Direct observation of reaction-limited aggregation on semiconductor surfaces

We report a discovery that the nucleation and growth of two-dimensional (2D) Ge islands at a Pb layer covered Si(111) surface are reaction limited. Using scanning tunneling microscopy, a compact-to-fractal island shape transition is observed as the; deposition flux is lowered, the temperature is raised, or at a low Ge coverage. This behavior is completely opposite to what was predicted from those theories based on diffusion-limited aggregation and previous experimental observations. Energy barriers are found to exist for the nucleation and growth of Ge islands, indicating that their growth behavior is exchange-reaction rate limited.