The complex reconstruction characteristic of annealed Si(111) surfaces is removed by the interaction of As with the clean Si(111) 7x7 surface. Surface-sensitive core-level spectroscopy measurements of the Si(111):As 1&1 surface reveal a well-ordered surface with threefoldcoordinated As atoms replacing the Si atoms in the outermost layer. The absence of dangling bonds in this structure leads to an unreconstructed surface which is highly resistant to contamination.Si(~&~) 4s:1x1Side view in the (110) plane of the Si(111):As 1 x 1 surface. The As atoms substitute for the top Si layer, resulting in a stable, unreconstructed surface.While many factors such as strain' and correlation energies combine with the electronic energy to determine the microscopic structure of a reconstructed surface such as Si(111)7 x 7, 2 the high energy of the ideal Si(111)surface arises from the existence of metallic dangling-bond orbitals. If these orbitals could be either removed or filled, the driving force for reconstruction would be removed, and a 1&&1 surface should result. We find that this situation is realized in practice by arsenic termination of the Si (111) surface. The atoms in bulk Si are tetrahedrally coordinated with sp3 hybridized bonding orbitals. At the (111)surface, Si atoms in the top layer have only three bonds, leading to instability and reconstruction. Arsenic, on the other hand, prefers a threefold coordination, with three orbitals taking part in bonding and the two remaining valence electrons in a nonbonding "lone-pair" state. If As atoms substitute for the surface Si atoms at the (Ill) surface, all atoms can be optimally bonded (see Fig. 1). Each Si atom in the bulk (b) is bonded to four Si atoms; each interface Si atom (a) is bonded to one Si and three As atoms; each As atom at the surface (shaded in Fig. 1) is bonded to three Si atoms. The result is an unreconstructed, stable Si(111):As 1 x 1 surface.This configuration is similar to the situation of As on Ge(111) which has been studied with angle-resolved photoemission spectroscopy. s However, in the germanium case the As atom is nearly the same size as the Ge atom it is replacing, so that terminating the surface with As does not lead to a large strain at the surface. For the silicon surface, the possibility exists that the larger As atom will lead to a strain-induced reconstruction. This does not happen, indicating that dangling-bond removal dominates over strain effects. It should be noted that As termination of the Si(111)surface is quite different from the 1 x 1 surface structure which results from chlorine adsorptions on Si(111)2x 1 or hydrogen adsorptions on Si(111)2&1 and Si(111) 7&7. In the case of Cl or H termination, the monovalent adatom passivates the surface by capping a single silicon dangling bond; in the case of As termination, we will show that the arsenic replaces the top silicon layer, bonding to three silicon atoms and terminating the surface with a nonreactive lone-pair orbital. We expect that the As atoms relax outwards from the bulk Si lat...

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Efficiencies for diffraction of 45-275-eV x rays into orders by interferometrically formed, electrodeposited, gold transmission gratings have been measured on the 4 degrees beam line at the Stanford Synchrotron Radiation Project (SSRP). Anomalous dispersion affects the observed efficiency since the gold is partially transmitting to x rays. Model calculations which include anomalous dispersion are in good agreement with observations. With a suitable choice of material and thickness, a grating can be optimized for a given wavelength range by reducing the zero order transmission and enhancing the higher orders. Even orders are suppressed for a grating with equal slit and wire sizes.

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