Browsing by Subject "Deformation potential"

Now showing 1 - 4 of 4

Open Access
Electronic structure of low dimensional semiconductor systems
(1992) Gülseren, Oğuz
Recent progress made in the growth techniques has led to the fabrication of the artificial semiconductor systems of lower dimension. Electrons and holes in these materials have quantization different from those of the three dimensional systems presenting unusual electronic properties and novel device applications. In this work, the important features of the free carriers in semiconductor superlattices are examined, and the electronic structure of some novel 2D semiconductor systems are investigated theoretically. This thesis studies various systems of lower dimensionality such as: the strained Si/Ge superlattices, i-doping. Si (100) surface and the tip-sample interaction in scanning tunneling microscopy (STM) study of this surface, and Wannier-Stark localization in finite length superlattices. The electronic energy structure of pseudomorphic Ge„i/Si„ superlattices is investigated by using the empirical tight binding method. Effects of the band offset, sublattice periodicity and the lateral lattice constant on the transition energies have been investigated. It is found that Ge„i/Si„ superlattices grown on Ge (001) can have a direct band gap, if m + n = 10 and m = 6. However, optical matrix elements for in-plane and perpendicular polarized light are negligible for the transition from the highest valence band to the lowest conduction band state at the center of the superlattice Brillouin zone. The electronic structure of the Si i-layer in germanium is explored by using the Green’s function formalism with layer orbitals. We found two dimensional parabolic subbands near the band edges. This approach is extended to treat the electronic structure of a single quantum well without invoking the periodically repeating models. Quantum well formation in Ge,„Si„ superlattices is also studied by using different number of ^-layers. Subband structure is observed by changing the height of the Si quantum well. The confinement of acoustical modes within 2DEG due to only the electronphonon interaction is proposed. The confined modes split out from the bulk phonons, if the 2DEG is created by means of modulation doping. This occurs even if the lattice has uniform parameters. The effect is more pronounced when the wave vector q of the modes increases and is maximum a,t q = 2kp {kp is the Fermi wave vector). In the case of several electron sheets the additional features of the confinement effect appear. Green’s function method is also applied to treat the modifications of electronic state density in STM. The tip-sample interaction in STM study of Si (100) surface is explored by calculating the Gieen’s function within the empirical tight binding method. Both of the proposed reconstruction models, buckled and symmetrical dimer model, is investigated. A dip occurs in the change of density of states of surface atoms at the energy of surface states for small tip-sample distances, and it decreases with increasing tip-sample separation. Although, in-plane tip position (above the up- or down-surface atom) affects the surface atoms differently in buckled dimer model, it influences the surface atoms symmetrically in symmetric dimer model. Recent experimental studies revealed the significant information on the Wannier-Stark localization. Following these experimental results, the WannierStark ladder is investigated by carrying out numerical calculations on a multiple quantum well structure under an applied electric field. The variation of the Wannier-Stark ladder energies and localization of the corresponding wave II function are examined for a wide range of applied electric field. Our results show that Wannier-Stark ladder do exist for finite but periodic system which consists of a large number of quantum well having multi-miniband structure. It is found that the miniband states are localized in the well regions with the applied electric field, while the continuum states preserve their extended character. Energies of the well states show a linear shift with the electric field except the small field values in which a nonlinear shift is resulted. Multiband calculations show that there is a mixing between the different band states although they are localized in different well regions.
Open Access
Scattering analysis of two-dimensional electrons in AlGaN/GaN with bulk related parameters extracted by simple parallel conduction extraction method
(American Institute of Physics, 2010-07-15) Lisesivdin, S. B.; Yildiz, A.; Balkan, N.; Kasap, M.; Ozcelik, S.; Özbay, Ekmel
We carried out the temperature (22-350 K) and magnetic field (0.05 and 1.4 T) dependent Hall mobility and carrier density measurements on Al 0.22Ga0.78N/GaN heterostructures with AlN interlayer grown by metal-organic chemical-vapor deposition. Hall data is analyzed with a simple parallel conduction extraction method and temperature dependent mobility and carrier densities of the bulk and two-dimensional (2D) electrons are extracted successfully. The results for the bulk carriers are discussed using a theoretical model that includes the most important scattering mechanisms that contribute to the mobility. In order to investigate the mobility of two-dimensional electron gas, we used a theoretical model that takes into account the polar optical phonon scattering, acoustic phonon scattering, background impurity scattering, and interface roughness scattering in 2D. In these calculations, the values are used for the deformation potential and ionized impurity density values were obtained from the bulk scattering analysis. Therefore, the number of fitting parameters was reduced from four to two. © 2010 American Institute of Physics.
Open Access
Strained band edge characteristics from hybrid density functional theory and empirical pseudopotentials: GaAs, GaSb, InAs and InSb
(Institute of Physics Publishing Ltd., 2016) Çakan, A.; Sevik, C.; Bulutay, C.
The properties of a semiconductor are drastically modified when the crystal point group symmetry is broken under an arbitrary strain. We investigate the family of semiconductors consisting of GaAs, GaSb, InAs and InSb, considering their electronic band structure and deformation potentials subject to various strains based on hybrid density functional theory. Guided by these first-principles results, we develop strain-compliant local pseudopotentials for use in the empirical pseudopotential method (EPM). We demonstrate that the newly proposed empirical pseudopotentials perform well close to band edges and under anisotropic crystal deformations. Using the EPM, we explore the heavy hole-light hole mixing characteristics under different stress directions, which may be useful in manipulating their transport properties and optical selection rules. The very low 5 Ry cutoff targeted in the generated pseudopotentials paves the way for large-scale EPM-based electronic structure computations involving these lattice mismatched constituents.
Open Access
Strained empirical pseudopotential generation from hybrid density functionals: GaAs, InAs,GaSb, InSb
(2015-08) Çakan, Aslı
Self-assembled quantum dots composed of III-V compounds receive considerable attention due to their potential applications on spintronics and quantum informa- tion processing. Here, lattice mismatch between two materials causes a remark- able strain and this subsequently affects not only carriers but also nuclear spins due to electric quadrupole interaction. In this thesis, the behavior of electronic band structure and deformation potentials under various strains are investigated in the family of semiconductors consisting of InAs, GaAs, InSb and GaSb. Com- putations are performed using semi-empirical pseudopotential method (EPM) by generating a new set of strain-compliant pseudopotentials. In order to both lead and validate EPM calculations, density functional theory based on hybrid functionals has been employed. Our results on hydrostatic and shear strain de- formation potentials obtained by either technique are in very good agreement with the experimental data. We demonstrate that the newly proposed empirical pseudopotentials perform well around band edges under anisotropic crystal de- formations. This paves the way for large-scale electronic structure computations involving lattice mismatched constituents.