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

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16 CHAPTER 2. MOLECULAR BEAM EPITAXY (MBE) Figure 2.1: The relationship between the fundamental units encountered in vacuum technology [56]. � Source: MBE vacuum.jpg ture in ultra-high-vacuum (UHV). Total pressures of the residual gas in the reactor below p ≤ 1.33 × 10 −7 Pa (10 −9 Torr) are called UHV. The composition of the grown epilayer and its doping level is adjusted via the relative arrival rates of the different constituents by con- trolling the beam fluxes of the various sources. Usual growth rates are in the range of about 1.5 monolayer (ML) per second (i.e. ∼ 1.5 ˚A/s). This moderate growth rate ensures sufficient surface migration of the impinging particles and therefore provide a smooth surface (also depending on the substrate temperature TS; higher TS enhances surface mobility). With the help of simple mechanical shutters in front of each of the beam sources the whole growth procedure or just the deposition of single constituents or dopants can be started, stopped or interrupted; that guarantees sharp borders or changes in composition and doping on an atomic scale, at least as long as segregation effects are thermally suppressed. In order to preserve one of the major characteristic features of MBE-growth – that is the beam nature of the mass flow towards the substrate – it is an indispensible requirement that ultra-high vacuum conditions are ensured. There are another two parameters closely related to the pressure p of the residual gas in the growth-chamber, namely the mean free path lmfp and the concentration nconc of the gas molecules travelling through the volume towards the target-substrate. Mean free path is the average distance of a molecule between two successive collisions, whereas the concentration is simply the number of species per unit volume. Large values for lmfp are not only necessary to avoid high scattering rates (that would destroy the beam-like nature), but also to minimize atoms from the residual gas to
2.1. MBE-GROWTH 17 hit the substrate surface and get incorporated into the epilayer. These contaminants and impurities would accumulate lattice imperfections or unintentional impurity levels, and could therefore also cause problems in the sensitive crystallization-process. All this would end up in unpredictable epilayers of low quality and usefulness. In Fig. 2.1 [56] the relationship between fundamental units in vacuum technology are compared. 2.1.2 Physical Processes in the Growth-Chamber The growth process in a MBE-chamber can be divided into three zones, where different physical reactions take place [57]: 1. In the first zone the molecular beams are generated under UHV conditions from electron- beam evaporators (for Si, Ge, partly for C) and sources of the Knudsen-type effusion cells (mostly used for dopants in SiGeC-MBE-technology). The temperatures of the effusion-cells are accurately controlled using proportional-integral-derivative (PID) con- trollers together with thermocouples for a feedback loop to enable flux-stabilization better than ± 1% [58]. By choosing appropriate temperatures for the effusion-cells and the substrate, the desired structural composition of the epilayers can be realized. 2. The second zone in a MBE vacuum reactor is the ”mixing” region in which the different molecular beams – originated at various sources – interpenetrate each other and form a more or less uniform beam. In case of a large enough mean free path there should not occur many collisions between the different species of molecules traversing towards the substrate, and interactions among these particles should be negligible. 3. The actual epitaxial growth process takes place on the substrate surface that forms the third zone. Here is a series of surface processes that can be distinguished: (a) Adsorption of the incoming molecules or constituent atoms impinging on the substrate surface (b) Surface migration and diffusion on the substrate (c) Incorporation of the adsorbed species into the crystal lattice of the substrate or grown epilayers (d) Thermal desorption of atoms that have not been incorporated into the lattice In Fig. 2.2 [12] the most important surface processes are illustrated. Atoms or molecules arriving at the substrate surface have an energy corresponding to the
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: Chapter 2 Molecular Beam Epitaxy (M
Page 29 and 30: 2.1. MBE-GROWTH 19 Figure 2.3: The
Page 31 and 32: 2.2. CLEANING PROCEDURE 21 rameter
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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66 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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