Fine structure in Raman spectroscopy and X-ray reflectometry and its uses for characterization of superlattices

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Abstract
Fine structure in the folded longitudinal acoustic (FLA) phonon peaks from high-resolution Raman spectra of Si/Si1−xGex superlattices has been observed. The observed FLA fine structure is attributed to fluctuations in the superlattice periodicity. A 0.2 cm−1 splitting in the FLA peaks is observed in the Raman spectrum of a 15-period superlattice. According to the Rytov theory for the FLA phonons, the peak splitting corresponds to a change in period of about four monolayers (~0.5 nm) during growth. X-ray reflectometry on these samples also shows a similar doublet formation on the main reflection peaks, consistent with a 2–3 monolayer period fluctuation. A test sample with a built-in period variation was also well characterized by both techniques. The good agreement between the two techniques, as well as between the theories and the experiments, demonstrates that both techniques are capable of high precision and allow detailed and quantitative characterization of the superlattice perfection. In addition to determining the phonon velocity, dispersion relation, layer thickness, period, and composition of the superlattices, these results demonstrate the possibility of measuring the fluctuation in the period and the interface quality, as well as the numbers of given periods in some special cases.

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