Femtosecond Far-Infrared Studies of Photoconductivity in a-Si:H and a-SiGe:H
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A5.9.1
Femtosecond Far-Infrared Studies of Photoconductivity in a-Si:H and a-SiGe:H A. V. V. Nampoothiri and S. L. Dexheimer Department of Physics, Washington State University, Pullman, WA 99164-2814
ABSTRACT We present femtosecond time-resolved studies of the frequency-dependent photoconductivity in the far-infrared spectral range (~ 1 – 10 meV) in PECVD a-SiGe:H and aSi:H thin films. The experiments are carried out using an optical pump / terahertz (THz) probe technique, in which a femtosecond pump pulse excites carriers into the extended states and a time-delayed probe pulse measures the resulting change in the far-infrared optical properties, which are directly related to the ac photoconductivity, as the carrier distribution evolves in time. We find that the frequency-dependent conductivity measured on picosecond time scales shows a strongly non-Drude behavior, with components of the response fitting to a power-law frequency dependence, reflecting processes associated with localized states.
INTRODUCTION Photoconductivity processes in a-Si:H and related amorphous semiconductors directly reflect the nature of the electronic states in these materials. Despite considerable previous efforts, fundamental issues remain for a full understanding of photoconductivity in these materials, including the physics of the initial carrier relaxation and localization. In this work, we report measurements of the ac conductivity of photoexcited carriers in the far-infrared spectral range on picosecond time scales. The measurements are carried out using an optical pump / terahertz (THz) probe technique, in which carriers are generated by a femtosecond optical pump pulse, and the far-infrared optical properties of the photoinduced carrier distribution are probed as a function of time following excitation. Since the optical properties of the photoexcited carriers are directly related to their ac conductivity, these measurements provide a non-contact probe of the photoconductivity. Far-infrared, or THz, frequencies are a key spectral range for understanding conductivity mechanisms, since they span typical inverse carrier scattering times in semiconductors and fall within the energy range of the band tail states in amorphous semiconductors. The high time resolution inherent in these measurements allows us to observe the evolution of the conductivity as carriers undergo localization on picosecond time scales. In our previous work on time-resolved far-infrared studies of carrier dynamics in these materials [1], we carried out optical pump / THz probe measurements of the time-dependent farinfrared absorption due to photoexcited carriers. These measurements were sensitive to the carrier absorption effectively averaged over the ~ 1 – 10 meV spectral range of the THz probe pulses. We found that measurements in the THz spectral range allowed us to detect a new component of the photoexcited carrier response that had not been resolved in a large body of earlier pump-probe studies using probe pulses at near-infrared or visible wavelengths. Our
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