Oxygen adsorption on GaAs

We will treat this system in some detail because it represents an excellent example of how the combination of kinetic measurements, photoemission, electron spectroscopic and surface structure studies in conjunction with theoretical calculations can produce a rather complete understanding of an adsorption process. However, the work has effectively been restricted to 110 surfaces for two reasons. Firstly, from an experimental viewpoint, it is the surface obtained by in situ cleavage and, until very recently, with the advent of molecular beam epitaxy [111], there was no alternative technique for preparing clean, damage-free surfaces. From a theoretical 

The basic effects with which we will be concerned are the manner in which the surface electronic structure influences adsorption and how it is modified by the adsorption process, thus defining the adsorption site. In purely kinetic terms, the important parameters are the sticking coefficient at zero coverage, the saturation coverage and the transition from oxygen adsorption to oxide growth. The amount of kinetic information is very limited, but this is more than compensated by the extent of ELS, UPS and XPS studies. 

The coverage dependence of the sticking coefficient could be described by an Elovich-type equation, viz. 

Somewhat different behaviour is observed if excited oxygen species are present (produced by a hot filament) as shown by the results in Fig. 29(b). The chemically shifted peak (AE = 2.9eV from the As 3d level) is present as before, but it does not increase beyond an exposure of 5 x 10s L excited oxygen. Instead, a second peak with a binding energy shift of 4.5 eV appears at about this exposure and eventually becomes the dominant As-associated peak. The Ga 3d peak also begins to broaden at this same exposure due to the formation of a shifted (AE= leV) peak, which increases concurrently with the shifted As peak. Finally, for very heavily oxidized surfaces, the shifted Ga peak is present together with two As peaks shifted by 0.4 and 3.2 eV with respect to the clean surface As peak, which is totally absent. 

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