Complex behavior in a simple system: low temperature Ag/Ag(100) growth revisited Yunsic Shim and Jacques G. Amar Department of Physics & Astronomy University of Toledo, Toledo, Ohio 43606, USA (Dated: June 2, 2009) The experimentally observed non-monotonic temperature-dependence of the surface roughness in Ag/Ag(100) growth over the temperature range T = 55 -180 K is examined. In general, we find that the surface roughness depends sensitively on a competition between a variety of low-barrier processes including downward funneling, edge-zipping and diffusion, atom-attraction, and interlayer diffusion at kinks. By taking these processes into account, along with the attraction of depositing atoms to microprotrusions, excellent agreement with experiment is obtained over the entire temperature range. PACS numbers: 68.55.-a, 81.15.Aa, 81.15.-z Recently, there has been a great deal of progress in un- derstanding the morphological evolution in epitaxial thin film growth (for a recent review see Ref. 1), and a variety of effects and processes have been shown to play an im- portant role. In addition to growth temperature, deposi- tion flux, and deposition angle, 2 these include the effects of crystal geometry, 3 the Ehrlich-Schwoebel (ES) barrier to interlayer diffusion, 4 edge- and corner-diffusion, 5,6 and the attraction of depositing atoms to the substrate. 2,7 Understanding these effects is important, since they can have a strong effect on a variety of important film prop- erties including the surface morphology. One case of particular interest is that of Ag/Ag(100) growth for which an unusually complex dependence of the surface roughness on deposition temperature has been observed over the temperature range T = 55 K - 300 K. 8 In particular, as the temperature was reduced below 300 K, the roughness of 25 monolayer films was found to first increase - with a peak at approximately 220 K - and then decrease as the temperature was further reduced. As the temperature was decreased below 135 K, the roughness again increased - with a second low-temperature peak at approximately 90 K - and then decreased again as the temperature was further reduced to 55 K. The non-monotonic behavior of the surface roughness at high temperature (T = 135 K - 300 K) has been ex- plained by Stoldt et al. 8 In particular, the increase in the surface roughness as the temperature is decreased from 300 to 220 K is due to the increased effect of the ES barrier to interlayer diffusion as the temperature is decreased over this temperature range. Similarly, the de- crease in the surface roughness as the temperature is fur- ther decreased from 220 K to 135 K has been explained by the increased role of downward funneling (DF) 9 of atoms deposited near step-edges, due to the increased island- and step-density as the monomer diffusion rate decreases. Using a simplified kinetic Monte Carlo (KMC) model which includes relatively small but non-zero bar- riers for DF of atoms deposited at non-fourfold hollow sites (e.g. ‘restricted’ DF), Stoldt et al 8 were able to partially explain the initial increase in roughness at low temperature (T< 135 K). However, the resulting model led to poor agreement with experiment and was also un- able to explain the decrease in the roughness below 90 K. In addition, it leads to predictions 8,10 for the low- temperature thin-film vacancy density which are more than an order of magnitude higher than the results of recent parallel temperature-accelerated dynamics (par- TAD) simulations. 11 Here we show that by taking into account the ex- istence of very-low barriers for edge-smoothing (‘edge- zipping’) and DF at three-fold hollow sites, along with the effects of SR attraction of depositing atoms to mi- croprotrusions, excellent quantitative agreement with ex- periment can be obtained over the temperature range T = 55 - 110 K. Furthermore, by taking into account the existence of low barriers for concerted interlayer diffu- sion at and near kinks, both qualitative and quantitative agreement with experiment can be obtained over the en- tire low-temperature range (T = 55 - 135 K) as well as at higher temperatures. We note that our model also leads to a negligible low-temperature vacancy density in good agreement with accelerated dynamics simulations of low-temperature Cu/Cu(100) growth. 11 In order to include the effects of short-range (SR) at- traction in our simulations, we have used a hybrid model which combines a one-atom molecular dynamics (MD) simulation of the deposition process with kinetic Monte Carlo (KMC) simulations of activated events. Following this method, the depositing atom is assumed to follow the trajectory determined by its interaction with the sub- strate (with the substrate atoms held fixed in their lattice positions) until its distance to the closest substrate atom is equal to the nearest-neighbor distance. The deposit- ing atom is then placed at the nearest fcc lattice site, and unless this site is a four-fold hollow site or other site with a barrier for DF, assumed to undergo DF until it reaches a four-fold hollow site or “trap” site with a barrier for DF. We note that in recent simulations of normal inci- dence Cu/Cu(100) growth at T = 160 K, 14 the surface roughness obtained using this method was found to be only slightly lower than that obtained by carrying out a full MD simulation of the depositing atom and surround- ing substrate. In order to check for the dependence on