Phase transition in epitaxial bismuth nanofilms

Raman and coherent phonon spectroscopies were used to investigate the thickness-dependent phononic properties of ultrathin single-crystal Bi films prepared by molecular beam epitaxy on Si(111) substrates. Both the A1g and Eg Raman peaks disappeared in the Raman spectra of a 4 nm Bi film, indicating a complete transition from the low-symmetry A7 structure to the high-symmetry A17 structure. Coherent phonon signals of the A1g mode also showed a strong dependence on the film thickness, where thin samples (≤15 nm) exhibited lower phonon frequency and shorter phonon lifetimes than the thick samples (≥30 nm). This difference is attributed to a shallower energy potential barrier caused by both a permanent phase transition, which is determined by the film thickness, and a temporary structural transition by photo-excited carriers. Our results not only provide evidence of a phase transition from the A7 to the A17 structure with the decreasing Bi film thickness but also reveal the influence of this phase transition on phonon dynamics. Understanding these material performance traits will facilitate modern application of Bi thin films in electronic devices.