enhanceddirectspace
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DevelopmentofEpitaxialAlN/GaN/GaInNPhotocathodeHeterostructuresforUV/BlueScintillationandCherenkovRadiationDetectionD.
J.
Leopolda,b,J.
Buckleya,W.
R.
Binnsa,P.
Hinka,andM.
H.
Israela(a)DepartmentofPhysicsandMcDonnellCenterfortheSpaceSciencesWashingtonUniversity,St.
Louis,MO63130(b)CenterforMolecularElectronics,UniversityofMissouri,St.
Louis,MO63121ABSTRACTAneedforextendingthesensitivityofphotondetectorstotheblueandUVwavebandscomesfromthefactthatbothCherenkovlightandscintillationlighttypicallyhaveanemissionspectrumthatispeakedatshortwavelengths.
Photocathodelayersconsistingofwide-band-gapAlN/GaN/GaInNheterostructuresarebeingcompositionallytailoredonanatomicscaleduringtheepitaxialcrystalgrowthprocesstocontrolphotoelectronabsorption,diffusion,andtransporttothenegativeelectronaffinitycathodesurface.
Thesenitridealloyheterostructurelayersarebeinggrowndirectlyontransparentsinglecrystalsapphiresubstratesinultra-highvacuumbymolecularbeamepitaxy.
ThistechnologyisexpectedtosignificantlyenhanceUV/blueradiationdetectionsensitivityinsinglephotoncountingimagingandnon-imagingdevices.
Directspace-basedimagingofrapidUV/blueastrophysicaltransientswillgreatlybenefitfromphotoncountingdetectorswithhighquantumefficiency.
AlsoX-ray,Gamma-ray,andcosmic-rayexperimentsthatemployscintillationlightdetectorsorCherenkovdetectorswouldbenefitgreatlyfromphotomultiplierswithhigherquantumefficiencies.
InthiscasetheneedforincreasingthesensitivityofphotondetectorsintheblueandUVwavebandscomesfromthefactthatbothCherenkovlightandscintillationlighthaveanemissionspectrumthatispeakedatshortwavelengths.
Photomultipliertubesinusetodayforhigh-energyparticledetectionapplicationshaveasignificantspectralmismatchwithtypicalsources.
ThispointisdemonstratedinFig.
1,wherewehavedisplayedatypicalCherenkovlightspectrumaswellastheemissionfromaplasticscintillatortogetherwiththedetectionsensitivitycurveofabialkaliphotomultipliertubeandatubeincorporatingastate-of-the-artGaAsPphotocathode.
TheshortwavelengthresponseoftheGaAsPtubeshowninFig.
1hasbeenenhancedthroughtheuseofawavelengthshiftercoatingonthewindow[1].
Photomultipliertubesarehigh-gainopticaldetectorscapableofphotondetectionoverawidespectralrange.
Theheartofsuchadeviceisthephotocathode.
Bythephotoelectriceffect,aphotonincidentonthephotocathodeejectsanelectronwhichthenisacceleratedbyanelectricalpotentialtoproduceanumberofsecondaryelectronseitherthroughacascadeofinteractionsinadynodechainorthroughadirectinteractionwithaphotodiode.
Thesesecondaryelectronsconstitutethemeasuredsignal.
Thequantumefficiencyofsuchadeviceisdefinedintermsofanopticaltoelectricalconversionpercentage,determinedbythenumberdensityofinitialphotoelectronsproducedperincidentphotonflux.
Figure1.
OurresearchonhighquantumefficiencyphotocathodesinvolvesthedesignandfabricationofpreciselytailoredheteroepitaxialsemiconductorstructuresthathavepeaksensitivityintheUV/bluespectralrange.
Afterconsideringtheoptoelecronicandstructuralpropertiesofdifferentwide-band-gapsemiconductoralloymaterialswehavedeterminedAlN/GaN/GaInNtobethemostsuitablecandidate.
Thebandgapofthissystemcanbetailoredoveranenergyrangefrom1.
9to6.
2eVandepitaxialthinfilmlayerscanbegrowndirectlyonopticallytransparentsapphiresubstrates[2].
GaAlN/GaInNhasalreadybeenrecognizedasaleadingmaterialinthefabricationofwide-band-gaplaserdiodedevices,makingitalogicalchoiceforheteroepitaxialphotocathodedevelopment.
TheAlN/GaN/GaInNheterostructuresdiscussedinthisworkNewPage1file:///C|/Nadia/space_detectors/leopold1.
htm(1of3)[8/21/20015:49:49PM]havebeenfabricatedinultra-highvacuumbymolecularbeamepitaxy(MBE).
TheuseofMBEforcrystalgrowthmakesitpossibletocontrolfilmcompositiononanatomicscaleandtofabricateabruptheteroepitaxialinterfaces.
Theultra-highvacuumconditionsandcryogenicallycooledchamberwallsallowforveryquickon/offswitchingofatomicandmolecularbeamsthroughtheuseofshuttersinfrontofeachthermalorelectronbeamsource.
Areflectionhighenergyelectrondiffraction(RHEED)systemmountedinsidethevacuumchamberallowsthesurfacecrystalqualitytobemonitoredandindividualatomiclayerstobecountedduringgrowthastheyareaddedtothesurfaceoneatatimebyexaminingsurfacereconstructiondiffractionpatterns.
Thisfeedbackprovidestheabsolutefinestcontroloftheheteroepitaxialcrystalgrowthprocess,therebymakingpossibleprecisefabricationofsemiconductorlayeredstructuresforuseinhighperformancedevicesAllAlN/GaN/GaInNheterostructuresusedinthesestudiesaregrownonsingle-crystalsapphiresubstrates.
ThemechanicalstrengthandUV/visibleopticaltransparencypropertiesofsapphiremakeitanexcellentchoiceasawindowmaterialforphotocathodestructures.
InordertoincreasethequantumefficiencyofAlN/GaN/GaInNphotocathodesacoupleofkeydesignfeaturesareincorporatedinourheteroepitaxiallayers.
ExamplesofthisareshowninFig.
2,whereconductionandvalencebandedgeenergyspatialprofilesaredisplayedfortwopossiblephotocathodedesigns.
Thisdiagramisbasicallyaplotofbandgapversusdepthintothephotocathodestructure.
ForclaritytheindividuallayerthicknessesshowninFig.
2arenotdrawntoscalebutinsteadshouldberegardedasaroughschematictoillustratethemaindesignfeatures.
AnAlNopticalantireflectionlayerinsertedbetweenthesapphireandtheGaInNphotocathoderegionservesasawide-band-gapbarriertopreventelectronicbackdiffusionintothesubstrateinterfacialregionwheredefectdensitiesareexpectedtobehigherandnonradiativerecombinationofphotoexcitedelectronslarger.
InsertingthiswidebandgapAlNbufferlayerinthestructureensuresthatphotoexcitedelectronsdonotdiffusebacktowardthesapphiresubstrateinterface,butinsteadarereflectedtowardthephotocathodeemissionsurface.
Figure2.
Itisknownthatanelectricfieldappliedinsideasemiconductorphotocathodelayerdriveselectronstowardtheemittingsurfaceandinsodoingcanincreasethequantumefficiencybyasmuchasafactoroftwo[3].
AsshownatthetopofFig.
2weplantocreatetheseinternalfieldsinthephotocathodelayerbygradingthealloycomposition,whichtiltstheconductionandvalencebandedgesEcandEv.
AstheInconcentrationinthelayerisincreasedtheenergygapbetweenvalenceandconductionbanddecreases,resultinginaslopingofthebandedges.
Althoughthefractionalamountofchangeintheconductionandvalencebandsaredifferent,theoveralleffectistotilttheconductionbandsincethep-typedopantincorporatedthroughoutthelayerallowsmobileholechargecarrierstodiffuseinamannerthatminimizestheenergy,leavingthevalencebandprofileEvflat.
Thetiltedconductionbanddrivesphotoexcitedelectronstowardthesurface,increasingtheirescapeprobabilityandthusthequantumefficiency.
Finally,anactivationlayerofCsOonthesurfacebendsthebandstoachieveanegativeelectronaffinity(NEA)condition,whichisvitalforhavingahighphotoelectronescapeprobability.
WearealsoinvestigatingnovelmeansofactivatingthephotocathodeNewPage1file:///C|/Nadia/space_detectors/leopold1.
htm(2of3)[8/21/20015:49:49PM]emittingsurfaceinordertoachievethenegativeelectronaffinitycondition.
SinceAlNandGaAlNwithhighAlconcentrationhaveanintrinsicnegativeelectronaffinitysurfaceweareexaminingthepossibilityofendingtheepitaxiallayerswithaSi-dopedGaAlNlayerinordertoachieveanNEAsurfacewithouttheneedforpost-growthCsOactivation.
ThisdesignisshownatthebottomofFig.
2.
ByincludingthesedesignfeaturesinlayersmadewithwidebandgapAlN/GaN/GaInNsemiconductormaterialsweexpecttoincreasethephotocathodequantumefficiencyresponseintheUV/bluespectralrange.
AtthepresenttimeAlN/GaInNphotocathodeshavebeendesignedandarebeingfabricatedonupto2-inchdiametersapphiresubstratesbyMBE.
Thestructural,optical,andelectronicpropertiesoftheseheterostructuresarebeingevaluated.
X-raydiffractionandTEMlatticeimagestudiesshowgoodregistryofepitaxialGaNandGaInNlayerswiththec-plane-oriented,single-crystalsapphiresubstrates.
OpticalabsorptionmeasurementsofGaNandGaInNconfirmtheexpectedbandgapshiftwithincreasingindiumconcentration.
Also,aphotoelectricemissionmeasurementstageisbeingdesignedforuseintheultra-high-vacuumMBEsystem.
Thisstagewillbeusedtomeasurethequantumefficiencyandspectralresponsivityofas-grownAlN/GaInNphotocathodeheterostructureswithoutbreakingvacuum.
Overallthisnewnitride-basedphotocathodetechnologyisexpectedtohaveasignificantimpactonCherenkovandscintillationradiationdetectionwhereemissionispeakedintheUV/bluespectralrange,andondirectspace-basedimagingofrapidUV/blueastrophysicaltransients.
ThisresearchissupportedbyNASAGrant#NAG5-8536undertheExplorerTechnologyProgram.
1.
S.
M.
Bradburyetal.
,Nucl.
Instr.
andMeth.
A387,45(1997).
2.
S.
Strite,M.
E.
Lin,andH.
Morkoc,ThinSolidFilms231,197(1993).
3.
L.
Guo.
J.
Li,andH.
Xun,Semicond.
Sci.
Technol.
4,498(1989).
NewPage1file:///C|/Nadia/space_detectors/leopold1.
htm(3of3)[8/21/20015:49:49PM]
J.
Leopolda,b,J.
Buckleya,W.
R.
Binnsa,P.
Hinka,andM.
H.
Israela(a)DepartmentofPhysicsandMcDonnellCenterfortheSpaceSciencesWashingtonUniversity,St.
Louis,MO63130(b)CenterforMolecularElectronics,UniversityofMissouri,St.
Louis,MO63121ABSTRACTAneedforextendingthesensitivityofphotondetectorstotheblueandUVwavebandscomesfromthefactthatbothCherenkovlightandscintillationlighttypicallyhaveanemissionspectrumthatispeakedatshortwavelengths.
Photocathodelayersconsistingofwide-band-gapAlN/GaN/GaInNheterostructuresarebeingcompositionallytailoredonanatomicscaleduringtheepitaxialcrystalgrowthprocesstocontrolphotoelectronabsorption,diffusion,andtransporttothenegativeelectronaffinitycathodesurface.
Thesenitridealloyheterostructurelayersarebeinggrowndirectlyontransparentsinglecrystalsapphiresubstratesinultra-highvacuumbymolecularbeamepitaxy.
ThistechnologyisexpectedtosignificantlyenhanceUV/blueradiationdetectionsensitivityinsinglephotoncountingimagingandnon-imagingdevices.
Directspace-basedimagingofrapidUV/blueastrophysicaltransientswillgreatlybenefitfromphotoncountingdetectorswithhighquantumefficiency.
AlsoX-ray,Gamma-ray,andcosmic-rayexperimentsthatemployscintillationlightdetectorsorCherenkovdetectorswouldbenefitgreatlyfromphotomultiplierswithhigherquantumefficiencies.
InthiscasetheneedforincreasingthesensitivityofphotondetectorsintheblueandUVwavebandscomesfromthefactthatbothCherenkovlightandscintillationlighthaveanemissionspectrumthatispeakedatshortwavelengths.
Photomultipliertubesinusetodayforhigh-energyparticledetectionapplicationshaveasignificantspectralmismatchwithtypicalsources.
ThispointisdemonstratedinFig.
1,wherewehavedisplayedatypicalCherenkovlightspectrumaswellastheemissionfromaplasticscintillatortogetherwiththedetectionsensitivitycurveofabialkaliphotomultipliertubeandatubeincorporatingastate-of-the-artGaAsPphotocathode.
TheshortwavelengthresponseoftheGaAsPtubeshowninFig.
1hasbeenenhancedthroughtheuseofawavelengthshiftercoatingonthewindow[1].
Photomultipliertubesarehigh-gainopticaldetectorscapableofphotondetectionoverawidespectralrange.
Theheartofsuchadeviceisthephotocathode.
Bythephotoelectriceffect,aphotonincidentonthephotocathodeejectsanelectronwhichthenisacceleratedbyanelectricalpotentialtoproduceanumberofsecondaryelectronseitherthroughacascadeofinteractionsinadynodechainorthroughadirectinteractionwithaphotodiode.
Thesesecondaryelectronsconstitutethemeasuredsignal.
Thequantumefficiencyofsuchadeviceisdefinedintermsofanopticaltoelectricalconversionpercentage,determinedbythenumberdensityofinitialphotoelectronsproducedperincidentphotonflux.
Figure1.
OurresearchonhighquantumefficiencyphotocathodesinvolvesthedesignandfabricationofpreciselytailoredheteroepitaxialsemiconductorstructuresthathavepeaksensitivityintheUV/bluespectralrange.
Afterconsideringtheoptoelecronicandstructuralpropertiesofdifferentwide-band-gapsemiconductoralloymaterialswehavedeterminedAlN/GaN/GaInNtobethemostsuitablecandidate.
Thebandgapofthissystemcanbetailoredoveranenergyrangefrom1.
9to6.
2eVandepitaxialthinfilmlayerscanbegrowndirectlyonopticallytransparentsapphiresubstrates[2].
GaAlN/GaInNhasalreadybeenrecognizedasaleadingmaterialinthefabricationofwide-band-gaplaserdiodedevices,makingitalogicalchoiceforheteroepitaxialphotocathodedevelopment.
TheAlN/GaN/GaInNheterostructuresdiscussedinthisworkNewPage1file:///C|/Nadia/space_detectors/leopold1.
htm(1of3)[8/21/20015:49:49PM]havebeenfabricatedinultra-highvacuumbymolecularbeamepitaxy(MBE).
TheuseofMBEforcrystalgrowthmakesitpossibletocontrolfilmcompositiononanatomicscaleandtofabricateabruptheteroepitaxialinterfaces.
Theultra-highvacuumconditionsandcryogenicallycooledchamberwallsallowforveryquickon/offswitchingofatomicandmolecularbeamsthroughtheuseofshuttersinfrontofeachthermalorelectronbeamsource.
Areflectionhighenergyelectrondiffraction(RHEED)systemmountedinsidethevacuumchamberallowsthesurfacecrystalqualitytobemonitoredandindividualatomiclayerstobecountedduringgrowthastheyareaddedtothesurfaceoneatatimebyexaminingsurfacereconstructiondiffractionpatterns.
Thisfeedbackprovidestheabsolutefinestcontroloftheheteroepitaxialcrystalgrowthprocess,therebymakingpossibleprecisefabricationofsemiconductorlayeredstructuresforuseinhighperformancedevicesAllAlN/GaN/GaInNheterostructuresusedinthesestudiesaregrownonsingle-crystalsapphiresubstrates.
ThemechanicalstrengthandUV/visibleopticaltransparencypropertiesofsapphiremakeitanexcellentchoiceasawindowmaterialforphotocathodestructures.
InordertoincreasethequantumefficiencyofAlN/GaN/GaInNphotocathodesacoupleofkeydesignfeaturesareincorporatedinourheteroepitaxiallayers.
ExamplesofthisareshowninFig.
2,whereconductionandvalencebandedgeenergyspatialprofilesaredisplayedfortwopossiblephotocathodedesigns.
Thisdiagramisbasicallyaplotofbandgapversusdepthintothephotocathodestructure.
ForclaritytheindividuallayerthicknessesshowninFig.
2arenotdrawntoscalebutinsteadshouldberegardedasaroughschematictoillustratethemaindesignfeatures.
AnAlNopticalantireflectionlayerinsertedbetweenthesapphireandtheGaInNphotocathoderegionservesasawide-band-gapbarriertopreventelectronicbackdiffusionintothesubstrateinterfacialregionwheredefectdensitiesareexpectedtobehigherandnonradiativerecombinationofphotoexcitedelectronslarger.
InsertingthiswidebandgapAlNbufferlayerinthestructureensuresthatphotoexcitedelectronsdonotdiffusebacktowardthesapphiresubstrateinterface,butinsteadarereflectedtowardthephotocathodeemissionsurface.
Figure2.
Itisknownthatanelectricfieldappliedinsideasemiconductorphotocathodelayerdriveselectronstowardtheemittingsurfaceandinsodoingcanincreasethequantumefficiencybyasmuchasafactoroftwo[3].
AsshownatthetopofFig.
2weplantocreatetheseinternalfieldsinthephotocathodelayerbygradingthealloycomposition,whichtiltstheconductionandvalencebandedgesEcandEv.
AstheInconcentrationinthelayerisincreasedtheenergygapbetweenvalenceandconductionbanddecreases,resultinginaslopingofthebandedges.
Althoughthefractionalamountofchangeintheconductionandvalencebandsaredifferent,theoveralleffectistotilttheconductionbandsincethep-typedopantincorporatedthroughoutthelayerallowsmobileholechargecarrierstodiffuseinamannerthatminimizestheenergy,leavingthevalencebandprofileEvflat.
Thetiltedconductionbanddrivesphotoexcitedelectronstowardthesurface,increasingtheirescapeprobabilityandthusthequantumefficiency.
Finally,anactivationlayerofCsOonthesurfacebendsthebandstoachieveanegativeelectronaffinity(NEA)condition,whichisvitalforhavingahighphotoelectronescapeprobability.
WearealsoinvestigatingnovelmeansofactivatingthephotocathodeNewPage1file:///C|/Nadia/space_detectors/leopold1.
htm(2of3)[8/21/20015:49:49PM]emittingsurfaceinordertoachievethenegativeelectronaffinitycondition.
SinceAlNandGaAlNwithhighAlconcentrationhaveanintrinsicnegativeelectronaffinitysurfaceweareexaminingthepossibilityofendingtheepitaxiallayerswithaSi-dopedGaAlNlayerinordertoachieveanNEAsurfacewithouttheneedforpost-growthCsOactivation.
ThisdesignisshownatthebottomofFig.
2.
ByincludingthesedesignfeaturesinlayersmadewithwidebandgapAlN/GaN/GaInNsemiconductormaterialsweexpecttoincreasethephotocathodequantumefficiencyresponseintheUV/bluespectralrange.
AtthepresenttimeAlN/GaInNphotocathodeshavebeendesignedandarebeingfabricatedonupto2-inchdiametersapphiresubstratesbyMBE.
Thestructural,optical,andelectronicpropertiesoftheseheterostructuresarebeingevaluated.
X-raydiffractionandTEMlatticeimagestudiesshowgoodregistryofepitaxialGaNandGaInNlayerswiththec-plane-oriented,single-crystalsapphiresubstrates.
OpticalabsorptionmeasurementsofGaNandGaInNconfirmtheexpectedbandgapshiftwithincreasingindiumconcentration.
Also,aphotoelectricemissionmeasurementstageisbeingdesignedforuseintheultra-high-vacuumMBEsystem.
Thisstagewillbeusedtomeasurethequantumefficiencyandspectralresponsivityofas-grownAlN/GaInNphotocathodeheterostructureswithoutbreakingvacuum.
Overallthisnewnitride-basedphotocathodetechnologyisexpectedtohaveasignificantimpactonCherenkovandscintillationradiationdetectionwhereemissionispeakedintheUV/bluespectralrange,andondirectspace-basedimagingofrapidUV/blueastrophysicaltransients.
ThisresearchissupportedbyNASAGrant#NAG5-8536undertheExplorerTechnologyProgram.
1.
S.
M.
Bradburyetal.
,Nucl.
Instr.
andMeth.
A387,45(1997).
2.
S.
Strite,M.
E.
Lin,andH.
Morkoc,ThinSolidFilms231,197(1993).
3.
L.
Guo.
J.
Li,andH.
Xun,Semicond.
Sci.
Technol.
4,498(1989).
NewPage1file:///C|/Nadia/space_detectors/leopold1.
htm(3of3)[8/21/20015:49:49PM]
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