Character of the reaction between molecular hydrog

1CharacterofthereactionbetweenmolecularhydrogenandsilicondanglingbondinamorphousSiO2F.Messina*,M.CannasDipartimentodiScienzeFisicheedAstronomiche-UniversitàdiPalermo.ViaArchirafi36,I-90123Palermo,Italy.*email:[e-mail]:ReactionbetweenH2andE’centerinsilica.CORRESPONDINGAUTHORFOOTNOTE:emailaddress:[e-mail];telephone:[phone];fax:[phone]ABSTRACT:ThepassivationbydiffusingH2ofsilicondanglingbonddefects(E’centers,O3≡Si•)inducedbylaserirradiationinamorphousSiO2(silica),isinvestigatedinsituatseveraltemperatures.ItisfoundthatthereactionbetweenE’centerandH2requiresanactivationenergyof0.38eV,andthatitskineticsisnotdiffusion-limited.Theresultsarecomparedwithpreviousfindingsontheotherfundamentalparamagneticpointdefectinsilica,thenonbridgingoxygenholecenter,whichfeaturescompletelydifferentreactionpropertieswithH2.Besides,acomparisonisproposedwithliteraturedataonthereactionpropertiesofsurfaceE’centers,ofE’centersembeddedinsilicafilms,andwiththeoreticalcalculations.Inparticular,thecloseagreementwiththereactionpropertiesofsurfaceE’centerswithH2leadstoconcludethatthebulkandsurfaceE’varietiesareindistinguishablefromtheirreactionpropertieswithmolecularhydrogen.21.INTRODUCTIONAmorphoussilica(a-SiO2)isamaterialofmajorscientificinterestforitsphysicalcharacteristicspeculiaroftheglassystateandforthemanytechnologicalapplications,particularlyinopticsandmicroelectronics.Generationofpointdefects,inducedbylaserorionizingradiation,limitstheapplicativeperformanceofthematerial,causinginparticularthereductionofitsnativehighopticaltransparencyintheultraviolet(UV).1Thestabilityofpointdefectsina-SiO2isoftenconditionedbymobilespecieslikehydrogenandoxygen,abletodiffuseinthematrixinawidetemperaturerange.1-5Themigrationofmobilespeciesinamorphoussolidsandtheirreactivitywithpointdefectsaretimelyresearchissuesmotivatedbybothafundamentalandanapplicativeinterest.4-7Inparticular,diffusionofhydrogeninSiO2hasbeenwidelydiscussedduetotheubiquitouspresenceofthisimpurityinthematerial.1-4Inthiscontext,oneofthebasicprocessesisthereactionbetweenH2andthesilicondanglingbond(O3≡Si•)defectinsilica,knowninthespecializedliteratureasE’center:1-2,8-17E’+H2→≡Si-H+H0(1)TheE’centerisaparamagneticdefectvirtuallyfoundineveryirradiateda-SiO2specimen,anditsmicroscopicstructureconsistsinanunpairedsp3electronlocalizedonathree-foldcoordinatedSiatom.Thisdefectfeaturesawideabsorptionbandpeakedat5.8eVwhichusuallydominatestheUVopticalabsorption(OA)profileoftheirradiatedmaterial.1,17InthepresenceofH2,reaction(1)causesthepartialdecay(passivation)ofE’centersinatimescaleofafewhoursatroomtemperature.8-9,15-17Thisprocessfindsaplaceinthegeneralissueofpassivationofmanydefectsdetrimentalforapplications;otherimportantexamplesarethePbcenter,(Si3≡Si•),6,18-19commonattheSi/SiO2interfaceinmetal-oxide-semiconductor(MOS)devices,andtheoxygendanglingbond(O3≡Si-O•),2,4whichistheotherfundamentalparamagneticdefectinsilicaglass,alsoknownasNonBridgingOxygenHoleCenter(NBOHC).3ThereactionpropertiesbetweenH2andNBOHCcentersinducedbylaserirradiationhavebeenclarifiedbyrecentexperimentalinvestigations,4-5whichconclusivelyestablishedthatthereactionkineticsisdiffusion-limited.Thisfindingagreeswithwhatiscommonlyexpectedforthereactionsofdiffusingspecieswithpointdefectsinsolids;indeed,inthesesystemsthediffusionprocessisusuallysoslowtobetheactualbottleneckcontrollingthereactionrate.Intheseconditions,thecharacteristicactivationenergyofthereactioncoincideswiththatofH2diffusioninsilica,∆E(H2)∼0.4eV;2,4consistently,itwasalsoshownthatthekineticsofthereactionbetweenNBOHCandH2isconditionedbythesite-to-sitestatisticaldistributionof∆E(H2)intheglass,2knowntobeafingerprintofthedisorderedstructureofamorphoussilica.2,4Incontrast,muchlessisknownaboutthereactionpropertiesofE’center.Infact,althoughthepassivationofE’centerbyH2hasbeendiscussedbymanyexperimentalandtheoreticalworks,aimingtoestimatethekineticparametersoftheprocess,itremainsdebatedwhetherthereactionkineticsisdiffusion-orreaction-limited.9-16Ontheonehand,severaltheoreticalstudieshaveproposedthereactiontorequireanactivationenergyofafewtenthofeV.10,12-14Ontheotherhand,untilnowdetailedexperimentalkineticstudieshavebeencarriedoutonlyforsurfaceE’centers,11andforE’centersinthinsilicafilms,8andeventhesetwoworksreportdisagreeingresults:themeasuredratesofreaction(1)atT=300Kdifferbytwoordersofmagnitude,whiletheactivationenergiesare0.4eVand0.3eVrespectively.8,11Mostimportant,scantyexperimentaldataareavailableforE’centersinbulksilica;indeed,whilethepassivationofE’centersonthesurface(orembeddedinafilm)canbedirectlyinvestigatedbyexposingthesampletogaseousH2,inthecaseofthebulkE’centerthetaskofstudyingexperimentallythereactionbecomesmoredifficult,sincetheconcentrationofavailableH2isnomoreanexternalcontrollableparameter,andthediffusionofH2inthebulkSiO2matrixbecomesanecessarysteptobringthereagentsincontact.Moreover,itisneededtomeasureinsituthekineticsofthetransientdefectsintemperature-controlledexperiments.Forthesereasons,evenifprocess