New features of the Si(100)-c(4 x 4) reconstruction observed with STM: suggestion of the structure with lowered symmetry

We report on the first scanning tunneling microscopy (STM) observations demonstrating that the Si(1 0 0)-c(4 x 4) surface structure may have a point group symmetry I in. Along with those, we were able to reproduce well known images of a point group symmetry 2 mm, which account for most of our STM experiments. We suggest, that the atoms belonging to the c(4 x 4) unit cell may occupy slightly non-equivalent positions - a difference which is not always detected, thus leading to the observation of a higher symmetry. A refined mixed ad-dimer model of the Si(1 0 0)-c(4 x 4) reconstruction is proposed and considered for explanation of obtained STM images. The corresponding atomic arrangement is found to be in the state of local energy minimum, not the global one, as confirmed by semi-empirical calculations. When compared with the 2 x 1 structure in the ad-layer of equal surface atomic density, the refined mixed ad-dimer model was found energetically grossly unfavorable. Thus, its validity can only be tested by more sophisticated ab initio calculations, possibly considering some impurity species. We also demonstrate that the Si(1 0 0)-c(4 x 4) surface has lower number of defects as the rate of cooling down after annealing at 650 degreesC decreases. Our data prompt us to think that the c(4 x 4) reconstruction is not a frozen metastable phase, but the basic factors which stabilize it in favor of a more familiar 2 x 1 reconstruction still remain unclear. (C) 2001 Elsevier Science B.V. All rights reserved.