Fine-structure features due to wave-function localization in coupled GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells

Photoluminescence (PL) measurements performed on nominally symmetric, coupled, double GaAs-${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As quantum wells grown by molecular-beam epitaxy (MBE) show fine-structure features within the observed excitonic transitions. This fine structure is interpreted in terms of wave-function-localization effects arising from deviations of the double wells from being perfectly symmetric. If the coupled-well structure is slightly asymmetric, with one well slightly larger than the other, the symmetric wave function is strongly localized in the wider well, while the antisymmetric wave function is strongly localized in the narrower well. Theory predicts that the energy spread covered by the fine-structure features resulting from the collapse of excitons associated with the symmetric combination of isolated electron and heavy-hole wave functions will be greater than that for the collapse of excitons associated with the antisymmetric combination of isolated electron and heavy-hole wave functions, in agreement with the experimental results reported here. Additionally, parity-forbidden as well as parity-allowed transitions are observed. The breakdown of the parity selection rule is believed to result from symmetry reduction arising from the presence of layer imperfections such as the clearly observed thickness variations. In some cases extrinsic transitions are also observed; magnetic-field-dependent PL measurements show these to be free-to-bound transitions (free hole to bound electron). Fine structure is also observed in the extrinsic transitions and is similar in nature to that observed for the intrinsic transitions. The asymmetry of the coupled wells is ascribed to surface kinetic phenomena occurring during MBE growth.