MODELS FOR DIFFUSION AND ISLAND GROWTH IN METAL MONOLAYERS

A model which describes self-diffusion, island nucleation and film gro wth on fee (001) metal substrates is presented. The parameters of the model are optimized to describe Cu diffusion on Cu(001) by comparing a ctivation energy barriers to a full set of barriers obtained from semi -empirical potentials via the embedded-atom method. It is found that t his model (model I), with only three parameters, provides a very good description of the full landscape of hopping-energy barriers. These en ergy barriers are grouped in foils main peaks. A reduced model (model II) with only two parameters is also presented. in which each peak is collapsed into a single energy value. From the results of our simulati ons, we find that this model still maintains the essential features of diffusion and growth oil this model surface. We Iind that hopping rat es along island edges are much higher than for isolated atoms (giving rise to compact island shapes), and that vacancy mobility is higher th an adatom mobility. We observe substantial dimer mobility (comparable to the single-atom mobility) as well as some mobility of trimers. The mobility of small islands affects the scaling of island density IV ver sus deposition rate F (N approximate to F-v) as well as the island siz e distribution. In the asymptotic limit of slow deposition. scaling ar guments and rate equations show that gamma=i/(2i+1), where i* is the size of the largest mobile island. Our Monte Carlo results, obtained f or a range of experimentally relevant conditions, show gamma=0.32 +/- 0.01 for the EAM barrier, 0.33 +/- 0.01 for the model I barrier and 0. 31+/-0.01 For the model II barrier. These results are loa rr than the anticipated value of gamma greater than or equal to 0.4 due to dimer ( and trimer) mobility. (C) 1998 Elsevier Science B.V.