The Interplay of Phonons and Subgap States Governs Ultrafast Carrier Dynamics in Layered HgPSe3.

Two-dimensional van der Waals materials display rich nonequilibrium physics due to their reduced dimensionality and strong light-matter interactions. Layered metal phosphorus trichalcogenides (MPX3) feature bandgaps spanning the visible to ultraviolet range and exhibit a complex interplay between excitons, phonons, and defects. Here we use static and time-resolved photoluminescence and femtosecond broadband transient reflectivity to investigate the temperature-dependent ultrafast carrier dynamics in layered HgPSe3, a semiconductor with a quasi-direct bandgap near 2 eV. We identify distinct sub-bandgap photoluminescence features that become prominent at low temperature. Their temperature and fluence dependence strongly indicate radiative recombination involving localized states. The transient reflectivity map displays three features associated with interband optical transitions in the visible range, as corroborated by density functional theory band structure calculations. These features exhibit fluence-dependent decay dynamics, indicating a competition between carrier localization into sub-bandgap states, state-filling effects that limit the available electronic phase space for hot-carrier relaxation, and phonon-bottleneck-limited carrier cooling. Our results provide new insights into the ultrafast carrier dynamics of HgPSe3 and establish a framework for understanding the role of sub-bandgap states in MPX3 semiconductors, with potential implications for broadband optoelectronic and photodetection applications.