FORMATION OF POLYCRYSTALLINE STRUCTURE IN METALLIC-FILMS IN THE EARLYSTAGES OF ZONE-1 GROWTH

The morphology of ultrathin Au and Pb films vapor deposited onto cryog enically cooled substrates has been studied by in situ Scanning Tunnel ing Microscopy (STM). At low substrate temperatures, 4 < T-S < 77 K, w here thermally activated grain growth processes are negligible, i.e. i n the Zone I regime, films with equivalent bulk mass deposited thickne sses, d greater than or equal to 0.9 nm uniformly cover the substrate and exhibit no discernible crystalline structure for d less than or eq ual to 1.6 nm for T-S=77 K. Thicker films exist in a polycrystalline p hase with a uniform grain size. It is argued that the observed polycry stalline structures do not form through normal thin film nucleation an d growth processes. Instead, they form from a transient amorphous phas e through athermal crystallization processes after a critical film thi ckness is reached. Results are also presented from annealing studies t hat support this view. (C) 1998 Acta Metallurgica Inc. Published by El sevier Science Ltd. All lights reserved.