Free-carrier relaxation and lattice heating in photoexcited bismuth thin films

TL;DR

This study investigates ultrafast carrier dynamics, heat transfer, and electron-hole recombination in photoexcited bismuth thin films, revealing key timescales and nonlinear behaviors relevant for understanding their optoelectronic properties.

Contribution

It provides detailed measurements of carrier relaxation, heat transfer, and recombination timescales in bismuth films, highlighting the nonlinear effects at high excitation densities.

Findings

Electron-hole recombination time is 12-26 ps.

Ambipolar diffusivity ranges from 18-40 cm²/s.

Carrier dynamics become nonlinear at excitation densities above 10^{20} cm^{-3}.

Abstract

We report ultrafast surface pump and interface probe experiments on photoexcited carrier transport across single crystal bismuth films on sapphire. The film thickness is sufficient to separate carrier dynamics from lattice heating and strain, allowing us to investigate the time-scales of momentum relaxation, heat transfer to the lattice and electron-hole recombination. The measured electron-hole ($e-h$) recombination time is 12--26 ps and ambipolar diffusivity is 18--40 cm$^{2}$/s for carrier excitation up to $\sim 10^{19} \text{cm}^{-3}$. By comparing the heating of the front and back sides of the film, we put lower limits on the rate of heat transfer to the lattice, and by observing the decay of the plasma at the back of the film, we estimate the timescale of electron-hole recombination. We interpret each of these timescales within a common framework of electron-phonon scattering and find qualitative agreement between the various relaxation times observed. We find that the carrier density is not determined by the $e-h$ plasma temperature after a few picoseconds. The diffusion and recombination become nonlinear with initial excitation $\gtrsim 10^{20} \text{cm}^{-3}$.