We report a quantitative atomic scale study of nucleation kinetics on an inhomogeneous substrate. Our model system, $\mathrm{Al}/\mathrm{Au}\left(111\right)-(\sqrt{3}\times 22)$, reveals a distinct nucleation transition due to the repulsive nature of surface dislocations. Whereas for $T<\mathrm{}200\mathrm{}\mathrm{K}$ Al adatoms are confined to quasipseudomorphic stacking areas experiencing a very small diffusion barrier $(30\pm 5\mathrm{meV})$, at $T>\mathrm{}200\mathrm{}\mathrm{K}$ surface dislocations, representing repulsive barriers of $\Delta E\approx 560\mathrm{meV}$, can be surmounted. The results illustrate the significance of surface dislocations as repulsive line defects in nucleation and growth.

• Received 29 June 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.1732