Disordered structure of cubic iron silicide films on Si(111)

Abstract

X-ray difFraction has been used to analyze a series of thin-film samples of Fe Si as a function of thickness and the composition variable x. Unannealed samples are found to be composed entirely of a CsC1type structure not present in the bulk phase diagram. A careful crystallographic analysis is used to show that the variable composition is accommodated by vacancies on the Fe sites. The unit cells of the film are nearly lattice matched to the substrate, which results in a rhombohedral distortion; this lateral strain is compressive for samples near the FeSi composition and tensile for samples near the Feo 5Si composition. Films annealed to 630 C are also lattice matched to the substrate but composed of the a-FeSi, structure.

Morphology of the asymmetric iron–silicon interfaces

“…The iron silicide layer at the Si-on-Fe interface shows a thickness of 1.4(1) nm and q ¼ 5:05ð10Þ g/cm 3 . This fitted density is similar to that of the stable stoichiometric -FeSi phase (q ¼ 5:19 g/cm 3 [29]), but it also lies between the value 4.73 g/cm 3 of the highly Fe-defective and epitaxially stabilized c-Fe 0.5 Si [30] and the close to stoichiometric c-Fe 0.88 Si [31], both with the CsCl structure. c-Fe 1Àx Si has been claimed to appear in the Fe/Si interface [18,19,12,15].…”

Short- and long-range order in iron and cobalt disilicides thin films investigated by the diffraction anomalous fine structure technique

“…FeSi 2 disilicide has the so-called ␣-FeSi 2 -derived tetragonal structure 11 instead. The various structures of the Fe metastable disilicides are well described in the paper of Whiteaker et al 12 Before annealing, the as-deposited thin iron disilicide film (Ͻ100-Å thick͒ has the CsCl-derived structure that can be viewed as a disordered version of the CaF 2 -type structure: the metallic atoms are randomly distributed into the fcc lattice positions and additional eightfold sites between the Si atoms, at the center of the cube with a unit cell of half the size. Upon annealing this silicide, a depletion of the interstitial sites occurs, which is accompanied by a change in the lattice symmetry, with a reduction of the lattice parameter c along the ͓001͔ direction.…”

“…Unlike the CoSi 2 -based thin films which are made of a totally ordered cubic CaF 2 -type phase, the FeSi 2 thin films present a tetragonal structure, with additional sites, characterized by their degree of vacancy. 11,12,14 The DAFS signal recorded on a superstructure peak is sensitive to that partial order through two effects: the occupation factor difference between the two types of metallic ͑001͒ planes and the relative positions of the planes in the unit cell.…”

“…A complete DAFS study performed on thin films of binary compounds whose structure is well known is now of a fundamental interest for a better understanding of the advantage of DAFS with respect to the XAFS technique. The samples we studied are transition-metal disilicide thin films co-deposited by molecular-beam epitaxy ͑MBE͒ on a Si͑111͒ substrate at room temperature and annealed above 900 K. For about ten years, new iron and cobalt silicide phases stabilized by the strain induced by epitaxy on Si͑111͒ substrates have been identified, by using either transmission-electron microscopy, 8,9 XRD, [10][11][12] or extended x-ray-absorption fine structure ͑EXAFS͒. [13][14][15] In this work, we are interested in the ordered disilicide phases that are prepared in thin films by codepositing at room temperature on Si͑111͒-or previously by solid phase epitaxy ͑SPE͒-and annealing at temperatures as high as 900 K. CoSi 2 grown in these conditions crystallizes in the common fluorite (CaF 2 ) cubic structure represented in Fig.…”

“…11,12 The structural information derived from XAFS experiments has also greatly contributed to accounting for the atomic structure of these novel phases of iron silicides. 14,15 The local order around metallic atoms in CaF 2 -type and ␣-FeSi 2 -derived phases have indeed their own EXAFS fingerprints.…”