  Magnetic layered ternary chalcogenides hold great promise for future
spin-optoelectronic devices in the two dimensional limit. Understanding how the
properties of the materials are impacted by magnetic ordering and the
spin-orbit interactions is critically needed information for the development of
applications. Ultrafast transient reflectance (TR) and photocurrent (PC)
spectroscopies are combined with ab initio density functional theory (DFT)
calculations to investigate the band structure and photoresponse of a layered
magnetic semiconductor CrSiTe3 (CST) nanosheet in the paramagnetic (PM, 300K)
and ferromagnetic (FM, 10 K) phases. We observe both a decrease of the direct
bandgap and emergence of a 120 meV splitting of the optical transition when the
FM phase is present. DFT band structure calculations suggest that the band
modifications are driven by a FM ordering-induced band splitting between the Te
p and the Cr d states at the valence and conduction band edges. We find that
the majority of carriers photoexcited at the direct gap recombine within
picoseconds through defect-mediated recombination, but that 2-3 % of the
electrons scatter into indirect conduction band valleys resulting in very
long-lived electrons and holes. Those long-lived carriers contribute to the
broadband PC response of CST devices that also features indirect absorption.
These results provide critical insights into the dynamics and energy landscape
of photoexcited electrons and holes, and how they are impacted by spin-ordering
effects in layered ferromagnets.
