Picosecond carrier dynamics in a-Si/sub 0.5/Ge/sub 0.5/:H measured with a free-electron laser

A picosecond time-resolved pump and probe experiment has been performed on a thin a-Si/sub 0.5/Ge/sub 0.5/:H film using the short optical pulses generated by the superconducting accelerator-pumped free-electron laser (FEL) at Stanford University. The FEL was tuned at 1.5 mu m. The probe beam was at the fundamental wavelength, and the pump beam was obtained after frequency doubling. The authors found that the transmission decreases due to photogeneration of carriers and then recovers on a 6 ps time scale. This recovery is the signature of ultrafast carrier recombination. It is faster than in a-Si:H for similar conditions. A possible origin of the difference is discussed.< >