dielectric breakdown is a process of local materials transitioningfrom insulating to conductive under a high external electric field.This phenomenon is well-known for gate dielectrics in MOSFETsand has been studied for a long time Recently, by site-specificstructural analysis using high resolution transmission electronmicroscopy (HR-TEM) with EELS, X Li et al [67] revealed thatafter dielectric breakdown, oxygen atoms in gate dielectric SiO2

were missing Substoichiometric silicon oxide SiOx (with x<2) wasformed, and the local energy gap was lowered with intermediatebonding state of silicon atoms Si1+, Si2+, and Si3+ in the percolationleakage path Also, chemical bond breakage and the local Jouleheating due to large current flowing through the percolation path arebelieved to be the main driving forces leading to the oxygendissociation

Similarly, in metal oxides, the electroforming/set process isinterpreted to be some kind of soft dielectric breakdown [60, 68-69]

J J Yang et al [70] claimed that they observed oxygen gas bubbles

at the electrode surface of their bipolar TiO2 memory devices in theelectroforming process They regarded the electroforming process inmetal oxide memory as an electro-reduction and Vo creation processcaused by high electric field, which is then enhanced by local Jouleheating During electroforming, Vo are created and drift towards thecathode, forming localized CFs in the oxide Simultaneously, oxygenions drift towards the anode where they discharge to evolve oxygengas M.-J Lee et al [71] investigated the composition change in

their unipolar NiO memory devices during the set process bysecondary ion mass spectroscopy (SIMS), as shown in Fig 4 Theauthors suggested that the oxygen atoms within the NiO layermigrated toward the Pt electrode after the switching, thus leaving the

Ni rich filaments in the NiO layer

Fig 4 (a) X-ray photoelectron spectroscopy (XPS) analysis of NiO memory; the

sample deposited at an oxygen partial pressure of 5% shows the coexistence of Ni (852.8 keV) and NiO (854.3 keV) peaks, which exhibited bistable resistive switching; the sample deposited at an oxygen partial pressure of 30% did not exhibit any stable resistive switching phenomenon, suggesting CFs may consist of metallic Ni precipitates (b) Secondary-ion mass spectroscopy (SIMS) data in NiO memory, showing the migration of oxygen atoms towards the electrodes after electrical switching Reprinted from [71]

Therefore, in both unipolar and bipolar switching metal oxidememory devices, the electroforming/set process is conjectured to beassociated with the generation of Vo and the migration of the oxygenatoms, generating CFs consist of either Vo or metal precipitates Infresh samples, usually the amount of Vo is small, thus a high voltage

is needed to trigger the electroforming process After electroforming,the devices have switched to LRS, and a sufficient amount of Vo ispresent In subsequent switching cycles, only a portion of the Vo, theones near the anode, can be recombined during the reset process.This is why the set voltage would be smaller than forming voltageand the resistance in HRS would be much smaller than the resistance

in IRS Through fitting of electrical parameters, the formation andrupture of the CFs is estimated to occur in a localized region (3–10

nm) thick near the anode [72] The remaining Vo rich region isreferred to as the “virtual cathode” [28]

Obviously, it is not desirable to have a large electroforming voltage

in practical applications Thus significant efforts have been made toachieve the so-called “forming-free” devices The forming voltage islinearly dependent on the thickness of the oxide film [30, 73-75] So

a thinner oxide film is effective in reducing the forming voltage H

Y Lee et al [30] claimed that the as-fabricated atomic layerdeposition (ALD) HfO2 device is free from the electroformingprocess as the thickness of the film is thinned to be 3 nm Besides,controlling the annealing ambient during deposition to obtainoxygen deficient films is also helpful in reducing the formingvoltage [76-78]

So far, we have stated that the nature of dielectric breakdown isthe formation of oxygen deficient percolation leakage paths Notethat gate dielectric breakdown is a permanent and irreversibleprocess, while the switching process in metal oxide memory is areversible process So the memory device should have a location tostore the missing oxygen atoms during the set process and then drivethem back during the reset process The concept of an “oxygenreservoir” is thus envisioned Usually, the anode electrode materialsact as the oxygen reservoir, and it also prevents the oxygen fromescaping from the device to the ambient The anode electrodematerial then provides a source of oxygen for reset process Fig 5illustrates how the oxygen reservoir works in a Ti/MnO2/Pt memorydevice [79] X-ray photoelectron spectroscopy (XPS) reveals that

amount of non-lattice oxygen became less in LRS than in HRS, andthe corresponding shift of Ti 2p peak indicates that an interfacialTiOx layer was formed This observation suggests that during the setprocess, oxygen ions moved toward the Ti anode when a positivebias was applied, where they reduced the Ti and formed TiOx Thisinterfacial TiOx layer is believed to act as an oxygen reservoir thatstores oxygen atoms When a negative bias was applied to the Tianode, oxygen ions moved away from the interfacial layer towardthe oxide bulk layer and recombine with Vo Therefore, the bipolarswitching characteristics are assumed to be the formation andrupture of the CFs consisting of Vo associated with oxygen ionmigration

Fig 5 XPS spectra of Ti/MnO 2 interface (a) The O 1s core levels spectra of the lattice peak (529.45 eV) and an additional non-lattice peak (531.33 eV), the change of non-lattice oxygen indicates oxygen ions migration during switching; (b) The spectra of Ti 2p, the shift of Ti 2p peak indicates an formation of interfacial TiO x layer, which can be regarded as an oxygen reservoir Reprinted from [79].

In the bipolar switching case, it is straightforward to conceive ofoxygen ions migrating away from the interface, since oxygen ionsare negative charged Under a negative bias, oxygen ions can bepushed toward the oxide bulk layer and then recombine with Vo andrupture the CFs if the CFs are made up of Vo C Yoshida et al [80]performed C-AFM writing experiments in 18O tracer gasatmosphere and related composition analysis by SIMS on the NiO

films Their results suggest that external oxygen can penetrate intothe oxide layer, and the composition change of the surface regionmay be responsible for the resistance change H Y Jeong et al [81]performed EELS oxygen element mapping experiments in HRS andLRS of Al/TiO2/Al structure The results suggest that the drift ofoxygen ions by the applied bias is the microscopic origin of thebistable resistivity switching So the reset mechanism for bipolarswitching is quite unambiguous in the literature [82-84] Recently,the nonlinear dynamical properties of bipolar switching TiO2 weremodeled by the drift and diffusion of ionized dopants (oxygenvacancies) in the oxide thin film [85-87] In essence, the descriptionusing oxygen vacancies is equivalent to the description using oxygenions here

In the unipolar switching case, the mainstream viewpoint of reset isdue to a thermal dissolution of CFs by local Joule heating [88-90].Electro-thermal calculation [88] suggests that the local temperaturecan rise to around 600 K during the switching process But thismodel is only phenomenological and does not reveal themicroscopic nature of the rupture of CFs Firstly, it does not addresswhether the loss of oxygen during set process can be recaptured insubsequent switching cycles or not, and the model does not addressthe question of whether the set process is a reversible process.Secondly, if the CFs are assumed to be simply “dissolved” by localJoule heating, then the CFs should rupture in the middle of thefilament where the temperature is highest [91], since the metalelectrodes work as a large heat sink The local Joule heating

conjecture contradicts with the many experimental observations thatswitching occurs in the region near the anode as mentioned before.Alternatively, M.-J Lee et al [71] regarded the reset process of theirunipolar NiO memory as an oxidation process of the metallic CFs,which is a consequence of the thermally activated oxygen atomsdiffusing from anode side followed by oxidation of the Ni-rich CFs.This argument acknowledges that the reset in unipolar switching is aJoule heating-assisted process At the same time, this argument isessentially consistent with the electrochemical ionic migrationmodel for bipolar switching devices The model provides an inherentlink between unipolar and bipolar switching In the next section, wediscuss another important question of why some materials exhibitunipolar characteristics, while some other materials exhibit bipolarcharacteristics before we make the final conclusion

13.2.3 The Effect of Electrode Materials on Switching Modes

There are many reports [92-94] on the effect of electrode materials

on the resistive switching characteristics of metal oxide memory Inmost cases, the switching mode is not an inherent property of theoxide film but an effect of the interaction between the oxide andelectrode materials R Waser et al pointed out that for bipolarswitching the MIM structure should have some asymmetry, such asdifferent electrode materials, while for uniploar switching the MIMstructure may be symmetric [27] Fig 6 shows two examples: thePt/ZnO/Pt [61] and Pt/NiO/Pt [20] structure show uniplolarswitching, while the TiN/ZnO/Pt [95] and Pt/NiO/SrRuO3 [96]

structure show bipolar switching Besides, many other metal oxidescan show either unipolar or bipolar behavior depending on whatelectrode materials are used Pt/ZrO2/Pt [97], Pt/TiO2/Pt [21],Pt/HfO2/Pt [62] all show unipolar switching, and Ti/ZrO2/Pt [97],Pt/TiO2/TiN [98], TiN/HfO2/Pt [99] all show bipolar switching Itshould be noted that the unipolar switching I-V curves are usuallysymmetric, since there is no distinction of either of the electrodes.

As a result, for unipolar switching devices, bipolar switchingbehaviors can also be demonstrated [100], which means the set can

be triggered at one polarity and reset can be triggered at the reversedpolarity Such case is also referred to “non-polar” switching [74].But in bipolar switching devices, there is always one polarity thatreset cannot occur when the same polarity bias as set process isapplied

Fig 6 (a) I-V characteristics of Pt/ZnO/Pt and TiN/ZnO/Pt devices, (b) Pt/NiO/Pt

and Pt/NiO/SrRuO 3 devices, showing the electrode materials effect on switching modes

Data are collected from:

Ref [1] W.-Y Chang, Y.-C Lai, T.-B Wu, S.-F Wang, F Chen, and M.-J Tsai,

Appl Phys Lett 92, 022110 (2008).

Ref [2] N Xu, L F Liu, X Sun, C Chen, Y Wang, D D Han, X Y Liu, R Q.

Han, J F Kang, and B Yu, Semicond Sci Technol 23, 075019 (2008).

Ref [3] S Seo, M J Lee, D H Seo, E J Jeoung, D.-S Suh, Y S Joung, and 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, p 5655 (2004).

Ref [4] J S Choi, J.-S Kim, I R Hwang, S H Hong, S H Jeon, S.-O Kang, B.

H Park, D C Kim, M J Lee, and S Seo, Appl Phys Lett 95, 022109 (2009).

The electrode materials can be roughly classified into two groups:one is noble metals, such as Pt, Au, Ru, etc, which are resistant tooxidation; the other one is non-noble metals, such as Ti, Al, Cu, Ni,

W, TiN, TaN, etc, which may suffer oxidation during the switchingprocess As observed above, devices using noble metals alwaysshow unipolar behaviors Even with the Pt/TiO2/TiN [98] structure,unipolar switching can occur at the Pt side However, the unipolarswitching cannot occur at TiN side, and only bipolar switching isobtained at the TiN side As mentioned before, during the setprocess, the oxygen ions migrate toward the anode Thus, the anodematerials may react with the oxygen and form an interfacial layer ifthe anode materials are oxidizable P Zhou et al [101] compared thedifferent roles played by the Pt electrode and the TaN electrode onthe switching modes of CuxO memory Fig 7 (a) and (b) showsdepth profile by Auger Electron Spectroscopy (AES) for Pt/CuxO/Custructure and TaN/CuxO/Cu structure, we can see that the oxygenconcentration has an obvious shift towards the TaN layer, whichindicates that a reaction occurs between the TaN electrode and oxidefilm This interfacial TaON layer was further confirmed by XPSbinding energy depth spectra The bright gray areas under the TaNelectrode in the TEM picture of Fig 7 (c) are assumed to be theTaON ultra-thin layer And Fig 7 (d) illustrates that with Pt as topelectrode, reset can occur at both bias polarities, showing a unipolarswitching behavior, while with TaN as top electrode, reset can occuronly under the reversed bias, showing a bipolar switching behavior

Therefore, we can infer that if noble metals like Pt are used for theelectrode, it hardly forms such interfacial oxide layer And withoutthe diffusion barrier, the thermally activated oxygen ions maydiffuse back during the reset process to the oxide bulk layer sincethere is a large concentration gradient of oxygen across the interfaceregion This may account for the unipolar reset mechanism Ifoxidizable materials are used for the electrode, it may form aninterfacial oxide layer between the electrode and oxide films, whichmay act as an oxygen diffusion barrier Thus, it is difficult for theoxygen ions to diffuse back through the thermal activation Thenonly by the acceleration of a reversed electric field can the oxygenions drift back to rupture the CFs This may account for the bipolarreset mechanism Numerical simulation [102] based on the oxygenions non-linear transport model is employed to support the aboveassumptions.

Fig 7 (a) Auger Electron Spectroscopy (AES) depth profile of (a) Pt/Cu x O/Cu structure and (b) TaN/Cu x O/Cu structure, indicating the existence of an interfacial TaON layer; (c) TEM picture of the TaN/Cu x O/Cu structure, showing the interfacial TaON layer; (d) I-V characteristics of Pt/Cu x O/Cu structure with unipolar switching behavior and TaN/Cu x O/Cu structure with bipolar switching behavior Reprinted from [101]

13.2.4 Summary of the Physical Mechanism for Resistive Switching in Metal Oxide Memory

Here, we would like to summarize the physical resistiveswitching mechanism in metal oxide memory It should be noted,however, while a self-consistent physical picture can be constructed

through the discussions above, the resistive switching phenomenon

in metal oxides involves a large variety of materials, so the modeldescribed here may not apply to all combinations of materials Basically, resistive switching is conjectured to be due to theformation and dissolution of conductive filaments localized at theanode interface, and is a reversible switching process Theconductive filaments may consist of oxygen vacancies or excessmetallic precipitates Multiple, parallel filaments may be formed.The filaments may preferentially be located at the grain boundaries.Experimental evidences suggest that the switching process is anelectrochemical process associated with oxygen migration, oxidationand reduction As shown in Fig 8, during the electroformingprocess, soft dielectric breakdown occurs and oxygen ions drift tothe anode interface by the high electric field, where they aredischarged as neutral non-lattice oxygen if the anode materials arenoble metals or react with the oxidizable anode materials to form aninterfacial oxide layer Meanwhile in the bulk, the resultant oxygenvacancies or metal precipitates form the highly conducting paths.During the reset process, oxygen ions migrate back to the bulk either

to recombine with the oxygen vacancies or to oxidize the metalprecipitates In the unipolar switching case, the Joule heatingthermally activates the diffusion of oxygen ions Oxygen ionsdiffuse against the electric field from the anode due to theconcentration gradient In the bipolar switching case, usuallyinterfacial layer between the anode and oxide acts as a diffusionbarrier, and oxygen ions can only drift back aided by the electric

field Usually only a part of the conducting paths is ruptured, leavingthe bottom part of the conducting paths to be a virtual cathode thatextend partially into the film Then during the subsequent setprocess, a process similar to the electroforming occurs but only inthe region near anode The physical picture presented here does notcontradict with the previous thermal model for unipolar switching orionic model for bipolar switching And it can at best be viewed as aphenomenological description of experimental observations Details

of the physics of switching remain an area of active research

Fig 8 Schematic of the switching process in metal oxide memory

13.3 Performances of Metal Oxide Memory Devices

Firstly, we would like to discuss the fabrication techniques briefly.Currently, the fabrication of metal oxide memory devices usesmainly conventional semiconductor fabrication processes.Sputtering of metals followed by annealing in oxygen ambient orreactive sputtering in oxygen ambient are the most commondeposition techniques for metal oxide layers Other methods usedare atomic layer deposition (ALD) [30, 43], pulsed laser deposition(PLD) [103], metal organic chemical vapor deposition (MOCVD)[26] and the sol-gel method [104] Several papers report resistiveswitching behavior of self-assembly grown nanostructure, e.g NiOnanowires [105-106] The thickness of the metal oxide layer isusually around 10-50 nm The substrate of the devices is mainlysilicon; however, there are several efforts to fabricate metal oxidememory on transparent substrate, e.g glass [107] or flexible

substrate, e.g polyethersulfone (PES) [108-109] In the rest of thissection, we will discuss the characteristics of metal oxide memorydevices including noise margin, scalability, power consumption,speed, reliability and uniformity

The noise margin for the read operation is characterized by theHRS/LRS resistance ratio Generally, a ratio >10 is desired for easiersense amplifier design Almost all the metal oxide memory devices

in the literature exceed this requirement

Scalability is one of the key concerns for any kind of memory.Because of the filamentary conduction nature, metal oxide memorydevices may be scaled down to the nanoscale Sub-100nm featuresize cells have been fabricated by 193-nm lithography with a contacthole structure [78], as well as by nanoimprint lithography or e-beamlithography with a crossbar arrays structure [51, 110-111] Recently,aggressively scaled 30 nm×30 nm HfO2 memory devices in 1Kbarray have been demonstrated with excellent yield [31] Resistiveswitching behavior has been successfully triggered by C-AFM tip on

a 10 nm × 10 nm region of NiO thin films [71], suggesting thepotential to scale the cell size even down to the sub-10 nm regime.Fig 9 plots the general scaling trend of HRS and LRS resistance.The leakage current in HRS is mainly due to bulk leakage current.Thus, the resistance of HRS increases as the inverse of the cell area,roughly following the Ohm’s law The conduction current in LRS ismainly filamentary conduction current, as discussed before So theresistance of LRS has only a slight dependency on the cell area Thistrend of increasing HRS/LRS resistance ratio as cell area decreases

is a benefit of device scaling.

Fig 9 HRS and LRS resistance versus cell area of metal oxide memory devices.

Data are collected from:

Ref [1] I G Baek, M S Lee, S Seo, M J Lee, D H Seo, D.-S Suh, J C Park,

S O Park, H S Kim, I K Yoo, U.-I Chung, and I T Moon, Tech Dig IEDM, p.

587 (2004).

Ref [2] H Y Lee, P S Chen, T Y Wu, Y S Chen, C C Wang, P J Tzeng, C H Lin, F Chen, C H Lien, and M.-J Tsai, Tech Dig IEDM, p 297 (2008).

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Although the read noise margin is improved in the scaled memorycell, the filamentary conduction nature of the memory cell results inincreasing power density as devices scale down The typicalswitching voltage in literature is around 1-5 V, while the switchingcurrent can be many orders different, depending on the material andthe device structure The key to reducing power consumption is toreduce the reset switching current The peak value of current inmetal oxide memory is the LRS current at the point when the resetprocess occurs, which is usually referred to as the reset current