Ultrafast carrier relaxation dynamics in HgSe nanoparticles probed by optical pump terahertz probe spectroscopy

Mercury selenide (HgSe) has gained considerable attention in the last few decades due to its promising optoelectronic applications facilitated by the high intrinsic carrier concentration, high mobility, and tunable semimetallic behavior. A comprehensive understanding of the ultrafast relaxation dynamics of the photoexcited carriers is essential to further assess its potential in device applications. Here, we study the ultrafast hot carrier relaxation dynamics in aggregated HgSe nanoparticles film, prepared by drop casting onto a quartz substrate using time-resolved optical pump terahertz probe spectroscopy. Our results show that the relaxation time (${\ensuremath{\tau}}_{\text{rel}}$) of the photoexcited carriers increases with the pump fluence $\ensuremath{\phi}$ and is independent of lattice temperature ${T}_{L}$. The observations of longer relaxation time associated with higher pump fluence show that carrier relaxation by emission of optical phonons is the dominant mechanism. We model the relaxation process of the coupled carrier-phonon system with rate equations incorporating electron-phonon scattering rate. The photoconductivity ($\mathrm{\ensuremath{\Delta}}\ensuremath{\sigma}={\ensuremath{\sigma}}_{\text{pump}\phantom{\rule{0.222222em}{0ex}}\text{on}}\ensuremath{-}{\ensuremath{\sigma}}_{\text{pump}\phantom{\rule{0.222222em}{0ex}}\text{off}}$) in both the time and frequency domain is calculated using the Boltzmann transport equation along with the hot carrier temperature (${T}_{e}$) and optical phonon occupation number (${n}_{q}$) evaluated in a self-consistent manner. This is achieved by taking into account the contributions of different charge scattering mechanisms such as neutral impurity scattering, Coulomb scattering, and optical phonon scattering. A quantitative comparison of the time-dependent photoconductivity and the model calculation yields an estimation of optical phonon lifetime, neutral and Coulomb impurity density, and electron-phonon coupling strength. Our study provides insight into the intraband conductivity of the photoexcited carriers in HgSe, essential for its optoelectronic applications.