INTERFACE STUDIES OF INAS/GASB SUPERLATTICES BY RAMAN-SCATTERING

We describe the first Raman measurements of optic phonon modes in InAs/GaSb superlattices. In this system there is no common anion or cation, and this results in two types of interface, either In/Sb or Ga/As. The similar molecular masses of GaSb and InAs lead to overlapping optical phonon branches in both materials. In contrast, interface pairs of atoms are either lighter (Ga/As) or heavier (In/Sb) producing interface modes above or below the superlattice optic modes, respectively. We have studied superlattices grown simultaneously on both [001], [111]A, and [111]B oriented GaAs substrates both with and without a buffer layer. Thick layers show bulk modes with the appropriate selection rules for the crystal orientations, confirming good crystalline quality for growth both with and without a buffer layer. As the period thickness decreases, interface modes appear on both the low and high energy sides of the optic phonons, and increase in intensity for thinner layers. The strongest features are on the higher (GaAs) side, but their relative intensity is found to be dependent on the substrate orientation. The behaviour for [001] samples is intermediate between the [111]A and [111]B cases. In short period structures grown without a buffer, additional sharp features are seen just below bulk GaAs optical phonon energies. These are attributed to microcrystal growth on the mismatched substrates which propagate upwards through the superlattice.