CW and pulsed electrically detected magnetic resonance spectroscopy at 263 GHz/12 T on operating amorphous silicon solar cells

1.3 MB·English

by  W. Akhtar

CW and pulsed electrically detected magnetic resonance spectroscopy at 263 GHz/12 T on
operating amorphous silicon solar cells
W.Akhtara,∗,A.Schnegga,∗,S.Veberb,c,C.Meierd,M.Fehra,K.Lipsa
aBerlinJointEPRLab,Institutfu¨rSilizium-Photovoltaik,HelmholtzZentrumBerlinfu¨rMaterialienundEnergie,Germany
bLaboratoryofMagneticResonance,InternationalTomographyCenterSBRAS,Russia
cNovosibirskStateUniversity,Russia
dBerlinJointEPRLab,FachbereichPhysik,FreieUniversita¨tBerlin,Germany
5
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0
2
Abstract
n
aHere we describe a new high frequency/high field continuous wave and pulsed electrically detected magnetic resonance (CW
J
EDMRandpEDMR)setup,operatingat263GHzandresonancefieldsbetween0and12T.Spindependenttransportinilluminated
4
hydrogenatedamorphoussiliconp-i-nsolarcellsat5Kand90Kwasstudiedbyinoperando263GHzCWandpEDMRalongside
1
withcomplementaryX-bandCWEDMR.Benefitingfromthesuperiorresolutionat263GHz,wewereabletobetterresolveEDMR

]signals originatingfrom spin dependenthoppingand recombinationprocesses. 5 K EDMR spectra were foundto be dominated
i
cbyconductionandvalencebandtalestatesinvolvedinspindependenthopping,withadditionalcontributionsfromtripletexciton
sstates. 90KEDMRspectracouldbeassignedtospinpairrecombinationinvolvingconductionbandtailstatesanddanglingbonds
-
lasdominatingspindependenttransportprocess,withadditionalcontributionsfromvalencebandtailandtripletexcitonstates.
r
t
mKeywords: Electricallydetectedmagneticresonance,conductionbandtail,valencebandtail,amorphoussilicon,solarcell
.
t
a
m1. INTRODUCTION However, the spectral resolution at this field is often not suf-
ficient to resolve g-tensor anisotropies and the resonances of
-
d Electrically detected magnetic resonance (EDMR) spec- different overlapping paramagnetic states, having close prox-
n
troscopy is a highly sensitive spectroscopic technique that imity in g-values. This restriction may be lifted by high fre-
o
cprobestheinvolvementofparamagneticstatesinchargetrans- quency/highfield EDMR, which is capableof resolvingsmall
[portandlossmechanisms,whileparamagneticbackgroundsig- g-tensoranisotropiesand overlappingEDMR resonanceswith

1 nals are eliminated [1–3]. EDMR is up to 8 ∼ 10 orders of slightlydifferentg-valueswhentheanisotropicZeemaninterac-
vmagnitudemoresensitivethanconventionalelectronparamag- tion,(∆g/giso)B0exceedsthefieldindependentinhomogeneous
5neticresonance(EPR)spectroscopy[4]andthusallowsforin- broadening,∆B,i.e. (∆g/giso)B0>∆B,where∆gistheg-value
8vestigatingasmallnumberofspinsinlowdimensionalmaterial difference,gisoisthemeang-valueandB0 istheexternalmag-
4systems and devices[4–8]. In addition, EDMRis regardedas neticfield.
3
one of the key technologiesto realize quantum computingal- A veryfirstreportonhighfrequency/highfield CWEDMR
0
.gorithmsbasedoncoherentmanipulationandreadoutofelec- appearedasearlyas1978in whichHoniget al. studiedphos-
1
tron spins in semiconductor samples [9–12]. Due to its high phorusdopedcrystallineSi(c-Si:P)at196GHz/7Tandatem-
0
5selectivity and sensitivity, EDMR is ideally suited for the as- peratureof1.4K [37]. Thisworkwasextendedin alater 240
1signmentandstructuralcharacterizationofparamagneticstates GHz/8.6TEDMRstudyonc-Si:PatT=2.8K[38,39],where
:determining charge transport and loss mechanisms in organic itwasshownthatthedominatingspin-dependenttransportpro-
v
iand Si solar cells [13–20]. Recently, it was shown that the cess at very high fields and low temperatures is a direct cap-
X
application range of EDMR as compared to continuous wave ture of conductionbandelectronsby phosphorusstates, while
r(CWEDMR)maybefurtherboostedbypulsed(pEDMR)de- spin-pair formation dominatesthe low-field EDMR spectra in
a
tection schemes [9, 11, 21–26], which greatly increased the thissystem [9]. Thisimportantfindingdemonstratesthathigh
selectivity to different spin-dependent transport mechanisms frequency/highfieldEDMRincreasesnotonlythespectralres-
and spin coupling parameters as well as the spectral resolu- olution,butalsotheselectivitytoparticulartransportprocesses.
tion [27–36]. Up to now most CW and pEDMR studies em- AfurtherW-band(94GHz/3.4T)EDMRstudyonadonor-
ployedconventionalX-band(9.4GHz/350mT)spectrometers. doped metal-oxide-semiconductor device employing a single
modecavityat5 K wasreportedbyLo et al. [12, 40]. Inthis
studysampledimensionswereofsub-millimeterrangetofita
∗Address: Kekule`straße5,12489Berlin,Germany,Phone: [phone]-
W-bandsinglemoderesonator. Dueto thesuperiorresolution
41353
ofhighfrequency/highfieldEDMR,thedonorandthetwodi-
Emailaddresses:[email protected](W.
Akhtar),[email protected](A.Schnegg) mensionalelectrongas(2-DEG)EDMRspectra,whichoverlap
PreprintsubmittedtoJournalofMagneticResonance January15,2015
Figure1:(coloronline).SchemeoftheCW/pulsed263GHzEPR/EDMRspectrometer.ThebasicspectrometerunitisanX-bandmwbridge(orangedashedblock)
whichdeterminesmwfrequency,phaseandtimestructure.AmixerblockconvertsX-bandtoQ-bandfrequencies,whicharefurthermultipliedtoachievethefinal
263GHzfrequency. 263GHzradiationisthanemittedintothequasi-optical bridge(greendashedblock)byhornantennas, whereitpropagatesasaGaussian
beam(solidpurpleline). Off-axisellipticalmirrors(M1andM2)directthe263GHzGaussianbeamintothecorrugatedwaveguideforsampleexcitationinside
themagnet. Wire-gridpolarizers(P1,P2)andaFaradayrotatorseparateinductionandreflectionmodeEPRsignals,whicharedownconvertedbysub-harmonic
mixers(SH1andSH2)toX-bandfrequenciesandfurtherprocessedbythedetectionelectronics(bluedottedblock). ForEDMRdetectionthecurrentthroughthe
solarcell(representedasadiodeinsidethemagnet)isdetectedbya(1)trans-impedanceamplifier,(2)ahigh-passfilter,(3)anamplifier,(4)alow-passfilterand
(5)anadditionalamplifier.Thefinalelectricalsignalisthanfedintothedetectionelectronics.
atX-bandfrequencies,couldbeseparated.VeryrecentlyMeier implemented CW EDMR/pEDMR extension. The depicted
et al. conducted a multi-frequency (X-band, Q-band and W- EDMRsetupisbasedonacommercialBrukerE-7801016,263
band)pEDMRstudyonmicro-crystalline(µc-Si:H)p-i-nsolar GHz/12TEPRspectrometer,whichcanbeoperatedinCWand
cells with a-Si:H n-layer [19]. Thereby, the g-valuesand line pulsed EPR and electron-nucleardouble resonance (ENDOR)
shapes of a-Si:H conductionband tail states and localized µc- modes. Microwave(mw)generationandmanipulationisdone
Si:Hconductionband(CE)statescouldbedeterminedwithin- in a heterodyne microwave bridge. The corresponding reso-
creased resolution. In orderto furtherpush the capabilitiesof nance fields are generated in a cryogen-free superconducting
very high frequency/highfield EDMR, with respect to the ac- magnet,whichconsistsofamainanda sweepcoil. Themain
cessible resonancefrequencies/fieldsand temperatures,within solenoidcoilcanbesweptbetween0and12T,whilethesweep
this work we developed a CW EDMR/pEDMR set-up based coilgeneratesadditionalfieldsof±0.15Twithhighaccuracy.
on a commercialBruker E-7801016EPR spectrometerwork- The latter can be driven in a special linearize mode to com-
ingat263GHz/12T.Herein,wegiveadetaileddescriptionof pensate for field nonlinearities. EDMR samples are placed in
thisinstrumentanddemonstratefirst263GHzCWEDMRand anon-resonantEPR/EDMRprobewhichisdepictedinFig.2,
pEMDRexperimentsonoperatinga-Si:Hp-i-nsolarcells. withaphotographoftheprobeinFig.2a. Fig.2bsketchesthe
cross section of the probe showing the corrugated waveguide
tapereddowntothesamplespace,thepositionofthelaserfiber
2. EDMRSPECTROMETER@263GHz
for light excitation of the sample, the modulation coil (maxi-
mummodulationamplitude50G@100kHz)andthesample
Fig. 1 shows the layout of the excitation and detection
position with the electrical contacts for EDMR detection. In-
scheme of the 263 GHz spectrometer including the newly
2
side the probe, sample temperaturecan be controlledbetween
4.5Kto300KusinganOxfordITC503temperaturecontroller.
Fig.2cshowsthep-i-nsolarcellsampleofdimension1×1
mm(activesolarcellarea)thatisattachedtothesampleholder.
Electricalconnectionstothesolarcellarerealizedbythingold
wires fixed using silver paste on the contact pads and finally
soldered to the coaxialline, which is used to applya constant
voltageandtomonitorthephotocurrentfromthesample. The
layer structure of the p-i-n solar cell is depicted in Fig. 2d. It
consists of a 1 µm thick intrinsic a-Si:H layer, which is sand-
wiched between thin p-type (20 nm) and n-type (30 nm) µc-
Si:H layers and Al doped ZnO layer, that act as transparent
conducting oxide contacts. We have used µc-Si:H as contact
layerstodistinguishsignalsfromthepandndopedlayersfrom
thatoftheintrinsica-Si:Hlayerasthetwomaterialshavedif-
ferentEDMRresonances[13, 14, 28]. Photoexcitationofthe
solar cellis achievedby lasersourcesorthelightfroma 250-
watt KL 2500 LCD halogen cold light source through a 1.22
mm optical fiber. The optical illumination and mw excitation
ofthesolarcellisdonethroughtheglasssubstrateasdepicted
in the Fig. 2d. The details of the current detection setup are
showninFig.1. Hereasteady-statephotocurrentisestablished Figure2: (coloronline). (a)Photographand(b)crosssectionofthe263GHz
underaconstantvoltagebias(V )andillumination.Theout- EDMRprobe. (c)PhotographoftheEDMRsampleholderwithmounteda-
bias
Si:Hp-i-nsolarcell. Theradiofrequency(rf)coilsshowninthefigurecanbe
put of the trans-impedance amplifier (I-V convertor) converts
usedforrfexcitationofthesample.(d)Layerstructureofthea-Si:Hp-i-nsolar
thephotocurrentintoavoltagewhichgoesthroughthefiltering cell. B0andB1indicatetheconstantexternalmagneticfieldandthemagnetic
andamplificationcircuit(shownintheFig.1inredblocks)to componentofthemwfield,respectively.
maximizethesignal-to-noiseratio. Thisset-upallowsforboth
CWEDMRaswellaspEDMRdetection. ForCWEDMRde-
tection, the current detection outputis connectedto either the uide)is≈ 1µs, whichcorrespondstoan excitationbandwidth
absorptionordispersionchanneloftheLock-Indetector.Since of≈1MHz.ThisputsarestrictiontoperformpEDMRonsam-
theoutputoftheLock-Inprovides4channels(0◦and90◦ofthe
ples with short relaxationtimes as well as on the detection of
EPRabsorptionanddispersionsignals,respectively),EPRand
electron spin echo envelopmodulation, ESEEM (nuclear Lar-
EDMR signals can be measuredsimultaneously. For pEDMR morfrequencyat9.4Tfor15N≈41MHz,29Si≈80MHz,31P
we directly connect the outputof the current detection to one ≈162MHzand1H≈400MHz). Infuture,changingthesam-
ofthechannelsoftheBrukerSpecJetdigitizer(bandwidth500
pledesignsuchthatthemwexcitationcanberealisedfromthe
MHz). ThemaximummwpoweravailableinCWaswellasin
front side, we will be able to avoid the loss due to glass sub-
pulsemodeis15mW.Thecurrenttransientafterthemwpulse
strate. With such a design shorter π/2 pulses will be possible
isboxcarintegratedandrecordedasafunctionofmagneticfield
thatwillincreasetheapplicationrangeofpEDMRexperiments.
toyieldthepEDMRspectrum[3].
The possibilityto measureEPR andEDMRsimultaneously
The current set-up allows for pulsed and CW EDMR with
inthecurrentset-upgreatlyimprovestheaccuracyoffieldcal-
magnetic field and mw amplitude modulationas well as elec-
ibrations,sincefortheprecisedeterminationofg-valuesrefer-
trically detected electron electron double resonance (EDEL-
enceEPRmeasurementswithfieldstandards(inourcaseMn2+
DOR) [25], electrically detectedelectronnucleardoublereso-
in MgO matrix [41] or N@C [42]) are required. These cal-
60
nance(EDENDOR)[26]andelectricallydetecteddoubleelec-
ibrations only yield optimum precision if the reference EPR
tron electron resonance (EDDEER) [36]. These experiments
sample can be placed exactly at the same position as the
carriedoutwithouttheemploymentofa resonatorholdsgreat
EPR/EDMR sample under study. Mounting and demounting
advantageas fully processed samplesof severalmm size may
ofthereferencesamplemayfailtoreproducethesampleposi-
bemounted,temperatureinducedinstabilitiesarelesspressing
tion. In orderto circumventthis problemwe attached a small
ascomparedtosinglemoderesonatorset-upsandinadditionit
amount of N@C underneath the solar cell and recorded its
60
providesmorefreedomtoaddadditionalelectricalconnections,
EPRalongsidewiththeEDMR.
rfcoilsorilluminationfacilities. However,omittingaresonator
significantly reducesthe mw field at the sample position. For
the solar cell used in this study, mw excitationhas to be done 3. EXPERIMENTALRESULTSANDDISCUSSION
throughtheglasssubstrate(seeFig.2d)whichfurtherleadsto
nearly10dBlossinmwpower.Evenwiththeabovementioned Fig. 3 depicts X-band CW and 263 GHz CW and pEDMR
losses,theminimumπ/2pulselengthachievableinthecurrent spectra obtained on illuminated a-Si:H solar cells under for-
EDMRset-up(15mWattheentranceofthecorrugatedwaveg- wardbias(V=+1V)at5Kand90K,respectively. Forbetter
3
comparability, all spectra are plotted againstthe same g-value
span (upperx-axes)and the correspondingmagneticfield val-
ues(bottomx-axes).Inthefollowing,wewillfirstqualitatively
describethecharacteristicsanddifferencesoftheEDMRspec-
trashowninFig.3. Basedonthesefindingswewillthancon-
cludeontheunderlyingspindependenttransportprocessesand
finallytesttheseassignmentsbyspectralsimulations.
X-band CW EDMR spectra measured at 5 K (Fig. 3a) and
90K(Fig.3b)areverysimilar. Bothspectraaredominatedby
asymmetriclinescenteredatg=2.0044andg=2.005,respec-
tively, which extend towards larger g-valuesaround g = 2.01.
In addition, we find a very broad resonance (width ≈ 20 mT)
centeredaroundg = 2.008,whichclearlyexceedsthespectral
windowdepictedinFig.3aand 3b. Thiscontributionismore
pronouncedin the 5 K spectrumas comparedto 90K. Differ-
ences between 5 K and 90 K CW EDMR spectra obtained on
thesamesolarcellsundersimilaroperatingconditionsstrongly
increaseuponincreasingtheresonancefrequencyfromX-band
to 263GHz. The263GHzCWEDMRspectrummeasuredat
5 K (Fig. 3c), now exhibits three well resolved contributions
around g = 2.01 and g = 2.0044 and a smaller signal at g =
1.996. Plotting 5 K X-band and 263 GHz CW EDMR spec-
tra on the same g-scale reveals a pronounced increase of the
spectralresolution,despiteofthefactthattheindividualspec-
tralcontributionssignificantlybroaden. Onthecontrary,90K
CW X-band and 263 GHz spectra (see Fig. 3b and 3d) have
verysimilarshapes. Theirmaindifferenceisanincreaseofthe Figure3:(coloronline).X-bandCWEDMR(firstrow),263GHzCWEDMR
absolutespectralwidthat263GHz. (secondrow)and263GHzpEDMR(thirdrow)spectraobtainedat5K(leftcol-
Observedtemperatureinduceddifferencesbetween5K and umn)and90K(rightcolumn),respectivelyona-Si:Hp-i-nsolarcell.Themw
frequencyfor(a)and(b)is9.6688and9.6644GHz,respectively, and263.32
90 K 263 GHz CW EDMR spectra are a clear indication that
GHzfor(c)to(f).CWspectraat5Kand90KweretakenwiththesameLock-
thespectraaredominatedbydifferentparamagneticstatesand inphase.Spectraareplottedvs.magneticfield(bottomaxis)andg-values(top
hence different spin dependenttransport processes, despite of axis). pEDMRspectrawerecorrectedformicrowaveinducedcurrentchanges
bythesubtractionofoff-resonancecurrenttransients. Linearbaselinecorrec-
theirsimilarity atX-bandfrequencies. Additionalinformation
tionwasappliedto263GHzCWEDMRspectra.Mainpeaksinthespectraare
ontheprevailingspindependenttransportprocessmaybeob-
markedbyverticaldoublesidedarrows.
tained from complementarypEDMR measurements. 5 K and
90K263GHzpEDMRspectraaredepictedinFig.3eand 3f,
respectively. 263 GHz pEDMR and CW EDMR spectra ex-
hibit identical spectral contributions. However, in addition to ture [48]. However, till date latter assignments have not been
temperature induced spectral changes 5 K and 90 K pEDMR confirmedbyhighfrequencyEPRorEDMR.
spectra exhibit different signs. At 5 K, 263 GHz pEDMR in- AtliquidHetemperaturesspindependenthoppingtransport
ducesacurrentenhancement(positivesignal),whereasat90K alongcbtandvbtgivesrisetoEDMRinducedcurrentenhance-
pEDMRcurrentquenching(negativesignal)isobserved. ment spectra [13, 16, 49, 50]. We therefore assign the 5 K
Wewillnowidentifytheobservedresonanceswithparamag- EDMR enhancing signals plotted in Fig. 3a, 3c and 3e, to
netic states and assign them to spin dependenttransportpath- two independent spin dependent hopping processes along cbt
ways. Intrinsic a-Si:H is a disordered semiconductorwith lo- (g=2.0044)andvbt(g=2.011)states,respectively.Thesmall
calizedstatesbelowtheconductionandabovethevalenceband, signalatg=1.996observedin5K263GHzCWEDMRspec-
whicharereferredtoasconductionbandtail(cbt)andvalence tra(Fig.3c)canbeassignedtoasimilarhoppingprocessalong
band tail (vbt) states. Under illuminationcbt and vbt become CE states in the n-doped µc-Si:H layer [17, 28, 51, 52]. An
paramagnetic[43–47].Forcbtstatesina-Si:H,multi-frequency additionalverybroadresonance(20 mT)centered aroundg =
pEDMRupto W-bandfrequenciesrevealeda purelyisotropic 2.008 has been observed by optically detected magnetic res-
g-value (g = 2.0047(2))[19]. For the vbt counterpart,the sit- onance [30, 53] and CW EDMR [49, 50] on undoped a-Si:H
uation is less clear. This state is typically associated with a layer and was associated to strongly coupled triplet electron-
resonanceatg=2.011[43–45].However,Q-bandEPRonillu- hole(e-h)pairs. Thistripletexcitonsignalmayaccountforthe
minatedintrinsica-Si:Hpowdersrevealedpronouncedg-tensor broad background observed in the X-band CW EDMR spec-
anisotropy for vbt states (g = 2.019, g = 2.012 and g = trum(Fig.3a).
xx yy zz
2.005) [47]. Furthermore, it was concluded that vbt g-values Intrinsica-Si:HexhibitsfurtherpronouncedEPRandEDMR
depend on the p-type doping level and the sample tempera- resonances, which have been associated to three fold coordi-
4
natedSiatoms,socalleddanglingbonds(dbs)[45,47,54,55].
At temperatures, between 70 K and room temperature, tun-
neling of trapped cbt electrons into neutral dbs leads to dou-
bly occupied db (db−) states which finally recombine with
holes [13, 16, 49, 50]. The same spin dependent recombina-
tion process givesrise to the 90 K EDMR spectra depicted in
Fig. 3b, 3d and 3f. The peak of the main resonance position
at g = 2.005, is a superposition of the db (g = 2.0055) and
cbt (g = 2.0044) resonances. In pEDMR, this process leads
to a currentquenchingsignal(see Fig. 3f). It shouldbe noted
here that the sign change between 5 K and 90 K is not evi-
dentintheCWEDMRspectramainlybecausethephaseofthe
CW EDMR spectra hascomplexdependenceonexperimental
conditions such as temperature, microwave power and modu-
lation frequency [56]. 90 K EDMR spectra still show contri-
butions from vbt resonances. This is most obvious in the 90
K X-band CW EDMR spectrum depicted in Fig. 3b, but can
bealsofoundinthe90K263GHzpEDMRcurrentquenching
spectrum (Fig. 3f). Similar signals were previously assigned
Figure4:(coloronline). Experimental(solidblue)andsimulated(reddashed)
to a recombination process, where the final electron-hole re-
CW EDMRspectra. 5K (a) 263 GHz and (c) X-band EDMRspectra were
combinationstepoccursbetweenadoublyoccupieddb− anda
simulated assuming a superposition of isotropic cbt (g = 2.0044, g-strain =
holetrappedinavbtstate. Thisstepmaybeenhancedbyspin- 0.0025, fieldindependent Lorentzian linewidth=1mT),vbt(g=2.011, g-
dependentdiffusionofholesviavbtstatestowardsthedb−[45]. strain=0.01,fieldindependentLorentzianlinewidth=1.5mT)andtriplet(g
=2.008, fieldindependent Gaussian linewidth=26mT)signals. 90K(b)
To further test these assignments, we performed numerical
263GHzand(d)X-bandEDMRspectraweresimulatedassumingidenticalg-
simulationsoftheexperimentalEDMRspectrausingEasyspin,
valuesandlineshapeparametersforcbt,vbtandtripletstatesandanadditional
aMatlabEPRsimulationtoolbox[57]. Inthesimulations,the db(gxx =2.0079, gyy =2.0061and gzz =2.0034, gxx-strain =0.0054, gyy-
individual EDMR resonances were modeled by isolated spin strain =0.0022, gzz-strain =0.0018, field independent Voigtian line of0.13
mTGaussianand0.43mTLorentzianlinewidth)contribution.Dottedorange,
centers, forwhichonlythe respectiveZeemaninteractionwas
black, magentaandgreenlines representcontributions fromcbt, vbt, dband
explicitlytakenintoaccountintheSpinHamiltonian.Anyspin- tripletstates,respectively.
spin interactionbetweendifferentcenterswasneglected. Line
width parameters for S = 1/2 centers (cbt, vbt and db) were
modeledusingsite-to-siteg-valuevariations(g-strain)incom-
bination with field independent Gaussian or Lorentzian lines. width-half-maximaof1mT(1.5mT),fielddependentg-strain
ThelattercombinationleadstoVoigtianlineshapesatX-band of 0.0025 (0.01) for cbt (vbt) lines and a 26 mT broad Gaus-
frequency,with increasing Gaussian contributionat 263 GHz. sianlineforthetripletexcitonsignal. Usingthesameg-values
Fortriplets(S=1),afieldindependentdipolarbroadenedGaus- and line width parameters, but differentweightsfor the single
sianlineshapewasassumed. Incaseswhereindependentpro- paramagneticstates, we were also able to reproducethe main
cesses contributetotheEDMRspectrum,e.g. forspin depen- featuresof5K X-bandCWEDMRspectrum(Fig.4c). How-
denthoppingamongvbtandcbtstates, theweightsoftheres- ever, in both cases we find deviations between simulated and
onancesweredeterminedindividually. However,forspectrain experimentalEDMR spectra. These differencesare most pro-
whichtworesonancescontributetothesameprocess,e.g. spin nounced at the low field edges of the spectra in the range of
dependent recombination from cbt to dbs, the same weights the vbt resonances. To test whether these discrepancies may
wereusedforbothspinpairpartners. originate from an anisotropic vbt g-tensor, we performed ad-
Since5Kand90KEDMRspectraareresultingfromdiffer- ditionalspectralfits to the 263GHz CW EDMRspectrumas-
entspindependentprocesses,differentmodelswereemployed suming isotropic cbtand triplet signals, but anisotropicvbtg-
to simulate low and high temperature EDMR spectra. Based values.Inthefitsallthreemaincomponentsofthevbtg-tensor
on the aboveassignments, we started to simulate the 5 K 263 and the isotropic cbt g-values were left open. A much bet-
GHz CW EDMR spectrum as a superpositionof isotropic cbt teragreementbetweenexperimentalandcalculatedspectra,as
(g=2.0044)andvbt(g=2.011)statesandanadditionalcon- comparedtothepurelyisotropicassumption,wereobtainedfor
tribution from triplet excitons (g = 2.008). To adjust the line acbtg-value=2.00445(6)andananisotropicvbtg-tensorwith
intensitiesandwidthsofthesimulationtotheexperimental5K gxx=2.015(1),gyy=2.009(1)andgzz=2.0058(2)(notshown).
263GHzCWEDMRspectrum,afittingroutinewasemployed The observedvbt g-tensoranisotropyis in accordancewith
in which the line widths and weights were left open. Fig. 4 previousQ-bandEPRexperimentsonilluminateda-Si:Hpow-
depicts simulated and experimental EDMR spectra alongside ders [47]. However, vbt g-values, extracted from spectral fits
withtheindividualcontributionsofdifferentspincenters. Best tothe263GHzCWEDMRspectra,significantlydeviatefrom
fits to the 5K 263 GHz CW EDMR spectrum (Fig. 4a) were those extracted by EPR [47]. Notably, the largest canonical
achieved using a field independent Lorentzian line with full- g-value, g = 2.015 deviates from g = 2.019, the value ob-
xx xx
5
tainedin Ref. [47]. Despite thefactthatour263GHz EDMR g-tensoror the contributionof additionalyetunassignedpara-
approachspreadsg-valuesoveramuchwiderspectralwindow magnetic states. A clear cut assignment of the involved cen-
andtherebypotentiallyprovidesmoreaccurateg-values,differ- tersrequiresadditionalpulsedmulti-frequencyEDMRexperi-
entreasonsmayberesponsiblefortheobserveddiscrepancy.It ments,whichwouldallowforadiscriminationbetweenS=1/2
may be dueto differentexperimentalconditionsanddetection andS=1spins[29,58]. Suchstudiesarecurrentlyontheway
techniques or the presence of additional yet unassigned para- in our lab. Additional evidence may be gained from comple-
magneticstatesinvolvedinotherspindependenttransportpro- mentaryadvanceddensityfunctionaltheorycalculations,which
cesses. Inparticular,thelatterpointcannotbeunambiguously became available only recently and were employed for the g-
discriminated froma vbt g-tensoranisotropybased on the ex- tensorcalculationsofcbtanddbina-Si:H[18,34,55]. Inthe
perimentaldatapresentedherein. presentcase,calculationsofvbtg-valuescouldprovideimpor-
Finally,wesimulatedtheCWEDMRspectraobtainedat90 tantadditionalinformationonwhethervbtstatesindeedexhibit
Kassumingspindependentrecombinationbetweencbtanddb anisotropic g-values. The presented results demonstrate that
statesasdominatingprocess,withadditionalcontributionsfrom multi-frequencyEDMR,includingveryhighfrequenciesupto
vbtstatesandtripletexcitons.g-values,g-strainsandfieldinde- 263 GHz, can help to resolve overlapping EDMR resonances
pendentlinewidthparametersofcbt,vbtandtripletstateswere andtoassigntransportprocessesinvolvingthesameordifferent
chosenidenticaltothoseextractedfromspectralfits(basedon paramagneticstates. Combing the high versatility of pEDMR
isotropic g-values of cbt and vbt states) to the 5 K 263 GHz with respect to mw excitation schemes with the ultimate sen-
EDMR spectrum(see above). The db g-tensor(g = 2.0079, sitivity of CW EDMR in a high frequency/high field EDMR
xx
g = 2.0061 and g = 2.0034) and strains were taken from experimentfurther boosts the potential of this approach. Due
yy zz
Ref.[55]. Tosimulatetheexperimental90K263GHzandX- toitsunmatcheddetectionsensitivity,suchstudiesarenotlim-
band CW EDMR spectrum, a fitting routine was employedin ited to cryogenic temperatures but may in the future also be
which the weightsof the individuallines were left open, with performedatroomtemperature,whichwillyieldfurtherhighly
the restrictions that cbt and db weights are assumed to be the desiredpiecesofinformationonspindependenttransportpro-
same. Fig. 4b and 4d depict experimental 263 GHz and X- cessesinoperatingelectronicdevices.
band90KCWEDMRspectra,respectivelyalongsidewiththe
contributions of the individual spin centers. Good match be-
tween simulated and experimentalEDMR spectra confirmthe 5. ACKNOWLEDGEMENT
assignmentofparamagneticstatescontributingtothespectrum.
Thiswork wassupportedby the GermanResearch Founda-
tion within SPP 1601. We thank F. Finger and O. Astakhov
4. CONCLUSION
(ForschungszentrumJu¨lich)forsupplyingthesolarcellswhich
In conclusion, we have developed a 263 GHz/12 T werepreparedthroughtheBMBFfundednetworkprojectEPR-
CW/pEDMRset-upwhichallowsforin-situmeasurementson Solar (03SF0328). We are also grateful to J. Behrends (FU
fullyprocessedelectronicdevicesunderlightandappliedbias Berlin/HZB)forhelpfuldiscussions.
voltage. With this set-up, EDMR measurements can be real-
ized from liquid He to room temperatures. Precise magnetic References
field calibration at the sample position can be obtained by si-
multaneousEPR measurementsof reference samples attached
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8