Exciton delocalization in thin double-barrier GaAs/AlAs/(Al,Ga)As quantum-well structures

Nonconventional GaAs/${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As quantum-well structures, with one or two monolayers of AlAs inserted between the GaAs layers and the ${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As barriers (double-barrier quantum wells), have been investigated to observe the quasi-three-dimensional behavior of excitons in thin quantum wells. These samples provide higher confinement energies than single quantum wells of the same thickness, in such a way that relatively thick wells produce shallow carrier subbands. An increase of the photoluminescence lifetime of excitons has been found when narrowing the well thickness. The analysis of intensity and shape of the photoluminescence spectra made us confident in interpreting this increase as an effect of the exciton delocalization, and a qualitative agreement with a simple model for the exciton radiative lifetime is obtained.