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*10 17 cm -3 to 4*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.