Carrier dynamics in femtosecond-laser-excited bismuth telluride

The carrier dynamics of $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ is studied using the femtosecond pump-probe technique. Three distinct processes, including free carrier absorption, band filling, and electron-hole recombination, are found to contribute to the reflectivity changes. The two-temperature model is used to describe the intraband energy relaxation process of carriers, and the Drude contribution well explains the intensity dependence of the peak values of the nonoscillatory component in the reflectivity signal. The combined effects of free carrier absorption and band filling result in a reflection minimum at about 2 ps after laser excitation. The nonzero background signal increases linearly with the pump fluence, which is attributed to the electron-hole recombination. Finally, our results provide an illustration of investigating the carrier dynamics in semiconductors from the ultrafast reflectivity spectra.

Figure 1

(a) Time-resolved reflectivity of $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ excited at $0.28\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^2$, together with the nonzero background signal and the peak value of the nonoscillatory component. (b) The best-fit results (the dark solid line) calculated by Eq. (4) combined with the contributions from free carrier absorption, band filling, and electron-hole recombination. (c) The changes in dielectric constant solely attributed to the Drude contribution.

Figure 2

Reflectivity spectra of $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ in the far-infrared region fitted by the Drude-Lorentz model (the red solid line).

Figure 3

The time-resolved reflectivity pumped by different laser fluences; the BG signal and peak value are exampled for the pump fluence of $1.27\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^2$.

Figure 4

Dependence of recombination rate upon the excited carrier density; the solid line shows the results calculated by Eq. (12).

Figure 5

Dependence of the peak value of the nonoscillatory component upon the pump fluence; the solid line demonstrates the contribution from free carrier absorption.