XPS for probing the dynamics of surface voltage and photovoltage in GaN

(1)

ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o ur na l ho me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

XPS

for

probing

the

dynamics

of

surface

voltage

and

photovoltage

in

GaN

Hikmet

Sezen

a

_,

_Ekmel

_Ozbay

b

_,

_Seﬁk

_Suzer

a,∗

a_Department_of_Chemistry,_Bilkent_University,₀₆₈₀₀_Ankara,_Turkey

b_Departments_of_Physics_and_Electrical_and_Electronics_Engineering,_Bilkent_University,₀₆₈₀₀_Ankara,_Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received10February2014

Receivedinrevisedform14June2014 Accepted14June2014

Availableonline20June2014 Keywords: Chargingdynamics Surfacephotovoltage XPS GaN Doping

a

b

s

t

r

a

c

t

WedescribeapplicationoftwodifferentdatagatheringtechniquesofXPSforprobingthedynamicsof surfacevoltageandsurfacephotovoltage(SPV)developedinmicrosecondstosecondstime-domain,in additiontotheconventionalsteady-statemeasurements.Forthelonger(secondstomilliseconds)regime, capturingthedatainthesnapshotfashionisused,butforthefasterone(downtomicroseconds),square wave(SQW)electricalpulsesatdifferentfrequenciesareutilizedtoinduceandprobethedynamicsof variousprocessescausingthesurfacevoltage,includingtheSPV,viathechangesinthepeakpositions. Thefrequencyrangecoversanywherefrom10−3to105_Hz_for_probing_changes_due_to_charging_(slow), dipolar(intermediate),andelectronic(fast)processesassociatedwiththeexternalstressesimposed.We demonstrateitspowerbyapplicationton-andp-GaN,anddiscussthechemical/physicalinformation derivedthereof.Inaddition,themethodallowsustodecomposeandidentifythepeakswithrespectto theirchargingnatureforacompositesamplecontainingbothn-andp-GaNmoieties.

1. Introduction

GaNisawideband-gapcompoundsemiconductorwith supe-rioroptoelectronicpropertiesutilizedinnumerousdevicesforhigh powerlightemittingdiodeandsensorapplications[1].Surfaceand defectstructuresofthematerialsusedcompletelydeterminemany ofthedeviceperformance,hencecontroloverthesepropertiesare ofutmostimportance.Surfacephotovoltage(SPV)[2,3],isoneofthe propertiesmeasuredtoevaluatethequalityandtheperformanceof suchmaterialsanddevices,andhasrecentlybeencombinedwith microscopictechniquessuchastheKelvinProbe(KP)[4,5].SPV measurementsrelateelectricalinformationtobandstructure,as wellastothenatureofsurfacemoieties, andmostimportantly tothenature of impurities and defects.However,even though asuperb (sub-micron)lateralresolutionisachievable,theKPis basedonelectricalmeasurements,hencedoesnothavechemical speciﬁcity.Ontheotherhand,X-rayPhotoelectronSpectroscopy (XPS),whichhasalsobeenusedforprobingSPV,provides excel-lentchemicalinformation,inadditiontotheelectricalpotentials developedonsurfacestructures,althoughtheprecisionfor mea-suringthelatterismuchlower(∼20meV)whencomparedtothat ofKPmeasurements(∼1meV)[2].

∗ Correspondingauthor.Tel.:+903122901476;fax:+903122664068. E-mailaddress:suzer@fen.bilkent.edu.tr(S.Suzer).

EarlierSPVmeasurementsweremostlysynchrotronbaseddue totheneededbrightnessofthesource[6–10],butcombinedwith X-raymicrofocusingandfastaccumulationtools,thenewgeneration commercialXPspectrometerscanalsoprovidecriticalinformation aboutsuchmaterialsanddevices[11,12].Inarecentpublication wereportedoncapturingofthetransientsurfacephotovoltagein n-andp-GaNbyXPSwherenewinformation aboutthe mecha-nismofthetransientsformedbyilluminationwitha∼50mWviolet (405nm)laserwasbroughtoutanddiscussed[13].However,faster measurementsarenotpossibleinthismode,therefore,wehadto introduceothermodulationtechniques[14–19].

XPSisawidelyusedchemicalanalysistechniqueforprobing chemicalandphysicalcompositionofsurfacestructures.In con-ventionaldatagatheringmodethesampleis grounded,and the positionaswellastheintensityofthepeaksarerecordedinastatic fashion[20].Althoughseveralreportshaveappearedinthe liter-aturerelatedtodynamicalXPSmeasurementsinthetwoextreme ends,veryfast(nanosecondstoattoseconds)[21–24],andveryslow (minutestohours)[25–27].Theintermediatedomain (microsec-ondstoseconds)hasbeenoverlooked.Theveryfastmeasurements require either synchrotron facilities and/or specialized instru-mentationwhich arenot easilyaccessibletomany researchers. Theintermediatedomainisimportantespeciallyfortestingand improvingperformanceofvariousmaterialsanddevicesusedfor charge storage, photovoltaics, ferroelectrics, chemical and bio-chemicalsensing,etc.Recently,ourgroupandothershavereported

http://dx.doi.org/10.1016/j.apsusc.2014.06.089

(2)

throughvariouspublications,thebasicprinciplesandtechniques forimplementingsuchmeasurementsemployingverysimple mod-iﬁcationtoconventionalinstruments[28–39].

Herein,we reportsuchXPSmeasurementsonn-andp-GaN samples,coveringtimeintervalsfromkilo-seconds(10−3Hz)down tomicrosecond(106_Hz),_and_discuss_the_results_obtained

pertain-ingtoelectricalpropertiesofthesampleasreflectedbythebinding energyshifts of theGa core-levels. Someof themeasurements includedinthepresentpaperhavealreadybeenpublishedinour previouspapers,buttheyarereproducedhereforthesakeof clar-ityandtoprovideacontinuousflowofthepresentation[13,40]. Thenextsectiondescribestheexperimentaldetailsandisfollowed byourresultsanddiscussions.Wefinishthepresentationbyour conclusions,tobefollowedbyacknowledgementsandreferences.

2. Experimental

GaNsamplesweregrownondoublepolishedc-planesapphire bylow-pressureMOCVD(AIX200/4RF-S).TheMgdopedp-and Si-dopedn-GaNsampleshaveconductivitiesof0.8and58S−1cm−1, respectively.AThermoFisherK-Alphaelectronspectrometerwith monochromatic AlK␣X-rays is used for XPS analysis,which is slightlymodiﬁedforimposingexternalvoltagestress(D.C.orA.C.) tothesampleduringdataacquisition. Thespectrometerisalso equippedwithalowenergyﬂood-gunfacilityforcharge neutraliza-tion,utilizingonlyelectronsorbothelectronsandAr+_ions._Sample

surfaceswerecleanedbysputtering withalowenergy(200eV) Ar+_ion_beam_for_avoiding_damage_by_the_Ar+_ions,_until_the_C1s

peakfellbelowdetectionlimits.Noannealingofthesampleswas performedaftercleaning.Toinducesurfacephotovoltagea50mW 405nm(violet)CrystaLaserlaserisemployedeitherintheC.W. mode,orthroughachopperwithafrequencyrangeof10−1–103_.

Forprobingthedynamicsofcharging/dischargingproperties,the dataisgatheredeitherinthefastsnapshotmodewith0.1stime resolutionorthesampleissubjectedtosquarewavepulses(SQW) of±10Vamplitudewithvaryingfrequenciesinthe10−3–105_Hz

rangeusingaStanfordResearchSystemDS345pulsegenerator, whilethedataisgatheredinnormalscanningmode.Ingeneralthe accuracyofourmeasurementsarebetterthan20meVinthepeak positions.However,sinceweallowthesamplestochargeand dis-charge,throughtheapplicationoftheSQWmodulation,thepeaks arebroader.Eveninsuchcase,wecanstillclaimanaccuracyof 20meV,sincethistimewemeasurethedifferencebetweentwo peakspositions,whichisinherentlymoreaccuratewhencompared toabsolutepeakpositionmeasurements.

3. Resultsanddiscussions

3.1. Photoresponseofn-andp-typeGaN 3.1.1. Steady-statemeasurements

Asurveyspectrumofthep-typeGaNhasanintenseGa2p3/2

peakat 1118.49eVand3s,3p,3dpeaksat161.41, 106.48,and 20.15eV,respectivelyasshowninFig.1.Inaddition,Gahasa num-berofLMMAugerlinesspanningtheintervalfrom380to630eV. Unfortunately,abroadLMMAugerlineofGaat397eVis overlap-pingwiththeN1speak,soitisnotpossibletofollowindividual N1sregionwithourXPSinstrumentusingAlK␣X-raysource.We generallyfollowtheGa2pifweneedhighintensity(inthesnapshot mode),buttheGa3dpeakisrecordedforbetterenergyresolution. Whenexposedtoelectromagneticradiation,materialsrespond inavarietyofwaysdependingonthewavelength,intensity,and durationof theexposure. TheGa2p_3/2 spectraof then-and p-GaNsamples,recordedwithandwithoutvioletlaserillumination, areshowninFig.2.Thehigheststeady-stateSPVforthen-and

Fig.1.AsurveyXPSspectrumofthep-GaNsampletogetherwiththeexperimental set-up.

p-GaN,aremeasuredas+0.15eVand−0.39eV,respectively,while thefloodgunisnotemployed.Whenthefloodgunisfunctioning, theresponseofespeciallythep-typeGaNissignificantlyaltered andprovidesadditionalinformationabouttheelectricalproperties ofthesemiconductor,aswasdiscussedinourpreviouspaper[13]. Whenthesamplesareintroducedintothespectrometer, elec-tronsflowfromthen-typeandholesfromthep-typeGaNsothat theyarechargeddifferently,duetotheirsignandextentofdoping. But,thechargesonthesurfacesarescreenedbybandbendingand byaccumulationofopposite-signchargesnearthesurface.When lightwithsufficientenergyisincidentonthesampleadditional chargecarriersareintroducedwhichnormallyreturnthebandsto theirflat-bandcondition[5,10,13,40–45].GaNhasaband-gapof 3.4eV,thereforethevioletlaser,whichhasonlyanenergyof3.1eV isnotexpectedtocauseanyband–bandexcitation.Presenceof sur-facestates,justabovethevalanceband,aswellasdefects,aidedby thermalenergyatroomtemperaturemustbethereasonforthis band-flattening.Asdepictedbytheschematicsshowninthelower panelofFig.2,theseprocessesmustinvolveanexcitationofan elec-tronandaholefromsurfacestatesintobulkforthen-andp-GaN, respectively,resultinginpartialflatteningofthebands.

Fig.2. Ga2p3/2regionofbothsamplesrecordedwithoutandunderillumination

withaC.W.VioletLaser(405nm)of∼50mWpower.Thelower panelsshow schematicallyﬂatteningoftheband-bendingunderillumination.

(3)

Fig.3.VariationofthepositionofGa2p3/2peakofp-GaNwithrespecttothepower

oftheilluminationsource,controlledbyNeutralDensityOpticalFilters(NDF).

Wehavealsocarriedoutmeasurementsbyvaryingtheintensity oftheexcitationsourcetoobtainarelationshipbetweenthe mea-suredvalueoftheSPVandthelogarithmoftheintensityoflightas giveninFig.3.Themostimportantﬁndingofthesemeasurements, isthatitrevealsasaturationbehaviortowardthehighintensity regionofthephotoexcitement.Thispowerdependencyofthe p-GaNindicatesthattheresponseswemeasuredaremostlydueto surface-statesanddefects’relatedprocesses,whichisalsoingood agreementwithourmechanisticexplanationofthepartialband ﬂatteningphenomenoninvolvingthesub-bandgapexcitations.

3.1.2. Transientmeasurementsusingthesnapshotmode

TheK-Alphaspectrometerallowsustorecord,withreasonably highsignal-to-noiseratio,anarrowspectralregionwithlessthan 0.1sintervals.InFig.4(a)and(b),thetime-resolvedGa2p3/2spectra

areshownforthetwosampleswhilethelaseristurnedonand offevery50s,whereweobservethattheSPVdevelopsinamuch shortertimeinterval(<0.1s)thanweareabletomeasure,butitgets severelyscreenedbytheflood-gunelectronsforthep-GaN.This indicatesthatwhenthefloodgunisoperativeanadditional mech-anismistriggeredjustafterthechangeofthestateofthelaser,asa resultofinteractionbetweentheelectronsofthefloodgunandthe surfaceofthep-GaNsample.Thiscanbeexplainedasfollows.The p-GaNdevelopsacertainmagnitudeofdownwardband-bending atitssurface.ThisdownwardbandbendingatthelaserOFFstate providesaproperwelltocapturetheflood-gunelectronsatornear theconductionband,asschematicallyshowninFig.4(c)tocause anoppositeshifttolowerbindingenergyasexpectedand mea-sured.WhenthestateofthelaserischangedtoON,thedownward band-bendingis immediatelyflattenedand thewelldisappears. Hence,theaccumulatedelectronsduring thepreviousstateare quicklysweptawaytothebulk,becausenowtheconductionband ofthebulkhasmanyfavorableenergylevelsfortheelectronsas alsoindicatedinFig.4(c).Thiseventisobservedasafurthershiftto ahigherbindingenergy.Moreover,thetimeconstantsofelectron accumulationandelectronsweepingawayfromthesurfaceofthe p-GaNsamplearedifferentandare6.3and0.85s,respectively.This differencearisesfromthedifferentnatureofthetwomechanisms. Theimpingingelectronstakelongertime tobeaccommodated, sinceeachaccumulatedelectroncontributestothebuild-upofthe negativesheetofchargeatthesurface,whichinturnsuppresses the rate of accumulation of further negative charge. However, whenthelaseristurnedon,thesweepingofaccumulatedelectrons tothebulkisveryeffectiveandquickduetoitslargevolumeof thebulkcomparedtothesurface.Noteinpassingthatwhenthe flood-gunisoperativetheSPVaresmearedout,andundercertain conditions, might be completely unobservable by the normal scanning mode of the spectrometer due to the time-averaged dataacquisition.Only,throughthecarefulimplementationofthe

Fig.4.VariationsofthecenteroftheGa2p3/2peaksrecordedwith0.1sintervalsusingthesnapshotmode,asthelaseristurnONandOFF,withtheﬂood-gunturned-offand

(4)

Fig.5.Ga3dpeakofthen-andp-GaNsamplerecorded;(i)grounded,andunder theSQWmodulationat(ii)2MHz,and(iii)10kHzfrequencies,withoutandunder photoillumination.

snapshotmodewithrespecttothetimewindowsweusedwere weableustorecoverthefullmagnitudeoftheSPV[13].

3.1.3. ApplicationofSQWpulses

Sinceneitherthesnap-shotmodenorutilizationof an opti-calchoppercanbeusedintheshortermicrosecondsregime,we makeuseofapplicationofelectricalvoltagestressesintheform ofSQWpulses.InFig.5,wedisplaytheGa3dpeakofthen-and thep-GaNsamplerecordedwhilethesamplesaregrounded,as wellasunder2MHzand10kHzSQWmodulations,andwithout andwithvioletlaserillumination.SQWmodulationofthesample resultsintwinningofallthepeaks,sincethesamplesexperience either+10Vor−10Velectricalstressduringtheup(positive)and thedown(negative)cycles,respectively.Ifthesampleis conduct-ingtheresultistrivialshiftsofthepeakpositionsto+10.00and

Fig.6. Measuredpeakpositionsat+10Vand−10VcyclesoftheGa3dpeakasa functionofthefrequencyoftheappliedSQWexcitation,withoutandunder photo-illumination.Thelowerpartplotsthedifferencebetweenthetwinnedpeaks.

−10.00eV,respectively,andwithabindingenergydifference mea-surementofexactly20.00eVbetweenthem.However,ifthesample is poorly conducting,a difference of less than 20.00eV results, sincechargeneutralizationisdifferentunder+10and−10Vbiasing cycles[14–19].Thismeasurednegativedeviationfromthe20.00eV islargeratlow frequenciessincemoretimeis allocatedforthe sampletochargeordischarge,andapproacheszeroastheSQW modulationfrequencyincreases,dependingonthemechanismof theoperatingprocesses. Hence,aplotof themeasuredbinding energydifferenceasafunctionoftheappliedSQWfrequenciesis highlyinformative(seebelowFig.6)[14–19].

Accordingly,asshowninFig.5,thebindingenergydifference iscloserto20.00eV,yetslightlylowerat2MHz,fromwhichwe

Fig.7. Ga3dpeakofacompositesampleformedbyplacingthen-andp-GaNsamplesside-by-side,recorded;(i)grounded,andundertheSQWmodulationat(ii)2MHz, and(iii)10kHzfrequencies,withoutandunderphotoillumination.Theinsetdepictstheexperimentalset-up.

(5)

cannowdeducethatn-GaNismuchmoreconductingcomparedto thep-GaNsample,inagreementwiththemeasuredvaluesgiven intheexperimentalsection.Thissimpleagreementisactuallyan importantproofofthevalidityofourmethodology,whichcould leadtonumerousanalyticalapplicationsforharvestingdielectric propertiesofbothsimpleandcompositesurfacestructures[28,46]. Inaddition,themeasureddifferenceof19.81eVatthelower fre-quencyof2MHzislowerthan19.98eVmeasuredat10kHz,while thelatteriswithintheexperimentaluncertaintyofthe theoreti-calvalueof20.00eV.Thesituationisverydifferentforthep-GaN, whichisdrasticallycharged,andthemeasuredvalueis16.74eV at2MHz, and approaches(19.93eV)to thetheoreticalvalueat 10kHz,butdoesnotquitereachit.Whatonelearnsfromthese mea-surementsisthat,especiallyforthep-GaN,charging/discharging propertiesarestronglyaffectedbyamultitudeofprocesses opera-tiveinthemicrosecondsallthewayupsecondsrange.

Inthesameﬁgure,datarecordedunderlightilluminationare alsogiven.For then-GaN,themagnitude ofthephotoresponse issmall,andisthesameirrespectiveofthemodeofdata gath-ering,whichrevealsthat photoresponseof thesampleis much faster(<10␮s)thanwecanmeasureevenifweemploytheSQW modulationuptothefrequencyof105_Hz._The_magnitude_of_the

photoresponseofthep-GaNismuchlarger,and,similartothecase oftheelectricalcharging,hasmultiple(slow,intermediateandfast) components,ascanbededucedfromthefactthatevenat10kHz modulation,themeasuredbindingenergydifferenceof19.97eV, underilluminationislargerthanthat(19.93eV)withoutit.

Acomprehensivesetof measurementsfor thep-GaNis dis-played in Fig.6, where themeasuredpositions of thetwinned peaksoftheGa3dpeak,aswellasthedifferencebetweenthem areplottedasafunctionoffrequencyoftheSQWpulses,bothwith andwithoutC.W.violetlaserillumination.Themeasuredcharging shiftsaresigniﬁcantlyalteredintwodomains;(i)0.2–20Hz,and (ii)0.2–10kHz,andcontinuetopersistevenathigherfrequencies. Wemustemphasizethatallthechargingshiftswehavereportedso farinourpreviousstudiesonvariousdielectricmaterialslikeSiO2,

CdSandpolymershavealwaysbeeninthelowerfrequencyrangeof 10−1–102_Hz,_and_this_is_the_ﬁrst_time_we_are_able_to_record_them_in

thekHzrange,whicharetoofastfortheusualchargecompensation processesinvolvingseveralcascadingprocesses,andmust some-howberelatedwithelectronicprocesses.Accordinglywepropose thatatleastforthep-GaN,wehavesuccessfullydemonstratedthat withacombinationofthesnap-shotmeasurementsandutilization ofSQWpulses,XPSisabletoprobeanddistinguishamongtheslow (ionic),intermediate(dipolar),andthefast(mostlikelydueto elec-tronic)chargingprocessesofmaterialsandtheirphotoresponses.

Asimple analyticalextensionofourmethodisdemonstrated inFig.7,whereweanalyzeacompositeregionconsistingofboth n-andp-GaNsamplesplacedside-by-sideinanad-hocp–n junc-tionfashion.TheGa3dpeakofthecompositesampleappearsas a singleone,when recorded withthesampleis grounded, and thephotoresponseisnotstrongenoughtodistinguishbetween thedifferentlydopeddomains.However,whentheSQWpulses with10kHzmodulationareapplied,thepeaksdecomposetotwo distinctcomponents,assignabletothen-orthep-domains, respec-tively.Atthelowerfrequencythedecompositionhappensonlyfor the+10Vcycle,whilestillonlyoneoverlappingcompositepeakis observableinthe−10Vcycle.Thephotoresponsesofthen-and thep-componentsare notasstrong,but neverthelessarealso indicativeofthenatureofthedopingascanalsobeseenfromﬁgure.

4. Conclusions

Byutilizingdifferentopticaland/orelectricalmodulation tech-niques, XPS spectroscopy, which is conventionally used in the

staticscanningfashion,canalsobeusedforprobingthe dynam-ics of surface structures. A combination of time-resolved XPS with0.1sintervals,incorporationofSQWmodulation,anduseof photoilluminationprovidesusnewwaysofinvestigatingsurface electronicstructureandothersurfacepropertiesof semiconduct-ing and dielectric materials in a chemically speciﬁcfashion, in themicrosecondstosecondstimedomains,which hasnotbeen reportedbefore.Oneparticularadvantageofthemethodisits abil-itytodistinguishandidentifythepeaksrepresentingtheirdoping natureforcompositesurfacestructures,usingtheirresponsesto electrical,electromagneticstresses,aswellastoacombinationof both.

Acknowledgements

ThisworkwassupportedbytheScientiﬁcandTechnological ResearchCouncilofTurkey(TUBITAK)GrantNo:212M051.

References

[1]M.Kocan,A.Rizzi,H.Luth,S.Keller,U.K.Mishra,Phys.StatusSolidiB234(2002) 773.

[2]L.Kronik,Y.Shapira,Surf.Sci.Rep.37(1999)1.

[3]D.K.Schroder,Meas.Sci.Technol.12(2001)R16.

[4]M.Foussekis,J.D.Ferguson,A.A.Baski,H.Morkoc,M.A.Reshchikov,Phys.B404 (2009)4892.

[5]M.A.Reshchikov,M.Foussekis,A.A.Baski,J.Appl.Phys.107(2010)113535.

[6]J.E.Demuth,B.N.J. Persson,A.J. Schell-Sorokin,Phys.Rev.Lett.51(1983) 2214.

[7]J.E.Demuth,W.J.Thompson,N.J.DiNardo,R.Imbihl,Phys.Rev.Lett.56(1986) 1408.

[8]A.Schellenberger,R.Schlaf,C.Pettenkofer,W.Jaegermann,Phys.Rev.B45 (1992)3538.

[9]R.Schlaf,A.Klein,C.Pettenkofer,W.Jaegermann,Phys.Rev.B48(1993) 14242.

[10]M.Moreno,H.Yang,M.Horicke,M.Alonso,J.A.Martin-Gago,R.Hey,K.Horn, J.L.Sacedon,K.H.Ploog,Phys.Rev.B57(1998)12314.

[11]H.Shpaisman,E.Salomon,G.Neher,A.Vilan,H.Cohen,A.Kahn,D.Cahen,J. Phys.Chem.C113(2009)3313.

[12]S.Schafer,S.A.Wyrzgol,R.Caterina,A.Jentsy,S.J.Schoell,M.Havecker,A. Knop-Gericke,J.A.Lercher,I.D.Sharp,M.Stutzmann,J.Am.Chem.Soc.136(2012) 12528.

[13]H.Sezen,E.Ozbay,O.Aktas,S.Suzer,Appl.Phys.Lett.98(2011)111901.

[14]H.Sezen,G.Ertas,A.Dana,S.Suzer,Macromolecules40(2007)4109.

[15]S.Suzer,H.Sezen,A.Dana,Anal.Chem.80(2008)3931.

[16]S.Suzer,H.Sezen,G.Ertas,A.Dana,J.ElectronSpectrosc.Relat.Phenom.176 (2010)52.

[17]H.Sezen,G.Ertas,S.Suzer,J.ElectronSpectrosc.Relat.Phenom.178-179(2010) 373.

[18]H.Sezen,S.Suzer,J.Vac.Sci.Technol.A28(2010)639.

[19]H.Sezen,S.Suzer,Surf.Sci.Lett.604(2010)L159.

[20]D.Briggs,M.P.Seah,PracticalSurfaceAnalysis,2nded.,Wiley,Chichester,1999.

[21]M.Marsi,R.Belkhou,C.Grupp,G.Panaccione,A.Taleb-Ibrahimi,L.Nahon,D. Garzella,D.Nutarelli,E.Renault,R.Roux,M.E.Couprie,M.Billardon,Phys.Rev. B61(2000)R5070.

[22]M.Marsi,M.E.Couprie,L.Nahon,D. Garzella,R. Bakker,A.Delboulbe,D. Nutarelli,R.Roux,B.Visentin,C.Grupp,G.Indlekofer,G.Panaccione,A. Taleb-Ibrahimi,M.Billardon,Nucl.Instrum.MethodsPhys.Res.A393(1997)548.

[23]M.Marsi,M.E.Couprie,L.Nahon,D.Garzella,T.Hara,R.Bakker,M. Billar-don,A.Delboulbe,G.Indlekofer,A.Taleb-Ibrahimi,Appl.Phys.Lett.70(1997) 895.

[24]M.Marsi,L.Nahon,M.E.Couprie,D.Garzella,T.Hara,R.Bakker,M. Billar-don,A.Delboulbe,G.Indlekofer,A.Taleb-Ibrahimi,J.ElectronSpectrosc.Relat. Phenom.94(1998)149.

[25]K.Hirose,M.Yamawaki,K.Torii,T.Kawahara,S.Kawashiri,T.Hattori,Appl. Surf.Sci.237(2004)411.

[26]K.Hirose,H.Nohira,K.Azuma,T.Hattori,Prog.Surf.Sci.82(2007)3.

[27]K.Hirose,J.ElectronSpectrosc.Relat.Phenom.176(2010)46.

[28]S.Suzer,Anal.Chem.75(2003)7026.

[29]F.Karadas,G.Ertas,S.Suzer,J.Phys.Chem.B108(2004)1515.

[30]S.Suzer,A.Dana,J.Phys.Chem.B110(2006)19112.

[31]G.Ertas,U.K.Demirok,A.Atalar,S.Suzer,Appl.Phys.Lett.86(2005)183110.

[32]G.Ertas,U.K.Demirok,S.Suzer,Appl.Surf.Sci.249(2005)12.

[33]S.Suzer,A.Dana,G.Ertas,Anal.Chem.79(2007)183.

[34]C.P.Conger,S.Suzer,Langmuir25(2009)1757.

[35]H.Cohen,R.Maoz,J.Sagiv,NanoLett.6(2006)2462.

[36]H.Cohen,S.K.Sarkar,G.Hodes,J.Phys.Chem.B110(2006)25508.

[37]A.Samokhvalov,R.W.Gurney,M.Lahav,S.Cohen,H.Cohen,R.Naaman,J.Phys. Chem.B107(2003)4245.

(6)

[39]H.Cohen,J.ElectronSpectrosc.Relat.Phenom.176(2010)24.

[40]H.Sezen,S.Suzer,J.Spectrosc.Dyn.2(2012)3.

[41]N.J.Halas,J.Bokor,Phys.Rev.Lett.62(1989)1679.

[42]J.P.Long,H.R.Sadeghi,J.C.Rife,M.N.Kabler,Phys.Rev.Lett.64(1990)1158.

[43]J.P.Long,V.M.Bermudez,Phys.Rev.B66(2002)121308.

[44]E.A.Kraut,R.W.Grant,J.R.Waldrop,S.P.Kowalczyk,Phys.Rev.Lett.44(1980) 1620.

[45]S.A.Ding,S.R.Barman,K.Horn,H.Yang,B.Yang,O.Brandt,K.Ploog,Appl.Phys. Lett.70(1997)2407.