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Full
Length
Article
Germanium
nitride
and
oxynitride
films
for
surface
passivation
of
Ge
radiation
detectors
G.
Maggioni
a,b,∗,
S.
Carturan
a,b,
L.
Fiorese
b,c,
N.
Pinto
d,e,
F.
Caproli
d,e,
D.R.
Napoli
b,
M.
Giarola
f,
G.
Mariotto
faDipartimentodiFisicaeAstronomiaG.Galilei,UniversitàdiPadova,ViaMarzolo8,I-35131Padova,Italy
bLaboratoriNazionalidiLegnaro,IstitutoNazionalediFisicaNucleare,Vialedell’Universita’2,I-35020Legnaro,Padova,Italy cDipartimentodiIngegneriadeiMaterialiedelleTecnologieIndustriali,UniversitàdiTrento,ViaMesiano77,I-38050Povo,Trento,Italy dScuoladiScienzeeTecnologie,SezionediFisica,UniversitàdiCamerino,ViaMadonnadelleCarceri9,Camerino,Italy
eINFN,SezionediPerugia,Perugia,Italy
fDipartimentodiInformatica—UniversitàdiVerona,StradaleGrazie15,I-37134Verona,Italy
a
r
t
i
c
l
e
i
n
f
o
Articlehistory: Received6June2016
Receivedinrevisedform9September2016 Accepted2October2016
Availableonline4October2016 Keywords:
Germaniumnitridelayer Germaniumoxynitridelayer Roomtemperaturedeposition Electricalresistivity Surfacepassivation
Hyperpuregermaniumdetector
a
b
s
t
r
a
c
t
Thisworkreportsadetailedinvestigationofthepropertiesofgermaniumnitrideandoxynitridefilms tobeappliedaspassivationlayerstoGeradiationdetectors.Allthesamplesweredepositedatroom temperaturebyreactiveRFmagnetronsputtering.Astrongcorrelationwasfoundbetweenthedeposition parameters,suchasdepositionrate,substratebiasandatmospherecomposition,andtheoxygenand nitrogencontentinthefilmmatrix.Wefoundthatallthefilmswereverypoorlycrystallized,consisting ofverysmallGenitrideandoxynitridenanocrystallites,andelectricallyinsulating,withtheresistivity changingfromthreetosixordersofmagnitudeasafunctionoftemperature.Apreliminarytestofthese filmsaspassivationlayerswassuccessfullyperformedbydepositingagermaniumnitridefilmonthe intrinsicsurfaceofahigh-puritygermanium(HPGe)diodeandmeasuringtheimprovedperformance,in termsofleakagecurrent,withrespecttoareferencepassivateddiode.Alltheseinterestingresultsallow ustoenvisagetheapplicationofthiscoatingtechnologytothesurfacepassivationofgermanium-based radiationdetectors.
©2016ElsevierB.V.Allrightsreserved.
1. Introduction
Germanium (Ge) has recently aroused renewed interest in microelectronicsowingtoitshigherholeandelectronmobilities ascomparedtosilicon(Si)[1],whileitshighabsorptioncoefficient inopticalcommunicationbandsmakesitagoodcandidateforthe developmentofGeopticaldevices[2].
OneofthemostseriousissuesinthefabricationofGedevices concerns thepassivationof Gesurfaces.Differentlyfrom Si,Ge forms unstable oxides:GeO2 is water-solubleand decomposes
underthermal treatmentaround420◦C[3]sothatit cannotbe usedinwetprocessesandthermaltreatmentsinULSIfabrication processes[4].Forthisreason,alternativepassivationcoatingshave beenstudiedanddevelopedandseveralGe-baseddielectric com-pounds(suchasGenitridesandoxynitrides)arenowconsidered
∗ Correspondingauthorat:DipartimentodiFisicaeAstronomiaG.Galilei, Univer-sitàdiPadova,ViaMarzolo8,I-35131Padova,Italy.
E-mailaddress:[email protected](G.Maggioni).
potentially interestingfortechnologicalapplications.In particu-lar,germaniumnitride(Ge3N4)exhibitsahighdielectricconstant,
iswater-insolubleanditsthermaldecompositiontemperatureis higherthanGeO2[5,6]sothatitcouldbeappliednotonlyasabuffer
layerfor high-kdielectricsgrown onGesubstratesbutalsofor thepassivationofGesurfaces.Germaniumoxynitrides(GeOxNy)
are alsopromising materials,because nitrogenincorporationin GeO2hasproventoimproveitschemicalandthermalstability[7]:
thedecompositiontemperatureofaGeOxNylayerwasfoundto
increaseupto550◦C,whichwasmorethan100◦Chigherthanthat ofpureGeO2[8,9].
Akeyrole totailorthepassivatingpropertiesofthisclassof materialsisplayedbythedepositiontechnique.Germaniumnitride andoxynitridelayersareusuallygrownbytechniquessuchas ther-malandplasmanitridationofGesurfaces[4,3,10],chemicalvapor deposition[11]andthermalammonolysisofGeO2[12,13].Reactive
sputteringtechnique,usingN2,NH3 orN2H4asreactivegas,has
beenalsoexploitedforthedepositionofgermaniumnitridelayers [14–18].Thesetechniquesusuallyrequireheatingthesubstrateup tosomehundredsofCelsiusdegreesduringthefilmgrowthinorder http://dx.doi.org/10.1016/j.apsusc.2016.10.006
Table1
Depositionparametersofgermaniumnitrideandoxynitridefilms.GedepositionrateandfilmcompositionweredeterminedbyRBS.TheerrorsontheNandOvaluesare around5%. Sample Target-substrate distance (cm) RFpower (W) Sample DCBias (V) Gas com-position Gasflow (sccm) Ultimate pressure(Pa) Watervapor bombardmentrate (1013molecules cm−2s−1) Gedeposition rate(1013atoms cm−2s−1) Filmcomposition A 14 40 – N2 40 8.5×10−5 3.1 10.7 Ge2O1.0N2.1 B 5 60 – N2 40 4.1×10−5 1.5 104 Ge3N4.6 C 5 60 – N2:Ar 25:15 3.4×10−5 1.2 184 Ge3N4.0 D 14 40 – N2:Ar 20:20 8.6×10−5 3.1 15.8 Ge2O1.2N1.9 E 5 60 – N2:Ar 20:20 3.3×10−5 1.2 171 Ge3N4.1 F 5 60 – N2:Ar 15:25 3.4×10−5 1.2 197 Ge3N4.1 G 5 60 −20 N2 40 1.1×10−4 4.0 104 Ge3N4.6 H 5 60 −40 N2 40 3.8×10−4 13.7 103 Ge3N4.2 I 5 60 −60 N2 40 1.1×10−4 4.0 103 Ge3N4.1 J 5 60 −80 N2 40 1.4×10−4 5.1 93 Ge3N4.1 K 5 60 −100 N2 40 1.9×10−4 6.8 88 Ge3N3.7
topromotethereactionbetweengermaniumandnitrogen/oxygen species.Thisrequirementcanbeadrawbackforthoseapplications, wherethesubstratecanbedamagedbyanexcessivelyhigh temper-ature.Thisisthecasewhenthegermanium(oxy)nitridecoatinghas tobeappliedasapassivationlayertotheintrinsicsurfaceofa high-purityGe(HPGe)detector[19–21].Infact,anexcessiveheatingcan promotetheunwanteddiffusionofcontaminantspeciesandofLi atoms(usedforthen+contactofGedetectors)thenjeopardizing
thedetectorperformance.
Aimingatthisspecificapplication,inthisworkweoptimized theroomtemperature(R.T.)depositionofgermaniumnitrideand oxynitridefilmsbymeansofreactiveRFmagnetronsputteringasa techniqueforproducingpassivationcoatingsforGe-baseddevices. Severalfilmswerepreparedunderdifferentdeposition parame-ters,suchasatmospherecompositionanddepositionrate,andtheir mainphysicalpropertieswerecharacterized.Onesetoffilmswas obtainedbyRFbiasingthesubstrates,inordertostudytheeffects oftheionbombardmentonthefilmproperties.
Asafinalstepofourstudy,agermaniumnitridecoatingwas depositedontotheintrinsicsurfaceofaplanarHPGediodeand thereverseleakagecurrentofthebiaseddiode,withandwithout coating,wasmeasuredandcompared.
2. Experimentaldetails
Theexperimentalequipmentusedforthecoatingsdeposition consistedofastainlesssteelvacuumchamberevacuatedbya tur-bomolecularpumptoabasepressurelowerthan5×10−4Pa.The glowdischargesustainingdevicewasa2-in.cylindricalmagnetron sputteringsourceconnectedtoaradiofrequencypower genera-tor(600W,13.56MHz),throughamatchingbox.Allfilmswere depositedattwovaluesoftheRFpower:40Wand60W.PureGe (99.999%)wasusedasatarget.SeveralN2-Argasmixtureswere
usedforthedepositions(seeTable1)byregulatingthegasflow ofeachgas.Thedifferentsubstrates(silicon,carbonandsapphire) wereplacedonasampleholderatdistancesof5cmand14cm.
Asetoffilmswaspreparedbybiasingthesampleswitha sec-ondRFpowergenerator(600W,13.56MHz),whichresultedinan averagedcvoltageacquiredbythesamplesandpurposelyfixedat valuesrangingfrom0Vto−100V,instepof−20V.Inthisway,a controlledion-bombardment-assisteddepositionwasachievedat aconstantaverageionenergy.Thetimeforeachdepositionrunwas variedinordertoachieveafilmthicknessbetween100and300nm. ForRamanspectroscopymeasurements,thedepositiontimeswere extendedinordertoget600nmthicksamples.Amass spectrome-ter(PrismaPlusQMG220,PfeifferVacuum)revealedwatervapor asthemainresidualcomponentinthedepositionchamber.
RutherfordBackscatteringSpectrometry(RBS)wasperformed using2.0MeV4He+ beamattheVandeGraafacceleratoratthe
LaboratoriNazionalidiLegnaro,atthescatteringangleof160◦,in ordertodeterminefilmdepositionrateandcomposition.Samples werecharacterizedbymeansofglancingx-raydiffraction(XRD) usingaPhilipsdiffractometerequippedwithglancing-incidence X-rayoptics.Theanalyseswereperformedat0.5◦incidenceusing CuK␣Nifilteredradiationat40kVand40mA.
Thevibrationaldynamicsof germaniumnitridesampleswas probed by either FT-IR or Raman spectroscopy measurements. Thetransmittance spectraofthe sampleswererecordedin the 4000–400cm−1range,usingaspectrometerJasco(modelFTIR660 Plus)witharesolutionof4cm−1.Micro-Ramanspectrawere car-riedoutinbackscatteringgeometry,atroomtemperature,under excitationat514.5nmor,alternatively,at647.1nmbymeansof atriple-axismonochromator(Horiba-JobinYvon,modelT64000), setindouble-subtractive/singleconfiguration,andequippedwith holographic gratingshaving1800lines/mm. Thescattered radi-ation was detected by a charge coupled device detector, with 1024×256 pixels, cooled by liquid nitrogen, and the spectral resolutionwasbetterthan0.6cm−1/pixel.Allthespectrawere cali-bratedinfrequencyusingtheemissionlinesofanArspectrallamp. Thesurfacemorphologyofthesampleswasinvestigatedeither byaSEM(TescanVega3XM)equippedwiththeenergydispersive spectrometry(EDS)optionorbyano-contact-modeAFMmodel C-21 (Danish Micro Engineering), mounting a DualScope Probe Scanner95-50.
Electricalresistivity,(T),offilmsdepositedonsapphire sub-strateswasmeasuredinaco-planarconfigurationasafunctionof thetemperaturefromabout300K–600K,byusingasmallfurnace operatinginvacuum(P<10−3Pa)andatdark.FourAuelectrodes, about5mmapart,weresputteredonthefilmsurface,nearits bor-ders.Duetotheextremelyhighresistanceexhibitedbyallthefilms, themeasurementswerecarriedoutinatwocontactsgeometry,by usingelectrometers(Keithleyeithermod.617or6517B) operat-ingintheV/Imode.Duringthisworktheexperimentalsetupwas updatedinordertomeasurehighervaluesofthesampleresistance. Dependingontheroomtemperatureresistance,theappliedbias rangedfrom10Vto100V,withatypicalvalueof50V.Thermal energyinthefilmwaschangedveryslowly,witharateofabout 0.5–1K/minand(T)collectedafterTstabilizationtobetterthan 1K(typically0.5K).Each(T)valuewasobtainedaveraging25–30 datapoints,collectedatafixedTvalue.Filmresistivitywas mea-suredcontinuously,fromroomtemperature(R.T.)upto600Kand thenbacktoR.T.
A planar,cylindricalHPGediode of 19mm (height)×39mm (diameter)waspreparedinourlaboratorytocheckthe passiva-tionpropertiesofgermaniumnitridecoatings.Thep+contactwas
obtainedbyBoronimplantationoveroneoftheplanarfacesofthe cylinderandthen+ contactbyLidiffusionontheoppositeside.
Furtherdetailsaboutdiodepreparationcanbefoundin[22].For themeasurementof theleakage current,thediodewithoutthe Ge3Nxcoatingwaspassivatedbythestandardmethanol
passiva-tion,consistinginanetchingina3:1HF40%:HNO365%solution
(reagentgrade,CarloErba)for3min,followedbydirectquenching inmethanol(Erbatron,CarloErba).Immediatelyafterthis treat-ment, thediodewasplaced in a commercialstandard cryostat, pumpeddowntoapressurelowerthan10−3Paandcooledtoabout 85K.Afterwards,theleakagecurrentwastestedbyaKeithley237 source-measurementunit(currentsensitivity≈10−14A)
reverse-biasingthe diodeat increasingvoltage,withstepsof 5V every 60sandameasurementevery5s,untiladriftofthecurrentwas observed.Inordertominimizetheeffectsofparasiticcurrentsand capacitiesonthemeasuredcurrent,triaxialcables wereusedto connecttheunittothediode.Ithastobepointedoutthatthe for-wardbiasregionwasnotinvestigated,sincethebulkcontribution tothecurrentobscuresthesurfacecurrentinthisregion.Forthe testofthecoateddiode,athinGe3Nxlayerwasdepositedwiththe
sameparametersofsampleC(seeTable1)onthelateralsurface ofthesamediode,byusingarotatingsampleholderaftercareful maskingofthetwocircularfaces.Immediatelyafterthedeposition, thediodewasplacedinsidethecryostat,pumpeddownandcooled forthemeasurement.Forbothconfigurationsnohysteresiseffect wasfound.
3. Resultsanddiscussion
3.1. Filmcompositionandstructure
Theexperimentalparameters usedforthedepositionof ger-maniumnitrideand oxynitridesamplesarereportedinTable1, together with the film deposition rates and compositions, as derivedbyRBS.SamplesfromAtoFweredepositedwithout bias-ingthesampleholder,whileforsamplesfromGtoKtheaverage dc bias was increasedfrom −20V to−100V. Data reportedin Table1 evidencehowthedepositionrateaffects thefilm com-position,independentlyof theatmospherecomposition. Infact, whenthedepositionrateislow(samplesAandD),theO/Geatomic ratiorangesfrom0.5to0.6andtheO/Nratiorangesfrom0.45to 0.60,whiletheN/Geratioisbetween0.97and1.05.Ontheother hand,whenthedepositionrateishigh(samplesB,CandfromE toK),theoxygencontentdrasticallydecreasesbelowthedetection thresholdandthenitrogencontentincreases(N/Gerangesfrom 1.21to1.53).Sincetheoxygenincorporatedintothefilmscomes mainlyfromtheresidualgasesdesorbedfromthedeposition cham-berwalls,thedifferenceintheoxygencontentcanbeascribedto thelargedifferenceintheratiobetweenthedepositionrateof Geatoms,rGe,andthatofoxygen-containingspeciesonthe
sub-stratesurface.Takingintoaccountthatthemaincomponentofthe residualatmosphereiswatervapor,asrevealedbymass spectrom-etrymeasurements(seeExperimentalsection),thebombardment rateofoxygen-containingspeciesontothesubstratesurfacecanbe approximatedtothatofwatervapor,rH2O(Table1).rH2Owas
calcu-latedfromtheresidualpressureofwatervaporimmediatelybefore thefilmdeposition[23].ForsamplesAandD,theratiobetweenthe depositionrateofGeatomsandthebombardmentrateofwater vapormolecules,rGe/rH2O,islessthanfiveGeatomsforeverywater
vapormolecule,whileforsamplesB,C,EandFrGe/rH2Oincreases
uptomorethansixtyanditiseasilyenvisagedthatitcanbefurther enhancedbybothincreasingtheGedepositionrateanddecreasing theultimatepressure.
Filmcompositionisalsostronglyaffectedbytheatmosphere composition,asexpected.WhenpureN2isused(sampleB),thefilm
isover-stoichiometricinnitrogen(N/Ge=1.53insteadof1.33).In contrast,withN2dilutioninAr(samplesC,EandF),astoichiometric
ratio(withintheexperimentalerror)willresult,inagreementwith dataofLewinetal.[18].
ThecompositionofsamplesfromGtoKevidencestheeffectsof ionbombardmentduringthefilmgrowth.Firstly,theoxygen con-tentisbelowtheRBSdetectionthresholdforallthebiasesranging from−20Vto−100V.Thisfindingshowstheprevailingeffectof theionbombardmentontheincorporationofcontaminantspecies inthefilmwithrespecttotheresidualpressureofwatervapor immediatelybeforethedeposition:infact,eveniftheratiorGe/rH2O
inthebiasedsamplesisintermediatebetweenthetwoprevious setsofunbiasedsamples(rangingfrom7.6to26,seeTable1),the oxygencontentisstillbelowthedetectionthreshold.Ion bombard-mentofthegrowinglayerpromotesthereleaseofweaklybonded, physisorbedwatermoleculesfromthefilmsurface,thusholding downtheoxygenincorporation.Asecondimportanteffectisthe trendofGe depositionrateatincreasingbias:it keepsconstant untilabiasof−60Vanddecreasesdowntoabout85%at−100V. ThistrendshowsthattheGere-sputteringfromthegrowingfilmis negligibleuntil−60Vandbecomesimportantstartingfrom−80V. ThelastimportantfindingconcernstheN/Geatomicratio,which decreases at increasing bias: thefilm is over-stoichiometric in nitrogen,atbiaseslower(lessnegative) than−60V,approaches thestoichiometry(within theexperimentalerror) at−60Vand −80Vandthendropstoasubstoichiometricvalueat−100V. Tak-ingintoaccountthetrendof rGe,thisbehavior indicatesthatN
resputteringbecomesimportantatlowerbiasvaluesthanforGe resputteringandkeepsmorepronouncedevenatthehigherbias (preferredNresputtering).Allthesefindingsevidencethe straight-forwardwaytofinelycontrolthefilmcompositionallowedbythe specificdepositiontechniqueusedinourstudy.
Concerningthestructureof depositedfilms,XRDshowsthat all thefilmsare very poorly crystallized(Fig.1)independently ofthedepositionparameters,withonlyabroadbandappearing between2=20◦and2=45◦.Inthisregionarelocatedsomeof themostintensepeaksofc-Ge3N4(␣-phase,JCPDS1169,and
-phase,JCPDS381374)andc-Ge2ON2[24]:theseareat2=29.26◦
(201),33.41◦(210)and36.81◦(211)for␣-phaseGe3N4,2=34.08◦
(210)for–phaseGe3N4and2=25.88◦(201),28.74◦(300),31.06◦
(020),33.77◦ (301)and36.69(220)forc-Ge2ON2.Inthespectra
ofGeOxNysamples(AandD)aweaksignalisalsofoundcloseto
2=10◦,whichcouldbeascribedtoac-Ge2ON2peak(2=9.49◦
(100))[24].Nootherfeaturecanbeidentified.Inthecaseofbiased samples,wecanobservea clearevolution inthebandshape at increasingbias(Fig.1):therelativeheightofthepeakplacedaround 2=37◦ decreaseswithrespecttotheleftpartoftheband.This trenddoesnotseemtobeduetoaphasetransformationbutrather toa rearrangementof thenanocrystallitespresent inthefilms, whosegrowthorientationbecomesmorerandom,thenresulting inanoverallmoredisorderedstructureatincreasingbias.
FT-IRanalysispointsoutasignificantdifferenceinthespectraof filmsdepositedatdifferentdepositionrate(Fig.2):asinglebroad peakisfoundintherange680to720cm−1forthehigh-deposition ratesamples(B,CandfromEtoK),whileforthelow-rateones (AandD)thepeakappearsaround770cm−1.Althoughpeaksof -phaseofc-Ge3N4arefoundat780and910cm−1and␣-phaseof
c-Ge3N4hasonepeakat730cm−1[25],a-Ge3Nxsamplesusually
exhibitapeakcenteredatabout700cm−1,whichisassignedto Ge-Nin-planeasymmetricstretchingvibration[26].Thisisthecase forthehigh-rategermaniumnitridesamples.Ontheotherhand, IRfeaturesoflow-ratesamplesareclearlyaffectedbythepresence ofoxygen:theabsorptionpeakaround770cm−1isassignedtothe stretchingmodeoftheO-Ge-Ngroup.Althoughtheexactposition ofthispeakincrystallinestoichiometricGe2ON2samplesisfound
Fig.1.XRDspectraofbiasedGe3Nxsamples(BandfromGtoK).ForcomparisonthespectrumofoneofthetwoGeOxNyfilms(D)hasbeenplottedaswell.TheotherGeOxNy
layer(A)showsasimilarspectrum.
Fig.2.FT-IRspectraofsampleI(−60V)andD(N2:Ar20:20,lowdepositionrate).
arisefromtheverypoorlycrystallinenatureofthesefilms,similarly tothegermaniumnitridesamples.
The almost amorphous structure of germanium nitride and oxynitridefilmsisconfirmedbyRamanscatteringmeasurements. AsanexampleFig.3reportsatypicalRamanspectrumcarriedout underexcitationat647.1nmfromthehighlynon-stoichiometric sample B. Ramanspectra of all the samples show in the low-wavenumberregion a paramount, broad and asymmetric band (seeinFig.3thecharacteristicprofilepeakedatabout100cm−1), whichistypicalofsystemswithoutalong-rangeorder,especially ofglasses[27],andisrelatedtoadensityofacousticvibrational statesofthesystem,usuallyreferredalsoasthe“bosonpeak”(BP)
[28].Inaddition,thesamefiguredisplaysanotherbroadspectral feature,centeredatabout290cm−1,appearingintheformofavery weakshoulderoftheBP,andthereforehardlyobservable,whichis relatedtothetranslationaldisorderofthesample.
TheArrheniusplotsoftheelectricalresistivity(T)ofthe dif-ferent filmsdeposited onsapphiresubstratescarriedoutin the temperaturerangebetween300Kand600KarereportedinFig.4. ThecurvesoffilmsC,GandHdonotexhibitvaluesatTlower than≈490K,≈450Kand≈420K,respectively,sincetheir resistiv-itywasmeasuredbeforetheupdateoftheexperimentalsetup(see theExperimentalsection).Itisworthwhilementioningthatallthe curvesshowninFig.4arethecoolingbranchesoftheresistivity
Fig.3.UnpolarizedRamanspectrumcarriedoutunderexcitationat647.1nmfromsampleB.Thesharppeakobservedat520cm−1isduetoc-Sisubstrate.
1.6
2.0
2.4
2.8
3.2
10
410
510
610
710
810
910
1010
11Dep B
Dep C
Dep D
Dep E
Dep G
Dep H
Dep I
Dep J
Dep K
cm
)
1000/ T
( K
-1)
600
500
400
T
(K)
300
Fig.4.Arrheniusplotsoftheresistivitymeasured,duringthecoolingrun,fortheGe3NxandGeOxNyfilms(seeTable1).Alltheexperimentaldatapointsareplotted
at≤1011cm,thisvaluebeingthesensitivitylimitofthemeasuringsetup.FilmsC,GandHhavebeencharacterizedbeforetheupdateofthemeasuringsetup(see
Experimental).
measurements,becausewefoundthat,duringthefirstcycle,the resistivitymeasuredduringthe heatingassumed values always lowerthanthosemeasuredinthecoolingrun.Thisbehaviorhas beenexplainedasareductionofinsulatingpropertiesoffilmsasa consequenceoftheirexposuretotheambientair[26,29].Thiseffect disappearedrepeatingthemeasurementunderthesamecondition, keepingthesampleinvacuumbetweentwomeasurementruns.
Theelectricalresistivitymeasurementsrevealthatallthe inves-tigatedfilmsarehighlyinsulatingaroundR.T.(Fig.4).Indetail,apart fromsampleK,whichistheleastresistiveoneamongthose mea-sured,alltheothersshowasaturationintheresistancevalueina TrangeextendingfromR.T.uptoabout350K÷450K,depending
onthefilm.TheresistancevaluedetectedinthisTrangeresults comparabletotheinsulationresistanceofthemeasuring appara-tus(≈5×1015at100Vofappliedbias)thussuggestingthatthe
realresistanceofthefilmscouldbeevenhigher.Thisimpliesthat theresistivityoftheselayersisunknownaroundR.T.
Thetemperaturedependenceoftheresistivityofallthesamples isthetypicaloneofinsulatingmaterialsanditchangesfromabout threetosixordersofmagnitude(Fig.4).Moreover,theArrhenius plotsoflayersresistivityexhibitsomeinterestingfeatures,like:i) the(T)curvesofthebiasedGe3Nxsamples(BandfromGtoK)
showadownwardshiftatbiasvalueslowerthan−20V.Thisis essentiallyrelatedtotwoconcomitanteffects:thecorresponding
Fig.5.AFMimageofthesurfaceofsampleB.
loweringofN/Geatomicratiointhefilm,asalreadyfoundbyother authorsinthecaseof understoichiometricGe3Nx films(x≤1.6)
[26][26andrefs.therein]andtheincreaseofdisorderinducedby theionbombardment,asshownbyXRDandFT-IRresults,which givesrisetoamoredefectivestructurewithahighernumberof localizedstatesinthebandgapandthentoareducedresistivity; ii)the(T)curvesofGe3NxfilmsdepositedwiththeAr+N2gas
mix-tures(i.e.filmsCandE)areverysimilarandpartiallyoverlapeach other.Thisresultdemonstratesahighreproducibilityofthefilm properties(atleastfromtheelectricalpointofview)andthetight relationbetweenresistivityandfilmcomposition.Itisworthwhile notingthatresistivityofthesefilmsisevenhigherthanthatofthe overstoichiometricones(BandG).iii)The(T)forthegermanium oxynitridesample(filmD)isthehighestamongthosemeasured inourexperiments.Acomparisonofourresultswiththoseinthe literatureforGe3Nxlayers[26,30]showsthattheresistivityofour
coatingsisseveralordersofmagnitudehigher,essentiallydueto thedifferentNcontent.Thisalsoinvolvesachangeofthe conduc-tionmechanism:inthecaseofstronglyunder-stoichiometricfilms, theconductionisdominatedbyavariablerangehopping mecha-nism[26,30],whileover-andstoichiometricfilmsshowathermally activatedconductionasdetailedinRef.[31].
Analysisofthe(T)curvesrevealstheexistenceofseveral acti-vationenergies,Ea,foreachlayer.TheEavaluesrangefrom≈0.6eV
around350Kto≈1.4eVaround600K,dependingonthefilm[31]. Consideredthattheband-gapinthisclassofmaterialsisexpected torangefrom4.5to5.5eV,wededucethatthemeasuredEavalues
mustberelatedtochargecarrierstransitionsinvolvinglocalized statesinsidetheenergy-gap,whoseoriginisduetothepresenceof defects.Duetothelackingofliteraturedataaboutthenatureand propertiesofelectronicstatesinGe3NxandGeOxNy,wehavetaken
intoaccountdefectstatespresentintheSi3Nxsystem[32],which
isverysimilartotheinvestigatedGe3Nxsystem.Theoretical
cal-culationsandexperimentsdemonstratethatintheSi3Nxsystema
paramagneticcentreforminga*stateislocated0.5eVabovethe valenceband(VB)edge[33].ChargecarriertransitionfromtheVB edgetothis*statecanaccountforthelowestEavaluesmeasured
around350K.AthigherT(≈600K)chargecarriertransitionsmay involvelocalizedstatesfartherfromtheVBedge,resultinginthe observationofhigherEavalues.
Adetailedanalysisoftheelectricalpropertiesrevealedthatall theinvestigatedGe3NxandGeOxNyfilmsfollowtheMeyer-Neldel
rule[34,31].
Finally,SEMandAFMimagesoftheasdepositedfilms(Fig.5) showaveryhomogeneousandsmoothsurfacewithatypical aver-ageroughness,Ra,below10nm.
3.2. Ge3Nx-coatedHPGediode
Atest tochecktheeffectivenessofGe3Nx filmsasasurface
passivationlayerhasbeencarriedout,fabricatingaplanarHPGe diodeandmeasuringitsreverseleakagecurrent,both withand without(seetheExperimental)aGe3Nxfilmdepositedontoits
lat-eralintrinsicsurface.TheGe3Nxlayerwasdepositedwiththesame
parametersofsampleC(seeTable1).Inordertoevidenceonlythe effectsofthesurfacepassivation,thesamediodewasusedforboth measurements.Inthiswaytheresultswerenotaffectedbypossible differencesinthebulkpropertiesarisingfromthehistoryof differ-entdiodes.Thecurrent-voltage(I–V)characteristics,measuredin boththeaboveconfigurations(i.e.:withandwithouttheGe3Nx
film)bybiasingthediodeuptoamaximumof80V,arereported inFig.6.Thereverseleakagecurrentkeepsverylow(below10pA) upto35V,forbothconfigurations,witha comparabletrend.At highervoltages,thetwocurvespartfromeachotherandthe leak-agecurrentoftheuncoateddevicestartstoexponentiallyincrease andgoesover1nAalreadyat60V.Thisbehaviourhighlightsthe defectivenatureofthis diode; infact,ifonetakesintoaccount thecharacteristicsofthisdevice,thedriftoftheleakagecurrent shouldstartatmuchhighervoltages[22].Thisoutcomeindicates that,inadditiontothesurfaceleakagecurrent,wehavetotake intoaccountastrongcontributionfrombulkcurrents,arisingfrom defectivecontacts.Thisfactpreventsusfromtestingthecoating propertiesatvoltageshigherthan80V.However,apositiveand necessarypremiseforfutureapplicationsisgivenbythebehaviour
Fig.6.I–VcharacteristicofaplanarHPGediodecoatedwithGe3Nxfilm(depositedunderthesamenominalconditionsofsampleC,seeTable1)comparedtothatofthe
samediodepassivatedwithmethanolsolution(seetheExperimental).
oftheGe3Nxcoateddiodeattheselowvoltages.Infact,Fig.6shows
thattheleakagecurrentstartstodriftatahighervoltagethanthe uncoatedone.At60Vthecurrentisaround100pAandisstillbelow 1nAat75V.ThisresultallowsusconcludingthattheGe3Nxcoating
doesnotincreasethesurfaceleakagecurrent,atleastatvoltages lowerthan80V.Takingintoaccountthatoneofthecontributions tothesurfaceleakagecurrentcomesfromtheconductioninthe passivatingfilm,evenbetterresultscouldbemaybereachedby usingGeOxNycoatings,sincetheresistivityoftheselastfilmsis
muchhigher(seee.g.curveDinFig.4).Itisclearthatthisresultis onlypreliminaryandthatmorecompletetestsonwell-performing diodesanddetectorsarestillrequiredinordertoquantitatively evaluatetheimprovedperformanceofthis classofmaterialsas passivatingcoatings.
4. Conclusions
In view of the application to the surface passivation of germanium-basedradiationdetectors,weproducedand investi-gatedindetailthephysicalpropertiesofgermaniumnitrideand oxynitride films, depositedat roomtemperaturebyreactiveRF magnetronsputtering.We foundthatthefilmcompositionwas strongly affected bythe depositionrate, ultimatepressure and depositionatmospherecomposition.Atlowdepositionrates ger-maniumoxynitridefilmswereobtained.Atincreasingdeposition rateanddecreasingultimatepressure,theoxygenincorporation inthefilmdrasticallydecreasedandgermaniumnitridecoatings weredeposited.Inthis case,thefilmswereover-stoichiometric in N when pure N2 was used as deposition gas, but became
stoichiometricwhenAr/N2gasmixtureswereused.Thefilm
sto-ichiometrywasstronglyaffectedbyanapplieddcsubstratebias, goingfromover-stoichiometryinnitrogenatlowbiasesdownto under-stoichiometryatincreasingbias.Inbiasedfilms,theoxygen contentwasalwaysbelowtheRBSdetectionthreshold.Allthefilms wereverypoorlycrystallizedandresultedtobehighlyinsulating atroomtemperature,theirresistivitybeingstrictlyrelatedtothe filmcompositionanddepositionparameters.
TheimprovedperformanceofaGe3Nx-coatedHPGediodewas
assessedbycomparingtheleakagecurrentofthecoateddiodewith
thatofadiodewithastandardsurfacepassivation.Thepromising resultsofthisfirsttestareanimportantpremiseforthe applica-tionofthesetreatmentsaspassivationroutesofHPGedetectors, whereinanextremelylowleakagecurrentisoffundamental impor-tancetoassureagoodenergyresolutioninthedetectionofgamma radiation.Furtherandexhaustivetestsaregoingtobecarriedout toidentifythebestperformingcoatingsandthenattaintheir com-pleteandquantitativevalidation.
Acknowledgements
The researchleading to theseresultsreceived fundingfrom the Third Scientific Commission of the Istituto Nazionale di Fisica Nucleare and from the European Union Seventh Frame-work Programme FP7/2007-2013 under Grant Agreement no. 262010—ENSAR.TheECisnotliableforanyusethatcanbemade ontheinformationcontainedherein.
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