Effects of defects and local thickness modulation on spin-polarization

JOURNALOFAPPLIEDPHYSICS108,094509 2010

Effectsofdefectsandlocalthicknessmodulationonspin-polarizationinphotocathodesbasedonGaAs/GaAsPstrainedsuperlattices

XiuguangJin,1,a YuyaMaeda,1ToshioSasaki,2ShigeoArai,2YoichiIshida,3

MasatakaKanda,3ShingoFuchi,1ToruUjihara,1TakashiSaka,3andYoshikazuTakeda1

1

DepartmentofCrystallineMaterialsScience,GraduateSchoolofEngineering,NagoyaUniversity,Furo-cho,Chikusa-ku,Nagoya464-8603,Japan2

HighVoltageElectronMicroscopeLaboratory,EcoTopiaScienceInstitute,NagoyaUniversity,Furo-cho,Chikusa-ku,Nagoya464-8603,Japan3

DepartmentofElectricalandElectronicEngineering,SchoolofEngineering,DaidoUniversity,Takiharu-cho10-3,Minami-ku,Nagoya457-8530,Japan

Received21June2010;accepted28September2010;publishedonline10November2010 Thespin-polarizationofelectronsfromtheGaAs/GaAsPsuperlatticeonaGaAssubstrate 90% ishigherthanthatfromthesamesuperlatticeonaGaPsubstrate 60% .TransmissionelectronmicroscopyandatomicforcemicroscopyobservationsrevealedthatstackingfaultswerethemaindefectsinthesuperlatticeontheGaAssubstrate,whilelocalthicknessmodulationofthesuperlatticelayerswasprominentinthesuperlatticeontheGaPsubstrate.Accordingtothedensityofstackingfaultsandthearealratioofthethicknessmodulation,itwasconcludedthatthethicknessmodulationinthesuperlatticewasthemainreasonforthespin-polarizationreductioninthephotocathodeontheGaPsubstrate.GrowthofathinGaAslayeronaGaPsubstratepriortosuperlatticegrowtheliminatedthethicknessmodulationandthespin-polarizationwasrecoveredto90%.©2010AmericanInstituteofPhysics. doi:10.1063/1.3506658

I.INTRODUCTION

Highlyspin-polarizedelectronsourcesbasedontheGaAs-typesemiconductorphotocathodehavebeendevel-opedmainlyforacceleratorsinthe eldofnuclearandpar-ticlephysics.1,2PhotocathodesbasedonaGaAs/GaAsPstrainedsuperlattice SL 3andanAlInGaAs/AlGaAsstrainedSL4providehighspin-polarizationsofapproxi-mately90%.Recently,thesephotocathodeshaveattractedconsiderableattentionaselectronsourcesforseveraltypesofelectronmicroscopesusedformagnetization-sensitiveimaging.5Forapplicationtoelectronmicroscopes,highbrightnessisrequiredforimagecontrastandamuchhigherimage-acquisitionrate.However,thebrightnessofconven-tionalphotocathodes 103–104Acm 2sr 1 islowerthanthatoftheLaB6thermionic-emitter 105–106Acm 2sr 1 ,whichisatypicalelectronsourceforelectronmicroscopes.Inourpreviousstudy,6,7wedevelopedanewhigh-brightnesstransmission-typeGaAs/GaAsPstrainedSLphotocathode.Inthisnovelsystem,asuperhighbrightnesselectronbeamwasachievedbyilluminatingasmallareaonthebacksideofthephotocathodewithapump-laserlightthatwaswell-focusedbyashort-focal-lengthlens.Torealizethissystem,aGaAs/GaAsPstrainedSLwasfabricatedonaGaPsubstrate band-gapenergy:2.26eV ,whichistransparenttothepump-laserbeam beamenergy:1.44–1.77eV ,insteadofaGaAs bandgapenergy:1.42eV substrate.Highspin-polarization 90% andasuperhighbrightness 1.3 107Acm 2sr 1 electronbeamwerereproduciblyachievedwiththistransmission-typephotocathode.7Thekeytechnologytoachievethehighspin-polarizationinthisphotocathodeisthe

a

introductionofa500nmGaAsinterlayer IL depositedontheGaPsubstrate.Thespin-polarizationofthephotocathodewithouttheGaAsILwasapproximately60%.

Inthisstudy,weinvestigatedthedefectsandstructuralchangeinphotocathodesgrownonaGaAssubstrate,GaPsubstrate,andaGaPsubstratewiththeGaAsIL,usingcross-sectionaltransmissionelectronmicroscopy TEM andatomicforcemicroscopy AFM .Theeffectsofthedefectsandstructuralchangesonthespin-polarizationintheGaAs/GaAsPstrainedSLphotocathodesarediscussed.II.EXPERIMENTAL

Electronicmail:jin@mercury.numse.nagoya-u.ac.jp.

Thephotocathodesweregrownusingalow-pressuremetalorganicvaporphaseepitaxysystemwithaverticalcold-wallquartzreactor.Triethylgallium,tertiarybutylphos-phine,andtertiarybutylarsinewereusedassourcematerials.ThesubstrateswereZn-dopedp-typeand 001 -orientedGaAsandGaP.Allsamplesweregrownat660°Cunderareactorpressureof76Torr.Threesubstrateswereplacedonthesusceptorandsamplesweregrownatthesametimeinthesamerun.Thesesampleswerethenusedforspin-polarizationmeasurements,x-raydiffraction,andTEMob-servations.

ThephotocathodestructuresareshowninFig.1.Fol-lowingthegrowthof1or2 mofGaAsPbufferlayerontheGaAssubstrate,GaPsubstrate,orGaPsubstratewitha500nmthickGaAsIL,a12-pairGaAs/GaAsPstrainedSLwasgrownwithZndopingtoaconcentrationof1.5 1018cm 3.ThedesignedthicknessoftheGaAsandGaAsPlayerswas4nm.Subsequently,eachSLstructurewascoatedwithahighly-doped5nmGaAslayerwithaZndopantconcentrationof6 1019cm 3.The nalsurfacewasactivatedwithaCs–Ocoatingfornegativeelectronaf nity

©2010AmericanInstituteofPhysics

0021-8979/2010/108 9 /094509/6/$30.00108,094509-1

094509-2Jinetal.

GaAs5nmZn6 1019cm-3GaAs4nm,GaAsP4nm

Zn1.512 pairs

1018cm-3

GaAsP2 mGaAsP2 mGaAsP1 mZn1.5 1018cm-3Zn1.5 1018cm-3

Zn1.5 1018cm-3GaAs0.5 mp-typeGaAs(001)

p-typeGaP(001)

p-typeGaP(001)

substratesubstrate

substrate

Spin-polarization:92%Spin-polarization:64%Spin-polarization:90%

FIG.1.Detailsofthephotocathodestructures.GaAs/GaAsPSLsweregrown

on

buffer

layerswithdifferentsubstrates:GaAs,GaP,andGaPsub-stratewithGaAsIL.Maximumspin-polarizationsobtainedfromeachsampleareshownatthebottomofeachstructure.

photocathodeformation.Thestructuraldetailsofthegrownphotocathodesobtainedbyx-raydiffractionmeasurementaresummarizedinTableI.Theslightdifferencesfromthede-signedvalueof4nmareduetothereproducibilitylimitsofthesourcegas owcontrollers.

Thespin-polarizationofthephotocathodeontheGaAssubstratewasmeasuredusingNPES-3,8whilethephotocath-odesontheGaPsubstrateweremeasuredusingJPES-1.6ThesesystemsweredesignedandfabricatedatNagoyaUni-versity.WithNPES-3,thepump-laserlightirradiatesthephotocathodesurfaceandelectronsareemittedfromthesameside.Ontheotherhand,withJPES-1,thepump-laserlightirradiatesthephotocathodebacksideandtheelectronbeamisextractedfromthephotocathodesurface.Thespin-polarizationsofthephotocathodesontheGaAssubstrateandtheGaPsubstratewiththeGaAsILwereover90%,whilethatontheGaPsubstratewasonly60%.

TEMobservationswereperformedatanelectronenergyof200keVusingaHitachiH-800electronmicroscopy.Cross-sectionalTEMspecimenswerepreparedbystandardsamplepreparationtechniquesasfollows: 1 twosmallpiecesofthesampleorientedalongthe 110 and 1¯10 directionswereadheredtoeachother; 2 thespecimenswerethinnedfrombothsidestoathicknessof20 musingaplanargrinderandthenpolishedwithAl3 Ar+2O3suspension; -ionmillingwascarriedoutat3.3keVunderanangle4°withanioncurrentof10 Auntilasmallperforationappearedinthecenterofthesample.Thesurfacemorphol-ogywasobservedusingAFM NanoScopeIII toevaluatethedistributionofdefectsandthestructuralchanges.

TABLEI.Structuraldetailsofphotocathodes.

PcompositionofGaAsPbufferlayerPeriodofSL

Sample

andGaAsPbarrierlayer

nm

SLonGaAssub0.348.0SLonGaPsub

0.388.6SLonGaPsubwithGaAsIL

0.3

8.6

J.Appl.Phys.108,094509 2010

(a)GaAsPbufferlayer/GaAssubstrate

(b)GaAsPbufferlayer/GaPsubstrate

(c)GaAsPbufferlayer/GaAsinter-layer/GaPsubstrate

FIG.2.Cross-sectionalTEMimagesoftheGaAsPbufferlayeron a GaAs, b GaP,and c aGaPsubstratewithGaAsIL.Stackingfaultsareintro-ducedintotheGaAsPbufferlayeronGaAs,whiledislocationsareintro-ducedonGaP.

III.RESULTS

A.TEMimageofGaAsPbufferlayeronvarioussubstrates

Figure2showscross-sectionalTEMimagesatthehet-erointerfacesbetweentheGaAsPbufferlayersandsub-strates.Regularlydistributedmis tdislocationarrayswereintroducedtorelievethemis t-strainintheGaAsPbufferlayersinallthreecases.Stackingfaultsextendfromthehet-erointerfaceintotheGaAsPbufferlayerontheGaAssub-strateonthe 111 plane,asshowninFig.2 a .Somestack-ingfaultsterminateatotherstackingfaults.Ontheotherhand,somedislocationsareobservedintheGaAsPbufferlayerontheGaPsubstrate,asshowninFig.2 b .Thedislo-cationdensitydecreaseswiththeincreaseinthethicknessoftheGaAsPbufferlayer.Weconsiderthatthedifferenceoftheintroduceddefectsisrelatedtothedifferenceofthestrain

094509-3Jinetal.(a)

(b)

FIG.3.Cross-sectionalTEMimagesofGaAs/GaAsPSLandGaAsPbufferlayeronaGaAssubstrate.Twoimageswereobtainedatdifferentpositionsfromthesamesample.Stackingfaults,threadingdislocations,andmis tdislocations

areevident.

relaxationprocessintheGaAsPbufferlayer:thecompres-siveandtensilestrainswererelievedintheGaAsPbufferlayerontheGaAssubstrateandtheGaPsubstrate,respec-tively.Inordertocontrolthestrainrelaxationprocess,aGaAsILwasgrownontheGaPsubstratepriortogrowthofthebufferlayer,asshownintherightofFig.1andinFig.2 c .StackingfaultsareinducedintheGaAsPbufferlayerontheGaAsIL,similartotheGaAsPbufferlayergrowndirectlyontheGaAssubstrate Fig.2 c .Thisindicatesthatstackingfaultsareduetotensilestrainrelaxationanddislo-cationsareinducedtorelaxthecompressivestrain.ManydislocationsareobservedintheGaAsILduetothelargelatticemismatchbetweenGaAsandGaP 3.7% .However,mostoftheseareterminatedattheinterfaceoftheGaAsPbufferlayerandtheGaAsIL.

B.TEMimagesoftheGaAs/GaAsPSLlayerandGaAsPbufferlayer

Figure3showscross-sectionalTEMimagesoftheSLlayerandtheGaAsPbufferlayerontheGaAssubstratesobtainedatdifferentpositionsonthesamesample.StackingfaultsextendedfromthebufferlayerintotheSLlayer,asshowninFig.3 a .Inaddition,afewmis tdislocationsandthreadingdislocationsareobservedinFig.3 b .TEMobser-vationrevealedthedefectdensitiestobeapproximately10–12stackingfaultsper10 m,4–5mis tdislocationsper10 m,and4–5threadingdislocationsper10 m.Thepe-riodoftheSLisregulatedverywell,althoughitbecomesslightlydisorderedaroundthestackingfaultsandthreadingdislocations.

Figure4showscross-sectionalTEMimagesoftheSLandGaAsPbufferlayerontheGaPsubstrate.Theremark-ablefeatureinthissampleisthebumpstructuresshowninFig.4 a .Thesizeofthebumpstructureisapproximately100nmwideand30nmhigh.ThelayerthicknessintheSLismodulatedinthebumpstructure.Thethicknessmodula-tionincreaseswiththeincreasingnumberofthelayerpairs gray/black andisespeciallyenhancedafterthe fthpair.AthickerGaAsPbarrier-layer gray isgrownonathinner

J.Appl.Phys.108,094509 2010

(a)

(b)

(c)

FIG.4.Cross-sectionalTEMimagesoftheGaAs/GaAsPSLandGaAsPbufferlayer

on

aGaPsubstrate.Threeimageswereobtainedatdifferentpositionsfromthesamesample. a ThicknessmodulationofSL, b thread-ingdislocation,and c mis tdislocationareshown.

GaAswell-layer black ,whileathinGaAsPbarrier-layerisgrownonathickerGaAswell-layer.Beneaththeconcavesurface,onlytheGaAsPbarrier-layergrowswithouttheGaAswell-layernearthesurface.Inotherareas,asshowninFigs.4 b and4 c ,afewthreadingdislocationsandmis tdislocationsareevident.Nostackingfaultswerefoundinthisphotocathode.

Across-sectionalTEMimageoftheSLandtheGaAsPbufferlayerontheGaPsubstratewiththeGaAsILisshowninFig.5.ThesurfaceoftheSLissmoothwithno

bump

(a)

(b)

FIG.5.Cross-sectionalTEMimagesofGaAs/GaAsPSLandGaAsPbufferlayeronaGaPsubstratewithGaAsIL.Twoimageswereobtainedatdif-ferentpositionsfromthesamesample.TheimageissimilartothatoftheSLontheGaAssubstrate Fig.3 .

094509-4Jinetal.J.Appl.Phys.108,094509 2010

(a)SL/GaAsPbuf./GaAssub.

(b)SL/GaAsPbuf./GaPsub.

(c)SL/GaAsPbuf./GaAsILFIG.6. Coloronline AFMimagesofGaAs/GaAsPSLandGaAsPbufferlayeron a GaAs, b GaP,and c aGaPsubstratewithGaAsIL.In b ,bumpstructuressurroundthehillockstructures.Notethatthelinepro leisverydifferentfrom a or c .

structure.TheimageissimilartothatoftheSLontheGaAssubstrate,i.e.,theSLlayersareperiodicandstackingfaultspenetratefromthebufferlayerintotheSL Fig.5 a .Inaddition,afewthreadingandmis tdislocationsareobserved Fig.5 b andthedensityofthedefectsissimilartothatintheSLontheGaAssubstrate.

C.SurfacemorphologyofGaAs/GaAsPSL

directions.Nobumpstructureswereobserved.ThestructuralfeaturesaresimilartotheSLgrownontheGaAssubstrate.IV.DISCUSSION

ThelayerthicknessintheSLbeneaththebumpstructureismodulated.ThespatialdistributionsofthebumpstructureswereevaluatedusingAFMandareshowninFig.6.ThesurfaceoftheSLontheGaAssubstratehassomehillocksofapproximately50nminheight,whichappearstobeweaklyrelatedwiththecrystalorientation.Inaddition,afewridgestructuresalongthe 110 directionsareobserved.TheridgestructuresareconsideredtoberelatedtothestackingfaultsshowninFig.3 a .ThesurfaceoftheSLontheGaPsub-stratealsohassomeanisotropichillocksof100nminheight,asshowninFig.6 b .However,noridgestructuresareob-served.Instead,manybumpstructuresarefoundtosurroundthehillocks.Thewidthofthebumpstructuresisapproxi-mately100nm,whichcorrespondstothesizeofthebumpstructuresintheTEMimageofFig.4 a .Thus,thethicknessmodulationsjustbeneaththebumpstructuresarepresentinapproximately30%oftheareainthisphotocathode.Figure6 c showsthesurfaceoftheSLontheGaPsubstratewiththeGaAsIL.Anisotropichillocksareevidentandmanyridgestructuresareformedalongthe 110 and 1¯10

Thedensitiesoftheseveraltypesofdefectsandtheareal

ratioofthethicknessmodulationareaaresummarizedinTableII.StackingfaultsarethemaindefectsintheSLontheGaAssubstrateandtheGaPsubstratewiththeGaAsIL,althoughthelocalthicknessmodulationoftheSLlayerswasonlyprominentintheSLontheGaPsubstrate.AccordingtotheTEMobservation,thedensitiesofstackingfaultsintheSLontheGaAssubstrateandGaPsubstratewithGaAsILareintherangeof10–15per10 m.Thethicknessmodu-lationintheSLontheGaPsubstrateispresentoverapproxi-mately30%ofthearea,accordingtotheAFMobservation.Thedensitiesofthreadingandmis tdislocationsinthethreephotocathodeswerealmostthesame.

A.Effectofstackingfaultsonthespin-polarization

Inspintronicsresearch,spin- ipbydefectshasbeenre-ported.Inspin-polarizedlight-emittingdiodes,9spin-polarizedelectronswereinjectedfromZnMnSeintoaGaAs/AlGaAsquantumwell,whereradiativerecombinationofthecarriersresultedintheemissionofcircularlypolarizedlight.Thespin-polarizationoftheinjectedelectronswasinverselycorrelatedwiththestackingfaultdensityattheZnMnSe/AlGaAsinterface.Spin- ipbydefectswasexplainedbyan

TABLEII.Densitiesofdefectsandarealratioofthicknessmodulation.

Numberofthedefectsper10 m

Sample

SLonGaAssubSLonGaPsub

SLonGaPsubwithGaAsIL

Stackingfault

10–12012–15

Threadingdislocation4–53–45–6

Mis tdislocation4–54–55–6

Arealratioofthicknessmodulation

%

0 300

094509-5Jinetal.

(b)ThicknessmodulatedJ.Appl.Phys.108,094509 2010

(a)Periodicarea

FIG.7.Cross-sectionalTEMimageofthe a periodicand b thicknessmodulatedareasofSLlayersonaGaPsubstrate.Intheperiodicarea,thethicknessesoftheGaAsandGaAsPlayersare3.6nmand5nm,respec-tively.InthethicknessmodulatedareaalongtheAA line,theaveragethicknessesoftheGaAsandGaAsPlayersare6nmand4nm,respectively,andalongBB linethethicknessesoftheGaAsandGaAsPlayersare8nmand2nm,respectively.

Elliot–Yafetmodel10thatshowstheasymmetricpotentialofthedefectresultsinstrongspin- ipscatteringintheforwarddirection.Accordingtothecorrelationbetweenthedensityandthespin-polarization,over90%spin-polarizationwasrealizedevenwithastackingfaultdensityof10–15per10 m.Thevalueofthespin-polarizationwasalmostthesameasthoseinthepresentstudy.Therefore,weconsiderthattheeffectofstackingfaults 10–15per10 m onthespin-polarizationisnotsigni cant.

B.EffectofthicknessmodulationoftheSLonspin-polarization

Inourpreviousstudy,11wespeculatedthatspin- ipduetodislocationsreducedthespin-polarizationintheSLontheGaPsubstrate.However,TEMobservationrevealedthatthedistinctivefeatureintheSLontheGaPsubstratewasthethicknessmodulationoftheSLandnotdislocations.Details

(a)Inperiodicarea

GaAsGaAsP3.6nm5nm

ofthemechanismfortheformationofthethicknessmodu-lation,andhencethebumpstructure,onlyintheSLontheGaPsubstratewillbeexplainedelsewhere.12

Thespincomponentsoftheelectronsexcitedbycircu-larlypolarizedphotonsfromheavyandlightholesareoppo-sitetoeachother.InthestrainedSLstructure,theheavyandthelightholemini-bandsaresplit,duetoboththequantumcon nementeffectandtheelasticstraineffect.Thespin-polarizationismaximizedbyadjustingtheexcitationphotonenergytotheenergygapfromtheheavyholeminibandtotheconductionminiband.

Theenergyofthebandgapissigni cantlydependentonthethicknessesoftheSLlayers.TEMimagesoftheperiodicandthicknessmodulationareasintheSLontheGaPsub-strateareshowninFig.7.Intheperiodicarea,thethick-nessesoftheGaAswell-layerandGaAsPbarrier-layerare3.6nmand5nm,respectively.Ontheotherhand,inthethicknessmodulationarea,thethicknessoftheGaAswell-layerandtheGaAsPbarrier-layeraresigni cantlydifferentfromthedesignedthicknessof4nm.InFig.7 b ,lineAA isalongthegrowthdirectionofthebumpstructureandlineBB isslightlyofflineAA .TheGaAswellthicknessesarewiderinthebumpstructureandtheelectronwavepacketmaylocalizeneartheBB line.Inordertoestimatetheef-fectsofthedifferenceinthicknessesalongtheAA andBB lines,thebandstructureswerecalculatedassumingaveragedGaAswell-layerandGaAsPbarrier-layerthicknesses.AlongtheAA line,theaveragethicknessoftheGaAswell-layeris6nmandthatoftheGaAsPbarrier-layeris4nm,andalongtheBB linetheaveragethicknessoftheGaAswell-layeris8nmandthatoftheGaAsPbarrier-layeris2

nm.

area(b)Inthicknessmodulationarea(c)Inthicknessmodulation

alongAA`alongBB`

GaAsGaAsP6nm4nm

GaAs8nm

GaAsP2nm

Conductionmini-bandPolarizedlightHeavyholemini-bandLightholemini-band

Bandsplit:110meV

Bandsplit:100meV

Bandsplit:90meV

Energybandgap:1580meV

Energybandgap:1545meV

Energybandgap:1520meV

(d)Inthicknessmodulationarea(e)Inthicknessmodulationarea

alongBB`.WhenhalfofthealongBB`.Whenhalfofthestrainrelieved.strainrelieved.

Conduction

mini-band

Energybandgap:1520meV

Energybandgap:1490meV

Heavyholemini-bandLightholemini-band

Bandsplit:60meVBandsplit:50meV

FIG.8.Energybandstructuresinthe a periodic,and b thicknessmodulatedareasalongAA ,and c alongBB notedFig.7 .Energybandstructuresalong d AA and e BB ,whenhalfofthestrainisassumedtoberelievedinthethicknessmodulatedarea.Inthethicknessmodulatedareas,bandgap,andbandsplitbecomesmaller.

094509-6Jinetal.SeveralenergybandstructureswerecalculatedusingModel-solid13andtheKronig-Pennymodel,14asshowninFig.8.IntheSLstructurealongtheBB line,theenergygapfromthelightholeminibandtotheconductionminibandisclosetothatfromtheheavyholebandtotheconductionminibandintheperiodicarea.Further,itisreportedthatthethicknessmodulationsareattributedtostrainrelaxation.15,16Thestrainrelaxationalsoreducestheenergybandgap.Thebandstructuresinthemodulatedarea,whenitisassumedthathalfofthestrainisrelieved,areshowninFigs.8 d and8 e .Itispossiblethatoppositelyspin-polarizedelectronsareexcitedfromtheheavyholeminibandintheperiodicareasandfromthelightholemini-bandinthethicknessmodulatedareasbythesameenergyoftheexitedphotons.Weconsiderthatthedegradationofspin-polarizationintheSLontheGaPsubstrateisrelatedtothethicknessmodulatedareas.

V.CONCLUSION

DefectsandstructuralchangeinGaAs/GaAsPSLsonaGaPsubstrate,GaAssubstrate,andaGaPsubstratewithaGaAsIL,asspin-polarizedphotocathodes,wereinvestigatedbyTEMandAFM.StackingfaultswereobservedintheSLonGaAs,whilebumpstructures,whicharerelatedwiththicknessmodulationsintheSLlayer,wereobservedintheSLonGaP.Theeffectofstackingfaults 10–15per10 m onthespin-polarizationwasnotsigni cant,whilethethick-nessmodulationintheSLresultedinlargechangestotheenergybandstructureanddegradationofthespin-polarization.GrowthofaGaAsthinlayerontheGaPsub-stratepriortoSLgrowtheliminatedthethicknessmodulationandthespin-polarizationwasrecoveredto90%.ACKNOWLEDGMENTS

ThisworkwaspartiallysupportedbytheTechnologyDevelopmentProgramforAdvancedMeasurementandAnalysisoftheJapanScienceandTechnologyAgency JST andResearchforPromotingTechnologySeeds.Oneoftheauthors X.G.Jin issupportedbyaResearchFellowshipfor

J.Appl.Phys.108,094509 2010

YoungScientistsfromtheJapanSocietyforthePromotionofScience JSPS .TheauthorsalsowishtothankProfessorsS.ZaimaandY.ShimuraoftheDepartmentofCrystalMateri-alsScienceatNagoyaUniversityforprovidingadvicere-gardingpreparationoftheTEMspecimens.

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