Formation of Metastable Solid Solutions in Bi-Ge Films during Low-Temperature Treatment

We investigated the mechanism and kinetics of the formation of metastable BiGe solid phases during the amorphous-to-crystalline transformation of Ge films in contact with Bi. Ge/Bi/Ge sandwich films with a Bi film between amorphous Ge films, which were fabricated by sequential deposition of the components in a vacuum, were used in this study. The total thickness and composition of the sandwich films varied in the range from 30 to 400 nm and from 22 to 48 wt% Bi, respectively. Electron diffraction, high-resolution (S)TEM imaging, EDX, and EEL spectroscopy were used for in situ and ex situ characterization of the morphology, composition, and structure of Ge/Bi/Ge films in the temperature range of 20–271 °C. We proved the formation of polycrystalline Ge films containing up to 28 wt% Bi during low-temperature treatment. The interaction process was activated at ≈150 °C, resulting in the crystallization of Ge with the simultaneous formation of a quasi-homogeneous supersaturated solid solution throughout the entire volume of the film at ≈210 °C. We showed that the formation of crystalline Ge films with an extended solid solubility of Bi depended mostly on the overall composition of the tri-layer film. The role of metal-induced crystallization of the amorphous germanium in the formation of the supersaturated solid phases is discussed.