Si(ill) surfaces quenched from 1100'C have been observed at around 470'C using hightemperature scanning tunneling microscopy. We closely investigated dynamic motion of Si adatoms in the metastable stacking-fault (SF) half cells of ll x ll and 13 x 13 dimer-adatom -stacking-fault (DAS) structures. On these SF half cells adatoms continually change their positions at high temperature of 470 C. Most of the interior adatoms escape from the SF half cells more easily than the corner or center adatoms. This indicates that the interior adatoms hardly contribute to the stabilization of n x n DAS structures.The Si(ill) surface has been extensively studied by a variety of techniques ' and the dimer-adatom -stackingfault (DAS) model of Takayanagi et al. is now widely accepted as a model for 7x 7 surface reconstruction. However, the role of Si adatoms in the stability of DAS structure is still unclear. Several proposals have been presented to answer this question. For example, Takayanagi et al. initially suggested that the stability of DAS structure stemmed from the reduced number of surface dangling bonds by the presence of adatoms. Payne proposed that dimer-adatom interaction stabilized DAS structure. Recently, using scanning tunneling microscopy (STM), we have studied the behavior of adatoms at the edge of a 7 x 7 DAS domain on a quenched Si(ill) surface. s The result shows that adatoms tend to adsorb to the sites adjacent to a dimer row and it has been concluded that adatom and dimer surely construct a stable structure. On the other hand, by a theoretical consideration Van-derbilt claimed that adatoms were not important for the DAS stabilization, but the formation of dimer-stackingfault (DS) part of the DAS structure, especially dimer row formation, was a Inore significant factor for the stabilization of 7 x 7 structure.The aim of this paper is to determine what the most important factor for the stabilization of DAS structure is and, in particular, to make clear whether the essential factor of DAS stabilization is the dimer-adatom interaction or the formation of DS part of the DAS structure. From the latter point of view, it should be possible to consider that DS structure stably exists without adatoms and adatoms play a minor role in stabilizing the DAS structure. One of the most eIII'ective ways to answer this question may be to remove all adatoms from a DAS structure and to examine the stability of the DS structure. The dimer-adatom stabilization model would be denied if the DAS structure remains without adatoms. However, the direct study of the DAS stability without dimer-adatom interaction has never been reported so far.In this paper, we present the STM study of the adatom contribution to the DAS stability by in situ observation of a quenched Si(ill) surface at around 470'C. The adatoms in the DAS structure can be classi6. ed into three types; corner, center, and interior. The corner adatom positions at each corner of stacking-fault (SF) or un-
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