MICROROUGHNESS AND EXCITON LOCALIZATION IN (AL,GA)AS GAAS QUANTUM-WELLS/

We report on microscopic photoluminescence and photoluminescence excit ation of thin Al0.3Ga0.7As/GaAs quantum wells grown on exactly oriente d (001) GaAs substrates. The experiments are done at low temperature b y selectively exciting a few mu m(2) of the sample with a low excitati on intensity. The photoluminescence spectrum performed under these con ditions shows sharp peaks appearing on the low-energy side of the main line of about 0.1-meV width. These features are attributed to localiz ed exciton states at fluctuations of the effective quantum well thickn ess. On the other hand, scanning tunneling microscopy performed on 2x4 reconstructed GaAs (001) surfaces clearly evidences the presence of o ne-monolayer-deep elongated ''holes'' regularly oriented along the [1( 1) over bar0$] direction with an average width of 6 nm. The same featu res are observed on 2x4 reconstructed Al0.3Ga0.7As surfaces (on which quantum wells are grown), though the width regularity is less pronounc ed. This strongly suggests that excitons at low temperature are locali zed 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.