Robust coherent phonon mode at GaP/Si(001) heterointerface

Lattice-matched GaP layers without extended defects can be grown on Si(001) substrate via a two-step growth procedure, consisting of low-temperature nucleation followed by high-temperature overgrowth. A transient reflectivity experiment on a thin, low-temperature nucleation layer discovered a previously unknown phonon mode at 2 THz upon below-bandgap optical excitation (Metteet al2025Adv. Mater. Interfaces2400573). Here we examine the influence of the two-step growth process on the ultrafast carrier and phonon dynamics of the GaP/Si interface. We find that the discrete electronic state, which governed the interfacial carrier dynamics of the thin nucleation layer, becomes suppressed when a thicker layer is formed by high-temperature overgrowth. The coherent 2 THz oscillation is observed also in the high-temperature overgrown structures, at the constant frequency regardless of the GaP layer thickness. Its resonance behavior closely follows that of the carrier dynamics at the respective growth stage. This supports its assignment to a phonon mode generated at the heterointerface and strongly coupled to the interfacial carriers. The phonon amplitude exhibits a non-monotonic dependence on the GaP layer thickness, and its optical polarization dependence is qualitatively altered by the high-temperature overgrowth, neither of which is accounted for by the carrier-phonon coupling alone. Our results demonstrate that the 2 THz interfacial phonon mode is robust against high-temperature overgrowth, while its amplitude is determined by both coupling to interfacial electronic transitions and atomic-scale structural reorganization at the interface.