InAs quantum dots in a single-crystal GaAs matrix

We directly synthesize InAs microclusters embedded in a single-crystal GaAs matrix by molecular-beam epitaxy. Fractional monolayers of InAs are deposited on terraced (001) GaAs surfaces and subsequently overgrown with GaAs. Growth conditions are adjusted in situ by reflection high-energy electron diffraction to those favoring step-flow nucleation of both Ga and In adatoms. The resulting microscopic structural configuration is studied by double-crystal x-ray diffractometry and high-resolution electron microscopy. These experiments reveal that InAs growth takes place in fact by nucleation of In adatoms on step edges. An array of isolated InAs clusters of subnanometer size (quantum dots) is thereby formed within the GaAs matrix. The interface of the InAs clusters is in registry with the surrounding GaAs matrix and is thus defect-free. Several spectroscopic techniques, such as transmission, cw photoluminescence, photoluminescence excitation, and picosecond photoluminescence, are applied to get insight into the optical properties of this system. We show that the optical response of excitons attached to the InAs dots is determined by the zero-dimensional symmetry of the system. This effect is most evident when comparing the spontaneous emission of InAs dots and InAs planes, which in either case results from the relaxation of excitons to the emitting state followed by their radiative recombination. The reduced translational symmetry causes a progressive release of wave-vector conservation, thus modifying the selection rules that uniquely determine the interaction of excitons with phonons (relaxation) and photons (recombination).