ADDITIONAL DIMER-ROW STRUCTURE OF 3C-SIC(001) SURFACES OBSERVED BY SCANNING-TUNNELING-MICROSCOPY

We present atomic-resolution images, obtained with scanning tunneling microscopy (STM), of 3C-SiC(001) surfaces with Si adlayers on the Si-terminated surface, formed by in situ cleaning. We propose a unified additional dimer-row model for the surface with extra Si atoms, which is consistent with structural features obtained by our STM and low-energy-electron-diffraction (LEED) observations. The unit cell of the Si-saturated (3X2) surface seen in LEED observations consists of a pair of additional Si dimers on the Si-terminated surface. The dimer pairs form a straight string along a direction through the centers of two constituent atoms of a dimer. Unit cells of (5X2) and a new reconstruction (7X2) also consists of a pair of additional Si dimers. The (5X2) and (7X2) surfaces have 5/3 and 7/3 wider distances between adjacent rows than that on the (3X2), respectively. This model predicts that even number phases such as (4X2), (6X2),..., which are so far unobserved, are unfavorable on the excess Si surfaces in terms of structural stability. Further, it is found that the saturation of Si adsorption on the (3X2) surface is derived from a steric hindrance caused by periodic dimer-vacancy strings between the adjacent dimer pairs.