Separation of interface and substrate carrier dynamics at a heterointerface based on coherent phonons

Transient reflectivity spectroscopy is widely used to study ultrafast carrier and phonon dynamics in semiconductors. In their heterostructures, it is often not straightforward to distinguish contributions to the signal from the various layers. In this work, we perform transient reflectivity measurements on lattice-matched GaP/Si(001) using a near-infrared pulse, to which GaP is transparent. The pump laser pulse can generate coherent longitudinal-optical phonons both in the GaP overlayer as well as in the Si substrate, which have distinct frequencies. This enables us to track the amplitude of the respective signal contributions as a function of the GaP layer thickness $d$. The Si phonon signal exhibits a drastic amplitude decrease and a sign change with increasing $d$, which can be quantitatively explained by the interference of the probe light reflected at the GaP/Si interface. Based on this knowledge, we can separate the interface contribution and the substrate contribution in the carrier-induced nonoscillatory transient reflectivity signal. The obtained signals reveal interfacial carrier dynamics that is dependent on the GaP layer thickness.