  The coupling between longitudinal optical (LO) phonons and plasmons plays a
fundamental role in determining the performance of doped semiconductor devices.
In this work, we report a comparative investigation into the dependence of the
coupling on temperature and doping in n- and p-type GaAs by using ultrafast
optical phonon spectroscopy. A suppression of coherent oscillations has been
observed in p-type GaAs at lower temperature, strikingly different from n-type
GaAs and other materials in which coherent oscillations are strongly enhanced
by cooling. We attribute this unexpected observation to a cooling-induced
elongation of the depth of the depletion layer which effectively increases the
screening time of surface field due to a slow diffusion of photoexcited
carriers in p-type GaAs. Such an increase breaks the requirement for the
generation of coherent LO phonons and, in turn, LO phonon-plasmon coupled modes
because of their delayed formation in time.
