First-principles calculations of the structural properties of ScSi_2 and the formation of ScSi_(2-x) on the Si(111) surface

摘要

Using first-principles total energy calculations, we have studied the structural properties of bulk ScSi_2, the two-dimensional arrangement of ScSi_2 on the Si(111) surface, and the formation of a few layers of ScSi_(1.7) on the same surface. ScSi_2 crystallizes in the hexagonal Th_3Pd_5 structure that can be obtained from the AlB-2 structure after relaxation around the Si vacancies within the (0001) plane. Our calculated geometry is in excellent agreement with experimental values. One monolayer of ScSi_2 on Si(111) results in a two-dimensional phase with (1 × 1) periodicity that consists of a layer of Sc atoms on T4 sites and a Si bilayer on top. This double layer of Si atoms is very close to the ideal Si(111)-(1 × 1) surface, but rotated 180° with respect to the rest of the crystal. More layers of scandium silicide epitaxially grown on Si(111) result in a hexagonal structure similar to bulk ScSi_2: graphitelike Si planes (with vacancies) intercalated with scandium planes and forming a (3~(1/2) × 3~(1/2) arrangement with a ScSi_(1.7) stoichiometry. The top Si layer does not contain vacancies and it does not present a graphitelike structure, but forms a bilayer arrangement as in bulk Si.