Ultrafast Carrier Dynamics in Silicon: A Two-Color Transient Reflection Grating Study on a(111)Surface

The dynamics of excited carriers generated near a silicon surface were characterized on femtosecond time scales using the transient grating technique in the reflection configuration. For electrons in the energy range $1.4\pm 0.6\mathrm{eV}$ above the conduction band edge and their corresponding holes, the lifetime for relaxation through phonon scattering at carrier densities below ${10}^{20}{\mathrm{cm}}^{-3\mathrm{}}$ was determined to be 240 fs. This relaxation time increased sharply for carrier densities higher than $5\times {10}^{20}{\mathrm{cm}}^{-3\mathrm{}}$, providing the first direct evidence for charge screening of carrier-phonon scattering in Si.