Light scattering in p-type GaAs:Ge

Abstract

We present in this work the results of a Raman spectroscopy study on the plasmon–phonon coupling in Ge-doped p-type gallium arsenide. A series of polarized Raman scattering experiments were carried out on epitaxial films grown by liquid-phase epitaxy on (100) GaAs substrates at 20, 100, and 300 K. The films were p type with free hole densities varying in the range of 5×1017–1×1020 cm−3. Under the scattering configurations employed, the longitudinal optical (LO) mode is forbidden for crossed polarization while the transverse optical (TO) mode is forbidden for both parallel and crossed polarizations. However, all the polarized Raman spectra showed two peaks with frequencies close to the TO and LO phonons of semi-insulating GaAs. The appearance of such forbidden modes was accounted for with a theoretical model which considers phonon–plasmon coupled modes with wave vectors much larger than those given by the regular q≊0 wave vector transferred by photons. Ionized acceptor impurities provide such additional wave vector transfer through elastic scattering of the photoexcited electrons and holes. It is demonstrated that the experimental values for position and linewidth of the peaks are well described by the theoretical calculations when Fröhlich-type and deformation potential mechanisms are considered as means of interaction.