Ultrafast dynamic conductivity and scattering rate saturation of photoexcited charge carriers in silicon investigated with a midinfrared continuum probe

TL;DR

This study uses ultrabroadband terahertz-midinfrared pulses to investigate the dynamic conductivity and scattering rate saturation of photoexcited charge carriers in silicon, revealing a saturation in scattering rate at high excitation densities.

Contribution

It provides the first direct measurement of plasma frequency range in silicon and demonstrates the saturation of scattering rates due to phase-space restrictions at high densities.

Findings

Scattering time decreases from 200 fs to 20 fs with increasing density.

Saturation of scattering rate occurs at high densities due to degeneracy effects.

Theoretical models based on generalized Drude approach accurately reproduce experimental results.

Abstract

We employ ultra-broadband terahertz-midinfrared probe pulses to characterize the optical response of photoinduced charge-carrier plasmas in high-resistivity silicon in a reflection geometry, over a wide range of excitation densities (10^{15}-10^{19} cm^{-3}) at room temperature. In contrast to conventional terahertz spectroscopy studies, this enables one to directly cover the frequency range encompassing the resultant plasma frequencies. The intensity reflection spectra of the thermalized plasma, measured using sum-frequency (up-conversion) detection of the probe pulses, can be modeled well by a standard Drude model with a density-dependent momentum scattering time of approx. 200 fs at low densities, reaching approx. 20 fs for densities of approx. 10^{19} cm^{-3}, where the increase of the scattering rate saturates. This behavior can be reproduced well with theoretical results based on the generalized Drude approach for the electron-hole scattering rate, where the saturation occurs due to phase-space restrictions as the plasma becomes degenerate. We also study the initial sub-picosecond temporal development of the Drude response, and discuss the observed rise in the scattering time in terms of initial charge-carrier relaxation, as well as the optical response of the photoexcited sample as predicted by finite-difference time-domain simulations.