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]
.