Nanostructures by Ion Beam Synthesis

Ion beams are widely used in research and development as well as in different fields of industrial applications, for example for doping of semiconductors in microelectronics. Furthermore, among other techniques ion beams exhibit a great potential for synthesis of new materials as well as for the self-organization of nanostructures. Two of the goals of materials research using ion beams are the ion beam synthesis (IBS) of nanocrystals (NC´s) and the self-organization of NCs under ion irradiation in a solid matrix, for example the formation of semiconducting or metallic NCs in insulating films for applications microelectronics and optoelectronics, respectively.
Semiconducting NCs embedded in the gate oxide of a MOS field effect transistor can be charged/decharged by tunnelling of electrons from/to the Si substrate, caused by an appropriate voltage pulse applied to the gate. There is a difference in the source/drain current for charged and neutral NCs which can be used for the storage of information. The basis of this memory concept is quite simple. However, the fabrication of tiny NCs (~2 nm) placed at the right position within the very thin (< 20 nm) gate oxide is a strong challenge. Different ion beam processes are being developed for realization: i) low energy ion implantation into the thin gate silicon oxide and ii) medium energy ion irradiation through the gate oxide. The two approaches will be described from the physical and technological point of view and experimental results of charge storage behaviour in NC MOS transistors fabricated by the two ion beam processes will be presented.
Another effort is currently devoted to the application of ion beam synthesized metallic NCs in optoelectronics. The fabrication of NCs by IBS requires usually a thermal treatment for phase separation of implanted impurity atoms and for damage annealing. This annealing leads inevitably to broad NC size distribution. Due to the broad size distribution of NCs fabricated by IBS, their size- depending characteristics, e.g. absorption and luminescence, can be obscured. Thus the potential for tailoring of size and size distribution of NCs by IBS is rather limited. It will be shown that size and size distribution can be tailored by high-energy ion irradiation through an ensemble of NCs.
Finally, in the outlook the lecture will address new trends of IBS for nanostructure fabrication, e.g. self-organization at ion implantation into templates and IBS of nanostructures by using focused ion beams (FIB).