Formation of multilayer two-dimensional Pb Islands on Si(111)7×7 at low temperature: From nucleation to growth

The morphology of Pb islands formed on Si(111)7×7 at low temperature has the characteristics of both three-dimensional (3D) and 2D structures as they are of multilayer thickness but have a flat top surface. We use scanning tunneling microscopy to observe the growth behavior from the incipient stage to the formation of islands. Our observations show that the growth is 3D (Stranski-Krastanov mode) right after the nucleation, but that there is a transition from 3D to 2D when the nucleus reaches a certain size. This transition is driven by the quantum size effect, and it changes the growth behavior of Pb islands to be similar to that of single-layer islands in homoepitaxy systems. This similarity allows us to apply the scaling theory to analyze the size distribution of Pb islands, and also to conclude the critical size to be three atoms at the nucleation stage.