Direct in situ characterization of Ge surface segregation in strained Si_1-xGe_x epitaxial thin films

Low-energy ion scattering spectrometry (LEISS) and x-ray photoelectron spectroscopy (XPS) have been employed to quantify in situ the near-surface composition of strained Si_1-xGe_x epitaxial thin films grown on Si(100) substrates using GeH₄ and Si₂H₆ as sources. The use of LEISS reveals the Ge concentration in essentially the first monolayer, whereas XPS is sensitive to several monolayers. We find that the extent of Ge surface segregation implied by each technique follows the trend: LEISS-Ge%≫XPS-Ge%≫bulk-Ge%. A two-site model (involving surface and bulk states) cannot account for both the XPS and LEISS results, rather a model invoking Ge enrichment in the subsurface layers is required to explain the data.