Growth of Ag on Pb Island with Si(111) Substrate^*

We study the growth of Ag on Pb island surface with low temperature scanning tunnelling microscopy. Two growth modes, the subsurface island mode and the surface alloy mode, are observed on the substrate at room temperature and at 100 K, respectively. In the surface alloy mode, the perfect alloy AgPb₂ is formed on the Pb island surface after annealing. The two growth modes at different substrate temperatures are attributed to the existence of an exchange barrier of Ag atoms on the Pb island surface. The modulation of the exchange barrier by the quantum well states is also observed on the Pb island.