Time-resolved reflectivity studies of coherent longitudinal acoustic phonon pulses in bulk III-nitride semiconductors

An interferometric technique employing femtosecond pump-probe reflectivity measurements has been used to study the direct generation, propagation, and detection of coherent longitudinal acoustic (LA) phonon pulses in bulk GaN and AlGaN thin films at frequencies in the 100 to 200 GHz range. Photoreflectance and resonant Raman studies suggest that the dominant mechanism in the generation of the LA phonon pulse is photoexcited carrier transport-induced screening of the near-surface electric field associated with residual strain in the incompletely relaxed GaN or AlGaN epilayers. This field screening component in the strain pulse generation can be as much as two orders of magnitude larger than the deformation potential component, in contrast to theoretical calculations for the multiple quantum well (MQW) case, for which the two components are of comparable magnitude. The technique has been used to directly measure the sound velocity in GaN and AlGaN of various Al contents. The results are in good agreement with those calculated from elastic moduli. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Wembeim.