The formation of a sharp metal-semiconductor interface for the growth of Quantum Size Effect islands

In order to form Quantum Size Effect (QSE) metal islands on semiconductors, a smooth island-substrate interface is necessary to set up the electron standing waves that lead to the new quantum confined states. How this occurs for the Pb-Si(111)7x7 system is a mystery because of the large lattice mismatch and the highly corrugated 7x7 reconstruction. To understand how QSE islands develop in this system we have performed structural Surface X-ray scattering measurements on the initial formation of Pb islands grown on Si(111). We show how a smooth Pb-semiconductor interface develops through a series of structural arrangements. Once a vertically disordered Pb monolayer is completed, second layer atoms nucleate fcc clusters. These clusters undergo a displacive transition lifting them above the Si adatoms. This allow the Pb islands to ``float'' above the Si substrate so that the first island layer is smooth, thus setting up the proper boundary condition for QSE.