Surface atomic structure determination of three-dimensional yttrium silicide epitaxially grown on Si(111)

The surface atomic structure of thin layers of three-dimensional yttrium silicide epitaxially grown on Si(111) 7×7 has been investigated by means of dynamical low-energy electron diffraction analysis. We determine the interlayer distances as well as the lateral and/or vertical relaxations of the atoms in the superficial planes. The epitaxial silicide consists of stacked hexagonal rare-earth planes and graphitelike Si planes with an ordered arrangement of Si vacancies. The ordered net of Si vacancies in the inner planes is responsible for the lateral relaxations of the surrounding Si atoms. The topmost layer does not present a graphitelike structure, forming a buckled Si layer with no vacancies. One of the three Si atoms in the lower plane of this bilayer is closer to the yttrium layer due to the presence of the vacancy in the last Si plane just below. This produces vertical relaxation in the termination layer.