Surface segregation in (Ga,In)As/GaAs quantum boxes

Three-dimensional coherent islands formed during the highly strained growth of ${\mathrm{In}}_{0.3}$ ${\mathrm{Ga}}_{0.7}$As on GaAs(001) are studied by scanning tunneling microscopy. High-resolution images evidence two different types of surface reconstructions between the top and the bottom of the islands. While a 2×4 GaAs(001)-like reconstruction is observed on the wetting layer, the top layer exhibits the (2×4)α2 phase, which is characteristic of the InAs(001) reconstructed surface. This is the consequence of In surface segregation leading to the formation of a monolayer of InAs at the island top. Finally, photoluminescence experiments exemplify the effect of segregation on the ${\mathrm{In}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As/GaAs quantum box optical properties.