DIFFUSIONAL ATTRACTIONS BETWEEN VOIDS ON A SI(111)7X7 SURFACE

Ostwald ripening or coarsening is a term which describes the growth of large domains at the expanse of small ones in the final stages of any phase separation process. Several theories were developed in order to explain this process. All of them are based on the Lifshitz, Slyozov, and Wagner (LSW) model which predicts an increase in the average radi us of domains (r) over bar(t) proportional to t(1/3) and a reduction o f the number density of domains as a function of time N(t) proportiona l to t(-1). The LSW model assumes static and circular domains, which a re distributed at random. These assumptions were found to be incorrect by many experiments. And more advanced theories were necessary to exp lain these observations. In our work, we study the coarsening of voids formed in a Si(111)7 x 7 surface covered by 0.8 bilayers (BL). Beside s the expected increase in the diameter of voids and the decrease in t he number density, very strong attractions and rapid motion of voids t owards each other were clearly observed. The result of the attraction between the voids is coagulation of voids and an increase in the spati al correlations between voids, as the coarsening process progresses, a s we indeed observed. This is contradictory to the ''ideal gas'' pictu re which is the basis of the LSW model. Basically, this can be explain ed by a concentration gradient which develops between two voids of dif ferent size and the growth of voids in the direction of the larger con centration. However, this model cannot explain the large diffusional m otions observed in our experiments. It is proposed that an amplificati on mechanism exists, whereby the arrival of a diffusing vacancy to the boundary of the void causes the release of several adatoms from the b oundary. This process causes a much larger motion than expected by a s ingle vacancy absorption. This picture, which is consistent with the s tructure of the steps, demonstrates how the microscopic details might have a significant affect on the global coarsening process. (C) 1997 A merican Vacuum Society. [S0734-211X(97)04203-0].