Dislocation Pipe Diffusion and Solute Segregation during the Growth of Metastable GeSn

Abstract

Epitaxial Sn-containing group-IV semiconductors (Si)GeSn relevant for optoelectronics are inherently strained and supersaturated in Sn and thus can suffer instabilities that are still to be fully elucidated. In this vein, in this work, the microstructure and atomic-scale composition of Ge0.82Sn0.18 are investigated at the onset of phase separation as the epitaxial growth aborts. In addition to the expected accumulation of Sn on the surface, leading to Sn-rich droplets and subsurface regions with the anticipated equilibrium Sn composition of 1.0 atom %, the diffusion of Sn atoms also yields conspicuous Sn-decorated filaments with nonuniform Sn content in the range of ∼1 to 11 atom %. The latter are attributed to the formation and propagation of dislocations, facilitating the Sn transport toward the surface through pipe diffusion. Furthermore, the interface between the Sn droplet and GeSn shows a distinct, defective layer with Sn content of ∼22 atom %. This layer is likely formed by the expelled excess equilibrium Ge as the Sn solidifies, and its content seems to be a consequence of strain minimization between tetragonal Sn-rich and cubic Ge-rich equilibrium phases. The elucidation of these phenomena is crucial to understand the stability of GeSn semiconductors and control their epitaxial growth at a uniform composition.