Photoreflectance spectroscopy of pseudomorphic Si 1−X Ge x (100) structures (x<0·26)

Photoreflectance spectra have been obtained both from uncapped epilayers of strained Si_1−XGe_x and from epilayers buried under a Si cap for 0·06 < × < 0·26. Fitting with Lorentzian line shapes yields E₁ and E₁ + ∆₁/E₀′ critical point transition energies in good agreement with those obtained by fitting to differentiated dielectric function spectra. This justifies the use of Aspnes’ low field theory in the analysis. A previously reported discontinuity in oscillator strength for 0 < × < 0·09 is confirmed and attributed to a crossing in the E₀′ and E₁ + ∆₁ transitions. Samples having a buried alloy layer exhibit a dominant Si peak not observed for the uncapped epilayers and a variation in SiGe line shape across a sample. The effect of interference on the Seraphin coefficients of the alloy layer is shown to be the cause of the line shape variation. The associated critical point transition energies, however, show a variation of <1% in equivalent Gefraction across a sample wafer.

MST/3296