Various interface structures formed between Si(lll) and a thick Ag overlayer are investigated by grazing-incidence x-ray diffraction. The (7x7) reconstruction of Si(lll) is preserved under a roomtemperature deposited Ag film. Upon annealing to 250°C the interface becomes (lxl). This is contrasted by the (V3xV3)/?30° structure formed by annealing a thin Ag film on Si(l 11). By depositing a thick Ag film on this (VJxV3)/?30° Ag/Si(lll) surface at room temperature, the (V3xV3)/?30° reconstruction is suppressed. PACS numbers: 68.35.Bs, 68.55.Bd, 68.55.JkIt is well known that a thin Ag film, on the order of 1-10 atomic layers thick, deposited on Si(l 1 1) will form a (V3xVJ)/?30° structure (root-3 structure, for short) upon annealing to temperatures above about 200 °C [1-3]. This is considered a prototypical, nonreactive metal-semiconductor system, and its structure has been the focal point of a large number of studies employing a variety of surface-science techniques (readers are referred to Ref.[4] for an up-to-date discussion). A basic question concerning this system, which is the main topic of this study, is whether or not this root-3 reconstruction represents an interface structure between Si and Ag. This question relates to a larger issue concerning interface structures in general, that is, whether or not the structure observed at monolayer coverage ranges can be taken as an indication of the true interface structure.Surface reconstructions can be observed quite easily with standard low-energy and medium-energy electrondiffraction techniques that are, however, totally inadequate for true interface structures involving a thick overlayer because of the very short penetration depth of the probing beam. Two techniques using high-energy, highly penetrating beams have been employed in recent years for interface studies: transmission electron microscopy (TEM) [5,6] and x-ray diffraction [7]. Both techniques involve substantial experimental difficulties. In the case of TEM, sample preparation is a significant problem. For x-ray diffraction, the interface signal is extremely weak because of the small scattering cross section of the interface. For these reasons, interface reconstructions for various material systems remain a relatively unexplored area.The present study employs grazing-incidence x-ray diffraction to determine the Ag/Si(lll) interface reconstruction. The diffraction experiment was carried out on beam line X-14 at the National Synchrotron Light Source, Brookhaven National Laboratory, and the sam-

Different Ag/Si(111) systems have been examined using synchrotron x-ray diffraction. Multi-atomiclayer deposition of Ag onto a Si(111)-(7X7)surface maintained at room temperature results in an unstrained, (111)-oriented film. The interface shows a Ag-modified (7 X 7) structure which when annealed above 200 -250'C transforms to a (1 X 1) structure. Although this is near the characteristic temperature for formation of the (&3 X&3)R30' surface reconstruction commonly observed for a monolayer of Ag adsorbed on Si(111),no evidence of this (&3X &3)R 30' reconstruction was found at the interface. A Ag monolayer (&3X&3)R30' surface, further covered by multilayer Ag deposition at room temperature, also shows no indication of the (&3X &3)R30 reconstruction at the interface. This indicates that the actual interface structure may or may not be related to the clean or adsorbed layer structures. The structure of the Ag-Si interface was further characterized by scans of the crystal truncation rods. Both the (7 X 7) interface prepared by room-temperature deposition and the annealed (1 X 1) interface show fairly sharp boundaries. The results suggest some intermixing occurs at the monolayer level for the annealed interface. The structure of the Ag film was also investigated.

Intensity distribution of x-ray scattering by thermal phonons in Si was recorded using synchrotron undulator radiation. A high-energy beam sent through a Si͑111͒ wafer in a transmission Laue geometry yielded a threefold symmetric pattern for the scattering cross section with rich details governed by phonon dispersion, population, and polarization. ͓S0163-1829͑99͒08105-9͔ PHYSICAL REVIEW B

The structure of a Si(111)-(7×7) surface capped by a 200 Å film of C60 was studied by grazing-incidence x-ray diffraction. The Si(111)-(7×7) reconstruction prepared in vacuum, including the loosely bonded ‘‘adatoms’’ on the surface, is preserved under the C60 overlayer. This result illustrates that C60 can be used as an inert cap for surfaces and suggests potentially interesting applications in surface science research and electronic device engineering.