Silicon-germanium nanostructures with quantum dots: Formation mechanisms and electrical properties

The generally accepted notions about the formation mechanisms for germanium islands with nanometer-scale sizes in a Ge-on-Si system are reviewed on th e basis of analysis of recent publications. The presence of elastic strains in the epilayers and in the three-dimensional Ge islands on Si is a key fa ctor that not only initiates a morphological transition from a planar film to an island-containing film (the Stranski-Krastanov mechanism) but also in fluences the subsequent stages of the islands' evolution, including their s hape, size, and spatial distribution. In many cases, this factor modifies a ppreciably the classical mechanisms of phase-formation and their sequence u p to the quasi-equilibrium coexistence of three-dimensional Ge nanoislands at the surface of the Si substrate. The methods for improving the degree of the ordering of nanoislands to attain the smallest possible sizes and larg e density of areal distribution of these islands are discussed. The publish ed data on optical absorption in the multilayered Ge-Si systems with quantu m dots are considered; these data are indicative of an anomalously large cr oss section of intraband absorption, which makes this class of nanostructur es promising for the development of photodetectors of the infrared region o f the spectrum. The results of original studies of electrical and optical p roperties of heterostructures that involve Ge quantum dots and are synthesi zed by molecular-beam epitaxy on the Si substrates are reported. (C) 2000 M AIK "Nauka/ Interperiodica".