Ultrafast carrier and coherent phonon dynamics in semi-insulated and n-type 4H-SiC

We investigated ultrafast carrier and phonon dynamics in semi-insulated (SI) and n-type 4H-SiC using time-resolved reflectivity measurements. In the n-type 4H-SiC, carriers are excited by an inter-conduction band transition, and thermalized by electron-electron scattering within 20 fs. We observed coherent phonons of folded phonon modes, which become Raman active as a result of the zone folding, and those of A1-symmetry longitudinal optical (A1-LO) phonon mode. In the n-type SiC, the A1-LO coherent phonon forms a coupled mode with a plasmon, resulting in an asymmetrically broadened Fourier transform spectrum. The polarization dependence and the sine-type initial phase indicate that the impulsive stimulated Raman scattering is the mechanism generating coherent phonons of E2-symmetry transverse optical and A1-LO modes in both the SI and n-type 4H-SiC.

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