Formation of polycrystalline structure in metallic films in the early stages of zone 1 growth

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