In-situ Real-Time Observation of Sn-Rich Dots and Wires During Annealing of GeSn Epitaxial Films

In this work, real-time morphological evolution occurring on the GeSn(111) surface during in-situ annealing in a N
₂
environment is examined using a phase contrast optical microscope equipped with a heating stage. Apart from isolated immobile Sn-rich dot-like islands, mobile teardrop-shaped islands are also found at the moving boundary separating the smooth and roughen regions on the surface. As the boundary grows, Sn-poor wire-like feature emerges. The rate of growth occurs with an activation energy of ~1.4eV and appears to be akin to liquid phase epitaxy where the Sn-rich droplets leads the formation of wire-like features and the movement is due to the directional flow of Ge atoms through the droplet. DFT calculations suggest that the morphological instability is thermodynamically driven, where the relaxation of the inherent strain occurs with Sn atoms preferentially segregating from the bulk-lattice and agglomerating at the surface. The formation of these heterogeneous surfaces would restrict the thermal budget for GeSn alloys to be monolithically integrated into Silicon or Germanium devices.