Shallow quantum well excitons: 2D or 3D?

A new regime is obtained in semiconductor heterostructures with constituents of nearly identical band gaps. Previously, it has been shown that even extremely shallow quantum wells (SHQWs) exhibit excitonic and electroabsorption properties typical of a 2D system, but 3D transport features. We show that, surprisingly, even when carriers are two-dimensionally confined in SHQWs, the hole spin relaxation is extremely fast (∼400 fsec) as in the bulk (3D) limit and that a 2D-3D transition in the hole spin dynamics in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As SHQWs takes place at x∼5%.