Tin surface segregation, desorption, and island formation during post-growth annealing of strained epitaxial Ge_(1-x)Sn_x layer on Ge(001) substrate

摘要

Annealing of strained Ge_(1-x)Sn_x epitaxial layers grown on Ge(001) substrate results in two distinctive regimes marked by changes in composition and morphology. Annealing at low temperatures (200-300 ℃ 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℃ to 500℃ (Regime-Ⅱ) leads to a decrease in the surface Sn composition. While the Ge_(1-x)Sn_x layer remains fully strained, a reduction in the bulk Sn composition is observed when the annealing temperature reaches 500 ℃. At this stage, surface roughening also occurs with formation of 3D islands. The island size increases as the annealing temperature is raised to 600 ℃. 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.