The effect of substrate orientation on the kinetics and thermodynamics of initial oxide-film growth on metals [Elektronische Ressource] / vorgelegt von Friederike Reichel
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The effect of substrate orientation on the kinetics and thermodynamics of initial oxide-film growth on metals [Elektronische Ressource] / vorgelegt von Friederike Reichel

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Max-Planck-Institut für Metallforschung Stuttgart The effect of substrate orientation on the kinetics and thermodynamics of initial oxide-film growth on metals Friederike Reichel Dissertation an der Universität Stuttgart Bericht Nr. 209 November 2007 The effect of substrate orientation on the kinetics and thermodynamics of initial oxide-film growth on metals Von der Fakultät Chemie der Universität Stuttgart zur Erlangung der Würde eines Doktors der Naturwissenschaften (Dr. rer. nat.) genehmigte Abhandlung vorgelegt von Friederike Reichel aus Aachen Hauptberichter: Prof. Dr. Ir. E. J. Mittemeijer Mitberichter: Prof. Dr. Dr. h. c. M. Rühle Prüfer: Prof. Dr. F. Aldinger Prüfungsvorsitzender: Prof. Dr. H. Bertagnolli Tag der Einreichung: 28.09.2007 Tag der mündlichen Prüfung: 19.11.2007 MAX-PLANCK-INSTITUT FÜR METALLFORSCHUNG STUTTGART INSTITUT FÜR METALLKUNDE DER UNIVERSITÄT STUTTGART Stuttgart 2007 Contents 1. General Introduction .............................................................................................7 1.1. The initial oxidation of bare metals......................................................................................... 8 1.1.1. Kinetics................................................................................................................................ 8 1.1.2.

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Max-Planck-Institut für Metallforschung
Stuttgart

The effect of substrate orientation on the kinetics and
thermodynamics of initial oxide-film growth on metals

Friederike Reichel
Dissertation
an der
Universität Stuttgart

Bericht Nr. 209
November 2007
The effect of substrate orientation on the kinetics and
thermodynamics of initial oxide-film growth on metals


Von der Fakultät Chemie der Universität Stuttgart
zur Erlangung der Würde eines Doktors der Naturwissenschaften (Dr. rer. nat.)
genehmigte Abhandlung


vorgelegt von

Friederike Reichel

aus Aachen

Hauptberichter: Prof. Dr. Ir. E. J. Mittemeijer
Mitberichter: Prof. Dr. Dr. h. c. M. Rühle
Prüfer: Prof. Dr. F. Aldinger
Prüfungsvorsitzender: Prof. Dr. H. Bertagnolli

Tag der Einreichung: 28.09.2007
Tag der mündlichen Prüfung: 19.11.2007



MAX-PLANCK-INSTITUT FÜR METALLFORSCHUNG STUTTGART
INSTITUT FÜR METALLKUNDE DER UNIVERSITÄT STUTTGART

Stuttgart 2007 Contents
1. General Introduction .............................................................................................7
1.1. The initial oxidation of bare metals......................................................................................... 8
1.1.1. Kinetics................................................................................................................................ 8
1.1.2. Thermodynamics................................................................................................................. 9
1.2. Thin oxide films on bare Al single-crystals.......................................................................... 10
1.2.1. Thermal oxidation of bare Al substrates ........................................................................... 10
1.2.2. Modifications of alumina ................................................................................................... 11
1.2.3. Applications of thin Al O films.......................................................................................... 11 2 3
1.3. Methods of characterization .................................................................................................. 12
1.3.1. Angle-Resolved X-ray Photoelectron Spectroscopy (AR-XPS)........................................ 12
1.3.2. Real-time In-situ Spectroscopic Ellipsometry (RISE)........................................................ 14
1.3.3. Low Energy Electron Diffraction (LEED)........................................................................... 15
1.3.4. High-Resolution Transmission Electron Microscopy (HR-TEM)....................................... 16
1.4. Outline...................................................................................................................................... 18
References...................................................................................................................................... 18
2. Thermodynamic model of oxide overgrowth on bare metals..........................23
2.1. Introduction ............................................................................................................................. 23
2.2. Theory ...................................................................................................................................... 25
2.2.1. Basis of the model............................................................................................................. 25
2.2.2. Interfacial energies............................................................................................................ 27
2.2.3. Misfit-dislocation energy ................................................................................................... 31
A) The Semi-infinite Overgrowth (SIO) approach ..................................................................... 31
B) The Large Dislocation Distance (LDD) approach................................................................. 35
C) The Extrapolation (EXTR) approach.................................................................................... 35
D) The First Approximation (APPR) approach.......................................................................... 36
E) The Ball approach ................................................................................................................ 37
F) The Volterra (VOLT) approach............................................................................................. 38
2.2.4. Minimization of γ ; numerical procedure .......................................................... 39
MM-Ox y
2.2.5. General remarks about the misfit-dislocation energy ....................................................... 40
2.3. Energetics of chromium-oxide films on chromium substrates.......................................... 40
2.3.1. Bulk Gibbs energies of the Cr O and Cr O cells................................................. 41
{ }23 23
2.3.2. Surface energies of the Cr O and Cr O cells...................................................... 43
{ }23 23
2.3.3. Interfacial energies of the Cr O and Cr O cells................................................... 46
{ }23 23
A) The interface energy of the crystalline-amorphous Cr −{Cr O } interface .................... 46 23
B) The interface energy of the crystalline-crystalline Cr − Cr O interface ...................... 47 23
C) Difference in interface energy of the crystalline and amorphous overgrowths .................... 57
2.4. Relative stabilities of amorphous and crystalline oxide films............................................ 59
2.4.1. Model predictions .............................................................................................................. 59
2.4.2. Experimental observations versus model predictions....................................................... 60
2.5. Conclusion ............................................................................................................................... 61
References ...................................................................................................................................... 63
3. The thermodynamic stability of amorphous oxide overgrowths on metals .. 65
3.1. Introduction.............................................................................................................................. 65
3.2. Theory and calculation ........................................................................................................... 67
3.2.1. Basics of the model........................................................................................................... 67
3.2.2. Bulk energy differences..................................................................................................... 69
3.2.3. Surface ener ............................................................................................... 70
3.2.4. Interface energy differences.............................................................................................. 71
A) The crystalline-amorphous interface energy ........................................................................ 71
B) The crystalline-crystalline interface energy .......................................................................... 72
3.3. Model predictions.................................................................................................................... 76
3.3.1. System specific details and results ................................................................................... 76
A) Al/Al O ................................................................................................................................. 76 2 3
B) Ni/NiO ................................................................................................................................... 79
C) Cu/CuO 81 2
D) Cr/Cr O. 81 2 3
E) Fe/FeO and Fe/Fe O ........................................................................................................... 82 3 4
F) Mg/MgO.......... 84
G) Zr/ZrO... 87 2
H) Ti/TiO.... 87 2
I) Si/SiO .................................................................................................................................... 88 2
3.3.2. Thermodynamic stability of amorphous oxide film on various metals............................... 89
3.4. Conclusions ............................................................................................................................. 92
Appendix 3.A. Estimation of the density of an amorphous oxide............................................. 94
Appendix 3.B. Estimation of the surface energies of the oxide overgrowths ......................... 95
3.B.1. Amorphous oxides ............................................................................................................ 95
3.B.1. Crystalline oxides.............................................................................................................. 97
Appendix 3.C. Enthalpy of mixing O in <M>.............................................................................. 101
References .................................................................................................................................... 102

4. The origin of high mismatch orientation relationships for ultra-thin oxide
overgrowths...........................................................................................................107
4.1. Introduction ........................................................................................................................... 107
4.2. Experimental............ 109
4.2.1. Material and surface preparation .................................................................................... 109
4.2.2. Oxidation ......................................................................................................................... 109
4.2.3. AR-XPS analysis and quantification ............................................................................... 110
4.2.4. HR-TEM sample preparation and analysis ..................................................................... 111
4.3. Thermodynamics of oxide overgrowths on metals; summary of theoretical background
........................................................................................................................................................ 112
4.4. Experimental results and discussion ................................................................................. 114
4.4.1. The oxidized Al{111} substrate ....................................................................................... 114
4.4.2. The oxidized Al{100} substrate 118
4.5. Experiment versus model predictions................................................................................ 124
4.6. Conclusions........................................................................................................................... 128
Appendix 4.A. Electron-radiation-induced changes of the oxide-film microstructure......... 129
References.................................................................................................................................... 131
5. The amorphous-to-crystalline transition for oxide overgrowths on Al
substrates ..............................................................................................................133
5.1. Introduction............. 133
5.2. Experimental details ............................................................................................................. 135
5.3. AR-XPS spectral reconstruction and quantification ......................................................... 137
5.3.1. Resolving the metallic Al 2p PZL intensity of the bare metal.......................................... 137
5.3.2. Resolving the metallic and oxidic Al 2p PZL intensities of the oxidized metal ............... 139
5.3.3. Resolving the O 1s PZL intensities of the oxidized metal............................................... 141
5.3.4. Quantification .................................................................................................................. 141
5.4. Oxide-film microstructure before and after annealing...................................................... 143
5.4.2. Oxide films on Al{111}..................................................................................................... 145
5.4.3. Oxide films on Al{100}... 147
5.4.4. Oxide films on Al{110}... 148
5.5. Discussion: the stability of amorphous oxide films.......................................................... 150
5.5.1. Amorphous oxide films on Al{111}.................................................................................. 150
5.5.2. Amorphous oxi{100} 152
5.5.3. Amorphous oxide films on Al{110} 153
5.6. Conclusions........................................................................................................................... 155
Appendix 5.A. Procedures for the AR-XPS quantification....................................................... 156

5.A.1. Expressions for the PZL photoelectron intensities ......................................................... 156
5.A.2. Calculation of oxide-film composition and thickness ...................................................... 158
Appendix 5.B. Calculation of effective depths of resolved species ....................................... 161
References .................................................................................................................................... 161
6. The effect of substrate orientation on the kinetics of ultra-thin oxide-film
growth on Al single-crystals................................................................................ 163
6.1. Introduction............................................................................................................................ 164
6.2. Experiment ............................................................................................................................. 166
6.3. Data evaluation...................................................................................................................... 166
6.4. Model description of the oxide-film growth kinetics ......................................................... 169
6.4.1. Theoretical background................................................................................................... 169
6.4.2. Application to oxide-film growth on Al ............................................................................. 171
6.5. Results and Discussion........................................................................................................ 174
6.5.1. Formation of a closed oxide film ..................................................................................... 174
6.5.2. Oxidation-induced reconstruction of the Al{110} face ..................................................... 177
6.5.3. Oxide-film growth kinetics ............................................................................................... 179
6.5.4. Oxide-film growth mechanisms ....................................................................................... 182
6.6. Conclusions ........................................................................................................................... 186
References .................................................................................................................................... 188
7. Summary ........................................................................................................... 191
8. Zusammenfassung in deutscher Sprache...................................................... 201
Symbols and abbreviations ................................................................................. 211
List of publications............................................................................................... 215
Danksagung .......................................................................................................... 217
Curriculum Vitae ................................................................................................... 219

Chapter 1
General Introduction
y
The chemical reaction of oxygen gas with a solid metal surface ( x⋅+M ⋅O M O ), x y22
leading to the growth of a thin (i.e. thickness < 10 nm) oxide film on top of the metal, is of
great scientific and technological interest, because thermally grown oxide films are employed
in many application areas such as catalysis [1-3], microelectronics [4-6] and surface coatings
for enhanced wear and corrosion resistance [7, 8]. The chemical and physical properties of
such oxide films (e.g. their electronic and thermal conductivity, chemical and mechanical
stability, corrosion resistance, adhesion properties, as well as its friction and wear resistance)
will depend on their microstructure, which comprises e.g. the chemical composition,
morphology, state-of-stress and crystallographic and defect structure of the oxide film.
Evidently, the microstructure of a thermally grown oxide film, in turn, depends on the growth
conditions such as the temperature (T), the partial pressure of oxygen ( p ), as well as the O2
cleanliness, roughness and crystallographic orientation of the parent metal substrate surface
[9].
To date, the technological potential to optimize and control the chemical and physical
properties of thin oxide films by tailoring their microstructure is still limited by a lack of
fundamental and comprehensive knowledge on the kinetics and thermodynamics of thin oxide
growth as function of the growth conditions [10]. For example, previous kinetic studies on the
thermal oxidation of bare metals surfaces have mainly addressed the empirical relationships
between the oxide-film growth rate, the developing oxide-film microstructure and the
oxidation conditions, but often failed to identify the governing mechanism(s) and rate-limiting
(or rate-determining) step(s) of the oxidation process. Further, only very recently, the first
thermodynamic studies on the microstructural evolution of the initial oxide overgrowth on a
bare metal have been reported, which not only acknowledge, but also strive to account for the
crucial role of surface and interfaces in such thin film systems [11].
This thesis addresses in particular the effect of substrate orientation on the kinetics and
thermodynamics of initial oxide overgrowth on bare (i.e. without native oxide film) single-
crystalline metals. To this end, the microstructural evolution of ultra-thin (< 5 nm) oxide
'8 Ch apter 1
overgrowths on different bare metals (Al, Ni, Cu, Cr, Fe, Mg, Zr and Ti) was calculated on a
thermodynamic basis as function of the metal-substrate orientation and the oxide growth
temperature, while accounting for the important role of surface and interface energetics in
such thin film systems (see Chapters 2 to 4). Experimental verification of the thermodynamic
model predictions for initial oxide overgrowth on Al{111}, Al{110} and Al{100} metal
surfaces was performed using a combined analytical approach by angle-resolved X-ray
Photoelectron Spectroscopy (AR-XPS), Low Energy Electron Diffraction (LEED) and High-
Resolution Transmission Electron Microscopy (HR-TEM) (see Chapters 4 and 5). Therefore,
-4bare Al single-crystalline substrates were exposed to pure oxygen gas at a p of 1×10 Pa in O2
the temperature range from 350 K to 700 K in an especially designed Ultra-High Vacuum
(UHV) system for specimen preparation, processing and in-situ analysis. At the same time,
the kinetics of oxide film growth on the bare Al{111}, Al{110} and Al{100} metal surfaces
were established by Real-time In-situ Spectroscopic Ellipsometry (RISE). Finally, the
governing growth mechanism and rate-limiting steps of the oxidation process of Al single-
crystalline substrates were revealed by modelling the oxide-film growth kinetics as function
of the growth conditions (see Chapter 6).
1.1. The initial oxidation of bare metals
1.1.1. Kinetics
The initial formation of a closed oxide film covering the entire metal surface involves a series
of concurrent steps, such as transport and subsequent physisorption of oxygen molecules to
the metal surface, (dissociative) chemisorption, oxide nucleation and growth. After formation
of a closed oxide film on the metal surface, further oxide-film growth is decelerated, because
the initial oxide film provides a diffusion barrier between the two reactants (i.e. parent metal
substrate and oxygen gas). It follows that continued oxide-film growth can only proceed if
(charged) reactant species (as, possibly, cations, anions, electrons, holes and vacancies) are
transported through the developing oxide film towards the reacting oxide/gas and/or
metal/oxide interfaces.
To describe the observed oxidation rates as function of the oxidation conditions, it is
usually sufficient to consider only the rate-determining steps of the oxidation process, which
correspond to those processes required to accurately describe the observed oxide-film growth
kinetics. If transport of a single charged species through the growing oxide film is much
slower than that of all the others, it is said to be the rate-limiting step of the oxidation process.