Cooling dynamics of photoexcited carriers in Si studied using optical pump and terahertz probe spectroscopy

Abstract

We investigated the photoexcited carrier dynamics in Si by using optical pump and terahertz probe spectroscopy in an energy range between 2 meV and 25 meV. The formation dynamics of excitons from unbound e-h pairs was studied through the emergence of the 1s-2p transition of excitons at 12 meV (3 THz). We revealed the thermalization mechanism of the photo-injected hot carriers (electrons and holes) in the low temperature lattice system by taking account of the interband and intraband scattering of carriers with acoustic and optical phonons. The overall cooling rate of electrons and holes was numerically calculated on the basis of a microscopic analysis of the phonon scattering processes, and the results well account for the experimentally observed carrier cooling dynamics. The long formation time of excitons in Si after the above-gap photoexcitation is reasonably accounted for by the thermalization process of photoexcited carriers.

Tailoring thermoelectric properties of nanostructured crystal silicon fabricated by infrared femtosecond laser direct writing

“…These results suggest that the irradiation of double‐pulse‐train may permit to control the electron excitation processes individually . Other possible cause is that although the thermalization of the electron occurs within 1 ps through the electron‐electron collisions, the delay time in this experiment is too shorter than the lifetime of the indirect excitons in c‐Si (~μs) . Figure b shows the shape change according to the time delay ( τ d ) between the orthogonally polarized femtosecond double‐pulse irradiation.…”

“…These results also indicate that the modified region was extended along with the polarization direction of the first arriving pulse. In addition, despite of positive and negative time delay, the length in the direction of the long axis was decreased during 200 ps time delay, and then became constant, corresponding to the long formation time of excitons in c‐Si after the photoexcitation by the phonon‐assisted indirect transitions .…”

Time-Resolved Laser Spectroscopy of Semiconductors - Physical Processes and Methods of Analysis

“…After the cooling phase a final equilibrium of all subsystems is reached by recombination of excitons and free carriers. This effect has been investigated in Si by Suzuki et al (Suzuki & Shimano 2011) using optical pump and THz probe measurements. They evaluated the time-dependent fraction of free carriers and excitons.…”

“…Recombination can happen within a free electron-hole pair or go via exciton recombination. The Saha equation (Kaindl et al, 2009, Suzuki & Shimano 2011, gives the equilibrium ratio between the density of excitons N ex and free carriers (i.e. when these two populations are reciprocally thermalized) in the Boltzmann limit.…”

“…In (Suzuki & Shimano 2011), cooling of the hot carriers was studied using a set of cooling rate expressions. A very flexible, but more computationally heavy alternative for studying this particular cooling dynamics would be to use a particle-based Monte Carlo (MC) simulation.…”