Nanosecond dynamics in intrinsic topological insulator ${\text{Bi}}_{2-x}{\text{Sb}}_x{\text{Se}}_3$ revealed by time-resolved optical reflectivity

Abstract

${\text{Bi}}_2{\text{Se}}_3$ is an ideal three-dimensional topological insulator in which the chemical potential can be brought into the bulk band gap with antimony doping. Here, we utilize ultrafast time-resolved transient reflectivity to characterize the photoexcited carrier decay in ${\text{Bi}}_{2-x}{\text{Sb}}_x{\text{Se}}_3$ nanoplatelets. We report a substantial slowing of the bulk carrier relaxation time in bulk-insulating ${\text{Bi}}_{2-x}{\text{Sb}}_x{\text{Se}}_3$ nanoplatelets as compared to $n$-type bulk-metallic ${\text{Bi}}_2{\text{Se}}_3$ at low temperatures, which approaches $3.3\text{ ns}$ in the zero pump fluence limit. This long-lived decay is correlated across different fluences and antimony concentrations, revealing unique decay dynamics not present in $n$-type ${\text{Bi}}_2{\text{Se}}_3$, namely the slow bimolecular recombination of bulk carriers.