Electromigration induced step instabilities on silicon surfaces.

摘要

In this thesis I will report on a sequence of experiments designed to address the complex step bunching behavior observed on Si(111). Prior to our studies it was proposed that increased step permeability [S. Stoyanov, Surf. Sci. 416, 200 (1998)] might be responsible for the change from step-down to step-up current bunching under sublimation conditions. Step permeability means that there is a significant direct flow of atoms from one terrace to another. According to this model, if steps are permeable enough then the surface will be unstable (stable) toward bunching for a step-up current under net sublimation (growth) conditions. By studying how Si deposition conditions affect step bunching during step-up direct current heating we find that this model cannot account for step bunching for step-up current in Regime II. We arrived at this conclusion by experimentally showing that there was no transition from step instability to stability when changing from net sublimation to net growth conditions in Regime II.;Another issue concerning step bunching on the Si(111) is whether the diffusion of atoms across terraces or the attachment/detachment of atoms to and from steps is rate limiting. We address this issue by measuring how the number of steps N and the minimum terrace width lmin in a bunch depend on initial surface miscut in all three temperature regimes. This is the first report of how bunching depends on surface miscut, and by comparing experiments to both analytical predictions and numerical simulations of the Stoyanov's sharp-step model we conclude that diffusion across terraces is rate limiting during step-down bunching in Regimes I and III for a wide range of surface miscut. This is contrary to previous beliefs that the attachment/detachment of atoms from step-edges was extremely slow compared to the diffusion on terraces. Because the original sharp-step model cannot explain step-up current bunching we were unable to conclude anything about Regime II, but it is interesting to note that we observe nearly the same dependencies of N and lmin on miscut in all three step bunching regimes. This suggests that the fundamental mechanism for step bunching may be similar at all temperatures. (Abstract shortened by UMI.).