New Models for the 7x7, 5x5, 2x8 Structures on Si(lll) and Ge(lll) Surfaces

On the basis of a detailed analysis of RHEED intensity distribution along reciprocal lattice rods taken from the Si(l11)7 x 7 structure, a new structure model has been obtained. The structure consists of twelve raised atoms and a vacancy in the unit mesh. The raised atoms are arranged in regular triangular forms with side 2a, a being the size of the bulk unit mesh and they can be classified into four kinds of atoms A, B, C and D. Among their heights of the displacements ZA, ZB, Zc and ZD, an inequality ZA=ZB=ZC=ZD has been consequently obtained. The Ge(lll)7x7-Sn surface has an identical structure with the Si(l11)7x7. Similarly the Ge(lll)2x8 structure consists of two of them, A and B, and the Ge(lll)5x5-Sn and Si(l11)5 x 5-Ge three of them, C, D and a vacancy.