Effects of exciton–acoustic-phonon scattering on optical line shapes and exciton dephasing in semiconductors and semiconductor quantum wells

The interaction of excitons with acoustic phonons in direct band-gap semiconductors gives the dominant contribution to the temperature-dependent part of the exciton homogeneous linewidths and to dephasing rates at lower temperatures, e.g., below 150 K in bulk GaAs. Experimental results have shown that this contribution increases substantially in going from GaAs quantum wells to bulk GaAs. A perturbation treatment of acoustic phonon scattering in a simple band exciton model—i.e., neglecting valence-band coupling and anisotropy of exciton dispersion—agrees with experimental results in narrow quantum wells. On the other hand, it fails by an order of magnitude for bulk GaAs and by a large factor for other materials. Here we give a thorough theoretical discussion of this problem. The exciton linewidth is calculated to lowest order in the exciton–acoustic-phonon coupling including valence-band interactions and anisotropic exciton dispersion. The effects of multiple scatterings of phonons in higher orders of the exciton-phonon interactions are calculated and previous work on these effects are discussed. The effects of impurity motion from acoustic phonons on the exciton linewidths are evaluated. We conclude that the temperature-dependent exciton linewidth is given reasonably well by the lowest-order phonon scatterings provided that the full anisotropic exciton dispersion is included. Higher-order phonon scatterings give a small contribution to the linewidth and can affect the line shape. These results agree well with available experimental results for the low-temperature exciton linewidths in bulk GaAs and ZnSe and in quantum wells from these materials.