Kinetic and Strain-Induced Self-Organization of SiGe ...

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20 CHAPTER 2. MOLECULAR BEAM EPITAXY (MBE) Figure 2.5: Pseudomorphic epilayer (left) and partially plastically relaxed epilayer introducing misfit dislocations (right) [12]. � Source: MBE relaxation.jpg the substrate material (e.g. in growing metals on insulators). In contrast, the layer-by-layer (Frank-Van der Merve) growth mode happens in the opposite case, i.e. when the impinging atoms are more strongly bound to the substrate than to each other. In consequence, the first atoms to condense form a complete monolayer (ML) on the surface, which is covered with ongoing deposition with a slightly less bound second layer. For the case in which – during further deposition and an increasing number of layers – the binding energy shows a monotonic decrease towards the value of the bulk crystal of the deposit, this layer-by-layer mode is obtained (e.g. often observed in semiconductor on semiconductor growth). The Stranski-Krastanov, or layer plus island growth, is an intermediate case. When the first or a few monolayers (”wetting-layers”) have been formed subsequent layer growth is energetically unfavorable and islands form on top of these intermediate layers. The accumulation of strain energy with increasing epilayer thickness disturbs the monotonic decrease in binding energy that is necessary for a layer-by-layer growth. Therefore the epilayer morphology gets three- dimensional to enhance stress relaxation at expense of surface energy. [55] 2.1.4 Heteroepitaxy versus Homoepitaxy There are two main categories distinguished in epitaxy, namely homoepitaxy and heteroepitaxy. In homoepitaxy the grown crystal consists of one main compound that can be additionally doped. In heteroepitaxy, which is exploited for bandstructure engineering, the crystal, or parts (layers) of it, is a mixture of different compounds (e.g. Si/SiGe). In heteroepitaxy the lateral lattice parameters of the underlying substrate or epilayers are continued in pseudomorphic overgrowth (see Fig. 2.5 [12]). Due to deviating lattice parameters (e.g. aGe > aSi) the pseudomorphic epilayers contain tensile or compressive strain, which can have significant influence on growth. For pseudomorpic growth the most relevant pa-
2.2. CLEANING PROCEDURE 21 rameter is the critical thickness tc. This tc is an equilibrium parameter that defines the film thickness at which the strain relaxation by the generation of misfit dislocations begins [67, 68]. Films with thicknesses below tc cannot relax, because the elastic energy stored in the strained layer is lower than the energy associated with the local distortion around a misfit dislocation. For films thicker than tc it becomes energetically favorable for the system to form misfit dislocations in order to provide partial strain relaxation (see Fig. 2.5 [12]) of the film. Under non-equilibrium conditions there is another critical thickness parameter t ∗ c, that defines a metastable thickness range between tc and t ∗ c. In this parameter range the nucleation and propagation of misfit dislocations is kinetically suppressed [69, 70]. Growth temperature has a strong influence on the maximum available critical thickness t ∗ c, as strained layers of metastable thickness can partly relax during later heat treatment. In Fig. 2.6 [68] the theoret- ical critical thickness for equilibrium tc, and the experimental critical thickness t ∗ c measured with films grown by MBE at 550 ◦ C (metastable limit) on Si are shown for Si1−xGex [6]. 2.1.5 Relevant Temperatures in MBE Temperature affects all aspects of the epitaxial crystal quality. Surface diffusion, incorpora- tion and redistribution of impurities and lattice defects strongly depend on temperature. Crystal growth by MBE systems is a non-equilibrium process. Usually growth temperatures are far below the melting temperatures Tm of the different constituents. For Si-MBE substrate temperatures around 550 ◦ C are used. But there is a wide temperature range in which layer-structures are grown, depending on the intended device properties. To achieve high crystal quality and to keep the defect-density in the epilayers low, higher temperatures up to ∼ 750 ◦ C are used. To minimize diffusion and segregation of dopants even lower temperatures down to ∼ 300 ◦ C can be of practical interest. The substrate temperature influences the whole surface kinetics and controls interface roughness and surface morphology [56]. Sometimes high-temperature steps up to 1000 ◦ C and more are introduced to growth procedures to heal out crystal defects or to adjust doping profiles via diffusion. Also, after chemical cleaning procedures outside the MBE-chamber the substrates are annealed in a short-time annealing step at 900–1000 ◦ C inside the MBE-system to remove silicon-oxides. 2.2 Cleaning Procedure It is obvious that a clean substrate surface is essential to make high quality epitaxial growth possible. A clean and smooth surface on an atomic scale is the requirement for perfect
Page 1: J O H A N N E S K E P L E R U N I V
Page 4 and 5: ii PREFACE Kurzfassung Eine kinetis
Page 6 and 7: iv PREFACE
Page 8 and 9: vi CONTENTS 2.1.3 Growth-Modes . .
Page 10 and 11: viii CONTENTS B General Physical Da
Page 13 and 14: Chapter 1 Silicon-Germanium Materia
Page 15 and 16: 1.1. STRUCTURAL PROPERTIES 5 Figure
Page 17 and 18: 1.2. VICINAL SI(001) 7 Vicinal subs
Page 19 and 20: 1.3. ELECTRONIC PROPERTIES 9 Monoat
Page 21 and 22: 1.3. ELECTRONIC PROPERTIES 11 Silic
Page 23 and 24: 1.4. SILICON GERMANIUM ALLOYS (SI1
Page 25 and 26: Chapter 2 Molecular Beam Epitaxy (M
Page 27 and 28: 2.1. MBE-GROWTH 17 hit the substrat
Page 29: 2.1. MBE-GROWTH 19 Figure 2.3: The
Page 33 and 34: 2.2. CLEANING PROCEDURE 23 H2SO4 :
Page 35 and 36: 2.3. MBE-SYSTEM 25 Figure 2.7: All-
Page 37 and 38: 2.3. MBE-SYSTEM 27 Figure 2.9: Sche
Page 39 and 40: Chapter 3 Methods of Investigation
Page 41 and 42: 3.2. SCANNING ELECTRON MICROSCOPY (
Page 43 and 44: 3.3. ATOMIC FORCE MICROSCOPY (AFM)
Page 51: Part II Main Research 41
Page 54 and 55: 44 CHAPTER 4. SELF-ORGANIZED GROWTH
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70 CHAPTER 5. SELF-ORGANIZED GROWTH
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86 CHAPTER 6. P-MODULATION DOPING A
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100 CHAPTER 6. P-MODULATION DOPING
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Chapter 7 Transient-Enhanced Si Dif
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7.3. EXPERIMENTAL RESULTS 117 > 100
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7.3. EXPERIMENTAL RESULTS 119 Figur
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7.4. DETAILED CHARACTERIZATION AND
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7.4. DETAILED CHARACTERIZATION AND
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Chapter 8 Germanium Source Reconstr
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Figure 8.2: Elevated front-view of
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water lines. Figure 8.4: Photograph
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Figure 8.7: Part 2 of final constru
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Appendix A Calibration and Characte
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A.1. CALIBRATIONS 135 Figure A.1: X
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A.1. CALIBRATIONS 137 Figure A.3: N
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A.2. PHOTOLUMINESCENCE 139 Sample 1
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A.2. PHOTOLUMINESCENCE 141 Figure A
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A.2. PHOTOLUMINESCENCE 143 Figure A
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Appendix B General Physical Data B.
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Bibliography [1] Herbert Lichtenber
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BIBLIOGRAPHY 149 [42] M. B. Prince,
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BIBLIOGRAPHY 151 [85] J. Zhu, K. Br
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BIBLIOGRAPHY 153 [130] C. S. Peng,
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BIBLIOGRAPHY 155 [174] G. Medeiros-
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BIBLIOGRAPHY 157 [214] G. Bastard,
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Postface Curriculum Vitae ⋄ ∗ 1
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POSTFACE 161 � D. Gruber, D. Pach
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POSTFACE 163 Abbreviations AFM Atom
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Kinetic and Strain-Induced Self-Organization of SiGe ...

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