  Effect of doping on the carrier-phonon interaction in wurtzite GaN is
investigated by pump-probe reflectivity measurements using 3.1 eV light in near
resonance with the fundamental band gap of 3.39 eV. Coherent modulations of the
reflectivity due to the E2 and the A1(LO) modes, as well as the 2A1(LO)
overtone are observed. Doping of acceptor and more so for donor atoms enhances
the dephasing of the polar A1(LO) phonon via coupling with plasmons, with the
effect of donors being stronger. Doping also enhances the relative amplitude of
the coherent A1(LO) phonon with respect to that of the high-frequency E2
phonon, though it does not affect the relative intensity in Raman spectroscopic
measurements. This enhanced coherent amplitude indicates that transient
depletion field screening (TDFS), in addition to impulsive stimulated Raman
scattering (ISRS), contribute to generation of the coherent polar phonons even
for sub-band gap excitation. Because the TDFS mechanism requires
photoexcitation of carriers, we argue that the interband transition is made
possible at the surface with photon energies below the bulk band gap through
the Franz-Keldysh effect.
