Self-assembling and ordering of Ge/Si(111) quantum dots: scanning microscopy probe studies

In order to exploit the unique electronic properties of semiconductor quantum dots (QDs) in novel quantum effect devices, the lateral dimensions of these structures have to be reduced to the order of tens of nanometres, which is the range of the de Broglie wavelength of electrons inside these materials. Moreover, millions of QDs should be packed in an orderly fashion in dense arrays to achieve the necessary active volume. So far, the most promising quantum structures have been fabricated using techniques based on direct crystal growth. This process has become very important due to its potential for creating damage-free and coherent nanocrystals. New scanning microscopy probes, such as scanning tunnelling microscopy and atomic force microscopy, are actually the most important analytical tools for checking the properties of these nanostructures. In the present paper we will review some of the results recently obtained on the Ge/Si system thanks to these techniques. Ge/Si is really appealing for future applications, in view of the integration of nanostructures with the present microelectronic technologies, but it is also at the origin of many general studies regarding atomic diffusion, intermixing, and island shape stabilization. The origin of the islands, their size and shape evolution, and the intermixing at the interface will be analysed starting from SPM data. The ordering of QDs on naturally or artificially nanostructured surfaces is one of the most recent objectives in Ge/Si research. For example, terraces created from the step bunching process on the Si(111) surface could be a way of controlling the spacing of the islands. Other possibilities are now opened by the increasing precision of electron-beam lithography and by new focused-ion-beam machines. These aspects, along with problems and drawbacks of self-assembling technology, will also be discussed.