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The growth of ultrathin silicide ͑ϳmonolayer͒ has been studied by scanning tunneling microscopy. Ni atoms react with the Si͑111͒-7ϫ7 surface to form clusters at room temperature. With increasing annealing temperature, they gradually coalesce and form monolayer islands surrounded by ͱ19ϫͱ19 ring structures. The surface structure shows ͱ3ϫͱ3-R30°at 400°C and 2ϫ2 at 600°C. The latter structure is a new finding for the Ni/Si system. It is suggested that the transformation of the surface structure on the monolayer island can be related to a change of the interfacial structure.

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