Phase discrimination of iron-silicides on Si(0 0 1) surfaces by three-dimensional reciprocal-lattice mapping

Abstract

Various iron-silicides are grown on clean Si(0 0 1) surfaces by solid phase epitaxy, a process which involves the deposition of iron and subsequent annealing [6]. Among them, we studied the structure of three-dimensional (3D) elongated islands, which are the major silicide type produced at lower Fe coverage (∼1 monolayer) and ⩾500–600 °C annealing. We applied a newly developed method of azimuth-scan reflection high-energy electron diffraction (RHEED) to obtain 3D reciprocal-lattice mapping. We succeeded in discriminating an α - FeSi 2 phase from controversial bulk phases of the islands, and we were also able to determine the orientation relation as α - FeSi 2 ( 1 1 0 ) ∥ Si ( 0 0 1 ) and α - FeSi 2 [ 1 ¯ 1 0 ] ∥ Si 〈 1 1 0 〉 , where the lattice mismatches are −1% in α - FeSi 2 [ 1 ¯ 1 0 ] direction and +34% in α - FeSi 2 [ 0 0 1 ] direction. The attenuation of the incident electron beam along the length direction of the islands leads to extremely weak spots in the RHEED pattern. We emphasize that such an analysis of the reciprocal-lattice mapping is also useful in studying other 3D island structures. Using scanning tunneling microscopy, we showed that the island’s elongated directions are perpendicular to the dimer rows of the substrate located under the islands. The islands are located near the S B step edges. The elongation lengths of the islands are almost the same as the widths of the Si substrate terraces. We discussed the formation mechanism of the 3D-elongated islands. From an atomic image of the facet and edge of a 3D-elongated island, we proposed an atomic-structure model of the island facet and edge: a Si adatom on the hollow site of four Si atoms of an α - FeSi 2 ( 1 1 0 ) unit, with [ 1 ¯ 1 0 ] ordering in the direction of the elongation, forming an α - FeSi 2 ( 1 1 1 ) facet locally.