Effects of thermal and non-thermal structural phase transitions on the reflectance of metavalent bonding PbGeTe

Metavalent bonding is known to contribute to the high reflectivity exhibited by the crystalline phase of phase change materials. To elucidate the impact of structural changes on metavalent bonding, we investigated the reflectivity changes of PbGeTe alloys accompanying their rhombohedral-to-cubic rocksalt transition using spectroscopic ellipsometry and ultrafast optical spectroscopy. In temperature-controlled ellipsometry, this transition is found to have a positive impact on the optical properties of PbGeTe alloys. Following femtosecond (fs)-laser excitation, coherent phonon oscillations resulting from the rhombohedral phase were observed at low temperatures. However, with an increase in excitation fluence, these oscillations disappeared immediately after excitation, suggesting that the nonthermal rhombohedral–cubic phase transition was induced by fs-laser irradiation. Despite the occurrence of such a phase transition, unfortunately the reflectivity eventually decreased without showing any increase. The reflectivity decrease induced by electron excitation and also temperature rise is considered to counteract the reflectivity increase expected from the stabilization of metavalent bonding induced by the transition to cubic phase.