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

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Heteroepitaxial Growth of III-V Semiconductor on Silicon Substrates 3.2.2 Basics Physical Processes in MBE Understanding kinetics, thermodynamics and how they interact and compete with each other would enable us to know how to control the growth of thin lm, which is of great important to all modern semiconductor technologies. However, in MBE growth, the beams from dierent sources intersect each other at the substrate surface, where the crystallization processes take place. A series of surface processes take place during MBE growth which are schematically summarized in Fig. 3.1. The surface processes occurring during MBE growth are characterized by a set of relevant kinetic parameters that describe them quantitatively. The arrival rate is described by the ux of the arriving species and gives the number of atoms impinging on the unit area of the surface per second. Evaporated atoms with temperature T i onto substrate surface, which has temperature T s , usually lower than T i at dierent positions are with dierent kinetics energies. Depending on the atom energy and the position at which it hits the substrate surface, the impinging atom could re-evaporate immediately, carrying with it an energy corresponding to temperature T e or exchange energy with atoms of the substrate at T s . A description of this process is possible by dening the thermal accommodation coecient (α)as [33]: α = T i − T e T i − T s (3.1) When T e equal to T s the accommodation coecient is unity. Thus, it is emerges as a measure of the extent to which the arriving atoms reach thermal equilibrium with the substrate. It is important to distinguish between the accommodation coecient (α) and the sticking coecient (S c ). The sticking coecient is dened as the ratio of the number of atoms adsorb (N ads ) or stick to the substrate surface, to the total number of atoms (N tot ) that impinge upon substrate surface during the same period of time, and mathematically expressed as below [33]: S c = N ads N tot (3.2) 24
3.2 The Concept of Epitaxy In many case S c is less than unity and it may be a small fraction in cases when the adsorption energy of atoms on the substrate is low, or the substrate temperature is high. Assuming α is unity, all the arriving atoms are accommodated on the substrate surface and achieve thermodynamics equilibrium. This does not mean, that they will stay there permanently. They still have a nite probability related to the substrate temperature of acquiring sucient energy to overcome the attractive forces and leave the substrate [33]. If aggregation of adatoms, does not occur, all the adatoms will eventually be re-evaporated. Thus, the S c can be almost zero even when α is unity. There are two main types of adsorption that can occur during MBE. The rst is physical adsorption, which refers to the case where there is no electron transfer between adsorbate and adsorbent by forming a van der Waal's bond with a surface atom "physisorption". The second is is resulted by forming a covalent or ionic bond with a surface atom, which refers to the case when electron transfer, i.e., chemical reaction, take places between adsorbate and adsorbent "chemisorption" [33]. The molecules stick to the surface via weak physisorption, which enables the migration. As the physisorbed state is only a precursor state, there is no way back from the chemisorbed state to the much weaker physisorbed state or even re-evaporation into the reactor vacuum. The rate at which adatoms adsorbed to the surface can be described by an exponential law (Eq. 3.3): R ads ∝ ν a exp −E ads K B T (3.3) Where ν a is the adsorption frequency, E ads describes the necessary energy to overcome the electrostatics potential, K B is Boltzmann constant and T is the substrate temperature [33]. However, most substrates have complicated reconstructions and the bonding is highly directional. Therefore, the probability of adsorption to some sites is higher than other. Assuming defect-free surfaces, a number of theoretical and experimental works has been carried out to nd the most stable adsorption sites. Adsorbed atoms may diuse from one site to another via thermally activated hopping , which can be expressed by Eq. 3.4: D ∝ a 2 K s ∝ a 2 exp −E d K B T (3.4) 25
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
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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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