Ultrafast carrier dynamics and layer-dependent carrier recombination rate in InSe.

InSe layered semiconductors with high mobility have advantages over transition-metal dichalcogenides in certain device applications. Understanding the dynamics of carriers, especially around the major bandgaps, is not only of fundamental interest but also important for improving the performance of devices. We investigated ultrafast carrier dynamics in exfoliated InSe near the bandgap and found that the presence of photocarriers led to shrinkage in the optical bandgap. In addition, we observed that the carrier recombination rate increased when the thickness of the InSe nanoflakes was reduced and the process was dominated by surface recombination. For the same flakes, the recombination rate became lower after the freshly exfoliated InSe was exposed to air and oxidized. Using a free carrier diffusion model, layer-dependent surface recombination velocities were obtained. Our investigation reveals that the surface condition and the thickness of few-layer InSe play important roles in carrier lifetimes.