半 导 体 学 报
CHINESEJOURNALOFSEMICONDUCTORS
Vol.26 No.12
Dec.,2005
EffectofThermalAnnealingonCharacteristics
ofPolycrystallineSilicon
RenBingyan1,GouXianfang1,2,MaLifen1,2,LiXudong2,XuYing2,andWangWenjing2
(1SemiconductorResearchInstitute,HebeiUniversityofTechnology,Tianjin 300130,China)(2BeijingSolarEnergyResearchInstitute,Beijing 100083,China)
Abstract:Oxygenandcarbonbehaviorsandminority2carrierlifetimesinmulti2crystallinesilicon(mc2Si)usedforsolarcellsareinvestigatedbyFTIRandQSSPCDbeforeandafterannealingat750～1150℃inN2andO2ambient.Forcomparison,theannealingofCZsiliconwithnearlythesameoxygenandcarbonconcentrationsisalsocarriedoutunderthesameconditions.Theresultsrevealthattheoxygenandcarbonconcentrationsofmc2SiandCZ2Sihavealesserdecrease,whichmeansoxygenprecipitatesarenotgenerated,andgrainboundariesinmc2Sidonotaffectcar2bonbehavior.Bulklifetimeofmc2SiincreasesinN2andO2ambientat850,950,and1150℃,andthelifetimeofmc2SiwafersannealedinO2arehigherthanthoseannealedinN2,whichshowsthatalotofimpuritiesinmc2Siathightemperatureannealingdiffusetograinboundaries,greatlyreducingrecombinationcenters.InterstitialSiatomsfillingvacanciesorrecombinationcentersincreaseslifetime.Keywords:polycrystallinesilicon;oxygen;lifetimeEEACC:2520C
CLCnumber:TN30411+2 Documentcode:A ArticleID:025324177(2005)1222294204
cells.Inthispaper,theeffectsofthermalannealing
1 Introduction
PolycrystallineSiwafershavebecomepreva2lentintherecentphotovoltaicmarket.However,theyneedfurtherqualityimprovementforhighlyefficient,low2costsolarcells.Firstwemustunder2standthebehaviorsofimpuritiesanddefectsinthepolycrystallineSiwafersinmoredetail.Becausetherearegrainboundariesandmoreimpuritiesanddefects,mc2Simaterialhasmorecomplicatedphysi2calbehaviorinhightemperatureannealingthanmono2crystallinesilicon.Oxygeninmc2Siisaveryimportantimpuritythataffectstheelectricalandmechanicalpropertiesofsiliconmaterialduringheattreatments[1].However,theformationofoxy2genprecipitates,thevarietyofminorcarrierlife2times,andtheinfluenceoftheannealingambientarelessinvestigatedforpolycrystallinesiliconsolar
onoxygenbehaviorandcarrierlifetimesforpoly2crystallineSiwafersareinvestigated.
2 Experiment
ThepolycrystallineSiwafersprovidedbyBa2yerSolarCorporationinthisexperimentwerep2
μmthick.Theinterstitialtype,019Ω・cm,and285
oxygenandsubstitutecarbonconcentrationsofthesampleswere813×1017and2×1017cm-3,respec2tively.Forcomparison,p2typeCZ2Sisampleswith
〈100〉orientation,1～3Ω・cm,athicknessof
μm,andalmostthesameoxygenconcentration330
werealsostudied.Thesampleswerecleanedwithchemicalsolution,andSioxidewasremovedinanHF(10%)solution.Thentheyweresubjectedtoheattreatmentat1260℃for1hinN2ambientsoastoeliminatetheinfluenceofthermalhistorybefore
Ζ2005ChineseInstituteofElectronics
Received10June2005
第12期RenBingyanetal.: EffectofThermalAnnealingonCharacteristicsofPolycrystallineSilicon
2295
annealing[2].Asinglestepheattreatmentwasthencarriedoutinthetemperaturerangeof750～1150℃for4hinN2andO2ambient.Interstitialox2ygenconcentrationsandminority2carrierlifetimesbeforeandafterannealingweredeterminedbyFT2IR(Fouriertransmissioninfraredspectroscopy)andQSSPCD(quasi2steadystatephotoconductancedecay).Finally,thesampleswereetchedbySirtlorWrightsolutionandwereexaminedwithanopticalmicroscopeandSEM.
Fig.2 Interstitialoxygenchangeswithannealingtem2
3 Results
3.1 Changeoftheinterstitialoxygenconcentra2
tions
peratureunderN2andO2surroundinginmc2Siwafers
3.2 Changeofsubstitutecarbonconcentration
Figures1and2showtheprofilesoftheinter2stitialoxygenconcentrationofCZ2Siandmc2Siduringsinglestepannealinginthetemperaturerangeof750～1150℃inN2andO2ambient.ItcanbeseenthattheoxygenconcentrationinCZ2Siandmc2Sislightlydecreases(exceptforthelightin2).Thechangeofoxygencreaseinmc2Siat950℃
concentrationisalmostuniforminN2andO2an2nealing.Thisindicatesthatthereisalmostnogen2erationofoxygenprecipitatesinCZ2Siandmc2SiwafersinN2orO2ambient.Thishaslessinfluenceonoxygenconcentrationduringsinglestepannea2ling.
CarboninCZsiliconhasbeenreportedtohaveasignificantinfluenceonoxygenprecipitationdur2ingheattreatments.Inaddition,thereisamuchhighercarbonconcentrationinmc2SithaninCZsilicon.Carboncanenhancethenucleationofoxy2
)andgenprecipitatesatlowtemperatures(<850℃doesnotaffecttheamountofoxygenprecipitates
)[2].Carboncon2athighertemperatures(>950℃
centrationafterannealingwasdeterminedbyFT2IR.AsshowninTable1,thecarbonconcentrationdoesnotaffecttheamountofoxygenprecipitateathighertemperaturesinsingleannealing,andgrainboundariesinmc2Sidonotaffectcarbonbehavior.
Table1 Changeofsubstitutecarbonconcentrationofmc2Si
T/℃
7502.05
8501.9
9502.15
10502.2
11501.6
[Cs]/1017cm-3N2ambient[Cs]/1017cm-3O2ambient
2.42.62.51.82.6
3.3 Changeofcarrierlifetime
Figures3and4showchangeofcarrierlifetime
Fig.1 Interstitialoxygenchangeswithannealingtem2peratureunderN2andO2surroundinginCZ2Siwafers
ofsiliconwafersduringsinglestepannealinginthetemperaturerangeof750～1150℃inN2andO2ambient.ThelifetimeofCZ2Siwafersafterannea2lingdecreasesdrasticallywithincreasingannealing
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半 导 体 学 报第26卷
temperaturebuthasarecoveryat950℃,whichprobablygeneratesmanydefectsandnewrecombi2nationcenters,andsomeinterstitialSiatomsfillingvacanciesorrecombinationcentersresultinlife2timerecoveryat950℃.Also,thelifetimeofCZ2SiwafersannealedinO2ambientislowerthanthoseannealedinN2.ThisphenomenonisprobablyduetothefactthatduringannealinginO2ambient,in2terstitialSiatomsaresuppliedfromagrowingSiO2/SiinterfacesothatexcessinterstitialSiat2omsmightrecreatenewrecombinationcenters[3].Thelifetimeofmc2Siwafersannealingat1150℃increases120%inO2ambientandthelifetimeofmc2SiwafersannealedinO2ambientishigherthanthoseannealedinN2.Thereasonfortheincreaseisprobablyduetothefactthatalotofimpuritiesinmc2Siathightemperatureannealingdiffusetograinboundaries,greatlyreducingrecombinationcenters.Ontheotherhand,interstitialSiatomsfillingvacanciesorrecombinationcentersresultsinalifetimeincrease.Fig.3 RatioofminoritycarrierlifetimechangeswithannealingtemperatureunderN2andO2surroundinginCZ2Siwafers
)(a)andopticalmicrographs(200Fig.5 SEM(500×
)(b)ofdefectsinthecleavageplaneofmc2Siafter×
annealingat1050℃
4 Conclusion
Fig.4 RatioofminoritycarrierlifetimechangeswithannealingtemperatureunderN2andO2surroundinginmc2Siwafers
Weconcludethattheoxygenconcentrationofmc2SiandCZ2Sihadaslightdecrease(exceptfora)inN2andO2am2lightincreaseinmc2Siat950℃
bientduringsingle2stepannealing,whichmeansoxygenprecipitatesweregenerated.Lowercarbonconcentrationdidnotaffecttheamountofoxygenprecipitatesat
highertemperatures,andgrain
boundariesinmc2Sididnotaffectcarbonbehavior.
AsshowninFig.4,thelifetimeofminoritycarriersinmc2Siwafershasgreatincreasesat850,950,and1150℃,respectively.Thechangesoflife2timeinN2andO2ambientarealmostthesame.
第12期RenBingyanetal.: EffectofThermalAnnealingonCharacteristicsofPolycrystallineSilicon
2297
Bulklifetimeofmc2SiincreasedwithtemperatureinN2ambientat850,950,1150℃,andannealinginO2showedbetterresultsthaninN2.Onthecontra2ry,thelifetimeofCZ2SiannealedinN2orO2de2creasedrapidly.ThechangesoflifetimeandoxygenconcentrationinN2andO2annealingwerealmostthesame.Thereasonforthelifetimeincreaseisprobablyduetothefactthatalotofimpuritiesinmc2Siathightemperatureannealingdiffusetograinboundaries,greatlyreducingrecombinationcenters.Ontheotherhand,interstitialSiatomsfillingvacanciesorrecombinationcentersresultsinalifetimeincrease.References
[1] YangD,LiDS.OxygeninCzochralskisiliconusedforsolar
cells.SolarEnergyMaterials&SolarCells,2002,72:133
[2] YangD,MoellerHJ.Effectofheattreatmentoncarbonin
multicrystallinesilicon.SolarEnergyMaterials&SolarCells,2002,72:542
[3] MimuraM,IshikawaS,SaitohT.Relationshipbetweenther2
maltreatmentconditionsandminoritycarrierlifetimesinp2type,FZSiwafers.SolarEnergyMaterials&SolarCells,2001,65:454热退火对多晶硅特性的影响
任丙彦1 勾宪芳1,2 马丽芬1,2 励旭东2 许 颖2 王文静2
(1河北工业大学半导体研究所,天津 300130)
(2北京太阳能研究所,北京 100083)
摘要:为研究热退火对太阳电池用多晶硅的影响,在750～1150℃,N2和O2环境下分别对硅片进行热处理.用傅里叶红外光谱仪和准稳态光电导衰减法测量退火前后的氧碳含量和少子寿命的变化.为了比较,对有相同氧碳含量的直拉硅片进行同样处理.结果发现:在多晶和单晶片中氧碳含量下降很小,意味着没有氧沉淀产生,晶界对碳行为影响不大.多晶硅片在N2和O2环境下,850、950和1150℃下退火,少子寿命都有很大提高,并且在O2中退火比N2中退火少子寿命上升得更多,可能由于在高温退火时大量杂质扩散到晶界处,减少了复合中心.另外,间隙硅原子填充了空位或复合中心从而导致寿命提高.关键词:多晶硅;氧;寿命
EEACC:2520C
中图分类号:TN30411+2 文献标识码:A 文章编号:025324177(2005)1222294204
2004206210收到Ζ2005中国电子学会
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