Dangling bond interface states

It should be mentioned that as well as for metals the passivation of semiconductors (particularly on Si, GaAs, InP) is also a subject of intense investigation. However, the goal is mostly not the suppression of corrosion but either the fonnation of a dielectric layer that can be exploited for devices (MIS stmctures) or the minimization of interface states (dangling bonds) on the semiconductor surface [63, 64]. 

Concerning the role of dimensionality we observed, in the three fully passivated cases, that the material is a semiconductor, as the band structure of the Si[i]-SiC>2 SL in Figure 39(a) shows, and that there is an opening of the gap as the thickness of the Si layer decreases. The band structure shows a gap which is slightly indirect for the presence of a state at the top of the valence band (mostly related to the Si atoms in the inner Si layer), that is partially due to the interaction between the interface Si and its double-bonded O atom. If we remove this extra O, leaving the two dangling bonds of the interface Si unsaturated, we find that the material is still a semiconductor with a new... 

Surface states on a semiconductor in a vacuum can sometimes be explained by means of the spare bonds that dangle from atoms on surfaces, or defects associated with dislocations. Neither of these mechanisms works at the semicon-ductor/solution interface. The dangling bonds will be expunged by adsorbed water, etc. Experiment shows that the concentration of surface states on semiconductors in solution is strongly potential dependent, and that defects in the crystal structure would not be potential dependent, at least until anodic dissolution of the substrate itself began. 

As to the nature of the surface states represented by Fig. 17, these are not likely to be calssical surface states provided, e.g., by dangling bonds. Such bonds (characteristic of the semiconductor-vacuum interface) are likely to have been removed by the adsorption of water from the solution. They would not have been expected to vary with potential (cf. Fig. 17). The surface states being measured here may result from adsorption of ions from solution. 

This system has been studied by Goldstein [294] and Levine [295], and seems to be an example of very site-specific adsorption in which the Cs atoms occupy four-fold coordination sites above the uppermost Si atoms. There is some similarity with other systems in that the Si dangling bond states are removed by Cs deposition to be replaced by Cs-induced gap states. There is, however, no evidence for interface instability effects. 

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