Volume 75A, number 3 PHYSICS LETTERS 7 January 1980

EFFECT OF FLUORINE ON THE ELECTRICAL CONDUCTIVITY OF AMORPHOUS Si

NGUYEN VAN DONG and C. THUAULTDépartementdePhysico-Chimie,CentreD’EtudesNucléairesdeSaclay,91190Gif-sur-Yvette,France

Received 23 October 1979

We report the effect of fluorine on the dc electrical conductivity of amorphous Si films prepared by cathodic sputtering.

Accordingto FischandLicciardello [1], hydrogen- I

atedamorphoussiliconcontainsa significant numberof Si—H—Si three-centerbondswhich act asdeepholetraps.Theseauthorshaveproposedsubstitutingfluorine ~ ®for hydrogento removethesecenters.More recently ~— -

Ovshinskyand Madan [2] havereportedthat slightlyhydrogenatedamorphousSi—Falloy films preparedby theglow dischargetechniquecontaina muchlowerdensityof gap statesat the Fermilevel thanamorphous ,~ - \ \alloy Si—H samples.However, theeffect of fluorine \ “~on theelectricalpropertiesof a-Si is expectedto be 4~differentfrom the caseof hydrogenation,becauseF a

atomsarestronglyelectronegativeandthereforecan \\ !\‘~~Ø

introduceacceptorstatesnearthe valenceband. 10’ - •\ ~ ‘~.

This paperreportstheeffect of fluorine incorpora- \ \ S

tion on thedc conductivityof sputtereda-Si films mea- ~suredup to 500 K. Furthermore,for a fluorinated \ A\

film, we haveobservedthat a deliberateadditionof ~ ‘.&,~®

a small amountof hydrogencanresult in a largede- \ \~,creasein theconductivity. A

The sampleswith thicknessesof about 1 pmweredepositedontoaluminiumoxidesubstratesin a dctriodesputteringsystemusinga plasmaof partialpres- 2

suresof argonandfluorine. The gaspurity is approxi-mately l0—~ppv for Ar and 10—2 ppv for F. The sub-strateswereheld at 220°Cduring the growthof the 8 _________________________________________

films. The conductivityand thermopowerwere mea- 3 3 4

suredin a planarsampleconfigurationwith nichrome (i-)electrodes.Thermopowermeasurementsnear roomtemperatureindicatethat all samplesare p-type.The . .

Fig. 1. Temperaturedependenceof theelectrical conductivitytemperaturedependenceof thedc conductivity for in sputtered a-Si for different partialpressuresofF (Torr):

different samplesis shown in fig. 1. For the lowest 1, PF = 0; 2, PF = 7 x iO~3, PF = 2 x 10~4. PF = 4pressureof F (PF 7 X l0~Torr), theconductivity x iO~

5’PF = 6 X

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Volume75A, number3 PHYSICSLETTERS 7 January 1980

I The activationenergyE0 is found to be about0.65eV and theu-curveextrapolatedto l/T = 0 gives for

thepre-exponentialfactor a0 5 X 103~2_1cm~io

6. commonlyobservedfor conductionin extended- states.With further additionof fluorine, themagni-

tude of a increasesrapidly. Thisgives rise to extrinsicp-typeconduction,similarly to the situationof borondoping.However, thetemperaturedependenceof afor an increasingamountof F atomssuggestsan in-creasein thehoppingof holesthroughacceptorstates.

7 With the highestfluorine pressure(PF 6 X l0~10 Torr), theconductivity follows Mott’s relation log

a ‘— T~I~overa wide temperaturerangeand theval-lie of the thermopoweris small, typically of metallic

conduction.o It is interestingto note that the lowestconductivi-

ty is achievedfor a film preparedby incorporating

simultaneouslyfluorine andhydrogen.As shown in10 - - fig. 2, theconductivity is singly activatedwith an ac-

tivationenergyF0 0.90 eV, whichis larger than

0 that for amorphousSi—H alloy films.

The authorswishto thankJ.Y. Le Ny for technicalassistanceduring the runningof theseexperiments.Theyexpresstheir gratitudeto Dr. R. Bourdonand

I Buy huy True for supplingthe mixtureof argonand(4-) fluorine.

Fig. 2. Temperature dependenceof theelectricalconductivityin a sputtereda-Si film prepared with PF 2.5 X i0~Torr ReferencesandpH 5 X iO-~Torr.

[1] R. FischandD.C. Licciardello,Phys.Rev.Lett. 41(1978)

which is muchlower thanthat of anunfluorinated 889.film, showsan activatedbehaviour.This fact clearly [21S.R. OvshinskyandA. Madan,Nature276 (1978)482.

showsthecompensationof danglingbondsby fluorine.

230