Effect of Mn doping on ultrafast carrier dynamics in thin films of the topological insulator Bi₂Se₃

Transient reflectivity (TR) measured at laser photon energy 1.51 eV from the indirectly intersurface-coupled topological insulator Bi_2−xMn_xSe₃ films (12 nm thick) revealed a strong dependence of the rise-time and initial decay-time constants on photoexcited carrier density and Mn content. In undoped samples (x  =  0), these time constants are exclusively governed by electron–electron and electron–phonon scattering, respectively, whereas in films with x  =  0.013–0.27 ultrafast carrier dynamics are completely controlled by photoexcited electron trapping by ionized Mn²⁺ acceptors and their dimers. The shortest decay-time (∼0.75 ps) measured for the film with x  =  0.27 suggests a great potential of Mn-doped Bi₂Se₃ films for applications in high-speed optoelectronic devices. Using Raman spectroscopy exploiting similar laser photon energy (1.58 eV), we demonstrate that due to indirect intersurface coupling in the films, the photoexcited electron trapping in the bulk enhances the electron–phonon interaction strength in Dirac surface states. (paper)