Mechanisms of mound coarsening in unstable epitaxial growth

摘要

The effects of various lsland and mound relaxation mechanisms on the mound morphology and asymptotic mound coarsening exponent n are investigated for growth on fcc/bcc(001) and fcc (111) metal surfaces. While the strenght of the diffusion bias (due for example to an Ehrlich-Schwoebel step barrier) affects the crossover time, the presence or absence of corner diffusion is found to play a crucial role in determining the asynmptotic value of the coarsening exponent. In the absence of island-relaxation or in the case of island-relaxation without rapid corner diffusion the asynmptotic exponent is found to satisfy n approx= 1/4. However, when rapid corner-diffusion is allowed, the coarsening exponent is found to approach 1/3. For the case of reversible siland growth a strong step-barrier leads to an effectiue corner diffusion so that a growth exponent of either 1/4 or 1/3 may be observed depending on the strength of the step-barrier. These results appear to account for recent experimental observations of a large coarsening exponent (n approx= 0.33) in epitaxial growth of Rh/Rh(111) at high temperature as well as of a smaller coarsening exponent (n approx= 1/4) observed in epitaxial growth of Fe/Fe(001) and Cu/Cu(001). An explanation for these results is also presented in terms of the effects of corner diffusion on the surface current and mound morphology.