Raman studies of heavily carbon doped GaAs

Raman spectroscopy is used to study GaAs heavily doped with carbon. Hole co ncentrations in these samples range from 2.3X10(19) to 1X10(20) cm(-3). Thr ee 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 ph onon and the free carriers. Only one allowed phonon-like coupled mode is ob served due to the large plasmon damping and high effective carrier masses. The coupled mode is seen to systematically redshift as carrier concentratio n increases. This behavior is described by a model which includes the effec ts 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 t o the carrier concentration p. Interestingly, the local mode intensity show s good correlation with that of the coupled plasmon-LO phonon mode as a fun ction of p. The ratio of the coupled plasmon-LO phonon mode intensity to th at of the LO phonon is found to be directly proportional to the carrier con centration. (C) 1999 American Institute of Physics. [S0021-8979(99)03710-X] .