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126 Discussion Δ G * [e V ] 2 0 1 5 1 0 5 2 0 0 1 5 0 1 0 0 5 0 0 -4 5 0 4 5 0 �� α�� i* [a to m s ] Figure 5.13: Calculated nucleation Gibbs energy ∆G ∗ (open circle) and critical cluster size i ∗ (solid circle) as a function of the tilt angle α from (100) towards (110) and (111) orientation. With this model the temperature dependence of the preferential orientation can be understood. The rates of thermally activated processes are higher for lower than for higher activation energies at lower temperatures. Thus at low annealing temperatures mainly (100) oriented clusters are formed. At higher annealing temperatures the nucleation rates for clusters with tilted orientations increases and cluster with other orientations stabilize as well. An alternative way to visualize this is referred to as nucleation attempt model. The critical clusters are formed from statistical agglomerations of atoms. When the initial, subcritical cluster is formed the tilt angle is already determined. Thus in Fig. 5.12 the position at the angle dependant axis is already determined and will not change throughout the attempt of the cluster to reach the critical size. The subcritical cluster agglomerates atoms and attempts to overcome the energy barrier at the maximum of ∆G. Especially if the (thermal) energy of the system is low it is much more likely for the subcritical cluster to overcome the energy barrier close to the (100) orientation. Using this model an idea for the interlayer dependance of the orientation of the resulting layer can be given with a little more information on aluminum oxide.
5.5 Preferential orientation model 127 Native oxide is amorphous but at temperatures above 450 °C a so called γ − Al2O3 phase is formed which is crystalline [100]. This structural change of the oxide layer much rather than the change in thickness is suggested to be the origin of the shift in preferential orientation with thermal oxide interlayers. Keeping the nucleation attempt model in mind, the amorphous oxide does probably not influence the number of attempts of any tilt angle. But the crystalline interface can offer defined bonds for the initial, subcritical cluster. And even though it is still a statistical process the number of nucleation attempts of a certain cluster orientation can be increased. The temperature dependance is not changed, but especially at high temperatures when the clusters of all types of orientations reach the critical cluster size the number of attempts can determine the final orientation of the film. This suggests that the thermal oxide (γAl2O3) increases the number of nucleation attempts close to the (111) orientation. In this chapter the layer exchange process is discussed. It is shown that the Si concentration within the Al is crucial for nucleation and growth. Below the saturation concentration CS neither nucleation nor growth of existing grains is possible. At concentrations larger than CS but below the critical concentration C ∗ grains grow but no new nuclei are formed. Only when C ∗ is exceeded nucleation is observed. The higher the concentration the higher the growth and nucleation rate. But the concentration is shown to be limited to a maximum concentration Cmax. The self- limited suppression of nucleation is elucidated for the ALILE process by showing the time dependence of the Si concentration within an Al/Si phase diagram in- cluding the above defined concentrations. Evaluation of the experimental results against the background of the model demonstrates the validity of the presented considerations in very simple qualitative terms. Furthermore a model for the origin of the preferential orientation elucidating both the temperature and interlayer dependence of the resulting poly-Si film is suggested. The formation of preferen- tially shaped clusters aligned at preferential nucleation sites is suggested to result in the observed preferential orientation.
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Nucleation and growth during the fo
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"Our ignorance is not so vast as ou
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Zusammenfassung Dünne Schichten au
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• Temperatureprofile mit hoher En
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3.2 In-situ optical microscopy . .
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1 Introduction ’Solar power is ho
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por deposition (PECVD) achieved par
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phous silicon interface layer is ex
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2 State of the art This chapter giv
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2.1 Kinetics of phase change 9 Figu
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2.1 Kinetics of phase change 11 [e
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2.2 Solid Phase Crystallization (SP
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2.2 Solid Phase Crystallization (SP
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2.3 Metal-Induced Crystallization (
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2.4 Aluminum-Induced Layer Exchange
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2.5 Current research involving ALIL
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3 Experimental This thesis is focus
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3.1 Sample preparation 39 does not
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3.1 Sample preparation 41 � �
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3.2 In-situ optical microscopy 43 m
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3.2 In-situ optical microscopy 45 B
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3.3 Electron Back Scatter Diffracti
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4 Results In order to understand an
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4.1 Interlayer 51 (a) (b) Figure 4.
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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
Page 91 and 92: 4.2 Temperature profiles 75 face la
Page 93 and 94: 4.2 Temperature profiles 77 � �
Page 95 and 96: 4.2 Temperature profiles 79 R C [%
Page 97 and 98: 4.2 Temperature profiles 81 a small
Page 99 and 100: 4.2 Temperature profiles 83 � �
Page 101 and 102: 4.2 Temperature profiles 85 (a) (b)
Page 107 and 108: 4.2 Temperature profiles 91 ��
Page 109 and 110: 4.2 Temperature profiles 93 in. Thu
Page 111 and 112: 4.2 Temperature profiles 95 ��
Page 113 and 114: 5 Discussion Two of the most import
Page 115 and 116: 5.1 Definition of three crucial Si
Page 125 and 126: 5.2 The ALILE process in the phase
Page 127 and 128: 5.2 The ALILE process in the phase
Page 129 and 130: 5.3 Depletion regions in the ALILE
Page 131 and 132: 5.4 Comparison of the qualitative m
Page 137 and 138: 5.5 Preferential orientation model
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Page 145 and 146: 6 Conclusions In the aluminum-induc
Page 147 and 148: A Preferential orientation calculat
Page 149 and 150: The area of the base of the pyramid
Page 151 and 152: B Abbreviations, symbols and units
Page 153 and 154: symbol unit meaning ∆G a eV activ
Page 155 and 156: Bibliography [1] M. Rogol, S. Doi,
Page 157 and 158: [20] R.W. Bower, R. Baron, J.W. May
Page 159 and 160: [38] J. Jang, J.Y. Oh, S.K. Kim, Y.
Page 161 and 162: [58] N.-P. Harder, T. Puzzer, P. Wi
Page 163 and 164: [72] G. Ekanayake, S. Summers, and
Page 165 and 166: [87] H.R. Philipp and E.A. Taft. An
Page 167 and 168: list of publications journal papers
Page 169 and 170: B. Rau, J. Klein, J. Schneider, E.
Page 171: (2002), 87. S. Gall, M. Muske, I. S
Page 174 and 175: tion. Special thank’s go to Dr. M
Page 176: 10/95-09/98 student electrical engi
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