Tin surface segregation, desorption, and island formation during post-growth annealing of strained epitaxial Ge1-xSnx layer on Ge(001) substrate

Annealing of strained Ge1-xSnx epitaxial layers grown on Ge(0 0 1) substrate results in two distinctive regimes marked by changes in composition and morphology. Annealing at low temperatures (200-300 degrees C or Regime-I) leads to surface enrichment of Sn due to Sn segregation, as indicated by X-ray photoelectron spectroscopy (XPS) results, while the bulk Sn composition (from X-ray diffraction (XRD)) and the surface morphology (from atomic force microscopy (AFM)) do not show discernible changes as compared to the as-grown sample. Annealing at temperatures ranging from 300 degrees C to 500 degrees C (Regime-II) leads to a decrease in the surface Sn composition. While the Ge1-xSnx layer remains fully strained, a reduction in the bulk Sn composition is observed when the annealing temperature reaches 500 degrees C. At this stage, surface roughening also occurs with formation of 3D islands. The island size increases as the annealing temperature is raised to 600 degrees C. The decrease in the Sn composition at the surface and in the bulk in Regime-II is attributed to additional thermally activated kinetic processes associated with Sn desorption and formation of Sn-rich 3D islands on the surface. (C) 2014 Elsevier B.V. All rights reserved.