Temperature-Dependent Trap-Assisted Ultrafast Carrier Dynamics in Amorphous and Crystalline In2Se3 Thin Films

The presence of trap states plays an important role in the performance of optoelectronic devices. In the present investigation, charge-carrier dynamics in amorphous and crystalline ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ thin films is studied using broadband femtosecond pump-probe spectroscopy in the temperature range of 5--300 K. Amorphous ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ thin films (approximately 300 nm thick) are deposited using a thermal-evaporation method, and undergo a transition to a crystalline phase under vacuum annealing at $250{\phantom{\rule{0.1em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. Ultrafast transient absorption studies suggest that the charge-carrier dynamics is dominated by the presence of inter-band-gap defect states in both types of film, and it is found to be slower in the amorphous than in the crystalline material due to the presence of deep defect states in the amorphous material. The carrier dynamics in the amorphous film is faster at 5 K than at 300 K, as the probability of free carriers becoming trapped in defect states is much higher at high temperatures due to their higher mobility. The change in resistance under light and with temperature is directly correlated with ultrafast spectroscopic data.