review in electroless electrochemical etching

Siliconnanostructuresfromelectrolesselectrochemicaletching

KurtW.Kolasinski

DepartmentofChemistry,UniversityofVirginia,P.O.Box400319,McCormickRoad,Charlottesville,VA22904-4319,UnitedStates

Abstract

RecentadvancesintheproductionofSinanostructuresfromelectrolessetchingarereviewed,includingstainetching,metal-assistedetchingandchemicalvapouretching.Abriefreviewoftheexplosioninapplicationsofporoussiliconoverthepast18monthsisalsogiven.Thestainfilmthatresultsfromtheetchingof(poly-orsingle-)crystallineSiiscomposedofaporousnetworkofnanocrystallinesilicon.Fewmechanisticstudiesofelectrolessetchinghavebeenperformed,butthemoreextensivelystudiedanodicetchingofsiliconinfluoridesolutionsprovidesmanycluesastohowporousfilmsareformed.Intriguingrecentresultshaveshownthatcontroloverthepropertiesofthefilmcanbeobtainedbyexercisingcontroloverthecompositionoftheetchant.Ó2006ElsevierLtd.Allrightsreserved.

Keywords:Stainetching;Poroussilicon;Nanostructure;HF;Nanowire

1.Introduction

Oneofthegreatchallengesofsolid-statephysicsistocontrolthebandstructureofphononsandelectronstoachievedesiredproperties:raisingtheTcandcriticalcur-rentofsuperconductors,producingsemiconductorswithwell-definedbandgaps,increasingtheefficiencyofthermo-electriccooling,alteringthecatalyticactivityofmetals,etc.Surfacescienceplaysafundamentalroleinaddressingthischallengebothinthatitprovidesameansforunderstand-ingtheunderlyinggrowthandetchingphenomena[1]thataffectthedesiredpropertyandinthatitprovidesasyn-theticmethodforproducingthematerialsthatexhibittheseproperties.Forexample,surfacetexturingisusedtoalignthegrainsofyttriumbariumcopperoxide(YBCO)insec-ond-generationsuperconductingwires.CVD[2–4]andMBE[5–7]areusedtoengineerthebandgapsofsemicon-ductingmultilayerstructures[6,8–12]toproduceanarrayofoptoelectronicdevices.DispersinggoldinnanoscaleclustersacrossanoxidesurfacetransformsthismostnobleofmetalsintooneofthefinestcatalystforCOoxidation[13].

E-mailaddress:Kolasinski@virginia.edu

1359-0286/$-seefrontmatterÓ2006ElsevierLtd.Allrightsreserved.doi:10.1016/j.cossms.2006.03.004

Intheworldofnanotechnology,therearetwograndschemesformakingstructuresanddevices:thetop-downandthebottom-upapproaches.Inaslightlyover-general-izedform,wecandefinethesetwoapproachesthusly:Thetop-downapproachrulestheroostintheproductionofintegratedcircuits.Itistheengineeringapproachthathasledustotheamazinglysuccessfulworldof$4billionfabs.Inthisworld,perfectionofindividualprocessingstepsissought.Onetriestokeeptheindividualstepssimplebutthecombinationisverycomplicated.Hundredsofstepsmaybeinvolved.Noamountofprocessingcomplex-ityistoogreataslongastheprocesscanbeturnedintoabatchjobthatcanberepeatedmillionsoftimesatlowcost.Makingalayerconsistsof,forinstance,takingaperfectlycleansiliconwaferinacleanroomenvironment,spincoat-ingapolymerresistontoit,bringinganintricatelydesignedmaskintocloseproximityoftheresist,exposingtheresistwithashortwavelengthphotonsources,chemicallytreatedtheexposedresisttoremoveeitherexposedorunexposedregionsofresist,applyingamaterialcomponenttothepat-ternthathasbeencreated,removingunwantedmaterialfromregionsoutsideofthepattern,rinse,lather,repeatforasmanyprocessingstepsasarerequired.Evensome-thingasseeminglysimpleas‘‘makingacleansiliconsur-face’’maytaketensofprocessingsteps.

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Inthebottom-upscenario,theprocessingstepsareessentiallyinfinitelycomplexbutrequiredlittleinterven-tiononthescientist’spart.Theideaistousechemistrytoperformerrorcheckingandcorrectingprocessesthatwillguidethemselvestotheendproduct:thedesiredstruc-ture.Theidealisinspiredbybiomimeticprocesses:selfassemblyandselforganization–theexploitationofnon-covalentinteractionstodirecttheformationofstructures.ItisinthisrealmthatwearetryingtocreateSinanostruc-turesbyelectrolessetching.Justhowfarcanwepushchemistrywithminimalprocessinterventiontoperformthetaskswewantsothattheprocessarrivesatarbitrarilycomplexstructures?

Specifictosilicon,turningbulkSiintonanocrystals[14,15]ornanocrystallinenetworks[16,17]changesSiintoabrilliantemitterinthevisiblebecauseoftheeffectsofquantumconfinement.Hencetherehasbeengreatinterestincontrollingandunderstandingtheformationofnano-crystallinesilicon[18].Wecanunderstandhowquantumconfinementincreasesthebandgapofananocrystallinesemiconductorbyanalogytotheparticleintheboxprob-lem.Asmallerboxleadstoagreaterspacingbetweenenergylevels,thusalargerbandgap.Poroussilicon(por-Si)isthemostintensivelystudiedvariantofnanocrystallinesilicon(nc-Si)andseveralreviewshavebeenpublishedregardingtheformation[19–21],properties[17,22,23],sur-facechemistry[24],andphotoluminescence(PL)[25–29]ofpor-Si.AmazingcontroloverporemorphologyhasbeendemonstratedbyGo¨seleandco-workers[30].

Electrochemicaletchingdifferentiatesitselffromchemi-caletchinginthatchargetransferisinvolvedintheformer.Hydroxideexhibitsbothchemicalandelectrochemicalpathways,whereasetchinginacidicfluorideisexclusivelyelectrochemical[1].Chemicaletchingofthetypeexten-sivelystudied,forinstance,byChabalandco-workerscanbeusedtoproducenearlyperfectlyflatandhydro-gen-terminatedSisurfaces[31–37].

Therearefourelectrochemicalroutestopor-Siallofwhichoccurinacidicfluoridesolutions:anodicetching,photoelectrochemicaletching,laser-assistedetchingandelectrolessetching.Theboundariesarenotcompletelydis-tinctandcombinedprocessesareknown.AnodicetchinginvolvestheattachmentofSi(theworkingelectrode)andacounterelectrode(usuallyPt)toapowersupply,whichisusedtoregulatethevoltageontheSicrystal.Theelectro-chemistrythatoccursiscontrolledbythevoltageandsolu-tioncomposition.Thisworksfineforp-typeSibutforn-typeSi,inadditiontoconnectingtheSiandcounterelec-trodetothepowersupply,freecarriersneedtobemadeavailablebyilluminationoftheSielectrode,hencethetermphotoelectrochemicaletching.Itispossibletodispensewiththepowersupplyandexternalcounterelectrode.ByirradiatingasmallspotonaSiwafer,freecarriersarepro-ducedandbandbendingisusedtoseparateholesfromelectrons.Inn-typeSi,holesareforcedtothesurfaceoftheirradiatedareaandpor-Siformsthere.Inp-typeSi,holesareforcedtotheunirradiatedareaandthatiswhere

por-Siformationoccurs[38].Mechanistically,thechemicaltransformationsthatoccurinthesethreetypesofetchingareverymuchthesame[*39].Inelectrolessetching,elec-trochemistryoccursspontaneouslywithouttheinterven-tionofapowersupplyorphotonsource.Threetypesofelectrolessetchingarereviewedhere:stainetching,metal-assistedetchingandchemicalvapouretching.

Stainetchingistheetchingthatresultsfromasolutioncomposedoffluorideandanoxidant.Inchemicalvapouretching,thevapourofsuchasolution,ratherthanthesolu-tionitselfisincontactwiththeSi.Inmetal-assistedetchingdepositedmetalparticlesarealsoinvolved.ElectrolessetchingofSitoformpor-Siisasimpleprocessthatrequirestheattachmentofnoelectrodesandcanbeperformedonobjectsofarbitraryshapeandsize.Nonetheless,theforma-tionofpor-Siviaelectrolessetchinghasreceivedmuchlesssystematicinvestigation[17,40].ThepreponderanceofworkhasconcentratedonuseofHFincombinationwithanitroxyoxidant,typicallyHNO3orNaNO2.Itwillbeshownthatthishasseverelylimitedtherangeoffilmsandtheirpropertiesandthattheuseofawiderrangeoffluoridecarriersandoxidantscanleadtogreatercontrolovermorphologyandproperties.

2.RecentadvancesinporoussiliconstructuresanddevicesWithinthepastyear,therehasbeenagreatproliferationofstudiesinvolvingpor-Siindevicestructures.Manyoftheseadvancesarechronicledintheproceedingsofthe4thInternationalConferenceonPorousSemiconductors–ScienceandTechnology(PSST-2004),whichwaspub-lishedin2005[41].Specificareasinwhichpor-Siisbeingimplementedtechnologicallyincludeoptoelectronics,sen-sors,massspectrometry,nanocrystalproduction,drugdelivery,biomaterials,fuelcellsandphotovoltaics.

EversinceCanham’sobservationofvisiblephotolumi-nescence[16]therehasbeengreatinterestinthepossibleapplicationofpor-Siinoptoelectronics[42].Theimplemen-tationofpor-Siinindustriallyrelevantprocesseshasproventobechallengingbutprogresscontinuestowardstheimple-mentationofSiphotonics[43]andtheoreticalunderstand-ingoftheelectronicstatesinnanocrystallineSi(nc-Si)continuestoadvance[44–46].Theproceedingsofthe2004SpringMeetingoftheEuropeanMaterialsResearchSoci-ety(E-MRS)titled‘‘Si-basedphotonics:towardstruemonolithicintegration’’makeforfascinatingreadingintotheadvancesmadeinpor-Si,nanocrystallineSiandSimul-tilayerstructuresforuseinwaveguides,photoniccrystals,opticalfilters,reflectors,photodetectors,lightemittingdevices,sensorsandevenlab-on-chipapplications.Tune-ablephotonicbandgapmaterialscanbemadefrompor-Sithatcanbethermallyorelectricallymodulated[47,48].Linnrosandco-workers[49,50]havemadegreatstridesinthefabricationofforestsoffree-standingSiquantumdottowers.Thehaveobserved[**51]thephotolumines-cencefromasingledotandfoundalinewidthasnarrowas2meVat35K,provingtheatomiclikenatureofthe

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emissionfromSinanocrystalsresultingfromquantumcon-finement.Thestudyofopticalgainandstimulatedemis-sioninnanocrystallineSicontinuestoattractmuchattention[52–55].Cloutieretal.[*56]havereportedthresh-oldbehaviour,opticalgain,longitudinalcavitymodesandlinewidthnarrowing,alongwithacollimatedfar-fieldpat-tern,allofwhichareindicativeofamplificationandstim-ulatedemissionat1278nmfromSipatternedwithanorderedarrayofnanopores.Yamamotoetal.[57]havereportedSi-basedallopticalswitchonthebasisofcircularpolarizationretention.TheobservationofballisticelectrontransportbyKoshidaandco-workers[58,59]hasledtothedevelopmentofasolid-statelightemittingdeviceinwhichorganicdyesareexcitedbyballisticelectronsproducedinpor-Si.DopingErandYbintopor-Sicanproduceawhitelightemitter[60].Byalteringthelayerstructureofpor-Sifilms,theiropticalbirefringencecanbetuned[61,62].Zenertunnellingoflighthasbeenobservedinopticalsuperlat-ticescomposedofpor-Si[63].

Theapplicationofpor-Sitosensortechnologyappearstobeparticularlypromising.Anammoniasensorbasedonpor-Si[]isnowcommerciallyavailable.Theversatil-ityofpor-Sibasedsensorsallowsforstructurestobemadethatcanexhibitlargeenhancementsinthesensitivitycom-paredto,forexample,conventionalsurfaceplasmonreso-nancedevices[*65].Ultrasonicelectronemissionenablesathree-dimensional(3D)imagesensor[66].Anumberofsensorapplicationshavebeenreportedinthepastyear:fororganicsolvents[67],anelectrical[68]orinterferomet-ric[69]sensorfordetectionofDNAhybridization,ahumiditysensorforrespirationmonitoring[70],aresis-tancebasedhydrogensensor[71],anopticalsensorforpes-ticidesinsolution[72],abiosensorforoligonucleotidesbasedoncapacitancemeasurements[73],aconductometricgassensorwithsub-ppmsensitivities[74],andachemilumi-nescencedetectorforbacteria[75].Anumberofdifferentapplicationsregardingbiosensorshavebeenreported[76–79].Bycombininglayerswithdifferentporesizes,adevicecanbemadethatbothseparatesbiomolecules,e.g.sucroseandbovineserumalbumin,andprovidesfordetectionviainterferometricmethods[*80].

Arecentadvanceinmatrixassistedlaserdesorption/ioni-zation(MALDI)istheuseofpor-Sisubstratesastheimmo-bilizationmatrix[81,82].ThisvariantofMALDIiscalleddesorption/ionizationonporoussiliconmassspectrometry(DIOS-MS)[83].AdenseforestofSinanowires[84],tips[85]orafilmwithacolumn/voidstructure[70]mayoffersomeadvantages,particularlywithapplicationtoproteomics.PorousSihasalsoappearedinanumberofenergyrelatedtechnologies.Poroussiliconhasbeenofgrowinginteresttosolartechnologybecauseofitspotentialtoincreasesolarcellefficiencythroughreducedreflectivityandrecombinationlosses,andexpandedspectralresponse[86].Aparticularlyinterestingdevelopmentinthisareaistheconstructionof3Dp–njunctionstructureforbetavol-taicsandphotovoltaicsbyFauchetandco-workers[**87].The3Dstructureexhibitsanorderofmagnitudeincrease

inefficiencycomparedtoaplanarstructure.Thegeometryofthese3Dporousdiodesshouldprovidesignificantenhancementintheperformanceofphotodetectorsandsolarcells.PorousSifilmshavebeeninvestigatedforanti-reflectioncoatings[88,].Anantireflectioncoatingofsil-iconnanowiresformedbymetal-assistedetchingexhibitsverylowreflectivitybutstillhasalowerefficiencythananuncoatedphotovoltaiccell,demonstratingthatprogressinimprovingcarrierlifetimesmuststillbemade[*90].MicroporesformedbychlorineplasmaetchingratherthanelectrochemistrycanalsobeusedtolowerthereflectivityofSisurfaces[91].Asacrificialpor-Silayerhasbeenusedtoimproveelectricalperformanceinsolarcellsbygetteringimpurities[92–95].CVDwasusedtodeposita20-lmthickc-Silayerontopofafree-standingpor-Sifilmtoproduceasolarcellwith9.6%efficiency[96].AnotheradvancehasbeenannouncedatIMEC[**97].Aburiedstackofalter-natinghigh(55%)andlow(22.5%)porositylayersisusedasaBraggreflectortoincreaseabsorptioninathinepitax-iallayerbyreflectinglightthathasbeentransmittedinafirstpassbackintotheepitaxiallayer.

Nanostructuredmaterialsareofgreatinterestforenergystorageandconversiondevices[98].PorousSihasbeeninvestigatedbyGoleandco-workers[*99]asanelectrodeforbatteries.TheproductionofminiaturefuelcellsusingaSisubstratepresentstheadvantagesofserialandparallelintegration.Micro-fuelcellshavebeenbuiltwithapor-Silayersupportinganelectrodeandprovidingchannelsforfuelflow[100],includingadirectethanolfuelcell[101],whichisdepictedinFig.1.PichonatandGauthier–Manuel[*102]havedevelopedaproton-conductingmembraneconsistingofpor-Siontowhichmoleculeswithacidgroupsaregrafted.Themembranescanbeoptimizednotonlybyadjustingthecharacteristicsofthepor-Sifilm(poresizeandstructure)butalsobychangingthenatureofthegraftedmolecules.Biologicalapplicationsofporoussiliconrelatenotonlytosensingbutalsotodrugdelivery,studiesofcell/surfaceinteractionsandbiomaterials.Por-Siisparticularlyinter-estinginabiologicalcontextbecauseitisbothbiocompat-ible[103]andbioresorbable[104,105].TheincorporationofSiintohydroxyapatitehasbeenstudiedinthecontextofdevelopingartificialbone[106]becauseitenhancesitsbiocompatibility.Sailor’sgroup[107]hasprobedhowthesurfacechemistryandporestructurecanbeoptimizedbothfortheuptakeandthereleaseofthesteroiddexametha-sone.Salonenandco-workershaveinvestigatedcarbonizedmesoporousSiparticlesfororaldosingofdrugs[108,109].Fonashandco-workers[70]haveshownpromotedattach-ment,differentiation,andproliferationofprokaryoticandeukaryoticcellsonpor-Siwithacolumn/voidstructureaswellaspreferentialadhesionofFL83Bhepatocytestopor-Sirelativetoborosilicateglass.3.Stainetching

RobinsandSchwartz[110–112]andTurner[*113]wereamongthefirsttostudysystematicallytheetchingofSiin

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Fuel Inlet AnodeMicro Capillaries (Fuel Channel) Contact (-) Contact (+) Platinum catalyst Nafion (PEM) Air Channels CathodeÓElsevier2004Fuel Chamber (Ethanol + Water) Fig.1.Cross-sectionalviewoftheporoussiliconbasedmicro-directethanolfuelcell(DEFC)stack.ThemacroporousSilayerprovideschannelsforcontactofairandfuelwiththeelectrodes.ReprintedwithpermissionfromS.Aravamudhan,A.R.A.Rahman,S.Bhansali,Sens.ActuatorsA2005,123–124,497.fluorideplusoxidantsolutions.RobinsandSchwartzcon-centratedontheelectropolishingregimeinwhichthesili-conisetchedisotropicallyleavingamoreorlesssmoothsurfaceandnoporousfilm.Turnerwasthefirsttostudystainetchinginwhichapor-Sifilmisproduced.Thisfol-lowedthediscoveryofpor-Siformationviaanelectro-chemicalroutebytheUhlirs[114].Sometimeelapsedbeforeitwasrealizedthatpor-Sifilmsproducedeitherelec-trochemically[16]orbystainetching[115,116]canproduceluminescentpor-Sifilmsaslongascrystallineorpolycrys-tallinesubstratesareused.VisibleluminescenceisnotobservedwhenamorphousSiisstainetched[117].

Turner[*113]proposedthatstainetchingisactuallyelectrochemicalinitsaction,thatis,thereareanodicandcathodicsitesonthesurfaceofthesemiconductorwithlocalcellcurrentsflowingbetweenthem.Thisassertionissupportedbystructuralstudies,suchasthoseofBealeetal.[118]andSchoisswohletal.[119],inwhichitwasreportedthatthestructureofanodicallyandstainetchedpor-Siissimilarand,therefore,theirformationmecha-nismsmustsharemuchincommon.Sigoesintosolutionattheanodicsiteswhiletheoxidantisreducedatthecatho-dicareas.Iftheetchingprocessisnon-preferentialandmaterialisremoveduniformly,anygivenareaonthesur-facecontinuallyalternatesbetweenbeinganodicandcathodic.Whenonespotisanodicmuchmorethanitiscathodicanetchpitwillform.Conversely,hillocksareformedonareasthatarecathodicmorethantheyareano-dic.Fromsuchanisotropy,poresdevelop.

Semiconductoretchingoccursbyoxidationofthesemi-conductorfollowedbyremovalofsurfaceatoms.However,oxidationisachemicallyambiguouswordandthishasledtosomemechanisticconfusion.Oftenetchingissaidtooccurbytheformationofanoxidefollowedbychemicaldissolutionoftheoxidelayer.ThisistrueofSietchingintheelectropolishingregime,(+0.7Vwithrespecttothenormalhydrogenelectrode[120]),whereanoxidegrows,itisremovedbyHF(aq)viachemicaletching[121–123]

andnopor-Siisformed.Butisthistrueinthepor-Sifor-mationregime?Alternatively,forpor-Sitoform,istherel-evantoxidationsteptheincreaseintheSioxidationstatetoamorepositivevaluewithouttheformationofanoxide?Itisnowknownthatpor-Siformationwhetherinitiatedelectrochemically,photoelectrochemicallyorbylaserirra-diation,isinitiatedbytheformationofvalencebandholesatthesiliconsurface.Anumberofmodelshavebeenpro-posedtoexplainetchingintheseregimesincludingthoseofKooijandVanmaekelburgh[*124];Gerischer,andcowork-´[127];LehmannandGoers[*125,126];KangandJorne¨sele

[*128];andKolasinski[*39].Turnerproposedthefollow-ingreactionsforetchantscomposedofHNO3,HFandH2O:

Anode:Siþ2H2Oþmhþ

!SiO2þ4Hþþð4ÀmÞeÀð1Þ

ð2ÞSiO2þ6HF!H2SiF6þ2H2O

Cathode:HNO3þ3Hþ

!NOþ2H2Oþ3hþð3ÞOverall:3Siþ4HNO3þ18HF

!3H2SiF6þ4NOþ8H2Oþ3ð4ÀmÞhþþ3ð4ÀmÞeÀ

ð4ÞThesearecompositereactionsratherthanelementarysteps.Whatisclearfromstudiestoelucidatetheelemen-tarystepsin(photo)electrochemicaletching[38,*125,129]isthatthefluoridespecieswhichcontrolthekineticsofetchingareHFandHFÀ2.

AsEq.(1)shows,holeproductioninthevalencebandisagainacrucialstepintheinitiationoftheetchingreaction.Here,nistheaveragenumberofholesrequiredtodissoci-ateoneSiatom.SinceTurner’stimethischemicalschemehasbeenacceptedalmostwithoutfurthercomment,eventhoughnoclearevidencehasbeenpresentedthatSiO2isformedasanintermediate[130]andonemustaskwhytheformationofpor-Sifromstainetchingissomuchdif-ferentfromthecommonmechanismthatdescribesanodic,

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photoelectrochemicalandlaser-assistedetchinginfluoridesolutions.InTurner’sreactionscheme,NOistheonlygasevolved.Ithasbeensuggested[131]thatNO+istheactivenitrogenspeciesorthatNOÀandrateofoxide2catalyzesthereaction[40].Theroleformationmustbecrucialtopor-Siformation.SiO2isetchedrapidlyandisotropicallybyacidicfluoridesolutions[132–134].IfauniformfilmofSiO2resultsfromtheinteractionoftheoxidantwiththesurface,thenthisfilmwillberemoveduniformlybytheactionofthefluoridecomponent.Electropolishingratherthanpor-Siformationensues.IfSiO2isformeddur-ingpor-Siformationitiscrucial,therefore,thattheSiO2beformedheterogeneouslyonthesurface,Ineffect,itmustonlyformatthebottomofpitssuchthatthesepitsdevelopintopores.Itsformationonsidewallsandhillocksmustsomehowbesuppressedtoavoidtheisotropicetchingofporesthatwouldleadtotheirremoval.IfSiO2formationisimportantinstainetching,thentheroleoftheoxidantistosomehowfacilitatetheformationofaninhomoge-neousoxidelayer.Onenecessaryconditionfortheforma-tionofthislayeristhattherateofoxidationmustnotexceedthecapacityofthefluoridesolutiontoetchtheoxide;otherwise,theoxidecoveragewillgrowuntilitcov-ersthesurfaceandisotropicetchingwillresult.

Itseemsunlikelythatasurfaceoxideplaysasignificantroleinpor-Siformationandthisideaisfurtherbolsteredbyexaminingetchinginalkalinesolutions[1].Etchingwithhydroxidedoesnotleadtopor-Siformation.PorousSiformsinafluoridesolutionbecausetheetchingreactionisself-limiting,whereasetchingwithOHÀisnotsimilarlyconstrained.ConsiderthemodelsofLehmannandGo¨sele[*128]andFrohnhoffetal.[135].Sietchinginacidicfluo-ridesolutionsisanelectrochemicalprocessandrespondstotheelectronicstructureoftheSi.QuantumconfinementeffectswidenthebandgapwhenSinanostructuresdropbelow$5nminsize[136].Thewallsoftheporesthembecomeeffectivelypassivatedbecausethereisadepletionofholeswithintheconfinedstructuresandholesarerequiredtoinitiatetheetchingsequence.Holesareinsteaddirectedtothebottomsofpores,whichareconnectedtounconfinedbulkSiandetchingproceedsthere.Hydroxidedoesnotexperienceasimilarquantumconfinementrelatedconstraintbecauseitcanetchviaachemicalpathway.Por-ousSiisnotstableinOHÀ(aq)andisquiteefficientlyremovedbyit.Intheabsenceofaself-limitingconstraintonoxideformationandsincethechemicaldissolutionofoxidebyHF(aq)isisotropic,thereisnodrivingforcetoinstigatetheformationofquantum-confinedstructuresastheresultofoxideformation.

Consequently,thewaytolookattheroleoftheoxidantistothinkofitpurelyasanelectrochemicaloxidantratherthanaproducerofsiliconoxides.ConsideringEq.(3)weseethatthecrucialroleoftheoxidantistoinjectholesintothevalenceband.Inthismannertheoxidant,ormorepre-ciselyitselectrochemicalpotential,takesontheroleofeitherthevoltageinelectrochemicaletchingorthephotonenergyinlaser-assistedetching.Therefore,crucialproper-

tiesoftheoxidantwillbeitselectrochemicalpotentialandtherateatwhichitcantransferchargewiththeSisur-face.Inordertoinjectholesintothevalenceband,anelec-tronacceptorleveloftheoxidantmustlieatorbelowthevalencebandmaximum(VBM),asshowninFig.2.Hence,theelectrochemicalpotentialoftheoxidantmustbesuffi-cientlypositive.Furthermore,byidentifyingtheroleoftheoxidantininitiatingetchingandtheimportanceofquantumconfinementtocreateself-limitingchargeinjec-tion,wecannowidentifytheelectrochemicalpotentialoftheoxidantasacontrolparameterthatcanbeusedtoinfluencepor-Siformation.

Kolasinski[*39]hasshownthattheholeisinjectedintoabulkbandandthatholeinjectiondirectlyintotheSi–Hbondisenergeticallyimpossibleundertheconditionsofmostelectrochemical,photoelectrochemical,laser-assistedetchingorstainetchingexperiments.ThusmodelsofstainetchinginvolvingholeinjectionintotheSi–Hbond[137]arenotfeasible.ThepresenceofavalencebandholechangestheeffectivestickingcoefficientofFÀ(aq)from65·10À11to$1.HoleinjectionistheswitchthatturnsonSietchingactivityinacidicfluoridesolutionswhereasOHÀistheinitiatorofreactivityinsolutionsnearandaboveneutralpH[*39].

NahidiandKolasinski[**138]haveusedanunder-standingofhowstainetchingisinitiatedtocontrolthephotoluminescencespectrumandbyimplicationthenano-crystallitesizedistribution.Theyusedoxidantswithdiffer-entelectrochemicalpotentialsE0(Fe(III),HNO3,MnO2À

foundthatthePLpeakwavelengthcorrelateswithE4)and0:amorepositivevalueofE0leadstobluerPLasshowninFig.3.

Furthermore,consistentwiththecontrollingroleoftheoxidantonthebasisofitsroleinholeinjectionintothevalencebandaccordingto(e.g.forFe(III)asoxidant)Cathode:Fe3þ!Fe2þþhþ

ð5Þ

Fig.2.HoleinjectioncanoccurviaanacceptorlevelsuchasA+,thatliesbelowtheVBMbutnotonesuchasB+thatliesaboveit.

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Fig.3.ThePLfromfilmsetchedwithFe3+(E0=0.47Vversusthestandardcalomelelectrode(SCE))peaksat696nm,comparedto654nmforHNO3(E0=0.66V)and604nmforMnOÀ4ðE0¼1:19VÞ.

theyhaveshown[**138]thatstainetchingcanproceedwithminimalbubbleformationandwithouttheneedtoin-vokesurfaceoxideformation.

Anode:Siþ6HFþhþ!H2SiF6þ4Hþþ3eÀ

ð6ÞOverall:SiþFe3þþ6HF!H2SiF6þFe2þþ4Hþþ3eÀ

ð7Þ

4.Metal-assistedetching

Dimova-Malinovskaetal.[139]demonstratedstainetchinginthepresenceofanevaporatedAllayercanpro-duceluminescentpor-Si.Kellyandco-workers[140,141]investigatedtheelectrochemistryofGalvaniccellforma-tionwhenanoblemetalisshortcircuitedtoSiinthepres-enceofanoxidantanddemonstratedthattheformationofmicroporousSiispossibleinaeratedHFandthatmacro-porousSicanbeproducedin5%HF+1%H2O2incom-binationwithpre-patterningofthesurfacewithinvertedpyramids.Metal-assistedelectrolessetchingtoformpor-SiwasdevelopedfurtherbyBohnandco-workers[*142–145]whosignificantlyimprovedtheprocessbyusingthinnerfilms.Au,PtorAu/Ptfilms(3nm6h620nm)weredepositedonaSisubstrate.EtchingwascarriedoutinanHF/H2O2/CH3OH(orCH3CH2OH)solution.DependingonthetypeofmetaldepositedandSidopingtypeanddopinglevel,por-Sifilmswithdifferentmorphol-ogiesandlight-emittingpropertieswereproducedonthetimescaleofsecondsinthedarkorunderambientlights,onbothp-andn-typeSi.

BypatterningthePtdeposit,theresultingporousfilmcanalsobepatterned.ChattopadhyayandBohn[145]haveusedaGa+focusedionbeamtodissociateanorganometal-licprecursoranddepositPtinsquareswith1.25–20lmedges.ThesquaresarecomposedofamixtureofC,Si,O,PtandGa.Etchingoccursunderandclosetothedepos-itsandresultsinahighlynon-uniformfilm.Alternatively[143],theSisubstratecanbecoveredwithanoctadecyltri-chlorosilane(OTS)monolayerpatternedusingmicro-con-tactprinting.PtisdepositedinareasnotcoveredwithOTSandpor-Sithatluminescencearound580nmuponUVilluminationisformedinthePt-coatedregions.

ThinmetalcoatingsfacilitatetheetchinginHFandH2O2,andofthemetalsinvestigated,Ptyieldsthefastestetchratesandproducespor-SiwiththemostintensePL.Gasevolutionfromthemetal-coatedareawasclearlyobserved,especiallyforPtandAu/Pd.Forthesemetalsnometaldissolutionwasobserved,incontrasttothebehaviourusingAl.Lietal.[*142]proposedareactionschemeinvolvinglocalcouplingofredoxreactionswiththemetaltoexplainthemetal-assistedetchingprocess.

Cathode:H2O2þ2Hþ!2H2Oþ2hþ

ð8Þ2Hþ2eÀ!2H2\"

ð9ÞReaction:Siþ4hþþ4HF!SiF4þ4Hþ

ð10Þ

SiF4þ2HF!H2SiF6ð11ÞOverall:SiþH2O2þ6HF!2H2OþH2SiF6þH2\"ð12ÞHoleinjectioninthiscaseisprovidedbythereactionofH2O2onthemetalparticle.TheholesareinjectedintotheSivalencebandandthendiffuseawayfromthemetalparticleexplainingwhyetchingisconfinedtothenear-par-ticlearea.DissolvedO2canalsoplaytheroleofoxidantbutleadstoetchingataverylowrate[146].

Gorostizaandco-workers[147–*149]havestudiedthedepositionofPtandNiinfluoridesolutionsontoSiwithregardtochargeexchangeandpor-Siformation.Intheabsenceofmetalions,theFermilevelofn-andp-typeSiliesclosetoE0(H+/H2)attheopencircuitpotential(OCP)asdepictedinFig.4.Thissteady-stateequilibriumcanbedescribedasadynamicequilibriumbetweentwooppositereactions:theanodiconebeingthedissolutionofthesubstrateandtheotherbeingthereductionofpro-

3002yteicoSlacimehCnaciremAÓFig.4.ExperimentalenergydiagramoftheinterfacebetweenSiandablankfluoridesolution(---)atpH2andfluoridesolution(—)inthepresenceof1mMPt2+/Pt.ReprintedwithpermissionfromP.Gorostiza,P.Allongue,R.Diaz,J.R.Morante,F.Sanz,J.Phys.Chem.B2003,107,54.

K.W.Kolasinski/CurrentOpinioninSolidStateandMaterialsScience9(2005)73–8379

tons(H2evolution)anddissolvedoxygen.Luminescentpor-SiisformedduringPtdeposition.Nobubblesareformedduringpor-Siformation.TheyinferthattheH2formedfromtheetchproducts’reactionwithwaterisreox-idizedtoprotons.TheelectronsproducedhaveenergiesclosetotheFermilevelofPtclusters.Theymayeitherbere-emittedfromthePtclustersintotheSiorrecombinewithholes.Thelatterprocessismorelikely,withtheinter-facestatesatthePt/Sicontactsactingasrecombinationcentres.NidepositsautocatalyticallyonSifromfluoridesolutionsofpH8,becausetheNi/Ni2+levellieswithintheSibandgap.DepositionisaccompaniedbyvisibleH2evolutionandnopor-Siformation.NoNidepositionorhydrogenevolutionisobservedatlowpH.

Anumberofstudieshaveappearedinthepastyearinvolvingelectrolessmetal-particle-assistedetching[150,151].Pengandco-workers(seeFig.5)haveusedetch-inginHF/AgNO3solutionstoformfilmscomposedofalignedSinanowires(SiNW)[*90,152–**155].Etchingdis-playslittlecrystallographicdependenceandcanbeper-formedoncrystallineorpolycrystallinesubstrates.Etchingfor20minat50°Ccreatesafilmapproximately10lmthick.AfteretchingAgparticlesremaininthefilm.Thefilmsexhibitverylowreflectivity,whichmakesthemattractiveforsolarcellapplications[*90].Agnanoparticlesdepositoutofsolutionontothesurfaceofthesubstrate.Theycatalyzetheetchingreaction,sinkbelowthesurfaceandleavebehindcolumnsofSinanostructures.IfonlyAgNO3isused,thenAgdendriteformationaccompaniesSiNWformation.ThesecanbeavoidedbythereplacingAgNO3afterashortperiodwithFe(NO3)3.

HF/H2O2mixtureshavebeenusedbyTsujinoandMat-sumura[156,157]toetchcylindricalandhelicalporesinc-Si.Ag,Pt,PdorCuparticlesaredepositedbyelectrolessplatingandthenexposedtothesolution.Thetopofthewaferiscoveredbyanupto3lmthickmicroporouslayer

4002HCV-yeliWÓFig.5.Across-sectionalelectronmicrographofapor-SifilmcomposedofverticalSinanowires.Thefilmwascreatedbyetchingina4.6moldmÀ3HF+0.02moldmÀ3AgNO3solutionfor60minat50°C.ReprintedwithpermissionfromK.Q.Peng,Z.P.Huang,J.Zhu,Adv.Mater.2004,16,73.

whenetchedina10:1(v:v)solutionof10%HFand30%H2O2andwithPtasthedepositedmetal.Thislayerexhib-itsvisiblePL.Themicroporouslayerisonly300nmthickwhenAgisused.WhatmoststrikinglydifferentiatesAgfromotherPdandCuisthatstraightmacroporesonSi(100)orinclinedmacroporesonSi(111)existbelowthemicroporousregion.SometimesforPt,cylindricalorheli-calporesarefoundbelowthemicroporousregion.ThehelicalmacroporesarealsosometimesobservedforAg.Switchingfromcylindricaltohelicalporesisaccomplishedbychangingthesolutionconcentrationsandthewallsofthemacroporesarelinedwithmicroporoussilicon.Agpar-ticlesarefoundatthebottomsofthesemacropores,withadiametermatchingthatofthepore.Iftheetchingtimeisextendedto10h,poresasdeepas500lmand$50nmindiameterarefound.

Cruzetal.[158]havestudiesHF/H2O2/CH3CH2OHetchingwithAuorPtparticlesandfoundthattheetchdepthandfilmmorphologyrespondtodopinglevelbutnotdopingtype.Themetalfilms(1nm6h68nm)weredepositedbyvacuumsputtering.Poremorphologyalsodependsonthemetal.AuisfoundtoformamorecolumnarstructureatahigherrateasopposedtoaspongystructureforPt.Theydidobservetheformationofsomestraightmacroporesbutalwaysinthepresenceofinterconnectinglateralpores.Etchdepthwasproportionaltoetchtimeupto3hatwhichpointitstopped.Increasedtemperature(40°Cversusroomtemperature)increasestheetchrate.5.Chemicalvapouretching

Aninterestingbutnotwellunderstoodvariationonstainetchingisthatofchemicalvapouretching(CVE)[159–163].InCVEasolutionismadeupfromconcentratedHFplusconcentratedHNO3.However,insteadofdippingtheSisubstrateinthesolution,thesubstrateisheldabovethesolutionandthetemperaturesofthesolution,thesub-strateandthetimeofexposurearecontrolled.Dependingontheseparameters,eitherapor-Silayeroralayercom-posedprimarilyof(NH4)2SiF6withathinpor-Sitransitionlayerisformed.Bothlayersarephotoluminescent.Kokeretal.[1]havedemonstratedthattheluminescenceasso-ciatedwithahexafluorosilicate/por-Siinterfaceisblue-shiftedcomparedtothePLfromthepurepor-Silayer.AsimilartrendwasfoundbySaadounetal.[162].Asammo-niumhexafluorosilicateiswatersolubleandpor-Siissolu-bleinalkalinesolutions,CVEcanbeusedtoformgroovesinSi,whichisofinterestfortheuseofpor-Siinsolarcelltechnology[94,95,159,160].

Themechanismofetchinginthisregimeisnotwellunderstood.Infraredspectroscopyrevealsacombinationofhydrogenterminationandoxidationofthesurface[159,161,162].Whentoomuchcondensationoccursanddropletsformonthesurface,por-Siformationissup-pressed[163].Capillarycondensationintheporesmaybeoccurringbeforedropletsareobservedontheexternalsur-face.IfEq.(2)weretheonlywaytoremoveSifromthe

80K.W.Kolasinski/CurrentOpinioninSolidStateandMaterialsScience9(2005)73–83

surface,thentherewouldbenowaytoetchSiintheabsenceofhexafluorosilicateformation.However,theini-tialetchproduct[1,*39]isavolatilecompoundsuchasHSiF3andthisprimaryproducthydrolyzestothehexaflu-orosilicate.Ifthehydrolysisreactionisslow,thevolatileprimaryetchproductcanbelosttothegasphasebeforeformingaproductthatcanprecipitateonthesurface.6.Conclusionsandperspectives

Overthelasttwoyearstherehasbeenanexplosionintheapplicationsofporoussilicontoavarietyoftechnolog-icalareasincludingsensors,energy,biomedicaltechnology,optoelectronicsandanalyticalchemistry.Progressintheseareasappearstobeaccelerating.Anodicetchingofsiliconremainsthemostintensivelystudiedandmostcontrollablesyntheticmethodforpor-Siproduction,particularlyinthatitcanbeusedtoproducemultilayerstacksofdifferentporosityorcombinedwithlithographicpositioningofporenucleationtocreatemacroporoussilicon.Nonetheless,electrolessetchingbecauseofitssimplicityandabilitytoetcharbitrarilyshapedobjectssuchasSipillars[165,166]offerssomeadvantages.Thereisstillmuchtolearnaboutthemechanismsinvolvedinelectrolessetching,particularlyformetal-assistedetching.Canmorphologicalcontrolbefurtherenhanced?Thefilmsproducedbymetal-assistedetchingpresentamuchdifferentstructurethanconven-tionalpor-Si.Someofthestainetchedfilmsshowevidenceforsuperstructuresimposedoncombinationsofmicro-andmacroporoussilicon.Greatermechanisticunderstandinginthefutureshouldleadtogreatercontroloverthestructureandpropertiesofthefilmsproduced.Acknowledgements

ItiswithpleasurethatIacknowledgeacriticalreadingofthismanuscriptbyJosephMaurerandGiovanniZangari.IalsothankShyamAravamudhanandShekharBhansaliforprovidingFig.1,FaustoSanzforprovidingFig.4,andKuiqingPengandJingZhuforprovidingFig.5.ThisworkwassupportedbytheUniversityofVirginiaandtheNationalScienceFoundationIGERTProgramunderGrant#9972790.References

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