Solid StateCommunications.Vol.33, pp. 1173—1176.PergamonPressLtd. 1980.Printedin GreatBritain.

ANNEALING BEHAVIOR AND ITS GLASS COMPOSITIONDEPENDENCEOFPHOTO-INDUCEDESRIN

MELT-QUENCHED Ge—S GLASSES

N. KumagaiandJ. Shirafuji

Faculty of Engineering,OsakaUniversity,Osaka,565,Japan

(Received24November1979 by Y. Toyozawa)

Annealingbehaviorsof photo-inducedESR andoptically-inducedabsorp-tionedgeshift havebeenstudiedin melt-quenchedGe—Sglasseswithvariouscompositions.Annealingcharacteristicof photo-inducedESRhasbeenfound to be quiteparallelto that of optically-inducededgeshift.Compositiondependenceof themis correlatedwith that of theglasstransitiontemperature.

AS-QUENCHEDandas-evaporatedGe—Sglassesexhi- photo-inducedESRin As2Se3 andAs2S3 glassesisbit a significantESRsignal, beingquite different from observedonly at low temperaturesaccompanyingaotherchalcogenideglassessuchasAs2S3, As2 Se3 and mid-gapabsorptionband.The optically-inducededgeGe—Se[1, 2]. Heattreatmentof theglassescauseda shift in As2 Se3 andAs2 S3,which ispreparedat lowlittle changein the ESRsignal.The ESRspectrumof temperaturesis still stableat room temperature.ThisGeS~glassislargely dependenton glasscomposition findinghasled to theconclusionthat thephotodarkeningand theshapeof the spectrumchangesdrasticallyat effect hasa completelydifferent origin from thephoto-the stoichiometriccompositionof GeS2.For glasses inducedESRwhich canbe causedby trappingof elec-with sulfurcompositionsx ~ 2, the ESRspectrumhas tronsor holesopticallygeneratedat unoccupiedandonly a singleline. On the otherhand,whenthesulfur doubly occupieddanglingbonds[2].compositionx exceeds2, the ESRsignalis composed We havestudiedthe annealingbehaviorsof bothof multi-lines [1]. photo-inducedESRandoptically-inducededgeshift

The origin of the ESRin Ge-richGe—S glasses in melt-quenchedGe—Sglasseswithvariouscompo-hasbeenrelatedto a danglingbond at a Ge site, sitionsincludingthestoichiometriccompositioninwhile themulti ESRlineshavebeenattributedto a orderto get a deepinsight into the origin of thedangling bondat a S atom [3]. photo-inducedESRandthephotodarkeningeffect

The intensityof theseESRsignalsis enhanced in chalcogenideglasses.by anillumination of light with photonenergies GeS~glasseswith sulfur compositions1.5 ~ xhigherthanthe opticalgap of the glasses.Thisopti- ~ 2.3were synthesizedby meltingGeand S withcally-inducedESRis annealedoutby heattreatment, relevantcompositionin evacuatedsilica tubesandfor exampleat 180°Cfor Ge~S70 evaporatedfilm by subsequentquenchingin water.For ESRmeasure-[4]. Shimizuetal. haveobserveda closecorrelation ment,melt-quenchedbulk glasseswere crushedintoof the annealingbehaviorsbetweenthe photo-induced smallgrainswith averagediameterof 1 mm andESRandthe optically-inducededgeshift [41.They sealedin fine silica tubesevacuatedto aboutI xproposeda model forESRcentersin Ge—Sglasses io~Torr to preventthem from surfaceattackthat a highdegreeof therandomnessof glassstruc- duringannealingprocess.ESRmeasurementwasturein Ge—Sglassesmay causea broadeningof elec- carriedout at about 150 K with an X-bandspectro-tronic levelsfor unoccupiedanddoubly occupied meter.Opticalillumination withband-gaplight atdanglingbonds,resultingin overlappingof thesetwo 150 K for 30 mm andsubsequentannealingfor 30electroniclevels andallowing the presenceof unpaired min at relevanttemperatureswere donein the micro-electronstatesevenin as-evaporatedGe—S films. The wave cavity of ESRspectrometer.The sampleforrandomnessenhancedby illuminationcausesthe opticalabsorptionmeasurementwasmechanicallyphotodarkeningeffect [5] andsimultaneouslythe polishedinto thin platewith thethicknessof aboutphoto-inducedESRthroughanenhancedbroadening 50 pm andwasmountedon a cold fingerof a metalof theelectroniclevelsof unoccupiedanddoubly cryostatwith opticalwindows.The opticalabsorptionoccupieddanglingbonds[6]. spectrumnearthe opticaledgewas measuredalso at

In contrastwith the caseof Ge—Sglasses,the 150 K. Illumination for optically-inducededgeshift

1173

1174 PHOTO-INDUCEDESR IN MELT-QUENCHED Ge—SGLASSES Vol. 33,No. 12

whereN andEg arethe densityof ESRcentersand12opticalgap,respectively,andsubscriptss, a andso• •representafter annealingat eachtemperature,well

E10 • annealedandafter illumination, respectively.It isU

seenfrom Fig. 2 thatboth the photo-inducedESR0

and the opticaledgeshift areannealedin a similarGeSx 150 K fashionthrougha singleannealingstage.Moreover,

>~ A thetemperatureof the annealingstageis greatly°

• illuminated dependenton the glasscomposition.The temperatureZ AU-I of the annealingstagebecomesmaximumat the04 stoichiometriccompositionof GeS2 - Forsamples

z annealed with glasscompositionsaway from thestoichiometric

“) 2 composition,the annealingstageof the photo-inducedo ~ ESRandopticaledgeshift shiftsto the lowertemper-

0 ature.In samplewith Ge-richcomposition,GeS15,C 1.5 18 1.9 2:0 2:3 bothof the photo-inducedESR andopticaledgeshift

S CONTENT X annealoutevenat roomtemperature.Theresult isFig. 1. Compositiondependenceof ESRcenterdensity consistentwith thatobservedin evaporatedGe30S70of well annealedandilluminatedsampleswith various films by Shimizueta!. [4] exceptfor themagnitudeofglasscompositions. thephoto-inducedESR.

The resultof the isothermalannealingexperi-andsubsequentannealingwere carriedoutin thesame mentfollowed neitherof first- nor second-ordercryostat.Thelight illumination for photo-inducedESR reactionwith singleactivationenergy.This fact mayandoptically-inducededgeshift were performedby a imply that the activationenergyhasa fluctuationtolight from ahigh pressuremercuryarc lamp through someextent.Whenwe assumetheGaussiandistri-an JRcut-filter. butionof the activationenergiesand thesimplefirst-

The ESRspectraquite similar to thoseobserved orderreactionwe obtainedthecentralactivationby Arai eta!. [1] andShimizueta!. [4] werealso energyof about 1.1 eV with thespreadof aboutobservedin our as-quenchedbulk samples.The an- 0.2eV for GeS18glasswith ajump frequencyofnealingof theas-quenchedsamplesdid notshow 10~ sec

1.remarkablechangein the ESRspectrumandits in- The moststriking featurein Fig. 2 is that thean-tensity.The photoenhancementof the ESRsignal nealingcharacteristicof the photo-inducedESR atalmostuniformly over thewhole ESR lineswasob- eachglasscompositionis quite in parallelto that ofservedin all sampleswith different compositions. thephoto-inducededgeshift, ashasbeenfirst observedHowever,themagnitudeof thephoto-inducedESR by Shimizu etaL [4] at a particularglasscomposition,signalwas largerin Ge-richglassesthanin S-rich glasses. Ge

30S70. This featureis definitely demonstratedinFigure 1 showsthecompositiondependenceof the Fig. 3, wherethevertical scaleplotsthe temperaturenumberof ESRcentersaftersufficient annealingor at which thephoto-inducedeffectsshow 50% anneal-after illumination with the samecondition. For compo- ing. The compositiondependenceof the 50%anneal-sitionx ~ 2, the densityof the photo-inducedESR ing temperaturehassomecorrelationto thatof thecentersis almostindependenton theglasscomposition glass transitiontemperature[7] as shownalso in Fig.andis muchhigherthan thoseof glasseswith compo- 3, althoughthe former is larger thanthe latter.Thesitionsx ~ 2. This indicatesthat the local structure correlationbetweenthe photo-inducedeffectsandof glassesresponsiblefor the photoenhancement~ theglass transitiontemperature,and theparallelismESRis different dependingonwhetherS composition betweentwo photo-inducedeffectsindicatestronglyx is higheror less thanthe stoichiometricvalue.This that thephoto-inducedeffectsare evidently associatedimplication is not incompatiblewith thefact that the with local displacementof atomspromotedby opticalESRsignalchangesin their form right at thestoichio- excitation.metric glasscomposition. The result of the presentexperimentsis compatible

Figure2. showstheannealingbehaviourof the with the modelproposedby Shimizueta!. [6]. How-photo-inducedESRand theoptically-inducededge ever,anotherpossibilitycannotbe ruled out.shift. Thevertical scaleplotsthe relativechangein Economoueta!. [8] haverecentlyproposedathephoto-inducedESR,(Ns~Na)/(Ngo~Na)or the specific model basedon theinteractionof electronsoptically-inducededgeshift, ~ (andholes)with the structuraldisordertunneling

Vol. 33, No. 12 PHOTO-INDUCEDESRIN MELT-QUENCHEDGe—SGLASSES 1175

•GeS15 ESR

I~ •GeS2‘GeS1~

V 0~ .\ ~viZLU < A-...~ ‘~01

b150K’, V s,\ \ ~v ‘\0.

shift “o~i~ ~ oGeS~5edge‘~0~l~wz o~3eS2 AC.-100 0 100 200

TEMPERATURE (°C

Fig. 2. Annealingbehaviorof photo-inducedESRandoptically-inducededgeshift.

are associatedwithsingly occupieddanglingbonds.The ideataking accountof danglingbondshasalso

~700 beenconsideredin the photo-inducedESRinGe~Se1—x glassesby StreetandBiegelson [101.~600 They observedtwo kindsof ESR centerswith a

a- half-widthmorethan 100 G whichhavedifferent~ ~500 annealingtemperatures.Thesecenterswere ascribedI- I-.- to danglingbondson Se andGesites,respectively.Z~

~400 ~ However,theydid not discusstherelationof photo-,— _J inducedESRto photo-structuraleffectssuchas~ ~300 _~.._.$ photodarkeningeffect. Thephoto-inducedESRrelated

to danglingbondshasbeenpostulatedin otherchalco-1.1) ° , , genideglasses[2].—~0 10 12 1.4 1.6 1.8 20 22 From thediscussionabove,it becomesclear that° L~ S CONTENT X thereare two microscopicmodelsfor explainingthe

Fig. 3. Compositiondependenceof 50%annealing photo-inducedESR;one needsthe existenceof thetemperatureof photo-inducedESRcentersandedge danglingbonds[6] andthe otherdoesnot [8]. Evenshift. Solid circlesindicatethe50%annealingtemp- in themodelwhich needsthe existenceof the danglingeratureof photo-induced. ESRandopencirclesindicatethe 50%annealingtemperatureof optically-induced bonds,a problemstill remains.It is not clearhow theedgeshift. Glasstransitiontemperature[7] is shown danglingbondsresponsiblefor the photo-inducedESRtogetherby hatchedcurve, are introduced.Theyexist inherentlyin samplesor can

be introducedby bondswitching [11] underopticalmodesin chalcogenideglasses.Moreover,they pointed excitation.outthat thesemodescantakesingly anddoubly charg- Onehasto providea directevidencewhethertheedstatesby captureingoneor two electrons(holes) photo-inducedESRis relatedto danglingbondsorandthat theelectron(hole)pairstatelie below(above) not.The finding by ShimizuetaL [3] thatan additionthe localizedsingleelectron(hole) statebecauseof of monovalentAg atomsinto GeS2.~ glasssuppresseseffectiveelectron—electron(hole—hole)attraction the ESRsignalmaygive a partial evidenceof partici-mediatedby thesemodes.Opticalirradiationexcites pation of danglingbondsthesingleelectronstateandthenthephoto-induced In As2Se3 glass,it hasbeenfoundthat thedensityESRis thusto be observed, of the photo-inducedESRcentersincreasesas thein-

Accordingto the modelby EconomouetaL [8], tensityof the photoluminescenceis fatigued [2]. Inthephoto-inducedESRis introducedthroughatomic addition, thephotoluminescenceintensityof Ge—Serearrangementmodesevenin aglasswithout dangling glasseshasbeenfound to be enhancedby annealingbonds,in contrastwith the modelproposedby Street underdeficientSe vaporpressure[12]. This factandMott [9]. On theotherhand,Shimizueta!. [6] also givesa partialsupportfor therole of danglinghavepostulatedthat thephoto-inducedESRcenters bondsin photo-inducedESR.

1176 PHOTO-INDUCEDESRIN MELT-QUENCHEDGe—S GLASSES Vol. 33, No. 12

In summary,it is observedthat the photo-induced 13, 1117(1973).ESRandthe optically-inducededgeshift haveparallel 2. S.G.Bishop,U. Strom & P.C. Taylor,Proc. 7thannealingbehaviorandthe temperatureof theanneal- mt. ConfAmorph.Liq. Semicond.(EditedbyW.E. Spear),p. 595.CICL Edinburgh(1977).ing stageshowsa glasscompositiondependencesimilar 3. I. Watanabe& T. Shimizu,SolidStateCommun.to theglasstransitiontemperature;this manifeststhat 25,705 (1978).the local displacementof atomsis directly responsible 4. T. Shimizu,M. Kumeda,I. Watanabe& Y.for thephoto-inducedESRand thephoto-induced Nakagaki,SolidStateCommun.27, 223 (1978).absorptionedgeshift. More elaborateexperimentis 5. K. Tanaka,Appi.Phys.Lett. 27,243(1975).necessaryfor revealingthe role of defectstatesin 6. T. Shimizu,SolidStateCommun.25,455 (1978).

7. A. Hruby, Czech.J.Phys.B23, 1263 (1973).photo-inducedESR. 8. K.L. Ngai,T.L. Reinecke& E.N. Economou,

Phys.Rev. B17, 790 (1978).Acknowledgement— The authorsaremuchindebted 9. R.A. Street& N.F.Mott, Phys.Rev.Lett. 35,to Prof. Y. Inuishi for his helpful discussionandcon. 1293 (1975).tinuousintereststhroughthecourseof the experiment. 10. R.A. Street& D.K. Biegelson,J. Non-Crystal.

Solids32,339 (1979).11. R.A. Street,SolidStateCommun.24,363

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1. K. Arai & H. Namikawa,SolidStateCommun. 67(1977).