Electronic growth of Pb islands on Si(111) at low temperature

The growth of Pb films on the Si(111)7x7 surface has been investigated at low temperatures using scanning tunneling microscopy. Flat-top Pb islands are formed and at low coverage the thickness of islands is confined in the range of four to nine atomic layers. Among these islands, those of seven-layer height are the most abundant. In low coverage limit, these multilayer islands prefer to grow in size instead of in thickness, showing a quasi-two-dimensional growth property. This growth behavior, different from the conventional growth modes, arises from the quantum size effect. At higher coverage, the growth also reveals layer-by-layer behavior. The Arrhenius plot of the island density versus temperature shows a linear relationship, indicating the formation of islands can be explained by the nucleation and growth theory. We also study the growth of Pb films on the incommensurate Pb/Si(111) surface at low temperatures. Flat Pb islands can be grown as well, but the threshold thickness is reduced to two atomic layers instead of four.