SELF-ORGANIZED REPLICATION OF 3D COHERENT ISLAND SIZE AND SHAPE IN MULTILAYER HETEROEPITAXIAL FILMS

Advances in modern science and technology continue to make electronic and optoelectronic devices faster and smaller. As its size approaches the nanometer scale, a crystal exhibits electronic and optoelectronic properties different from those of the bulk, leading to potential new applications, such as “quantum dot” (QD) structures in which nanometer-size clusters of one material are embedded into the matrix of a second one. The growth of three-dimensional (3D) islands in strained films via the Stranski-Krastanov (SK) growth mode provides a possible route to the fabrication of QDs. After forming a thin wetting layer, strained layers may spontaneously form 3D islands to relieve the misfit strain. Under appropriate growth conditions, these islands achieve nanometer size and are coherent with the substrate lattice (i.e., free of dislocations) [1] but, in general, are not uniform in size, shape, or spacing. However, uniformity in size and shape is a prerequisite for their potential use in QD electronic or optoelectronic devices, for which precise knowledge of the electronic energy levels is essential. It has been discovered that multilayering improves island uniformity: With increasing number of bilayers [the spacer layer (same as the substrate) plus the SK layer], the 3D islands become more uniform in size, shape, and spacing [2–11]. For sparse initial island arrays, typical for III-V systems, individual vertical columns of islands are formed with islands in each column converging to a stable size and shape [2–7]. For dense initial island arrays, typical for the SiGe system, multilayering leads both to uniform island size and shape and to uniform spacing [8–11]. Theoretical models, focusing on island nucleation, have been proposed to explain the vertical self-organization [2,8]. The 3D islands buried by the spacer layer produce a tensile region above themselves (assuming the original SK growth occurs for a compressed layer, as in all systems so far investigated). When the next SK layer is deposited, this tensile region induces a preferential nucleation of new 3D islands above the buried ones by a strain-directed diffusion [2] and/or by lowering the overall lattice misfit [2,8]. The repetition of layers creates columns of 3D islands. A model [8] also shows that the interaction of the localized strain fields induced by the buried islands at