Boron Doping of Hydrogenated Amorphous
Silion Prepared by rf-o-Sputtering
M.M.de LimaJr. 1
, F.L. Freire Jr. 2
, and F.C. Marques 1
1
Institutode F
isia\GlebWataghin",UniversidadeEstadual deCampinas
Uniamp,13083-970 Campinas,SP,Brasil
2
Pontif
iiaUniversidadeCatoliadoRiode Janeiro,
PUC-RJ,Riode Janeiro-RJ,Brazil
Reeivedon23April,2001
Thispaperaddressesthe dopingmehanismof amorphoussemiondutors throughthe
investiga-tionof borondoped rfo-sputteredamorphous hydrogenated silion. The ativation energyand
roomtemperatureondutivityvariedfrom 0.9 to 0.3 eVandfrom 10 12
to 10 4
Ohm 1
.m 1
,
respetively,byrangingtheborononentrationfrom0to3at.%. Theserangesofeletroni
prop-erties areofthe sameorderofthosereportedfor samplespreparedby plasmaenhanedhemial
vapordeposition(PECVD). Inspiteofthesesimilarities,therearesomerelevantdierenes. The
doping eÆieny, at low boron onentrations, is muh lower than that of PECVD samples. In
addition,thereationof deepdefets(danglingbonds)doesnotfollowthesquarerootpowerlaw
dependeneonthe borononentration as proposed by Street andobservedinPECVDsamples.
Thesedierenesareassoiatedwiththedensityofdefets,whihismuhhigherinlmsprepared
bysputtering. Theinreaseinthedeepdefetdensityismorelikelyrelatedtotopologialdisorder,
introduedbythepreseneofahighonentrationofinativeimpurities.
I Introdution
Theobservationofdopingina-Si:Hthinlms,in1975,
by Spearand Le Comber[1℄was animportant
break-throughintheeldofamorphoussemiondutors,sine
atthattimemostpeopleusetobelievethatdisordered
materials ould not be doped at all. Indeed, due to
the lak of onstraints of an amorphous network, the
8-NMott'srule[2℄preditsthatalltheelements
intro-dued into those materials would be bonded at their
ownonguration. Thus, theimpuritiesould notat
as donors or aeptors. A modied Mott's rule
(in-luding harged atoms) was then proposed by Street
[3℄ in order to explain the mehanism that allows the
experimentallyobserveddopingofamorphous
semion-dutors. This model managed to explain the lowand
variabledopingeÆienyandthereationofadditional
danglingbondsinhydrogenatedamorphoussilionand
germaniumpreparedbyplasmaenhanedhemial
va-pordeposition (PECVD), doped either withboron or
phosphorous. This model is supposed to be very
gen-eral: independent of deposition tehnique, and valid
for silion and germanium doped with anyelementof
olumns III and V.Nevertheless,arseni dopeda-Si:H
deposited by PECVD [4℄, as well as a-Ge:H prepared
byrf-sputteringusingolumn III andVdopantsother
indiationthatother mehanism,besidesharge
inje-tion, is also taking plae. Though alot of researhes
havebeenperformedandseveraltehnologialdevies
have been developed using doped amorphous silion,
the atual doping mehanism of amorphous
semion-dutors is, until now, not ompletely understood. In
thiswork,borondopeda-Si:Hlmswasstudiedinorder
toinvestigatethevalidityofStreet'smodelinlms
pre-paredbyrf-sputtering,whihhavemuhhigher
onen-trationofdeepdefetsthanlmspreparedbyPECVD.
II Experimental details
The samples were prepared in a Leybold Z400
rf-sputteringsystem o-sputtering small piees of boron
on a 3" diameter silion target. The boron to
sil-ion area ratio (from 0 to 13 %) was the only
pa-rameter modied. All the other deposition
parame-ters were kept xed (bias of 1000V; 250 o
C substrate
temperature; H2 ux of 8sm; Ar + H2pressure of
8x10 2
mbar; and 120 min of deposition time). The
lms, of about 0.6-0.8m thik, were deposited onto
rystalline silion and quartz substrates, for the
mea-surements of optial properties and the onentration
ov-measurements.
Theatual onentrationofboronin thelmswas
determinedusingthenulearreation 11
B(p,a)2a[6℄.
A quite linear orrelation between the boron area in
the target and the boron ontent in the lm was
ob-served. Theonentrationofbondedhydrogenwas
de-termined by infrared absorbane using a onventional
Niolet FTIR spetrometer and the well-known
ali-bration onstant for the hydrogen-silion wagging
vi-bration (at 640 m 1
) [7℄. The absorption oeÆient
andpseudo-gap weredetermined bytheproedure
de-velopedbySwanepoel[8℄usingthetransmittanedata
on the visible region. Photothermal Deetion
Spe-trosopy(PDS)wasusedtoobtainthedensityof
dan-glingbonds andthe Urbah energy. Theondutivity
measurements wereperformedusing asandwihsetup
(the top ontatswere evaporated aluminum dots) in
ordertoavoidsurfaeeets,whihareusuallypresent
inmeasurementsperformedusingonventionalparallel
ontats. Themeasurementsweretakeninthe300-450
Ktemperaturerange at arate of 3K/min,during the
oolingyle. All sampleswere measuredat thesame,
xed,voltageandintheohmiregime.
III Results
Fig. 1showsthe resultsof the ondutivity
measure-mentsasafuntion of theboron ontent. Fig. 1a
dis-playstheroomtemperatureondutivity,whileFig.1b
shows the ativation energy. For omparison, Fig. 1
also inludes data from Spear and Le Comber [9℄ for
dopeda-Si:HpreparedbyPECVDusingdiboraneasa
soureofboron;andbyKalbitzeretal. [10℄fora-Si:H
also preparedby PECVD,but introduing boron into
thelms byimplantation. Onean notie that in our
samples we manage to hange the room temperature
ondutivity, as well as the ativation energy, in the
samerangeobtainedfor PECVDlms. Themost
im-portantdiereneisthelargeamountofboronrequired,
in thesputteredlms,to ahievethesameparameters
obtainedforglowdishargelms.
Fig. 2presentsthevariationofthedefets density,
inferredbyPDSspetrosopy,asafuntionoftheboron
ontent. Thedotted line, withalog-logslope of 0.65,
is a powerlaw t to the data. The lms with boron
ontent higher than 3x10 20
atoms/m 3
were exluded
fromthisttingbeauseataboutthisonentrationthe
optialandstruturalpropertiesstarttohange
signif-iantly. Inpartiular, thehydrogenonentrationand
bandgapdereaseastheborononentrationinreases
Figure 1. Ativation energy and room temperature
on-dutivityofborondopeda-Si:Hasafuntionoftheboron
onentration. Thefullirlesareo-sputteredsamples;the
openirlesareboronimplantedPECVDsamples;andthe
solidlineistheresultobtainedforPECVDlmsusing
dib-oraneasasoureofboron.
500
1000
1500
2000
2500
3000
3500
4000
For all BN samples
F(N
2
): 44
F(B
2
H
6
): 44
B
2
H
6
/N
2
: 1x10
-2
rf-power: 0.5 W/cm
2
N-H
h-B-N
h-B-N
WAVENUMBER (cm
-1
)
ABSORBANCE (a.u.)
340
o
C
180
o
C
RT
o
C
Figure 2. Deepdefetdensity as afuntionof boron
on-entration. Thesolidlineisalineart,whoseslopeis0.65.
Fig. 3 shows the relationbetween the Urbah
en-ergyandthedensityofdanglingbonds. Datafrom
lit-erature, used by Stutzmann [11℄ to support his model
ofspontaneousonversionofweakbondsintodangling
bonds,werealsoaddedtothegureforthesakeof
Figure 3. Deepdefetdensity as afuntionofUrbah
en-ergy. Theirlesare data fromliterature and thesquares
aredatafromthiswork(o-sputteredsamples). Thedashed
line is alulated aording to the weak bond to dangling
bondonversionmodelproposedbyStutzmann.
IV Disussion
The Street's model laims that the ratio between the
four-fold and three-fold oordinated silion atoms is
given,onequilibrium, bythehemialreation:
Si 0
4 +X
0
3 Si
+
3 +X
4
where the subsripts are the oordination number
andthesupersriptsarethehargestate. Xisany
im-purity from III and V periodi table olumns. From
this simple hemial reation, using the mass ation
law,oneanndthedanglingbondsdensity(N
D ),as
wellasthedopingeÆieny(),asafuntionofthe
im-purityonentration. Thedanglingbonddensityvaries
diretlywith thesquare rootof impurityontent (N
D
[X℄ 1
2
), while the doping eÆieny is inversely
pro-portionaltothesquarerootoftheimpurityontent(
[X℄ 1
2
).
Fig. 2 shows that, in our samples, N
D [B℄
0:65
;
i.e., itdoesnotfollowthesquareroot powerlaw
fun-tionpreditedbytheStreet'smodel. Thisdisagreement
mustberelatedwiththeeletronistruturedierenes
betweenthe sputteredand PECVDsamples. First of
all,thedensityofdeepdefetsontheintrinsisputtered
deposited by PECVD. Besides, the Urbah energy, of
about80meV, isalsomuhhigherthanthatobtained
fromgoodPECVDsamples,whihisabout50meVor
lower. Indeed,asitiswellknow,ahighdensityofdeep
defetsmustpintheFermilevelnearthemidgap. Asa
onsequeneonemustinorporatemuhmoreboronin
sputteredlmsthaninPECVDonesinordertoobtain
thesameativation energy variation. Forinstane, in
Fig. 1one annotie that in order to reahvalues of
Ea~0,4 eV and C
RT ~10
7
W 1
m 1
, the boron
in-orporationinthesputteredlmsisalmost 3ordersof
magnitude higherthan that of PECVD lms.
There-fore,theeetofdistortionaroundtheimpurity
viin-ity due tothe inorporationof suh ahigh
onentra-tionofatomswithoordination3annotbenegleted.
In this proess, the amount of Si-Si weak bonds
in-reasesandaertainquantityof danglingbonds must
bereated. This statementissupported bytheresults
obtainedforthe Urbahenergy,sineits relationwith
the dangling bonds density (Fig. 3) is quantitatively
ingoodagreementwith themodel proposed by
Stutz-mann [11℄. This model explains how an inrease of
weakbonds, expressedin termsofUrbahenergy,an
produean exess of dangling bonds. Thus, the
dan-glingbond produtionin boron doped amorphous
sili-onpreparedbysputteringannotbeexplainedonlyin
termsof hargeinjetionStreet's model asin PECVD
samples. Other features suh as topologial disorder
reatedby thelarge amount of boron introdued into
thelmsmustbetakenintoaount.
V Conlusion
In summary, we managed to dope amorphous silion
with boron, using ao-sputtering tehnique, with
a-tivation energyand ondutivityof the same order of
thoseobtainedforPECVDlms. However,theamount
of boron required to shift the Fermi level is about
3 orders of magnitude higher than that neessary in
PECVDsamples. Itwasalsoobservedthatthe
harge-induedmodeldoesnotdesribesatisfatorilythe
pro-dutionofdanglingbonds. However,thedatatsvery
wellto the Stutzmannmodelbasedon thereationof
topologialdisorderduetotheinorporationofalarge
onentration of 3-fold oordinated boron atoms into
thesilionnetwork.
Aknowledgements
ThisworkwassupportedbyFAPESPand CNPq.
Referenes
[1℄ W.E. Spear and P. G.Le ComberSolid State
Com-muniation17,1193(1975).
[4℄ M.Stutzmann,K.D.Biegelsen,andR.A.Street,Phys.
Rev.B35,5666(1987).
[5℄ I. Chambouleyron and D. Comedi, J. of
Non-CrystallineSol.411-417,227(1998).
[6℄ M. Vollmer, J.D. Meyer, R. W.Mihelmann, and K.
Bethge,Nulear Instrumentsand MethodsinPhysis
ResearhB117,21(1996).
[7℄ H.Shanks,C.J.Fang, L.Ley,M. Cardona,J.F.
De-mond,andS.Kalbitzer,Phys.StatusSolidiB100,43
(1980).
[8℄ R.Swanepoel,JournalofPhysisE16,1214 (1983).
[9℄ W.E.SpearandP.G.LeComber,SolidSt.Commun.
17,1193(1976).
[10℄ S.Kalbitzer,G.Muller, P.G.LeComber, andW.E.
Spear,Phil.Mag.B41(4),439(1980).
