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

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Theoretical Background of Semiconductor Nanostructures level can enhance material functions and characteristics, leading to considerable energy savings and environmental load reduction. However, according to this approach which is based on the monolithic integration of nanostructures semiconductor, such as (quantum dots QDs, quantum wells QWs, etc.) on silicon substrates. Nanostructures, such as quantum dots (QDs), can be fabricated with either a top-down technique or bottom-up technique. Top-down techniques are great for generating a uniform distribution of diameters. Unfortunately, top-down approaches like lithography are limited by the diraction limit and implies material damage and defects. The most common way to create a QD is through a bottom-up approach. This can be done either with metal-organic chemical vapor deposition (MOCVD) or MBE. Self-assembled quantum dots (SAQDs) nucleate spontaneously under certain conditions during MBE and metal-organic vapor phase epitaxy (MOVPE), when a material is grown on a substrate to which it is not lattice-matched. The resulting strain produces coherently strained islands on top of a two-dimensional wetting layer. This growth mode is known as Stranski-Krastanov (SK) growth (like InAs QDs on GaAs system). The islands can be subsequently buried to form the quantum dot. Therefore, the fundamental understanding of low-dimensional semiconductor physics and the electronic connement inside these structures is essential for the growth and characterization of these systems including their new structural and optical properties, etc. The next section will bring the focus on the underlaying physics, which explains these interesting nanostructures. 2.3 Low-Dimensional Semiconductor (Nanostrtuctures) With the advent of epitaxial growth techniques like molecular beam epitaxy (MBE), the realization of low-dimensional structures became feasible. However, when one of the three spatial dimensions of a bulk solid, like in semiconductor material, is of a size comparable to de Broglie wavelength (λ B ) (Eq. 2.1) of the electrons or smaller, this means we are dealing with a material or a structure of low-dimensionality (nanostructure) [18], which is referred as two-dimension 12
2.3 Low-Dimensional Semiconductor (Nanostrtuctures) (2D) system known as quantum well (QW), where the electrons are conned in one-dimension and the degree of freedom for this system in this case is two. λ B = h p = h m ∗ v (2.1) It is well known from quantum mechanics that for an electron of momentum p can be described by a wave function with a wavelength given by the de Broglie wavelength (λ B ). However, m ∗ is the electron eective mass. From solid state physics, we know that inside a semiconductor, electrons behave dynamically as if their mass was m ∗ , instead of the mass m 0 of the electron in vacuum. This observation is very important because for many interesting semiconductors, like GaAs, m ∗ is much smaller than m 0 , for example m ∗ GaAs is equal to 0.067 m 0 . Therefore, that the smaller the value of m ∗ , the easier will be to observe the size quantum eects in nanostructures. This is the case of semiconductors in comparison with most metals, for which the conduction electrons behave as quasi-free [18]. The connement of electrons and holes (charge carriers) in these nanostructures modies the electronic as well as the optical and vibrational properties of the material. Semiconductor QWs, where narrow-bandgap semiconductor material is sandwitched between dierent wide bandgap materials by means of hetrojunctions causes the electron connement in two-dimensions. This two-dimensional connement results in new optical properties that are dierent from bulk material (3D) structure. However, in semiconductors (λ B ) usually ranges between 10 and 100 nm and therefore we have to deal with solids of size in the nanometer range in order to observe quantum eects in nanostructures of a given size [19]. Therefore, low-dimensional materials are classied according to the number of spatial dimensions of nanometric size or the degree of electronic connement. Table 2.1 summarizes the dierent types of nanostructures and the degree of connement of each structure. If only one of the three-dimensions is small enough, then the structure is called a quantum well (2D). In the case of quantum wires (1D), two of the dimensions are in the nanometer range, and nally in the case that the three dimensions of the structure are comparable to (λ B ) the structure is called a quantum dot (0D). 13
Page 1: Molecular Beam Epitaxial Growth of
Page 4 and 5: Gutachter (Supervisors): 1. Prof. D
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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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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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