Raman studies of heavily carbon doped GaAs

Abstract

Raman spectroscopy is used to study GaAs heavily doped with carbon. Hole concentrations in these samples range from 2.3×1019 to 1×1020 cm−3. Three main Raman features are investigated: the longitudinal-optic (LO) phonon mode, the substitutional carbon-at-arsenic local-vibrational mode, and the coupled plasmon–LO phonon present due to the interaction between the LO phonon and the free carriers. Only one allowed phonon-like coupled mode is observed due to the large plasmon damping and high effective carrier masses. The coupled mode is seen to systematically redshift as carrier concentration increases. This behavior is described by a model which includes the effects of high hole concentrations on the dielectric function and an additional shift in the optic phonon we tentatively attribute to carbon size effect. The local vibrational mode intensity is found to be directly proportional to the carrier concentration p. Interestingly, the local mode intensity shows good correlation with that of the coupled plasmon–LO phonon mode as a function of p. The ratio of the coupled plasmon–LO phonon mode intensity to that of the LO phonon is found to be directly proportional to the carrier concentration.