Ultrafast carrier relaxation in SnSe_x (x=1, 2) thin films observed using femtosecond time-resolved transient absorption spectroscopy

In this study, SnSe_x (x = 1, 2) films are prepared using pulsed laser deposition (PLD) and characterized using X-ray diffraction spectroscopy (XRD) and X-ray photoelectron spectroscopy (XPS). The crystal structure and valence state of SnSe_x films can be observed using XRD patterns and XPS spectra. The carrier dynamics are investigated using femtosecond time-resolved TA spectroscopy. The carrier relaxation process is strongly influenced by thin film stoichiometry. The TA spectra observed in the SnSe and SnSe₂ films associated with photoinduced absorption, free carrier absorption, and state filling effects. The carrier ultrafast dynamics of two films are well fitted using a tri-exponential decay model with fast (several picoseconds), medium (tens of picoseconds), and slow (several nanoseconds) components. These components are associated with carrier scattering, Auger recombination, and diffusion, respectively. The component with medium lifetime (Auger recombination) in the SnSe₂ film is faster than that of SnSe film because defect state trapping. The slow relaxation component in SnSe₂ film, on the other hand, is much longer than that of SnSe film. This is also due to trapping by defect states in SnSe₂, which affects the diffusion of the photo-induced carriers. We investigated the carrier relaxation processes in SnSe_x (x = 1, 2) films and offered an insight in the ultrafast dynamic mechanism in the two tin selenides.