Influence of growth conditions for GaAs quantum wells and composition fluctuations in Al(Ga)As barriers on exciton and electron localization

Abstract

Quantum wells (QWs) with short-period superlattice barriers are grown by molecular-beam epitaxy in two-dimensional island nucleation or step-flow growth mode. A pronounced roughening of step edges as imaged by atomic force microscopy for samples grown either at high As 4-partial pressure or on misoriented substrates with As-terminated steps is observed. Micro-photoluminescence spectra of the GaAs wells and their barriers reveal a rich fine structure. The narrow lines are attributed to exciton localization by potential fluctuations at the interface. Since the fine structure is observed in ternary barriers too, it is concluded that compositional fluctuations at QW interfaces are crucial. The interface roughness correlation lengths are measured by diffuse X-ray scattering. Another localization effect is connected with the appearance of heavy-mass X-electrons in the short-period superlattice barriers on both sides of the QW. Thus, their high screening capability results in unexpected high mobilities at high carrier densities. Transport correlation lengths are compared with those of the interface roughness.