ZnO based materials hydrothermal synthesis, material properties and a study on hydrogen effect

Z
Z
Z
ZN
N
N
NO-
O-
O-
O-BASED
BASED
BASED
BASEDM
M
M
MATERIALS
ATERIALS
ATERIALS
ATERIALS:
:
:
:H
H
H
HYDROTHERMAL
YDROTHERMAL
YDROTHERMAL
YDROTHERMALS
S
S
SYNTHESIS

YNTHESIS
YNTHESIS
YNTHESIS,
,
,
,
M
M
M
MATERIAL
ATERIAL
ATERIAL
ATERIALP
P
P
PROPERTIES
ROPERTIES
ROPERTIES
ROPERTIES
AND
AND
AND
AND
A
A
A
A
S
S
S

STUDY
TUDY
TUDY
TUDYON
ON
ON
ONH
H
H
HYDROGEN
YDROGEN
YDROGEN
YDROGENE
E
E
EFFECT
FFECT
FFECT
FFECT
L
L
L
LI
I
I
IT
T
T
TONG
ONG

ONG
ONG
(B.
(B.
(B.
(B.E.,
E.,
E.,
E.,T
T
T
TONG
ONG
ONG
ONGJ
J
J
JI
I
I
IU
U
U
UNIVERSITY
NIVERSITY
NIVERSITY
NIVERSITY,
,
,
,C

C
C
CHINA
HINA
HINA
HINA)
)
)
)
A
A
A
A
T
T
T
THESIS
HESIS
HESIS
HESISSUBMITTED
SUBMITTED
SUBMITTED
SUBMITTED
FOR
FOR
FOR
FORTHE
THE
THE
THED

D
D
DEGREE
EGREE
EGREE
EGREEOF
OF
OF
OFD
D
D
DOCTOR
OCTOR
OCTOR
OCTOROF
OF
OF
OFP
P
P
PHILOSOPHY
HILOSOPHY
HILOSOPHY
HILOSOPHY
D
D
D
DEPARTMENT
EPARTMENT
EPARTMENT

EPARTMENTOF
OF
OF
OFM
M
M
MATERIALS
ATERIALS
ATERIALS
ATERIALSS
S
S
SCIENCE
CIENCE
CIENCE
CIENCEAND
AND
AND
ANDE
E
E
ENGINEERING
NGINEERING
NGINEERING
NGINEERING
2012
2012
2012
2012
I

D
D
D
DECLARATION
ECLARATION
ECLARATION
ECLARATION
Iherebydeclarethatthisthesisismyoriginalworkandithasbeenwrittenbyme
initsentirety.
Ihavedulyacknowledgedallthesourcesofinformationwhichhavebeenusedin
thethesis.
Thisthesishasalsonotbeensubmittedforanydegreeinanyuniversity
previously.
LiTong
31/07/2012
II
A
A
A
ACKNOWLEDGEMENTS
CKNOWLEDGEMENTS
CKNOWLEDGEMENTS
CKNOWLEDGEMENTS
FirstandforemostIwouldliketoexpressmysincereappreciationtomy
supervisor,Prof.DingJun,forhisguidanceandencouragementthroughoutmyPhD
study.Hispatience,enthusiasm,creativeideasandimmenseknowledgehelpedmein
allthetimeofresearchworkandwritingofthisthesis.
IalsowouldlikemakeagratefulacknowledgementtoProf.FengYuanpingand
MrOngChinShenforconductingthefirst-principlescalculationsofZnO.Besides,I
wouldliketothankDrHerngTunSengandDrYiJiabao,whohelpedmerevisemy

manuscriptsandgaveinsightfulcomments,fromwhichIbenefitedalot.Inparticular,
IamgratefultoDrFanHaimingforenlighteningmethefirstglanceofmyresearch
work.
Moreover,IgreatlyappreciatethekindhelpfromMsBaoNinaforoperating
pulsedlaserdepositionmachineandconductingSQUIDmeasurement.Iwouldliketo
acknowledgeallmyresearchgroupmembersfortheirkindassistanceinvarious
aspects.
A
specialmentionisgiventothelabofficersinDepartmentofMaterialsScience
andEngineeringfortheirtechnicalsupportinsamplecharacterization.
Inaddition,Iwouldliketooffermydeepgratitudetothefinancialsupport
providedbytheNationalUniversityofSingapore.
Lastbutnotleast,Iwouldlikethanktomyfamily:myparentsforgivingbirthto
meandsupportingmethroughoutmylife;andmyhusband,JinJianfeng,forhis
accompanyingalltheway.
III
L
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OF
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PUBLICATIONS
UBLICATIONS

UBLICATIONS
UBLICATIONS
(1)
T.
T.
T.
T.
Li
Li
Li
Li,H.M.Fan,J.M.Xue,J.Ding,"SynthesisofHighly-texturedZnOFilmson
DifferentSubstratesbyHydrothermalRoute",
ThinSolidFilms
,518,e114(2010).
(2)
T.
T.
T.
T.
Li
Li
Li
Li,H.M.Fan,J.B.Yi,
T.
S.Herng,
Y.
W.
Ma,X.L.Huang,J.M.Xue,J.Ding,
"StructuralandMagneticStudiesofCu-dopedZnOFilmsSynthesizedviaa
HydrothermalRoute",

JournalofMaterialsChemistry
,20,5756(2010).
(3)
T.
T.
T.
T.
Li
Li
Li
Li,C.S.Ong,
T.
S.Herng,J.B.Yi,N.N.Bao,J.M.Xue,
Y.
P.
Feng,J.Ding,
"SurfaceFerromagnetisminHydrogenated-ZnOFilm",
AppliedPhysicsLetters
,
98,152505(2011).
(4)
T.
T.
T.
T.
Li
Li
Li
Li,
T.

S.Herng,H.K.Liang,N.N.Bao,
T.
P.Chen,J.I.Wong,J.M.Xue,J.
Ding,"StrongGreenEmissioninZnOFilmafterH
2
SurfaceTreatment",
Journal
ofPhysicsD:AppliedPhysics
,45,185102(2012).
(5)
T.
T.
T.
T.
Li
Li
Li
Li,
W.
Xiao,
T.
S.Herng,N.N.Bao,J.Ding,"MagneticandOpticalStudiesof
HydrogenatedCu-dopedZnOFilm",
JournalofKoreanPhysicalSociety
,under
review.
(6)
Y.
W.
Ma,X.L.Huang,X.Liu,J.B.Yi,K.C.Leong,LapChan,

T.
T.
T.
T.
Li
Li
Li
Li,N.N.Bao,
J.Ding,"MagneticandTransportPropertiesofn-typeFeDopedIn2O3andZnO
Films",
NanoscienceandNanotechnologyLetters
,4,641(2012).
IV
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ABLE
ABLE
ABLE
ABLEOF
OF
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OFC
C
C
CONTENTS
ONTENTS
ONTENTS
ONTENTS

D
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DECLARATION
ECLARATION
ECLARATION
ECLARATION


I
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A
ACKNOWLEDGEMENTS
CKNOWLEDGEMENTS
CKNOWLEDGEMENTS
CKNOWLEDGEMENTS


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OF
OFP
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PUBLICATIONS
UBLICATIONS
UBLICATIONS
UBLICATIONS


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ONTENTS
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S
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SUMMARY
UMMARY
UMMARY
UMMARY


VIII
VIII
VIII
VIII
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ISTOF
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IGURES
IGURES
IGURES


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ABLES
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ABLES


XVIII
XVIII
XVIII
XVIII
CHAPTER
CHAPTER
CHAPTER
CHAPTER1:
1:
1:
1:Introduction
Introduction
Introduction
Introduction


1
1
1
1
1.1OverviewofZnO-basedMaterials 1
1.2ZnO-basedDilutedMagneticSemiconductors(DMSs) 3
1.2.1ReviewofFerromagnetisminZnO-basedDMSs 3
1.2.2TheOriginofFerromagnetism 7

1.3OpticalPropertiesofZnO 13
1.3.1PhotoluminescenceStudyofZnO 14
1.3.2ReviewofDefectEmissioninZnO 18
1.4HydrogeninZnO 24
1.4.1ExistingFormsofHydrogeninZnOLattice 25
1.4.2RoleofHydrogeninZnOProperties 27
1.5ZnOGrowthTechniques 31
1.5.1ZnOCrystalStructuresandGrowthStructures 31
1.5.2OverviewofSynthesisMethods 33
1.5.3ReviewofHydrothermalSynthesisofZnO 35
1.6MotivationsandObjectives 40
REFERENCE 46
CHAPTER
CHAPTER
CHAPTER
CHAPTER2
2
2
2:
:
:
:Characterization
Characterization
Characterization
CharacterizationTechniques
Techniques
Techniques
Techniques

61
61
61
61
2.1StructuralCharacterization 62
2.1.1X-rayDiffraction(XRD) 62
2.1.2ScanningElectronMicroscopy(SEM) 64
V
2.1.3Energy-dispersiveX-raySpectrometer(EDS) 66
2.1.4TransmissionElectronMicroscopy(TEM) 67
2.1.5AtomicForceMicroscope(AFM) 70
2.1.6X-rayPhotoelectronSpectroscopy(XPS) 72
2.1.7RamanSpectroscopy 73
2.2MagneticPropertyCharacterization 75
2.2.1VibratingSampleMagnetometer(VSM) 75
2.2.2SuperconductingQuantumInterfaceDevice(SQUID) 76
2.3OpticalPropertyCharacterization 78
2.3.1Ultraviolet-visibleSpectroscopy(UV-vis) 78
2.3.2Photoluminescence(PL) 80
REFERENCE 82
CHAPTER
CHAPTER
CHAPTER
CHAPTER3
3
3
3:
:
:
:Hydrothermal

Hydrothermal
Hydrothermal
HydrothermalSynthesis
Synthesis
Synthesis
Synthesisof
of
of
ofZnO
ZnO
ZnO
ZnONanostructures
Nanostructures
Nanostructures
Nanostructuresand
and
and
andFilms
Films
Films
Films


84
84
84
84
3.1Introduction 84
3.2Experimental 85
3.2.1Set-upofHydrothermalSystem 85

3.2.2ExperimentalDetails 86
3.3ResultsandDiscussion 87
3.3.1BasicCharacterizationofZnO 87
3.3.2EffectofReactionTemperatureandTime 92
3.3.3EffectofPHvalue 94
3.3.4EffectofAdditivesinPrecursorsolution 96
3.3.5EffectofSubstrate 97
3.4Summary 102
REFERENCE 104
CHAPTER
CHAPTER
CHAPTER
CHAPTER4
4
4
4:
:
:
:ZnO
ZnO
ZnO
ZnOFilms
Films
Films
FilmsDoped
Doped
Doped
Dopedwith
with
with

withNon-transition
Non-transition
Non-transition
Non-transitionMetal
Metal
Metal
MetalElements
Elements
Elements
Elements(Na,
(Na,
(Na,
(Na,Mg
Mg
Mg
Mg
and
and
and
andAl)
Al)
Al)
Al)via
via
via
viaa
a
a
aHydrothermal
Hydrothermal

Hydrothermal
HydrothermalRoute
Route
Route
Route


106
106
106
106
4.1Introduction 106
4.2Experimental 107
4.3InvestigationonNa-dopedZnOFilm 108
VI
4.3.1StructuralCharacterizationofNa-dopedZnOFilm 108
4.3.2TransportPropertiesofNa-dopedZnOFilm 110
4.3.3FerromagnetismofNa-dopedZnOFilm 112
4.3.4FerromagnetismOriginofNa-dopedZnOFilm 114
4.4InvestigationonOtherElementsDopedZnOFilms 115
4.4.1Mg-dopedZnOFilm 116
4.4.2Al-dopedZnOFilm 119
4.5Summary 122
REFERENCE 123
CHAPTER
CHAPTER
CHAPTER
CHAPTER5
5
5

5:
:
:
:ZnO
ZnO
ZnO
ZnOFilms
Films
Films
FilmsDoped
Doped
Doped
Dopedwith
with
with
withTransition
Transition
Transition
TransitionMetal
Metal
Metal
MetalElements
Elements
Elements
Elements(Cu)
(Cu)
(Cu)
(Cu)via
via
via

viaa
a
a
a
Hydrothermal
Hydrothermal
Hydrothermal
HydrothermalRoute
Route
Route
Route


125
125
125
125
5.1Introduction 125
5.2Experimental 126
5.3StructuralandMorphologyCharacterizationofCu-dopedZnOFilm 127
5.3.1Cu-dopedZnOFilmsonDifferentSubstrates 127
5.3.2Cu-dopedZnO/quartzFilmswithDifferentDopingConcentration131
5.4FerromagnetismofCu-dopedZnOFilm 135
5.4.1DopingConcentration 135
5.4.2EffectofAnnealingConditions 138
5.5FerromagnetismOriginofCu-dopedZnOFilm 143
5.6Summary 149
REFERENCE 151
CHAPTER
CHAPTER

CHAPTER
CHAPTER6
6
6
6:
:
:
:Effect
Effect
Effect
Effectof
of
of
ofHydrogen
Hydrogen
Hydrogen
Hydrogenon
on
on
onZnO
ZnO
ZnO
ZnOFe
Fe
Fe
Ferromagnetism
rromagnetism
rromagnetism
rromagnetism

154
154
154
154
6.1Introduction 154
6.2Experimental 155
6.2.1ThinFilmFabrication:PulsedLaserDeposition(PLD) 155
6.2.2HydrogenationProcess 157
6.3SurfaceFerromagnetisminHydrogenatedZnOFilm 158
6.3.1Experimental 158
6.3.2StructuralCharacterizationofHydrogenatedZnOFilm 159
VII
6.3.3FerromagnetismofHydrogenatedZnOFilm 161
6.3.4FerromagnetismOrigin-First-principlesCalculation 165
6.3.5Summary 170
6.4FerromagnetisminHydrogenatedCu-dopedZnOFilm 170
6.4.1Experimental 170
6.4.2StructuralCharacterizationofHydrogenatedCu-dopedZnOFilm.171
6.4.3FerromagnetismofHydrogenatedCu-dopedZnOFilm 172
6.4.4FerromagnetismOriginofHydrogenatedCu-dopedZnOFilm 174
6.4.5Summary 178
6.5Summary 179
REFERENCE 181
CHAPTER
CHAPTER
CHAPTER
CHAPTER7
7
7

7:
:
:
:Effect
Effect
Effect
Effectof
of
of
ofHydrogen
Hydrogen
Hydrogen
Hydrogenon
on
on
onZnO
ZnO
ZnO
ZnOLuminescence
Luminescence
Luminescence
Luminescence


183
183
183
183
7.1Introduction 183
7.2Experimental 184

7.3PhotoluminescenceStudy 185
7.3.1HydrogenEnhancedGreenEmissioninZnOFilm 185
7.3.2StabilityofGreenEmission 187
7.3.3AnnealingTemperatureandTimeEffectonGreenEmission 190
7.3.4Low-TemperaturePhotoluminescenceStudy 191
7.4StructuralandMorphologyCharacterizationofHydrogenatedZnOFilm 193
7.4.1StructuralandMorphologyStudy 193
7.4.2InfluenceofStartingMaterialsonGreenEmission 197
7.5GreenRandomLasinginHydrogenatedZnOFilm 199
7.6Large-scaleGreenEmissionZnOFabricationviaMicro-sizePattern 201
7.7Summary 201
REFERENCE 203
CHAPTER
CHAPTER
CHAPTER
CHAPTER8:
8:
8:
8:Conclusions
Conclusions
Conclusions
Conclusionsand
and
and
andFuture
Future
Future
FutureWork
Work
Work

Work


205
205
205
205
8.1Conclusions 205
8.2PossibleImprovementsforFutureWork 211
REFERENCE 214
VIII
S
S
S
SUMMARY
UMMARY
UMMARY
UMMARY
EventhoughresearchfocusingonZnOgoesbackmanydecades,therenewed
interestisfueledandfannedbyitsprospectsinspintronicsandoptoelectronics
applications.Therefore,aresearchintothesenovelapplication-relatedunique
propertiesofZnOisoneofthemostimportantissuesinZnOresearchcommunity.
Furthermore,anexplorationofasimpleandefficientZnOgrowthtechniqueis
desirableforbettermaterializationofthesepotentialZnO-baseddevices.Besides,as
hydrogen(H)isaninevitableelementinZnO,abettercontrolandunderstandingofH
effectinZnOisofgreattechnologicalinterest.Thisthesisfocusedonhydrothermal
synthesis,ferromagneticandluminescentproperties,andhydrogeneffectofZnO.
Basedonthedetailedinvestigation,thecontributionoftheworkissummarized
below:
(1)Low-temperaturehydrothermalroutewasdemonstratedtobeasimple,

efficient,andenvironmentallyfriendlygrowthmethodforsynthesisofhigh-quality
ZnOnanostructures/films.
A
betterunderstandingofZnOmorphologycontrolwas
achieved.Mostimportantly,byusingpulsedlaserdepositiontechniquederivedZnO
seedlayers,itovercamethelimitationoftypicaltwo-stephydrothermalmethodon
substrateselection.Highly-texturedZnOfilmsgrownondifferentsubstratesincluding
silicon,glass,sapphireandquartzwereobtained.Besides,thismethodwasalso
demonstratedtobeapplicabletodopingofvariouselementsintoZnOlattice[Na(I
A-group),Mg(IIA-group),Al(IIIA-group)andCu(transitionmetal)inthisstudy]
andtheresultantmaterialsexhibitedexcellentperformance.
IX
(2)Hightemperatureferromagnetismwasachievedinundoped,nonmagnetic
transitionmetalelements(e.g.Cu)dopedandnontransitionmetalelements(e.g.Na)
dopedZnOsystems,whichwereproposedaspromisinghostmaterialsforspintronics
devices.Inthisstudy,nointentionalintroductionofmagneticelementsintoZnO
excludedanypossibilitiesofferromagnetisminducedbytheprecipitatesorphase
segregationofmagneticdopants,whichfavoursabetterunderstandingofintrinsic
ferromagneticpropertyandrealizationofagenuinedilutedmagneticsemiconductor.
Mostimportantly,althoughtheferromagneticoriginmaybedifferentfordifferent
systemsandalsofordifferentfabricationconditions,itwasverifiedthattheobserved
ferromagnetisminZnO-basedmaterialsisgenerallycorrelatedwithandcanalsobe
tunedbydefects,includingnativedefects(e.g.oxygenvacancy),dopantsand
hydrogen.Defectengineeringtechniqueforferromagnetismimprovementofcertain
ZnO-basedmaterialwasdeveloped.
(3)BasedontheinvestigationofhydrogeneffectonZnOferromagnetism,a
2-dimensionalferromagnetismmodelassociatedwithOHattachmentwasfirstly
proposed.FerromagneticorderingofundopedZnOcouldbeswitchedbetween“on”
and“off”statesbyintroducingandremovingOHattachmentonZnOsurface,
respectively.First-principlescalculationsconfirmedthatOH-terminatedZnOsurface

hasthelowestformationenergyof-2.97eVandamagneticmomentof0.30μ
B
per
OH.TheoriginofFMinhydrogenatedundopedZnOwasattributedtotheunpaired
magneticmomentofelectronsoccupyingtheO2
p
orbitalatthesurface.
(4)BasedontheinvestigationofhydrogeneffectonZnOluminescence,a
X
large-scalegreenemissionwithhighthermal/chemicalstabilitywasachievedinZnO
thinfilm,accompaniedbyarandomlasing-likeactivity.Concerningtheoriginofthe
greenemission,itwasfoundthattheintrinsicnativedefectsthemselves(i.e.Oxygen
vacancies)orHincorporationcannotproducethestronggreenemissionwith
coexistenceofrandomlasing-likeactivity.Thecoexistenceofporousmorphologyand
complexdefect(s)inducedbyhydrogentreatmentwassuggestedtofavorthestrong
greenemission.Thestabilityandeasilyachievedmaskingpatternofstronggreen
emissiondemonstratespotentialapplicationsofresultantZnOfilmsascomponentsin
noveloptoelectronicdevices,suchasgreenlightemittingdiodes.
XI
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OF
OFF
F

F
FIGURES
IGURES
IGURES
IGURES
Figure
Figure
Figure
Figure1.
1.
1.
1.1
1
1
1.
.
.
.ThetheoreticalvaluesofCurietemperature(T
c
)forvarious
p
-type
semiconductorscontaining5%Mnand3.5×10
20
holespercm
3
.Thedashedline
indicatesroomtemperature(300K).(ModifiedfromRef.[20])
Figure
Figure

Figure
Figure1.
1.
1.
1.2
2
2
2.
.
.
.Illustrationofmagneticpolarons.Thecationsitesarerepresentedby
smallcircles.Oxygenisnotshownandunoccupiedoxygensitesarerepresentedby
squares.(ModifiedfromRef.[62])
Figure
Figure
Figure
Figure1.
1.
1.
1.3
3
3
3.
.
.
.TypicalwurtzitestructureofZnOlattice.Theblueandpurplespheres
denoteZnandOatoms,respectively.
Figure
Figure
Figure

Figure2.1.
2.1.
2.1.
2.1.SchematicillustrationofBragg'slaw.
Figure
Figure
Figure
Figure2.2.
2.2.
2.2.
2.2.SchematicillustrationofTEMbrightfieldimaging.
Figure
Figure
Figure
Figure2.3.
2.3.
2.3.
2.3.SchematicillustrationofTEMdarkfieldimaging.
Figure
Figure
Figure
Figure2.4.
2.4.
2.4.
2.4.SchematicdiagramofAFMsystem.
Figure
Figure
Figure
Figure2.
2.

2.
2.5
5
5
5.
.
.
.EnergyleveldiagramofsignalsshowinginRaman.
Figure
Figure
Figure
Figure2.
2.
2.
2.6
6
6
6.
.
.
.
A
schematicillustrationofVSMset-up.
Figure
Figure
Figure
Figure2.
2.
2.
2.7

7
7
7.
.
.
.
A
schematicdiagramofSQUIDsystem.
Figure
Figure
Figure
Figure2.
2.
2.
2.8
8
8
8.
.
.
.
A
schematicdiagramofphotoluminescenceprocess.
Figure
Figure
Figure
Figure3.1.
3.1.
3.1.
3.1.Schematicdiagramofanautoclaveusedinhydrothermalmethod.

Figure
Figure
Figure
Figure3.2.
3.2.
3.2.
3.2.(a)XRDspectrumofZnOpowdersynthesizedbythehydrothermal
method(specimen11,powder);(b)XRDspectraforZnOseedlayer(quartzsubstrate)
preparedbyPLDandthesubsequenthydrothermallygrownZnOfilm(specimen11,
film).
Figure
Figure
Figure
Figure3.
3.
3.
3.3
3
3
3.
.
.
.XPSspectrumforZnOfilmsynthesizedbythehydrothermalmethod
(specimen11,film).
Figure
Figure
Figure
Figure3.4.
3.4.
3.4.

3.4.TheUV-visibleabsorptionspectraofZnO/quartzfilms(specimen11,film)
andtheinsetshows(α
hν
)
2
plottedagainstthephotonenergy
hv
.
Figure
Figure
Figure
Figure3.5.
3.5.
3.5.
3.5.(a)TheroomtemperaturePLspectra;(b)Ramanspectraforas-prepared
ZnOfilm(specimen11,film)andthefilmannealedinAr80%-O
2
20%atfrom400
o
Cto700
o
C.
Figure
Figure
Figure
Figure3.
3.
3.
3.6
6

6
6.
.
.
.FieldemissionSEMimagesof(a)specimen1,powder(60
o
C);(b)
specimen2,powder(75
o
C)and(c)specimen3,powder(90
o
C).
Figure
Figure
Figure
Figure3.
3.
3.
3.7
7
7
7.
.
.
.FieldemissionSEMimagesofthesurfacemorphologyofZnOfilms(a)
specimen4(15min);(b)specimen5(1h);(c)specimen6(6h);and(d)specimen7
XII
(12h).Theinsetin(a)iscorrespondingpowderproductsofspecimen4.Otherinsets
showthecorrespondingcross-sectionalmorphologyofeachfilm.(e)showsthe
variationindiameterandlengthofZnOnanorodswithincreasingreactiontime.

Figure
Figure
Figure
Figure3.
3.
3.
3.8
8
8
8.
.
.
.FieldemissionSEMimagesofZnO(a)specimen8,powder(PH=7.5);
(b)specimen9,powder(PH=10.7).Theinsetsin(a)and(b)aretheenlargedSEM
images.(c)and(d)representsurfacemorphologyandcross-sectionalmorphologyof
specimen9,film,respectively.
Figure
Figure
Figure
Figure3.
3.
3.
3.9
9
9
9.
.
.
.FieldemissionSEMimagesofZnO(a)specimen10,film(PH=10.7
withsodiumcitrate);(b)specimen10,powder(PH=10.7withsodiumcitrate).

F
F
F
Figure
igure
igure
igure3.10.
3.10.
3.10.
3.10.FieldemissionSEMimagesofthesurfacemorphologyof(a)specimen
11,ZnO/quartzfilms;(b)specimen12,ZnO/glassfilms;(c)specimen13,ZnO/Si
films;and(d)specimen14,ZnO/sapphirefilms.Theinsetsshowthecorresponding
cross-sectionalmorphologyofeachfilm.
Figure
Figure
Figure
Figure3.11.
3.11.
3.11.
3.11.(a)θ-2θXRDpatternsofZnOseedlayersonquartz,silicon,sapphire
andglasssubstrates.(b)XRDrockingcurvesonthe(002)reflectionofZnO/quartz,
ZnO/sapphire,ZnO/Si,ZnO/glassfilms(dashlines)comparedwiththeirseedlayers
(fulllines)fabricatedbyPLD,respectively.
Figure
Figure
Figure
Figure3.12.
3.12.
3.12.
3.12.θ-2θXRDpatternsofZnOfilmonamorphousZnO/quartzsubstrates.

TheinsetsshowtheXRDrockingcurvesonits(002)reflectionandthesurfaceand
crosssectionalmorphologyofthefilm.
Figure
Figure
Figure
Figure3.13.
3.13.
3.13.
3.13.Off-axisΦ-scanfor(101)planeof(1)ZnO/sapphireand(2)ZnO/quartz
films.
Figure
Figure
Figure
Figure4
4
4
4.1.
.1.
.1.
.1.θ-2θXRDpatternof1%Na-dopedZnOfilm.TheinsetshowstheXRD
rockingcurvesonits(002)reflection.
Figure
Figure
Figure
Figure4
4
4
4.
.
.

.2
2
2
2.
.
.
.FieldemissionSEMimagesof(a)surfacemorphologyand(b)cross
sectionalmorphologyof1%Na-dopedZnOfilm.
Figure
Figure
Figure
Figure4
4
4
4.
.
.
.3
3
3
3.
.
.
.Na1sXPSspectrumfor(a)1%NadopedZnOfilmwithp-type
conductivityand(b)2%NadopedZnOfilmwithn-typeconductivity.
Figure
Figure
Figure
Figure4
4

4
4.
.
.
.4
4
4
4.
.
.
.(a)SIMSpeaklistofdifferentelementsin1%Na-dopedZnO;(b)SIMS
profilesofelementsZn,NaandOinthefilm.
Figure
Figure
Figure
Figure4
4
4
4.
.
.
.5
5
5
5.
.
.
.(a)M-HcurvesoffilmswithdifferentNadopingconcentration.(b)The
saturationmagnetizationdependentonNadopingconcentration.(c)Hysteresisloops
of1%Na-dopedZnOat5Kand300K.Theinsetisthesaturationmagnetizationof

thesampledependentontemperature.
Figure
Figure
Figure
Figure4
4
4
4.6.
.6.
.6.
.6.M-Hloopsfor(a)Mg-dopedZnOfilmand(b)Al-dopedZnOfilm.The
insetsareM-Hcurvesbeforesubtractionofthesubstratesignals.
Figure
Figure
Figure
Figure4
4
4
4.7.
.7.
.7.
.7.Dopingcontentinfilmsasafunctionofthatintheprecursorsolutionfor
(a)Mgcontentand(b)Alcontent.
XIII
Figure
Figure
Figure
Figure4
4
4

4.8.
.8.
.8.
.8.(a)θ-2θXRDpatternsofZn
1-x
Mg
x
Ofilms;(b)Mgconcentration
dependenceofthelatticeconstantc.Theinsetin(b)showstheXPSspectrumofMg
2plevelfor4%MgdopedZnOfilm.
Figure
Figure
Figure
Figure4
4
4
4.9.
.9.
.9.
.9.ResistivityofZn
1-x
Mg
x
OfilmswithdifferentMgcontent.
Figure
Figure
Figure
Figure4
4
4

4.10.
.10.
.10.
.10.(a)(α
hν
)
2
plottedagainstthephotonenergy
hv
forZn
1-x
Mg
x
Ofilms;(b)
EnergybandgapasafunctionofMgcontent.
Figure
Figure
Figure
Figure4
4
4
4.
.
.
.
1
1
1
1
1.

1.
1.
1.(a)θ-2θXRDpatternsofZn
1-x
Al
x
Ofilms;(b)Alconcentration
dependenceofthelatticeconstantc.Theinsetin(b)showstheXPSspectrumofAl
2plevelfor3.5%AldopedZnOfilm.
Figure
Figure
Figure
Figure4
4
4
4.
.
.
.1
1
1
12.
2.
2.
2.Resistivity,carriermobility,andcarrierconcentrationasafunctionofAl
dopingconcentrationinZn
1-x
Al
x
Ofilms.

Figure
Figure
Figure
Figure4
4
4
4.
.
.
.1
1
1
13.
3.
3.
3.(a)(α
hν
)
2
plottedagainstthephotonenergy
hv
forZn
1-x
Al
x
Ofilms;(b)
EnergybandgapasafunctionofAlcontent.
Figure
Figure
Figure

Figure5
5
5
5.1.
.1.
.1.
.1.FieldemissionSEMimagesof(a)surfacemorphologyand(b)cross
sectionofZn
0.98
Cu
0.02
O/quartzfilms,(c)surfacemorphologyand(d)crosssectionof
Zn
0.98
Cu
0.02
O/Sifilms,and(e)surfacemorphologyand(f)crosssectionof
Zn
0.98
Cu
0.02
O/Sapphirefilms.Theinsetsin(a),(c)and(e)areEDXspectraof
Zn
0.98
Cu
0.02
O/quartz,Zn
0.98
Cu
0.02

O/SiandZn
0.98
Cu
0.02
O/Sapphirefilms,respectively.
Figure
Figure
Figure
Figure5
5
5
5.
.
.
.2.
2.
2.
2.θ-2θXRDpatternsforZn
0.98
Cu
0.02
Ofilmsonquartz,Siandsapphire
substrates.
Figure
Figure
Figure
Figure5
5
5
5.

.
.
.3.
3.
3.
3.VisibleRamanspectraforZn
0.98
Cu
0.02
Ofilmson(1)quartz,(2)sapphire
and(3)Sisubstrates.
Figure
Figure
Figure
Figure5
5
5
5.
.
.
.4
4
4
4.
.
.
.θ-2θXRDpatternsofZn
1-x
Cu
x

O/quartzfilms.TheinsetshowstheXRD
rockingcurvesonthe(002)reflectionofZn
1-x
Cu
x
OfilmandthepureZnOseedlayer
fabricatedbyPLD.(1),(2),(3),and(4)representtheZn
1-x
Cu
x
Ofilmswithx=0,0.01,
0.02and0.05,respectively.
Figure
Figure
Figure
Figure5
5
5
5.5.
.5.
.5.
.5.XPSspectrumforZn
0.98
Cu
0.02
Ofilm.
Figure
Figure
Figure
Figure5

5
5
5.
.
.
.6
6
6
6.
.
.
.TheUV-visibleabsorptionspectraandnormalizedroom-temperaturePL
spectraofZn
1-x
Cu
x
Ofilms.(1),(2),(3),and(4)representtheZn
1-x
Cu
x
Ofilmswith
x=0,0.01,0.02and0.05,respectively.
Figure
Figure
Figure
Figure5
5
5
5.
.

.
.7
7
7
7.
.
.
.Magnetizationversusmagneticfield(M-H)loopsforZn
1-x
Cu
x
Ofilmsat
roomtemperature.ThebottomrightinsetshowstheM-Hloopsfor
Zn
0.98
Cu
0.02
O/quartzfilmsat5Kand300K.(1),(2),(3),and(4)representthe
Zn1-xCuxOfilmswithx=0,0.01,0.02and0.05,respectively.
Figure
Figure
Figure
Figure5
5
5
5.
.
.
.8
8

8
8.
.
.
.ThetemperaturedependenceofFCandZFCmagnetizationsfor
Zn
0.98
Cu
0.02
O/quartzfilms.
Figure
Figure
Figure
Figure5
5
5
5.
.
.
.9
9
9
9.
.
.
.Room-temperaturePLspectraforas-preparedZn
0.98
Cu
0.02
O/quartzfilm,

thefilmannealedat600
o
CinH
2
/Ar(2.5%/97.5%)atmospherefor1handthefilm
XIV
annealedat600
o
CinO
2
/Ar(20%/80%)atmospherefor1h.
Figure
Figure
Figure
Figure5
5
5
5.
.
.
.10
10
10
10.
.
.
.VisibleRamanspectraofZn
0.98
Cu
0.02

O/quartzfilm,thefilmannealedat
600
o
CinH
2
/Ar(2.5%/97.5%)atmospherefor1handthefilmannealedat600
o
Cin
O
2
/Ar(20%/80%)atmospherefor1h.
Figure
Figure
Figure
Figure5
5
5
5.
.
.
.
1
1
1
1
1
1
1
1.
.

.
.Magnetizationversusmagneticfield(M-H)loopsforas-prepared
Zn
0.98
Cu
0.02
O/quartzfilm,thefilmannealedat600
o
CinH
2
/Ar(2.5%/97.5%)
atmospherefor1handthefilmannealedat600
o
CinO
2
/Ar(20%/80%)atmosphere
for1h.
Figur
Figur
Figur
Figure
e
e
e5
5
5
5.
.
.
.1

1
1
12
2
2
2.
.
.
.VariationofsaturationmagnetizationofZn
0.98
Cu
0.02
Ofilmwhileheated
itat600
o
CinO
2
/Ar(20%/80%)for1handtheninH
2
/Ar(2.5%/97.5%)at600
o
Cfor
1h.TheinsetshowsthecorrespondingM-Hloops.Here,representsthe
as-preparedsamples,representsthesamplesheatedinO
2
/Arandrepresents
thesamplesheatedinO
2
/AratmosphereandtheninH
2

/Aratmosphere.
Figure
Figure
Figure
Figure5
5
5
5.
.
.
.1
1
1
13
3
3
3.
.
.
.VariationofsaturationmagnetizationofZn
0.98
Cu
0.02
Ofilmwhileheated
itat600
o
CinO
2
/Ar(20%/80%)for1handtheninpureAratmosphereat600
o

Cfor
1h.TheinsetshowsthecorrespondingM-Hloops.Here,representsthe
as-preparedsamples,representsthesamplesheatedinO
2
/Aratmosphereand
representsthesamplesheatedinO
2
/AratmosphereandtheninpureAratmosphere.
Figure
Figure
Figure
Figure5
5
5
5.1
.1
.1
.14
4
4
4.
.
.
.(a)Zn2pXPSspectrafortheas-preparedZn
0.98
Cu
0.02
Ofilmsandthe
filmsannealedinH
2

/Ar(2.5%/97.5%)atmosphereat500
o
Cand(b)Cu2pXPS
spectraforthefilmsannealedinH
2
/Ar(2.5%/97.5%)atmosphereat500
o
C.
Figure
Figure
Figure
Figure5
5
5
5.1
.1
.1
.15
5
5
5.
.
.
.O1sXPSspectrafor(a)theas-preparedZn
0.98
Cu
0.02
Ofilms,(b)the
filmsannealedinO
2

/Ar(20%/80%)atmosphereand(c)thefilmsannealedinO
2
/Ar
(20%/80%)atmosphereandsubsequentlyinH
2
/Ar(2.5%/97.5%)environment.
Figure
Figure
Figure
Figure5
5
5
5.1
.1
.1
.16
6
6
6.
.
.
.(a)SEMimagesofZn
0.98
Cu
0.02
Opowderspreparedbyhydrothermal
routewithPH=7.5.(b)XRDspectraofZn
0.98
Cu
0.02

Opowderswhileheatedin
differentatmosphere.(c)VariationofsaturatedmagnetizationofZn
0.98
Cu
0.02
O
powderswhileheatedindifferentatmosphere.TheinsetinFig.7(c)showsthe
correspondingM-Hloops.Here,representstheas-preparedpowders,
representspowdersheatedat600
o
CinO
2
/Ar(20%/80%)for1handrepresents
thepowdersheatedat600
o
CinO
2
/Ar(20%/80%)atmosphereandtheninH
2
/Ar
(2.5%/97.5%)atmosphereat600
o
Cfor1h.(d)Room-temperaturePLspectraof
as-preparedZn
0.98
Cu
0.02
Opowdersandthepowdersheatedat600
o
CinO

2
/Ar
(20%/80%)atmosphereandtheninH
2
/Ar(2.5%/97.5%)at600
o
Cfor1h.
Figure
Figure
Figure
Figure5
5
5
5.1
.1
.1
.17
7
7
7.
.
.
.Themagnetizationversusmagneticfield(M-H)loopsfor
Zn
0.98
Cu
0.02
O/quartzfilmpreparedinprecursorsolutionwithPHvalueof11.The
insetshowstheSEMimageofthefilm.
Figure

Figure
Figure
Figure6
6
6
6.1.
.1.
.1.
.1.SchematicdiagramofPLDsystemset-up.
Figure
Figure
Figure
Figure6
6
6
6.
.
.
.2
2
2
2.
.
.
.Set-upoftubefurnaceforhydrogenationprocess.
XV
Figure
Figure
Figure
Figure6

6
6
6.
.
.
.3
3
3
3.
.
.
.θ–2θXRDpatternsofas-depositedZnOfilmandthefilmannealedin
95%Ar-5%H
2
atmosphereat500°Cfor1h.Theinsetisoff-axis
Φ
-scanfor(101)
planeofZnOfilm.
Figure
Figure
Figure
Figure6
6
6
6.
.
.
.4
4
4

4.
.
.
.XPSspectraofZn2ppeaksforas-depositedZnOfilmandthefilm
annealedin95%Ar-5%H
2
atmosphereat500°Cfor1h.
Figure
Figure
Figure
Figure6
6
6
6.5.
.5.
.5.
.5.(a)M
s
independenceofH
2
-annealingtemperatureof300nmthickZnO
films;(b)thicknessdependenceofM
s
afterannealinginAr-H
2
at500°Cfor1h.
Figure
Figure
Figure
Figure6

6
6
6.6.
.6.
.6.
.6.Magnetizationversuscyclingannealingprocess.Theprocessis,annealing
as-deposited300nm-thickfilm(ZnO)inAr-H
2
at500°Cfor1h(1
st
ZnO:H)followed
byaprolongedpureArannealingat500°C(ZnO:H~Ar),andsubsequently
conductingthe2
nd
hydrogenationprocess(2
nd
ZnO:H).Theinsetsarecorresponding
M-Hloops.
Figure
Figure
Figure
Figure6
6
6
6.7.
.7.
.7.
.7.O1sXPSspectrafor(a)theas-depositedZnOlm,(b)thelmannealed
inAr-H
2

at50
o
C,(c)thefilmannealedinAr-H
2
at500
o
C;and(d)thelmannealed
inAr-H
2
atmosphereat500
o
CandsubsequentlyinpureArat500
o
C.
Figure
Figure
Figure
Figure6
6
6
6.8.
.8.
.8.
.8.VariationofM
s
whenthehydrogenated-ZnOfilmisdippedintoHCl
solution(PH≈6).
Figure
Figure
Figure

Figure6
6
6
6.9.
.9.
.9.
.9.Diagram(a)showsthechargedensityofaparticular(3×3)OH-terminated
ZnO(001)surfacesuperposedwiththe
p
31
m
symmetryoftheOHarrangement.
Diagram(b)showsthespindensityofthesamesurfaceanddiagram(c)showsthe
spindensityasviewedfromtheside,i.e.[100]or[010]directions.Indiagram(a),the
yellowcolourrepresentselectronchargedensity.Indiagrams(b)and(c),itrepresents
spindensity.
Figure
Figure
Figure
Figure6.10.
6.10.
6.10.
6.10.ExamplesofconfigurationsofotherOH-terminatedsurfaceswithhigher
formationenergy.
Figure
Figure
Figure
Figure6
6
6

6.
.
.
.11
11
11
11.
.
.
.θ–2θXRDpatternsofas-deposited2%Cu:ZnOfilm(50nm)andthe
filmannealedin95%Ar-5%H
2
atmosphereat500°Cfor1h.
Figure
Figure
Figure
Figure6
6
6
6.
.
.
.12
12
12
12.
.
.
.(a)M
s

independenceofH
2
-annealingtemperatureofZnOfilm(upper)
and2%Cu-dopedZnOfilm(bottom)withaconstantthicknessof50nm.(c)Film
thicknessdependenceofarealM
s
forZnOfilm(upper)and2%Cu-dopedZnOfilm
(bottom)afterH
2
annealingat500°Cfor1h.
Figure
Figure
Figure
Figure6
6
6
6.
.
.
.13
13
13
13.
.
.
.TheM-Hloopsforhydrogenated2%Cu-dopedZnOfilm(50nm)at5K
and300K.
Figure
Figure
Figure

Figure6
6
6
6.
.
.
.14
14
14
14.
.
.
.(a)O1
s
XPSspectraforas-deposited2%Cu-dopedZnOfilm,thefilm
annealedinAr-H
2
atmosphereat500
o
CandthefilmsubsequentlyannealedinAror
O
2
atmosphere.(b)Cu2
p
XPSspectraforas-deposited2%Cu-dopedZnOfilm,the
filmannealedinAr-H
2
atmosphereat500
o
CandthefilmsubsequentlyannealedinO

2
atmosphere.
XVI
Figure
Figure
Figure
Figure6
6
6
6.
.
.
.15
15
15
15.
.
.
.Magnetization
versus
cyclingannealingprocessforZnOand2%
Cu-dopedZnOfilms.Theprocessis,annealingas-depositedZnO/Cu:ZnOinAr-H
2
at
500
o
Cfor1hfollowedbyaprolongedannealingat500
o
CinpureArorpureO
2

atmosphere.
Figure
Figure
Figure
Figure6
6
6
6.
.
.
.16
16
16
16.
.
.
.Ramanspectraforas-deposited2%Cu-dopedZnOfilm,thefilm
annealedinAr-H
2
atmosphereat500
o
CandthefilmsubsequentlyannealedinAror
O
2
atmosphere.
Figure
Figure
Figure
Figure7.1.
7.1.

7.1.
7.1.TheroomtemperaturePLas-depositedandannealedZnOfilm.Dashlines
Gaussianfittingofgreenband.
Figure
Figure
Figure
Figure7.
7.
7.
7.2
2
2
2.
.
.
.UV-vistransmittancespectraforas-depositedZnOfilmandthefilm
undergoingAr-H
2
heattreatment.
Figure
Figure
Figure
Figure7.
7.
7.
7.3
3
3
3.
.

.
.(a)PLstabilitystudyofgreenemissionbandforArannealedZnOfilms,
whensubsequentlyannealedtheminO
2
atmosphere.(b)PLstabilitystudyofgreen
emissionbandforAr-H
2
annealedZnOfilms,whensubsequentlyannealedtheminO
2
atmosphere.
Figure
Figure
Figure
Figure7.
7.
7.
7.4
4
4
4.
.
.
.TheroomtemperaturePLofas-depositedZnOfilm,thefilmannealedin
O
2
at1000
o
Cfor1handthefilmannealedinArat1000
o
Cfor1h.

Figure
Figure
Figure
Figure7.
7.
7.
7.5
5
5
5.
.
.
.TheroomtemperaturePLspectraofZnOfilmannealedinAr-H
2
at500
o
Cfor1handthatundergoingacetoneimmersionfor24h.Theinsetshowsthe
correspondingimagestakenundera365nmUVlamp.
Figure
Figure
Figure
Figure7.
7.
7.
7.6
6
6
6.
.
.

.(a)TheroomtemperaturePLspectraofZnOfilmannealedinAr-H
2
for1
hatdifferenttemperature.(b)Absoluteintensityofgreenbandasfunctionof
annealingtemperature.
Figure
Figure
Figure
Figure7.
7.
7.
7.7.
7.
7.
7.(a)TheroomtemperaturePLspectraofZnOfilmannealedinAr-H
2
at
500
o
Cfordifferentannealingtime.(b)Absoluteintensityofgreenbandasfunction
ofannealingtime.
Figure
Figure
Figure
Figure7.8.
7.8.
7.8.
7.8.Thetemperature-dependent(6K~300K)photoluminescenceof
hydrogenatedZnOfilm(500
o

C);spectraareoffsetforclarity.
Figure
Figure
Figure
Figure7.9.
7.9.
7.9.
7.9.(a)Low-temperaturePLspectrameasuredat10K.(b)showsenlargedPL
spectraoftheboundexcitonregion.
Figure
Figure
Figure
Figure7.
7.
7.
7.10
10
10
10.
.
.
.SEMimagesof(a)as-depositedZnOfilmandthefilms(b)when
annealedinpureArat500
o
Cfor1h;(c)whenannealedinArat1000
o
Cfor1h;(d)
whenannealedinO
2
at500

o
Cfor1h;(e)whenannealedinO
2
at1000
o
Cfor1h;(f)
whenannealedinAr-H
2
at100
o
C;(g)whenannealedinAr-H
2
at300
o
C;(h)when
annealedinAr-H
2
at500
o
Cfor15min;(i)whenannealedinAr-H
2
at500
o
Cfor1h;
(j)thefilmwhichwasfirstlyannealedinAr-H
2
at500
o
Cfor60minandtheninO
2

at
700
o
Cfor1h;(k)thefilmwhichwasfirstlyannealedinAr-H
2
at500
o
Cfor1hand
theninO
2
at1000
o
Cfor1h.Theinsetin(a)showstheTEMimagesofas-deposited
ZnOfilm.
Figure
Figure
Figure
Figure7.1
7.1
7.1
7.11
1
1
1.
.
.
.(a)RoomtemperaturePLspectraofhydrogenatedZnOfilmandthe
XVII
hydrogenatedfilmwhichissubsequentlyimmersedinHClsolution(PH=2.5and5)
for5seconds;(b),(c)and(d)showthecorrespondingSEMimageofthe

hydrogenatedfilm,thehydrogenatedfilmimmersedinHClwithPH=5and2.5,
respectively.
Figure
Figure
Figure
Figure7.1
7.1
7.1
7.12
2
2
2.
.
.
.RoomtemperaturePLspectraofas-depositedZnO/sapphirefilmandthe
hydrogenatedZnO/sapphirefilm.TheinsetshowstheSEMimageofhydrogenated
ZnO/sapphirefilm.
Figure
Figure
Figure
Figure7.1
7.1
7.1
7.13
3
3
3.
.
.
.(a)Room-temperaturePLspectraofhydrothermalroutepreparedZnO

filmandAr-H
2
annealedfilms.(b)SEMimageofhydrothermalrouteas-prepared
ZnOfilmandthestructuralinfluenceofAr-H
2
atmosphereat500
o
Cfor1hisshowed
in(c).
Figure
Figure
Figure
Figure7.1
7.1
7.1
7.14
4
4
4.
.
.
.(a)Room-temperaturelasing-likeemissionspectrumofPLDprepared
ZnOfilmafterH
2
surfacetreatment.(b)ThecorrespondingFouriertransform
spectrumwiththeestimatedcavitydiameterof~1.3µm.
Figure
Figure
Figure
Figure7.1

7.1
7.1
7.15
5
5
5.
.
.
.
A
schematicdiagramofmaskingpatternofgreenemission.Imageof
remarkablegreen“NUS”logowastakenunderexcitationof365nmUVlight.
XVIII
L
L
L
LIST
IST
IST
ISTOF
OF
OF
OF
T
T
T
T
ABLES
ABLES
ABLES

ABLES
Table
Table
Table
Table1.1
1.1
1.1
1.1.
.
.
.PositionsandproposedoriginofreportedboundexcitonlinesinZnO.I
0
to
I
11
arelabeledaccordingtoRef.[80].
Table
Table
Table
Table1.2
1.2
1.2
1.2.
.
.
.CalculatedenergylevelofnativedefectsinZnO.(V
Zn
2-
,V
Zn

-
,andV
Zn
representdoublycharged,singlycharged,andneutralzincvacancies,respectively.Zn
i
andZn
i
+
representneutralandsinglychargedzincinterstitials,respectively.V
O
and
V
O
+
denotesneutralandsinglychargedoxygenvacancies,respectively.O
i
isoxygen
interstitial.O
Zn
meansantisiteoxygenandV
O
Zn
i
representsdefectcomplexofoxygen
vacancyandzincinterstitial.)
Table
Table
Table
Table1.
1.

1.
1.3.
3.
3.
3.Typicaltwo-stephydrothermalrouteaccordingtoRef.[194].
Table
Table
Table
Table2.1.
2.1.
2.1.
2.1.Instrumentsforcharacterization.
Table
Table
Table
Table3
3
3
3.
.
.
.1
1
1
1.
.
.
.ExperimentalProceduresofhydrothermalsynthesis.
Table
Table

Table
Table3
3
3
3.
.
.
.2
2
2
2.
.
.
.Synthesisconditionsforeachspecimen.
Table
Table
Table
Table4
4
4
4.
.
.
.1
1
1
1.
.
.
.TransportpropertiesofNa-dopedZnOFilms.

Table
Table
Table
Table5
5
5
5.
.
.
.1
1
1
1.
.
.
.Resistivity,carrierconcentrationandcarriermobilityofZn
0.98
Cu
0.02
Ofilms
onquartz,Siandsapphiresubstrate.
T
T
T
T
a
a
a
able
ble

ble
ble5
5
5
5.
.
.
.2
2
2
2.
.
.
.NominalandactualCucontentinZn
1-x
Cu
x
O/quartzfilms.
Table
Table
Table
Table5
5
5
5.
.
.
.3
3
3

3.
.
.
.Resistivity,carrierconcentrationandcarriermobilityofZn
1-x
Cu
x
O/quartz
filmswithx=0,1%,2%and5%.
Table
Table
Table
Table5
5
5
5.
.
.
.4
4
4
4.
.
.
.Resistivity,carrierconcentrationandcarriermobilityofZn
0.98
Cu
0.02
Ofilms
underdifferentannealingconditions.

Table
Table
Table
Table5
5
5
5.
.
.
.5
5
5
5.
.
.
.Resistivity,carrierconcentrationandcarriermobilityandsaturation
magnetizationofZn
0.98
Cu
0.02
OfilmspreparedinprecursorsolutionofPH=7.5or11.
Table
Table
Table
Table6
6
6
6.1.
.1.
.1.

.1.CalculatedformationenergyandmagneticmomentofpossibleH-related
speciesinZnO.
Table
Table
Table
Table7.1.
7.1.
7.1.
7.1.Detailedinformationofgreenemissionbandinroom-temperaturePL
spectrabasedonGaussianfitting.
Table
Table
Table
Table7.
7.
7.
7.2
2
2
2.
.
.
.Thetopographyandsurfaceroughnessanalysisofas-depositedZnOfilm
andthefilmsannealedindifferentatmospheres.
1
CHAPTER
CHAPTER
CHAPTER
CHAPTER1:
1:

1:
1:Introduction
Introduction
Introduction
Introduction
A
surgeinresearchintooxide-basedfunctionalmaterialshasbeenobservedand
continuestoexpandduetotheiruniqueandnovelapplications.Amongthese
functionaloxides,zincoxide(ZnO)hasdrawnconsiderableattentionbecauseof
richestnanostructuresanddistinguishedperformance,whichrenderitsuitablefor
variousapplications.EventhoughresearchfocusingonZnOgoesbackmanydecades,
therenewedinterestisfueledandfannedbyitsprospectsinspintronicsand
optoelectronicsapplicationsowningtoreportsofroomtemperatureferromagnetic
behaviou
r
,itsdirectwidebandgap(E
g
~3.3eVat300K)andhighexciton-binding
energy(60meV)[1].Therefore,theinvestigationonthesenovelapplication-related
uniquepropertiesofZnOisoneofthemostimportantissuesinZnOresearch
community.Furthermore,hydrogen(H)tailoredZnOpropertiesrecentlybecomes
anotherpertinentissuebecauseHcouldbeinevitablyincorporatedintoZnOlattice
duringcrystalgrowthand/ordeviceprocessing[2].
ThisChapterpresentsanin-depthoverviewofZnO-basedmaterials,which
chieflyfocusesonZnO-baseddilutedmagneticsemiconductors(DMSs)andZnO
luminescence.TherestofthereviewisdevotedtotherolesofHinZnO,followedby
growthmethodsofZnOsystem.
1.1
1.1
1.1

1.1Overview
Overview
Overview
Overviewof
of
of
ofZnO-based
ZnO-based
ZnO-based
ZnO-basedMaterials
Materials
Materials
Materials
Oxidesarethebasisoffunctionalmaterialsandsmartdevices.Functionaloxides
haverecentlyattractedextensiveattentionduetotheirdiverseandvariedstructures
2
withpropertiescoveringalmostallaspectsofmaterialscienceandphysics.
Amongthesefunctionaloxides,ZnOisparticularlyoutstanding,asseenfroma
surgeofarelevantnumberofpublicationsinZnOresearchfield.Thisisfirstly
becauseZnOisamaterialthathastherichestfamilyofnanostructuresamongall
materialsevenincludingcarbonnanotubes[3].
A
varietyofZnOnanostructureshave
beenrealized,suchasnanodots,nanorods,nanowires,nanobelts,nanotubes,
nanosheets,nanocombs,nanocages,nanowalls,nanohelixes,nanoringsaswellasthin
films[4-11].Thisopensupnewprospectstostudyontheirsize/morphologyrelated
uniqueoptical,magneticandelectricalproperties.
AnotheradvantageofZnOisthatithassimpleandefficientgrowthtechniques,
resultinginapotentiallylowercostforZnO-baseddevices.ZnOnanostructurescan
begrowneitherinsolutionorfromgaseousphase.Inmostofthegaseousphase

methods,somespecialexperimentalconditionssuchashightemperature,electric
fieldorhighpressurearerequired.
To
overcomethesedrawbacks,thesolutionphase
methodsuchashydrothermalprocesshasgainedimmensepopularityduetoits
simplicityandtolerablegrowthconditionsaswellasitsgreenenvironmentprotection.
Mostimportantly,versatilepropertiesinelectrical,magneticandopticalaspects
reportedrenderZnO-basedmaterialspromisingforavarietyofpracticalapplications.
ZnOhasbeenwidelyusedasadditivesinrubberandconcreteindustry[12].In
addition,itssensitivitytovariousgasspecies,suchasethanol,carbonmonoxideand
acetylene,enablesitssensingapplications[13].
W
orthnotingisalsoitsstrong
piezoelectricpropertyresultingfromnon-centrosymmetricwurtzitestructure,which
3
makesitsuitableformechanicalactuatorsandpiezoelectricsensors[4].Lastbutnot
least,ZnOisbio-safeandthussuitableforbiomedicalapplicationssuchas
UV-blockerinsunlotion[4,14].
Particularly,thepromisingapplicationsofZnO-basedmaterialstospintronicsand
optoelectronicsdeviceshaveputZnOstudyintorenewedinterest.Themagneticand
opticalpropertiesofZnOrelatedwiththesepotentialapplicationsarethusfascinating
tobeexplored.Therelevantissuesinthisresearchareawillbedetailedlyreviewedin
therestofthischapter.
1.
1.
1.
1.2
2
2
2ZnO-based

ZnO-based
ZnO-based
ZnO-basedDiluted
Diluted
Diluted
DilutedMagnetic
Magnetic
Magnetic
MagneticSemiconductors
Semiconductors
Semiconductors
Semiconductors(DMSs)
(DMSs)
(DMSs)
(DMSs)
Thissectionstartswithabriefreviewofdevelopmentinspintronicsandprimarily
offersasummaryoftheknowledgeaboutanupsurgeofinterestinZnO-basedDMS
materials.Inaddition,areviewofliteraturepertinenttostudiesonroomtemperature
ferromagnetism(RTFM)inZnO-basedDMSsispresented.Furthermore,anin-depth
understandingofFMoriginisacriticalissueforfurtherdesignandfabricationof
practicalspintronicsdevices.Therefore,inthelatterpartofthissection,several
modelsproposedsofarregardingthenatureofFMorderinginZnO-basedDMSsare
introducedandevaluated.
1.2.1
1.2.1
1.2.1
1.2.1Review
Review
Review
Reviewof

of
of
ofFerromagnetism
Ferromagnetism
Ferromagnetism
Ferromagnetismin
in
in
inZnO-
ZnO-
ZnO-
ZnO-b
b
b
based
ased
ased
asedDMSs
DMSs
DMSs
DMSs
Electronshavetwoimportantfeatures:spinsandcharges.Thesuccessof
traditionalsemiconductorelectronicshasbeenestablishedonthechargedegreeof
freedomofelectronswhichusuallyignoredthespins.However,duetothecontinuous
4
shrinkingofworkingdimensionofsemiconductorsinrecentyears,aspin-dependent
interactionamongcarrierscannolongerbeignored.Inaddition,theadvancementin
semiconductorscienceandtechnologyenablesthecontrolandmanipulationofthe
spindegreeoffreedominsemiconductors[15,16].Thediscoveryofthegiant
magnetoresistance(GMR)byGrünbergandFertin1988[17,18],whichwasawarded

theNobelPrizeforPhysicsin2007,especiallyinitiatedasurgeinresearchinto
spin-electronics(spintronics)andguidedafirst-generationdevice.
A
specificfeature
ofspintronicsisthejointactionofchargeandspindegreesoffreedomofelectrons.
Onecanthusexpectthatspintronicstechnologymaypermitfabricationofnovel
deviceswithdualfunctionalitiesinthefuture-processinginformationandstoringdata
atthesametime.Comparedtoconventionalsemiconductordevices,another
advantageofspintronicsdevicesisthelowerpowerconsumptionandhigherdata
processingspeed.Thisisbecausetheflipofelectronspinsrespondsfastertoa
magneticfieldwithmuchlowerenergyconsumptionthanthedriftofelectroncharges
inresponsetoanelectricfield[16,19].
A
keyissuefortherealizationoffunctionalspintronicsdevicesliesinselectinga
rangeofsuitableandpromisingmaterials.Thereareseveralmajorcriteriafor
spintronicsmaterialsselection:(1)longspinlifetime,(2)highspininjection
efficiency,(3)compatiblewithexistingsemiconductormaterialsand(4)
ferromagneticorderingshouldberetainedatpracticaltemperatures(aboveroom
temperature).Ferromagneticsemiconductorispredictedtobeanidealchoice[20].
Thisisduetoitsgreatpotentialasasourceforspin-polarizedcarriers[21-23]and
5
capacitytointegratewithexistingsemiconductortechnology.Inaddition,electrical
spininjectioninaferromagneticsemiconductorheterostructurewasconfirmedby
Ohno
etal
[24].SincethefirstdemonstrationofferromagnetisminMn-dopedGaAs
[25],mostofthepastattentionwaspaidtothetransitionmetal(TM)elementsdoped
III-arsenideferromagneticsemiconductors,whichwereconsideredastheprototypical
DMSsforspintronicsdevices[26-28].Thesufficientlylongspinlifetimeand
coherencetimeinGaAs[29]andtheabilitytoachievespintransferthrougha

heterointerface[24,30]havebeendemonstrated.However,thereisanoticeablegap
thatreportedCurietemperatures(T
c
)fortheseconventionalDMSsystemsaretoolow
(sofarupto110K[31])tohavesignificanttechnologicalimpact.Therefore,
searchingforanewDMSmaterialwithhighT
c
becomesanurgentissue.
ThedevelopmentofthenewZnO-basedDMSswasmotivatedbythetheoretical
predictionbyDietl
etal
(2000)[20]whichclaimedthatZnO-basedDMSsshow
stableFMwithhighT
c
above300K.Soonafterthat,SatoandKatayama-Yoshida
et
al
[32]employedfirst-principlescalculationstodemonstratethathightemperature
FMcanbeachievedinTMelements(
V,
Cr,Fe,CoandNi-)dopedZnOandthus
boostedtheattemptstofabricateTM-dopedZnODMSs.Followingtheinitial
experimentalobservationofRTFMinCo-dopedZnObyUeda
etal
in2001[33],
consecutivetheoreticalandexperimentalreportshaverevealedthatTM-dopedZnO
exhibitedRTFM.Sofar,considerableamountofTM-dopedZnOsystemswithRTFM
havebeenobtained,suchasZnOdopedwithSc[34],Mn[35],Ti[34,36],V[37],Cr
[34,38],Fe[39],Co[40,41],Ni[42],Cu[43]andco-dopedwithCoFe[44]and
6

MnCo[45].Theserecentinvestigationshavefueledhopesthatthesematerialswill
indeedprovideafundamentalbasisforpracticalspintronicsdevices.However,afirst
keyissueinmanyofthepublishedreportsisthatitisdifficulttounambiguously
clarifythattheFMbehaviourisintrinsicratherthanextrinsic.Thereisalsoaschool
ofthoughtwhichevensuggeststhattheobservedFMisnotintrinsictoTM-doped
ZnO.Ithasbeenreportedthatprecipitates,clusteringorsecondaryphaseofdoped
TMelements[46,47],oxygenvacancies[48,49],Zninterstitials[50]orZnvacancies
[51]mightberesponsiblefortheobservedFM.Secondly,althoughaconsiderable
amountofexperimentaldatasupportingRTFMinTM-dopedZnOhasbeen
accumulated,severalgroupshavereportedthatTM-dopedZnOmaterialspossessFM
orderingwithamuchlowerT
c
,suchas83K[52]and110K[53]forMn-dopedZnO.
Inaddition,someevenhavereportedthatZnOfilmsdopedwithTMelements[39,54,
55]canonlyexhibitparamagnetismorsuperparamagnetism.Thesecontroversies
amongresearchgroupsmaylieinthefactthatthemagneticbehaviourofTM-doped
ZnOcouldbeinfluencedbymanyfactorsincludingmagneticdopants,microstructure,
preparationparameters,localstructureandelectronicstructure.
Furthermore,inrecentyears,RTFMhasbeenobservedinundopedZnO[56,57]
aswellasZnOdopedwithnonmagnetic/nontransitionelements(i.e.,ZnO:C,ZnO:Li,
ZnO:Cu,ZnO:Ga[58-61]).ItthusintentionallyexcludedanypossibilityofFM
arisingfromthepresenceofmagneticprecipitatesorsecondaryphases.Inlightof
thesediscoveries,itisgenerallyagreedthattheexactgrowthconditionsarecrucialin
determiningthemagneticpropertiesofZnO-basedsystem.ThehighsensitivityofFM