Study of zone-folding effects on phonons in alternating monolayers of GaAs-AlAs

Samples prepared with one, two, and four atomic layers of GaAs alternating with similar layers of AlAs have been studied by Raman and infrared spectroscopy. The new lattice periodicity along the growth direction is predicted to cause gaps in the phonon spectra and certain new zero-wave-vector modes. New modes are observed in the spectra in the range 150-380 ${\mathrm{cm}}^{-1\mathrm{}}$ which are ascribed to the layering effects. Comparisons with random alloy samples and with detailed model calculations of mode frequencies and strengths show that layering is present in the samples, and allow a semiquantitative determination of the degree of order of the structures.