STRUCTURES OF MEDIUM-SIZED SILICON CLUSTERS

Silicon is the most important semiconducting material in the microelec tronics industry, If current miniaturization trends continue, minimum device features will soon approach the size of atomic clusters. In thi s size regime, the structure and properties of materials often differ dramatically from those of the hulk, An enormous effort has been devot ed to determining the structures of free silicon clusters(1-22). Altho ugh progress has been made for Si-n with n < 8, theoretical prediction s for larger clusters are contradictory(2-22) and none enjoy any compe lling experimental support, Here we report geometries calculated for m edium-sized silicon clusters using an unbiased global search with a ge netic algorithm. Ion mobilities(23) determined for these geometries by trajectory calculations are in excellent agreement,vith the values th at we measure experimentally, The cluster geometries that we obtain do not correspond to fragments of the hulk, For n = 12-18 they are built on a structural motif consisting of a stack of Si-9 tricapped trigona l prisms, For n greater than or equal to 19, our calculations predict that near-spherical cage structures become the most stable, The transi tion to these more spherical geometries occurs in the measured mobilit ies for slightly larger clusters than in the calculations, possibly be cause of entropic effects.