Stacking and Ordering in Self-Organized Quantum Dot Multilayer Structures

Abstract

Strained-layer heteroepitaxy has become a powerful tool for the fabrication of self-assembled semiconductor nanostructures [1–4]. It is based on the natural tendency of highly strained layers to spontaneously form coherent, i.e., dislocation-free three-dimensional (3D) nanoislands on the surface of a thin wetting layer [4–7]. This Stranski–Krastanow islanding is driven by the highly efficient elastic strain relaxation within the islands due to lateral elastic expansion or compression in the directions of their free side faces [4–11]. For islands larger than a certain critical size, the relaxed elastic energy thus gained outweighs the corresponding increase in surface energy, leading to an effective lowering of the total free energy of the system [4–10]. Therefore, this Stranski–Krastanow growth transition occurs in most high-misfit heteroepitaxial systems. When the 3D surface islands are embedded in a higher energy band gap matrix material, self-assembled quantum dots with sharp, atomiclike electronic transitions are formed [12–14]. Owing to the statistical nature of growth, however, ensembles of self-assembled dots exhibit considerable variations in size and shape. This results in a large inhomogeneous broadening of the energy levels as well as of the related optical transitions [1–3, 12]. In addition, there is little control over the lateral arrangement and position of the nanoislands. Both factors pose considerable limitations to device applications.