In situ TEM heating experiments on thin epitaxial GeSn layers: Modes of phase separation

The thermal stability of GeSn epitaxial thin films was investigated via in situ transmission electron microscopy (TEM). Samples were grown with a similar layer structure and 10 at.% Sn content by either molecular beam epitaxy or chemical vapor deposition. Despite the same layer thickness and concentration, the decomposition mode differs dramatically for each GeSn sample during annealing experiments. We observed that the sample with a Ge buffer on a Ge substrate is structurally stable up to 500 °C, while above this temperature, β-Sn precipitates appear, indicating a decomposition mechanism of solid-state precipitation. On the other hand, the second sample exhibited high susceptibility to Ga ion incorporation during the focused ion beam TEM specimen preparation, which is attributed to a high defect density owing to an atypically thin Ge buffer layer grown on a Si substrate. In this case, the efficient phase separation in the sample was facilitated by Ga contamination, promoting the appearance of a GaSn-based liquid phase at a temperature as low as 200 °C. The decomposition temperatures found and the occurrence of the two different decomposition modes are discussed in relation to the experimental methods used.