The ordering of Bi on a Si(111)-7&&7 surface was studied as a function of overlayer coverage and deposition conditions using low-energy electron diff'raction (LEED) and Auger electron spectroscopy.We observed a one-third-monolayer and a saturated one-monolayer phase. Both phases displayed identical J3XJ3-R30' LEED symmetries. LEED intensity data were used to differentiate between the two phases and to determine a quantitative atomic geometry via a thorough dynamical LEED analysis.The structural characterization of column-III and -V elements deposited on semiconductor substrates continues to be a major subject of study. This interest is driven in part by the desire to improve III-V epitaxy on silicon for application to the manufacture of microelectronics. ' The Si(111)surface is one of the most thoroughly studied substrates. It is known that atomic reconstruction occurs for the free surface of Si(111) and that this reconstruction can be significantly modified by the presence of adatom impurities even at trace, submonolayer concentration. The physical origin of surface reconstruction can be understood as a minimization of the surface free energy.The Si(111)-1x 1 surface with all atoms located at their ideal bulk positions is not realized since it would have too many energetically costly dangling bonds. The 2X 1 and 7X7 reconstructions are stabilized electronically by the removal of surface dangling bonds, leading to a subtle change in the surface atomic geometry. The introduction of covalently bonded adatoms introduces an additional term in the surface free energy, that passivates the surface dangling bonds. Column-III (Al, Ga, In) adatoms generally induce a J3x J3-R30' (called J3Xv 3 hereafter) reconstruction on the Si(111) surface by bonding at the T4 site directly above second-layer Si atoms. The column-V metal As, on the other hand, stabilizes a 1 x 1 surface cell by substituting for the topmost (absent) Si atom. One might expect As-like reconstructions to occur upon adsorbing other column-V elements such as Sb and Bi. Instead, both Sb and Bi adatoms induce a J3XJ3 structure; no well-ordered 1 x 1 structure has been reported. Recent x-ray-diff'raction and scanning-tunnel microscope (STM) studies show that the J3XJ3 reconstruction involves adatom trimers at each 43 x J3 site, at one monolayer (ML) coverage.This trirner model disagrees with the model of 3 -ML saturated coverage proposed earlier by Kawazu et al. However, x-ray-diffraction results are ambiguous with respect to a 180 rotation of the Si(111) surface. ' Therefore, the atomic configuration responsible for the J3XJ3 reconstruction on an atomic scale has not been determined. In this paper, we present results from low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) studies which show that two distinct phases form the v3X&3 superlattice in the Bi/Si(111) system. The Bi coverage necessary for the completion of each phase was determined to be about -, ' and 1 ML and the substrate temperatures corresponding to each phase are -360 C an...

Arrays of phase-locked Josephson junctions have been developed and tested which deliver over 1 μW of power, via superconducting microstrip, to loads with resistances ranging from 20 to 60 Ω. The arrays, designed to operate at 350 GHz, typically are continuously tunable from 350 to 450 GHz or higher, the upper limit being set by the onset of loss in the lead alloy microstrip.

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