EXCITON RADIATIVE LIFETIME IN SHORT-PERIOD GAAS-ALAS SUPERLATTICES OF TYPE-II

From an extensive study of time-resolved photoluminescence in the temperature range 1.8-50 K on a dozen of samples of type II GaAs-AlAs(001) superlattices, we have identified the region where the states derived from Xz-valleys are lower than those derived from Xxy-valleys. This is in agreement with a simple model involving the competition between (i) the difference in the confinement energies of X-type states in AlAs layers related to their effective mass anisotropy, and (ii) X-valley splitting of about 19 meV caused by the residual uniaxial stress in AlAs layers related to the small lattice mismatch with GaAs. We present an analysis of the luminescence decay curves in order to determine the variation of radiative lifetime of excitons involving Xz- or Xxy-electrons, as a function of AlAs and GaAs layer thicknesses. The parity of Xz-states in the superlattice and the mixing of Xx- and Xz-components into the wavefunction of these excitons localized by interface disorder play a significant role in this analysis; they contribute both to non-exponential decay. © 1990.