Hot carriers in silicon: femtosecond reflectivity studies.

The observation of hot carrier relaxation in silicon obtained by time-resolved reflectivity is reported. The measurements were made under low carrier density (approx. 1017 cm-3) for which carrier-carrier processes are negligible. Carriers were injected into the 0.5-μm thick film of silicon with 70-fs optical pulses at 625 nm (2.0 eV) from a dispersion-compensated colliding-pulse-mode-locked dye laser. Initially, these carriers have substantial excess kinetic energy (approx. 0.85 eV), since the bandgap of silicon is only 1.15 eV at room temperature. Changes in reflectivity of a probe beam at the same wavelength were measured as a function of time with approx. 100-fs temporal resolution and sensitivity better than one part in 106. The samples used in this study were approx. 0.5-μm silicon-on-sapphire wafers. Results are reported showing variations in reflectivity as well as in constancy across the wafer. A simple analysis considering only two reflections explains this reflectance variation and predicts the substantial signal enhancement seen in the thin silicon film.