Vibrational characterization of the oxidation products on Si(111)-(7 x 7)

The oxidation products on Si(111)-(7x7) are investigated at 82 K by means of high-resolution electron energy loss spectroscopy. The isotope-labeled vibrational spectra with 16O2, 18O2, and 16O 18O show that, in the initial stage of the oxidation, an O2 molecule dissociates to form a metastable product with an O atom bonding on top of the Si adatom and the other inserted into the backbond. The metastable product is observed as a dark site in the topographic scanning tunneling microscopy (STM) image and can be transformed to a stable product by the STM manipulation. Our results are in good agreement with recent theoretical calculations.     

Electron-phonon interaction at the Si(111)-7 x 7 surface

It is shown that electron-phonon interaction provides a natural explanation for the unusual band dispersion of the metallic surface states at the Si(111)-(7 x 7) surface. Angle-resolved photoemission reveals a discontinuity of the adatom band at a binding energy close to the dominant surface phonon mode at h(omega0) = 70 meV. This mode has been assigned to adatom vibrations by molecular dynamics calculations. A calculation of the spectral function for electron-phonon interaction with this well-defined Einstein mode matches the data. Two independent determinations of the electron-phonon coupling parameter from the band dispersion and from the temperature-dependent phonon broadening yield similar values of lambda = 1.09 and lambda = 1.06.     

Electron spin resonance observation of the Si(111)-(7x7) surface and its oxidation process

Electron spin resonance (ESR) observation of dangling-bond states on the Si(111)-(7x7) surface is demonstrated for the first time. The ESR spectra clearly show that a reaction of molecular oxygen with the Si(111)-(7x7) surface is associated with the appearance of a new dangling-bond center at unreacted Si adatoms. Most of the oxidized surface sites do not show ESR signals, but in a minor part of the surface another new type of surface defect is detected. The well known P(b) center at the SiO2/Si interface is found to evolve at an oxide thickness as thin as 0.3 nm.     

Low temperature scanning force microscopy of the Si(111)-(7x7) surface

A low temperature scanning force microscope (SFM) operating in a dynamic mode in ultrahigh vacuum was used to study the Si(111)- (7x7) surface at 7.2 K. Not only the twelve adatoms but also the six rest atoms of the unit cell are clearly resolved for the first time with SFM. In addition, the first measurements of the short range chemical bonding forces above specific atomic sites are presented. The data are in good agreement with first principles computations and indicate that the nearest atoms in the tip and sample relax significantly when the tip is within a few A of the surface.     

A superstructural 2D-phase diagram for Ga on the Si(111)- 7x7 system

The structural modifications of an Si(111)- 7x7 reconstructed surface and the evolution of growth induced by Ga adsorption in the submonolayer regime at various substrate temperatures ranging from room temperature (RT) to 600 °C, with a low Ga flux rate of 0.1 ML/min (1 ML∼6.8×10¹⁴ atoms/cm²) have been studied in-situ in Ultra High Vacuum (UHV) using Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED) and Electron Energy Loss Spectroscopy (EELS) as characterization probes. Ga grows in the Stranski-Krastanov (SK) growth mode for temperatures ranging from RT to 350 °C, where 3D-islands form after one and two flat monolayers of Ga adsorption, while for higher temperatures ranging from 450 to 550 °C, Ga grows in the Volmer-Weber (VW) growth mode. A comprehensive 2D-phase diagram for this Ga/Si(111) system for adsorption, which provides pathways to attain the observed superstructural phases, viz., √3x√3-R30°, 6.3x6.3, 6.3√3x6.3√3-R30° and 11x11, has been investigated. The characteristic EELS spectrum for each superstructural phase is also reported in this study.