Adsorption of Atoms and the 7x7 Reconstruction Pattern

We see that the effects on the silicon (111) surface of adding atoms from columns I and 7 of the Solid State Table are very much the same. This also appears to be true if the added atoms are from columns 2 and 6 likewise if they are from columns 3 and 5 adsorption from columns n and 8 - ii is similar. 

An added atom from columns 3 or 5 neutralizes the three atoms below it Just as added atoms from columns 1 and 7 neutralize the single atoms below them. This would leave the alternate inward-outward reconstruction between such added titoms. We would expect the three back atoms to relax in order to reduce the angular discrepancy. It is possible that they move into the surface, tending to raise the atoms around them. The covertige of such atoms would certainly be limited to one added atom for each three surface atoms, and might well be limited further hy the elastic repulsion between the extended distortion patterns. 

We turn finally to added atoms from column 4, with the addition of a silicon atom to silicon being the most important case, since it will occur naturally from difl usion across the surface. We can expect the most stable position for an added atom outside (he surface to be above three surface atoms, ju.st as for columns 3 and 5. The additional atom bonds with three dangling hybrids, leaving a single dangling hybrid surrounded by an alternate in-out reconstruction. It would be reasonable to expect that such an added atom would have higher energy than those in the undisturbed iii-out reconstruction, and that thc.se added atoms would 

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