Visible and ultraviolet Raman scattering studies of Si1ÀxGex alloys
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We report Raman studies of the Si–Si phonon band in Si12xGex alloys, where the excitation is by visible and ultraviolet ~351 nm! light. At a wavelength 351 nm, the optical penetration depth is extremely shallow ~'5 nm!. By varying the excitation from 351 to 514 nm, the optical penetration depth spans from 5 to 300 nm. Two sets of samples were examined. Thin layers grown using molecular beam epitaxy were coherently strained to match the lattice constant of the silicon substrate. Thick layers grown using organo–metallic chemical vapor deposition were strain relaxed. For the thin, strained layers, visible excitation produces a spectrum, which is a superposition of the substrate and the epilayer phonon bands. Reducing the wavelength ~and, consequently, penetration depth! allows us to isolate the epilayer spectrum. Phonon energies obtained using all excitation wavelengths agree. We conclude that Raman scattering from these alloys using 351 nm laser light gives us bulk alloy properties pertinent to the near-surface composition and strain. The epilayers show no evidence of compositional variance or strain relaxation near the surface.