Investigation of Interface-Induced Helicity-Dependent Photocurrent and High-<i>T</i>_C Ferromagnetism in Wafer-Scale 2D Ferromagnetic Fe₄GeTe₂/Bi₂Te₃ Heterostructures

The helicity-dependent photocurrent (HDPC) of Fe₄GeTe₂ (3, 5, 8, 10 nm)/Bi₂Te₃ (8 nm) heterostructures grown on sapphire substrates was systematically investigated. It is revealed that the HDPC is induced by the interface coupling between the Fe₄GeTe₂ and Bi₂Te₃ films, and it is dominated by the circular photogalvanic effect (CPGE) rather than by the circular photodrag effect (circular photon drag effect). As the tensile strain increases, the CPGE current decreases, which can be attributed to the decrease of the interface-induced spin-orbit coupling with increasing tensile strain. In addition, it is demonstrated that by applying appropriate tensile strain, the 5 nm Fe₄GeTe₂/Bi₂Te₃ sample can be used to detect the circular polarization state of a light. Finally, Fe₄GeTe₂ (5, 8, and 10 nm)/Bi₂Te₃ (8 nm) heterostructures show a <i>T</i>_C larger than 390 K. The dependence of the CPGE on the film thickness of Fe₄GeTe₂ is different from that of Curie temperature, indicating that the enhanced exchange interaction induced by the interface coupling may be the dominant mechanism for the high-<i>T</i>_C ferromagnetism. The large interface-induced CPGE in the Fe₄GeTe₂/Bi₂Te₃ suggests that Fe₄GeTe₂/Bi₂Te₃ heterostructures may provide a good platform for designing novel opto-spintronic devices.