Carrier and Coherent Lattice Dynamics of Si Probed with Ultrafast Spectroscopy

We have studied the ultrafast near‐IR reflectivity of native‐oxide covered Si(100) using 28 fs pulses from an 800 nm Ti:sapphire oscillator. Initial carrier excitation density is (5.5 ± 0.3) × 1018 cm−3. The reflectivity variations reveal both carrier and phonon dynamics on times scales from a few 10’s of fs to several hundred ps. Carrier‐dynamics contributions to the reflectivity variations include polarization‐grating, anisotropic‐distribution, state‐filling, and free‐carrier terms. From these contributions we obtain a momentum relaxation time of 32 ± 5 fs, an energy relaxation time of 260 ± 30 fs, and a surface‐recombination velocity of (3 ± 1) × 104 cm sec−1. The measurement also excites and detects coherent response of the Si zone‐center phonon. The phase of the oscillations indicates impulsive stimulated Raman scattering (ISRS) as the excitation mechanism of the coherent vibrations. The 64.2 ± 0.1 fs period of the oscillation is in excellent agreement with standard Raman scattering measurements. The coherent phonon decay time of 2.9 ± 0.1 ps is significantly shorter than 3.4 ± 0.2 ps deduced from Raman‐scattering data from the same samples, but is in agreement with data from heavily doped p‐type samples