Abstract

The ultrafast carrier dynamics in a-Si_1-x-Ge_x:H alloys (energy gap 1.0 to 1.8 eV) were examined using femtosecond time-resolved spectroscopy. Optical pulses from an amplified CPM laser (photon energy = 2 eV and pulse duration d fsec) were used to create a large density (10¹⁸ to 10²¹ cm³) of excited carriers. The time dependence of changes in the reflectivity and transmission were measured with a probe pulse obtained from a white light continuum. By varying the energy of the probe pulse photons to be above or below the energy gap in these materials, the ultrafast electronic and thermal effects¹ can be isolated. The effective lifetime of carriers near the mobility edge was found to depend strongly upon the carrier density, becoming as short as 1 ps for the highest densities. The ultrafast rise in lattice temperature indicates that electrons and holes are recombining nonradiatively. The carrier lifetime scales with density in a manner that suggests that the mechanism is Auger recombination² of geminate electron-hole pairs.

© 1990 Optical Society of America