Influence of Cation Choice on magnetic behavior of III-N Dilute Magnetic semiconductors

2005·3.1 MB·English

by  Frazier, Rachel Marian

INFLUENCEOFCATIONCHOICEONMAGNETICBEHAVIOROFIILNDILUTE
MAGNETICSEMICONDUCTORS
By
RACHELMARIANFRAZIER
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
2005
ACKNOWLEDGMENTS
Manypeoplehelpedsupporttheresearchpresentedinthisdissertation,toomanyto
listindividually.Iappreciateallthateachpersoncontributed.Inparticular,Ithankeach
memberofmyresearchgroup,especiallythosewhotaughtmethebasics,requirements
forsuccessinmyresearch.Ialsothankeachmemberofmycommitteefortheinspiring
thoughtsandinsightfulconversations.Ihavemetmanyandlearnedmuchduringmy
graduatecareeratUF.
11
TABLEOFCONTENTS
page
ACKNOWLEDGMENTS
ii
LISTOFTABLES v
LISTOFFIGURES vi
CHAPTER
1 INTRODUCTIONTODILUTEMAGNETICSEMICONDUCTORS 1
2 THEORETICALANDEXPERIMENTALBACKGROUNDAND
INVESTIGATIONS 4
ExperimentalProgressinDMS 4
FocusonHI-As 5
ExtensiontoIII-Ns 5
TheoreticalBackground 7
FreeCarrierMediatedModelofFerromagnetisminDMS 7
PercolationPictureofFerromagnetisminDMS 8
3 SURVEYOFPOTENTIALDILUTEMAGNETICSEMICONDUCTORSWITH
IONIMPLANTATION 10
IonImplantationMethodDescription 10
SurveyofIonImplantedA1N 11
ExperimentalCharacterizationandResultsofIonImplantation 12
4 THINFILMALLUMINUMNITRIDEDILUTEMAGNETIC
SEMICONDUCTORGROWTHANDOPTIMIZATIONOFMAGNETIC
PROPERTIES 22
GrowthbyMolecularBeamEpitaxy 22
GrowthofThinFilmAlMnN 24
CrystallinityandPhaseStructure 25
MagneticProperties 26
MagneticMechanism 27
GrowthofThinFilmAlCrN 28
CrystallinityandPhaseStructure 29
in
RoleofV/IIIonGrowthandMagneticProperties 30
EffectofDopantonFerromagnetism 30
5 THERMALSTABILITYINVESTIGATION 42
6 ALNANDGANBASEDDILUTEMAGNETICSEMICONDUCTOR
APPLICATIONS 50
SpinFilterApplicationsofA1NDMS 50
DeviceStructureandDesign 50
DeviceTestingandResults 52
7 INVESTIGATIONOFGADOLINIUMASAMAGNETICDOPANT 64
IntroductionofaNewMagneticDopant 64
GrowthofGaN:Gd 64
MagneticPropertiesofGaN:Gd 65
ThermalStabilityInvestigationofGaN:Gd 67
8 CONCLUSION 82
AIN-basedDMSSurvey 82
ImpurityComparison 83
ThermalInstability 84
DeviceApplications 84
SummaryandFutureWork 85
APPENDIX
SUPERCONDUCTINGQUANTUMINTERFERENCEDEVICE
MAGNETOMETRY 86
SampleMeasurementMethod 87
BackgroundSubtraction 88
LISTOFREFERENCES 89
BIOGRAPHICALSKETCH 92
IV
TABLE
Table page
4.1. TablefeaturingthelatticeconstantandelectricalpropertiesofAlMnNand
AlCrNfilms 32
LISTOFFIGURES
Figure page
3.1 Powderx-raydiffractionscanofA1NimplantedwithMn.Noadditionalphases
areobservedascomparedtoundopedA1N 15
3.2 Powderx-raydiffractionscanofA1NimplantedwithCr.Additionalphasesas
comparedtotheA1Nareshowninthefigure 16
3.3 Powderx-raydiffractionscanofA1NimplantedwithCo.Additionalphasesas
comparedtotheA1Nareshowninthefigure 17
3.4 MagnetizationversusappliedfieldtraceofMn+-implantedA1Natatemperature
of100K 18
3.5 MagnetizationversusappliedfieldtraceofCr+-implantedA1Natatemperature
of300K 19
3.6 MagnetizationversusappliedfieldtraceofC0+-implantedA1Natatemperature
of300K 20
3.7 MagnetizationasafunctionoftemperatureforCo+-implantedA1N.Amagnetic
fieldof500Oewasappliedduringthemeasurement 21
4.1 Estimatedsaturationmagnetizationandestimatedremanentmagnetizationof
AlMnN.ThefilmwasgrownwithMncelltemperatureof650°Candundera
nitrogenflowof1.3seem 33
4.2 MagnetizationversusappliedmagneticfieldforAlMnNattemperaturesof10K
and300K.ThefilmwasgrownwithaMncelltemperatureof650°Candundera
nitrogenflowof1.3seem 34
4.3 Magnetizationvs.appliedfieldoftheundopedA1Nshowingparamagnetic
behavior 35
4.4 MagnetizationversustemperatureforfilmsofeitherundopedA1N,singlephase
AlMnN,orM114N 36
4.5 ReflectionhighenergyelectrondiffractionphotoofAlCrNfilmgrownataCr
celltemperatureof992°C.Thephotodepictsa2D/3Dpatternwithonebythree
reconstruction 37
VI
4.6 Atomicforcemicroscopyimage(1urnby1um)ofAlCrNwithanrmsroughness
valueofapproximately11nm 38
4.7 MagnetizationversusnitrogenflowforAlCrNfilmsgrownatasubstrate
temperatureof780°CandwithaCrcelltemperatureof987°C 39
4.8 Magnetizationvs.appliedfieldfortheoptimalAlCrNandAlMnNfilms 40
4.9 Magnetizationvs.temperatureinthetemperaturerangefrom10K-50Kfor
AlMnNandAlCrN.Adifferenceincurvatureisseenforthetwomaterials 41
5.1 EstimatedsaturationmagnetizationforAlCrNateachannealtemperature 46
5.2 Magnetizationversusappliedfieldmeasurementscomparingtheas-grown
AlCrNtothepost-annealAlCrN.Verylittlemagnetizationisleftafterananneal
oftheAlCrNat700°C 47
5.3 Magnetizationversustemperaturetakenunderanappliedfieldof250Oe
comparingasgrownAlCrNtopost-annealAlCrN 48
5.4 Powderx-raydiffractionscanscomparingasgrownAlCrNtopost-annealed
AlCrN.Nosecondphasesareapparentafterthe700°Canneal 49
6.1 Schematicoftheallsemiconductortunnelingmagneto-resistancestack.The
referencestackcontainedundopedA1NinplaceoftheAlMnNlayer,andall
thicknessedremainedthesame.Thedarksquaresrepresentohmiccontactsmade
tothetopGaN:SilayerandtotheunderlyingMOCVDGaNbuffer 55
6.3 Maskdesignusedforfabricationofallsemiconductordevice.Alignmentmarks
arefoundateachcornerandinthemiddleofthemask.Thelargerdarkbars
representtheareaofthemesaofthedevice.Theopen,orlightareas,represent
whereohmiccontactwasmadetothedevice 56
6.4 Scanningelectronmicrographofallsemiconductordevice.Topviewshowstop
andbottomTi/Auohmiccontact,thetopofthemesaandtheetchedvalley
showingtheMOCVDGaNbuffer 57
6.5 Current-voltagemeasurementofallsemiconductortunnelingmagneto-resistance
devicewithandwithoutandappliedmagneticfield.OpencirclesrepresenttheI-
Vmeasurementtakenwithoutanappliedfield.DarksquaresrepresentI-V
measurementtakenafterapplicationofa4000Oefield.Notethattunneling
increasesafterthefieldisapplied 58
6.6 Resistancevs.appliedfieldmeasurementtakenat5Kfortheallsemiconductor
referencedevice 59
6.7 Resistancevs.appliedfieldtakenat300Kfortheallsemiconductortunneling
magneto-resistancedevice 60
vn
1
6.8 DarkfieldZSTEMimagetakenoftheallsemiconductortunnelingmagneto-
resistancedevice.ThedarkAlMnNlayershowsroughnessindicatingpoor
growthqualityattheinterface 61
6.9 Selectedareadiffractionpatterntunnelingelectronmicrographtakenoftheall
semiconductortunnelingmagneto-resistancedevice.TheAlMnNspinfilter
layerisindicatedwithanarrow.StraincanbeseenwithintheAlMnNlayeras
indicatedinthephoto 62
6.10 Scanningelectronmicrographofatunnelingmagneto-resistancedevicewith
FeNiasaspininjector.Fromthephoto,thedegradationoftheFeNiisvisibleon
thecontactpad.Thisdegradationrepresentsnearly90%ofthedevices.The
FeNidegradedduringphotolithography,mostlikelyduetotheuseofsolvents
duringprocessing 63
7.1 ReflectionhighenergyelectrondiffractionpatternofGaN:Gd,Tod=1050°C.
Thepictureshowsa2D/3Dpatternwith1x3reconstruction 70
7.2 AtomicforcemicroscopyimagerepresentingGaN:Gdwithrmsroughnessof
1.541ran 71
7.3 Magnetizationvs.appliedfieldlooptakenat50KforGaN:Gd,withTea=
950°C.Hysteresisisobservedat50K,butnotnecessarilyat300Kor350K 72
7.4 Magnetizationvs.appliedfieldtakenat50KforGaN:GdcorrespondingtoTed=
1000°C-1100°C 73
7.5 Estimatedsaturationmagnetizationvs.inverseofGdcelltemperatureplot
whichshowstheoptimalTed 74
7.6 Magnetizationvs.appliedfieldloopstakenat50K,300K,and350Kfor
GaN:GdwithTGd=1050°C 75
7.7 Magnetizationvs.temperatureforGaN:GdwithTGd=1050°C.Notethatthe
magnetizationissignificantlydecreasednearroomtemperature 76
7.8 Estimatedsaturationmagnetizationvs.annealtemperatureforGaN:GdwithTod
=1050°C 77
7.9 Magnetizationvs.appliedfieldcomparingtheasgrownGaN:Gdtothat
annealedat600°C 78
7.10 Magnetizationvs.temperatureofGaN:Gdafter700°Canneal 79
7.1 Magnetizationvs.appliedfieldat50Kand350KforGaN:Gdannealedat
700°C 80
7.12 Magnetizationvs.appliedfieldat300KforGaN:Gdannealedat700°C 81
vni
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegreeofDoctorofPhilosophy
INFLUENCEOFCATIONCHOICEONMAGNETICBEHAVIOROFIII-NDILUTE
MAGNETICSEMICONDUCTORS
By
RachelMarianFrazier
August2005
Chair: CammyAbernathy
MajorDepartment: MaterialsScienceandEngineering
Withtheincreasinginterestinspintronics,manyattemptshavebeenmadeat
incorporatingspin-basedfunctionalityintoexistingsemiconductortechnology.One
approach,utilizingdilutemagneticsemiconductors(DMS)formedviaintroductionof
transitionmetalionsintoIll-Nitridehosts,wouldallowforintegrationofspinbased
phenomenaintocurrentwidebandgapdevicetechnology.
Toaccomplishsuchdevicestructures,itisnecessarytoachievesinglephase
transitionmetaldopedGaNandA1Nwhichexhibitroomtemperaturemagneticbehavior.
Ionimplantationisaneffectivesurveymethodforintroductionofvarioustransition
metalsintoA1N.InionimplantedA1N,theCoandCrdopedfilmsshowedhysteresisat
300KwhiletheMndopedmaterialdidnot. However,itisnotatechniquewhichwill
allowforthedevelopmentofadvancedspinbaseddevices.Suchdeviceswillrequire
epitaxialmethodsofthesortcurrentlyusedforsynthesisofIll-Nitrideoptoelectronics.
IX
Onesuchtechnique,GasSourceMolecularBeamEpitaxy(GSMBE),hasbeen
usedtosynthesizeA1NfilmsdopedwithCrandMn.Roomtemperatureferromagnetism
hasbeenobservedforAlMnNandAlCrNgrownbyGSMBE.Inbothcases,themagnetic
signalwasfoundtodependonthefluxofthedopant.ThemagnetizationoftheAlCrN
wasfoundtobeanorderofmagnitudegreaterthanintheAlMnN.Thetemperature
dependentmagneticbehaviorofAlCrNwasalsosuperiortoAlMnN;however,the
AlCrNwasnotresistanttothermaldegradation.
Anall-semiconductortunnelingmagnetoresistivedevice(TMR)wasgrownwith
GaMnNasaspininjectorandAlMnNasaspinfilter.Theresistanceofthedeviceshould
changewithappliedmagneticfielddependingonthemagnetizationoftheinjectorand
filter.However,duetotheimpuritybandsfoundintheAlMnN,theresistancewasfound
tochangeverylittlewithmagneticfield.
ToovercomesuchobstaclesasfoundinthetransitionmetaldopedA1N,another
dopantmustbeused.OneviabledopantisGd,whichduetothelowconcentration
incorporatedinthesemiconductormatrixshouldprovideasingleimpuritylevelwithin
theDMSinsteadofanimpurityband.TheincorporationofGdintoGaNandA1Nmaybe
theultimatedopantfortheseIII-NbasedDMS.