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

TL;DR

This study uses transient reflectivity spectroscopy to distinguish and analyze carrier and phonon dynamics at a GaP/Si heterointerface, revealing ultrafast carrier injection and interface-specific signals through interference effects.

Contribution

The paper introduces a method to separate interface and substrate carrier dynamics in heterostructures using coherent optical phonons and interference effects in transient reflectivity.

Findings

Distinct phonon frequencies enable layer-specific signal tracking.

Interference effects explain oscillatory behavior of Si phonon amplitude.

Ultrafast carrier injection from Si to GaP is evidenced by interface signals.

Abstract

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 (LO) 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 GaP layer thickness $d$. The Si phonon amplitude in the signal exhibits an oscillatory behavior with increasing $d$. This can be quantitatively explained by the interference of the probe light reflected at the air/GaP/Si heterointerface. Based on this knowledge, we can then separate the interface- and the substrate-contributions in the carrier-induced non-oscillatory transient reflectivity signal. The obtained interface signal provides evidence for ultrafast carrier injection from the Si substrate into the GaP overlayer. This is also corroborated by examining the deviation of the polarization-dependence of the GaP coherent optical phonon signal from that of the bulk semiconductor.