Section 1. Defect studies in vitreous silica and …

1 Section 1. Defect studies in vitreous silica and related materialsOptically active oxygen -de ciency- related centers inamorphous silicon dioxideLinards Skuja*Institute of Solid State Physics, University of Latvia, 8 Kengaraga Str., LV-1063 Riga, LatviaAbstractThe spectroscopic properties, structure and interconversions of optically active oxygen -de ciency- related point de-fects in vitreous silica are reviewed. These defects, the E0-centers ( oxygen vacancies with a trapped hole or 3-fold-co-ordinated silicons), di erent variants of diamagnetic `ODCs' ( oxygen -de ciency centers), and their Ge- related analogsplay a key role in the ber-optic Bragg grating writing processes. The controversy surrounding the structural models forthe Si- and Ge- related ODCs is discussed and the similarity between the bulk and surface point defects in silica isemphasized. The possible interconversion mechanisms between 2-fold-coordinated Si, neutral oxygen vacancies and E0-centers are discussed.

2 The e ects of glassy disorder have a profound e ect on Defect properties and interconversionprocesses in silica . 1998 Elsevier Science All rights IntroductionSince the discovery of the writing of Bragggratings in germanosilicate optical ber wave-guides by ultraviolet light [1], spectacular advancestowards developing this nding into useful opto-electronic devices have been made [2]. It is wellestablished that the photosensitivity is connectedto a light-induced transformation of pre-existingpoint defects in glass matrix or to a creation of newones. Particular roles are played by speci c defects,connected to partially reduced Si or Ge species, theso-called oxygen -de ciency centers (ODCs).Despite the considerable progress in under-standing of many aspects of photo-induced gratingphysics, a few basic questions still remain unan-swered. The mechanisms of photo-induced trans-formations of point defects responsible for therefractive index changes are not well understoodon the atomic-scale level.

3 First of all, the structuralmodels of Si- and Ge- related ODCs remain main reason for this situation is simple:ODCs are diamagnetic and thus invisible to elec-tron paramagnetic resonance (EPR) has provided so far most of the structuralinformation on defects in SiO2-based glasses. Thethree fundamental centers which form the basis ofthe present understanding of defects in vitreoussilica, the E0-center [3], the peroxy radical (POR)[4] and the non-bridging oxygen hole center(NBOHC) [5] have been all discovered and char-acterized by being of prime importance for applica-tions of fused silica , the optical activity of defectshas been often only of a secondary role as a toolfor Defect structure studies . The amount of thestructural information derived directly fromoptical methods is limited in most cases by theJournalofNon-CrystallineSolids239(199 8)16 seefrontmatter :S0022-3093(98)00720-0electron phononinteractionandbyinhomoge-neousbroa deninge ciency-relateddefects,withthefollowingem phasispoints: advanceswhichhaveoccurredafter1991,theti meofthelastreviewontheopticalpropertieso fdefectsinsilica[6], a ectoftheglassydisorder:inhomogeneousbroa deningandcross-correlatione ects, ciency-relatedcentersinthesimplestoxideg lass,non-dopedvitreoussilicaand asfarassimilaritiesexist ctothemorecomplexbinarySiO2:GeO2glassesh avebeenre-centlyreviewedelsewhere[7].

4 EctoftheamorphousstateThemajorityofthepa ststudiesofcolorcentersinsilicaarebasedo nasimpli edmodel,whichassumesthat,exceptfortheran domorientation, ,morerecentstudiesindicatethatthreeaddit ionalphenomenaareessentialandmustbetaken intoaccount:theinhomogeneousbroad-ening, thecross-correlatione ectsandthepresenceofdefectsspeci phononcoupling,opticalspectraconsistgene rallyofbroadbands, `broaden-ing'isapropertyoftheeachsinglec enteranditrepeatsexactlythesameway(`homo geneously')ineveryotherinstanceofthisdef ectinacrystal, ,toagreaterextent,inglass,thesite-to-sit enon-equivalenceofdi erentinstancesofthesamedefectcausestheop ticaltransitionenergiestodi `inhomogeneous' ,di erentvarietiesofE0-centers[8],NBOHCs[5]a ndPORs[4], ttedwithcomputer-simulatedspectra,explic itlytakingintoaccountthee ectsofinhomogeneousbroad-eningong-factor sandhyper neinteractions(thetechniquesaredescribed in, [9,10].Itwasshownthatforacorrectinterpre tationofEPRspectratheinclusionoftheinhom ogeneousbroadeninge , edapproachisthatthecontributionoftheinho mogeneousbroadening( )isoftenhardtodetectonthebackgroundofthe muchlargerhomogeneousbroadening( 1eV).)

5 However,forsomeofthedefectsinsilica,such asNBOHCsorODCs,thedisordere ectssigni cantlya phononcouplinginanopticaltransitionissma ll(Huang Rhysfactorisbelow 4), ecttheshapeofthespectrum, ectscanbepartiallysuppressedbyusingsite- selectivelu-minescenceorspectralhole-bur ningtechniques[11].Thesetechniqueshavepr ovedtobeusefulinthestudiesofNBOHC sinsilica[12,13].Exceptforthecaseoflumin escenceofinterstitialO2insilica[14],site -selectivetechniquesaretheonlypresentlyc on `Thecross-correlationse ect'Inmostcases, (1998)16 4817spectraalone,andparallelinvestigatio nsofthesamedefectsbydi bysynthesisconditions,irradiation,orther maltreatments andtosearchfortheinter-relatedchangesinm easurableproperties,suchasopticalabsorpt ion,luminescence, ndingthedefectstructureontheatomicscale, themostcommontaskofthiskindisestablishin gtheEPR ,thenumberofreportedunambiguousEPR (ornotbelonging) x1;x2;..;xN ,wherexirepresents, ,peaken-ergiesofopticalbands,luminescenc equantumyield,excitedstatelifetimes, uctu-ations,asimilarcenteringlassmustbed escribedinsteadbyaprobabilityfunctionP x1;x2.

6 ;xN .Forsomearbitrarypropertyxk(16k6N),thepr obabilitydistributionfunctionp(xk)andthe averagevaluexavkarethenobtainedbythestan -dardexpressionsp xk ZP x1;..;xN dx1;..dxk 1;dxk 1;..;dxN; 1 xavk Zxkp xk dxk; 2 whereintegralsaretakenoverallpossiblexis andRP x xj;xk ,thatis,disorder-inducedsite-to-site uctuationsofxjdonotautomaticallycausefun ctionallycorrelatedchangesinxk,thenp xj;xk p xj p xk andtheprobabilitydistributionofxkisexact lythesamefordi erentsiteshavingdi ,however,merelyaconvenientsimpli uctuationsofdi erentpropertiesofthecenteraremutuallycro ss-correlated,forexample,oscillatorstren gth,excitedstatelifetimeandtransitionene rgy,transitionenergyandthee ciencyofphotochemicaltransformations,com ponentsofg-matrixinEPRspectrabetweenthem selvesandwithexcitedstateenergies[9].The secross-corre-lationscangiverisetotwokin dsofproblems: Thedistributionfunctions,p(xk),andaverag evaluesxavkofdefectpropertiesmaychangedu r-ingisochronalannealing,photobleachingo ranyotherexperiment, ,a ectingthedefectconcentration(activatione nergies,reactionrates,photo-bleachinge ciency)arecorrelatedwith,say,op-ticalabs orptionoscillatorstrength,transitionener giesorluminescencequantumyield,theex-pec tedlinearcorrelationbetweenopticalab-sor ption,luminescenceandEPRsignalsmaybedist orted,andthespectralbandsmayexperi-ence` anomalous'peakshiftsawayfromtheirorigina lpositions,ortheshapeofthephoto-bleached partofanabsorptionbandmaydi erfromtheshapeofthewholeabsorptionband.

7 Everyexperimentaltechniqueisselectiveins omeway,andthemathematicallycorrectaver-a ging,assumingsummingoverallpossiblexsinE qs.(1)and(2) `sees' ,thesubsetofthedefectswithlargerab-sorpt ioncross-sectionprevailsintheabsorptions pectra, ,thesimpleone-to-onerelationshipsobserve dbetweendi (1998)16 48entpropertiesofdefectsinmonocrystalsma yturntoapproximateones, erentopticalorEPRsignalintensitiesinglas sdonotautomaticallyprovethatthesignalsor iginatefromdi erentformsofSiO2:vacancies,interstitials ,anddanglingbondsinbulkandonsurfacesSinc emostcrystallineandamorphousformsofSiO2h avethesameshortrangeorderofcorner-shared SiO4tetrahedra,similaritiesbetweenpointd efectsinthesedi ,ithasturnedout,thatapartfromtheE0-cente randtheinterstitialoxygenmolecules,simi- laritiesbetweenthedefectsina-quartzandsi licaaresurprisinglyfew[15].Evidently,the a ectoftheglassystategoesbeyondmerelyintro ducingisotropyandthein-homogeneousbroade ninge ,describedinthetermsofvacancies,intersti tialsandtheiraggregates,undercoordinated atoms(danglingbonds),overcoordinatedatom sand`wrongbonds'( ,peroxybridge,O ObondinginSiO2) ,net-work(lattice)defectsofSiO2canbedivi ded,ap-proximately,intothreegroups.

8 (I)Defects,whosepropertiesaredeterminedi nthe ,NBOHC,POR,3-fold-coordinatedSi(`surface variant'ofE0-center)and2-fold-coordinate dSi(oneofthealternativemodelsforODC(II)[ 16])fallinthiscategory.(II)Defects,whose propertiesaredetermined,tothe rstorder, (theE0-center)andtherelaxeddiamagneticox ygenvacancy,suggestedasmodelforODC(I)[16 ]. glasscomparisonshavebeenusefulinunder-st andingthestructureoftheE0-center,anditca nbehopedthatthismaybehelpfulinthecaseofd iamagneticvacanciestoo.(III)Extendeddefe cts,whosepropertiesaredetermined,evenint hemostbasicapproximation,byinteractionsb etween3ormoreSiO4tetrahedra,forinstance, oxygendivacancies,`non-relaxed'oxygenvac ancies(alternativemodelsforODC(II))orE0- centersinthecasewhenthe`weak'super-hyper ,whilecausing uctuationsofthemutualorientationsofthene ighboringtetrahedra,itcanbeexpectedthatt heinhomogeneousbroadeninge ectstendtoin-creasewhengoingfromgroup(I) togroup(III).Thisargumentassumesthatthes tructureofsilicaisdescribedbycontinuousr andomnetwork,andthatnanometersizedregion sofmoreorderedstructure(crystallites) [17], (group(I))anddefectsofgroups(II)and(III) canbedi cult, (isolated-coordinatedSi) (I) ,theintrinsicdefectslikeNBOHC,PORandODC( II),whichdominatethevisibleandnearultrav iolet(UV)partoftheopticalabsorptionspect rumofvitreoussilica( ) [17 19]onlyafterirradiationbyfastparticles,u suallyneutrons, (1998)16 ,thesedefectshaveneverbeenobservedinquar tzmonocrystalsirradiatedbyc-rays, , ,however,thereisnosuchagreementonODC(II) .

9 Whilethecrystal glassanalogyisthusoflittleuseinstudiesof dangling-bondtypedefects,an-otherextreme lypowerfulavenueofinvestigationsexists:d anglingbondsarepresentassurfacede-fectsi nmechanicallygroundsilicaglassorinhigh-s urface-areasilicas( ).Becauseofthesurfacelocationofthedefect s,theyareeasilyac-cessiblebygasmolecules forpurposefulchemicalmodi cations, ,during,andafterthemodi cationscanbemonitored(whenapplicable)byE PR,infrared,photoluminescence,di usere- ectancespectroscopies[20]andopticalabsor p-tion[21].ThesurfacevariantsofE0centers ,NBOHCs, ,new`ex-otic'surfaceradicalswerefound,wh oseexistencehavenotyetbeencon rmedinthebulksamples:@Si@O(oxygendoubleb ondorsilanonegroup)[22]and@ :threeoxygen-excessdefects,NBOHC[29],POR [30]andinterstitialO3(ozone) (top)andofcentersonthesurfacesofSiO2(bot -tom). , (1998)16 48 Table1 Characteristicsofthemajoropticallyactive defectsinsilicaCommonlyuseddefectname(s) oracronym(s)Suggestedstructuralmodel(s)P eakpositionsoftheopticalabsorption/excit ationbands(eV)Halfwidth(eV)atT (cm2)PeaksofPLbands(eV) ciency-relateddefectsE0centers:E0a,E0b,E 0c,;BSi.

10 (SiB) [8,55,61,62]E0dSiO4vacancy?giso (surface)BSi [54]SiODC(II),B2-center,B2(Si),Si02,oxyg envacancy,oxygendivacancyoxygenvacancy,o xygendivacancy, (?)None(diamagneticcenter)[76,77] [101] 10 [118][104]Ge(ODC),B2(Ge),Ge02,oxygenvaca ncydivacancy,Ge2 oxygenvacancy,oxygendivacancy, (?) 9nsNone(diamagneticcenter)[102 104] [106,107] 10 [122 125]Si Sibond,oxygenvacancySiODC(I)BSi 10 17cm2 SiODC(II)emissionbandsNone(diamagneticce nter)[58,59,75][76,78,80,85]H(I)center,` 'BSi H 6(photodissociation) [27,73,109,111]BSi H SiBAiso(H) [100]H(II)center,` 'BGe H ?(dissociationby5eVlight)Notobservedgiso [27,104]BGe H SiBAiso(H) [100] (1998)16 4821 Table1(Continued)Commonlyuseddefectname( s)oracronym(s)Suggestedstructuralmodel(s )Peakpositionsoftheopticalabsorption/exc itationbands(eV)Halfwidth(eV)atT (cm2)PeaksofPLbands(eV) (NBOHC)BSi O (10 20)lsg1 [29] 4 10 4g2 [12,13,17]g3 (POR)BSi O O 10 4 Notobservedg1 [184] (?)