Lateral Aligment of Epitaxial Quantum Dots

Part of the series Nano Science and Technolgy pp 49-76

Routes Toward Lateral Self-Organization of Quantum Dots: the Model System SiGe on Si(001)

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Because its lattice constant is 4.2% larger, Ge grows on Si in the Stranski–Krastanov (SK) growth mode, that is, with formation of coherent threedimensional (3D) nanocrystals after completion of a two-dimensional (2D) wetting layer of one or several monolayers (MLs) [1]. The formation of such 3D nanocrystals in SK growth was convincingly demonstrated for the first time in 1990, using scanning tunneling microscopy (STM) of Ge grown on the technologically relevant Si(001) surface [2]. At the same time it was shown, using cross-sectional transmission electron microscopy (TEM), that these nanocrystals are coherent with the substrate and therefore free of interfacial (misfit) dislocations [3]. Some years earlier, 3D morphologies arising in strained-layer growth had been observed using TEM [4,5], but with questionable interpretations. Mo et al. were able to image the facet structure of Ge nanocrystals growing on Si(001) with atomic resolution and proposed a model for the atomic structure of the facets [2]. The corresponding STM image is reproduced in Fig. 2.1a. The nanocrystal has a rectangular base with edges in <110> directions and four {105} facets, two of which are trapezoidal and two triangular, giving a prism-like shape with canted ends. The shape is best described as a hut conformation, resulting in the appellation “hut cluster” [2]. Hut shapes can vary from square to rectangular, with the elongation, if it exists, along either the [100] or the [010] direction, and with aspect ratios of up to about 4.