Effect of fluorine on the electrical conductivity of amorphous Si
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Volume 75A, number 3 PHYSICS LETTERS 7 January 1980 EFFECT OF FLUORINE ON THE ELECTRICAL CONDUCTIVITY OF AMORPHOUS Si NGUYEN VAN DONG and C. THUAULT Département de Physico-Chimie, Centre D’Etudes Nucléaires de Saclay, 91190 Gif-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. According to Fisch and Licciardello [1], hydrogen- I ated amorphous silicon contains a significant number of Si—H—Si three-center bonds which act as deep hole traps. These authors have proposed substituting fluorine ~ ® for hydrogen to remove these centers. More recently ~— - Ovshinsky and Madan [2] have reported that slightly hydrogenated amorphous Si—F alloy films prepared by the glow discharge technique contain a much lower density of gap states at the Fermi level than amorphous ,~ - \ \ alloy Si—H samples. However, the effect of fluorine \ “~ on the electrical properties of a-Si is expected to be 4~ different from the case of hydrogenation, because F a atoms are strongly electronegative and therefore can \\ !\‘~~Ø introduce acceptor states near the valence band. 10’ - •\ ~ ‘~. This paper reports the effect of fluorine incorpora- \ \ S tion on the dc conductivity of sputtered a-Si films mea- ~ sured up to 500 K. Furthermore, for a fluorinated \ A\ film, we have observed that a deliberate addition of ~ ‘.&,~ ® a small amount of hydrogen can result in a large de- \ \~, crease in the conductivity. A The samples with thicknesses of about 1 pm were deposited onto aluminium oxide substrates in a dc triode sputtering system using a plasma of partial pres- 2 sures of argon and fluorine. The gas purity is approxi- mately l0—~ ppv for Ar and 10—2 ppv for F. The sub- strates were held at 220°C during the growth of the 8 _________________________________________ films. The conductivity and thermopower were mea- 3 3 4 sured in a planar sample configuration with nichrome (i-) electrodes. Thermopower measurements near room temperature indicate that all samples are p-type. The . . Fig. 1. Temperature dependence of the electrical conductivity temperature dependence of the dc conductivity for in sputtered a-Si for different partial pressures ofF (Torr): different samples is shown in fig. 1. For the lowest 1, PF = 0; 2, PF = 7 x iO~ 3, PF = 2 x 10~ 4. PF = 4 pressure of F (PF 7 X l0~ Torr), the conductivity x iO~ 5’PF = 6 X 229
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Transcript of Effect of fluorine on the electrical conductivity of amorphous Si
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
229
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
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