Tin Oxide Nanowires: The Influence of Trap States on Ultrafast Carrier Relaxation

We have studied the optical properties and carrier dynamics in SnO₂ nanowires (NWs) with an average radius of 50 nm that were grown via the vapor–liquid solid method. Transient differential absorption measurements have been employed to investigate the ultrafast relaxation dynamics of photogenerated carriers in the SnO₂ NWs. Steady state transmission measurements revealed that the band gap of these NWs is 3.77 eV and contains two broad absorption bands. The first is located below the band edge (shallow traps) and the second near the center of the band gap (deep traps). Both of these absorption bands seem to play a crucial role in the relaxation of the photogenerated carriers. Time resolved measurements suggest that the photogenerated carriers take a few picoseconds to move into the shallow trap states whereas they take ~70 ps to move from the shallow to the deep trap states. Furthermore the recombination process of electrons in these trap states with holes in the valence band takes ~2 ns. Auger recombination appears to be important at the highest fluence used in this study (500 μJ/cm²); however, it has negligible effect for fluences below 50 μJ/cm². The Auger coefficient for the SnO₂ NWs was estimated to be 7.5 ± 2.5 × 10⁻³¹ cm⁶/s.