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

Abstract

The diffraction anomalous fine-structure technique is applied to the characterization of the local order around metallic atoms located in binary and ternary iron and cobalt disilicides thin films prepared by molecular-beam epitaxy on Si(111). The study is first performed on binary compounds, namely, cubic ${\mathrm{CoSi}}_{2}$ $({\mathrm{CaF}}_{2}$ type), and metastable tetragonal ${\mathrm{FeSi}}_{2}$ $(\ensuremath{\alpha}\ensuremath{-}{\mathrm{FeSi}}_{2}$-derived type). With the crystallographic structure of both phases known, the first-order data analysis proposed by Proietti et al. [Phys. Rev. $59,$ 5479 (1999)] is used to reduce the experimental data. The analysis of the Co K-edge fine structure collected on the $111\ifmmode\bar\else\textasciimacron\fi{}$ fundamental diffraction peak of the standard ${\mathrm{CoSi}}_{2}$ allows the determination of the short-range order around the unique metallic site, in the same way as extended x-ray-absorption fine structure. In addition, it is shown that recording the Fe K-edge oscillations on a single superstructure peak of iron disilicide can provide further information about the long-range order in the $\ensuremath{\alpha}\ensuremath{-}{\mathrm{FeSi}}_{2}$-derived-type structure. The value of the long-range-order $\ensuremath{\eta}$ parameter is found to be $0.9\ifmmode\pm\else\textpm\fi{}0.05.$ Finally, the method is applied to an epitaxial layer grown at room temperature with the ratio Fe:Si of 0.7:2 and Co:Si of 0.3:2 Si on Si(111) and annealed at 930 K. The analysis of Fe and Co K-edge fine structure collected on the $111\ifmmode\bar\else\textasciimacron\fi{}$ diffraction peak permits to differentiate between Co and Fe local environments in ternary ${\mathrm{Co}}_{0.6}{\mathrm{Fe}}_{0.4}{\mathrm{Si}}_{2}$ grains having a ${\mathrm{CaF}}_{2}$-type structure. These nanostructures are embedded in a ${\mathrm{Co}}_{0.05}{\mathrm{Fe}}_{0.95}{\mathrm{Si}}_{2}$ matrix whose tetragonal lattice is very close to the $\ensuremath{\alpha}\ensuremath{-}{\mathrm{FeSi}}_{2}$-derived one. The value of the $\ensuremath{\eta}$ parameter derived from the analysis of the Fe K-edge oscillations on the 001 superstructure peak is found to be $0.8\ifmmode\pm\else\textpm\fi{}0.05.$