Thermal stability, atomic vibrational dynamics, and superheating of confined interfacial Sn layers in Sn∕Si multilayers

Multilayers composed of materials with low (Sn) and high (Si) bulk melting points were grown at room temperature by ultrahigh vacuum deposition. ¹¹⁹Sn Mössbauer 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 ¹¹⁹Sn nuclei in ultrathin (10 Å thick) α-like Sn layers embedded between 50 Å thick Si layers. The f factor was found to be nonzero with a value of 0.036±0.009 even at 450 °C. This provides unequivocal proof of the solid state of the confined α-like Sn layers at least up to 450 °C. Melting can only be achieved by superheating to T>450 °C. This temperature is significantly higher than the melting temperature of bulk β-Sn (231.9 °C) and of a nonconfined epitaxial α-Sn single layer grown on InSb(111) (170 °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 α-Sn starts at ∼380 °C and is complete at ∼530 °C according to the Lindemann criterion. Since we still observe the solid state at 450 °C for the confined α-like Sn films, considerable superheating is observed for this system. The stability of the ultrathin confined α-like Sn layers arises from electronic interactions with the surrounding Si layers, as evidenced by the Mössbauer chemical shift.