Electronic, optical, and transport properties of widegap II-VI semiconductors

172 Pages·1997·5.8 MB·English

by  Li, Jianzhong

ELECTRONIC, OPTICAL,ANDTRANSPORT
PROPERTIESOFWEDEGAPII-VISEMICONDUCTORS
By
J1ANZHONG LI
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAFNPARTIALFULFILLMENT
OFTHEREQUIREMENTS FORTHEDEGREEOF
DOCTOR OFPHILOSOPHY
UNIVERSITY OFFLORIDA
1997
Dedicatedtomy belovedChinaandfamily
ACKNOWLEDGMENTS
Theauthorwouldliketothankhisthesisadvisor,ProfessorC.J.Stanton,forguidance
andencouragementthroughoutthecourseofthisthesis. Alsotheauthorwouldliketo
thankProfessorK.A.MuttalibforwholeheartedsupportduringhisyearsattheUniversity
ofFlorida.
Theauthorishonoredandgrateful tohaveProfessorsJohnR. Klauder, DavidH.
Reitze,andJosephH.Simmonsserveonhissupervisorycommittee.
Professors J. H. Simmons, R. M. Park and P. H. Holloway are instrumental to
thisthesisbyprovidingexperimentaldataanddiscussions. Dr. G.D. Sandersoffered
immeasurableassistancewheneverneeded. Dr. V.J.Sosagreatlyimprovedtheaccuracy
oftheXPS data.
The author appreciates a dissertation fellowship from the Threadgill family and
enjoyed every minute during his stay at the Mikroelektronik Centret (MIC) at the
Technical University ofDenmark.
Gratitudegoestothosefriendswhoplayedsoccerandbridgewiththeauthorduring
theseyears;tothoseofficemateswhosharedtheirthoughtswiththeauthor;andtothose
secretariesinthePhysicsoffices,inparticular,JulieSmithandCathyKnudsen,without
whose technical assistance it would have been impossible for the author to make it
through. SpecialthanksgototheGraduateSeminarandpizzas.
Lastbutnottheleast,theauthorwouldliketoextendhisappreciationtohisfamily's
unlimitedandinvaluableencouragementandsupport.
Ill
TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS
Hi
LISTOFTABLES vii
LISTOFFIGURES viii
ABSTRACT xjjj
PARTS
I INTRODUCTION 2
1. OVERVIEW 2
2. IEB-VIACOiMPOUNDSEMICONDUCTORS 8
ZnS 8
ZnSe 12
CdTe 13
II OPTICALPROPERTIESANDXPSSPECTRUMOFZnS 17
3. INTRODUCTION .' 17
4. THEORIES 19
X-rayPhotoemissionSpec—troscopy(XPS) 19
LinearOpticalProperties InterbankTransitionTheory 21
5. RESULTSANDDISCUSSION 24
6. CONCLUSIONS 35
III QUANTUMCONFINEMENTEFFECTSINCdTe 37
7. INTRODUCTION 37
8. THEORIES 44
TruncatedCrystalApproximation 44
EnvelopFunctionApproximation 47
HamiltonianModels 49
3D30x30k pTheory 51
ID2x2k pModel 52
IDWhite-ShamModel 52
IV
9. RESULTS 54
TruncatedCrystalApproximation 55
EnvelopFunctionApproximation 59
10. DISCUSSION 66
TruncatedCrystalApproximation 66
EnvelopFunctionApproximation 68
11. CONCLUSIONS 70
IV STATICSCREENINGANDMOBILITYINn-ZnSe 72
12. INTRODUCTION 72
13. BACKGROUND 75
14. THEORIES 79
ChemicalPotential 79
ElectronicMobilityduetoIonizedImpurityScatterings 82
15. RESULTSANDANALYSIS 86
SeriesA 87
SeriesBandC 101
ExperimentalResults 117
16. DISCUSSION 124
17. CONCLUSIONS 129
V CONCLUDINGREMARKS 132
18. SUMMARY 132
APPENDICES
A. EMPIRICAL30x30k-pTHEORY 135
ReducedSchrddingerEquation 135
BasisFunctions 136
MatrixElements 137
B. GILAT-RAUBENHEIMERSCHEME 141
C. GENERALIZEDDEBYETHEORY 144
Basics 144
ApplicationsofGeneralizedDebyeTheory 146
DegenerateElectronGasin3D 146
ClassicalElectronGas 147
V
SinglyChargedDonors 147
BIBLIOGRAPHY 148
BIOGRAPHICALSKETCH 155
VI
.
LIST OFTABLES
Table Page
1 CollectionofImportantParametersofZnS,ZnSe,andCdTe,, 9
2. k pResultsforZnSComparedwithExperimentalValues 24
3. 30x30k-pMethodInputParametersforZnS,ZnSe,andCdTe 25
4. QDSizeswith200meVDiscrepancybetweenCalculatedAbsorptionEdges
by30x30k pModelandApproximateBandModels 39
5. k-pResultsforCdTeComparedwithReportedValuesinLiterature 51
6. ResultsofCdTeQDsandCalibratedAbsorptionEdgesbyPotter 54
7. ZnSeMaterialParametersUsedintheCalculationsofPartIV 80
8. InputParametersUsedintheCalculationsofPartIV 87
9. DonorandAcceptorConcentrationsintheExperiments 117
10. Empirical30x30k-pTheoryBasisStates 137
11. LinearMomentumParametersfor30x30k pTheory 139
12. AntisymmetricPotentialParametersfor30x30k-pTheory 139
13. Spin-orbitalCouplingParametersfor30x30k-pTheory 140
vll
LIST OFFIGURES
Figure Page
1. UnitcelloftheFCClatticeofzincblendestructure 4
2. ConnectionsofthePresentDoctoralResearch 7
3. Schematicdiagramofthethin-filmMISIMstructureandbandedgediagramof
TFELDunderhighelectricfield. Indicatedarethemainprocesses: tunnel
emissionfrominterfacestates,impactexcitationofactivators,impact
ionizationofelectron-holepairs,andcaptureatarrivalattheanodeinterface.. 10
4. BandstructureofzincblendeZnSbytheempirical30x30k pmethod. ...26
5. DensityofstatesforzincblendeZnS,calculatedbyGRscheme 28
6. XPSspectrum(darkcurve)andtheoreticalDOScalculationofZnS 29
7. ConvolutedtheoreticalDOS(darkcurve)ofZnSwithaGaussiandistribution
ofFWHM=1.25eV 30
8. DielectricconstantsandreflectivityforzincblendeZnS.Realpartofthe
dielectricconstantpredominantlycontributestothereflectivity 32
9. Theoreticalreflectivitycomparedwithexperiment. Themeasurementcovers
upto9.5eVandthepeakat3.4eVisduetoFabry-Peroteffect 34
10. Comparisonofnon-parabolicmodelwith30x30k pmodelforconduction
band. Clearly,previousmodelisunsuitabletodescribecarrierenergieseven
at5%offTpoint 40
11. ComparisonofLuttingermodelwith30x30k pmodel. Asshown,
LuttingermodelisdeficientinpredictingcarrierenergiesoffTpoint 41
Vlll
12. MeasuredabsorptionedgeofCdTequantumdotscomparedwithcalculated
absorptionedge. Theblue-shifteffect,duetokineticconfinement,onthe
absorptionedgeisabout 1.0eVat24Aandslightlyoffsetbythered-shift
effectduetoCoulombinteraction 43
13. BandstructureofzincblendeCdTecomputedbytheempirical30x30k p
method 50
14.TruncatedCrystalApproximation(TCA)resultof30ACdTequantumfilm.
TheTCAmethodmapsthequantizedcrosssectionsoftheelectronicstructure
ontotheunaffectedplane. Shownarethefirstthreesubbandsfromeachband
inthebulk. Reflectedclearlyarethewarpingnatureofthevalencebandsand
theliftofthedoubledegeneracyatthetopofthevalencebands 56
15. PredictedbandedgesatsymmetricalpointsofCdTequantumcubebythe
TCAmethod. Noinversesquarebehaviorsarefoundandasymptotic
saturationofquantumconfinementeffectonthebandedgesispredicted. ... 57
16. PredictedtransitionenergiesofCdTequantumcubebytheTCAmethod. T
pointistheabsorptionedgeabove 16Aandthenreplacedbyanunidentified
pointinsomegeneraldirection 58
17. ExperimentaldatacomparedwithTCApredictionsforCdTequantumdots.
Theexperimentaldataarecorrectedbyconsideringsizedistributionand
tunnelingintofiniteglassbarriers. DiscrepancyatlargeLlimitisbecausethe
TCApredictionsareforQDsatzerotemperature 60
18. ComparisonbetweennumericalEFAandTCAresults. SolidlinesareEFA
calculationswith4meshpointsinfinitedifferencescheme. Insetshows ID
2x2k pmodel,whichisasymptoticallyparabolicforbothbands 62
19. ComparisonbetweennumericalEFAandTCAresults. SoiidlinesareforEFA
calculationswith20meshpoints. ComparedtoEFAresultswith4meshpoints
(Fig. 18)andTCAresults,noagreementsbetweenEFAresultswithdifferent
meshpointsandbetweenEFAandTCAresultshavebeenachieved 63
20. Analytical,numericalandTCAresultsoftheWhite-Shammodel. Straight
solidlinesareforviewingguide. Theinsetshowsthebandsaspredictedby
theWhite-Shammodel. Excellentagreementshavebeenachievedamongall
results 65
21. Derivativeoftheionizeddonorconcentrationwithrespecttothechemical
potentialforSeriesA.Itindicatesthescreeningcapabilityoftheionized
donors. Atlowtemperature,compensationincreasesthecapabilityasthe
filledstatesbecomepartiallyoccupied. Sharptransitionoccursat 26.5K.At
hightemperature,the{3factorkicksinandreducesthecapability 90
22. Derivativeofthefreeelectronconcentrationwithrespecttothechemical
potentialforSeriesA.Atlowtemperature,thermalexcitationincreasesthe
screeningcapabilityofelectronsforsystemsoflowcompensation. Forhighly
compensatedsystems,duetoMItransition,theyshowthebehaviorof
degenerateelectrons. The(3factoraffectsthescreeninginalltemperature
rangebutdominatesathightemperature 91
23.DebyeLength(inA)versustemperatureforSeriesA.Atthecritical
temperatureof 26.5K,MItransitionduetostaticscreeningoccursatlow
compensation. Thetransitiontemperaturedecreaseswiththeacceptor
concentrationandvanishesabovethecriticalacceptorconcentrationwhere
thesystembecomesmetallic. Belowthecriticaltemperature,screeningeffect
ispredominatedbycompensation. The(3factoreffectweaklyincreasesthe
Debyelengthinnon-transitionalregime 92
24. BindingenergyofdonorsversustemperatureforSeriesA.Belowthecritical
temperature,donorlevelisreducedasacceptorconcentrationincreases. Away
fromthetransitionregion,itincreasesduetothe(3factoreffect 94
25. IonizeddonorconcentrationversustemperatureforSeriesA.Atlow
compensation,thermalionizationofdonorsincreasestheionizedpopulation
exponentially. Increaseinacceptorconcentrationaccordinglycreatesthesame
amountofionizeddonorsatlowtemperature. Abovethecriticalacceptor
concentration,ionizeddonorconcentrationincreasesweaklywithtemperature.
Thereisslightchangeintheionizeddonorconcentrationabovethecritical
temperature 95
26.ChemicalpotentialasafunctionoftemperatureforSeriesA.MItransition
occursatacriticalacceptorconcentrationof5x1015cm~3forcompensated
systemsoratacriticaltemperatureof26.5Kforuncompensatedsystems.
Belowthecriticaltemperatureandthecriticalacceptorconcentration,
chemicalpotentialisfrozenaround-13meV.Athightemperature,the
chemicalpotentialdecreasestowardsthemiddleofthebandgap 97