Shape Transition of InAs Islands on InP(111)A
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FIG. I Evolution of InAs surface morphology characterized by a transition from round quantum dots to triangular islands with increasing InAs coverage. (a) 1.5 ML, (b) 2.5 ML, and (c) 3.5 ML.
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analyzed ex situ using atomic force microscopy (AFM) in the contact mode, which was performed in a Nanoscope EHAFM (Digital Instruments) system with silicon nitride tips. Planview and cross-sectional transmission electron microscopy (TEM) measurements were also used for structural characterization in some samples, which were performed using the conventional dark-field technique. TEM specimens were prepared by mechanical thinning followed by ion milling. RESULTS AND DISCUSSION The AFM images in Figure 1 display the evolution of InAs surface morphologies grown on InP (11 )A. The average island densities and sizes are presented in Table 1. When the InAs coverage just exceeds 1.5 ML, coherent round-shaped QDs of -26 nm in base diameter appear on the surface (to show the round shape, we use a high magnification). After 2.5 ML (Fig.lb), triangular-shaped islands with height -20 nm and lateral dimensions around 70 nm appeared, i.e., round QDs and triangular islands coexist. TEM studies show that the round QDs are coherent, while the large triangular islands are relaxed. A distinct feature of the triangular islands is that their edges are exclusively parallel to the threefold I 1101 directions. Triangular islands rotated by 180' cannot be seen, which is different from the results of InGaAs triangular islands grown on GaAs (11 )B.7 With further InAs deposition, the size of the triangular islands increases further, and their height and lateral dimensions are 50 nm and 180 nm, respectively (Fig.lc). Meanwhile, the size of small QDs remains almost unchanged, in sharp contrast to the dramatically increased size of triangular islands. After 4.5 ML, almost all the islands formed are triangular islands (the AFM image is not shown here). The density is three orders of magnitude smaller than the 1.5 ML samples. The height and side length are increased to about 55 nm and 300-500 nm, respectively. Moreover, the triangular islands exhibit a highly faceted pyramid shape. AFM line profile measurements show that the side facets are all inclined by
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