Miniband dispersion in (In,Ga)As-GaAs strained-layer superlattices

Abstract

Photoluminescence excitation (PLE) spectroscopy has been used to characterize miniband formation in (In,Ga)As-GaAs superlattices with nominally 50-Å-wide wells and barriers between 50 and 200 Å. The nominal composition of the alloy layers was ∼0.06. The observed exciton features are consistent with theoretical predictions of the Δn=0 and Δn≠0 (i.e., e1-hh2) intersubband transitions both at the center of the superlattice minizone and at the zone edge. The observation of these features allows us to map the evolution of the superlattice minibands at energies both above and below the top of the superlattice potential, with decreasing barrier thickness. Furthermore, as the GaAs thickness is varied, a dramatic change in shape of PLE spectra occurs in the region of the first free-electron–to–heavy-hole subband continuum. For samples in which the electron miniband width is comparable with the exciton binding energy, a significant enhancement of the absorption strength is observed, brought about by a redistribution of the oscillator strengths of the continuum states due to the electron-hole, Coulomb interaction. The best agreement with the measured exciton positions is found using a band-offset ratio of ∼67:33.