Thermal stability of Al nanocluster arrays onSi (111)-7 7 surfaces.

By means of in situ high-temperature scanning tunnelling microscopy(STM), the thermal stability of highly ordered artificial Al nanoclusterarrays, so-called two-dimensional nanocluster crystals, on vicinalSi (111)-7 7 surfaces has been investigated. It was found that Al nanocluster crystals are stable below500 C.Above 500 C, the Al nanocluster crystal transforms into a surface phase with singular triangle shapes on down-terraces (the down-step side of aterrace). However, on up-terraces(the up-step side of a terrace), Al nanoclusters can move onSi (111)-7 7 surfaces over a large distance and the Al nanocluster crystal retracts toward the phase until the bare Si (111)-7 7 surface is completely recovered on up-terraces. The thermallyactivated motion of Al nanoclusters was not observed apparently below400 C, though Al nanocluster crystals can be formed even at200 C. This indicates that it is atom diffusion, not cluster diffusion, whichis responsible for the formation process of metal cluster crystals onSi (111)-7 7 surfaces. [ABSTRACT FROM AUTHOR]