A channeled ion energy loss study of the surfactant-mediated growth of Ge on Si(100)

222 Pages·1996·8.1 MB·English

by  Boshart, Mark Andrew, 1969-

ACHANNELEDIONENERGYLOSSSTUDY
OFTHESURFACTANT-MEDIATEDGROWTHOFGEONSI(100)
By
MARKANDREWBOSHART
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
1996
UNIVERSITYOFFLORIDALIBRARIES
ACKNOWLEDGEMENTS
Isimplycouldnothavemadeitthisfarwithoutthehelpandsupportofmany
people. Iwouldfirstliketothankmyparentswhohaveprovidedinfinitepatience
andunderstandingthroughout my life. Mygrandparentshavealso alwaysbeen
there andprovidedforme, and “BigPop” has always shown aninterest in my
research. Iwouldalsoliketothankmysisters,MaryEllenandDebbie, andmy
nephewJasonforlotsofgoodtimes.
Ofcourse,myadvisorLizSeiberlingdeservesspecialthanks. Heradviceand
guidancearethereasonwhythisdissertationhasbeenwritten. Shehasprovided
anexcellent workingenvironment, and has alwaysbeenextremely approachable
whendifficultiesarose. IhaveenjoyedworkingwithMarkGrant,whotaughtme
theropes,AllisonBailes,RickPiciullo,andJanaeAdams. AndrzejDygoinstilled
inmetheneedtobeaspreciseaspossible. RolandJohns,DonMcNeill,andBob
Litwinswerealsoextremelyhelpful.
Finally, Iwanttothankmywife, MarthaKosa,whohasbeenashoulderto
leanonformyentiregraduatecareer,especiallyduringthenumeroustimeswhen
IthoughtthatIcouldn’tgoon.
n
TABLEOFCONTENTS
page
ACKNOWLEDGEMENTS
ii
ABSTRACT vi
CHAPTERS
1 INTRODUCTION 1
ChanneledIonEnergyLoss 1
Surfactant-MediatedEpitaxialGrowth 2
Motivation 5
AdvantagesofUsingTransmissionIonChanneling 5
AdvantagesofUsingaMonteCarloSimulationofChanneling . . 5
AdvantagesofUsingEnergyDistributions 7
2 TRANSMISSIONIONCHANNELING 14
Equipment 14
MeVIonScattering 16
RutherfordBackscatteringSpectrometry 17
ChannelingandtheContinuumModel 27
YieldVersusTiltforAdsorbateLocation 30
3 ENERGYLOSSOFCHANNELEDMeVLIGHTIONSINSILICON 33
VandeGraaffCalibration 33
BeamtoSampleAlignment 35
HighVacuumScattering 35
AutoScan 36
BeamDirectionAlignment ... 37
GoniometerCalibrationandCoordinateTransformations.... 39
BeamtoGoniometerMisalignmentandSampleNormal ... 42
MonteCarloSimulationofChannelingandDataReduction 43
InteractionZones 45
ImpactParameterandIonDeflections 50
AdditionalFeatures 53
iii
SimulatedEnergyDistributions 54
ExperimentalEnergyDistributions 55
EnergyLossModels 59
Three-ComponentModel 61
Semi-ClassicalApproximation 62
MeanExcitationEnergyApproximation 67
ValenceElectrons 70
H+Nearthe(100)and(110)Axes 75
Experiment 76
ResultsandDiscussion 79
He+ Nearthe(100)and(110)Axes 100
Experiment 101
ResultsandDiscussion 103
4 DATAACQUISTIONANDREDUCTION Ill
BeamtoSampleAlignment Ill
TransmissionEnergyAxisCalibration 114
ExperimentalEnergyDistributions 118
LinearBackgroundFit 121
5 SITEDETERMINATIONUSINGENERGYDISTRIBUTIONS. . 124
SimulationofAdsorbateEnergyDistributions 124
ObtainingMonteCarloOutput 124
ObtainingTrialSiteCoordinates 125
OverlapofTrialPositionwithMonteCarloOutput 135
ComparisonofSimulationandExperiment 137
PositionsintheSurfacePlane 138
PositionsNormaltotheSurfacePlane 140
AdsorbateVibrations 141
SiteProgramTest: SbonSi(100) 142
Experiment 143
CalculationofSiteDependentEnergyDistributions 148
ResultsandDiscussion 153
6 SURFACTANT-MEDIATEDGROWTHOFGeONSi(100)... 166
ModificationstoSiteDetermination 166
GeandSbPositionsAfterSiteExchange 169
Experiment 171
ResultsandDiscussion 176
GeandSbintheEarlyStagesofSurfactant-MediatedGrowth . . 179
Experiment 181
ResultsandDiscussion 183
IV
7 CONCLUSIONS 190
ChanneledIonEnergyLoss 190
SiteDeterminationofSbonSi(100) 191
Surfactant-MediatedGrowthofGeonSi(100) 192
APPENDICES
A OXYGENBEAMIMPURITY 194
B PREPARATIONOFSILICONTHINWINDOWS 198
BoronDiffusion 198
ChemicalEtch 201
CleaningProcedure 201
REFERENCES 204
BIOGRAPHICALSKETCH 213
AbstractofDissertationPresentedtotheGraduateSchool
OftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegreeofDoctorofPhilosophy
ACHANNELEDIONENERGYLOSSSTUDY
OFTHESURFACTANT-MEDIATEDGROWTHOFGEONSI(100)
By
MarkAndrewBoshart
August1996
Chairman: LucyElizabethSeiberling
MajorDepartment: Physics
Transmission ion channeling is used to investigate the surfactant-mediated
growth ofGe on Si(100). The experimental scattered ion energy distributions
for this system are comparedwith the simulated distributions for several mod-
els. ThesimulateddistributionsareobtainedusingaMonteCarlosimulationof
channelingthatincorporateschanneledMeVlightionenergyloss. A trialadsor-
batesiteisthenoverlappedwiththeMonteCarlooutput(ionpositionandenergy
withinthechannelattheexitsurface).
Toobtainamodelforchanneledionenergyloss,theenergydistributionsfor
625keVH+and2.5MeVHe+ionsincidentonthin(~8000A)Sisinglecrystalsare
studied. Detailedangularscansaretakenalonglow-indexdirections. Scatteringis
fromanamorphousAulayer(~8A)onthebeam-exitsurface. Theexperimental
distributionsarereproducedbyaMonteCarlosimulationusingthesemi-classical
vi
approximationmodelforenergylosstocoreelectrons(scaledup12%)andthe2-
componentfree-electrongasmodelforenergylosstovalenceelectrons(scaleddown
6%andusingthesolid-statedensity).
AprogramforgeneratingsimulatedenergydistributionsfromtheMonteCarlo
outputwasdevelopedbystudyingSbonSi(100). TheSbsitefoundagreeswith
previousexperimentsonthissystem,amodifiedbridgesitewithabondlengthof
2.80db0.10Aandadistanceabovethebulk-extrapolatedsiliconsurfaceof1.63±
0.10A. TheSbvibrationsappearanisotropic.
Finally,thesurfactant-mediatedgrowthofthesystemGe/Sb/Si(100)isinves-
tigated. Coveragesof0.15monolayer(ML)and0.68MLofGedepositedatroom
temperature(RT)onSb-terminatedSi(100)arestudied,bothbeforeandafteran-
nealingat350°C.WefindthatRTdepositionofGeforbothcoveragesisconsistent
withamodeloflooselybondedGedimersadsorbedbetweenundisturbedSbdimer
rows. Afterannealing,weobservebulk-likeGeunderneathSbdimersfor0.68ML
Ge. For1MLGedepositedat350°,theenergydistributionsclearlyshowthatGe
isbulk-like,andthequantitativesitesintheliteratureareconsistentwiththedata.
Vll
CHAPTER
1
INTRODUCTION
ChanneledIonEnergyLoss
Thestoppingofenergetic ions in crystallinematerialsis knownto strongly
depend onthe directionofincidence. Inparticular, ionsmoving alongamajor
crystallographicaxislosetheirenergyataratethatcanbeaslowas50%orlessof
theso-callednormalorrandomvalue. Thisreducedstoppingisoneofthedominant
featuresofthechannelingphenomenon[Morg73],andhasfueledintenseinterestin
themechanismsofenergylossofchanneledions(i.e.,forimplantationapplications).
Incontrast, the (well-studiedin amorphoustargets) randomstoppingoccursfor
directionsfarfromanysignificantaxialorplanarchannels,wherethecrystallattice
appearstotheionbeamasarandom-likearrangementofatoms.
Thereducedstoppingofchanneledionsiseasilyunderstoodintermsofthe
impact-parameterdependenceoftheenergylossthatoccursinasingleion-atom
collision. Thechanneledionsmaintainlargedistancesfromtheatomrows and,
therefore,canonlymakesmallenergytransferstotheatomiccores. Similarly,the
contributionofvalence(outer-shell)electronstothestoppingisdiminished,astheir
densityinthecentralregionofthechannelisreduced.
Investigations ofenergy-loss processesfor ions interacting in single crystals
havefocusedprimarilyonmajoraxialandplanardirections[Satt65;Satt67;Dell72;
Eise75;Melv73;Cemb77;Carn78;Gehr85;Kita72;Dett74;Koma74;Dett75;Desa77;
Esbe78; Bure80; Craw83; Kuhr83; Khod83; Gras85; Asco86; Gold86; Nami88;
1
2
Mort91], as thelargest reductioninenergyis achievedwhen aligned withthese
highsymmetrydirections[Gemm74]. Theprimarymeansofanalysisoftheseearly
studieshasbeenthehighenergypeakorleadingedgevalueoftheobservedfinalion
energies(theenergydistributionoftheexitingions)[Kuma81]. Thisgivesanar-
rowviewoftheimpact-parameterdependenceoftheenergylossasessentiallyonly
impactparametersclosetothechannelradius(thebestchanneledions)axebeing
explored. Inaddition,suchcomparisonsareoftenambiguousduetotheinability
toresolvevariouscontributionstotheenergyloss[Belo78]. Further,anextraction
oftheleadingedgevaluefromexperimentaldistributionsissubjecttouncertainties
relatedtoenergystragglingaridenergyresolutionfactors[Stee81].
Amuchbettermethod is to modelindividual trajectories inthecrystalby
the Monte Carlotechnique, attemptingtofit thefullexperimentaldistributions
[Murt93; Dijk94], not just peak or leading edge values, allowing a study over
abroaderrangeofimpactparameters. Furthermore, bytiltingthecrystalaway
fromtheaxial(planar)direction,smallerimpactparametersplayanincreasingly
importantrole,andcomplexstructurescanbeobtainedintheenergydistributions
[Dygo94e], Theenergylossmodelthusproposedshouldbe validforimpact pa-
rametersthat spanthechannelradiusratherthanjust nearthe channel center.
AnaccuratedeterminationoftheenergylossprocessesforMeVlightionsforim-
pact parameters spanningthechannelcanthenbeemployedbytechniques that
involvechanneling.
Surfactant-MediatedEpitaxialGrowth
To understand the importance ofsurfactant-mediatedepitaxial growth, one
mustfirstunderstandtheimportanceofandproblemsassociatedwithstrainedlayer
3
superlattices. Strainedlayersuperlattices(suchasGe/Siheterostructures)areof
extremeinterest astheycanbeusedas toolsformodifyingthebandstructures
ofmaterials. However, >2% strain(lattice mismatch) is requiredfor significant
bandstructurealteration[Pear90]. Further,interfacesmustbelargelydefectfree,
thin(~100A),andofuniformcomposition. Suchsuperlatticescanthenbetaken
advantageofinlasers,bipolartransistorsandfieldeffecttransistorstotunelaser
emissionwavelengthsandtoimprovetransistorperformancebytheconfinementof
carriers[Pear90].
Togrowstrainedlayersuperlatticeswiththerequiredstrainanduniformityis
notaneasyfeat. Growingonematerialonanotherofcourseintroducesstrainas
thetwomaterialshavedifferent latticeconstants. Ifthesubstratefreeenergyis
largerthantheinterfaceplusgrownlayerfreeenergy,thenthiselasticstraincan,
ingeneral,beaccomodatedwithoutdefectformationtoacertainpoint,asittakes
energytocreatedislocations. Thus, ifthe thicknessofthegrownlayerissmall
enough,theenergyrequiredtorelievethestrainbydefectformationislargerthan
simplymaintainingtheelasticstrain. Thistwo-dimensionalgrowthisreferredtoas
Frank-vanderMerwegrowth. Whenacriticalthicknessisreached,defectswillbe
producedtorelievethestrain. Onewayofrelievingthestrainisforthematerialto
growinclusters,orislands,withitsnormallatticeconstant,ruiningtheuniformity
ofthelayer. Thistwo-dimensionalfollowedbythree-dimensionalgrowthiscalled
Stranski-Krastanovgrowth.
Molecularbeam epitaxy (MBE) has been extremely successful at achieving
two-dimensionalgrowthforupto4%latticemismatches,morethanadequatefor
theapplicationsnotedabove[Pear90]. However,defectproduction(islanding)limits