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

More documents

Recommendations

Info

Theoretical Background of Semiconductor Nanostructures the order of 0.01 eV . In contrast, Frenkel excitons are found in materials with a small dielectric constant, the Coulomb interaction between an electron and a hole may be strong and the excitons thus tend to be small, of the same order as the size of the unit cell. Molecular excitons may even be entirely located on the same molecule. These Frenkel excitons has a typical binding energy on the order of 0.1 to 1 eV and found mostly in insulator and molecular crystals [23]. They are also other exciton types like surface excitons, where the hole is inside the solid and the electron is in the vacuum. These electron-hole pairs (surface-excitons) can only move along the surface. Alternatively, an exciton may be thought of as an excited state of an atom, ion, or molecule, the excitation wandering from one cell of the lattice to another this later type called atomic or molecular excitons. An electron is said to be found in the lowest unoccupied orbital and an electron hole in the highest occupied molecular orbital, and since they are found within the same molecular orbital manifold, the electron-hole state is said to be bound. Molecular excitons typically have characteristic lifetimes on the order of nanoseconds, after which the ground electronic state is restored and the molecule undergoes photon or phonon emission [25]. The eect of quantum connement on excitons in semiconductors of lowdimensions have been intensively investigated for many years [26, 27, 28]. Two factors are responsible for the exciton properties in a quantum dot. The rst is the Coulomb interaction between the electron and hole. The second is the connement by the quantum dot three-dimensional potential. The connement is ruled by the size and shape of the dot as well as by the dot and barrier material to produce various bands osets. In the quantum dots, the connement also inuences binding energy. Therefore, both factors inuence the energy and oscillator strength of exciton in a complex way [23]. As the size of the quantum dots approaches the Bohr radius of bulk exciton, quantum connement eects become apparent. There are two limiting cases depending upon the ratio between the radius of the quantum dot (r) and the eective Bohr radius of the bulk exciton (B eff ) described in Eq. 2.7: B eff = 2 ɛ m ∗ e 2 (2.7) 18
2.3 Low-Dimensional Semiconductor (Nanostrtuctures) where ɛ is the dielectric constant of the material, e is the electron charge, m ∗ is the eective reduced mass of the exciton 1/m ∗ = 1/m e + 1/m h , with m e and m h being the electron and hole eective masses, respectively. When the size of the quantum dot is smaller than the critical characteristic length or Exciton Bohr radius (Eq. 2.7), the electrons crowding lead to the splitting of the original energy levels into smaller ones with smaller gaps between each successive level. The Exciton Bohr radius is larger than the Bohr radius due to the eect of dielectric screening and the inuence of periodic lattice structure of the crystal. The quantum dots that have radii larger than the Exciton Bohr radius are said to be in the (weak connement regime). On the other hand, the ones that have radii smaller than the Exciton Bohr radius are said to be in the (strong connement regime) [22]. The strong connement of excitons in the quantum dots enhances the electron and hole interaction due to increased overlap of electron and hole wave functions in two ways. First, the overlap is enhanced because of reduced-dimensionality in 0D dots. Second, the electron and hole overlap is determined by the size and shape of the quantum dot, and the barrier height. Both eects signicantly aect the exciton binding energy. Therefore, the energy of the emitted photon is determined by the size of the quantum dot due to quantum connement eects. As it was mentioned in the thesis approach, this thesis will bring the focus in the next chapters to the growth and characterization of the III-V QDs on silicon substrate via molecular beam epitaxy (MBE) technique. 19
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
Page 10 and 11: viii
Page 12 and 13: CONTENTS 3.4.4 Planar Defects Assoc
Page 14 and 15: CONTENTS xii
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
Page 36 and 37: Heteroepitaxial Growth of III-V Sem
Page 64 and 65: Experimental Growth and Characteriz
Page 78 and 79: MBE Growth of Self-Assembled InAs a
Page 80 and 81: MBE Growth of Self-Assembled InAs a
Page 82 and 83:
MBE Growth of Self-Assembled InAs a
Page 84 and 85:
Page 86 and 87:
Page 88 and 89:
Page 90 and 91:
Page 92 and 93:
Page 94 and 95:
Page 96 and 97:
Page 98 and 99:
Page 100 and 101:
Page 102 and 103:
Page 104 and 105:
Page 106 and 107:
Page 108 and 109:
Page 110 and 111:
Page 112 and 113:
Page 114 and 115:
Page 116 and 117:
Page 118 and 119:
MBE Growth of InAs and InGaAs Quant
Page 120 and 121:
Page 122 and 123:
Page 124 and 125:
Page 126 and 127:
Page 128 and 129:
Page 130 and 131:
Optimization of MEE and MBE growth
Page 132 and 133:
Page 134 and 135:
Page 136 and 137:
Page 138 and 139:
Page 140 and 141:
Page 142 and 143:
Conclusions substrates, respectivel
Page 144 and 145:
REFERENCES [8] J. M. Gerard, O. Cab
Page 146 and 147:
REFERENCES [29] http://www.wmi.badw
Page 148 and 149:
REFERENCES [49] M. R. Brenner: GaP/
Page 150 and 151:
REFERENCES [66] Y. Ishida, T. Takah
Page 152 and 153:
REFERENCES [84] A. Garcia, C. M. Ma
Page 154 and 155:
REFERENCES [100] M. Felici, P. Gall
Page 156 and 157:
REFERENCES [116] H. Usui, H. Yasuda
Page 158 and 159:
REFERENCES [133] Grassman, T. J. Br
Page 160 and 161:
Conferences Contributions: Tariq Al
Page 162 and 163:
Nomenclature
Page 164 and 165:
REFERENCES symbol Descriptions γ f
Page 166 and 167:
like to thank all member of the Ins
Page 168:
REFERENCES Erklärung Hiermit versi
show all

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

Create successful ePaper yourself

Delete template?

Save as template?