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

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Theoretical Background of Semiconductor Nanostructures discrete and the DOS consists of a series of sharp (delta-function-like) peaks. Eq. 2.6 describes the corresponding discrete eigenenergies of the electrons. Due to the nite life time of electronic states, the peaks are broadened and the DOS is a sum of Lorentzian functions. In real samples not all QDs are of the same size. Dierent sizes mean dierent eigenenergies. Accordingly, the electronic energy states of an ensemble of quantum dots are distributed around a mean energy related to the average QD size. In many applications, the active device material contains a large ensemble QDs. Their density of states then includes a statistical broadening characterized by a Gaussian function, this broadening is often called inhomogeneous broadening [21]. Being zero-dimensional, quantum dots have a sharper density of states than higher-dimensional structures. As a result, they have superior optical properties, as they are already in use by III-V based QDs laser diodes, ampliers, and biological sensors markets. 2.3.2 Carriers Connement in Semiconductor Nanostructures Quantum connement in low-dimensional heterostructures strongly modies the basic properties of a semiconductor crystal and the bandgap energy. In a quantum well (QW), carriers are spatially conned in the transverse direction and move freely in its plane. In a quantum wire (QWR), carriers are spatially conned in two transverse directions and move freely along the wire direction. Hence, the carrier energy spectra in both QWs and QWRs are continuous within wide sub-bands of allowed states. Fundamentally, in all cases, quantum connement pushes away the allowed energies eectively increasing the bandgap. The enlargement of the quantum conned bandgap increases as the nanoparticle size becomes smaller. It also increases as the characteristic dimensionality of the quantum connement increases (from 1D to 2D to 3D). Therefore, quantum connement may be used to tune the energy of the emitted light in nanoscale optical devices based on the nanoparticle size and shape. In a quantum dot (QD), carriers are three-dimensionally conned and the modication of electronic properties is most strongly pronounced; 16
2.3 Low-Dimensional Semiconductor (Nanostrtuctures) the energy levels are discrete. For this reason, QDs are also referred to as superatoms or articial atoms. A QD of typical size (several nanometers to several tens of nanometers) contains several ten thousands to several tens of million atoms. Quantum dots have generated much interest as a new class of human-made materials with tunable (by varying both the composition and size) energies of discrete atomic-like states [22]. 2.3.3 The Excitons Connement in Quantum Dots An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. Excitons are the main mechanism for light emission in semiconductors. An exciton can form when a photon is absorbed by a semiconductor. This excites an electron from the valence band into the conduction band. The Coulomb force in excitons act attractively between the two particles, negatively and positively charged, respectively, and a stable state can be formed between electron-hole pair. However, this attraction provides a stabilizing energy balance. Consequently, the exciton has slightly less energy than the unbound electron and hole. The binding energy is much smaller and the particle's size much larger than a hydrogen atom. This is because of both the screening of the Coulomb force by other electrons in the semiconductor ( i.e., its dielectric constant), and the small eective masses of the excited electron and hole [23, 24]. In general, excitons can be classied in two basic types: Wannier-Mott excitons also called free excitons, and Frenkel excitons which are tightly bound excitons. Wannier-Mott excitons are typically found in semiconductor crystals with small energy gaps and high dielectric constants, but have also been identied in liquids, such as liquid xenon [24]. In semiconductors, the dielectric constant is generally large. Consequently, electric eld screening tends to reduce the Coulomb interaction between electrons and holes. The result is a Wannier exciton, which has a radius larger than the lattice spacing. As a result, the eect of the lattice potential can be incorporated into the eective masses of the electron and hole. Likewise, because of the lower masses and the screened Coulomb interaction, the binding energy is usually much less than that of a hydrogen atom, typically on 17
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
Page 8 and 9: (MEE = migration enhance epitaxy) u
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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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