Nucleation and stacking faults on the Iridium (111) surface [Elektronische Ressource] = Nukleation und Stapelfehler auf der Iridium-(III)-Oberfläche / vorgelegt von Carsten Busse

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• Physik

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• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2003
• Nombre de lectures: 9
• Langue: English
• Poids de l'ouvrage: 5 Mo

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Nucleation and stacking-faults on the
Iridium (111) surface
(Nukleation und Stapelfehler auf der Iridium (111)
Oberfla¨che)
Von der Fakult¨at fur¨ Mathematik, Informatik und
Naturwissenschaften der Rheinisch-Westf¨alischen Tech-
nischen Hochschule Aachen zur Erlangung des akademis-
chen Grades eines Doktors der Naturwissenschaften
genehmigte Dissertation
vorgelegt von
Diplom-Physiker Carsten Busse
aus Olpe
Berichter: apl. Prof. Dr. rer. nat. Thomas Michely
apl. Prof. Dr. rer. nat. Hans Paul Bonzel
Tag der mundlic¨ hen Prufung:¨ 1. August 2003
Diese Dissertation ist auf den Internetseiten der Hochschulbib-
liothek online verfu¨gbar.Angefertigt im I. Physikalischen Institut der RWTH Aachen.
Abgabe der Arbeit: 21. Mai 2003.
Prufungsk¨ ommission:
Prof. Thomas Michely (Betreuer und Berichter)
Prof. Hans Paul Bonzel (Berichter)
Prof. Uwe Kreibig (Vorsitzender)
Prof. Walter Selke
Datum der Prufung:¨ 1. August 2003.
2Abstract
By analyzing the saturation island density during nucleation in homoepitax-
ial growth on Ir(111) using scanning tunneling microscopy (STM), param-
eters of adatom diffusion could be determined. Furthermore, for the first
time also the diffusion barrier for the dimer was determined by this method.
These parameters are in perfect agreement with the respective results from
filed ion microscopy (FIM).
In extension of these measurements, an important quantity in surface
science, namely, the binding energy of an adsorbed dimer, was determined.
ComparingthesystemsIr(111), Al(111), andPt(111)showsthatthebinding
energyscaleswiththecohesiveenergy(E = (0.11±0.01)E ),asexpectedb,2 coh
in a simple model of nearest-neighbor bonds. In addition it was shown,
that the binding energies determined by FIM are in error due to neglected
processes at step edges.
Theformationofstacking-faultswasobservedinthesystemIr/Ir(111)un-
derawiderangeofdepositionparameters. Aquantitativemodelcanexplain
the observations: Stacking-faults form out of small clusters that can occupy
faulted sites with significant probability in thermal equilibrium. Metastable
areas in the wrong stacking sequence then grow out of these clusters by suf-
ficiently fast addition of adatoms.
Uponfurthergrowth, islandsinthesamestackingcoalesce, butislandsin
different stacking sequences do not. In the latter case, atoms can, however,
move to the energetically favorable, regular stacking via a kink-flip process
(self-healing). In the ideal case this leads to a complete disappearance of the
wrong stacking and a defect-free film evolves. This effect can be observed in
situ by annealing experiments.
The electronic structure of the two phases is very different, as can be
shown using voltage-dependent STM. Theoretical calculations support the
respective experiments.Contents
1 Introduction 11
2 Background 15
2.1 Structure of the fcc(111) surface . . . . . . . . . . . . . . . . . 15
2.2 Atomic processes . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.3 Nucleation of adatom islands. . . . . . . . . . . . . . . . . . . 25
3 Experimental 31
3.1 The vacuum system . . . . . . . . . . . . . . . . . . . . . . . 31
3.2 The sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.3 The evaporator . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.4 The scanning tunneling microscope . . . . . . . . . . . . . . . 43
3.5 Experimental procedure . . . . . . . . . . . . . . . . . . . . . 44
4 Nucleation on Ir(111) 47
4.1 Experimental results . . . . . . . . . . . . . . . . . . . . . . . 48
4.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.3 Experimental results on other fcc(111) surfaces. . . . . . . . . 55
4.4 Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.5 Dissociation on terrace vs. dissociation at steps . . . . . . . . 58
4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
5 Stacking-fault islands on Ir(111) 65
5.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.2 Experimental results . . . . . . . . . . . . . . . . . . . . . . . 67
5.3 Model of stacking-fault formation . . . . . . . . . . . . . . . . 70
5.4 Mean-field approach . . . . . . . . . . . . . . . . . . . . . . . 83
5.5 Discussion and conclusion . . . . . . . . . . . . . . . . . . . . 88
6 Self-healing of stacking-faults 91
6.1 Completion of the first monolayer . . . . . . . . . . . . . . . . 91
5Contents
6.2 Atomic resolution of the fcc and the hcp phase . . . . . . . . 93
6.3 Coalescence and assimilation . . . . . . . . . . . . . . . . . . . 94
6.4 Atomic processes during self-healing . . . . . . . . . . . . . . 98
6.5 Self-healing upon annealing . . . . . . . . . . . . . . . . . . . 109
6.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
7 Electronic structure of the fcc and the hcp phase 117
7.1 Experimental results . . . . . . . . . . . . . . . . . . . . . . . 117
7.2 Density functional calculations . . . . . . . . . . . . . . . . . . 118
7.3 Comparison between experiment and theory . . . . . . . . . . 121
7.4 Height of decoration rows . . . . . . . . . . . . . . . . . . . . 123
7.5 Discussion and conclusion . . . . . . . . . . . . . . . . . . . . 124
8 Summary and outlook 125
8.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
8.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
Bibliography 129
A Technical 135
A.1 Calculating the evaporation rate . . . . . . . . . . . . . . . . 135
A.2 Pyrometer measurements . . . . . . . . . . . . . . . . . . . . 138
A.3 Measuring the ion flux . . . . . . . . . . . . . . . . . . . . . . 139
A.4 STM-arcing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
B Publications 141
C Curriculum vitae (Lebenslauf) 143
D German summary (Deutsche Kurzfassung) 145
E Acknowledgements (Danksagungen) 147
6Contents
Frequently used symbols
Symbols
˚a nearest-neighbor distance, a (Ir)=2.71 Ann nn
D diffusion coefficient
D prefactor0
D diffusion coefficient for clusters of size ii
sD coefficient for of size i in stacking si
d jump width of atomic process jj
E binding energyb
E of the dimerb,2
aE binding energy of dimer assuming ν =νb,2 0,diss 0,1
bE of dimer ν =k T/hb,2 0,diss B
∗
∗E binding energy of critical nucleus ib,i
E cohesive energy of a crystalcoh
E energy barrier in atomic processd
fE effective energy barrier for movement from fcc to hcpd
hE effective energy barrier for movement from hcp to fccd
E energy barrier of atomic process jd,j
E difference between initial and final configuration ofif
atomic process
E kinetic energy barrier for atomic processkb
E energy barrier per bond in nn-modelk−nn
E nearest-neighbor bond energy in nn-modelnn
F deposition rate
F free energy of cluster of size ii
fF free energy of of size i in fcc stackingi
hF free energy of cluster of size i in hcp stackingi
h Planck’s constant
h distance between consecutive (111) layers in fcclayer
i size of cluster
I tunneling current
∗i critical nucleus
†i largest mobile cluster
I filament current in Ir-evaporatorfil
k Boltzmann’s constantB
n cluster density
n coordination number in final configuration of atomic processfin
n number density of regular areasfcc
7Contents
Symbols
n number density of stacking-fault areashcp
n density of clusters of size ii
sny of clusters of size i in stacking si
n coordinationnumberininitialconfigurationofatomicprocessin
n density of clusters of size i>xx
∗p vapor pressure
P background pressure during evaporationevap
P probability of finding a cluster in fcc stackingfcc
P proy of a in hcp stackinghcp
R distance between evaporator and sample
r effective radius of filament in Ir-evaporatoreff
R resistance of filament in Ir-evaporatorfil
s stacking, s=f (fcc), h (hcp)
0 0s stacking f (fcc), h (hcp), s =s
S entropy in adsorption position of atomic processa
S entropy in saddle position of atomic processs
T temperature
T temperature of depositiondep
T temperature of filament in Ir-evaporatorfil
T melting temperaturemelt
T onset temperature of atomic processonset
T sample temperaturesample
T temperature while scanningSTM
U voltage drop across filament in Ir-evaporatorfil
U tunneling voltage with respect to the tip
z height of tip above surface
Γ dissociation rate of cluster of size ii
ΔE difference in binding energy between regular fcc crystal and1ML
crystal with first ML in stacking-fault geometry
ΔE difference in binding energy for cluster of size i in fcc and hcpb,i
stacking
ΔF difference in free energy for cluster of size ii
Δh apparent height difference between fcc and hcp phase
ΔS differenceinentropyforclusterofsizeiinfccandhcpstackingb,i
θ layer coverage in first layer1
Θ coverage
Θ coverage in fcc stackingfcc
Θ coverage in hcp stackinghcp
ν overall effective rate of atomic processes
8
6Contents
Symbols
ν effective attempt frequency for atomic process0
0ν attempt frequency of half-step of atomic process0
ν at frequency of dimer dissociation0,diss
ν attempt frequency of atomic process j0,j
fν at frequency of half-step of atomic process leading fro