Development and application of the k.p method to investigate spin and optical properties of semiconductor nanostructures

• Autor(a) principal: Faria Júnior, Paulo Eduardo de
• Data de Publicação: 2016
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: http://www.teses.usp.br/teses/disponiveis/76/76132/tde-21102016-160111/
• topic: k.p method; Lasers de spin; Método k.p; Politipismo; Polytypism; Semiconductors; Semicondutores; Spin lasers; Wurtzita; Wurtzite

Many observable properties of semiconductor systems, such as transport and optical transitions, are manifestations of their underlying electronic band structures, i. e., the energy levels that electrons may have in the semiconductor. Among the theoretical approaches to calculate the band structure, the k.p method is a versatile framework that can be extended to deal with confined systems, overcoming the computational limitations of first principles methods. In this thesis, we develop and apply k.p Hamiltonians to investigate spin and optical physical phenomena in unconventional semiconductor systems. Specifically, we addressed three different topics: spin lasers, polytypism in III-V semiconductors and spin-orbit coupling effects in wurtzite materials. For spin lasers, we investigate the behavior of their active region, in a VCSEL geometry, based on GaAs/AlGaAs zinc-blende quantum wells by calculating the spin-dependent gain coefficient. Assuming spin polarized electrons, our calculations showed the spin-filtering and the threshold reduction features found in experiments and by the conventional rate equation approach in the steady-state operation. Motivated by experimental evidence of enhanced dynamic operation for light polarization because of anisotropies in the semiconductor system, we calculate the birefringence coefficient of the active region under uniaxial strain. Our calculations showed that, even for a small value of applied strain, the birefringence coefficient can easily exceed 200 GHz. In fact, our predictions were experimentally demonstrated for values up to 250 GHz in similar GaAs/AlGaAs spin VCSELs. For the polytypism topic, we develop a k.p model combined with the envelope function approximation to investigate the polytypismin III-V semiconductor systems with mixed zinc-blende and wurtzite crystal structures. We apply our model for InP polytypic quantum wells to investigate quantum confinement and strain effects. We then extended this polytypic model to include the explicit coupling between the conduction and the valence bands in order to investigate optical properties in InP polytypic superlattices. For pure phase nanowires, modeled with bulk calculations and the optical confinement, we can see the same experimental trends regarding the light polarization, i. e., zinc-blende phase favors light polarization along the nanowire axis while wurtzite phase favors the polarization perpendicular to the axis. Including the crystal phase mixing and the quantum confinement effects, we obtain the degree of light polarization ranging from pure zinc-blende to pure wurtzite nanowires and, more specifically, that this degree of polarization is very sensitive to the size of zinc-blende regions, a feature that is also observed in photoluminescence measurements. Finally, we develop a realistic k.p Hamiltonian, with parameters obtained from ab initio band structures, to investigate electronic properties and spin-orbit coupling effects in InAs and InP semiconductors with wurtzite structure. Our 8×8 k.p model describes the conduction and the valence bands, including spin, around the energy gap. We also include the k-dependent spin-orbit coupling term, usually neglected in the literature, to correctly describe the bulk inversion asymmetry of wurtzite structure. We show that all the investigated energy bands have a spin expectation value that follows a Rashba-like spin texture, with either clockwise or counter clockwise orientation. We emphasize that all the ab initio features of band structure, spin splittings and spin orientation were systematically checked to provide the best parameter sets. Using the 8×8 k.p Hamiltonian, we calculated the density of states and predicted the carrier density as a function of the Fermi energy. We also provide an analytical approach for conduction band and a compact description for the valence bands, however, the 8×8 Hamiltonian is the best approach to recover the ab initio calculations around a large region of the first Brillouin zone.