FINITE-SIZE EFFECTS ON SUPERLATTICE ACOUSTIC PHONONS

The high-resolution Raman spectrum of a Si/Si1-xGex superlattice with N = 15 periods has been studied. It is shown that the principal, secondary, and fine-structure features in the acoustic-phonon spectrum can all be consistently understood within a linear-chain model using first-principles force constants. The physical appearance of the features is very sensitive to N for N < 20. It is also pointed out that simpler theories (e.g., Rytov model) would fail to predict the secondary structure and the fine structure, and the superlattice layer parameters fitted to the observed principal spectrum would be different from the true values. Thus it is important to use a detailed theory in using Raman spectra as a tool for the reliable characterization of microstructures.