Ionizing Radiation and Hot Carrier
Eets in SiC MOS Devies
E.A. de Vasonelos, E. F. daSilvaJr.,
Departamento deF
isia,Universidade FederaldePernambuo,
CidadeUniversitaria, 50570-901Reife,Pernambuo, Brazil
T. Katsube, and S. Yoshida
Graduate ShoolofSieneandEngineering,SaitamaUniversity
255Shimo-Okubo, Urawa-shi,Saitama-ken, 338-8570,Japan.
Reeivedon23April,2001
Weperformedomparativestudiesofradiation-induedinterfaetrapgenerationatn-6H-SiC/SiO2
andSi/SiO
2
interfaesforhighdosesoflowenergyx-raysintherange0to200Mrad(Si). Wefound
thatradiation-induedinterfaetrapgenerationatthen-6H-SiC/SiO2interfaeismuhlowerthan
at theSi/SiO
2
interfae. Theexperimentsprovideevidene of interfaetrap generationnear the
SiCondutionbandassoiatedwithoxidetraps.
A mirosopi model of radiation-indued defets
in SiC MOSdevies is still missing and further
inves-tigation is neessary [1℄. In this work, we performed
omparativeinvestigationsofionizingradiationandhot
arriereets in SiC andSi MOSdevies. Wewill
re-portonexperimentsinvolvinginterfaeandoxidetrap
generation in theoxideand brieydisuss the
meha-nismsresponsiblefortheseeets.
Si MOS apaitors were fabriated using n-type 2
in., (100) silion wafers of resistivity 1 -m. The
waferswereleanedbyamodiedRCAproessas
de-sribedelsewhere,yieldingdevieswithradiation
hard-ness improved by up to one order of magnitude [2℄.
Thermal oxides were grown in dry O
2
at 1000 Æ
C,
fol-lowed by an in situ dry N
2
annealing for 30 minutes.
Aluminumlmsapproximately200nmthikwere
ther-mally evaporatedto form gateeletrodes. After
bak-sidemetallization,thewaferswereannealedinforming
gasat400 Æ
Cfor30minutes. Thisisanoptimized
fabri-ationproessyieldingdevieswithimprovedradiation
hardness. Ontheotherhand,theproessingoftheSiC
MOSdevieswasnotoptimizedandthedeviesarenot
expetedto havethebest radiationhardnesspossible.
SiCMOSapaitorswerefabriatedona3.5 Æ
o-angled
n-type 6H-SiC wafer (Si fae) with a 5-mirons-thik
epitaxiallayer. Thenominaldopingdensityofthe
epi-layer is 610 15
m 3
. Thewaferswere ultrasonially
leaned in suessive, ve-minstepsin methanol,
ae-tone and 10% HF with a two-min rinse in deionized
dryO
2
at 1000 Æ
Cfor 13hours, followed byan in situ
dryN
2
annealingfor30minutesatthegrowth
tempera-ture.Aluminumwasevaporatedtoformgateeletrodes
withadiameterof1mmandalargeareabakontat.
Thepost-metallizationannealingwasdoneat400 Æ
Cin
a mixture of 75% Ar-25%H
2
during 30 minutes.
Ir-radiation soure was a x-ray beam generated from a
Cutargetbombardedby40keVeletrons(20mA)and
thedose ratewas6Mrad(Si)/h. Mostoftheabsorbed
doseresultsfromtheradiationnear10keV.Thex-ray
dosewas previouslyevaluatedbymeansofdosimeters,
whihonsistinSiphotodiodeswithanaluminumgate
ofspeiedthikness(200nm). Irradiatedsamples
al-ways havealuminum gates with sameeletrode
thik-ness(200 nm), so that the eetive x-ray dose in the
oxide is known. The apaitors were in open-iruit
onditionduringirradiation. Carrierinjetionwas
per-formed by means of a HP 4155A Semiondutor
Pa-rameterAnalyzer.
Exposureto ionizingradiation orhotarrier
inje-tionauseinreasingdegradationofSi-andSiC-based
deviesdueto thegenerationofinterfaetrapand
ox-idedefets,asdepitedingures1and2. Fig. 1shows
that after Fowler-Nordheim injetion of eletronsinto
theoxide,theurrentdensityinreasesinthediretand
Fowler-Nordheimtunnelingregimes. Thisinrementin
urrentismostlyassoiatedwiththegeneration of
ox-ide traps, whih assist the tunneling proess. Oxide
Figure1. CurrentharateristisofSiCandSiMOS
apa-itors afterrepeated positive voltage rampstressing (seven
yles,urrentlimitsetto1A).
arerespetively: 50nmand3.0210 3
m 2
(urves(a)
and (b)) and 10 nm and 3.6 10 3
m 2
(urves ()
and(d)). Fig. 2showsroomtemperature
apaitane-voltage harateristis of Si and SiC MOS apaitors
irradiatedwithsimilar x-raydoses. Oxidethiknessof
then-6H-SiCMOSapaitor(urves(a)and(b))is85
nm,gateareais7.8510 3
m 2
. Oxidethiknessofthe
Si MOS apaitor (urves() and (d)) is 50 nm, gate
areais1.2210 2
m 2
. InFig.1,oneanseearedued
leakageurrentgenerationin theSiCMOSdevieand
analyzingthehigh-frequenyapaitane-voltage(CV)
urves in Fig. 2, it is lear that the negative voltage
shift,reetingthegeneration ofpositiveoxideharge
and thestreth-outofthe urves,reetingthe
gener-ation of interfaestates, are muh reduedin the SiC
MOS apaitor. This striking dierene in behavior
after exposure to ionizing radiation was the main
fo-us of this investigation. In order to estimate the
to-talinterfaetrapdensity,weusedboththestreth-out
tehnique and the photo-apaitane-voltage method
(also known as the Jenq tehnique) [3℄, by
illuminat-ing the apaitors with intense white light from ber
opti illuminatorsfor5to10minutes. Weused
room-temperature quasistati CV urves to hek the
qual-ityof theoxideand toget aqualitativepitureofthe
interfae-trap generation. At room temperature, due
to thelarge band gap ofSiC, minority arrier
genera-tionisverylowandSiCbehavesinsimilarwayasSiat
lowertemperatures,sothattheenergyrangeprobedby
thehigh-lowtehniqueisreduedtoenergiesverylose
to SiC ondution band, orequivalently, verylose to
aumulation. This region of the spetrum has been
Figure2. Capaitane-voltage harateristisofa SiMOS
and a SiC MOS apaitor irradiated with similar x-ray
doses.
Figure3. Capaitane-voltageurvesofa6H-SiCMOS
a-paitorexposedtodierentx-raydoses. Oxidethiknessis
9nm and gatearea is7.8510 3
m 2
. Doses inMrad(Si)
are: (a)0,(b)14, ()110.
Fig. 3 shows high frequeny and quasistati CV
urves of a SiC MOS apaitor exposed to dierent
x-rays doses. As in Fig. 2, the negative voltage shift
of the high-frequeny CV urves, reeting the
gen-eration of positive oxide harge is low. Most of the
variationinthequasistatiurvesappearsnearthe
a-umulation region. This variation is probably due to
thegenerationofinterfaestatesintheupperrangeof
the SiC band gap, sine the energy range probed by
the tehnique is redued to energiesverylose to SiC
ondution band. Therefore, thevariationobservedin
thequasistatiurvesimpliesdefetgenerationnearthe
Currentpeaksandmisalignmentarealsoobserved.
Current peaks near atband are observed in devies
with oxidesheavilyontaminatedwith mobileions[5℄.
Atelevatedtemperatures,whentheoxideeldreverses
sign,theionsmoveandapeakinthedisplaement
ur-rentappears. At room temperature, movementof the
ions isreduedandweareled tobelievethat the
ur-rentpeaksarepossiblyrelatedtooxide-trappedharges
generated by irradiation and exhanging harge with
the substrate, the so-alled border traps or,
equiva-lently, swithing oxidetraps [6℄. Further investigation
is needed to separate unambiguously the eet of the
oxidetraps fromtheeetofthemobileions.
Figure4.Interfaetrapgenerationinn-type6H-SiCandSi
MOS apaitors. Curve(a)is apreviously reported result
obtained inn-typeSiunder similarirradiationonditions,
as disussed inthe text. Oxidethikness is 23nm (urve
(a)),85nm(urve(b))and9nm(urve()).
Fig. 4showsinterfaetrapgeneration (D
it )as a
funtion of dose ompared with a previouslyreported
result obtained in n-type Si under similar irradiation
onditions (urve(a)). Curve (a) is the response of a
ommerial radiation-hard devie (23 nm gate oxide)
exposed to 10 keV x-rays at a dose rate of 18 Mrad
(Si)/hwithgatevoltageequaltozero[4℄. Itisknown,
forSidevies,thatthenumberofradiation-indued
in-terfaetrapsinreasesas theoxidethiknessinreases.
Suh adependene onoxidethiknessis alsoobserved
for the SiC devies, as shown in Fig. 4. However, it
is lear that the SiC MOS apaitor has lower
inter-fae trap buildup, despite its oxide being almost four
timesthiker(85nm)thantheoxideoftheommerial
hardened(23nm)Sidevie. TheSiCMOSis
radiation-dures. These results are onsistent with previous
re-ports, performedusing lowerdoses ofradiation,
show-ing promising radiation hardness for SiC devies [7℄.
Radiation-indued holes and hydrogen ions migrating
towards the Si/SiO
2
interfae play a very important
role in the formation of interfae traps. It is known
that a relatively thik (20 nm) negative harge layer
hasbeenobservedin 6H-SIC MOS apaitors[7℄. So,
onepossibilityto explainimprovedradiationhardness
inSiCMOSdeviesistoadmitthatamehanismof
an-nihilationorhargeompensationoursnearoratthe
SiC/SiO
2
interfae,thereforepreventingtheformation
ofinterfaetrapsbyholesandhydrogenions. The
ox-idetrapsgeneratedbytheseompensation/annihilation
reationsneartheinterfaeouldatasoxide-trapped
hargesthat exhange hargewith the substrate,
pos-siblyausingpeaksinthequasistatiCVurves.
In summary,omparative x-ray irradiation of
epi-taxial6H-SiCMOSapaitors andSi MOSapaitors
showed thatinterfaetrapgenerationrateis muh
re-duedin SiC MOSdevies. Evideneofinterfaetrap
generation near the SiC ondution band assoiated
withswithingoxidetrapsouldbeobserved. The
ra-diationresponseof SiC devies,under x-rayexposure,
indiatedamuhhigherradiationimmunitythaninSi
devies,despitethefat thatthe SiCdevieswerenot
speiallyproessedto beradiationhard.
Aknowledgments
The authors are thankful to the nanial
sup-port reeived during the development of this work
from MONBUSHO # 08455160 (Japan), CNPq,
PADCT/FINEP under ontrat# 77.97.1120.00, and
CTPETRO/FINEPunderontrat#65.00.02.80.00.
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