G. Rossi

Structure of the Ag on Si(111) 7 × 7 interface by means of surface exafs

Polarization dependent surface extended X-ray absorption fine structure (SEXAFS) measurements are used to determine the structure of the Ag on Si(111)7 × 7 system at the early stages (< 3 monolayers (ML)) of interface formation. At room temperature (RT) Ag is found to initially (< 0.5 ML) chemisorb in the threefold hollow site, approximately 0.7 A above the outermost Si layer with an average Ag-Si distance of 2.48±0.05 A. Above monolayer coverage the SEXAFS spectrum is dominated by the Ag-Ag distance indicating Ag island formation on the surface. Upon heating (200 ⩽T⩽ 600°C) a (√3 ×√3)R30° LEED pattern is observed. At the lowest coverage ( < 0.7 ML) this pattern is determined to arise from Ag atoms which are embedded in the threefold hollows, ~ 0.7 A below the first and above the second Si layer, with a Ag-Si distance of 2.48 ± 0.04 A. At higher coverage (

Nature of the valence states in silicon transition metal interfaces

1 ML) Ag clusters are found to grow on this interface with the same Ag-Ag distance as in Ag metal. Our results are discussed in the context of previous experimental and theoretical results.

Surface science lettersChemical reaction at the Ge(111)-Ag and Si(111)-Ag interfaces for small Ag coverages

Abstract Photoemission spectroscopy using synchrotron radiation and excitation energies around the Cooper minimum have been applied to studies of the nature of the bonding in SiPd and SiPt interfaces. Evidence is presented that breaking of the tetrahedral coordination in Si and the Si-metal interaction introduces new valence states in the interface region. The energy positron and the orbital character of these Si-metal mixed valence states are determined. A suggestion is made how these results can be generalized to other Si-transition metal interfaces.

Intermixing at the early stage of the Si(111)/Ag interface growth

We present a detailed synchrotron radiation photoemission study of the elemental semiconductor-Ag interfaces for very small coverages (up to 2.5 monolayers). Our results provide the first evidence of a chemical reaction and intermixing between Ag and the (IV) semiconductors at room temperature.

Formation of the PtSi(111) interface

Abstract A synchrotron radiation (SR) photoemission investigation of the Si(111) cleaved surface+Ag thin overlayers (0.25–2.5 monolayers (ML) has been carried out at room temperature with the Cooper minimum method and at liquid nitrogen temperature (LNT) for a critical coverage (0.5 ML) with the energy dependent escape depth method. The two experiments give independent evidences of intermixing, following chemical reaction, between Ag and Si at the very low coverages, indicating that the very early stage of the Si(111)/Ag junction is not a sharp interface but an intermixed region extended to at least two layers of material.

Reactive germanium/transition metal interfaces investigated with synchrotron radiation photoemission: Ge/Ni and Ge/Pd

Abstract We have studied the atomic structure and some of the electronic properties of the PtSi(111) interface as it forms under evaporation of Pt onto a clean silicon surface near room temperature, using Auger electron spectroscopy (AES), photoemission with synchrotron radiation, low energy electron diffraction (LEED), and Auger/sputter profiling. Growth of a single homogeneous Pt monolayer is observed at sufficiently low rates of deposition of Pt. For Pt coverages above one monolayer a reaction occurs between Pt and Si resulting in silicide-like features in the Si(L 23 VV) Auger spectrum and in valence band and core level photoemission excited with synchrotron radiation. An inward diffusion of Pt is observed in cases of elevated substrate temperatures and during ion bombardment. For slow deposition of Pt on a 7 × 7 surface, a change of the 7 × 7 pattern into a weak √3 × √3 pattern is observed. Further experiments were conducted to investigate the annealing behaviour of monolayer deposits with LEED and AES. With annealing to ∼ 800°C sharp √3 × √3 patterns are found. At intermediate annealing temperatures (320°C). a √7 × √7 pattern is sometimes observed.

The Ge/Au interface investigated with photoemission at the cooper minimum

Abstract We report the first photoemission studies of the electronic structure at the interface between Ge(111) and d-metal overlayers at monolayer coverages. Two systems are discussed, Ge(111)/Pd and Ge(111)/Ni. In the case of Ge(111)/Pd a broad, reacted germanide-like phase is formed with similar electronic features to the Si(111)/Pd interface. In the Ge(111) Ni case an evolution of the valence band structure versus coverage from a reacted situation to a metal rich situation is found, indicating a strong gradient of concentration. The interface is narrower than for Ge/Pd. Photoemission spectra of the valence band at hv =80 eV are presented as well as core line data of the Ge 3d which show the trends in binding energy shifts and the changes in the relative intensities as a function of metal coverage. The discussion is carried out on the differences in the growth of these interfaces and comparisons are made to the correspondent silicide/metal systems.

Surface science lettersPhotoemission investigation on the oxidation of Si(111)-Ag interfaces and its relation to the interface structure

Abstract The formation of the Au/Ge interface at room temperature is followed for increasing Au coverages and studied by energy dependent photoemission. The experimental EDCs are discussed and compared with density of states calculations for Au 3 Ge. The evidence for Au 5d-Ge 4p mixed states indicates that the Au/Ge interface is similar to the near-noble metal germanides. A note on the photoemission branching ratio in semi-isolated Au and in thick Au layers is added.

Chemical reaction at the Ge(111)-Ag and Si(111)-Ag interfaces for small Ag coverages

We give the first photoemission results on the enhancement of Si reactivity to oxygen when a noble metal (Ag) is present. The tunability of synchrotron radiation (SR) has been used to get high surface sensitivity and to take advantage of cross section energy dependence. We show that when one monolayer of Ag is deposited onto Si(111), the exposure to oxygen (30×106L) originates the overgrowth of an oxide phase which is basically SiO2. This indicates that Ag breaks the sp3 configuration of Si atoms with a consequent dramatic increase in the Si reactivity. This behaviour rules out the model of Ag adsorbed on top of Si with an atomically abrupt interface.

Compound formation and bonding configuration at the Si-Cu interface

Abstract We present a detailed synchrotron radiation photoemission study of the elemental semiconductor-Ag interfaces for very small coverages (up to 2.5 monolayers). Our results provide the first evidence of a chemical reaction and intermixing between Ag and the (IV) semiconductors at room temperature.