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年度总结11范文 Article DirectionalSolidification ofMulticrystalline SiliconUsing theAelerated Crucible Rotation Technique?Abstract?Full TextHTML?Hi-Res PDF556KB?PDF w/Links191KBFigures?R.Bairava Ganesh?,Hitoshi Matsuo?,Yoshihiro Kangawa?,Koj iArafune,Yoshio Ohshita,and KoichiKakimoto*?References MasafumiYamaguchiGraduate Schoolof Engineering,Kyushu University,6-1,Kasuga-Koen,Kasuga,Fukuoka816-8580,Japan,Crystal GrowthCentre,Anna University,Chennai600025,India,Research Institutefor AppliedMechanics,Kyushu University,6-1,Kasuga-Koen,Kasuga,Fukuoka816-8580,Japan,and Toyota Technological Institute,2-12-1,Hisakata,Tempaku-ku,Nagoya468-8511,Japan Cryst.Growth Des.,xx,8 (7),pp25252527DOI:10.1021/cg800160g PublicationDate(Web):May28,xxCopyright?xxAmerican ChemicalSociety*Corresponding author:Mailing address:RIAM,Kyushu University,6-1,Kasuga-Koen,Kasuga,Fukuoka816-8580,Japan.Tel:+81-92-583-7741.Fax:+81-92-583-7743.E-mail address:kakimotoriam.kyushu-u.ac.j p.,?Graduate Schoolof Engineering,Kyushu University.,?Anna University.,Research Institutefor AppliedMechanics,Kyushu University.,Toyota TechnologicalInstitute.Abstract Employingthe aelerated crucible rotation technique,we grewmulticrystalline siliconby the directional solidification process.The distribution of carbon concentration determined by Fourier transform infraredspectroscopy demonstratedthat applicationof aeleratedcrucible rotationhomogenized the carbon concentration in the grown ingot.Attempts weremade toexplain theeffect ofcrucible rotationon homogenization of carbon concentration interms ofsegregation phenomena.Moreover,growth striations induced by the crucible rotation wereobserved in the axial direction of the ingot.?o Topof Pageo Introductiono Aelerated Crucible Rotation Technique oExperimental Sectiono Results and Discussiono Conclusiono References1Introduction In the photovoltaicindustry,more than90%of annualsolar cellproduction isbased onsilicon in the formof singlecrystalline,multicrystalline waferand thinfilm modules. (1)The marketshare ofmulticrystalline silicon(mc-Si)has increasedremarkably inrecent years. (2)The directional solidification(DS)process isa cost-effective techniquefor producingmc-Si.For reducingcost,low-quality feedstock,which containsmany lightimpurities suchas oxygen,carbon andnitrogen,is used in theDS process. (3)The segregationof theseimpurities in mc-Si isa seriousproblem,which degradesthe qualityof thegrown ingot.(4-6)For lesstime consumption,the mc-Si isbeing grown by arapid solidification process usinghigh growthrates;hence,the concentration of impuritiesis highand inhomogeneousthroughout thevolume of the ingot. (7)Among theseimpurities,carbon originatesprimarily from the feedstock,graphite crucible,insulation andheating elementsin the furnace.Carbon issubstitutionally dissolvedin siliconcrystals andtetrahedrally bondedto silicon atoms,resulting inprecipitation in the formof siliconcarbide(SiC)if theconcentration exceedsthe solubilitylimit. (8)These SiCprecipitates cancause severeohmic shuntsin solarcells. (9)Furthermore,during thewafer sawingprocess,these precipitatesin thewafer damagethe wiresaw. (10)Therefore,an effectivemethod forhomogenizing the carbon concentration inmc-Si isneeded.In thispaper,we describethe directional solidification of mc-Si by using theaeleratedcruciblerotationtechnique(ACRT).Fouriertransform infrared(FTIR)spectroscopy was used todetermine the carbon concentration in thegrown ingot.The effectsof the ACRT on the directionalsolidification of mc-Si and the homogenization of carbon concentration in thegrown ingot arediscussed.The growth striations induced by the ACRT arealso discussedin detail.?o Topof Pageo Introductiono AeleratedCrucible RotationTechnique oExperimental Sectiono Resultsand Discussiono Conclusiono References2AeleratedCrucible RotationTechniqueGenerally,the ACRT is usedto produceforced convectionin the melt andthereby enhancemixing of the melt. (11)The basicidea isto transfermomentum of a cruciblewall to the melt by modifyingthe rotation rate of a crucibleas afunction oftime.As therotation ratechanges,an Ekmanflow layeris builtup anddrives the melt floweither spin-up orspin-down. (12)With thebination of spin-up andspin-down basedon Ekmanflow cycles,the ACRThas beenwidely usedfor mixingthe meltand thusfor enhancingmass transfer. (13)The constitutional supercooling phenomenonin themelt growthmay leadto morphologicalinstability throughinterface breakdown.The ACRTeliminates theconstitutionalsupercooling,thus restrictingnucleation toone crystalor afew crystalsin thecase ofnonseeded growth. (14)It hasbeen reportedthat cruciblerotation indirectionalsolidificationofmc-Si isadvantageous forreducing theSiC particleprecipitation. (15)Inthe present experiment,the durationof the ACRT was determined bycalculating thespin-up timeusing thefollowing equation:where Eandare the Ekman numberand maximumcruciblerotation rate,respectively.(16,17)The calculatedvalues areE=0.00132,=5rpm and=329s.However,for experimentalfeasibility weused15s asaeleration,holding anddeceleration times,respectively,with maximumrotationrate of5rpm.The periodof oneACRT cyclewas3min,andthe ACRT was applied36times with15min intervalsduring solidification.The appliedACRT cycleis shown in Figure1.Figure1.ACRT cycleusedin thepresentexperiment.?o Topof Pageo Introductiono AeleratedCrucible RotationTechnique oExperimental Sectiono Resultsand Discussiono Conclusiono References3Experimental SectionThe growthexperiment wascarried outin adirectionalsolidificationfurnace atToyotaTechnologicalInstitute.Off-grade siliconfeedstock withgallium dopant(1017atoms/cm3)was placedin acylindrical quartzcrucible of10cm indiameter.The cruciblehad beencoated withSi3N4,which preventsthe ingotfrom stickingto thecrucible.The materialswere heatedup to1550C,j ustabove themelting pointof silicon,in anargon atmosphereinside thefurnace.After melting,the temperaturewas loweredto1450C.Then heatermovement wasinitiated from the bottomtothe top of thefurnaceat arate of12mm/h forsolidification.The ACRT wasappliedsimultaneously with the heatermovement during solidification.After theheater hadalmost reachedthetopof thefurnace,the temperaturewas loweredat arateof300deg/h toroom temperature.The entiregrowth cycleis shownin Figure2.The grown ingot wassliced verticallyparallel tothe growthdirection.The waferswere roughlyetched with a solutionof hydrofluoricacid andnitric acidin aratio of1:13for10min toremove thesawing damage.To obtain a mirrorlikesurface,the mc-Si waferswere polishedwith diamondparticles of0.5and0.3?m ingrain size,coordinated withwater.Then,the waferswere polishedwith aluminapowder particlesof0.1?mingrain sizeand deionizedwater.Finally,each waferwas washedwith acetone,methanol andwater sequentially.After polishing,the waferswere etchedfor3min usingthe sameetchant asthat describedabove toremove thepolishing damage.Figure2.Temperature profileand heatermovement during the growth ofmc-Si.A JASCO(MFT-2000)FTIR spectrophotometerwas usedto measurethe carbon concentration insilicon wafers.The measurementwas doneat roomtemperature inair atmosphere.The thickness of thesamples forFTIR spectroscopywas0.5mm.The measurementrange ofwavenumber wasset from500to1200cm?1with aresolution of4cm?1.A nondopedCzochralski silicon sample wasused asa reference.Both thetest andreference sampleswere carefullyprepared bykeeping thethickness variationless than0.005mm withidentical surfacepreparation.The two-phonon latticebands ofsiliconat610cm?1are superimposedon theband of carbon andaffect theintensity of the carbonpeak.Hence,these siliconlattice vibrationswere subtractedusing anFTIR spectrumofapure Czochralskisiliconsample.The opticalabsorption lineat605cm?1,which isassociated with the vibrationof carbonin thesubstitutional position,wasusedfor analyzingthe carbon concentration. (18)The carbon concentration wasdeterminedbyASTM standards. (19)?o Topof Pageo Introductiono AeleratedCrucible RotationTechnique oExperimental Sectiono Resultsand Discussiono Conclusiono References4Resultsand Discussion Inorder todetermine theeffect of the ACRTon thehomogenization of carbonconcentration,the resultswere paredwith thosefor anotheringot grownunder similargrowth conditionswithout the ACRT.Figures3(a)and(b)show cross-sectional imagesof the ingot andcarbonconcentration distributionofmc-Si grownwith andwithout the ACRT,respectively.The carbonconcentrationdistributionat theupper regionof the ingot grown by the ACRT isnot shownin Figure3(a).This isbecause thecarrier concentration in theupper regionis high,and wewere thereforenot ableto measurethecarbonconcentrationinthis region.In Figure3(b),it can be seenthe ingotgrown without the ACRThas aninhomogeneous distributionofcarbonconcentration alongradial andaxial directionswithahigh localconcentrationin the centralarea.Along the radial direction,the variationofcarbonconcentrationin Figure3(a)is lessthan that inFigure3(b).The resultsimply thateffective mixing ofthemeltby the ACRT resulted inhomogenizationofthecarbonconcentration along theradial direction,though asmall variationalongtheradialdirectiondue tothe back-melting processduringthe ACRTwasobserved asshowninFigure3(a).Stirring by convection causesa relativelyhomogeneous distributionof positionin theinterior part ofthezone,whereas concentrationgradients areformed inregions adjacent tothe interfaces.Hence,thecarbonconcentration washigh at the centerand toppartofthe ingotgrown withoutthe ACRT.Figure3.(a)Photograph ofthe ACRT-grown mc-Si waferwith correspondingcarbonconcentrationdistribution.(b)Photograph ofa conventionallygrown mc-Si waferwith correspondingcarbonconcentrationdistribution.When the ACRT isused,theEkmanlayer flowhas astrong effectonthemixingofthemelt.The binationofspin-up andspin-down processeschanges theabsolute valueoftheconcentrationofthe speciesattheinterface.Figure4shows thecarbonconcentrationprofiles inthe axialdirection ofthe mc-Si grownbytheACRT andthat grown withouttheACRT.In addition,duringsolidification,the imbalancebetween rejection ofimpurities fromthe growingcrystal surfaceand masstransfer ofimpurities awayfromthecrystal surfaceresults inthe formationofaconcentration boundary layer infront ofthe solid?liquid interface.Application oftheACRTreduces thethicknessofthe impurityboundarylayerby effectivemixing andthus results in lessradial segregationinthegrown ingot.Another possiblereason for the ourrenceof radial segregation isinplete mixingbyconvection. (20)Application oftheACRTresults inforced convectioninacrucible,which effectivelydecreases theradialsegregation.Figure4.Carbon concentrationprofiles inthe axialdirection ofmc-Si grownbytheACRT andgrownwithouttheACRT.The ACRThas numerousadvantages overconventional methods,but italso hassome drawbacks.Due tothe oscillatorynature oftheACRT,it caninduce growthstriationsinthe growingcrystal.ACRT-induced growthstriations havebeen reportedby manyauthors.(21,22)We observedgrowthstriationsinducedbytheACRTinthe axialdirection ofthegrown ingot.Striation patternswere visibleonly atthe timewhen theACRTisswitched on,possibly due to asudden changeinthecrucibles angularvelocity.Due tothe periodicfluctuation inthe growthrate,the ingotexperiences back-melting.Some ofthe missedstriations couldaount for the back-melting.The striations were formedin aordancewiththetime differencebetween theACRT cycles.Moreover,the missingstriations arealso originatedby unsteadyflow ofthemeltwhich makesback-melting. (23)The formationof thesestriations isthought tobe suppressedby choosingthe properinterval oftheACRT.The ACRTcanbeimplemented withthe directionalsolidificationprocessforthegrowthoflarge sizemc-Si ingots.It canbe achievedby usinga suitablerotational assemblywiththegrowth system.Moreover,a slowrotationratewould beenough toobtain betterresultsinthe industrialgrowth system.?o Topof Pageo Introductiono AeleratedCrucible RotationTechnique oExperimental Sectiono Resultsand Discussiono Conclusiono References5Conclusion EmployingtheACRTtechnique,mc-Si wasgrownbythedirectionalsolidificationprocess.Carbon concentrationinthegrown ingotwasdeterminedbyusingFTIR spectroscopy.The carbonconcentrationintheingotgrownbytheACRTwas morehomogeneous thanthatina conventionallygrowningot.This impliesthat applicationoftheACRTresultedinthehomogenizationofcarbonconcentrationinthegrowningot.Weak growthstriationswereobserved intheaxialdirectionoftheingotduetothe oscillatorynature oftheACRT.?o Topof Pageo Introductiono AeleratedCrucibleRotationTechnique oExperimental Sectiono Resultsand Discussiono Conclusiono ReferencesAcknowledgment Oneoftheauthors(R.B.G.)thanks the Ministry of Education,Culture,Sports,Science andTechnology,Japan,fortheaward ofJapanese GovernmentScholarship throughtheMinistryof HumanResource andDevelopment,India.He isalso thankfulto AnnaUniversity,India,to carryout thiswork inJapan.This workwas supportedby aNEDO project,a Grant-in-Aid forScientific Research(B)06990707-0fromtheJapanese MinistryofEducation,Culture,Sports,Science andTechnology.?o Topof Pageo Introductiono AeleratedCrucibleRotationTechnique oExperimental Sectiono ResultsandDiscussiono Conclusiono ReferencesReferences Thisarticle references23other publications.1.1.M ller,A.;Ghosh,M.;Sonnenschein,R.;Woditsch,P.Mater.S ci.E ng.,Bxx134257262CrossRef2.2.M ller,H.J.;Funke,C.;Rinio,M.;Scholz,S.Thin S olid Filmsxx487179187CrossRef3.3.Newman,R.C.Mater.S ci.E ng.,B199636112CrossRef4.4.Matsuo,H.;Bairava Ganesh,R.;Nakano,S.;Liu,L.;Arafu