Slide vật liệu nano và màng mỏng material science engeering r

MaterialsRRAM devices are usually composed of a storage layersandwichedbytwoelectrodes.Accordingly,thematerialsinvolved in RRAMs can be classified into storage media and electrodematerial

Recent progress in resistive random access memories: Materials,

F Pan * , S Gao, C Chen, C Song, F Zeng

Contents

1 Introduction 2

2 Materials 3

2.1 Storagemedia 3

2.1.1 Inorganicstoragemedia 4

2.1.2 Organicstoragemedia 5

2.2 Electrodematerials 5

3 Switchingmechanisms 6

3.1 Ionmigration 6

3.1.1 Cationmigration 6

3.1.2 Anionmigration 13

3.2 Chargetrapping/de-trapping 17

3.2.1 Interfacialchargetraps 17

3.2.2 Chargetrapsprovidedbyamiddlenanoparticlelayer 17

3.2.3 Randomlydistributedchargetraps 18

3.3 Thermochemicalreaction 20

3.3.1 Thermochemicalreactioninsemiconductingmetaloxides 20

3.3.2 Thermochemicalreactioninorganics 20

3.4 Exclusivemechanismsininorganics 21

3.4.1 Insulator-to-metaltransitioninMottinsulators 21

3.4.2 Thesp2/sp3conversioninamorphouscarbon 22

Keywords:

Memristor

Thisreviewarticleattemptstoprovideacomprehensivereviewoftherecentprogressintheso-called resistiverandom accessmemories(RRAMs).First,abriefintroductionispresentedto describethe construction and development of RRAMs, their potential for broad applications in the fields of nonvolatilememory,unconventionalcomputingandlogicdevices,andthefocusofresearchconcerning RRAMs over the past decade Second, both inorganic and organic materials used in RRAMs are summarized,andtheirrespectiveadvantagesandshortcomingsarediscussed.Third,theimportant switchingmechanismsarediscussedindepthandareclassifiedintoionmigration,charge trapping/de-trapping,thermochemicalreaction,exclusivemechanismsininorganics,andexclusivemechanismsin organics.Fourth,attentionisgiventotheapplicationofRRAMsfordatastorage,includingtheircurrent performance,methods forperformanceenhancement,sneak-pathissueandpossiblesolutions,and demonstrations of 2-D and 3-D crossbar arrays Fifth, prospective applications of RRAMs in unconventional computing, as well as logic devices and multi-functionalization of RRAMs, are comprehensivelysummarizedandthoroughlydiscussed.Thepresentreviewarticleendswithashort discussionconcerningthechallengesandfutureprospectsoftheRRAMs

ß2014ElsevierB.V.Allrightsreserved

ContentslistsavailableatScienceDirect

j o urn a l hom e pa ge : ww w e l s e v i e r c om/ l o ca t e / ms e r

http://dx.doi.org/10.1016/j.mser.2014.06.002

cuu duong than cong com

3.5 Exclusivemechanismsinorganics 22

3.5.1 Chargetransfer 22

3.5.2 Conformationalchange 23

4 RRAMsfordatastorage 24

4.1 Currentperformance 24

4.2 Methodsforperformanceenhancement 25

4.2.1 Doping 25

4.2.2 Electrodeengineering 27

4.2.3 Interfaceengineering 29

4.2.4 Optimizationofdevicestructureandmeasurementcircuit 31

4.2.5 Multilevelstorageandconductancequantization 33

4.3 Sneak-pathissueandpossiblesolutions 37

4.3.1 Diode 38

4.3.2 Bidirectionalselector 39

4.3.3 Self-rectification 41

4.3.4 Complementaryresistiveswitch 41

4.4 Demonstrationsof2-Dand3-Dcrossbararrays 43

5 Prospectiveapplicationsandmulti-functionalizationofRRAMs 44

5.1 RRAMsforunconventionalcomputing 44

5.2 RRAMsforlogicapplication 47

5.2.1 ReconfigurableswitchesinFPGAs 47

5.2.2 Logicgates 48

5.2.3 Materialimplicationlogic 48

5.3 Multi-functionalizationofRRAMs 50

5.3.1 InvolvementofspinsinRRAMs 50

5.3.2 InteractionsbetweenphotonsandelectronsinRRAMs 51

5.3.3 Acombinationofresistiveswitchingandsuperconductingbehavior 51

6 Challengesandprospects 52

Acknowledgements 54

References 54

1 Introduction

Silicon-based Flash memories, consisting of a

metal-oxide-semiconductorfield-effect-transistorwithanadditionalfloating

gateineachmemorycell,representthestate-of-the-art

nonvola-tilememoryandrepresentthelion’sshareofthecurrentsecondary

memorymarketduetotheirhighdensityandlowcost.However,

Flashmemoriessufferfromseveralobviousdisadvantagessuchas

low operation speed (write/erase time: 1ms/0.1ms), poor

endurance (106 write/erase cycles) and high write voltage

(>10V)[1].Moreover,Flashmemorieswillreachtheir

miniaturi-zationlimitinthenearfuture,notfortechnicalreasons,butfor

physicallimitationssuchaslargeleakagecurrents.Toovercome

theshortcomingsofFlashmemories,fouremergingrandomaccess

memories (RAMs) have been proposed: ferroelectric RAMs

(FRAMs),magneticRAMs(MRAMs),phase-changeRAMs(PRAMs)

andresistiveRAMs(RRAMs).Amongthesememories,FRAMsand

MRAMsalsofacetheminiaturizationissuebecauseoftheirlarge

memorycellsize[1].For PRAMs,thelarge power consumption

duringthereversiblephasetransitionbetweentheamorphousand

crystallinephases would be the mostserious obstacle to their

commercialization [1] Fortunately, RRAMs have been

demon-strated to exhibit excellent miniaturization potential down to

<10nm [2] and to offer sub-ns operation speed [3,4], <0.1pJ

energyconsumption[5,6]andhigh-endurance(>1012switching

cycles)[7].Therefore,RRAMsarealsoapotentialalternativetothe

currentmainmemory,i.e.,dynamicRAMs(DRAMs)

Ingeneral,aRRAMcelliscomposedofaconductor/insulator(or

semiconductor)/conductorsandwichstructure,asshowninFig.1a

Thissimplestructureenablesittobeeasilyintegratedinpassive

crossbararrayswithasmallsizeof4F2(Fistheminimumfeature

size),andthesizecanbefurtherreducedto4F2/nwithinvertically

stackedthree-dimensional(3-D)architectures(nisthestacking

layernumber of the crossbar array) [8] The intrinsic physical

phenomenon behind RRAMs is resistive switching (RS), which

means that the device can be freely programmed into a high resistancestate(HRS,orOFFstate)oralowresistancestate(LRS,or

ON state) under external electrical stimuli In most cases, the current flows uniformly through the device in theHRS and is restricted toa localregion with highconductance known as a conductingfilament(CF)intheLRS[9].Theswitchingeventfrom theHRStotheLRSandthecorrespondingvoltagearedenotedas setprocessandVset,respectively.Incontrast,theswitchingevent fromtheLRStotheHRSandthecorrespondingvoltagearedenoted

asresetprocessandVreset,respectively.Thereareusuallytwotypes

ofswitchingmodes:unipolarandbipolarswitching.Theformer requires the same electrical polarity during the set and reset processes,whereasthelatterrequiresoppositeelectrical polari-ties,asshown bytheschematiccurrent–voltage(I–V)curvesin

Fig.1 andc,respectively

Ithasbeenalmostahalfcenturysincetheinitialexperimental observationsofRS.In1962,Hickmott[10]observedlargenegative differentialresistanceinfivethinanodicoxidefilmsincludingSiOx,

Al2O3,Ta2O5,ZrO2andTiO2.Subsequently,morematerialswere demonstratedtoshowRS,andtheswitchingmechanismsstarted

tobe explored as well[11–13].Strong research interest in RS, however,onlylastedapproximatelyadecadeowingtothefactthat theobservedRSatthatmomentwasnot sufficientlyrobustfor memory application,andalsodue totheprosperityofSi-based integratedcircuittechnology.Sincethelate1990s,interestinRS begantorevivebecauseofthesearchforanalternativetoSi-based memories.ThefirstpracticalapplicationofRRAMswasreportedby Zhuanget al [14] Theseresearchers fabricateda 64-bit RRAM arrayusingPr0.7Ca0.3MnO3viaa 0.5-mmcomplementary metal-oxide-semiconductor (CMOS)process The deviceshowed good performance with low operation voltage (<5V), fast speed (10ns) and a large memory window (>103) Meanwhile, organics-based RRAMs were introduced by Yang’s group [15], greatlyenrichingtherangeofusablematerials.In2004,Baeketal

[16] successfully demonstrated the world’s premier binary

2

cuu duong than cong com

transition-metal-oxide-based RRAMs with operation below 3V

and 2mA, 106 set/reset operations and 1012 reading cycles,

triggeringasurgeofstudiesinthisresearchtopic.Recently,RRAMs

have been identified as the physical realization of the fourth

fundamentalpassivecircuitelementnamedmemristor[17],and

have also been suggested for new application fields including

unconventionalcomputing[18]andlogicdevices[19].Moreover,

combinations of RRAMs and ferromagnetic, optical, and even

superconductingpropertieshaverecentlybeenreported aswell

attentionfromacademiaandfromapplication-orientedsocieties,

demonstratedbytheincreasingnumberofpublicationsperyear,

asshowninFig.2

ThecommercializationofRRAMsreliesheavilyonathorough

understandingoftheunderlyingswitchingmechanisms.Inmost

cases,however,itisnotablydifficulttolocatethetinyswitching

regionexactlyduetoitsrandomnature,nottomentionclarifying

the complicated switching process Thanks to advances in the

fabrication and the characterization of nanoscale materials(includingtheuseoffocusedionbeam(FIB)systemsandinsitutransmissionelectronmicroscopes(TEM)),significantprogresshasbeenmadeoverthepastdecadeinunderstandingtheswitchingmechanismsbehindRRAMs.Forexample,thedetailednucleationandgrowthprocessesofconductingfilamentshavebeendirectlyrecordedbyinsituTEMinsomematerials[23–26].Guidedbythein-depthunderstandingofswitchingmechanisms,manyeffectivemethods have been developed to optimizethe performance ofRRAMs, includingdoping[27,28],electrodeengineering[29,30],interfaceengineering[31,32],etc

Although passive crossbar arrays are the most attractivearchitectures for future practical use of RRAMs due to theirultra-high integration density,they facea troublesome obstaclecalledthesneak-pathissue,whichcansignificantlyreducetheread-outsensemargin,increasethepowerconsumption,andlimitthearraysize[33].Howtosolvethesneak-pathissuehasbeenanotherfocusofresearchoverthepastdecade.AsimplesolutionistodirectlyconnectanonlinearcircuitelementsuchasatransistororadiodetoeachRRAMcell.BecausetheconventionalSi-basedtransistorsanddiodesareundesirable,primarilybecauseoftheircomplicatedandhigh-temperaturefabricationprocedures,variousoxide-andevenorganics-baseddiodes[34,35]andbidirectionalselectors[36]havebeen designed and characterized Meanwhile, as alternativesolutions,RRAMs withself-rectification[37]andcomplementaryresistiveswitches[33]areattractingincreasingattentionbecausetheydonotneedadditionalnonlinearcircuitelements

To improve the storage density of passive crossbar arraysfurther,multilevelstorageoperationinasinglememorycellhasbeencomprehensivelyexploredoverthepastdecade.ThecommonmethodsformultilevelstorageoperationaresettingvariousIcomp

valuesduringthesetprocess[38]andsettingvariousstopvoltagesduring the reset process [39] Recently, a new method calledconductance quantization has been suggested and intensivelystudied[22,40–42].ConductancequantizationoriginatesfromthequantumsizeeffectsoftheCFsandmeansthattheconductanceof

aRRAMcellinLRSisanintegralmultipleofasingleatomicpointcontact.Conductancequantizationwasinitiallyobservedincationmigration-based RRAMs [40] and has been extended to anionmigration-based RRAMs [22,41], thereby acting as a notablypromisingmethodforachievingultra-highdensitystorage

Itshouldbenotedthatseveral reviewsofRRAMshavebeensuccessivelyreportedinthepastseveralyears[9,43–46].GiventheveryrapiddevelopmentofRRAMs,areviewarticlethatcoversthemostrecentadvancesinthisresearchfieldisurgentlyneeded.Inthiswork,weprovideacomprehensivereviewofrecentprogress

inRRAMsbasedonbothinorganicandorganicmaterials.Afterabrief introduction of RRAMs, materials,switching mechanisms,RRAMsfordatastorage,andprospectiveapplicationsandmulti-functionalizationofRRAMsarediscussedindetailinSections2,3,

4and5,respectively.Finally,thechallengesandfutureprospectsofRRAMsarepresented.Inthiswork,itshouldbenotedthatsomereferences may have been overlooked owing to the rapiddevelopmentofRRAMs

2 MaterialsRRAM devices are usually composed of a storage layersandwichedbytwoelectrodes.Accordingly,thematerialsinvolved

in RRAMs can be classified into storage media and electrodematerials,whichwillbediscussedseparatelybelow

2.1 Storagemedia

AgreatmanymaterialshavebeenexploredasstoragemediaforRRAMsoverthepastdecades.Inthiswork,thestoragemediaare

cuu duong than cong com

categorizedintoinorganicandorganic storagemedia.Generally

speaking,inorganicstoragemediahavearemarkableadvantage

overorganiconesinswitchingstability,whileorganiconesstand

outintermsofhigh-mechanicalflexibility,simplefabrication,and

lowcost.Theswitchingcharacteristicsandfabricationmethodsof

commonmaterialsbelongingtoeachcategoryarelistedindetail

anddiscussedindepthbelow.Itshouldbementionedherethat

therearealsoseveral otherclassificationmethods.Forexample,

basedondimensionality,storagemediacanbegroupedinto

zero-dimensional(0-D)nanoparticlessuchasNiOnanoparticles[47],

one-dimensional(1-D)nanowiressuchasZnOnanowires[48]and

two-dimensional(2-D)thinfilmssuchasAlNfilms[49].On the

otherhand,basedontheswitchingpolarity,storagemediacanbe

sortedintostoragemediashowingunipolarswitchingsuchasNiO

[47]andstorage mediashowingbipolarswitchingsuchasa-Si

[50]

2.1.1 Inorganicstoragemedia

InorganicstoragemediaforRRAMscanbelooselygroupedinto

binaryoxides(e.g.,SiOx[4],TiOx[17],NiOx[47],TaOx[3]andHfOx

[6]), ternary and more complex oxides (e.g., SrTiO3 [51],

La0.7Sr0.3MnO3 [52] and BiFeO3 [53]), chalcogenides (e.g., Ag2S

[54]and GexSey[55]), nitrides(e.g.,AlN[49]and SiN[56])and

others(includinga-C[57],a-Si[50],etc.).AdetailedsummaryofthecommoninorganicstorageaccompaniedbythecorrespondingswitchingcharacteristicsisprovidedinTable1.Onecanseethatbinaryoxidesarethemostabundantandshowthebestswitchingcharacteristicsincludingultra-highON/OFFratioof>109inGeOxand TaOx, sub-ns operation speed in SiOx, HfOx and TaOx, andextreme endurance of >1012 cycles in TaOx In addition, theirsimplecompositionenablesthemtobeeasilyfabricatedandtheirthermal stability is also satisfactory Hence, binary oxides,especially AlOx, SiOx, TiOx, CoOx, NiOx, CuOx, ZnOx, ZrOx, HfOx,TaOx and WOx, have been thefocus of both the academicandindustrial communities over the past decade Among theseextensively studiedbinary oxides,CuOxand WOxare themostcompatiblewiththeconventionalCMOSprocessbecausetheycan

befabricatedbyasimpleadditionaloxidationstepoftheCuorWvia/plug [80,134] To understand the carrier type-dependentswitchingkinetics,muchattentionhasbeenpaidbytheacademiccommunitytoNiOxandTiOx,whicharetypicalp-typeandn-typesemiconductors,respectively[135,136].Recently,researchinter-estinHfOxandTaOxhasbeenextremelyhighsincetheybothresult

insub-nsoperationspeedandextremeenduranceof>1010cyclesandconsequentlymaybethemostpromisingstoragemediaforRRAMsinthenearfuture

The inorganic storage media can be fabricated by various

methods, mainly including magnetron sputtering [137,138],

pulsed laser deposition (PLD) [116], atomic layer deposition

(ALD) [139], thermal oxidation [80,134], plasma oxidation

[140,141]andthesol–gel method[91].Magnetronsputteringis

a high-rate, high-efficiency film deposition technology and is

becomingincreasinglypopularbecauseofitshigh-yieldand

low-cost production of uniform films over large areas Magnetron

sputteringcanbeutilizedtodepositmostoftheinorganicstorage

media,includingbinaryand ternaryoxides(e.g.,TiOx[66],NiOx

[136],ZnOx[138],TaOx[137]andLa0.7Sr0.3MnO3 [142]),

chalco-genides(e.g.,GeSxandGexSey[143]),nitrides(e.g.,AlN[49]andSiN

[56]), a-Si [18], etc PLD is an optimal deposition method for

complexmaterialsowingtoitsstoichiometrictransferofmaterials

fromthetarget Accordingly,PLDisoftenusedtogrowstorage

mediasuch as ternaryperovskite oxides and multi-component

materials (e.g., SrTiO3 [114], BaTiO3 [116] and Pr0.7Ca0.3MnO3

[119]) However, PLD is not widely used in thesemiconductor

industrydue toitshigh-costand thesmalluniform area ofthe

depositedfilm.OxidefilmspreparedbyALDhavebeenextensively

studiedas gate oxides in the metal-oxide-semiconductor

field-effect-transistorstructure and as dielectricmediain theDRAM

stack capacitors [144] Recently, ALD is attracting increasing

attentionforthedepositionofstoragemediaforRRAMsduetoits

superiorabilitytopreciselycontrolthethicknessanduniformityof

thefilms[66,145,146].Althoughthedepositionofa fewoxides

such as TiO2 and Al2O3 by ALD is mature, the reproducible

depositionofotheroxidesbyALDisstillindevelopment [144]

Thermal oxidation and plasma oxidation are often utilized to

prepare binary oxides, such as CuOx and WOx, which can be

obtainedbyasimpleadditionaloxidationoftheCMOScompatible

Cu/W plugs [80,134] The sol–gel methodis a chemical liquid

deposition methodwhichrequires considerablyless equipment

andispotentiallylessexpensive[147].Itisusuallyusedtodeposit

oxidestoragemediasuchasTiO2[148]andZnO[149].However,

thesol–gelprocessisoftentime-consumingwhenpreciseaging

anddryingarerequired.Moreover,theproblemsofshrinkageand

stresscrackingthatoccurduringthedryingofthefilmsprepared

bythesol–gelmethoddorequirecarefulattention.Basedonthe

abovediscussion,itcanbeobservedthatmagnetronsputteringisthemostextensivelyused filmdeposition forinorganicstoragemedia, while ALD may bethemost promising onein thenearfuture

2.1.2 OrganicstoragemediaExcept for graphene oxide and its derivatives [150–153],organicstoragemediaforRRAMscanbesimplysortedintosmallmolecules,suchas2-amino-4,5-imidazoledicarbonitrile(AIDCN)

157],andpolymers,suchaspoly(3-hexylthiophene)(P3HT)[158–160],poly(methylmethacrylate)(PMMA)[161]andpoly(N-vinyl-carbazole)(PVK)[162,163].Itshouldbenotedherethat,inthiswork,thecompositestoragemediaareclassifiedbasedontheirmatrixes.Smallmoleculeshavelowmolecularweightandcanbedirectlydepositedunderhighvacuumwithoutdecompositionbythermalevaporation,andthiscaneasilyleadtouniformfilmwithalargearea[15,156].Incontrast,polymersconsistofmuchlargermoleculeswithlongchainsofrepeatingmonomerunitsandwilldecomposeduringthermalevaporation.Hencesolutionprocesses,forexample,spin-coating,areoftenadoptedtofabricatepolymerstoragemedia[164,165]

AdetailedsummaryofthecommonorganicstoragemediaandcorrespondingRScharacteristicsisprovidedinTable2.Thetablereveals that satisfactory switching characteristics have alreadybeendemonstrated,suchasultra-highON/OFFratioof>5109inPVK, fast operation speed of <5ns in graphene oxide-relatedmaterialsandhighenduranceperformanceof>7.2107inRoseBengal Although it must be admitted that most switchingcharacteristicsoforganicstoragemediaarestillnotcomparable

to thoseof inorganicstorage media, organic storage mediaareattractingmoreandmoreattentionduetotheireaseoffabrication,low costand, especially,high-mechanicalflexibilitythat enablethemtobeextremelypromisingforfutureflexibleelectronics.2.2 Electrodematerials

Electrodesinconventionalelectronicdevicesactprimarilyastransport pathsfor carriers, whereas in RRAMs theycan often

cuu duong than cong com

example, stable bipolar RS behavior was observed in a Cu/

P3HT:PCBM/indium-tin-oxide(ITO)structure,butitdisappeared

aftertheCuelectrodewasreplacedwithaPtelectrode[21].To

date, a great number of materials have been explored as

electrodesforRRAMs,includingpuremetalssuchasCu,Ag,Pt

andAu[196],carbonmaterialssuchasgraphene[47]andcarbon

nanotubes[197],conductiveoxidessuchasITO[21]andnitrides

suchasTiN[140],p-andn-typeSi[198],andsoon.Inthispaper,

twomethodsareusedtoclassifytheelectrodematerialsforthe

RRAMs

On the one hand, the common electrode materials can be

grouped into five categories based on composition, including

elementarysubstance electrodes,alloy electrodes,silicon-based

electrodes,nitride-basedelectrodesandoxide-basedelectrodes

Theelementarysubstanceelectrodesarethemostabundantand

arewidelyused,includinggraphene[47],carbonnanotubes[197],

Al[15],Ti[199],Cr[200],Co[201],Ni[202],Cu[21],Zr[201],Nb

[22],Ru[139],Pd[203],Ag[42],Hf[201],Ta[41],W[137],Ir[204],

Pt[49],Au[205],andsoon.AlloyelectrodesmainlyincludeCu–Ti

intentionallydesigned to stabilize RS behavior For instance, a

Cu/TiO2/Ptcellexhibitedonly400endurancecycleswithasmall

initialON/OFFratioof50,whileaCu0.36Ti0.64/TiO2/Ptcellshowed

improvedenduranceperformanceofover1000cycleswithalarge

initialON/OFFratioof200[206].Forsilicon-basedelectrodes,

p-Siandn-Siaretheonlytwotypes[198].TiNandTaNarethemost

common nitride-based electrodes [140,141] The oxide-based

electrodesarerelativelyabundant,includingITO[130],F-doped

SnO2(FTO)[208],Al-dopedZnO(AZO)[209],Ga-dopedZnO(GZO)

[210],SrRuO3[211],Nb-SrTiO3[212],LaNiO3[213],YBa2Cu3O7x

[214],andsoon.Inparticular,ITO,FTO,AZOandGZOhavebeen

intensivelystudied as electrodes for future transparent RRAMs

[130,208–210]

Conversely,thecommonelectrodematerialscanbesortedinto

fourtypesbasedontheircontributiontotheRSbehavior.First,the

electrodesactprimarilyasatransportpathforcarriersandhave

almost no effect on theRS behavior Such electrode materials

mainlyincludeinertmetalssuchasPd[203],Ir[204]andPt[49]

Second,theelectrodesarehelpfulfortheformationofCFs,andthis

isusuallyobservedinanionmigration-basedRRAMs[199,201].In

theseRRAMs,theCFsareformedviamigrationandaccumulation

of anion vacancies whose concentrations can be significantly

affectedbytheelectrodes.Asaresult,aproperchoiceofelectrode

materialforagivenstoragemediumcanleadtothegenerationof

an appropriate concentration of anion vacancies, thus being

helpful for the formation of CFs and the stability of the RS

behavior.For example,Chenetal.[201]recentlyfoundthat the

Ta2O5-basedRRAMswithrelativelychemicallyactiveelectrodes

(AlandTi)showsmallervariationsinswitchingparametersthan

those with chemicallyinert electrodes (Ni and Co)or highly

chemicallyactiveelectrodes(HfandZr).Third,theelectrodesare

responsiblefortheformationofCFs,whichisexactlythecasein

cationmigration-basedRRAMs[21,215].IntheseRRAMs,theCFs

areformedviaelectrochemicaldissolutionanddepositionofthe

electrochemicallyactivemetalelectrodesincludingmainlyCu

andAg.Forinstance,thedissolutionoftheAgelectrodeandthe

subsequentgrowthofAgfilamentshavebeenunambiguously

confirmedinaplanarAg/PEDOT:PSS/Ptstructure[215].Finally,

theelectrodematerialsaredeliberatelychosenforsomespecial

purposes,suchas theITO fortransparentelectrodeinanITO/

AlN/ITOstructure[127],theITO/Ag/ITOmultilayeredstructure

fortransparentaswellasflexibleelectrodeinanITO/ZnO/ITO/

Ag/ITOstructure[216],andthen-Sielectrodeforthe

construc-tionofRRAMswithself-rectificationinaCu/SiOx/n-Sistructure

[198]

3 SwitchingmechanismsThe exact switching mechanism in a sandwich structuredependsnotonlyonthechoiceofelectrodeandstoragematerialsbutalsoontheadoptedoperationmode.Consequently,therearemanypossibleswitchingmechanismsmainlyduetothediversity

of electrode and storage materials In this work, the commonswitchingmechanismsareclassifiedintofivetypesincludingionmigration,chargetrapping/de-trapping,thermochemicalreaction,exclusivemechanismsininorganicsandexclusivemechanismsinorganics, each of which will be discussed separately in thefollowingsections

3.1 Ionmigration

In an ion migration-based RRAM cell, a forming process isusuallyrequiredtotriggerstableRSbehavior,duringwhichCFsform and short-circuit the memory cell Subsequently, localruptureandre-formationoftheCFs occurduringtheresetandsetprocesses,respectively,leadingtothealternationbetweentheHRSandtheLRS.Basedonthepolaritiesofthecharges,therearetwo types of ions— cations and anions — in nature,and theymigrate in opposite directions under an external electric field.Accordingly, ion migration is sorted into cation and anionmigration, andthedetailed formationand ruptureprocesses oftheCFsbasedoneachofthesearediscussedseparatelyasfollows.3.1.1 Cationmigration

In a cation migration-based RRAM cell, there usually is anelectrochemicallyactiveelectrode(AE),suchasAgorCu,andanelectrochemicallyinertcounterelectrode(CE),suchasPt,AuorW

[217].TheCFsareformedviaelectrochemicaldissolutionandthenredepositionoftheactivemetalatoms.Therefore,suchRRAMsareoftencalledelectrochemicalmetallization(ECM)memories,andare also referred to as conductive bridging RAMs (CBRAMs),programmablemetallizationcells(PMCs)orgapless-typeatomicswitchesinsomeliteratures[218–220].WhenAE,usingAgasanexample,ispositivelybiasedduringtheformingandsetprocesses,thefollowingstepsareinvolved:(i)anodicdissolutionoftheAgelectrodeinaccordancewithAg!Ag++e;(ii)migrationofAg+

ions along fast diffusion channels (e.g., grain boundaries inpolycrystallinefilmsandsurfacesofnanowires[48])towardthe

CEdrivenbyexternalelectricfield;(iii)reductionofAg+ionsinaccordance with Ag++e!Ag, and (iv) growth of the Agfilaments.OncetheAgfilamentsshort-circuittheAEandCE,theECM cell will have been switched from the HRS to the LRS.Subsequently,under negativevoltage,theexisting AgfilamentswillbeelectrochemicallydissolvedwiththehelpofJouleheating

attheirthinnestparts,therebyresettingtheECMcellbacktotheHRS

TheinitialexplorationofECMcellsdatesbackto1976.HiroseandHirose[221]reportedbipolarRSbehaviorintheAg/Ag–As2S3/

Mo sandwichstructure To clarify theswitching mechanism, aplanarAg/Ag–As2S3/Austructurewasfabricatedatthesametime,andthegrowthofAgfilamentsfromtheCE(Auelectrode)totheAE(Agelectrode)wasconfirmedunambiguouslybytheuseofopticalmicroscopy.Todate,avarietyofmaterialshavebeenexploredforECM memories On the one hand, in addition to conventionalchalcogenideelectrolytessuchasAg2S[54]andGexSey[55],agreatnumber of other storage media have been explored, includingoxidessuchasSiO2[204]andZnO[83],halidessuchasAgI[133],amorphoussilicon[50]andcarbon[57],smallorganicmoleculessuchasfluorene[222],polymerssuchasP3HT[159],andsoon.Ontheotherhand,avarietyofmetalshavebeenreportedtoserveaselectrochemicallyactiveelectrodes,includingAg[42],Cu[21],Ni

to the following two reasons First, their standard electrode

potentials,E0(Ag+/Ag)=0.7993VandE0(Cu2+/Cu)=0.339V,are

nottoolargewhencomparedtoAuwithE0(Au+/Au)=1.69V,thus

enabling them to be easily electrochemically dissolved [226]

Second,theirstandardGibbsfreeenergyofformationofoxides,

DfG8(Ag2O)=11.21kJ/mol and DfG8(CuO)=129.7kJ/mol, are

muchsmallerthan those forNi (DfG8(NiO)=211.7kJ/mol),Al

(DfG8(1 / 3Al2O3)=527.4kJ/mol),and so on [227] Consequently,

thegenerated Ag+ and Cu2+ only showweak interactions with

anionsinthestoragemediaandcanmigrateeasilythroughthem

Although there is no doubt about the existence of metal

filamentsincationmigration-basedRRAMs,thedirectobservation

ofthesemetalfilamentsand,especially,theirdynamicgrowthand

rupture processes has aroused great interest in academia and

application-oriented societies In the beginning, only ex situ

observationsof metal filamentsin planarmicroscale ECM cells

werereported.In recent years,however,thankstoadvancesin

fabricationandcharacterizationofnanomaterials,bothexsituand

insituobservationsofmetalfilamentsinverticalnanoscaleECM

cellshavebeenextensivelyexplored.Adetailedsummaryofthe

directobservationsofmetalfilaments,inanascendingorderofthe

yearofpublication,ispresentedinTable3.Itcanbeobservedthat

the focus of the research has been moved from conventional

chalcogenideelectrolytestooxidesandorganics.Forthegrowth

modes of metal filaments, three different modes have been

reported,and eachof themisdiscussed separatelyinfollowing

paragraphs

3.1.1.1 Metal filaments grow from the CE to the AE The direct

observationofsuchagrowthmodeofAgfilamentsinaplanarAg/

Ag–As2S3/Austructurewasreportedduringtheinitialexploration

ofECMcellsin1976[221].Thesamegrowthmodehadalsobeen

confirmedunambiguouslyinmanyotherECMcellsbeforetheyear

2012,includinginAg/Ag–Ge–Se/Ni(orAl,W)[228,229],Cu/Ta2O5/

Pt [99], Ag (Cu)/H2O/Pt (Cu) [230,237], Ag/Ag2S/W [231,237],

Cu/Cu–GeTe/Pt–Ir [23]and Pt–Ir/Cu–GeS/Pt–Ir [236]ECM cells.Notethat,exceptforTa2O5,allofthereportedstoragemediaareeitherconventionalsolidelectrolytesorH2O,whichpossesshighcationmobility.Inthissituation,thecationsoriginatingfromtheanodicdissolutionoftheAEcaneasilymigratethroughthestoragemediaandthenbereducedintoatomsatthesurfaceoftheCE,thusleadingtothegrowthofmetalfilamentsfromtheCEtotheAE

Fig.3presentsinsituscanningTEM(STEM)resultsthatshowthedynamicgrowthandruptureprocessesof CufilamentsinaverticalCu/Cu–GeTe/Pt–Irstructure[23].Theexperimentalset-up

isschematicallydepictedinFig.3a.Anexternalvoltageisapplied

tothePt–Irtip(CE)withtheCuelectrode(AE)beinggroundedallthetime.StartingfromtheinitialHRS,anegativevoltageofupto

0.8Vwith0.1Vstepswasapplied,andthenapositivevoltage

of+0.4Vwasapplied(Fig.3b).Cross-sectionalZ-contrast(Z:theatomicnumber)STEMimagesobtainedaftervoltageapplications

of0,0.4,0.8,and0.4VareshowninFig.3c–f,respectively.ToprovideaclearerpictureoftheCufilaments,therawSTEMimages

inFig.3c–fwereconvertedintoblack-and-whiteimagesinFig.3g–

j,respectively.BycarefullyexaminingFig.3g–i,onecanconcludeunambiguouslythatmultipleCufilamentsgrowfromtheCEtothe

AEduringthesetprocesswiththeirthinnestpartsneartheAE.Afterthevoltageapplicationof0.4V,theECMcellwasresetintotheHRS(Fig.3b)duetotheruptureoftheexistingCufilamentsattheirthinnestparts(Fig.3j).BecausethegrowthmodefromtheCE

totheAEisconsistentwellwiththeclassicallawsofistry,andnoothergrowthmodehadbeenreportedbefore2012,suchagrowthmodewasonceconsideredtobeapplicabletoallECMcells

electrochem-3.1.1.2 Metal filaments grow from the AE to the CE The widelyacceptedgrowthmodefromtheCEtotheAEhasbeenchallengedsince 2012 First, Peng et al [241]systematically analyzed theasymmetryoftheI–VcurvesintheHRSofCu/ZnO/PtandCu/ZnO/AZOECMcells,andinferredthattheCufilamentismostlikelytohaveaconicalshapewithitswiderandnarrowerdiametersnear

cuu duong than cong com

growthofCufilamentwassuggestedtostartattheAEandstopat

theCEduetothefactthatthediffusioncoefficientofCuionsinZnO

is smaller than that in conventional solid electrolytes such as

consulfidesandselenides.Soonafterthat,insituTEMexperiments

wereconductedinAg(orCu,Ni)/ZrO2/Pt[24,202],Ag/a-Si/W[25]

andAu/ZnO/Au[205]ECMcells.Themetalfilamentsinallofthese

experimentswereunambiguouslyobservedtogrowfromtheAEto

theCE.Inaddition,thesamegrowthmodehasalsobeenproved

indirectlyinCu/P3HT:PCBM/ITO(orCu)[21,239]andAg–Pd/PZT/

Ag–Pd[242]structures

Here,aninsituTEMstudyofaAg/a-Si/WECMcell[25]isused

asanexampletodescribethegrowthmodeofmetalfilaments

fromtheAEtotheCE.Theexperimentalset-upisshowninFig.4a

Aconformala-SifilmwasfirstdepositedonaWprobevia

plasma-enhanced chemical vapor deposition Then the W probe was

mountedonthemoveableendofasingle-tiltTEMholder.Toform

theAg/a-Si/WECMcell,theWprobewasmovedtoachievecontact

withahigh-purityAgwirefixedtotheotherendoftheTEMholder

The ECM cell underwent a forming process under a constant

externalvoltageof12VappliedtotheAgelectrodewithrespectto

theWelectrode,andthecorrespondingI–tcurveisdisplayedin

Fig.4b.OnecanclearlyseethattheECMcelltransformedfromthe

HRStotheLRSat490s.Fig.4c–garerepresentativeTEMimages

oftheECMcellatdifferentswitchingstagesthataredenotedby

datapointsc–ginFig.4b.Basedonthesefigures,itisevidentthat

AgfilamentsgrowfromtheAE(Ag)totheCE(W),andtheyfeature

aconicalshapewiththebroadestbaseneartheAE.Theseimagesalsorevealthatthebase,growingfromtheAE(Fig.4d),servesasareservoiroftheAgionsforsubsequentgrowthoftheAgfilaments,anddiscreteAgparticlesarecreatedsubsequentlyasthefilamentsextendtowardtheCE(Fig.4e–g).Inaddition,insituTEMstudyofthe reset process of the Ag/a-Si/W ECM cell revealed that theexisting Ag filaments rupture initially at their thinnest partslocatedneartheCE andthefilamentmaterialisredepositedontheAE

BecausetheresetprocessisinducedbyJouleheating-assistedelectrochemicaldissolutionoftheexistingmetalfilaments,there

isnodoubtthatallofthemetalfilaments,irrespectiveoftheirgrowthmodes,ruptureinitiallyattheirthinnestpartsduetothelargestJoule heating occurringat those locations.The growthmodefromtheAE totheCEis notexpectedfromtheexistingECMtheorydeveloped forelectrolyte-basedmemory cellsandcannotbe explainedby theclassicallaws ofelectrochemistry.However,theseinsituTEMexperimentalresultsareinperfectaccordancewithPeng’sinferenceandserveasstrongevidenceforthesuggestionthatthegrowthmodeofthemetalfilaments

is closely related tomobility of cationsin thestorage media.Suchagrowthmodecanbeexplainedwellfromtheviewpoint

ofkinetics.Givenalowmobilityina-Siandinmostoxides,the

8

cuu duong than cong com

cationsthatoriginatefrom anodicdissolution oftheAEcan

onlymigrateanextremelyshortdistancebeforebeingreduced

by the oncoming electrons The precipitated atoms in the

vicinityoftheAEsharealmostthesameelectrostaticpotential

as the AE and therefore act as an extension of the AE

Subsequently, the extended AE will be dissolved again and

reducednearby,thusresultinginthegrowthofmetalfilaments

fromtheAEtotheCE

3.1.1.3 Metalfilamentsnucleateinitiallynearthemiddleregionofthememorycellandthenextendtowardbothelectrodes GiventhatbothgrowthmodesofmetalfilamentsfromtheCEtotheAEandfromtheAEtotheCEexist,onemayaskwhethermetalfilamentscannucleateinitiallynearthemiddleregionofthedeviceand thenextend toward both electrodes.From an experimental point ofview,suchagrowthmodewouldactasaperfectcomplementtothe growth kinetics of metal filaments Interestingly, direct

cuu duong than cong com

evidencefortheexistenceofsucha growthmode wasrecently

reportedbyourgroupinaplanarAg/PEDOT:PSS/PtECMcell[215],

asdemonstratedinFig.5.Aconstantvoltageof1Vwasappliedto

theAgelectrodewithrespecttothePtelectrodeinfourpristine

cellswithaperiodof2,4,10,and 130s.ThecorrespondingI–t

curvesforeachperiodaredisplayedinFig.5a–d.Itcanbeobserved

thatthedevicewithalongerdurationofvoltagestressmaintainsa

lowerresistanceduetotheformationofstrongerAgfilaments.In

particular,asuddenjumpinthecurrentappearsat127sinFig.3d,

indicating that thedevice hasbeenset into theLRS Fig 5e–h

present SEM images that were obtained after the I–t tests in

Fig.5a–d,respectively.OnecanseefromFig.5ethatAgclusters

form initially near the middle region of the PEDOT:PSS With

longervoltagestresstime,moreAgclustersemergeandsmallAg

clusters start to aggregate (Fig 5f and g) Consequently, the

chainsofAgclusters(Agfilaments)becomestrongerandextend

towardbothelectrodesatthesametime.Finally,thesetprocess

occursoncetheAgfilamentsarestrongenoughtoshort-circuit

theECM cell,as shown inFig.5 andh Thisspecialgrowth

modeofAg filamentsonceagainchallengestheexistingECM

theorythatwasdevelopedforelectrolyte-basedmemorycells

andfurthersupportsthesuggestion thatthegrowthmode of

metal filaments is closely related to cation mobility in the

storagemedia

Toprovideauniversalmodelforallgrowthmodesofthemetal

filaments, we have conducted an in-depth analysis of the

relationshipsbetweenthewaittime(tw)andtheelectricfieldin

Ag/a-Si/Pt [25], Ag/PEDOT:PSS/Pt [215] and Ag/SiO2/Pt [25]

structures.Notethattwisdefinedasthedurationbeforeasharp

resistance-switchingeventwhenafreshdeviceisunderaconstant

voltage.Inotherwords, twreferstothetimeneededtoforma

completemetalfilamentinafreshdeviceunderaconstantvoltage

Hence,foragiventw,alargerelectricfieldcorrespondstoalower

mobilityofAg+ionsinthestoragemedium.Itisfoundthat,when

tw10s,thecorrespondingelectricfieldvaluesinAg/a-Si/Pt,Ag/

PEDOT:PSS/Pt and Ag/SiO2/Pt structures are 1.1, 0.011 and

0.22MV/cm,respectively,thusrevealingthatthemobilityofAg+

ionsinSiO2islowerthanthatinPEDOT:PSSbuthigherthanthatin

a-Si However, if the growth mode of metal filaments is only

determinedbycationmobilityinthestoragemedia,themobilityof

Ag+ ions in SiO2 should be higher than that in both a-Si and

PEDOT:PSS based on the fact that the Ag filaments initially

nucleateneartheAgelectrode,nearthemiddleregionofthedevice

andnearthePtelectrodeinAg/a-Si/Pt,Ag/PEDOT:PSS/PtandAg/

SiO2/Ptstructures,respectively.Thisobviouscontradiction

indi-catesthatthecationmobilityisnotthesolefactorthataffectsthe

growthmodeofmetalfilaments

Althoughithasbeenacknowledgedthatthegrowthkineticsof

CFsismainlyaffectedby(i)thetransportofcationsthroughthe

storagemediaand(ii)thereductionofcationsandthesubsequent

nucleationofmetalfilaments,theeffectoftheformerfactoronthe

growthmodeofmetalfilamentsisstillnotcompletelyclearand

thatofthelatterfactoris oftenoverlooked.Here,weconducta

detailed2-DsimulationbasedonthekineticMonteCarlo(KMC)

method[243,244]toshedlightontheeffectsofthesefactorsonthe

growthmodeofmetalfilaments.Notethatthefeasibilityofusing

theKMCmethodtosimulatethegrowthofmetalfilamentshas

beendemonstratedin[237].Thedeviceconfigurationadoptedis

showninFig.6a.TheAEandtheCEareconsideredtobeAgandPt,

respectively The storage medium is represented by a square

matrixoforder20,andaninterfaciallayerrepresentedbyamatrix

with20 rowsand1 columnis locatedbetween theAEand the

storagemedium.Eachelementofthematrixesshouldberegarded

asasiteatwhichaAg+ionoraAgatomcanlocate.Thedistance

betweenthecentersoftwoadjacentsitesisassumedtobe0.4nm

Hence,underanexternalelectricfieldof1MV/cm(0.1V/nm),the

voltagedropbetweenthecentersoftwoadjacentsitesalongthefieldiscalculatedtobe0.04V

Fig.6aalsoshowsthephysicalandchemicalprocessesthatareinvolvedinthesimulation,includingthegeneration(process1),migration(processes2–5)andreduction(process6)ofAg+ionsandtheoxidationofAgatoms(process7).ThetransitionrateofeachprocesscanbeexpressedbyGi=n0exp(Ea,i/(kT)),wheren0

isafrequencyfactorthatisassumedtobeconstantforalloftheprocesses, Ea,i is the activation energy for process i,

k=1.381023J/K is the Boltzmann constant, and T=300K isthe absolute temperature Based on the basic theory of KMCmethod, the exact value of n0 here will not affect the finalconfigurationofAg+ionsandAgatoms.Therefore,then0inthissimulationistakentobe1toacceleratethesimulationspeed,thusleadingtoGi=exp(Ea,i/(kT)).Forsimplicity,theshapeoftheAgelectrode is assumed to be unchanged through the entiresimulation process, and the ionization of the Ag electrode toproduceAg+ionsisreplacedbythegenerationofAg+ionswith

Ea,1=0.35eVatthenullsitesintheinterfaciallayer.EachAg+ion,

10

cuu duong than cong com

sitesaroundit.Duetothepresenceoftheexternalelectricfield,

Ea,3=Ea,4=Ea,l=Ea,20.02eV=Ea,5+0.02eV,whereEa,lrefersto

theactivationenergyforAg+ionmigrationperpendiculartothe

electricfield.Redoxreactions,includingthereductionofAg+ions

and oxidation of Ag atoms, are assumed to be able to occur

anywhereinthestoragemedium.Thisassumptionisreasonable

basedonpreviousstudies[24,25,205,215].Theactionenergyforthe

reductionofaAg+ion(Ea,6)ortheoxidationofaAgatom(Ea,7)is

closelyrelatedtothenumber(n)ofAgatomssurroundingit.Here,

basedon [237], weconsider thatEa,6(n=0)=Ea,7(n=0)0.2eV,

Ea,6(n)=Ea,6(n+1)+0.05eV and Ea,7(n)=Ea,7(n+1)0.05eV In

addition,it should bementioned herethat:(i)the migrationof

Agatomsisignoredbecause,underanexternalelectricfield,the

migrationofatomsismuchslowerthanthatofions;(ii)theinjection

ofelectronstomaintainelectricneutralityandtheirmigrationare

alsoignoredforsimplicity

Initially, each site of the matrixes is null.The simulation is

performedbasedontheflowchartpresentedinFig.6b.Basedon

theflowchart,weshouldfirstdeterminethevalueofEa,i.Infact,we

onlyneedtodetermineEa,landEa,6(n=0)becauseEa,1isassumed

tobe0.35eVduringtheentiresimulationprocessandallofthe

otherscancalculatedusingtheformulasinthepreviousparagraph

Then,weneedtocalculatethesum(Gsum)ofthetransitionratesofallprocessesthatmayoccurinthenextstep.Ifallofthesitesaremarked oneby one by a number j(j=1,2,3, ), Gsum can beexpressed by Gsum¼SjSiGij, where Gij is the meaningfultransitionrateofprocessiatsitej.NotethatGijiscloselyrelated

tothecurrentstateofsitej.Forexample,ifaAg+ionislocatedatsitej,itmaymigrateorbereducedinthenextstep.Inthiscase,onlyGijwithi=2,3,4,5,and6maybemeaningful,whereasGij

withi=1 and7is meaninglessandshouldbeassignedtobe0.Third,weneedtorandomlychooseaprocesstooccurduringthenext step according to Sl10 Gk<RNGsumSl0Gk, where Gk

equalstoGijwhenk=7(j1)+i,lisanaturalnumber,andRNisarandom number between 0 and 1 generated by thecomputer.Fourth,weupdatetheconfigurationofAg+ionsandAgatomsandcalculatethetotalnumber(N)ofAgatoms.OnceNincreasesto10,thesimulationwillendandoutputtheconfigurationsofAg+ionsandAgatomswhenN=1,4,7and10.TheAgatomshereshouldbeequatedwiththenucleiofAgfilaments.Thereasonwhywedonotsimulate the growth of complete Ag filaments is because theassumptionofauniformelectricfieldisapplicableonlywhenNismuchsmallerthanthetotalnumberofallofthesites,i.e.,420.Giventhateachgrowthmodeofmetalfilamentsischaracterized

by a uniqueinitialnucleationlocation,theeffects oftheabove

cuu duong than cong com

be unambiguously revealed by their effects on the initial

nucleationlocationofmetalfilaments

Notethatthetransportofcationsthroughthestoragemediais

relatedtoEa,l,whereasthereductionofcationsandthesubsequent

nucleationofmetalfilamentsarerelatedtoEa,6(n=0).Wefirstly

conductedsimulationswithvariousEa,lvaluesbutwithaconstant

Ea,6(n=0)of0.40eV.ForeachEa,l,thesimulationwasrepeated

manytimesandshowedgoodrepeatability.Fig.7a–cshowstypical

evolutionprocessesoftheconfigurationsofAg+ionsandAgatoms

withEa,lof0.25,0.30and0.35eV,respectively.Notethatasmaller

Ea,lcorrespondstoahighermobilityofAg+ions.In thecaseof

Ea,l=0.25eV, the generated Ag+ ions rapidly pass through the

storagemediumduetohighmobilityandthenaggregateatthe

storagemedium/PtinterfacewheretheyarereducedintoAgatoms

(Fig.7a).Thiscorrespondstothegrowthmodeofmetalfilaments

fromtheCEtotheAE.However,whenEa,lisincreasedto0.30eV,

thereisnoobviousaggregationofthegeneratedAg+ionsduetothe

appropriatemobility,leadingtoarandomdistributionoftheinitial

Agatoms inthewholestoragemedium(Fig.7b).Thissituation

correspondsqualitativelytothegrowthmodeofmetalfilaments

withaninitialnucleationlocationnearthemiddleregionofthe

device.Finally,whenEa,listakentobe0.35eV,thegeneratedAg+

ionsaggregateinthevicinityof theAg electrodeowingtolow

mobility, and consequently, they are initially reduced into Ag

atomshere(Fig.7c).Thiscorrespondstothegrowthmodeofmetal

filaments from the AE to the CE Furthermore, the statistical

distributionofAgatomsasafunctionofpositionundervariousEa,l

valuesisshowninFig.8a.Thisclearlyrevealsthatunderaconstant

Ea,6(n=0),thegrowthmodeofAgfilamentsfromtheCEtotheAEgraduallychangesintothatfromtheAEtotheCEasthemobilityof

Ag+ionsdecreases

Subsequently, we conducted simulations with various

Ea,6(n=0)valuesbutconstantEa,lof0.30eV.Notethatasmaller

Ea,6(n=0)correspondsqualitativelytoalowernucleationenergy

ofAgfilaments.TheresultingstatisticaldistributionofAgatomsas

afunctionofpositionundervariousEa,6(n=0)valuesisshownin

Fig.8b.Fromthisfigure,itcanbeconcludedthatunderaconstant

Ea,l, the growth mode of Ag filaments from the CE to the AEgraduallychangesintothatfromtheAEtotheCEasthenucleationenergyofAgfilamentsdecreases

Basedon theabovesimulation results,weconcludethatthegrowthmode of metalfilamentsis affected byboth thecationmobility and the nucleation energy of metal filaments, asschematicallyshowninFig.9.Froma qualitativepointofview,this figureperfectly matches the above simulation results, i.e.,under a given cation mobility (nucleation energy of metalfilaments),thegrowthmode ofmetalfilamentsfromtheCE totheAEgraduallychangesintothatfromtheAEtotheCEasthenucleationenergyofmetalfilaments(cationmobility)decreases

In addition,basedon this figure,theobserveddifferentgrowthmodesofAgfilamentsinAg/a-Si/Pt[25],Ag/PEDOT:PSS/Pt[215]

and Ag/SiO2/Pt [25] structures can be reasonably explained asfollows.ThechangeofthegrowthmodefromtheCEtotheAEintheAg/SiO2/PtstructureintothatfromtheAEtotheCEintheAg/a-Si/PtstructureismainlyduetoadecreaseinmobilityofAg+ions.For the Ag/PEDOT:PSS/Pt structure, although it hasthehighestmobilityofAg+ions,itmostlikelyhasthelowestnucleationenergy

ofAgfilaments

12

cuu duong than cong com

3.1.2 Anionmigration

In anion migration-based RRAM cells, the common storage

mediaareoxidessuchasTiOx[245],NiOx[75],HfOx[199],TaOx

[137]andgrapheneoxide[161]andtherestarenitridessuchas

AlN[246]andNiN[247].Forsimplicity,themigrationofanionsis

generallydescribedbythemigration oftheirpositivelycharged

counterparts,i.e.,oxygenvacancies(VOs)inoxidesandnitrogen

vacancies(VNs)innitrides.DuringRScycles,themigrationofthese

positivelychargedvacancies,drivenmainlybytheexternalelectric

field,canresultinavalencechangeofthecationsinthestorage

media.Therefore,anionmigration-basedRRAMsareusuallycalled

valencechangememories(VCMs)intheliterature[9,248].Here,

theVCMsareroughlygroupedintofilament-andinterface-type

VCMsbased on the cellarea dependence of theLRS resistance

(RLRS)tosimplifythediscussion

3.1.2.1 Filament-typeVCMs Infilament-typeVCMs,theRS

behav-iororiginatesfromtheformationandruptureofCFsinthestorage

media,therebyexhibitingnoorveryweakdependencebetween

thecellareaandRLRS[9].Ithasalreadybeendemonstratedthatthe

exact formation and rupture processes of the CFs are closely

related tothecarriertype in semiconductorsand to theinitial

distributionofanionvacanciesininsulators,asdiscussedinthe

followingparagraphs

Switchingkineticsinsemiconductors.Themostcommon

semicon-ducting storage media are NiO1+x [75,249–251], CoO1+x [252],

TiO2x[245]andZnO1x(x>0)[249].Theformertwocasesare

p-typesemiconductorswithabundantcationvacancies,whilethe

latter two are n-type semiconductors with abundant anion

vacancies.Inthesesemiconductors,theconductioncharacteristics

oftheCFscanbeeithersemiconductingormetallic,basedonthe

exactIcompvalueduringthesetprocess.Ingeneral,smallandlarge

IcompvaluesresultinsemiconductingandmetallicCFs,

respective-ly.Forinstance,inaPt/NiOnanowire/Ptcell,thecurrentintheLRS

(obtainedwithaIcompof1010A)tendstodecreaseinareducing

atmosphereandtoincreaseinanoxidizingatmosphere,indicatingtheformationofp-typesemiconductingCFs[250].Incontrast,inaPt/NiO/Pt cell, theresistance in the LRS (obtained witha Icomp

of103A)increaseswithincreasingtemperature,implyingtheformationofmetallicCFs[253,254].Notethattheformationandrupture ofsemiconducting CFsare primarily dominatedby anionmigration,whereasthoseofmetallicCFsareprimarilydominatedbythermochemicalreactions.Here,thefocusisontheswitchingkinetics

ofRRAMswithsemiconductingCFs,whiletheswitchingkineticsofRRAMswithmetallicCFswillbediscussedinSection3.3.1

In 2010,Kinoshtia etal [249]conductedconducting atomicforcemicroscope(C-AFM)measurementsonaNiOfilm,whichisap-type semiconductor, and a GZO film, which is an n-typesemiconductor.During measurements,theRh-coatedSitip(Rh-Tip) was grounded, and a bias voltage (V) was appliedto thebottomelectrode(BE),asseeninFig.10candd.Forthep-typeNiOfilm,theHRSwaswrittenwithanegativebiasandtheLRSwithapositivebias(Fig.10a).However,forthen-typeGZOfilm,theHRSwaswrittenwithapositivebiasandtheLRSwithanegativebias(Fig.10b),whichisoppositetothatforthep-typeNiOfilm.Thisphenomenon can be explained based on the anion migration-inducedredoxreaction.Forp-typeNiO,anegativebiasontheBEwill lead tothemigration of O2ions away fromthe BE, thusresultingintheformationofastoichiometric(insulating)NiOneartheBEandaconsequentHRS(Fig.10c).Onthecontrary,apositivebiasontheBEisneededtocausetheformationofastoichiometric(insulating) GZO near the BE and a consequent HRS, asschematically shown in Fig.10d Thiswork suggeststhat withthesameactiveRSregion,theswitchingpolaritiesinp-typeandn-typesemiconductorsareoppositetoeachother.Inotherwords,ifp-type and n-type semiconductors show the same switchingpolarity,theactiveRSregionsin themwillbeopposite toeachother

Subsequently, to study the switching kinetics of migration-induced RS behaviors in semiconductors, Oka et al

cuu duong than cong com

NiOnanowire/Ptjunctions,asshowninFig.11a.Thebasictypeis

anuncoveredPt/NiOnanowire/PtjunctiondenotedasTypeIII.The

Pt/NiOnanowire/Ptjunctions passivated byanamorphous SiO2

layerattheanodeside,atthecathode sideandover theentire

regionaredenotedasTypeI,TypeIIandTypeIV,respectively.Note

thatthedefinitionofanodeandcathodedependsonthepolarityof

external voltage and consequently reverses in set and reset

processes of bipolar RS behavior To ensure a thorough and

coherentdiscussion,anodeandcathodeinthismanuscriptalways

refertotheelectrodesfromwhichthecurrentflowsintoandoutof

theRRAMdeviceduringforming(orset)process,respectively.The

pristineresistancevalues werealmostidenticalinalljunctions

Afteraformingprocess,alljunctionsexhibitedstablebipolarRS,

and typicalI–Vcurves of TypeI, TypeII,Type III, and TypeIV

junctions are shown in Fig 11b–e, respectively The most

remarkabledifference between Fig 11d and e is that thereset

voltagehasbeensignificantlyloweredsolelybythepresenceofthe

passivationlayer over the entire region When thepassivation

layer is only at the anode side, however, the RS is almost

unaffected,asshowninFig.11b.Therefore,itisnaturaltoconclude

thattheactiveRSregionislocatednearthecathode.Indeed,this

conclusionhasbeenconfirmedunambiguouslybythefactthatthe

resetvoltagehasalsobeensignificantlyloweredbythepresenceof

thepassivationlayeratthecathodeside(Fig.11c).Meanwhile,the

sameconclusionhasalsobeendrawnbyNagashimaetal.basedon

aPt/cobaltoxidenanowire/Ptstructure[252].Inaddition,given

thattheactiveRSregionsinp-typeandn-typesemiconductorswill

beoppositetoeachotheriftheyshowthesameswitchingpolarity,

itcanbededucedhereinthattheactiveRSregionshouldbelocated

neartheanodeinn-typesemiconductors

Basedontheaboveexperimentalresults,the

anion-migration-dominatedswitching kineticsin semiconductorscan be

under-stoodasfollows.Inap-typestoragemedium,theremustbesome

mobileoxygenionsthatarepresentnearcrystaldefectssuchas

VOsand grainboundaries, as schematically shown in Fig 12a

When thetop electrode (TE) is positivelybiased, thesemobile

oxygenionswillmigratetowardtheTEandthenaccumulateinits

vicinity,thuscreatingabundantcationvacanciestherein(Fig.12b)

Notethatcationvacanciesthatcancreateanacceptorlevelnear

the valence band are the source of hole carriers in p-type

semiconductors Hence, these newly created cation vacanciescan develop into the nuclei of p-type semiconducting CFs.Subsequently, these nuclei will act as extensions of the anodeandlargernucleiwillgrowpreferentiallybecausetheycanmoreeffectively concentrate the electric field Once a full p-typesemiconductingCF forms,thememorycellwill switchintotheLRS (Fig 12c) With the growth mode from the anode to thecathode,it isnatural thatthethinnestpartoftheformedCFislocatednearthecathode.WhentheTEisnegativelybiased,mostoftheJouleheatwillbegeneratedatthethinnestpartoftheCF,thussignificantlyacceleratingthemobilityofoxygenionsinthisregion.Drivenbytheelectricfield,mobileoxygenionsinthisregionwillrapidlymigratetowardtheBEandthenbestoredatthestoragemedium/BEinterface or thegrainboundariesof theBE Conse-quently,theconcentrationofcationvacanciesinthethinnestpart

ofthep-typesemiconductingCFwillbesignificantlydecreased,resultingintheruptureoftheCFatthatlocation(Fig.12d).Forann-typestoragemedium,themediumalreadyhasmany

VOs,asschematicallyshowninFig.12e.WhentheTEispositivelybiased,theseVOswillmigratetowardtheBEandthenaccumulate

initsvicinity(Fig.12f).BecauseVOsthatcancreateanacceptorlevelneartheconductionbandarethesourceofelectroncarriersinn-typesemiconductors,theseaccumulatedVOswilldevelopintothe nuclei of n-type semiconducting CFs Subsequently, thesenuclei will act as extensions of the cathode and grow towardtheanode.Onceacompleten-typesemiconductingCFforms,thememorycellwillswitchintotheLRS(Fig.12g).Withthegrowthmode from thecathode tothe anode,the thinnest partof theformed CF should be located near theanode When the TE isnegativelybiased,mostoftheJouleheatwillbegeneratedatthethinnestpartoftheCF,thussignificantlyacceleratingthemobility

ofVOsinthisregion.Drivenbytheelectricfield,VOsinthisregionwillrapidlymigratetowardtheTEandthenbeannihilatedbytheoxygenionsthatarestoredatthestoragemedium/TEinterfaceorthegrainboundariesoftheTEduringtheformingprocess.Asaresult,theconcentrationofVOsinthethinnestpartofthen-typesemiconductingCFwillbesignificantlydecreased,leadingtotheruptureoftheCFatthatlocation(Fig.12h)

Hereinitisnecessaryandmeaningfultoexplainwhytheactive

RSregionofacertaintypeofsemiconductorinFig.10isoppositetothatinFig.12.Thediscussionisfocusedonthep-typeNiOfilmand

14

cuu duong than cong com

NiO film in Fig 10a shows an intermediate state, indicating

theexistenceofaweakCF.SincethisweakCFisformedduringthe

fabrication process,a cylindrical shape can be expected.When

the BE is negatively biased withthe TE grounded, there is no

preferentialmigrationofoxygenionsalongthisweakCFbecauseof

itsuniformityindiameter.Therefore,themostseriousdepletionof

oxygenionsappearsnaturallyinthevicinityoftheBE(Fig.10c),

i.e.,theactiveRSregionlocatesneartheelectrodethatisnegatively

biased during the reset process In contrast, the formed CF in

Fig.12chasaconicalshapewithitswiderandnarrowerdiameters

near the TE and BE, respectively During the subsequent reset

process withthe TE negatively biased and the BE grounded, a

preferentialmigrationofoxygenionsappearsinthevicinityofthe

BE becausemost of the Jouleheat is generated in this region

(Fig.12d).Consequently,themostseriousdepletionofoxygenions

appearsnaturallyinthevicinityoftheBE,i.e.,theactiveRSregion

locates near the electrode that is grounded during the reset

process,whichisjustoppositetowhatshowninFig.10c

Switching kinetics in insulators The common insulators in

filament-typeVCMsareTa2O5[7,137,255–257],HfO2[96,97],ZrO2

[89,90], etc The switching kinetics in these insulators can be

significantlyaffectedbytheinitialdistributionofanionvacancies

inthem Here thefocusis on Ta2O5,and thediscussionis also

applicabletoalloftheotherinsulators.Notethatmanymetastable

suboxides such as TaO2 and TaO are usually inevitable in the

fabricatedTa2O5films,althoughTa2O5isthemoststabletantalumoxide Hence, the more exact expression of Ta2O5 should be

Ta2O5xorTaOx,asshowninmostpublishedpapers[7,137,255–257]

AnRRAMcellcomposedofaTa2O5xlayersandwichedbetweentwo inertelectrodes,suchasPtorW,showsan initialuniformdistributionofVOsintheTa2O5xlayer.Suchamemorycellcanexhibitbothbipolar[137]andunipolar[255]switchingbehaviors.Thegrowthkineticsof theCFs is independentof theswitchingpolarityandisidenticaltothatoftheanion-migration-dominatedbipolar switching in n-type semiconductors (Fig 12e–g) Incontrast,therupturekineticsoftheCFsiscloselyrelatedtotheswitchingpolarity.Indetail,theruptureoftheCFsduringbipolarswitchingis causedmainlyby theion-migration-inducedredoxreaction[137],whichisidenticaltothatshowninFig.12h,whiletheruptureoftheCFsduringunipolarswitchingiscausedmainly

bytheJouleheat-inducedthermalmeltingattheirthinnestparts

[255]

AninitialnonuniformdistributionofVOsinaTa2O5xstoragelayerisoftenobtainedbytheuseofatopmetalelectrodewithahighoxygen affinity (e.g.,Ta, Tior Al) [201]or by deliberatelyintroducingaVO-richconductinglayer(e.g.,TaO2x)betweenthetopelectrodeandtheTa2O5xlayer[7].Itisworthpointingouthere that such a nonuniform distribution of VOs is usuallybeneficial to the RS, including decreasing the forming voltage(Vform), improvingtheswitchingstabilityandendurance ability,

cuu duong than cong com

of VOs in a Pt/TaO2x/Ta2O5x/Pt cell The VO-rich conducting

TaO2xlayercanserveasareservoirofVOs,anditcanalsoactasa

seriesresistortoreplacetheIcompduringthesetprocess.Whenthe

TEispositivelybiased,therichVOsintheTaO2xlayerwillmigrate

towardtheBEanddevelopintoCFs(Fig.13b).OncetheCFsconnect

totheBE,thePt/TaO2x/Ta2O5x/PtcellwillswitchintotheLRS

(Fig.13c).Such a growthmode oftheCFs hasbeenverifiedby

simulations[256],andhasalsobeenrecentlyconfirmedbyPark

etal using in situ TEMin a Pt/TaO2x(30nm)/Ta2O5x(10nm)/

SiO2(1.5nm)/Ptstructure[257].TorupturetheCFs,abipolarreset

processisneeded,asschematicallyshowninFig.13d,becausea

positivevoltagewillfurtherenhancethestrengthoftheexisting

CFsbyprovidingmoreVOstotheCFs[258]

3.1.2.2 Interface-type VCMs.In interface-type VCMs, the

resis-tance in the LRS is inversely proportional to the cell area,

suggestingthattheentirecellareais involvedintheswitching

behavior[9,259].Aninterface-typeVCMcellisusuallycomposed

ofasemiconductorlayerthatissandwichedbetweenoneelectrode

withan Ohmiccontact anda secondelectrode witha Schottky

contact [9,259] Based on energy band theory, Ohmiccontacts

showing linear I–V behavior form between n-type (p-type)

semiconductorsandmetalswithlow(high)workfunction,while

SchottkycontactsshowingrectifyingI–Vbehaviorformbetween

n-type(p-type)semiconductorsandmetalswithhigh(low)work

function

Fig.14a andbshowsI–Vcurves ofM/Pr0.7Ca0.3MnO3/SrRuO3

(M/PCMO/SRO) and M/SrTi0.99Nb0.01O3/Ag (M/Nb:STO/Ag) cells,respectively.HeretheMrepresentsatopelectrodeofTi,Au,orSRO,withworkfunctionsof4.3,5.1and5.3eV,respectively.The SRO and Ag bottom electrodesare chosen toform OhmiccontactswiththePCMOand Nb:STO,respectively Onecanseeclearly that the contact resistance of the M/PCMO (p-type)interface decreases withthe workfunction ofM, whereasthat

of the M/Nb:STO (n-type) interface increases with the workfunctionofM.Moreimportantly,theI–VcurvesoftheTi/PCMO,Au/Nb:STO and SRO/Nb:STO interfaces show rectifying I–Vbehavior,thus revealingthepresence ofSchottky contacts.The

RSphenomenaonlyappearattheseinterfaces,suggestingacrucialroleofSchottkycontactsintheseinterface-typeVCMs.Further-more,theseexperimentsprovideinsightintothedirectionalityofsuchRSphenomena,meaningthattheset(reset)processoccurswhenaforward(reverse)biasvoltageisappliedtotheSchottkyinterface

NotethatthedirectionalityofsuchRSphenomenaisoppositetothat ofthefilament-typeVCMs.Here, thefocus inon theSRO/Nb:STO/Agstructure.Asdescribedpreviously,thegrowthofCFsinn-type semiconductors starts from the cathode due to theaccumulationoftheVOs.Inthiscase,theVOswillaccumulateattheSRO/Nb:STOinterfacewhenapositivevoltageisappliedtothe

Agelectrode.Asaresult,theSchottkybarrierwillbedegradedandtheresistanceoftheSRO/Nb:STO/Agstructurewilldecrease,whicharecontrarytoFig.14b.Tosolvethiscontradiction,Waseretal

[260]recentlysuggestedareasonablemodel.ThekeypointoftheirmodelisthatthereisneitherasupplyofVOsfrom,noranuptakeof

VOs by, the bottom electrode A negative voltage at the topelectrodewilldriveVOstotheupperinterface.BecausethebottomelectrodecannotcontributetoafurthersupplyofVOs,asufficientlyhighelectricfieldwillresultinaVO-depletedregionbeneaththe

VO-richtop interfacelayer, resettingthedeviceintotheHRS ApositivevoltageonthetopelectrodewillreversethisprocessandeliminatetheVO-depletedregion,thussettingthedeviceintotheLRS.Asmentionedbytheauthorsthemselves,however,themodelwillhavetobeconfirmedbyfurtherexperimentsandshouldnot

beregardedasacompleteandfinalmodeltoexplainthispeculiarswitchingbehavior

16

cuu duong than cong com

3.2 Chargetrapping/de-trapping

Thechargecarriersinjectedfromtheelectrodesmaybetrapped

bychargetrapsinastoragemediumtoformaspacecharge.The

spacechargecaneithermodulatethebarrierstoinjectionofthe

chargecarriersfromtheelectrodesoraffectthetransportprocess

ofchargecarriersthroughthestoragemedium,therebyresulting

intheRSbehavior.Basedonthedistributionstate,chargetrapscan

begroupedintothreecategories,asdiscussedbelow

3.2.1 Interfacialchargetraps

Inadditiontotheproposedanion-migrationmodelinSection

3.1.2,thebipolarRSphenomenaoftheSchottkyjunctionscanalso

be explained by charge trapping/de-trapping [261,262] The

trappingstateofthechargetraps attheSchottkyjunctionscan

be changed by external voltages Accordingly, the barriers to

injectionofchargecarriersfromtheelectrodeswillbemodulated,

resultinginthebipolarRSbehavior.Ingeneral,thechargetrapping

processtakesplaceunderareversebiasandleadstotheHRS,while

thechargede-trappingprocesstakesplaceunderaforwardbias

andleadstotheLRS.Forexample,ithasbeenconfirmedthatthe

Au/Nb:STOjunctioncanbesetintotheLRSandresetintotheHRS

when the Au electrode is positively and negatively biased,

respectively [262] The HRS shows rectifying I–V behavior

indicating the recovery of the Schottky barrier, while the LRS

showslinear I–Vbehaviorindicating a collapse oftheSchottky

barrier, as displayed in Fig 15a Schematics of the electron

detrappingandtrappingprocessesintheAu/Nb:STOjunctionare

showninFig.15bandc,respectively.Thechargetrapshavebeen

provedtobespacedefectsintroducedduringthedepositionofAu

electrodesbyionsputtering.Whenalargeforwardbiasisapplied

totheSchottkyjunction,electronsaredrawnoutfromthetraps,

leavingbehindpositivelychargedtraps(Fig.15b).Suchpositively

chargedempty traps can provide an additional potential, thus

reducing the build-in potential and the depletion width for

maintainingtheFermienergybalancebetweentheAuandNb:STO

Asaresult,theAu/Nb:STOjunctionissetintotheLRS.Incontrast,

whentheSchottkyjunctionintheLRSisreverselybiased,electrons

willbeinjectedintoandthentrappedbytheemptytraps,resulting

inneutraloccupiedtraps(Fig.15c).Consequently,thecollapsed

Schottkybarrier recoversto itsvirginstateand theAu/Nb:STO

junctionisresetbackintotheHRS.Inaddition,theshortretention

time(250s)oftheLRSinaplanarPt/Ba0.7Sr0.3TiO3/Ptdevicecan

serveaspowerfulevidenceforthevalidityofthechargetrapping/de-trappingmodel[263]

3.2.2 Chargetrapsprovidedbyamiddlenanoparticlelayer

An RRAM cell with charge traps provided by a middlenanoparticle layerisschematically shownin Fig.16a ThehoststoragemediainsuchcellscanvaryfrominorganicssuchasZnO

[264]toorganicssuchasAlq3[265].Thenanoparticlelayersareoftenformedbydepositingasinglelayerwithanominalthicknessbelow10nm[266].Inthiscase,theinitialislandgrowthprocesshas not been completed, thus preventing the islands fromcoalescingtoformacontinuousfilm.Thecommonnanoparticlelayers are metals suchas Cu and Al,and theothers are somesemiconductingmaterialssuchascopperphthalocyanine(CuPc)

[266].TherearetwotypesofRSbehaviorsintheseRRAMcells,i.e.,conventional bipolarswitching andpeculiarunipolar switching

Fig 16b shows the conventional bipolar switching of a Cu/ZnO(20nm)/Cu(3nm)/ZnO(20nm)/Ptcell[264].Anexternalvoltage was applied tothe Cu electrode withthe Pt electrodegrounded.OnecanseethatstablebipolarswitchingwithaVsetof

1.7VandaVresetof1.2Visobtainedafteraformingprocessof

3.5V To understand the conduction behavior and switchingmechanismofthememorycell,thepositivepartoftheswitchingcurveisre-plottedusingdouble-logarithmiccoordinates(Fig.16c).Based on the fitting results, the I–V characteristic of the HRSfollowsalinearOhmicbehavioratlowvoltagewiththeadditionof

aquadratictermathighervoltage,whichistypicalofaninsulatorwith shallow traps and space charge limited current (SCLC)injection.For theLRS,theI–Vcharacteristicstillconsistsoftwoportions:anOhmicregion(I/V)andaChild’slawregion(I/V2),whichcanbeexplainedbythetrap-filledSCLC.Hence,thebipolarswitching behavior can be explained by a charge trapping/de-trappingprocess,calledthetrap-controlledSCLCmodel.Indetail,chargetrappingunderapositivevoltageleadstothesetprocess,whereaschargede-trappingunderanegativevoltageresultsintheresetprocess

Fig 16d shows the peculiar unipolar switching of an Al/Alq3(50nm)/Al(5nm)/Alq3(50nm)/Alcell[265].Theword‘pecu-liar’herereferstothephenomenonthattheVresetislargerthanthe

Vset,whichdiffersfromtheconventionalunipolarswitchingwherethe Vreset is smaller than the Vset (Fig 1b) The detailedcharacteristics of such switching behavior can be summarized

asfollows

(i) WhenthecellisintheLRS(ONstate),anN-shapeI–Vcurveisobtainedduringthepositivevoltagesweep,i.e.,thereisalocalmaximumcurrentatVmax,followedbya regionofnegativedifferentialresistance(NDR)andalocalminimumcurrentat

Vmin.(ii)IfavoltagenearVminisappliedandthenrapidlysetbackto0V,thedeviceisleftintheHRS(OFFstate)

(iii)WhenthecellisintheHRS,thecurrentremainslowuntilathresholdvoltage(Vth)isreached,atwhichtimethecellissetintotheLRS

(iv)TheVthiscomparableto,butslightlylessthan,Vmax.(v)IfavoltageclosetoVmaxandaboveVthisappliedandthenreducedto0V,thedeviceisleftintheLRS

(vi)MultilevelstoragecanbeobtainedbysettingthevoltageintheNDRregion,asdemonstratedinFig.16e

Such peculiarunipolar switchingis almostidenticaltowhatwasobservedinan electroformedAu/SiO/Aldiodeby Simmonsand Verderber [12].Theyalso proposeda model, calledtheSVmodel,toexplaintheirexperimentalresults.Onthebasisofthismodel, the initial LRS of the electroformed Au/SiO/Al diode is

cuu duong than cong com

SiOlayerduringtheelectroformingprocess.Theseatomscannot

only act as electron traps but also provide available sites for

tunnelingofelectrons.TheNDRregioniscausedbythetunneling

ofelectronsintotrapsitesandthesubsequentestablishmentofa

space charge field, which opposes the applied field and thus

reducesthecurrent.IftheexternalvoltageneartheVminisrapidly

reducedtozero,thetrapped electronsare leftin theinsulator,

leadingtotheHRS.Incontrast,iftheexternalvoltageneartheVmin

isslowlyreduced tozero,thetrappedelectronshave sufficient

timetoleakoutfromtheinsulator,leadingtotheLRS.Theabrupt

currentjumpatVthisduetothede-trappingprocessofthetrapped

electrons.ForfurtherdescriptionoftheSVmodel,thereadersare

encouragedtorefertotheoriginalpaper.Hence,byequatingthe

nanoparticletrapswiththeAutraps,theSVmodelcanserveasa

goodexplanationforthepeculiarunipolarswitchingofRRAMcells

withthechargetrapsprovidedbyamiddlenanoparticlelayer.It

mustbenotedhere,however,thatsuchRRAMcellsareinitiallyin

theHRS,whichdiffersfromtheinitialLRSoftheelectroformedAu/

SiO/Aldiode.Thisdiscrepancymayimplytheexistenceofother

possible switching mechanisms and that further attention is

neededinthisresearchfield.Moreover,thedecisivefactorforthe

appearanceofeachRSbehavior(conventionalbipolarswitchingor

peculiar unipolar switching)alsoneeds to beconfirmed in the

future

3.2.3 Randomlydistributedchargetraps

AnRRAMcellwithrandomlydistributedchargetrapsisshown

schematicallyinFig.17a.Basedonthesize,thechargetrapscanbe

groupedintonanoparticle-andatom-leveltraps.The

nanoparticle-level traps include metal nanoparticles(e.g., Au and Ag

nano-particles[266])andtheirderivatives(e.g.,Aunanoparticlescapped

with1-dodecanethiol[267]), semiconductor nanoparticles (e.g.,

ZnO,ZnS,andCdSenanoparticles[268]),carbonnanotubes[189],

18

cuu duong than cong com

showRS behaviorsthat areidenticaltothose of thecells with

chargetrapsthatareprovidedbyamiddlenanoparticlelayer,i.e.,

theconventionalbipolarswitching(e.g.,theAl/PS+Au-NPs/Alcell

peculiarunipolarswitching (e.g.,theAl/xBP9F+Au-NPs/ITOcell

[266])explainedbytheSVmodel.Therefore,thediscussionbelow

isfocusedontheRRAMcellswithrandomlydistributedatom-level

traps

Therearetwotypesofatom-leveltraps.Thefirsttyperefersto

the intrinsic crystal defects such as vacancies, which lead to

conventionalbipolarswitchingbehaviorthatcanbeexplainedby

thetrap-controlledSCLCmodel[270,271].Theothertypeof

atom-leveltrapswasfirstintroducedbyChenetal.[272]insixsolid

solutionsthatwerehostedbyinsulatorsanddopedbyelectronic

conductors If named with the form of insulator:conductor,

these six solid solutions wereSiO2:Pt, Si3N4:Pt, LaAlO3:LaNiO3,

LaAlO3:SrRuO3, CaZrO3:LaNiO3 and CaZrO3:SrRuO3 The word

‘atom-level’meansnoPtclustersinthefirsttwosolidsolutions

andno ordering or phaseseparationin the other four cases,as

verified by the UV reflectivity test and cross-sectional

high-resolutionTEM(HR-TEM)analysis,respectively.Inthisstudy,we

considerSiO2:Ptasanexampletodiscusstheswitchingbehavior

and underlying mechanism of these RRAM cells Based on the

straightforward criterion for distinguishing between random

insulators and conductors, provided by Anderson in 1958, the

diffusiondistancezforelectronsat0Kisfiniteininsulators,whileit

isinfiniteinconductors[272].Becausethelateralpercolationlimit

oftheSiO2:Ptisf0.38(f:molarfractionofPtintheSiO2:Pt),the

SiO2:0.2Ptmusthaveafinitez.Theexactvalueofzcanbeobtained

bytestingtheresistanceoffilmswithvariousthicknessesd,thatis,filmswithd zaremetallicacrossthethickness,whilefilmswith

d zareinsulating.Fig.17bshowstheinitialR–VcurvesofthePt/SiO2:0.2Pt/MocellswithvariousSiO2:0.2Ptlayerthicknesses.OnecanseethatthevirginSiO2:0.2PtfilmisanOhmicconductorwithalow resistanceup to d=16nm, whileit becomesa non-Ohmicinsulatorwithahighresistanceatd=21nm.Theseresultssuggest

16nm<z<21nm.Mostremarkably,peculiarbipolar switchingwasobservedinthePt/SiO2:0.2Pt/Mocellswithadfrom7nmto

16nm.Theword‘peculiar’ referstothefactthatthese cellsareinitiallyintheLRS,whichiscontrarytotheconventionalbipolarswitchingwithaninitialHRS.Incontrast,thePt/SiO2:0.2Pt/Mocellwith d=21nm(d=5nm) stayed in the HRS (LRS) all the time.Moreover, three interesting characteristics about this peculiarbipolar switchingwere found: (i)the Vresetofthe Pt/SiO2:Pt/Mocellwasindependentoffandd;(ii)atlowertemperature,theR–Vhysteresis loop expanded vertically, while the Vreset remainedunchanged(Fig.17c);and(iii)UVirradiationwasabletoswitchthePt/SiO2:Pt/MocellfromtheHRStotheLRS(Fig.17d).Allofthesecharacteristics argue against a thermally activated switchingmechanism such as ion migration and suggest the chargetrapping/de-trapping mechanism.Thetransitionfrom theLRStotheHRScanbeunderstoodbytheinjectionofelectronsbyFowler–Nordheimtunnelingfromtheelectrodewithlowerworkfunction.These injected electrons are trapped at sites near the originalconductingpathwaysandthenprovideCoulombrepulsiontoblockfurther electron transport along these conducting pathways.TheabsenceofRSbehaviorinthe Pt/SiO2:0.2Pt(5nm)/Mocellis

cuu duong than cong com

insuchathinlayerisnotenoughtoblockfurtherelectrontransport

along the original conducting pathways Under a reverse bias,

successivede-trappingofthesetrappedelectronsthroughthesame

tunnelingbarriertypicallyleavesbehindaseriesofintermediate

states,whichisbeneficialformultilevelstorage

3.3 Thermochemicalreaction

In thermochemical reaction-dominated RRAMs, the forming

andsetprocessescorrespondtothethermaldecompositionofthe

storagemediaandconsequentformationoftheCFs,andthereset

processistriggeredbythermalmeltingoftheexistingCFs[9].The

formed CFs usually show metallic conduction behavior, as

demonstrated in Pt/NiO/Pt [253,254], Pt/ZnO/Pt [26] and W/

polystyrene/W[273].BecauseJouleheatingisindependentofthe

electricpolarity,bothunipolar andbipolaroperationmodesare

valid in such RRAM cells Accordingly, nonpolar switching is

introducedinsomeliteraturestodescribethisswitchingbehavior

The discussion of the switching kinetics of thermochemical

reaction-based RRAMs below is classified into two categories

basedonthestoragemedia

3.3.1 Thermochemicalreactioninsemiconductingmetaloxides

With a Icomp of 103A, semiconducting metal oxides

sandwiched between two inert electrodes, such as Pt/NiO/Pt

[136,253,254,274],Pt/CoO/Pt[274],Pt/TiO2/Pt[135,274] andPt/

ZnO/Pt[26],frequentlyshowthermochemicalreaction-dominated

switchingbehavior.Thedrivingforcefortheformationofmetallic

CFsistheenergeticallyfavoredlowervalencestatesofthesemetal

oxidesathightemperature,asseenfromtheEllinghamdiagramin

[9]

To clarify the switching kinetics, a three-terminal device

configurationwasoftenemployed[135,136,274],asschematically

illustratedinFig.18a Theoperationprocessofthisdeviceisas

follows First, a forming process between electrode A and

electrode C (A–C configuration) is performed by applying a

positivebiasonelectrodeAwithelectrodeCgrounded.Second,a

unipolarresetprocessisconductedwiththeA–Cconfiguration

Note that the A–C configuration can be regarded as a serial

connection of the memory cell beneath electrodeA and the

memorycellbeneathelectrodeC.Finally,theresistancestates

beneathelectrodesAandCareexaminedbymeasuringtheI–V

curvesof theA–B andB–Cconfigurations,respectively.Using

this three-terminal device, Nagashima et al [274] recently

studiedtwop-type oxides(NiOxand CoOx)and three n-type

oxides(TiO2x,yttriastabilizedzirconia(YSZ)andSnO2x),and

theobtainedresults areshown inFig.18b–e.Basedonthese

results,itcanbeunambiguouslyconcludedthattheactive RS

regionislocatednearthecathodeinp-typeoxidesbutnearthe

anodeinn-typeoxides.Inaddition,itcanbefurtherdeduced

that,duringtheformingprocess,theCFsgrowfromtheanodeto

thecathodeinp-typeoxidesbutfromthecathodetotheanodein

n-typeoxides

ForthecompositionofthesemetallicCFs,itisnaturaltodeduce

thattheyshouldbecomposedofsuboxidesorevenmetalatoms

Toconfirmthisdeduction,manyattemptshavebeenmadeover

thepastfewyearsandmanysignificantresultshavebeenmade

thusfar.In2007,Park etal.[275]systematically examined the

structuralchangesofpolycrystallineNiOx(x=1–1.5)filmin the

HRS,theLRSandtheswitchingfailedstate(irreversibleLRS)by

HR-TEMandelectronenergy-lossspectroscopy(EELS).Theyfound

thattheCFsinNiOxarecomposedofNiatomsandarelocatedat

grainboundaries.In2010,byusingHR-TEMandinsituI–Vaswell

as low-temperature (130K) conductivity measurements,

Hwang’sgroup[276]confirmedthattheCFsinTiO2arecomposed

of TinO2n1 (so-called Magne´li phase) and they grow from thecathode totheanodewiththeir thinnestpartsnear theanode.Recently,Chenetal.[26]useinsituTEMtoshowthattheCFsinZnO are composedof Zn-dominatedZnO1x Moreimportantly,theysuccessfullyrecordedtherealtime formationand ruptureprocesses of these CFs, as shown in Fig 19 The I–Vcurves offormingandsubsequentresetprocessesaredisplayedinFig.19f

Fig.19a–dareaseriesofTEMimagescorrespondingtodatapointsa–dinFig.19f,revealingthattheCFsgrowfromthecathodetotheanodeduringtheformingprocess.Duringtheresetprocess,theCFswereconfirmedtoruptureneartheanode(Fig.19e),whichaccordswellwiththeconclusionmadebasedontheI–Vmeasurementsinthepreviousparagraph

All of the above experimental results can be reasonablyexplainedby theuniversalmodelfor thermochemicalreaction-dominatednonpolarswitchingbehavior proposedby Kimetal

[277],as shown in Fig 20 For a p-type semiconducting metaloxide,taking NiOasanexample,localizedholeinjection attheanodicinterfaceduringtheformingprocessispossibleduetothelowerSchottkybarrierforsomereasons(forexample,largesurfaceroughnessornon-uniformityoftheNiOlayerandpenetrationofelectrodeintotheNiOlayer),asschematicallyshowninFig.20a.As

aresult,athermochemicalreactionandtheaccompanyingoxygenlossoccurattheselocalareasbecauseoftheseriousJouleheating,leadingtothegenerationofNiinterstitials(Ni00

i)inaccordancewithNiO!Ni00i þ2e00þ1=2O2ðgÞ.Onthisoccasion,thelocationofholeinjectionmightserveasasourceofNi00i ions.Whenthegenerated

Ni00i ionsareabundantenough,theycombinetoformmetallicNifilaments,resultinginthegrowthofNifilamentsfromtheanodetothecathode(Fig.20b).Withsuchagrowthmode,thethinnestparts

oftheNifilaments,i.e.,theactiveRSregions,areexpectedtobelocatednearthecathode.Incontrast,inann-typesemiconductingmetal oxide such as TiO2, electrons are injected locally at thecathodic interface at a location where the interfacial Schottkybarrierislower(Fig.20c).TheconsequentJouleheatingeffectcansignificantly promote thelocalgeneration ofVOs atthe anodicinterface and their drift and diffusiontoward the cathode Thegenerated,abundantVOsaremostlikelydraggedtothelocationwhereelectroninjectionoccursduetoelectrostaticforceandthenaccumulate there, leading to the preferential nucleation of

TinO2n1filamentsatthecathodicinterface.Hence,theTinO2n1

filaments will grow from the cathode to the anode in n-typesemiconductingmetaloxides(Fig.20d)andshowaconicalshapewithwiderand narrowerdiameters atthecathodicand anodicinterfaces,respectively

3.3.2 Thermochemicalreactioninorganics

In an organic RRAM cell with thermochemical dominated RS behavior, the carbon-rich filaments are formedduringtheformingprocessbylocalthermaldecompositionoftheorganicfilm.Subsequently,thelocalruptureandre-formationofthesecarbon-richfilamentsresultinalternationbetweentheLRSandtheHRS.Asearlyasinthe1970s,suchRSbehaviorhadbeenreportedinsomepolymersformedbyaglow-dischargetechnique

reaction-[273,278].Forexample,whentheW/polystyrene/WcellwasintheLRS, Segui et al [273]found that:(i) it showed an OhmicI–Vcharacteristic;(ii)itsresistancedecreasedfrom10to6Vwhenthetemperaturewaschangedfrom293to77K;and(iii)itscurrentwasindependentoftheareaoftheelectrode.Allofthesepropertieswereinaccordancewiththeexistenceofthefilamentarycarbon-rich paths Moreover, theemission of hydrogen was inevitableduringthethermaldecompositionofthepolystyrenefilm.Thishadbeenprovedunambiguouslybythefactthat,afterswitching,anadditionalhydrogenpeakappearedinthespectrumobtainedbychromatographicanalysisinthevaporphaseofthegasaroundtheRRAMcell

20

cuu duong than cong com

3.4 Exclusivemechanismsininorganics

3.4.1 Insulator-to-metaltransitioninMottinsulators

SomeMottinsulatorsshowinganinsulator-to-metaltransition

(IMT) have also been introducedas storage mediafor RRAMs

Basedon theoriginof theRSbehavior,theseinsulatorscan be

groupedintotwocategories.ThefirstcategoryincludesVO2[279],

Ca2RuO4[280],NbO2[281],andsoon,inwhichtheRSbehaviorhas

beenuniversallyacknowledgedtooriginatefromtheelectric

field-inducedJouleheatingeffect.Eachofthesematerialshasaspecific

thresholdtemperatureofIMT(TIMT),forexample,TIMT=340Kfor

VO2[279],and357KforCa2RuO4[280].Thematerialwillchange

from an insulator to a conductor as its temperature increases

acrossthe TIMT, and vice versa Hence, thesematerials usually

exhibitthresholdswitching(i.e.,theLRScanholdonlywhenthe

externalvoltageislargerthanathresholdvoltageVth)andserveas

media in switch devices for RRAM crossbar arrays [77,281],

althoughnonvolatileRSbehaviorinVO2hasalsobeenreported

[282] Alternatively, the second category refers to the ternary

chalcogenidesAM4X8(A=Ga,Ge;M=V,Nb,Ta;X=S,Se),inwhich

theRSbehavior isattributedtoelectric-field-induced electronic

phaseseparationatthenanoscale[283].Thediscussionbelowis

focusedontheswitchingbehaviorandunderlyingmechanismof

AM4X8

Fig.21aisaperspectiveviewofthecrystalstructureofAM4X8,

where the M4 tetrahedral clusters are presented as blue

tetrahedra [284] This structure is called as a deficient spinel

cubicstructure,andcanbederivedfromtheregularspineltypeby

shifting the metal atoms off the centers of the chalcogen

octahedraalong[111]toformtheM4clusters.Theintracluster

M–Mdistancesarecompatiblewiththeformationofmolecular

bonds,thusleadingtotheformationofmolecular-likeelectronicstates within the clusters, while the larger intercluster M–Mdistancespreventmetal–metalbonding.AM4X8usuallyexhibitsavery smallMott–Hubbard gap(Eg)rangingfrom 0.1to0.3eV,which makes it very sensitive to external perturbations Forexample,dopingandexternalpressurecaneasilyinducetheIMT

inAM4X8[285,286].Moreimportantly,electricfield-inducedIMT

inAM4X8isattractinggreatattentionrecentlyduenotonlytotheinterestingphysicalphenomenonitselfbutalsotoitspotentialapplicationinRRAMs.In2008,Carioetal.[287]providedthefirstexperimental evidence for both volatile and nonvolatile RSbehaviors in bulk single crystalline GaTa4Se8 Afterwards, thesamephenomenawerereportedinothercompoundsbelongingtothe AM4X8 family [283] In 2011, Besland et al [288] firstdemonstratednonvolatileRSbehaviorinapolycrystallineAM4X8

film by using a Au/GaV4S8(1mm)/Au structure Under 10mspulsesof2.5V,aswitchingratioof33%wasobtained,asshown

inFig.21b

Ingeneral,theAM4X8showsvolatileandnonvolatileswitchingwhentheexternalelectricfieldisslightlyandmuchlargerthanathresholdvalue(Eth)ofafewkV/cm,respectively[283,285].TheEth

hasbeenprovedtoincreaseasapowerlawoftheEg,whichisingoodagreementwiththeuniversallawofEth/E2:5

g reportedforavalanchebreakdowninsemiconductors,demonstratingthatthevolatileswitchingofAM4X8originatesfromavalanchebreakdown

[285].ToclarifytheoriginofthenonvolatileswitchingofAM4X8,Corrazeetal.[289]conductedadetailedTEMstudyonaGaNb4Se8

crystalafteritwasswitchedintheLRS.Totheirdisappointment,all

oftheexperimentalresults,includingthehighresolutionimagesand the convergent beam electron diffraction experiments(performedwithregularstepsof20nmovera2mmlongsegment

cuu duong than cong com

witha typicalbeamsizeof 1nm),didnotrevealany

crystallo-graphic symmetry breaking or amorphization between the

electrodesatthenanoscale.In contrast,thescanning tunneling

microscopy(STM)studyof aGaTa4Se8 crystalprovided alot of

usefulinformation[290].Fig.21cisahistogramofthegapwidths

forapristineGaTa4Se8crystal,establishedfromthegapwidthmap

shownintheinset.ThisindicatesthatthepristineGaTa4Se8hasa

homogeneousinsulatingelectronicphasewithanEgof200meV

Surprisingly, the GaTa4Se8 crystal in the LRS shows a very

inhomogeneouselectronicphase,asrevealedbyFig.21d There

aretwotypesof30–50nmdomainsembeddedinthe

electroni-cally unchanged crystal volume The first type of domains (in

white) are clearly metallic with gap values peaking at Eg=0,

whereasthesecondtypeofdomains(inblack)aresuper-insulating

correspondingtoacontinuumoflargergapsdistributedalmost

homogeneouslybetween 400 and 700meV Itshould be

men-tioned that in a Mott insulator, negative pressure (volume

expansion) is expected to lead to a continuous increase in Eg,

whilepositivepressure(volumecompression)canleadtoa

first-ordertransitionfromaMott insulatortoa correlatedmetal,as

shownby a schematictemperature–pressure phase diagram of

GaTa4Se8 in its pristine statein Fig 21e Given the significant

similarity between the electric field- and pressure-induced

metallic states of AM4Q8 [286], and thestrong electron–lattice

couplinginAM4Q8duringnonvolatileswitchingrevealedbylocal

sampledeformation[291],thenonvolatileswitchingofAM4Q8can

bereasonably explainedbytheelectric field-inducedelectronicphase separation at the nanoscale concomitant with a latticedeformation

3.4.2 Thesp2/sp3conversioninamorphouscarbon

Itiswell-knownthatcarboncanexistsinvariousforms,amongwhichthemostprominentonesarethesp2-dominatedgraphiticformwithhighconductivityandthesp3-dominateddiamondformshowing low conductivity The reversible conversion betweenthese two forms in amorphous carbon (a-C) with inert metalelectrodes (such as Pt and W) has been recently utilized forunipolarRRAMapplications[131,292,293].Theresearchinterest

in suchRRAMs is keen because,compared with other storagemedia, carbon exhibits advantages due to its simple chemicalcomposition,lowcost,compatibilitywithCMOSprocesses,andthepotentialforrealizingfuture‘all-carbondevices’bycombinationwiththehighlyconductiveinterconnectingcarbonmaterials.Thesetprocesscan beunderstood asthesp3!sp2conversionandconsequent formation of conductive sp2-rich carbon filamentsinduced mainly by Joule heating This explanation is stronglysupported by real-time TEM observation of the formation oftubules from a-C nanowires under high-bias-induced Jouleheating [294] To explain the reset process, the temperaturedistributionofacarbonfilamentwitha5nmdiameteraftera1nscurrent pulse with a current density of 1.7GA/cm2 has beensimulated[292].Theresultshowsapeaktemperatureof3928K,which is sufficient to rupture the existing sp2-rich carbonfilaments.Althoughthesp2/sp3conversioncanalmostperfectlyexplaintheRSbehaviorofa-C,itshouldbenotedthatanothermodel,basedontherearrangementofsp2carbonatomswithinthe sp3 matrix, has also been proposed [295] This model issupportedbyasignificantsimilaritybetweenthenear-edgeX-rayabsorption fine structure spectra obtained before and afterswitching, meaning that the sp3 to sp2 ratio remains almostunchanged.Hence,topromotethepracticaluseofRRAMsbased

ona-C,furtherstudyisneededtoprovideaclearerpictureoftheunderlyingswitchingmechanism

3.5 Exclusivemechanismsinorganics3.5.1 Chargetransfer

Underanexternalelectricfield,acomplexcontainingelectrondonorsandelectronacceptorsusuallyundergoesachargetransferprocess,during which electrons in theelectron donor moieties(such as carbazolegroups and tetrathiafulvalenes) are partiallytransferredtotheelectronacceptormoieties(suchasfullerenes,graphenesandgoldnanoparticles)[187,188,296,297].Afterchargetransfer, a partially filled highest occupied molecular orbital(HOMO) of a polymercontaining electrondonor moieties,or apartiallyfilledlowestunoccupiedmolecularorbital(LUMO)ofapolymer containing electron acceptor moieties, or both willappear, thus leading to an increase in the conductivity of thecomplex.Forinstance,afunctionalPVK-C60polymer,containingcarbazole moieties (electron donors) and fullerene moieties(electron acceptors) in a molar ratio of approximately 100:1,wassynthesizedbyLingetal.[296]viacovalenttetheringofC60toPVK.TheHOMOandLUMOenergylevelsofthesynthesizedPVK-

C60relativetothevacuumlevelweredemonstratedusingcyclicvoltammetry method to be 5.57 and 3.56eV, respectively.Interestingly,bipolarswitchingbehaviorwasobservedwhenthePVK-C60film was sandwichedbetween an Al electrode and anindiumtinoxide(ITO)electrode,asshowninFig.22a.Duringthetest,externalvoltageswereappliedtotheAlelectrodewiththeITOelectrodegroundedallthetime.Given theworkfunctionsofAl(4.28eV)andITO(4.8eV),theenergybarriersoftheAl/PVK-C60

andPVK-C60/ITOinterfacesunderpositive(negative)voltageare

from Ref [277] Copyright ß 2011, IOP Publishing Ltd.

22

cuu duong than cong com

charge injections during the negative voltage sweep are more

favorable than during the positive voltage sweep, which can

reasonablyexplainwhythevirginAl/PVK-C60/ITOcellintheHRS

canbeswitchedintotheLRSonlybythenegativevoltagesweep

TheswitchingfromtheHRStotheLRSiscausedbytheelectric

field-inducedelectrontransferfromthecarbazolemoietiestothe

fullerene moieties and the consequent heavy p-doping of the

polymer,asschematicallyshowninFig.22b.Becauseofthestrong

electron-withdrawing ability of C60 with a high LUMO of

3.13eV and the strong dipole moment of VK-C60 (3.32D),

theelectronstrappedinC60moietiescanberetainedandcoexist

withthesurroundingpositivelychargedcarbazolemoieties,thus

leading to thenonvolatility of theLRS During the subsequent

positivevoltagesweep,thenegatively chargedC60moietieswill

lose thetrapped electrons toneutralize the positively charged

carbazolemoieties.Consequently,thedevicewasresetbackinto

itsvirginHRS

3.5.2 ConformationalchangeTheconformationalchangeofsomeorganicmoleculescanalso

be utilized for memory applications [298–301] Each of thesemolecules hasat leasttwo distinct isomers,and thechange inconformation is usually concomitant with a change in theconduction One representative group of such molecules ispolymerscontainingcarbazolependant groups, suchas poly(2-(9H-carbazol-9-yl)ethyl methacrylate) (PCz) [299], poly(2-(N-carbazolyl)ethylmethacrylate)(PMCz)[300],poly(9-(2-((4-vinyl-benzyl)oxy)ethyl)-9H-carbazole) (PVBCz)[300],poly(N-vinylcar-bazole)-phenylfluorene(PVK-PF)[301],etc.Itiswell-knownthatthecarbazolegroupisanelectron-donorandhole-transporter,andhasatendencytoformapartialorfullface-to-faceconformationwith the neighboring carbazole groups to result in extendedelectrondelocalization.Inthepristinestate,thesepolymersareinthe HRS because the carbazole groups included are randomlyoriented Under an external electric field, oxidation of thecarbazolegroupsoccursinitiallyinthevicinityoftheelectrodes

[290] Copyright ß 2013, American Chemical Society (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

cuu duong than cong com

from which holes are injected into the polymers, leading to

positivelychargedcarbazolegroups.Asaneffectiveelectrondonor,

thenearbyneutralcarbazolegroupsundergochargetransferor

donor–acceptorinteractions withthealreadypositivelycharged

carbazolegroupstoformpartialorfullface-to-faceconformation

withtheneighboringcarbazolegroups.Consequently,thepositive

charges are delocalized to the neighboring, ordered carbazole

groups.Asthedurationofstressingwiththeexternalelectricfield

lengthens,thisprocesswillgraduallypropagatethroughthewhole

thicknessofthepolymerfilm.Onceasignificantfractionof the

carbazolegroups hasundergonesucha conformationalchange,

charges can be easily transported through the neighboring,

ordered carbazole groups either on the same or neighboring

polymerchains(intrachainorinterchainhopping),thusleadingto

theappearanceoftheLRS.Fig.23aschematicallyshowssucha

conformational change of PVK-PF More importantly, such a

conformationalchange of PMCz hasbeen directly captured by

HR-TEM.Fig.23bisatypicalHR-TEMimageofPMCzinthepristine

state(HRS),fromwhichonecanseeonlyamorphousstructure.In

contrast,theHR-TEMimageofPMCzintheLRS(Fig.23c)hasclear

ordereddomains, whichresemblesthepolycrystallinestructure

observedintheHR-TEMimageofPVKwithhighconductiondueto

orderedcarbazolegroups.Thestabilityoftheorderedcarbazole

groupsinduced by theelectric field is dependent on the exact

chemicalstructureofthepolymers.Asthestabilityoftheordered

carbazolegroupsimproves,theswitchingbehaviorcanvaryfrom

volatileswitchinginPVBCz[300],tononvolatilebipolarswitching

in PVK-PF [301], to nonvolatile write-once-read-many-times(WORM)memorybehaviorinPCz[299]andPMCz[300]

4 RRAMsfordatastorage4.1 CurrentperformanceAfterdecadesofefforts,manyexcitingrecordsregardingtheperformance ofRRAMshave beenset thus far.First,scalabilitydownto10nmincellsizehasbeendemonstratedinHfOx-and

WOx-based RRAMs [2,6] Second, ultra-fast switching speed of

100pshasbeenreportedinTaOx-andPt-dispersedSiO2-based

24

cuu duong than cong com

proved in Ni/GeO/HfON/TaN and TiN/Hf/HfO2/TiN sandwich

structures[5,6].Moreover,extremeenduranceof>1012switching

cycleshasbeensuccessfullyrealizedinaPt/Ta2O5x/TaO2x/Ptcell

[7].Finally,satisfactoryretentionofseveralmonths

(experimen-tallydemonstrated) and even over 10 years (extrapolated) has

often been observed in RRAMs [42] Note that each record

performanceisfarsuperiortothatofFlashmemoriesandeven

comparabletothatofDRAMs.However,itmustbeadmittedthat

these exciting records are mostly observed in different RRAM

systems, and tointegrate them intoa singleRRAM cell is still

impossible.Inaddition,theperformancevariancefromswitching

cycletocycleandfromdevicetodeviceisanothertroublesome

issuethatmustbesettledbeforepracticaluseoftheRRAMs

4.2 Methodsforperformanceenhancement

ToenhancetheperformanceofagivenRRAMsystem,various

methodshavebeenproposedandtriedoverthepastdecades.The

discussion below is focused on the common methods for

performanceenhancementofRRAMs,includingdoping,electrode

engineering,interface engineering,optimizationofdevice

struc-tureandmeasurementcircuit, and multilevelstorageoperation

andconductancequantization

4.2.1 Doping

Itiswell-knownthatdopingisaveryeffectivewaytomodulate

thepropertiessuchaselectricalconductivityandmagnetismofa

given material.In theresearchfield ofRRAMs, ithasalsobeendemonstratedthatdopingactsasapowerfulmethodtoenhancetheperformanceofRRAMs.Forexample,thecommonbipolarRSbehavior of cation migration-based RRAMs is a troublesomeobstacleifwe wanttointegratesuchdevices into3-Dcrossbararrays withasimple one-diode/one-resistor(1D1R)structuretoovercomethesneak-pathissue.Thisisbecausethereversecurrentdensity of a diode is usually too low to reset a RRAM device.Fortunately, the bipolar RS behavior of cation migration-basedRRAMscanbechangedtothenonpolaronebysimplydopingthestorage media with the corresponding active electrode metals.Chenetal.[28]havesuccessfullyobtainednonpolarRSbehaviorin

a Cuionmigration-basedRRAMdevicewithAlNasthestoragemedium.ThedopingofCuintotheAlNfilmwasrealizedbyrapidlyannealinganAlN(50nm)/Cu(3nm)/AlN(50nm)tri-layerat7008C

inN2ambientfor60s.AnX-raydiffraction(XRD)patternoftheAlN:CufilmonaPt/Ti/SiO2/Si(100)substrateisshowninFig.24a,which revealsonlya wurtziteAlN(002)peak.Theinsetofthisfigurerepresentsalowmagnificationcross-sectionalTEMimageofthe Pt/AlN:Cu/Pt device, which clearly shows the tri-layerstructure The initial Pt/AlN:Cu/Pt device is in the LRS, andconsequentlynoformingprocessisneededtotriggerthestableRSbehavior.ThisisbecausetheinsertedCulayercaneasilydiffuseintotheAlNfilm,especiallyalongthegrainboundariesbetweenadjacentcolumnarAlNgrainsduringtheannealingprocess,which

ispracticallyathermalformingprocess.ItisinterestingthatthePt/AlN:Cu/PtdeviceshowsanonpolarRSbehavior(Fig.24b)thatisremarkablydifferentfromthebipolarRSbehaviorintheCu/AlN/Pt

cuu duong than cong com

uniformi-ty between different devices (Fig 24c), excellent endurance

propertyupto 103 (Fig.24d)and retention performanceup to

10years(extrapolateddata,Fig.24e),andfastswitchingspeedof

100ns(Fig.24f),thusactingasastrongcandidateforhigh-density

1D1R memory applications In addition, similar nonpolar RS

behaviorhasalsobeenreported inRRAMdevicesbased on

Cu-dopedSiO2[302],Cu-dopedZrO2[88],Cu-dopedHfO2[38],etc

Conversely,dopingcangreatlyimpacttheconcentration and

mobilityofVOsandcanconsequentlymodulatetheperformanceof

oxygenionmigration-basedRRAMdevices[27,303].Zhangetal

[27]systematicallystudiedtheeffectofchemicalvalenceandthe

radiusofdopantiononformationenergy(Ef)ofneutralVOinZrO2

via first principle calculations The VO in ZrO2 was built by

removingoneoxygenatomfromamonoclinicZrO2supercellof96

atoms,asshowninFig.25a.Tosimulatethedopingeffect,oneZr

atomwasreplacedbyadopant(Al,Ti,orLa),andtheVOanddopant

wereassumedtoformnearestneighbors(Fig.25b).Basedonthe

simulation results in Fig 25c, they found that the Ef (VO) is

significantlyinfluencedbythechemicalvalence,whiletheimpact

ofthedopantionradiusisratherweak.TheobviousreductionofEf

(VO)causedbytrivalentdopantcouldbeattributedtotheCoulomb

interaction of dipoles formed between dopants (negatively

chargedacceptor) andVOs(positivelychargeddonor).Based on

theseresults,theypredictedthat thedopingoftrivalentmetals

suchasAlandLaintotheZrO2 layerwouldhelptocontrolthe

formationofVOandresistiveswitchingbehaviors.Todemonstrate

this,theysummarized thedifferencesin RS behaviorsbetween

undopedandAl-dopedZrO2RRAMdevices,asshowninFig.25d

ande.NotonlywasthehighmeanVformof9.89V(withastandarddeviation of 3.86V) reduced to that of 3.80V (with standarddeviationof1.15V),butthelargedispersionsofRHRSandRLRSwerealso significantly suppressed Moreover, when compared toundoped HfO2, enhanced performances were also achieved inGd-dopedHfO2RRAMdevicesincludingimproveduniformityofswitchingparameters,enlargedmemorywindow,andincreasedswitchingspeedwithoutobviousreliabilitydegradation[303].All

oftheseexperimentalworkshaveprovedthecorrectnessoftheirprediction,whichcouldberegardedasaguideforoptimizingtheperformanceofoxide-basedRRAMdevicesbydoping

Abroaddefinitionofdopingshouldincludetheintroductionofnanoparticlesintothestoragemedia.NoblemetalnanoparticlessuchasPt[304–307],Ru[308],andNi[309]nanoparticlesareusually intentionally selected to avoid chemical interactionbetweenthenanoparticlesandthestoragemedia.Thepresence

ofthesenanoparticleswillleadnotonlytoaslightreductionoftheeffectivethicknessofthestoragemediabutalsotolocallyconcentratedandenhancedelectricfieldsbelowandabovethemetalnanoparticles,asrevealedbysimulationresultsin[307].Thesetwoeffectswillfinallyresultinthereductionofthresholdvoltages(suchasinPt-NPs-dopedSrZrO3[305]andSiO2[306]

andNi-NPs-dopedNiO[308])andtheguidedformationofCFsandconsequentimproved stabilityof RS behavior(suchas inPt-NPs-dopedTiO2[304]andNiO[305]andRu-NPs-dopedAl2O3

[307]), thus demonstrating the feasibility of optimizing theperformance of RRAM devices by doping nanoparticles intostoragemedia.Itshouldbenotedthatthebriefdescriptionofdopingnanoparticlesinstoragemediahereisbecauseitseffect

26

cuu duong than cong com

decorationofelectrodeswithnanoparticles(discussedinSection

4.2.2)

4.2.2 Electrodeengineering

IthasbeendemonstratedthattheRSbehaviorofagivenstorage

mediumcanbesignificantlyaffectedbythechoiceofelectrode

materials.Forexample,itwasreportedthattheON/OFFratioofaTE/Mn-dopedZnO/Ptstructuredecreasesfrom104to10asthe

TEchangesfromCutoTiN[310,311].SimilarbehaviorwasalsoobservedinAlN-basedRRAMs[49,246].Alloftheseresultsindicatethat, fora givenstorage medium,a large ON/OFFratiois morelikely to be obtained with cation-migration-based switchingmechanism

LLC.

cuu duong than cong com

To provide a rule for electrode selectionfor O2

migration-basedRRAMdevices,Chenetal.[201]systematicallystudiedthe

effect of TE materials on the RS behavior of the TE/Ta2O5/Pt

structure.Sixmetalelectrodeswereadopted,i.e.,Ni,Co,Al,Ti,Zr

andHf.Fig.26aandbshowsthestatisticalresultsofVset/Vresetand

RLRS/RHRSoftheTE/Ta2O5/Ptstructures,respectively.Duringallof

theswitchingcycles,externalvoltageswereappliedtotheTEwith

thePtelectrodegrounded,andtheIcompwassetas1mA.Itcanbe

easilyseen that:(i)theNi/Ta2O5/Pt and Co/Ta2O5/Ptstructures

showveryscattereddistributionsofVset,RLRS,andRHRS;(ii)theZr/

Ta2O5/PtandHf/Ta2O5/PtstructureshavethelargestVsetandVreset

withalowerscatterinthedistribution;and(iii)theAl/Ta2O5/Pt

andTi/Ta2O5/PtstructuresexhibitthebestRSpropertywithvery

concentrated distributions and appropriate amplitudes of all

parameters.TheseresultssuggestthatthechoiceoftheTEhasa

significantinfluenceontheperformanceoftheTE/Ta2O5/PtRRAM

device.TounderstandthereasonwhytheRSbehavioroftheTE/

Ta2O5/Ptstructure varies withthechoice ofTE, Augerelectron

spectroscopy (AES) depth profiles of these structures were

obtained.Asrepresentatives,memorydevices withTEof Ni,Al,

and Hf were delivered to AES analysis after 100 successive

switchingcycles, and thecorresponding curves are depicted in

Fig 26c–e, respectively The out-diffusion of oxygen ions are

highlightedbytheblackboxineachfigure.Basedonthesefigures,

onecanclearlyseethat:(i)theoxygensignalincreasessharplyat

theNi/Ta2O5interface,indicatingweakout-diffusionoftheoxygen

ions;(ii)slightout-diffusionofoxygenionsintotheAlTEoccursat

the Al/Ta2O5 interface; and (iii) very obvious out-diffusion of

oxygenionsintotheHfTEtakesplaceattheHf/Ta2O5interface

Theout-diffusionofoxygenionsleadedtotheoxidationofTEsand

the consequent formation of interfacial layers, which was

confirmedbythedirectTEMobservationofa2nmamorphous

AlOxatthelayerAl/Ta2O5interfaceanda7nmamorphousHfOx

layer at the Hf/Ta2O5 interface Note that the out-diffusion of

oxygen ions will spontaneously leave VOs in the Ta2O5 layer

Therefore,theformationofinterfacialsub-oxidelayersandVOsin

the Ta2O5 layers controls the RS behavior of the TE/Ta2O5/Pt

structures

Itiswell-knownthattheabilityofametaltoabsorboxygen

atoms(i.e.,oxygenaffinity)canbeapproximatelyrevealedbythe

magnitudeofthestandardGibbsfreeenergyofformationofthe

corresponding stable metal oxide According to [227], Df

G8(2-NiO)=–423.4kJ/mol,DfG8(2CoO)=–428kJ/mol,DfG8(2 / 3Al2O3)=–

1054.9kJ/mol, DfG8(TiO2)=–888.8kJ/mol, DfG8(ZrO2)=–

1042.8kJ/mol, DfG8(HfO2)=–1088.2kJ/mol and DfG8((2/

5)Ta2O5)=764.4kJ/mol.NotethatalowerDfG8correspondsto

ahigheroxygenaffinity.ForNiandCo,theiroxygenaffinitiesare

farhigherthanthatofTa.Asaresult,fewoxygenionsareabsorbed

intotheTEfromtheTa2O5layer,asschematicallyshowninFig.26f

TheentireTa2O5filmisalmoststoichiometricandmanifestshigh

resistivity,thusoftenrequiringarelativelyhighvoltagetosetthe

deviceand leading to thelowest RLRS In thesubsequent reset

process,averylargepeakcurrentof10mAwillgenerateagreat

amountofJouleheattorupturetheexistingVOfilaments[255]

DuetotherandomnatureoftheJouleheat-dominatedRSbehavior,

theNi/Ta2O5/PtandCo/Ta2O5/Ptstructureswillinvoluntaryshow

characteristicssuchasveryscatteredVsetvoltage,RLRSandRHRS

Simultaneously,theTEswithlowoxygenaffinitymayalsocause

theevolution of O2 gasduring the forming and setprocesses,

causingphysicaldegenerationoftheRRAMcells.Conversely,the

oxygenaffinitiesofZrandHfarefarlowerthanthatofTa,leading

totheformationofthickinterfaciallayers(Fig.26h).Theinterfacial

layersmayserveasseriesresistancesduringthesetprocessand

blocktheback-diffusionofoxygenionsintotheTa2O5layerduring

the reset process, thus resulting in the largest Vset and Vreset

Finally,forTiandAl,theiroxygenaffinitiesarecomparabletothat

ofTa,whichwillleadtotheformationofthininterfaciallayersandthegenerationofappropriateamountofVOs(Fig.26g).Duringthesuccessive switching cycles, the interfacial layers will serve asreservoirsofoxygenions,andthegeneratedVOswillfacilitatetheset process Consequently, the Ti/Ta2O5/Pt and Al/Ta2O5/Ptstructures show the best RS property with very concentrateddistributions and appropriate amplitudes of all parameters.Therefore, it can be suggested that the metal electrodes withcomparableoxygenaffinitiestothemetalintheswitchinglayerare the best choices to optimize O2 migration-based RRAMdevices.ThisrulecanactasaguideforelectrodeselectioninthefutureandmayalsoprovideaperfectexplanationfortheobservedexcellentenduranceintheTiN/TiO2/Pt(>2106cycles)[65],Hf/HfO2/TiN(>1010cycles)[97]andTa/TaOx/Pt(>1010cycles)[312]

structures

Ithasalsobeendemonstratedthatthealloyingofelectrodescanenhancetheswitchingstability,retentionpropertyandeventhermal stability of RRAM cells [29,206,207,313] For instance,based on a Pt (50nm)/CuxTe1x(50nm)/Al2O3(3nm)/n-Si struc-ture,Goux et al [29] have systematically studiedthe effect ofalloying of the electrochemically active electrode on the RSpropertyofanECMcell.Threemaincompositionregionscanbedistinguished, associated withthe typical I–Vcurves shown in

Fig.27c.Fortheregion(1)x<0.5,veryanunstableCufilamentisgeneratedduringthesetprocess,asimpliedbytheverylargeRLRSandtheverysmallIRESETinFig.27aandb,respectively.ThelowerxvaluesusuallyresultinvolatileRSphenomenon(Fig.27c),whichindicatesthattheCufilamentformedduringthesetprocesswillspontaneouslyruptureaftertheremovaloftheexternalvoltage.Fortheregion(3)x>0.7,astableCufilamentisformedduringthesetprocess,andduringthesubsequentresetprocess,averylargeresetcurrentisrequiredtorupturetheexistingCufilament(i.e.,

Ireset IcompshowninFig.27b),whichissimilartotheRSbehavior

ofthedevicewithapureCuelectrode.Interestingly,attractiveRSbehavior with IresetIcomp is observed for the region (2)0.5<x<0.7 Remarkable resistance uniformity during >103

switching cycles and excellent retention property of >104s at

858C have also been demonstrated in this region The abovephenomenacanbeexplainedbasedonthefactthatintheCuxTe1x,theenergybarrierforCudiffusiondecreaseswithincreasingCuconcentrationduetolargerbondingenergyofCu–TecomparedtoCu–Cu[29].Fortheregion(1)x<0.5,thebetterstabilityoftheCuTeandTephasesprobablydoesnotallowtheformationofanystable Cuphase, so theCu atomsinjected intothe Al2O3 layerduringthesetprocesswillreadilydiffusebacktotheelectrodeaftertheremovaloftheexternalvoltage,leadingtothevolatileRSproperty (Fig 27d) In contrast,for region (3) x>0.7, theeasyinjectionofCuatomsintotheAl2O3layerusually resultsintheovergrowth of the Cu filament during the set process Conse-quently, a very large reset current (Ireset Icomp)is needed torupture theexisting strong Cu filament duringthe subsequentresetprocess.Finally,thewell-controlledCufilamentformationinregion(2)0.5<x<0.7maybeexplainedbythestabilityoftheCu-deficient Cu2dTe phases in this composition range, which cansuppress the spontaneous rupture and overgrowth of the Cufilamentandconsequentlyenhancetheswitchinguniformity.The decoration of electrodes with nanoparticles is anotherpowerfulway toenhanceRRAM performance[30,233,314,315].For example, Shi et al [30] have significantly improved theswitchinguniformityofAg/ZnO(100nm)/Ptdevicesbydecorat-ingthePtelectrodeswithAgparticles,asshowninFig.28.TheAgparticleswerefabricatedbyPLDunderapureAratmosphereof

10Pa By comparing AFM images of the Pt electrode with AgparticlesinFig.28bandthebarePtelectrodeinFig.28a,onecanclearlyseearandomdistributionofAgparticleswithadiameterof

20nmandaparticledensityof120mm2onthecommercialPt

28

cuu duong than cong com

substrate with an average grain size of 100nm Typical I–V

characters of the Ag/ZnO/Pt devices with the commercial Pt

substrate and the Pt substrate with Ag particles are shown in

Fig.28c and d,respectively Toprovide moreclearinformation

abouttheeffectofAgnanoparticlesontheRS,statisticalanalyses

ontheRLRS/RHRSandVset/Vresetwereperformedanddisplayedin

Fig.28eandf,respectively.Basedonthesefigures,onecanseethat,

forthedevicewithoutAgparticles,bothVsetandVresetscatterina

widerangefrom0.1to2Vandfrom0.2to0.8V,respectively

Meanwhile,RLRSvariesfrom10to102VandRHRSvariesfrom

106to108V.Incontrast,thedevicewithAgparticlesshowsa

muchnarrowerdistributionofVset/Vreset,i.e.,from0.1to0.23Vfor

Vset and 0.13 to 0.23V for Vreset Correspondingly, RLRS is

between 10 and 20Vand RHRS is between 0.8106 and

3107V.Hence,asignificantimprovementoftheRSpropertyis

clearlyrevealedinthedevicewiththeAgparticles.Thisisbecause

theAgparticlescanactasseedsforthegrowthoftheCFs,thus

significantlyreducingthedispersionandrearrangementoftheCFs

duringrepeatedswitchingcycles

4.2.3 Interfaceengineering

Interface engineering mainly means enhancing the

perfor-mancesofRRAMdevicesbyintroducinganadditionalthinlayerat

eitherorbothofthetwoelectrode/storagemediuminterfaces.The

commonadditionalthinlayersareoxidessuchasAlOx[31]and

SiOx[316].Themostobviousroleplayedbyanadditionalthinlayer

is a seriesresistor[31,316,317] For example,Chenet al [316]

observedareductionoftheoperatingcurrentinTiN/V:SiO2(17nm)/

Ptafterinsertinga3nma-SilayerattheTiN/V:SiO2interface.TheyfoundthattheconductionmechanismoftheLRSchangedfromOhmic conduction followed by Poole–Frenkelemission in theTiN/V:SiO2/PtdevicetoOhmicconductionfollowedbySchottkyemission in the TiN/a-Si/V:SiO2/Pt device Consequently,they suggested that the reduction of the operation current ismainlycausedbytheseriesresistanceoftheSiO2layerformedbyoxidationofthea-Silayerduringthesetprocess.Choetal.[31]

observed an enlargement of the memory window in an Al/PI:PCBM(20nm)/Aldevice bycarefullycontrollingthethick-nessofanadditionalAlOxlayeratthePI:PCBM/Alinterface.ThedevicelayoutisshowninFig.29a.TheadditionalAlOxlayerwascreatedbyO2plasmatreatment,anditsthicknessincreaseswiththetotalO2plasmatreatmenttime(Fig.29b).Fig.29cshowsthetypical I–V characteristics of the Al/PI:PCBM/Al devices withdifferentlengthsofO2plasmatreatmenttime,demonstratingthereduction of operating current with increasing AlOx layerthickness.TheONandOFFresistancesand theON/OFFratioas

afunctionoftheO2plasmatreatmenttimeareshowninFig.29d.OnecanseethatboththeONandOFFresistancevaluesgraduallyincreasewiththeO2plasmatreatmenttime.Furthermore,whencomparedwiththeONresistance,arelativelylargeincreaseinOFFresistancecontributestoahigherON/OFFratiointhedeviceswiththeO2plasmatreatment.TheseresultsindicatethattheadditionalAlOxlayerservesasaseriesresistorandgreatlyaffectstheinitialOFF resistance The OFF resistance does not seem to increase

Fig 27 R LRS (a) and I RESET (b) as a function of x in Cu x Te 1x , both extracted after set switching using I comp = 100mA (full circles) or I comp = 5mA (empty squares) (c) Typical I–V

cuu duong than cong com