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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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We have performed a comprehensive investigation of n-type quantum dot infrared photodetectors ͑QDIPs͒ based on InAs/GaAs epitaxical island quantum dots ͑QDs͒ grown via the innovative punctuated island growth technique. The structural properties of the QDs were investigated with cross-sectional transmission electron microscopy and atomic force microscopy. The electronic properties of the QDs inserted in QDIP devices were investigated with photoluminescence ͑PL͒, PL excitation, and intra-and inter-band photocurrent spectroscopy. The influence of AlGaAs layers inserted into the QDIP active regions on the performance of dark current and inter-and intra-band photocurrent was examined. Initial results on intra-band responsivity and detectivity of these QDIPs at 77 K with undoped active region show promise for application.

Excitation transfer processes within self-organized quantum dot ͑QD͒ pairs in bilayer InAs/GaAs QD samples are investigated. QDs in samples with a 1.74-ML InAs seed layer and a 2.00 ML InAs second layer are found to self-organize in pairs of unequal sized QDs with clearly discernible ground-state transition energy. Photoluminescence ͑PL͒ and PL excitation results for such asymmetric QD pairs provide evidence for nonresonant energy transfer from the smaller QDs in the seed layer to the larger QDs in the second layer. Variations in the optical behavior as a function of the spacer thickness and composition are attributed to the barrier-dependent tunnel probability. Tunneling times down to 20 ps ͑36 ML GaAs spacer͒ are estimated, depending exponentially on the GaAs spacer thickness. ͓S0163-1829͑98͒52640-9͔

A method to independently manipulate the density and size of stress-driven self-assembled quantum dots (QDs) is demonstrated in the InAs/GaAs material system. In bilayer stacks, different InAs deposition amounts in the initial (seed) and second layer are shown to enable independent control, respectively, of the density and the size distribution of the second layer QDs. The approach allows enhancing the average volume and improving the uniformity of InAs QDs, resulting in, respectively, low and room temperature photoluminescence at 1.028 eV (∼1.2 μm) and 0.955 eV (∼1.3 μm) with remarkably narrow linewidths of 25 and 29 meV for 1.74 ML (seed)/3.00 ML InAs stacking.

The later stages of the evolution of epitaxical island quantum dots are examined systematically for InAs depositions on GaAs(001) following the conventional continuous deposition mode and an approach introduced here called punctuated island growth (PIG). The comparative study provides clear structural and optical evidence for a change in InAs island shape at a self-limiting lateral size, first reached for depositions ∼2 ML. The PIG approach has also allowed realization of the narrowest reported inhomogeneous linewidth of 23 meV for low temperature photoluminescence from a single layer of binary InAs/GaAs quantum dots.