Theory of the electronic and optical properties of GaAs/AlGaAs quantum wells under uniaxial stress

A comprehensive theoretical study of the effects of in-plane uniaxial stress on the excitonic absorption spectra of GaAs/A1xGa1-xAs quantum wells is presented. In particular, stress is used to investigate optical features of excitonic mixing effects. This analysis yields a detailed understanding of the various origins and underlying mechanisms of exciton mixing in quasi-two-dimensional systems. State-of-the-art calculations of realistic excitonic absorption spectra under stress are performed that take valence-band mixing fully into account within the Γ8 subspace. To include the stress-induced anisotropy of the bandstructure, a full band-mixing model is developed that goes beyond the standard axial approximation. Moreover a very efficient technique is presented that allows solving the exciton equation despite its enormous numerical intricacy on an ordinary personal computer. Two important aspects of in-plane uniaxial stress are identified, each of which affects exciton mixing in a different way. On the one hand, the natural quantisation direction gets rotated by stress from the confinement direction to the stress direction. This leads to a marked polarisation dependence of the absorption spectrum and a characteristic non-linear energy shift of the excitonic ground state. Both features can be explained within a simple model of single-particle zone-centre states. On the other hand uniaxial stress also allows the energy alignments of the valence-states to be varied substantially. Thereby it is possible to influence the exciton mixing considerably, in particular between the lowest Is light-hole exciton and the p-continuum of the second heavy-hole exciton. As the 1s exciton is shifted towards the p-continuum a doublet structure occurs that reveals anticrossing behaviour. This anticrossing, is ascribed to an unusually strong Fano-related interference. The prominent asymmetric line shape of the doublet, which is caused by a mechanism different from the classic Fano resonance case, and the characteristic dependence of the doublet splitting energy on the quantum well width offer striking evidence for the strong underlying valence-band mixing. Moreover, this proves that previous explanations of similar doublets in experimental data, employing simple two-state models, are incomplete. Excellent agreement between theoretical results and available experimental data is achieved with respect to the exciton line shapes, the polarisation dependence of the transition intensities and the stress-dependence of the exciton energies. This powerfully demonstrates the high accuracy of the performed calculations. Finally, an alternative method is presented for determining the hydrostatic and shear deformation potential of the valence-band in III-V semiconductors from the stress dependence of the quantum well bandgap only. Analysis of experimental data confirms the validity of the proposed method. (author)