Observation of a structural transition during the low-temperature growth of the Si(111)7×7?Pb interface

The low-temperature Si(111)7×7–Pb interface has been investigated using surface X-ray diffraction. The Schottky barrier height of the Pb/Si diode is known to depend on the atomic reconstruction of the interface. We have studied the structure of the evolving interface using X-ray growth oscillations and by interrupting the growth and measuring the specular reflectivity. Both techniques reveal that the film is initially highly disordered with irregular oscillations in the reflected intensity during growth. At a critical coverage of approximately five Pb layers, the whole overlayer crystallizes to a well-ordered Pb(111) structure with subsequent growth being layer-by-layer. Detailed models of the mature interface are consistent with the preservation, after burial, of the Si(111)7×7–Pb reconstruction originally formed during growth of the first monolayer.