Ultrafast carrier relaxation in radiation-damaged silicon on sapphire studied by optical-pump–terahertz-probe experiments

We investigate the relaxation of photogenerated carriers in radiation-damaged silicon on sapphire using a collinear optical-pump–terahertz-probe arrangement. Carrier densities greater than ${10}^{20} {\mathrm{cm}}^{\mathrm{-3}}$ are injected using 400 nm, 100 fs pump pulses, and the change in transmission of the terahertz-probe pulse is measured as a function of pump–probe delay. The time-dependent carrier density is deduced using a thin-film Drude model. A carrier mobility of $\text{422±17\hspace{0.05em}}{\mathrm{cm}}^2/\mathrm{V s}$ is measured, and single-exponential carrier relaxation times of 4 ps at low fluence and 6 ps at high fluence are observed.