Charge Coupled Device detectors with high quantum efficiency at UV wavelengths

0.41 MB·

by  Erika T. Hamden

CCD detectors with high QE at UV wavelengths
ErikaT.Hamden1,AprilD.Jewell2,CharlesA.Shapiro2,SamuelR.Cheng2,TimM.Goodsall2,
JohnHennessy2,MichaelHoenk2,ToddJones2,SamGordon3,HweiRuOng4,DavidSchiminovich4,
D.ChristopherMartin1,ShoulehNikzad2
1DepartmentofAstronomy,CaliforniaInstituteofTechnology,1200EastCaliforniaBlvd,Pasadena,CA91125,USA
2JetPropulsionLaboratory,CaliforniaInstituteofTechnology,M/S302-304,Pasadena,CA91109,USA
3ArizonaStateUniversity,SchoolofEarthandSpaceExploration,Tempe,AZ,85287USA
4DepartmentofAstronomy,ColumbiaUniversity,550W120thSt,NewYork,NY10025,USA
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Abstract.
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Wereportonmulti-layerhighefficiencyanti-reflectioncoating(ARC)designanddevelopmentforuseatUVwavelengthsonCCDsandother
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Si-baseddetectors.Wehavepreviouslydemonstratedasetofsingle-layercoatingswhichachieve>50%quantumefficiency(QE)in4bandsfrom
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130to300nm. Wenowpresentmulti-layercoatingdesignsthatsignificantlyoutperformourpreviousworkbetween195to215nm. Usingupto

n 11layers,wepresentseveralmodeldesignstoreachQEabove80%.Wealsodemonstratethesuccessfulperformanceof5and11layerARCson
a siliconandfusedsilicasubstrates. Finally,wepresenta5layercoatingdepositedontoathinned,delta-dopedCCDanddemonstrateexternalQE
J greaterthan60%between202and208nm,withapeakof67.6%at206nm.

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Keywords:UV,Anti-reflectioncoatings,thin-films,CCDs.
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Addressallcorrespondenceto:ErikaT.Hamden,CaliforniaInstituteofTechnology,1200EastCaliforniaBlvd,MC278-17,Pasadena,CA,USA,

] 91125;Tel:[phone];E-mail: [email protected]
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.
h 1 Introduction
p
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o Since their invention at Bell Labs in 1969, charged coupled devices (CCDs) have become ubiquitous in astronomy.
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t
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These silicon-based imagers have had great success in both ground and space-based applications at wavelengths as
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varied as the X-ray, visible, and near IR. Until recently, CCDs have not been effective in the UV (100-350 nm),
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v
3 with low to zero quantum efficiency (QE). This low efficiency is due to both the very shallow absorption depth of
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7 UV photons and the absorptive nature of the poly-silicon front side circuitry. A combination of thinning, backside
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illumination, and a process called delta doping can bring the QE of CCDs up to the reflection limit of silicon at all
.
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wavelengthsinastableandconsistentway;1–3intheUVthisbringstheQEtoaround30%between150and300nm.
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: NASA’s Jet Propulsion Laboratory (JPL) has had great success in the development of delta-doping technology.
v
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X This technique passivates the back surface of a thinned silicon CCD, complementary metal-oxide semiconductor
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(CMOS)imagers,orp-type/intrinsic/n-type(PIN)diodearrayandresultsinnearly100%internalQEfromtheextreme
UV to the IR.1 The shallow absorption depth of UV photons in silicon (< 10 nm) requires the use of both back-
illuminated and thinned CCDs, to avoid both absorption in the front-side circuitry and recombination loses in the
backside surface layer. Without some type of passivation, such as delta doping, traps will form in the surface layer
betweenSi-SiO andinteractwiththeshallowlygeneratedcharges. ThisinteractioncausespoorQE,hysteresis,and
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highdarkcurrent.4 Theinsertionofasingleatomiclayerofheavilydopedsilicon, depositedusingmolecularbeam
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epitaxy (MBE) and capped with pure silicon, passivates the surface layer. This ‘delta-doped’ layer eliminates traps
and allows for stable and efficient charge collection within the CCD.2 Delta doping provides a superior passivation
to other methods that can be destructive to the silicon crystal or have only short-lived effects. While delta doping
modifiestheinternalelectricfieldofthedevice,thesurfaceitselfremainsassiliconcoveredbyitsnativeoxide. The
QEofadelta-dopeddeviceislimitedbyreflectionfromthesiliconsurface. Lossesduetoreflectioncanbereduced
withtheapplicationofasuitableanti-reflectioncoating(ARC).
Single layer ARCs are often used to increase transmission over wide band passes in the visible (e.g. HfO for
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increased blue sensitivity5). We have previously shown >50% QE at UV wavelengths (100-300 nm) using single
layerARcoateddelta-dopeddetectors,6,7anunprecedentedQEperformanceforanyUVdetector,especiallybetween
100 and 250 nm. These high-quality ARCs were made possible with atomic layer deposition (ALD, see Ref. 8 for
anexcellentintroductiontoALDandit’sapplications). WorkbyRef.9usingMBEtopassivatethebacksurfaceofa
thinnedCCDhasyieldedhighQE(∼50%at100nm),buttestingfocusedprimiarlyontheextremeUV(<110nm)
anddidnotincludeARCs. Morestandard, commerciallyavailableblueenhancingcoatingsonbacksideilluminated
CCDsareabletoachievehighQEsabove250nm.
For higher transmission, quarter wave stacks and other configurations will minimize reflectance to nearly zero
over a limited range of wavelengths and have been well explored in the visible.10 These multi-layer techniques can
be extended to UV wavelengths, but are constrained by both the limited range of UV-transmissive materials and by
thechangingindexofrefractionofsilicon(varyingfrom0.5to5.5between100nmand350nm). Siliconitselfisan
absorbingsubstratewithahighcomplexrefractiveindexand,therefore,theARCrequirementsaredifferentthanfora
moreconventionaltransmissivecoating. Furthercomplicatingmattersarethetrade-offsbetweenachievingahighQE
throughtheuseofmulti-layers, narrowingofthebandpass, andthepotentialforincreasedabsorptionintheUVas
theoverallcoatingthicknessgrows.
Forthiswork, wefocusondesigningmulti-layercoatingsforuseintheUVandoptimizedoveranarrowwave-
lengthrange(195-215nm). This20nmbandisadvantageousfromamaterialsstandpoint,astheindexofrefractionof
siliconisrelativelyunchangingoverthisnarrowrange. Furthermore,thenatureofthemulti-layerARCmakeswider
bandpassesmoredifficulttodesign. AnumberofmaterialscanbedepositedviaALDwithlimitedabsorptioninthis
bandpass,providingseveraloptionsforgoodARCdesigns. Mostimportantly,thiswavelengthrangeoffersparticular
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advantagessinceitcorrespondstoanatmosphericwindowbetweenO andO absorptionbands,11 permittingobser-
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vationsintheUVatballoonandrocketaltitudes. Theidealtestbedforathinned,delta-dopedandARcoatedCCDis
theFaintIntergalacticRedshiftedExperimentBalloon(FIREBall,Ref.12)experiment,settolaunchin2016,which
willmeasureemissioninthe200-210nmrangeandbenefitsfromhighQE,UV-optimizedARC.
The rest of this paper is organized as follows: in Section 2, we describe techniques for model creation and the
modelselectedfordeposition. Section3detailsdepositionrecipesandtechniques, alongwithgrowthratesforeach
material.Section4presentsthereflectanceandtransmissionmeasurementsofthecoatingsoninertsubstrates.Section
5 covers deposition, testing and results of a coating deposited on a delta-doped CCD. In Section 6, we discuss the
implicationsofourresultsandprospectsforfuturework.
2 ModelDevelopment
WehavedevelopedasuiteofpossiblecoatingsdesignedtomaximizeQEwithintheFIREBallband.ARCperformance
was modeled using TFcalc(cid:13)R(Software Spectra, Inc.); materials under initial consideration included MgO, MgF2,
Al O , and SiO . HfO , often used for visible ARCs, was rejected due strong absorption below 220 nm. ARC
2 3 2 2
performancewasmodeledbasedonopticalconstantsfromRef.13,14,aswellasvacuumellipsometrymeasurements
performedinhousebyJ.A.Woollam(VUV-VASE)usingsamplespreparedatJPL.Thesubstratewasmodeledusing
the measured indices for delta-doped silicon, which differs slightly from that of un-doped silicon (see Appendix).
Figure1showsthedifferenceinreflectionbetweenbulksiliconanddelta-dopedsilicon;whilethedifferenceisslight
atmostwavelengths, thereflectancecanchangebyseveralpercentatUVwavelengths. Ourmodelsalsoincludeda
thin(2nm)nativeoxideatthesubstrateinterface.
OurARCdesignsaresummarizedinTable1andplottedversuswavelengthinFigure3;wepresentpeakQE,the
correspondingwavelength,materialsused,andthewidthofbandat50%QE.Forsimplicity,initialARCdesignswere
restricted to three layers and only two materials; this provided a basis for further optimization with additional layer
pairs.Fourofthesixstarterdesignsyieldedpotentialtransmissionabove80%at205nm.Ofthese,3Cwasselectedas
thebasisofmulti-layercoatingswithmorethanthreelayers,usingSiO andAl O inlargepartbecausetheirgrowth
2 2 3
isstraightforwardandrepeatableusingALD.Table1alsoincludesthedetailsfortwomorecomplexcoatings,using
five(5A)andeleven(11A)layers. 5Aissimilarto3C,butwithahigherpeakandnarrowerwidth. 11Ahastwoclose
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peakswithlowertransmittancethanthesinglepeakofcoating5A,buthighertransmittanceattheedgesoftheband
(above70%between195-215nm),asshowninFigure2.
More complex ARC designs with three or more materials may offer higher QE or broader peaks than those dis-
cussedinthispaper;however,thefeasibilityofmorecomplexmodelsislimitedbyphysicalconstraints. Forexample,
Al O beginsabsorbingat∼240nm,dependingonfilmpreparation,andisstronglyabsorbingbelow180nm;thiswill
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impacttransmissionasthenumberandthicknessoftheAl O layersincrease. Othermaterials,suchasHfO ,MgO,
2 3 2
andTiO havewellknownabsorptioncutoffsnearUVwavelengths,andsoarenotasusefuloveralargewavelength
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range.Evenso,weshowherethatthetransmittancepeakcanbeshiftedtonearlyanywavelengthabovetheabsorption
cutoffbymodifyingthefilmthicknessesorthematerialsused. FurtherdiscussionofthiscutoffforarangeofUVfilm
materialscanbefoundinRef.6.
3 DepositionTechniques
ALDenablesconformal,atomiclevelcontrolovercoatinggrowthandallowswell-controlled,repeatabledepositions.
Spatial uniformity of ALD films can be affected by a variety of parameters, including system pressure, pulse and
purge times, substrate temperature and surface preparation (i.e., cleanliness). For example, incomplete purging or
non-uniformheatingmayleadtoparasiticCVD-likegrowth, resultinginnon-uniformcoatings. Thegrowthrecipes
used in in this work showed repeatable ALD growth behavior, and film quality was verified prior to deposition on
live devices. Initial ALD recipes for Al O came from Ref. 15, MgO from a custom reaction developed at JPL by
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theauthors, SiO fromRef.16andHfO fromRef.17. Growthrecipesrequiredmodificationstoaccommodatethe
2 2
equipment used. Film composition and uniformity were validated using X-ray photo-electron spectroscopy (XPS),
transmissionelectronmicroscopy(TEM),secondaryionmassspectrometry(SIMS),spectroscopicellipsometry,ora
combinationofthesetechniques. Growthswereconductedat200◦C. AdditionaldetailsregardingtheALDgrowth
parameters can be found in Ref. 18. A Beneq TFS200 ALD system at JPL was used for all depositions. Our work
withALDspansseveralyearsandsystems.
Coatings were deposited on 1 inch <100> 1-20 ohm-cm silicon wafers (to test reflectance) and on 3-mm thick
fused silica window with λ/10 peak-valley flatness at 635 nm (to test transmission/absorption). For simplicity, all
measurements(andcorrespondingmodels)plottedinthefollowingfiguresarebasedonasiliconsubstratewithoutthe
dopedlayer;previousworkondelta-dopedCCDshasshowngoodfidelitybetweenbothmodelpredictionsandresults
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fromsiliconwafersubstrates.6,7 Filmthicknesseswereverifiedbyspectroscopicellipsometry(usingeitheraSentech
SE850orHoribaUVISEL2).
OurpreviousworkmeasuringQE,describedinRef.7andRef.6,consistedalmostentirelyofsingle-layercoating
deposits on silicon substrates. In general, ALD deposition rate calibration is done by measuring the thickness of a
singlelayeronasiliconsubstrateafteracertainnumberofcycles. However,ALDfilmnucleationandgrowthratesfor
agivenmaterialwilloftenvarydependingonthesubstrate. Forthemulti-layercoatingsdiscussedinthispaper,only
the first layer is deposited directly on silicon; subsequent layers were deposited onto other ALD films. As a result,
wefoundthattheSiO layerswithinthestackwereconsistently3-4nmthinnerthandesired. Additionalexperiments
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revealedthatthenucleationrateofSiO onAl O isdifferentthanthatonSi-thatisthenumberofcyclesrequired
2 2 3
to achieve good surface nucleation is greater. Figure 4 shows the measured thickness for growths of ALD-SiO on
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baresiliconandonsiliconwitha7nmlayerofAl O . Inallcases,theSiO grownontheAl O basewasthinner
2 3 2 2 3
for the same number of cycles. The nucleation delay can be seen as an offset in the y-intercept. With good surface
nucleation,thedepositionrateswerenearlyidentical(1.12vs1.15A˚ percycle). Inpractice,severaladditionalcycles
were required when growing SiO on Al O versus silicon or its native oxide. We found no measurable difference
2 2 3
intheindicesofrefraction(nandk)forSiO grownonAl O versussilicon,norforAL O grownonSiO versus
2 2 3 2 3 2
silicon.
4 ReflectanceandTransmittanceTesting
Before applying these ARCs to functional CCDs, we first tested them out on inert substrates for performance veri-
fication. This provides a consistency check for coating performance prior to the more complicated direct QE mea-
surements. Wewantedtoverifythatthecoatingaloneperformedasexpectedwithoutthecomplicatingfactorsofthe
CCDoperation. Oncetheseopticaltestsmatchedthemodelperformance,thenweappliedseveralARCstofunctional
CCDs. AnymajordiscrepanciesbetweentheQEmeasurementsandthemodelarelikelytheresultofdeviceoperation
andnotthecoatingperformance.
Reflectance and transmittance tests were conducted with the same system at Columbia University. The samples
were placed in a vacuum chamber maintained at less than 1 × 10−4 Torr for the duration of the measurement. An
ActonResearchCorpVM-504monochromator,fedbyafocuseddeuteriumlamp,providesilluminationtothesamples.
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Thelightisreflectedfromsiliconsubstratesformeasuringreflectanceorisdirectedthroughtransparentsubstratesfor
measuringtransmission.
Reflectancemeasurementswereperformedat5−10◦fromnormalincidencewithsiliconsubstrates. Werecorded
intensityusingaPMT(R6095)withascintillatorandlightpipeassembly(McPhersonModel658). Foreachsample,
the following set of measurements were made: direct intensity from the lamp, reflected intensity from the sample,
reflectedintensityfromabaresiliconstandard, reflectedambientintensitynotdirectlyinthepathofthelight. This
lastmeasurementisfromanopenfilterpositionandservesasabackgroundmeasurement. Thesemeasurementswere
thenusedtocalculatedirectreflectancefromthesampleandsilicon(asastandard).
Transmissionmeasurementswereperformedatnormalincidencewithfusedsilicasubstrates.Withthesameset-up
asforreflectancemeasurements, werecordedthefollowingmeasurements: directintensityfromthelamp, reflected
intensityfromthesample,directtransmittancethroughthesample,reflectedintensityfromabarefusedsilicawindow,
directtransmittancethroughabarefusedsilicawindow,andreflectedambientintensityasabackgroundmeasurement.
We also re-modeled our selected ARCs with fused silica substrates to provide models of expected transmission and
reflectance,sincethiswilldiffersignificantlyfrommodelswithasiliconsubstrate.
Transmission into the Si is calculated by measuring both reflectance from the ARC with a silicon substrate and
verifying reflectance and transmission from the same ARC applied to a fused silica window. While fused silica is
an incoherent substrate with a non-negligible thickness, we felt that absorption/transmission measurements on an
alternativesubstratecouldprovideamorenuancedviewofthecoatingperformance,despitenotbeingaperfectmatch
tothesiliconsubstrate. Themodelsusedtoverifytheperformanceofthecoatingonfusedsilicatakeintoaccountthe
thicknessofthewindowandusetheexpectedindicesofrefractionfortheindividualcoatinglayers. Measurementsof
reflectanceandtransmissionthroughtheARConfusedsilicamatchthemodelwithgoodfidelity(seebottompanelsof
Figures5and6)andconfirmthatthebehavioroftheARConadifferentsubstrateisasexpectedinbothtransmittance
andreflectance.
Toprovideabetterideaofexpectedtransmissionbasedonourtesting,weusethefusedsilicasubstratemeasure-
mentstoprovideanideaofabsorptionattributabletothecoatingitself. Thisprocessisinlieuofmeasuringabsorption
directly, which can only be done via QE testing. To estimate absorption in the layer, we make several assumptions.
First,weassumethattheabsorptionofamulti-layerARConSiwillbenearlythesameastheARConfusedsilica,
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and thus we can use one to approximate the other. The models show this to be a reasonable assumption. We also
assumethatthereisnotsignificantabsorptionintheARCatourwavelengthofinterestandabove(absorptionshould
belessthan10%above195nm). Wealsotreatthecoatingasasinglebodytosimplifythecalculation. Sincepotential
transmittance is multiplicative through several layers,10 we assume that the potential transmittance of the ARC on
fused silica is equal to the potential transmittance of the ARC times the potential transmittance of the fused silica,
wherepotentialtransmittanceisdefinedas:
T
ψ = j (1)
j (1−R )
j
foralayerj. ThustodeterminetheabsorptionoftheARCalone,wedividethemeasuredabsorptionofthelayer
and window by the measured absorption of only the window. This does not well approximate the absorption of the
layer at very low wavelengths (< 160 nm), where the fused silica is very absorbing. However, since our region of
interest is at 195 nm and longer, we don’t expect this will significantly change our results. For shorter wavelength
targets, a MgF window can be used. Estimated transmission is calculated by subtracting this measured absorption
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andpreviouslymeasuredreflectanceofacoatedsiliconsubstratefrom100%,asshowninEquations2and3.
(1−A )
(1−A )= coating,fusedSi (2)
coating (1−A )
fusedSi
T =1.0−R −A , (3)
calc Si,coating coating
whereA iscalculatedabsorptionduetothemulti-layercoatingonly,A ismeasuredabsorp-
coating coating,fusedSi
tionduetoboththecoatingandthefusedsilicawindow,A ismeasuredabsorptionofanuncoatedfusedsilica
fusedSi
window,T isthecalculatedtransmissionoftheoverallcoating,andR isthemeasuredreflectionoffthecoated
calc Si
silicon substrate. For small absorbances in the ARC, (i.e. low values for A ), Equation 2 is valid given the
coating
assumptionsdescribedaboveandprovidesagoodfitwhencomparedtotheexpectedtransmission.
In this paper we preset results from two coating designs, 5A and 11A. For coating 5A, the overall performance
matched the model with high fidelity, as shown in Figure 5. The minimum reflectance point originally expected
at 205nm was shifted towards lower wavelengths, due to the SiO growth discrepancy described in Section 3. We
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modified the thickness of the components in the initial model to create a version that best matches the measured
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coating performance. We are able to model the new coating with thinner SiO layers, and find that it matches the
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observedreflectanceandexpectedtransmission.
Tofurtherstudytheeffectofincreasedmulti-layerstacks,wealsotestedan11-layercoating(model11A),which
is designed to have a double peak, as shown in the model (Figure 2) and measurements (Figure 6). This coating
waspreparedincorporatingtheratesofSiO growthonanAl O baselayer. ARCdesign11Ahasaslightlylower
2 2 3
theoretical peak QE (84% vs. 90%) but has higher average transmission in the 20 nm band pass. The estimated
transmissionissomewhatofffromthemodel, withabsorptioncalculatedasdescribedaboveforthe5-layercoating.
Thisslightchangeislikelytheresultofafewnanometersdifferenceinthetoporbottomlayerthickness. Changesof
onlyananometerortwocanbeenoughtochangethepeakheightsbyseveralpercentagepoints.
5 CCDARCdepositionandQEmeasurements
Afterourinitialtestsonsiliconsubstrates,weappliedavariationofcoating5A(accountingfortheSiO growthrate
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onAl O asdiscussedearlier),describedinSection4,toathinnedanddelta-dopede2vCCD201-20s.Thepreparation
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anddelta-dopingprocessaredescribedinmoredetailpreviously.3,7,19 Forourwork,deviceswereprocessedatwafer
level,butthesameprocessingcanbeperformedonindividualdie. Wafersarefirstbondedtoacarrierwafertoprovide
supportafterthinning.ThisbondingprocessisperformedbyZiptronix.Bondedwafersarethenthinnedtotheepitaxial
layerusingacustomprocessdevelopedatJPL.Afterthinning,thewafersarecleanedandpreparedfordelta-doping.
Afterdelta-dopingthewafercaneitherbedicedintoindividualdevicesforcustomARCdepositionorasingleARC
canbedepositedontheentirewaferandthendiced. Ineithercase,individualdevicesarethencleaned,inspected,and
packagedfortesting. Operationofthesedevicesfollowsthestandardsrecommendedbythemanufacturer.
Thecharacterizationset-uphasalsobeenpreviouslydescribedindetail.20 Tobrieflysummarize,thisset-upusesa
carefulcalibrationsystemtodirectlymeasureQEfromarangeofdevices. Adeuteriumlampisusedforwavelengths
below 350 nm and a quartz tungsten halogen lamp above 350 nm and into the IR. A set of long pass and band pass
filters minimize leakage from bluer wavelengths in the case of the deuterium lamp, and redder wavelengths for the
quartz tungsten lamp. These filters are sufficient to reduce out of band light to less than 2 percent, and below 0.5
percent at UV wavelengths. The filtered light passes through a vacuum monochromator that has two output slits
selectablebyaflipmirror. Oneslitfeedseitherthecalibratedphotodiode(PD)oracooledCCDforQEtesting. The
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otherslitfeedsasecond”concurrentphotodiode”thattakesfluxmeasurementsinterleavedintimetoaccountforlamp
variability. TheentireslitimageiscapturedbyeachPDorCCDwithoutoverfilling.
To measure QE, the CCD and a fast shutter are used to image the slit at each wavelength, and calibration dark
frames are taken with the same exposure times as the slit images. Interleaved photocurrent measurements I(λ) are
taken with the concurrent PD. The calibrated PD is used to measure photocurrent at the same wavelengths, also
interleavedwiththeconcurrentPD.WedefineR(λ)astheratioofthetwoPDmeasurements,whichonlydependson
thethroughputratioofthe2outputslitsandtheQEratioofthe2PDs. CCDconversiongainK (e−/DigitalNumber,
DN)iscalculatedusingthestandardphoton-transfercurveprocedure.21 Equation4summarizestheQEcalculation,
definedasthetotalCCDsignaline−dividedbytheexpectednumberofphotonsintheexposure:
S(λ)QE (λ)KC
QE (λ)= PD , (4)
CCD I(λ)R(λ) ∆t
whereS(λ)istheintegratedsignalontheCCD(DN),QE (λ)istheQEofthecalibratedPD,Cistheelementary
PD
chargeinCoulombs,and∆tistheexposuretimeoftheCCDimage. KnowingQE ,anobserverusingtheCCD
CCD
inthefieldcanestimateasource’sphotonfluxfromtheCCDsignaline−.
Atwavelengthsbelow400nm,photo-chargescanbeproducedwithenoughkineticenergytosendadditionale−
into the conduction band through collisions. While the QE in Equation 4 above correctly relates the detector signal
to the incident photon flux, obtaining the fraction of incident photons detected requires a correction to account for
thegenerationofmultipleelectron/holepairsperphoton.7 Theaveragenumberofelectron/holepairsgeneratedper
interacting photon is known as quantum yield (QY), and random variation around the average is known as Fano
noise.21–23 TheequationforcorrectingforQYisasfollows:
QE (λ)
QE (λ)= CCD , (5)
corrected QY(λ)
where QE is the QE as calculated using Equation 4, QY is the quantum yield, and QE is the true
CCD corrected
measureofthefractionofinteractingphotons.
Measurements of QY in silicon use one of two methods. The first method, used by Kuschnerus, is to compare
the detector QE to the measured or modeled transmission, assuming that QE accounts for any discrepancy between
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them.24,25 This method provides an upper limit of QY, since it relies on the assumption that the photodiodes have
smallinternallossesandthatthesilicon-oxideinterfacehaslimitedabsorption. Theassumptionregardingabsorption
of the oxide fails below 160 nm, but these results provide reasonable QY for longer wavelengths. An alternative
method21,26 usesaphotontransfercurve(PTC)takenatrelevantwavelengths. Thismethodexploitsthefactthatthe
meanandvarianceofthedetectorsignalscaleasQYandQY2 respectively,violatingthebehaviorofthephotonshot
noise,whichisPoisson-distributed. PTCmeasurementsofQEareaccurateatveryhighenergies(e.g. X-ray)where
thee/hpairsgeneratedperphotonarestronglypeakedaroundameanvalue,i.e. Fanonoiseisrelativelylow. Inthe
UV,Fanonoisecontributessignificantlytothesignalvariance,whichbiasestheQYmeasurement(seeequation30in
McCullough23). Quantumyieldscalculatedinthiswayarecanbewellbelowthosecalculatedusingthereflectance
method(1.1electrons/photonvs. 1.3electrons/photonat200nm,forexample)andcanchangetheapparentQEofa
detectorsignificantly,asseeninFigure7.
For the purposes of this paper, we use the values derived by Kuschnerus24 as a conservative measure of QE.
KuschnerusreportsthemeanenergyWtoproduceanelectron-holepairinsiliconinFigure6oftheirpaper. Wethen
estimateQYas:
E hc
QY = photon = (6)
W λW
These QY values are higher than those measured using a PTC, and provide a lower limit of the possible QE
measured. ToprovideanideaoftherangeinQEvalues,wealsoincludeQYvaluesmeasuredusingthePTCmethod
atJPL(greenlineinFigure7), butwerestrictourdiscussiontoQEderivedwiththeKuschnerusvalues. Adetailed
studyofQYintheUV,variationsbetweendetectors,andtheexactcontributionofFanonoiseisneededtoprovidea
moreexactvalueforquantumyieldandisbeyondthescopeofthispaper.
The measured QE is compared to the expected QE based on the transmission model in Figure 7. The peak is
locatedat206nm,withamaximumQEof67.6%atthiswavelength. QEisabove50%acrosstheFIREBallbandpass
(200-210nm)andabove60%between202and208nm. Structureinthemodel,includingthewidthofthepeakat205
nmandtheincreasingQEatlongerwavelengths,areclearlypresentinthedata.
Previoustestsofdelta-dopeddeviceshaveshownnodeteriorationwithtime.2 Thecoatinganddevicepresented
here have been tested several times since the initial QE test presented here. We found no significant degradation in
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