From interface-limited to Auger-dominated carrier dynamics in $π$-SnS
Metastable cubic tin(II) sulfide ($π$-SnS) is an earth-abundant semiconductor whose three-dimensionally bonded chiral lattice may overcome the short minority-carrier lifetime of orthorhombic SnS while maintaining a near-ideal bandgap for tandem photovoltaics. Despite its promise, ultrafast carrier cooling and recombination mechanisms over illumination density remain poorly constrained. We use core-level extreme-ultraviolet attosecond transient absorption spectroscopy at the Sn $4d$ edge to track carrier injection, cooling, and recombination in $π$-SnS with element- and orbital-specific sensitivity. Following femtosecond near-infrared excitation, the Sn $4d\rightarrow$CB onset exhibits conduction-band state filling and a carrier-induced edge shift, enabling extraction of density-dependent kinetics. The transient response follows a biexponential decay with a fast hot-carrier cooling component and a slower recombination component. At low carrier densities, recombination is consistent with interface-limited processes, whereas above $\sim1\times10^{20}$ cm$^{-3}$ both cooling and recombination accelerate, indicating a crossover to carrier-carrier interaction-dominated dynamics. Coherent phonon oscillations with a period of $\sim188$ fs reveal coupling between electronic excitation and lattice motion. These results provide a comprehensive picture of nonequilibrium carrier and phonon dynamics in cubic SnS, reveal a change of mechanisms over a range of carrier densities, and establish the value of using attosecond transient absorption spectroscopy to study ultrafast processes in complex semiconductors that have optoelectronic and energy-conversion applications.