Molecular beam epitaxial growth of III-V semiconductor ... - KOBRA

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Heteroepitaxial Growth of III-V Semiconductor on Silicon Substrates Figure 3.7: Strained layer above the critical thickness. (a) Conned dislocations at the overlayer-substrate interface (desirable mode of epitaxy). (b) Penetrating dislocations in the overlayer structure (undesirable mode of epitaxy and harmful for optoelectronic devices) [31]. limited by kinetic barriers and dislocation-dislocation interactions [46, 47]. • Dislocations Confinement at the Interface : Experimentally, the point in growth where dislocations are generated is not so clear and depends upon the growth conditions, surface conditions, dislocation kinetics, etc. However, one may use the criteria given by Eq. 3.11 for loosely characterizing two regions of overlayer thickness for a given lattice mismatch. Below critical thickness, the overlayer grows without dislocations and the lm is under elastic strain. Under ideal conditions above critical thickness, the lm has a dislocation array, and after the dislocation array are generated, the overlayer grows without strain with its free lattice constant [48], and they could propagate in the overlayer structure as shown in Fig. 3.7(b). Mist and threading dislocation segments can be nucleated in a variety of ways in mismatched heteroepitaxy above the equilibrium critical thickness. At very high mist strain, mist dislocation loops can form spontaneously via homogeneous nucleation, although this mechanism rarely occurs in practical growth scenarios. Much more common is heterogeneous nucleation of dislocation loops at the interface and surface imperfections or at point defect in the epitaxial layer [14]. A great deal and challenge of 38
3.4 Challenges of Heteroepitaxial Growth of III-V on Silicon this work, has been done to study and ensure that the dislocations generated stay near the overlayer-substrate interface and do not propagate into the overlayer as shown in Fig. 3.7(a). 3.4.3 Large Thermal-Mismatch In lattice-mismatched heteroepitaxial layers, most of the mismatch may be accommodated by mist dislocations during growth, even if kinetic factors are important. Therefore, the grown layer will be nearly relaxed at the growth temperature. However, the strain measured at room temperature may be quite dierent if the epitaxial layer and substrate have dierent thermal expansion coecients like in the case of III-V epitaxy on silicon (see Table 3.1). Therefore, a thermal strain will be introduced during the cool down to room temperature. The thermal expansion coecient of a given material is related to the expansion or contraction of the crystal lattice as the material's temperature changes. Commonly, the lattice expands under increased temperature, and returns to its original dimensions as the temperature is reduced. Moreover, thermal cycling during device operation will result in a temperature dependence of the built-in strain [31]. At room temperature conditions, the mismatch between GaP and Si is 0.37 %, while at a typical MBE growth temperature of 550 ◦ C, the mismatch is increased to 0.48 %, due to the fact that the GaP with higher thermal expansion coecient will expands more than the Si substrate. This change in lattice mismatch increases the mist strain, reducing the critical thickness of the GaP epilayer at growth temperature, and must be taken into account [49]. On the other hand, the thermal expansion coecient of GaAs is almost 60 % larger than Si expansion coecient at room temperature. After the relaxation process at the growth temperature during MBE epitaxy, the GaAs layer will shrink much more than the Si substrate during the cooling down after the growth. Assuming that the mismatched GaAs layer has completely relaxed at the growth temperature, a tensile strain ɛ t will thus be developed during the cooling process in the layer, which will be proportional to the total change in temperature of the system: ɛ t = ∫ T T 0 [α S − α L ] dT (3.16) 39
Page 1: Molecular Beam Epitaxial Growth of
Page 4 and 5: Gutachter (Supervisors): 1. Prof. D
Page 6 and 7: patterns indicating a 2D smooth sur
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Page 12 and 13: CONTENTS 3.4.4 Planar Defects Assoc
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Page 16 and 17: Introduction thesis a detailed stud
Page 18 and 19: Introduction 1.1.2 Thesis Approach
Page 20 and 21: Introduction 1.2 Thesis Outline The
Page 22 and 23: Theoretical Background of Semicondu
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Page 64 and 65: Experimental Growth and Characteriz
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MBE Growth of Self-Assembled InAs a
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MBE Growth of InAs and InGaAs Quant
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Optimization of MEE and MBE growth
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Conclusions substrates, respectivel
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REFERENCES [8] J. M. Gerard, O. Cab
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REFERENCES [29] http://www.wmi.badw
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REFERENCES [49] M. R. Brenner: GaP/
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REFERENCES [66] Y. Ishida, T. Takah
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REFERENCES [84] A. Garcia, C. M. Ma
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REFERENCES [100] M. Felici, P. Gall
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REFERENCES [116] H. Usui, H. Yasuda
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REFERENCES [133] Grassman, T. J. Br
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Conferences Contributions: Tariq Al
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Nomenclature
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REFERENCES symbol Descriptions γ f
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like to thank all member of the Ins
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REFERENCES Erklärung Hiermit versi
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Molecular beam epitaxial growth of III-V semiconductor ... - KOBRA

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