Breaking the symmetry : spin splitting of hole subbands in strained quantum wells

Sammanfattning: Properties of the subband structure and related propertiesof quantum wells (QWs) are investigated, within the frameworkof the multi-band envelope function method. The subbands of aQW are replaced by discrete Landau levels when a magnetic fieldis applied perpendicular to the plane of the well. Calculationsare made self-consistently in the axial approximation.The spin splitting in the hole subbands is investigated inInx-GA1-xAs/InxGa1-xAsyP1-ylattice-matched quantum wells on different InxGa1-xAsyP1-ysubstrates, allowing for both tensile andcompressive strain. In the first place the contributions frommicroscopic and mesoscopic inversion asymmetry are considered,as well as their dependence on biaxial strain in the absence ofa magnetic field. It is shown that the spin splitting isdominated by the asymmetry of the mesoscopic potential (theband edges), except for large values of biaxial tension wherethe microscopic inversion asymmetry dominates. The spinsplitting is reduced by a factor of 6 - compared to theunstrained case -In a magnetic field the spin degeneracy is lifted even whenthe QW is symmetric under inversion. The contributions to spinsplitting due to microscopic and mesoscopic inversion asymmetryare investigated as a function of strain. The mesoscopicasymmetry has negligible effect on the spin splitting. Themicroscopic asymmetry becomes important at biaxial tension.Many-body effects in ap-type QW with heavy delta-doping in the well areinvestigated theoretically and experimentally. Excitons arefound to remain in the metallic limit. Ann-type triple QW is designed for second harmonicgeneration at λ = 10.6 µ m showing a simpler way totune the energy levels than previous designs.Key words:Quantum well, hole subband, spin splitting,strain, Landau levels, doping, many-body effects, secondharmonic generation, envelope functions