Microroughness and exciton localization in (Al,Ga)As/GaAs quantum wells

We report on microscopic photoluminescence and photoluminescence excitation of thin ${\mathrm{Al}}_{0.3}$ ${\mathrm{Ga}}_{0.7}$ As/GaAs quantum wells grown on exactly oriented (001) GaAs substrates. The experiments are done at low temperature by selectively exciting a few μ${\mathrm{m}}^2$ of the sample with a low excitation intensity. The photoluminescence spectrum performed under these conditions shows sharp peaks appearing on the low-energy side of the main line of about 0.1-meV width. These features are attributed to localized exciton states at fluctuations of the effective quantum well thickness. On the other hand, scanning tunneling microscopy performed on 2×4 reconstructed GaAs (001) surfaces clearly evidences the presence of one-monolayer-deep elongated 'holes' regularly oriented along the [11-bar0] direction with an average width of 6 nm. The same features are observed on 2×4 reconstructed ${\mathrm{Al}}_{0.3}$ ${\mathrm{Ga}}_{0.7}$ As surfaces (on which quantum wells are grown), though the width regularity is less pronounced. This strongly suggests that excitons at low temperature are localized in such boxes. We also discuss the very different results obtained using both techniques (micro-photoluminescence and scanning tunneling microscopy) on quantum wells grown on vicinal (001) substrates.