Interface-Specific Excitation of Coherent Phonons at the Buried GaP/Si(001) Heterointerface

Ultrafast charge-carrier and phonon dynamics at the buried heterointerface of GaP/Si(001) are investigated by means of two-color pump-probe reflectivity measurements. The carrier-induced reflectivity signal exhibits a resonant enhancement at a pump-photon energy of 1.4 eV, which is assigned to an optical transition at the interface. In addition, the transient reflectivity is modulated by a coherent oscillation at 2 THz, whose amplitude also becomes maximum at 1.4 eV. The observed resonant behavior of the oscillation, in combination with the characteristic wavelength-dependencies of its frequency and its initial phase, strongly indicates that the 2-THz mode is a difference-combination mode between a GaP-like and a Si-like phonon at the heterointerface and that the corresponding second-order Raman scattering process can be enhanced by a double resonance involving the interfacial electronic states.