Origin of O 1s core-level shifts on oxygen adsorbed Si(111)-(7x7)

Atomic force microscopy identification of Al-sites on ultrathin aluminum oxide film on NiAl(110)

Ultrathin alumina film formed by oxidation of NiAl(110) was studied by non-contact atomic force microscopy in an ultra high vacuum at room temperature with the quest to provide the ultimate understanding of structure and bonding of this complicated interface. Using a very stiff Si cantilever with significantly improved resolution, we have obtained images of this system with unprecedented resolution, surpassing all the previous results. In particular, we were able to unambiguously resolve all the differently coordinated aluminum atoms. This is of importance as the previous images provide very different image patterns, which cannot easily be reconciled with the existing structural models. Experiments are supported by extensive density functional theory modeling. We find that the system is strongly ionic and the atomic force microscopy images can reliably be understood from the electrostatic potential which provides an image model in excellent agreement with the experiments. However, in order to resolve the finer contrast features we have proposed a more sophisticated model based on more realistic approximants to the incommensurable alumina interface.

Adsorption and dissociation of oxygen molecules on Si(111)-(7×7) surface

The adsorption and dissociation of O2 molecules on Si(111)-(7×7) surface have been studied by first-principles calculations. Our results show that all the O2 molecular species adsorbed on Si(111)-(7×7) surface are unstable and dissociate into atomic species with a small energy barrier about 0.1 eV. The single O2 molecule adsorption tends to form an ins×2 or a new metastable ins×2* structure on the Si adatom sites and the further coming O2 molecules adsorb on those structures to produce an ad-ins×3 structure. The ad-ins×3 structure is indeed highly stable and kinetically limited for diving into the subsurface layer to form the ins×3-tri structure by a large barrier of 1.3 eV. Unlike the previous views, we find that all the ad-ins, ins×2, and ad-ins×3 structures show bright images, while the ins×2*, ins×3, and ins×3-tri structures show dark images. The proposed oxidation pathways and simulated scanning tunneling microscope images account well for the experimental results and resolve the long-standing confusion and issue about the adsorption and reaction of O2 molecules on Si(111) surface.

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.