X-ray studies of metal thin film growth on semiconductors

Real time in situ synchrotron x-ray studies of continuous Pb deposition on Si(111)-(7 x 7) at 180 K reveal an unusual growth behavior. A wetting layer forms first to cover the entire surface. Then islands of a fairly uniform height of about five monolayers form on top of the wetting layer and grow to fill the surface. The growth then switches to a layer-by-layer mode upon further deposition. This behavior of alternating layer and island growth can be attributed to spontaneous quantum phase separation based on a first principles calculation of the system energy.;X-ray reflectivity measurements have been performed for an investigation of the structure of Ag films deposited in situ on Ge(111) over the thickness range of 3--15 monolayers. The films deposited at a substrate temperature of 110 K are not well ordered, but become well ordered upon annealing, as evidenced by substantial changes in the x-ray reflectivity data. The thickness distribution for each annealed film, deduced from a fit to the reflectivity data, is remarkably narrow, with just two or three adjacent discrete thicknesses present, despite the large lattice mismatch between Ag and Ge. In some cases, the film thickness is nearly atomically uniform. The results are discussed in connection with recent models and theories of electronic effects on the growth of ultrathin metal films.