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

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Heteroepitaxial Growth of III-V Semiconductor on Silicon Substrates hand, epi-layers with lattices constant larger than the substrate lattice constant (f< 0) will undergo compressive strain. In heteroepitaxial systems with a low amount of mismatch (|f|< 1), the initial growth tends to be coherent, or pseudomorphic as illustrated in Fig. 3.5(a). Therefore, a pseudomorphic (full strained) layer exhibits an in-plane strain (ɛ || ) equal to the lattice mismatch (f) as described in Eq. 3.9 [31]. ɛ || = f (P seudomorphic) (3.9) The residual strain in highly strained materials is generally a function of the mismatch and layer thickness. However, the strain can be calculated based on a thermodynamic model, as long as the growth occurs near thermal equilibrium. However, there are kinetic barriers to the lattice relaxation. These are associated with the generation and movement of dislocations. Kinetic models have been devised to explain and predict the lattice relaxation behavior in these situations [31]. These models predict that the residual strain in the layer will depend on the growth conditions and post growth thermal annealing (PGA) applied after the growth, as well as the mismatch and layer thickness. However, with higher amount of mismatch, or thicker epitaxial layers, the mist strain will increase until it exceeds the elastic strength of the coherent semiconductor-substrate bonds. At this point, the lm will undergo a plastic deformation resulting in the formation of broken bonds and non-coherent growth fashion with mist dislocations at the epitaxial lm-substrate interface as shown in Fig. 3.5(b). This mist dislocation is associated with an extra half-plane of atoms in the substrate. The pseudomorphic layer matches the substrate lattice constant in the plane of the interface and therefore experiences in-plane biaxial compression. Beyond the critical layer thickness, therefore, part of the mismatch is accommodated by mist dislocations (plastic strain) and the balance by elastic strain. In this case, using the denition for the mismatch adopted here, the in-plane strain (ɛ || ) is expressed as in Eq. 3.10. ɛ || = f − δ (P artially relaxed layer) (3.10) Where δ is the lattice relaxation. In the pseudomorphic layer, for which no lattice relaxation has occurred (δ=0), the result will be a fully strained system as 34
3.4 Challenges of Heteroepitaxial Growth of III-V on Silicon Figure 3.5: Growth of heteroepitaxial layer on a mismatched substrate (a) pseudomorphic (fully strained) layer below the critical thickness (b) Partially relaxed layer above the critical thickness with mist dislocations generation in red color. Figure modied according to reference [31]. discribed in Eq. 3.9. The introduction of crystal dislocations and other defects is an important aspect of lattice-mismatched heteroepitaxy. The mist dislocations located at the heterointerface will degrade the performance of any device whose operation depends on it. On the other hand, any device fabricated in the heteroepitaxial layer will tend to be compromised by the presence of threading dislocations in this layer. The threading dislocations are associated with the mis- t dislocations and are introduced during the relaxation process. Whereas the mist dislocations are expected to be present in partially relaxed layers under the condition of thermal equilibrium, threading dislocations are nonequilibrium defects [42]. Threading dislocation are 1D crystallographic structures, they act as non-radiative recombination centers and their presence is not welcome for III-V heteroepitaxial structures on Si. There are important dierences between low-mismatch and high-mismatch heteroepitaxial systems, which are not simply a matter of degree. The actual mechanisms of strain relaxation and defect introduction have been found to be dierent. This is due, at least in part, to the three-dimensional nucleation mode of highly mismatched heteroepitaxial layers. It is often expected that a heteroepitaxial layer will take on the same crystal orientation as its substrate. In practice, both pseudomorphic and partly relaxed layers often exhibit small misorientations 35
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
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Page 22 and 23: Theoretical Background of Semicondu
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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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