Band Splitting and Long-lived Carrier Recombination in Ferromagnetic CrSiTe3 Nanosheets

Magnetic layered ternary chalcogenides hold great promise for future spin-optoelectronic devices in the two-dimensional (2D) limit. Insights about magnetic ordering and the spin-orbit interactions that affect the properties of these materials are critically needed information for the development of applications. Here, using ultrafast transient reflectance (TR) and photocurrent (PC) spectroscopies in conjunction with ab initio density functional theory (DFT) calculations, we investigate the band structure and photoresponse of a layered ferromagnetic (FM) semiconductor CrSiTe3 (CST) nanosheet in the absence (at 300 K) and presence (at 10 K) of the FM phase. We observe a surprising decrease of the direct bandgap and an emergence of additional 120 meV higher-lying direct gap when the FM phase is present. We infer by comparison with density functional theory band structure calculations that the induced band modifications are driven by ferromagnetic ordering-induced band splitting between the "Te" p and the "Cr" d states at the valence and conduction band edges. The dynamics of photoexcited carriers are dominated by defect-mediated recombination of the majority of hot carriers within a few ps followed by long-lived residual carriers that recombine through the indirect valleys. 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 spin-ordering effects in layered ferromagnets and show the great potential of CST devices for broadband spin-optoelectronic applications.