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A4.4.1

Femtosecond far-infrared studies of carrier dynamics in hydrogenated amorphous silicon and silicon-germanium alloys A. V. V. Nampoothiri,1 B. P. Nelson,2 and S. L. Dexheimer1 1 2

Department of Physics, Washington State University, Pullman, WA National Renewable Energy Laboratory, Golden, CO

ABSTRACT We present femtosecond time-resolved studies of the photoexcited carrier response in the far-infrared spectral range in PECVD a-Si:H and a-SiGe:H thin films. The experiments are carried out using an optical pump / terahertz (THz) probe technique, in which a femtosecond pump pulse excites carriers in the sample and a time-delayed probe pulse measures the resulting change in the far-infrared optical properties as a function of time delay following the excitation. These measurements are sensitive to carrier processes at low energy, corresponding to a range of approximately 1 - 10 meV, a key energy scale in these materials. We find that the observed photoexcited carrier dynamics are consistent with trapping of carriers into band tail states on a picosecond time scale. INTRODUCTION Despite considerable previous work on time-resolved optical measurements on amorphous silicon and related materials, important questions have remained regarding fundamental carrier processes in these materials, including the time scale and nature of the initial carrier localization processes. Most experiments on picosecond and femtosecond time scales have been carried out using a pump-probe technique, in which a short optical pump pulse excites carriers into the extended states, and a short, time-delayed probe pulse measures the resulting change in optical properties during the time evolution of the photoexcited carrier distribution. In a large body of previous work, the photoexcited carrier distribution has been probed in the near-infrared and visible spectral ranges, corresponding to relatively large transition energies. The response observed under these conditions is dominated by an induced absorption from the photoexcited carriers with a decay consistent with simple bimolecular recombination kinetics, though the recombination mechanism is still not well understood [1-3]. More recently, we have used femtosecond pump-probe measurements in the visible and nearinfrared spectral ranges to resolve the initial carrier thermalization dynamics, in which carriers that are initially photoexcited into extended states in the conduction and valence bands relax in energy toward the band edge via phonon emission, and we have established that this process takes place on a time scale of ~ 150 fs [4]. In the work we report here, we have used recently developed methods for generating and detecting femtosecond pulses in the far-infrared, or terahertz (THz), spectral range to directly probe the photoexcited carrier distribution at energies of ~ 1 – 10 meV, a key energy scale for these systems, and one which has remained essentially unexplored in these materials. We find that measurements in the THz spectral range allow us to detect a new component of the c