FORMATION OF BR-TERMINATED SI-6 RINGS DURING ETCHING OF SI(111)-7X7

Scanning tunneling microscopy was used to study the morphological evol ution of Si(111)-7 x 7 exposed to molecular bromine. Etching at 900 K results in the removal of adatoms and conversion from the 7 x 7 surfac e to a 1 x 1 Br-terminated structure. Removal is dominated by bilayer step retreat with edge profiles that reflect etching anisotropies. Uni que structures attributed to regrowth of Si released during step etchi ng were consistent with six-membered Si rings terminated with Br. The distribution of such rings near the steps reflects the local bonding a nd etching anisotropies for the steps. Theoretical analyses of these r ings terminated with Cl determine bond lengths, bond angles, and charg e transfer within the ring.