Spectromicroscopic characterisation of the formation of complex interfaces [Elektronische Ressource] / Florian C. Maier. Betreuer: Eberhard Umbach

Florian C. Maier

Sujets

• Physik

Informations

• Publié par: julius-maximilians-universitat_wurzburg
• Publié le: 01 janvier 2011
• Nombre de lectures: 22
• Langue: English
• Poids de l'ouvrage: 25 Mo

Extrait

Spectromicroscopic characterisation
of the formation of complex
interfaces
Dissertation zur Erlangung des
naturwissenschaftlichen Doktorgrades
der Julius–Maximilians Universität Würzburg
vorgelegt von
Florian C. Maier
aus Nürnberg
Berlin, Würzburg 2010Eingereicht am: 29. Oktober 2010
bei der Fakultät für Physik und Astronomie
1. Gutachter – Prof. Dr. Eberhard Umbach
2.hter – Priv.-Doz. Dr. Jörg Schäfer
der Dissertation.
1. Prüfer – Prof. Dr. Eberhard Umbach
2. – Priv.-Doz. Dr. Jörg Schäfer
3. Prüfer – Dr. Reinhold F. Fink
im Promotionskolloquium.
Tag des Promotionskolloquiums: 09. Dezember 2010
Doktorurkunde ausgehändigt am ...Contents
Summary 1
Zusammenfassung 5
1 Introduction 9
2 Experimental Methodology 13
2.1 Spectro–microscopeSMART . . . . . . . . . . . . . . . . . . . . . 13
2.1.1 Instrumental setup of the microscope . . . . . . . . . . . . . 14
2.1.2 Modes of operation . . . . . . . . . . . . . . . . . . . . . . . 16
2.2 Principles of applicable methods . . . . . . . . . . . . . . . . . . . . 18
2.2.1 Electron Diffraction . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.2 Spectroscopy . . . . . . . . . . . . . . . . . . . . . 20
2.2.3 Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.3 Synchrotron radiation source . . . . . . . . . . . . . . . . . . . . . . 34
2.3.1 BESSY II – Soft X–ray source UE49PGMc . . . . . . . . . . 34
2.3.2 High flux–density by demagnified beam . . . . . . . . . . . . 36
2.3.3 Ultrahigh flux–density in 3D-space . . . . . . . . . . . . . . 37
2.4 Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . . 38
3 Low T growth of PTCDA/Ag(111) 41
3.1 Introduction to PTCDA on Ag(111) . . . . . . . . . . . . . . . . . . 42
3.1.1 Ag(111) specimen preparation . . . . . . . . . . . . . . . . . 42
3.1.2 PTCDA growth conditions . . . . . . . . . . . . . . . . . . . 45
3.2 PTCDA growth mode transition . . . . . . . . . . . . . . . . . . . . 45
3.2.1 PTCDA growth below RT . . . . . . . . . . . . . . . . . . . 45
3.2.2 Growth mode transitions . . . . . . . . . . . . . . . . . . . . 46
st nd
3.3 Growth behaviour of the 1 and 2 layer . . . . . . . . . . . . . . 50
st
3.3.1 Substrate morphology determines 1 layer growth . . . . . . 52
nd
3.3.2 2 layer– growth limitations, shape and kind . . . . . . . . 55
3.3.3 Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.4 Second layer PTCDA ripple–phase . . . . . . . . . . . . . . . . . . 64
iContents
nd
3.4.1 Several phases in the 2 layer . . . . . . . . . . . . . . . . . 64
3.4.2 Temperature range . . . . . . . . . . . . . . . . . . . . . . . 64
3.4.3 Growth behaviour of the ripple phase . . . . . . . . . . . . . 67
3.4.4 Commensurate ripple phase . . . . . . . . . . . . . . . . . . 71
3.4.5 Model to explain the ripple phase . . . . . . . . . . . . . . . 75
3.4.6 Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4 CdSe(Te)/ZnSe Quantum dots 87
4.1 The MBE grown sample stack . . . . . . . . . . . . . . . . . . . . . 90
4.1.1 Stack composition . . . . . . . . . . . . . . . . . . . . . . . . 90
4.1.2 Transport precautions . . . . . . . . . . . . . . . . . . . . . 91
4.1.3 Contamination evaluation . . . . . . . . . . . . . . . . . . . 91
4.1.4 α–Te desorption . . . . . . . . . . . . . . . . . . . . . . . . . 94
4.2 Inhomogeneous distribution and order of cap–Te . . . . . . . . . . . 95
4.2.1 Structural investigation . . . . . . . . . . . . . . . . . . . . . 95
4.2.2 Nano–spectroscopy of the α–Te cap . . . . . . . . . . . . . . 98
4.2.3 Complementary AFM data reveal topography . . . . . . . . 119
4.2.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
4.3 Cap and substrate structure influence QD formation . . . . . . . . . 122
4.3.1 Real–time observation of QD formation . . . . . . . . . . . . 123
4.3.2 Dots form between holes and Te–crystallites . . . . . . . . . 126
4.3.3 Correlation of real space with LEED–structure . . . . . . . . 131
4.3.4 First spectromicroscopic results of the CdSe/ZnSe QD surface 134
4.4 Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . 140
A Acronyms 145
B Data acquisition, intensity and analysis 149
B.1 Intensity Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . 149
B.1.1 XPS intensity depends on objective focus . . . . . . . . . . . 149
B.1.2 Background removal for quantitative analysis . . . . . . . . 152
B.2 Scaling and accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . 154
B.2.1 Time scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
B.2.2 Temperature measurement and scale . . . . . . . . . . . . . 155
B.2.3 Length scale — real space . . . . . . . . . . . . . . . . . . . 155
B.3 Electron bombardment and C contamination . . . . . . . . . . . . . 156
B.4 Information content of images . . . . . . . . . . . . . . . . . . . . . 159
B.4.1 Common information content . . . . . . . . . . . . . . . . . 159
B.4.2 How to read the XPEEM stack figures . . . . . . . . . . . . 159
Bibliography 161
iiContents
List of Figures 173
List of Tables 175
iiiivSummary
Spectromicroscopic characterisation of the formation of complex
interfaces
Within the framework of this thesis the mechanisms of growth and reorganisa-
tion were investigated that are the basis for the fabrication of high quality thin
films and interfaces. The majority of the measurements was performed with the
recently developed low energy electron spectromicroscope SMART [1], the first
double–aberration corrected instrument of its kind [2]. Comprehensive methods
(LEEM/PEEM, μ–LEED, μ–XPS) integrated in this system were utilised to study
in–situ and in real time the formation processes on surfaces and to determine the
morphology, local structure and local chemical composition of the resulting thin film.
Complementarily, a commercial AFM [3] was used ex–situ to get a direct measure
of the morphology and the absolute height of surface objects. XPEEM and μ–XPS
measurements were made possible by attaching SMART to a high flux density
beamline of the soft–X–ray source BESSY–II [4] which included the development of
proper alignment strategies.
Depending on the application, stacked homogeneous layers or the controlled forma-
tion of semiconductor nanostructures are desired. Such quantum structures may
offer new properties as, e.g., the trapping of carriers for enhanced emission or even
show size dependent quantum effects.
Two quite different model systems were chosen to study details of the growth and
reorganisation process of thin films and to demonstrate the power of the aberration
corrected spectromicroscope at the same time. Here the measurements benefit
especially from the enhanced transmission of the microscope and also from its
improved resolution.
PTCDA/Ag(111) – Growth and structure of the first two layers
Although PTCDA/Ag(111) is one of the most intensely studied model systems for
the growth of organic semiconductor thin films, it still offers new insights into a
complex growth behaviour. Hence the presented studies enlighten the temperature
dependant influence of morphological features as small as monatomic Ag steps on
1Summary
the growth process of the first two layers.
At sufficiently low temperatures of the substrate, single steps act as diffusion barriers
for the migrating PTCDA molecules in the first layer. This barrier is reduced as
soon as the Ag is covered by PTCDA, which allows interdiffusion between adjacent
Ag terraces. Nevertheless domain boundaries in the first PTCDA–layers persist as
boundaries for crystallite growth in the second layer. This leads to different growth
regimes in the second layer.
The first and the common second layer grow differently in respect to the expanding
domains. Whereas the first layer islands are more compact, the more dendritic
development of the second layer indicates a reduced interaction strength between
nd st
2 and 1 layer.
These findings are explained by two effects: First, the reduced substrate – layer
interaction in case of second layer molecules allows enhanced diffusion, which is
also observed across former barriers. Second, the structural difference between
neighbouring domains in the first layer prevents the overgrowth by single coherent
nd
2 layer domains.
The second part of the PTCDA study reveals a variety of phases that appears
if only two layers are deposited. Besides the six known, rotational domains of the
interfacesystemPTCDA/Ag(111)[5], afurthermanifoldofstructureswasdiscovered,
which was not reported before. Besides a surprising striped image contrast, the
second layer also grows in an elongated way along the so–called ’ripples’. The latter
show a rather large period of 40 nm and were found in a temperature range between
210 and 280 K. Additionally the μ–LEED pattern of such a domain shows a new
super–superstructure as well.
This phase is explained by a structural model that introduces a rotated, more relaxed
domain in the second layer that does not exist in the first layer. Its structural
parameters are similar to those of the bulk unitcells of PTCDA.
This approach for the stacking is confirmed by the obse