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4 Introduction Figure 1.1: Schematic ALILE process. The layer stack before (left) and after (right) the layer exchange are shown. The oxide interface layer remains in position during the process. layers can be used as templates for subsequent epitaxial thickening at low temper- atures. To do so a continuous smooth layer of crystallized silicon is needed. In 1998 O. Nast et al. [9] reported on a process which is now known as aluminum- induced layer exchange process (ALILE). In this process an aluminum and amor- phous silicon bi-layer on a glass substrate exchange their position with a con- current crystallization of the silicon. The resulting polycrystalline silicon layer is smooth, continuous and features large grains (> 10 µm) with a preferential (100) orientation. Thus ALILE layers are eligible as a template (seed layer) for subsequent low temperature epitaxy. The aluminum-induced layer exchange process (ALILE) is shown schematically in Fig. 1.1. The initial glass/Al/oxide/a-Si layer stack (left side) is transformed into a glass/poly-Si/oxide/Al(+Si) layer stack (right side). The initial a-Si needs to be slightly thicker than the initial Al layer in order to form a continuous poly-Si layer. The Si excess remains in the final Al layer and forms c-Si islands. The interlayer between the initial Al and a-Si layer plays a crucial role in the process. Usually a native Al oxide layer is formed on top of the Al prior to a-Si deposition. This oxide layer remains in position and separates top and bottom layer throughout the layer exchange process. The ALILE process is the topic of this work. In order to optimize the poly-Si seed layers formed in the ALILE process a profound understanding of the driving forces and mechanisms in the process is required. The role of the initial aluminum amor-
phous silicon interface layer is examined and compared to the influence of the annealing temperature. Limits of the resulting nucleation density and grain size are investigated and the origin of a self-limited nucleation suppression within the ALILE process is elucidated. The influence of the interlayer and the annealing temperature on the resulting orientation of the film is studied. The possibilities of non-isothermal annealing to optimize the process results are tested. The obtained results are discussed against the background of thermodynamics and existing mod- els. A possible explanation for the self-limited suppression of nucleation and the formation of the preferential (100) orientation is suggested. And a route to an optimized poly-Si film made by the ALILE process is proposed. The thesis is divided into following chapters: Chapter 2: ’State of the art’ links this thesis within the context of existing research. An overview is given introducing the history of research on crystallization in general and the background of solid phase crystallization, metal-induced crystallization and the aluminum-induced layer exchange process in particular. Chapter 3: ’Experimental’ lists the sample preparation parameters. Oxidation and annealing processes are depicted. The characterization methods are explained including important evaluation methods. Chapter 4: ’Results’ shows the influence of the two main parameters under inves- tigation within this thesis. A thin membrane between aluminum and silicon is crucial for the layer exchange process. Here the influence of altering this membrane is investigated. Naturally, the annealing temperature determines the process char- acteristics strongly. Non isothermal annealing is investigated for improving the process and revealing the mechanism behind the layer exchange. Chapter 5: ’Discussion’ elucidates the findings of chapter 4. The parameters influence on nucleation and growth of the Si grains is discussed. A model is suggested which explains the origin of the self-limited nucleation suppression and the preferential (100) orientation. Chapter 6: ’Conclusion’ summarizes the most important results and closes with 5
Page 1: Nucleation and growth during the fo
Page 5: "Our ignorance is not so vast as ou
Page 9 and 10: Zusammenfassung Dünne Schichten au
Page 11: • Temperatureprofile mit hoher En
Page 14 and 15: 3.2 In-situ optical microscopy . .
Page 17 and 18: 1 Introduction ’Solar power is ho
Page 19: por deposition (PECVD) achieved par
Page 23 and 24: 2 State of the art This chapter giv
Page 25 and 26: 2.1 Kinetics of phase change 9 Figu
Page 27 and 28: 2.1 Kinetics of phase change 11 [e
Page 29 and 30: 2.2 Solid Phase Crystallization (SP
Page 31 and 32: 2.2 Solid Phase Crystallization (SP
Page 33 and 34: 2.3 Metal-Induced Crystallization (
Page 39 and 40: 2.4 Aluminum-Induced Layer Exchange
Page 45 and 46: 2.5 Current research involving ALIL
Page 53 and 54: 3 Experimental This thesis is focus
Page 55 and 56: 3.1 Sample preparation 39 does not
Page 57 and 58: 3.1 Sample preparation 41 � �
Page 59 and 60: 3.2 In-situ optical microscopy 43 m
Page 61 and 62: 3.2 In-situ optical microscopy 45 B
Page 63 and 64: 3.3 Electron Back Scatter Diffracti
Page 65 and 66: 4 Results In order to understand an
Page 67 and 68: 4.1 Interlayer 51 (a) (b) Figure 4.
Page 69 and 70: 4.1 Interlayer 53 Figure 4.2: TEM c
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4.1 Interlayer 55 c o u n ts [a .u
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4.1 Interlayer 57 R C [% ] 1 0 0 8
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4.1 Interlayer 59 � � ��
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4.1 Interlayer 61 � � ��
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4.1 Interlayer 63 -2 ] N G [m m 8 6
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4.1 Interlayer 65 TA = 450 °C TA =
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4.1 Interlayer 67 (111) (100) (110)
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4.1 Interlayer 69 4.1.2 Molybdenum
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4.1 Interlayer 71 Fig. 4.17 optical
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4.1 Interlayer 73 R C [% ] 1 0 0 8
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4.2 Temperature profiles 75 face la
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4.2 Temperature profiles 77 � �
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4.2 Temperature profiles 79 R C [%
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4.2 Temperature profiles 81 a small
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4.2 Temperature profiles 83 � �
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4.2 Temperature profiles 85 (a) (b)
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4.2 Temperature profiles 91 ��
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4.2 Temperature profiles 93 in. Thu
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4.2 Temperature profiles 95 ��
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5 Discussion Two of the most import
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5.1 Definition of three crucial Si
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5.2 The ALILE process in the phase
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5.2 The ALILE process in the phase
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5.3 Depletion regions in the ALILE
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5.4 Comparison of the qualitative m
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5.5 Preferential orientation model
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6 Conclusions In the aluminum-induc
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A Preferential orientation calculat
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The area of the base of the pyramid
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B Abbreviations, symbols and units
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symbol unit meaning ∆G a eV activ
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Bibliography [1] M. Rogol, S. Doi,
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[20] R.W. Bower, R. Baron, J.W. May
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[38] J. Jang, J.Y. Oh, S.K. Kim, Y.
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[58] N.-P. Harder, T. Puzzer, P. Wi
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[72] G. Ekanayake, S. Summers, and
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[87] H.R. Philipp and E.A. Taft. An
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list of publications journal papers
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B. Rau, J. Klein, J. Schneider, E.
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(2002), 87. S. Gall, M. Muske, I. S
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tion. Special thank’s go to Dr. M
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10/95-09/98 student electrical engi
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