Defects in Hydrogenated Amorphous Silicon and Its Based Alloy Films

(1)

Defects in Hydrogenated Amorphous Silicon and Its Based Alloy Films

著者徐希翔

journal or

publication title

博士学位論文要旨論文内容の要旨および論文審査結果の要旨／金沢大学大学院自然科学研究科

number 平成2年6月

page range 41‑45

year 1990‑02‑01

URL http://hdl.handle.net/2297/33151

(2)

氏名徐希翔

学位の種類学位記番号学位授与の日付学位授与の要件学位授与の題目

論文審査委員

学術博士学博甲第17号平成2年3月25日

博士課程終了（学位規則第5条第1項）

DefectsinHydrogenatedAmorphousSiliconand

ItsBasedAlloyFilms

（水素化アモルファスシリコン及びシリコン系合金薄膜

における欠陥）

( 主査）清水立生 ( 副査）久米田稔 ( 副査）長谷川誠一・ ( 副査）渡辺一郎 ( 副査）樋渡保秋

学位論文要旨

Theoriginandnatureofthedefectsinhydrogenatedamorphoussiliconfilm ( a ‑ S i : H ) a n d i t s b a s e d a l l o y s , s u c h a s s i l i c o n ‑ c a r b o n , s i l i c o n ‑ n i t r o g e n a n d s i l i c o n ‑ germaniumalloysareinvestigatedinthisworkbyelectronspinresonance, constantphotocurrentmethodaswellasotherelectricalandopticalmethods.

Itisfoundthatalloyinga‑Si:HwithCorNincreasesbulkdefectdensityby ashighasthreeordersofmagnitudewhilethesurfacedefectdensityremains unchangedwithinafactorof3‑4.Thennalequilibriumphenomenahavebeen

firstobservedina‑Sibasedalloys・Boththesurfacedefectsandchargeddefects

inundopeda‑Si:Handitsalloysseemnottobedirectlyinvolvedinthethermal equilibrationalthoughtheneutralbulkdefectdensityincreaseswithtemperature above"thermalequilibriumtemperature''Tb.Thermallyinducedreversible changerelaxesfollowingastretchedexponentialform.Aninterestingcorrelation

betweentheThandtheopticalbandgapisfoundtoexistforthesethreekinds

ofalloysandthephysicalmechanismunderlyingtherelationisdiscussed,The resultsonthedefectsrelatedtodopingandalloyingarealsodescribed.

Fromtheviewpointofbothfundamentalresearchandapplicationperspectives,

itisextremelyimportanttounderstandtheoriginandnatureofthedefects

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2 inhydrogenatedamorphous silicon(a‑Si:H)anditsrelated

alloys・Thisworkismainly

devotedtodescribeseveralim‑

portantbutcontroversialissues relatedtothedefectsina‑Si:H anditsalloys,suchasa‑Si,̲jrCz:

H,a‑Si,̲zN":Handa‑Si,一垂Ge":

H,includingsurfaceandbulk defects,neutralandchalgedde‑

fects,thermally‑induceddefects andtheirthermalequilibrium process,defectsinducedby dopingandalloying,etc.The mainresultsarebrienydes‑

１

︵Ｎ●や匡○Ｎｂ↑×︶︒︑之

〆

_{一一ｊｒｆ}

0

500 1000 1500

d ( ム）

0

FigurelThicknessdependenceofNsafora‑Si:H filmsinsmallthicknessregion.Closedcirclesare experimentaldatafromESRmeasurements.Fitted resultisrepresentedassolidline.Dashedlineis theextrapolationofthelinearpartofthesolidline toshowthesurfacespindensity.

cribedasfollows:

1)ESRandCPMstudiesshowthatthereisasurfacedefectivelayerona‑

Si:Hfilmswithatypicalthicknessaround200A(seeFig.1).Experimentalresults

obtainedfromthea‑Si:Hfilmswithvariousthicknessesandsurfacetreatments demonstratethatthefreesu㎡aceismuchmoredefectivethantheinterfaceand thesurfacedefectshaveacloserelationwithnaturaloxidization.

2)Alloyinga‑Si:HwithCorNincreasesthebulkdefectdensitybynearly threeordersofmagnitude,fromlO'5cm‑3tolO'8cm‑3,whileitonlyincreasesthe surfacedefectdensitybyafactorof3,varyingwithinl‑4×10'2Cm‑2.

3)Su㎡acedefectscanbedescribedbyanexponentialdistributioninthe nearsurfaceregion・Theoptimalfittinggivesl20Aofacharacteristicsurface t h i c k n e s s a n d l . 2 × 1 0 ' 8 c m ‑ 3 o f t h e s u r f a c e d e f e c t d e n s i t y f o r a ‑ S i : H ( F i t t e d r e s u l t

isshownbyadashedlineinFig.1).

4)Thedefectdensityinundopeda‑Si:Hcanreachthermalequilibriumstate

above7h(around200A).Thermalequilibriumdefectdensityincreaseswith

temperature,whichisclarifiedbyboththemeasurementofthefrozen‑indefect

densityandtheinsitumeasurementofthedefectdensityatelevated

temperatures.Areversibleincreaseinthebulkdefectdensityat400Aina‑Si:

Hisaboutafactorof4‑5,asshowninFigure2.However,boththedensities ofsurfacedefectsandchargeddefectsdidnotshowappreciablechangewith

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EB(ev) 0 . 5 1 ． 0 1 ． 5 2 ． 0

700

〃

／

β

／ノノ

ノ▲

ノ

／霧

ノ ▲ ノ

▲ ′^ノ^ノ戸

ノ

TA(･C)

400300200150(。C)

1 0 8

600

四一コ10フ且､［ 500

７Ｊ１

︵面０戸匡Ｕ︶

ユーコ1087レC

４３００００

︵エ︶岻戸

ノ

６

ｊ

ｌ

麦一両仁①ロ仁一Ｑ︑

／

△ ご宝 ' O . 9 f し O O 6

− A

／

／ 200 ノ

ノノ

｡g 5 . 0 0 9 3 0 y O C

− O

ノ

100 ノノ

1015 ^ノ

〃 0

0 0 ． 5 1 . 0 1 . 5 2 . 0 2 . 5

E｡pt(eV)

Figure30pticalbandgapEbp:dependence ofthennalequilibriumtemperature庇．

Solidcir℃lesareourdataandtheopen cil℃leisfromtheresultsofPrincetongroup.

TrianglesshownversusEb,theactivation energyofrelaxationtime.

1 . 5 2 . 0 2 . 5

1000/TA(KF')

Figure2Dependenceofbulkspindensities ofthefrozen‑indefectsonannealling temperatureZAfOrfivehydrogenateda‑Si basedalloyfilms.Thean℃wsindicatethe thermalequilibriumtemperaturert.

temperature,implyingthatbothmaynotbedirectlyinvolvedinthennal equilibriumprocess,whichalsopartiallysupportstheassumptionthat

thermally‑inducedincreaseinthedefectdensityisnotduetoeitherthe

conversionbetweenchargeddefectsandneutralonesorthechangeinthe distributionbetweenbulkregionandsurfacelayer.

5)Measurementsofthermallymetastabledefectsina‑Si,̲zC":Handa‑Si,̲z Nr;Hprovidethefirstreporttoextendthestudyofthermalequilibrium phenomenatoa‑Sibasedalloys.

Thedefectdensityinthesewide‑bandgapalloysalsoincreaseswith temperatureandthealloyingwithCorNmakes7bshifttoahighertemperature side・Similarstudiesonnarrow‑bandgapa‑Si,̲drGer:Halloyscouldnotobserve anappreciableincreaseinthedefectdensityalthoughbothdark‑andphoto‑

conductivitiesdecreasereversiblybyafactorof5‑7after250℃FC,startingfrom l45℃fora‑Sio.62Geo.38:H.

6)Thefrozen‑inchangeina‑Si:HbasedalloysisalsofOundtofollowa

(5)

stretchedexponentialrelaxationlawasin

E

a‑Si:H・Fora‑Si,−zC":Handa‑Si,−必Nz:."､20

"‑､10

H , r e l a x a t i o n d a t a o f t h e f r o z e n ‑ i n i n c r e a s e

^{炉 8}_､gld

l 6 ! 9

inthedefectdensityshowthat,with m

z

increasingthealloycontent,alongertime

, d 8

isrequiredtorelaxtheseexcessdefectsand

,(I7

theactivationenergyoftherelaxationtime

alsoincreases,froml.4eVfora‑Si:Hto g2005

厚2004

about1．6eVfora‑Sio､80C0．20:H.However, _6,2003 thethermally‑inducedchangeofconducti‑ 30

vitiesina‑Si,̲zGez:HshowsashorterrR relaxationtimeandasmalleractivation

220

エ

energy,0．9eVfOra‑Sio.62Ge0.38:H.To d l O measuretheHdiffUsionconstantandto

O

seeifthereisacolTelationbetweentheHUo.51.0

X

diffisionenergyandtheactivationenergyFigure4ThespindensityM,g‑value

fortherelaxationtimecanfurtherverifyand△必,asfunctionofxina‑Si,̲"Cz:

w h e t h e r " H ‑ g l a s s m o d e l ' ' c a n b e e x t e n d e d g m : W S A d " 急ぎ s 脇馳獺

toa‑Sibasedalloys. CirclesarefOrsamplespreparedat 350℃andtrianglesareforsamples preparedatroomtemperature.

7)Itisfoundthatthereisaverysimple

correlationbetweenthethermalequilibriumtemperature7bandtheoptical bandgapEbpfforbothwide‑bandgapandnarrow‑bandgapa‑Sibasedalloys.

AsshowninFig､3,ThincreaseswithEbp!froml45℃to320℃approximately inalinearwayovertherangeofEbp!from1．45eVfora‑Sio.62Ge0.38:Htoabout 2.05eVfora‑Sio.80C0.20:H,bothfollowinganequation,Th=Ebp'/40AItis interestingthattheextrapolatingavailableexperimentaldatatovanishingE.

makes7b(Eb"!=0)OK・TheproportionalrelationbetweenThandEb"'should betestedinawiderrangeofthebandgapinhigher‑alloy‑contenta‑Sibased

alloys,evenina‑Ge:Handa‑C:H.

8)P‑relatedhype㎡inecentersarestudiedbyESRinbothP‑dopeda‑Si:H

anda‑Si,̲"C":Halloys.Hyperfinesplittingisfoundtodecreasewiththedecrease

inPdopinglevelfrom245Gto203G,butincreasesupto280Gwithdecreasing nforP‑dopeda‑Si:H.InP‑dopeda‑Si,̲zCz:H,HFSincreaseswithCcontent,

althoughEi,shiftsdownwardsfromtheconductionbandedgeinallthreecases.

TheseresultsaredifficulttoexplainbyP:modelwhichexpectsthatHFS

increaseswiththedownwardshiftofafromtheconductionbandedge.The weakSi‑PbondmodeltakingaccountoftheinfluenceofHcanexplainthese

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experimentalresults.ItisalsonoteworthythatP!modelcanactuallybeincluded

intotheweakSi‑Pbondmodelastheextremeoftheweakbond.

9)Ina‑Si,−重C.z:Hpreparedbyusing!3C,thelinewidthofESRsignalswas prominentlyincreasedwithincreasingCcontentx,fromabout7Gto30GfOr

a Sio.45Co.55:H(seeFig.4).Intherangeofasmallxuptoaboutx=0.3,itis

possiblethatSidanglingbondswith'3Catbackbondsaretheoriginofthe wideESRsignal,whichmeansthedominantdefectsareSidanglingbonds・In thesampleswithalargex,]3Cdanglingbondsshouldexist・Thefactthatno

splittingoftheESRsignalwasobservedinanycaseinvestigatedimplieseither Cdanglingbondsmaybeclusteredorthes‑characterinCdanglingbondstate isdistributed.

論文審査の結果の要旨

平成2年2月6日の口頭発表の結果をふまえ，その後論文審査委員会を開催し，協議の結果，以下の通り判定した。

本論文ではアモルファス太陽電池や液晶ディスフ・レー用薄膜トランジスタなどへの応用がなされている水素化アモルファスシリコンとその合金薄膜(SiにC,N,Geなどを合金化したもの）の構造欠陥について主としてESRを用いて研究を行っている。これらの薄膜材料は今後電子材料として幅広く利用されることが期待されているが，その光電特性の向上のためには欠陥密度の低減が極めて重要である。本論文ではこのような欠陥の成因を明らかにする目的で，欠陥密度と熱平衡状態との関係を詳しく調べ，次のような極めて重要な事実を明らかにしている。即ち，200℃以上に温度を上げると可逆的に欠陥密度が上昇し，急冷によってその欠陥密度が凍結される。このような振る舞いは水素化アモルファスシリコンだけでなく，ずっと欠陥密度の大きなSi‑CやSi‑Nなどの合金薄膜においても見られる｡これらの結果はいくつかの国際会議においても報告され，

国際的にも注目され，高い評価を得ている。今後，もっと光電特性の優れた欠陥密度の少ない膜を作るための基礎として，本論文の結果は重要な意味を持っている。この他にも表面欠陥の正体を明らかにしたり，不純物Pに関係した欠陥に関する新しい知見を得るなど，重要な成果を得ている。以上のように本論文は極めて優れた内容を有しており，

学術博士の学位に値するものと判定する。

Defects in Hydrogenated Amorphous Silicon and Its Based Alloy Films

Defects in Hydrogenated Amorphous Silicon and Its Based Alloy Films

DefectsinHydrogenatedAmorphousSiliconand

（水素化アモルファスシリコン及びシリコン系合金薄膜

Itisfoundthatalloyinga‑Si:HwithCorNincreasesbulkdefectdensityby ashighasthreeordersofmagnitudewhilethesurfacedefectdensityremains unchangedwithinafactorof3‑4.Thennalequilibriumphenomenahavebeen

inundopeda‑Si:Handitsalloysseemnottobedirectlyinvolvedinthethermal equilibrationalthoughtheneutralbulkdefectdensityincreaseswithtemperature above"thermalequilibriumtemperature''Tb.Thermallyinducedreversible changerelaxesfollowingastretchedexponentialform.Aninterestingcorrelation

ofalloysandthephysicalmechanismunderlyingtherelationisdiscussed,The resultsonthedefectsrelatedtodopingandalloyingarealsodescribed.

itisextremelyimportanttounderstandtheoriginandnatureofthedefects

alloys・Thisworkismainly

〆

d ( ム ）

1)ESRandCPMstudiesshowthatthereisasurfacedefectivelayerona‑

Si:Hfilmswithatypicalthicknessaround200A(seeFig.1).Experimentalresults

2)Alloyinga‑Si:HwithCorNincreasesthebulkdefectdensitybynearly threeordersofmagnitude,fromlO'5cm‑3tolO'8cm‑3,whileitonlyincreasesthe surfacedefectdensitybyafactorof3,varyingwithinl‑4×10'2Cm‑2.

3)Su㎡acedefectscanbedescribedbyanexponentialdistributioninthe nearsurfaceregion・Theoptimalfittinggivesl20Aofacharacteristicsurface t h i c k n e s s a n d l . 2 × 1 0 ' 8 c m ‑ 3 o f t h e s u r f a c e d e f e c t d e n s i t y f o r a ‑ S i : H ( F i t t e d r e s u l t

above7h(around200A).Thermalequilibriumdefectdensityincreaseswith

densityandtheinsitumeasurementofthedefectdensityatelevated

〃

／

／ 霧

1 0 8

４３ ００００

ｊ

△ ご 宝 ' O . 9 f し O O 6

｡g 5 . 0 0 9 3 0 y O C

thermally‑inducedincreaseinthedefectdensityisnotduetoeitherthe

E

H , r e l a x a t i o n d a t a o f t h e f r o z e n ‑ i n i n c r e a s e

l 6 ! 9

, d 8

220

seeifthereisacolTelationbetweentheHUo.51.0

fortherelaxationtimecanfurtherverifyand△必,asfunctionofxina‑Si,̲"Cz:

w h e t h e r " H ‑ g l a s s m o d e l ' ' c a n b e e x t e n d e d g m : W S A d " 急 ぎ s 脇 馳 獺

AsshowninFig､3,ThincreaseswithEbp!froml45℃to320℃approximately inalinearwayovertherangeofEbp!from1．45eVfora‑Sio.62Ge0.38:Htoabout 2.05eVfora‑Sio.80C0.20:H,bothfollowinganequation,Th=Ebp'/40AItis interestingthattheextrapolatingavailableexperimentaldatatovanishingE.

alloys,evenina‑Ge:Handa‑C:H.

8)P‑relatedhype㎡inecentersarestudiedbyESRinbothP‑dopeda‑Si:H

inPdopinglevelfrom245Gto203G,butincreasesupto280Gwithdecreasing nforP‑dopeda‑Si:H.InP‑dopeda‑Si,̲zCz:H,HFSincreaseswithCcontent,

TheseresultsaredifficulttoexplainbyP:modelwhichexpectsthatHFS

experimentalresults.ItisalsonoteworthythatP!modelcanactuallybeincluded

9)Ina‑Si,−重C.z:Hpreparedbyusing!3C,thelinewidthofESRsignalswas prominentlyincreasedwithincreasingCcontentx,fromabout7Gto30GfOr

possiblethatSidanglingbondswith'3Catbackbondsaretheoriginofthe wideESRsignal,whichmeansthedominantdefectsareSidanglingbonds・In thesampleswithalargex,]3Cdanglingbondsshouldexist・Thefactthatno

d ( ム）

／霧

４３００００

△ ご宝 ' O . 9 f し O O 6

w h e t h e r " H ‑ g l a s s m o d e l ' ' c a n b e e x t e n d e d g m : W S A d " 急ぎ s 脇馳獺