The Sn/Si͑111͒ )ϫ) surface has been studied by photoelectron spectroscopy, low-energy electron diffraction ͑LEED͒, and scanning tunneling microscopy. Unlike Sn/Ge͑111͒, the Sn/Si͑111͒ surface shows a )ϫ) LEED pattern at low temperature also ͑70 K͒. The electronic structure, however, is inconsistent with a pure )ϫ) phase. Sn 4d spectra exhibit two major components and the valence band shows two surface bands. These features have been associated with the low-temperature 3ϫ3 phase in the case of Sn/Ge͑111͒. The similarity in the electronic structure points to stabilization of a low-temperature phase for Sn/Si͑111͒ also, but at a significantly lower temperature ͑Ͻ70 K͒.Phase transitions in low-dimensional systems have recently attracted a lot of experimental and theoretical interest. A striking example is the transition that occurs on the 1 Pb/ Ge͑111͒ and 2 Sn/Ge͑111͒ surfaces. The room-temperature )ϫ) reconstruction, with 1 3 monolayer of Pb or Sn adatoms, changes gradually to a 3ϫ3 phase when the temperature is lowered. As determined by surface x-ray diffraction, 3,4 the transition to the 3ϫ3 phase involves vertical atomic displacements in the adatom layer which give rise to sharp 3ϫ3 low-energy electron diffraction ͑LEED͒ spots. Scanning tunneling microscopy ͑STM͒ images of these surfaces show a transition from a )ϫ) to a 3ϫ3 unit cell, which has been attributed to the formation of a commensurate charge-density wave.1,2 Other electronic structure studies, concentrated on the Sn/Ge͑111͒ system, have been done by photoelectron spectroscopy.5-7 An interesting and rather puzzling result is that the electronic structures of the )ϫ) and 3ϫ3 surfaces are qualitatively quite similar. The two major Sn 4d components and the two surface-state bands that are observed find a natural explanation in a 3ϫ3 surface phase but are not directly accounted for in a )ϫ) periodicity.Although the Pb/Si͑111͒ and Sn/Si͑111͒ systems can be expected to behave in a similar way to their Ge͑111͒ counterparts, they have been much less studied. The fact that there is no report of a )ϫ) to 3ϫ3 transition on the Sn/Si͑111͒ surface seems to be reflected in the lower number of publications for this system. It is known, however, that the Sn 4d core level of the Sn/Si͑111͒ )ϫ) surface shows an unexpected second component.8 Inspired by this situation, we have used several techniques to address the interesting atomic and electronic structure of Sn/Si͑111͒.The Sn/Si͑111͒ )ϫ) surface has been studied using photoelectron spectroscopy, LEED, and STM. Various Sn coverages were investigated in order to find the optimum preparation of the )ϫ) surface. The use of roomtemperature STM allowed us to check the quality of the surfaces and to characterize the different types of defects that are present. In contrast to the Sn/Ge͑111͒ system, we do not observe any transition to a 3ϫ3 phase in LEED at temperatures down to 70 K, which was the lowest temperature in this study. Despite this difference we find that both the Sn 4d core-level and valence-band spectra show the...

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We report a direct and reliable way to produce the Si͑100͒-c(4ϫ4) reconstruction by submonolayer deposition from a SiC source and subsequent annealing. Auger electron spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy ͑STM͒ investigations reveal that a C amount equivalent to 0.07 monolayers ͑ML's͒ is sufficient to obtain full coverage of the c(4ϫ4)reconstruction. A deposition of 0.035 ML's C produces a c(4ϫ4)coverage of only 19%, indicating that C is not only present in the c(4ϫ4)areas, but also in the 2ϫ1 areas. There is not enough C to make it a regular part of the c(4ϫ4) reconstruction and we therefore conclude that the c(4ϫ4) reconstruction is strain induced. We find that a combination of the mixed ad-dimer and buckled ad-dimer models explains all main features observed in the STM images. Images of freshly prepared c(4ϫ4) surfaces exhibit a decoration of approximately 50% of the unit cells, which is attributed to perpendicular ad-dimers. Long exposures (Ͼ20 h) to the UHV background gas quench these features and the c(4ϫ4) reconstruction appears as if more homogeneous.

, Biasdependent scanning tunneling microscopy study of the oxygen-adsorbed Si (111) We have observed the initial stage of oxygen adsorption on a Si(111)-(7ϫ7) surface using scanning tunneling microscopy. Among the bright sites observed after exposing the surface to oxygen in occupied state images, there are differences in the bias dependence of the brightness. Taking into account the local density of states of the oxygen-adsorbed Si(111)-(7ϫ7) surface, we conclude that the sites appearing brightly only with a tip bias of уϩ2.1 V are the molecular oxygen. The preferred adsorption site of this molecular species is a corner adatom, which has an oxygen atom adsorbed into its backbond, of the faulted half of the (7ϫ7) unit cell.

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