Structure and vibrational dynamics of interfacial Sn layers in Sn/Si multilayers

Abstract

The structure and vibrational dynamics of room-temperature-grown nanoscale Sn/amorphous $(a$-)Si multilayers have been studied by x-ray diffraction, Raman scattering, ${}^{119}\mathrm{Sn}$ M\"ossbauer spectroscopy, and ${}^{119}\mathrm{Sn}$ nuclear-resonant inelastic x-ray scattering (NRIXS) of synchrotron radiation. With increasing Sn-layer thickness, the formation of $\ensuremath{\beta}$-Sn was observed, except at the Sn/Si interfaces, where a 10-\AA{}-thick metastable pure amorphous-$\ensuremath{\alpha}$-Sn-like layer remains stabilized. By means of NRIXS we have measured the Sn-projected vibrational density of states (VDOS) in these multilayers (in particular, at the interfaces), and in 500-\AA{}-thick epitaxial $\ensuremath{\alpha}$-Sn films on InSb(001) as a reference. Further, the Sn-specific Lamb-M\"ossbauer factor $(f$ factor), mean kinetic energy per atom, mean atomic force constant, and vibrational entropy per atom were obtained. The VDOS of the amorphous-$\ensuremath{\alpha}$-Sn-like interface layer is observed to be distinctly different from that of (bulk) $\ensuremath{\alpha}$-Sn and $\ensuremath{\beta}$-Sn, and its prominent vibrational energies are found to scale with those of amorphous Ge and Si. The observed small difference in vibrational entropy $(\ensuremath{\Delta}{S/k}_{B}=+0.17\ifmmode\pm\else\textpm\fi{}0.05$ per atom) between $\ensuremath{\alpha}$-Sn and interfacial amorphous-$\ensuremath{\alpha}$-like Sn does not account for the stability of the latter phase.