RAMAN-SCATTERING BY PLASMON PHONON MODES IN HIGHLY DOPED N-INAS GROWN BY MOLECULAR-BEAM EPITAXY

Raman scattering by coupled plasmon-phonon modes is studied with Si-doped InAs epilayers grown by MBE with carrier concentrations from 7.5 x 10(17) cm-3 to 4 x 10(19) cm-3. Unexpectedly, an unscreened LO line is observed throughout the whole carrier concentration range together with a low frequency (L-) line arising from wavevector dependent LO phonon-plasmon coupling. The frequency of the L- branch lies between the LO and TO phonon frequencies and approaches the TO frequency asymptotically from the high-frequency side as the carrier concentration increases. This behaviour is attributed to competition between screening (dominant in the high-density limit) and large-wavevector induced decoupling. The L+ branch of the plasmon-phonon system is observed for the first time in Raman experiments with InAs.