Ultrafast time-resolved carrier dynamics in tellurium nanowires using optical pump terahertz probe spectroscopy

We report carrier relaxation dynamics in semiconducting tellurium nanowires (average diameter ∼ 10 nm) using ultrafast time-resolved terahertz spectroscopy. After photoexcitation using an 800 nm pump pulse, we observed an initial increase in the THz conductivity due to the absorption of THz radiation by photoexcited carriers. The time evolution of the differential conductivity (Δσ(τ_pp) = σ_{pump on}(τ_pp) - σ_{pump off}) shows a bi-exponential relaxation with the initial fast decay time scale of τ₁ ∼ 25 ps followed by a longer relaxation time constant of τ₂ ∼ 100 ps. Interestingly, the two time scales depend on the amount of the capping agent present on the surface of TeNWs, showing a faster relaxation of the photoexcited carriers as the percentage of capping decreases. This is physically interpreted as the surface state mediated relaxation mechanism of the photo-pumped carriers depending on the density of available surface states. A quantitative understanding is obtained using a coupled rate equation model taking into account the decay mechanisms determined from the surface mediated relaxation rate (D_S) and direct recombination rate (D_R) of the electron-hole pairs. Furthermore, the measured lattice temperature (T_L) dependent dynamics, showing a faster relaxation at lower temperature, is understood using the same rate equation model, giving a power law dependence of the electron-hole recombination rate (D_R) on T_L as D_R ∝ T_L^-1/2. This is explained by estimating D_R using the van Roosbroeck-Shockley theory taking into account the density of states () of one-dimensional nanowires. Furthermore, to understand the measured frequency-dependent THz photoconductivity, we model Δσ(ω) using the Boltzmann transport equation taking into account the energy-dependent scattering rates showing the dominant role of short range (Γ_sr) and Coulomb scattering (Γ_C) rates in the relaxation process, which further provides a measure of the charged and neutral impurity concentrations.