Excited states and energy relaxation processes are studied for stacked InAs/GaAs QD's with GaAs cap layers grown by migration enhanced epitaxy. Photoluminescence excitation ͑PLE͒ spectra reveal the excited state spectrum as a function of size for self-assembled InAs QD's in multilayered samples with 36-ML spacers. The observed energy shifts and splittings are consistent with those of hole states numerically calculated for pyramidal QD's supporting assignment to the transition between the electron ground ͉000͘ and the ͉001͘ excited hole state. The optical results suggest the island shape uniformity to improve in multilayered samples, which is attributed to the contribution of the buried islands to the surface strain altering the island formation kinetics and energetics that also underlie vertical self-organization. Time-resolved photoluminescence ͑TRPL͒ results yield a lifetime of 40 ps for the first excited ͉001͘ hole state, attributed to multiphonon relaxation processes bridging the approximately 100 meV level separation, and ground-state lifetimes around 700 ps independent of the detection energy. At high excitation densities saturation of QD states leads to long-living excited-state PL and up to 1 ns delay in the ground-state PL decay, showing radiative decay to be the dominant recombination process in the QD's. The results presented contribute to the understanding of PLE spectra of an inhomogeneous QD ensemble, which is argued to be sensitive to the shape uniformity, the excited-state spectrum, and competing recombination processes. ͓S0163-1829͑98͒03115-4͔

The two-dimensional (2D) to three-dimensional (3D) transition in highly strained growth of InAs of GaAs(001) is investigated using in situ scanning tunneling microscopy and photoluminescence spectroscopy. Remarkably, InAs structural features up to five monolayers (ML) high appear at ϳ1.25 ML, disappear, and reappear prior to the onset of well-developed 3D islands at 1.57 ML, thus manifesting a hitherto unrecognized reentrant behavior in the formation of 3D islands. The results provide new insights into the long-standing problem of the kinetic aspects of 2D to 3D morphology change not embodied in the widely encountered Stranski-Krastanow growth mode. [S0031-9007(97)03235-3] PACS numbers: 68.35.Bs, 61.16.Ch, 78.66.FdThe surface morphology of overlayers having a high lattice mismatch with substrates has, for a wide variety of combinations, been found to change from an initially two-dimensional (2D) to a three-dimensional (3D) islandlike nature beyond a (growth condition dependent) critical amount of material deposition (film thickness) [1]. Such a growth mode is referred to as the Stranski-Krastanow growth mode [2]. For nearly five decades it was assumed that the change from the planar 2D to 3D island morphology is accompanied by the formation of defects (such as dislocations). Indeed, a school of thought attributed the initiation of 3D islands itself to the appearance of dislocations [3]. However, the reports some six years ago that, in the semiconductor systems InGaAs on GaAs [4] and Ge on Si [5], coherent (i.e., defect-free) 3D islands can form have led to intensive efforts towards a better atomistic and kinetic understanding of the actual pathway from 2D to 3D morphology [6-12]. On the pragmatic side, the coherent nature of the 3D islands has, in the past three years, caused explosive growth in the examination of their optical behavior as quantum boxes [dubbed quantum dots (QDs)] [10,13-17] and of their potential for QD-based injection lasers [18,19]. Indeed, understanding the atomistic mechanism of strain-induced evolution of the 3D islands is fundamental for exploiting concepts of self-assembly and/or self-ordering [20] in order to realize the desired electronic and optical properties of the QDs. This demands careful and systematic atomic level structural [such as provided by a scanning tunneling microscope (STM)] and optical studies carried out on comparable samples. In this Letter we report on the results of such a study undertaken for the InAs͞GaAs(001) system (lattice mismatch ϳ7%) for InAs depositions from submonolayer to just above 2 monolayers (ML). The STM results reveal, and the optical results independently confirm, a highly unexpected reentrant nature of the formation of 3D-like features with increasing InAs deposition, in which 3D-like features appear well in advance of the stage of 3D island formation, disappear, and then reappear just prior to the onset of coherent 3D island formation. The observations thus provide clear evidence that the change from 2D to 3D morphology in highly strained growth ...

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In-situ, ultra high vacuum combined scanning tunneling microscope/atomic force microscope (STM/AFM) studies were undertaken to examine the initiation of 3D InAs islands on GaAs (100) and their density and size distribution as a function of growth conditions. A decreasing island density with increasing As4 pressure is observed and points to the significance of strain in affecting In migration and As4 incorporation. A stack of InAs islands separated by GaAs spacer layers exhibit a vertically self-organized growth. Through analysis of a phenomenological model, this is shown to be a consequence of a directional In adatom migration caused by the islandinduced nonuniform strain fields.