Thermal stability, atomic vibrational dynamics, and superheating of confined interfacial Sn layers inSn∕Simultilayers

Abstract

Multilayers composed of materials with low (Sn) and high (Si) bulk melting points were grown at room temperature by ultrahigh vacuum deposition. $^{119}\mathrm{Sn}$ M\"ossbauer spectroscopy has been used to investigate the temperature dependence of the Debye-Waller factor $f$, the mean-square displacement, and the mean-square velocity of $^{119}\mathrm{Sn}$ nuclei in ultrathin ($10\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ thick) $\ensuremath{\alpha}$-like Sn layers embedded between $50\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ thick Si layers. The $f$ factor was found to be nonzero with a value of $0.036\ifmmode\pm\else\textpm\fi{}0.009$ even at $450\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. This provides unequivocal proof of the solid state of the confined $\ensuremath{\alpha}$-like Sn layers at least up to $450\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Melting can only be achieved by superheating to $T>450\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. This temperature is significantly higher than the melting temperature of bulk $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Sn}$ $(231.9\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ and of a nonconfined epitaxial $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}$ single layer grown on InSb(111) $(170\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ previously reported in the literature [T. Osaka et al., Phys. Rev. B 50, 7567 (1994)]. Our molecular dynamics calculations show that melting of bulk-like $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}$ starts at $\ensuremath{\sim}380\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and is complete at $\ensuremath{\sim}530\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ according to the Lindemann criterion. Since we still observe the solid state at $450\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ for the confined $\ensuremath{\alpha}$-like Sn films, considerable superheating is observed for this system. The stability of the ultrathin confined $\ensuremath{\alpha}$-like Sn layers arises from electronic interactions with the surrounding Si layers, as evidenced by the M\"ossbauer chemical shift.