Ultrafast Dynamics of Photoexcitations in HWCVD Hydrogenated Amorphous Silicon Alloys
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Ultrafast Dynamics of Photoexcitations in HWCVD Hydrogenated Amorphous Silicon Alloys J.E. Young,1 B.P. Nelson,2 and S.L. Dexheimer1 1
Department of Physics and Materials Science Program, Washington State University, Pullman, WA 2 National Renewable Energy Laboratory, Golden, CO ABSTRACT We present femtosecond studies of carrier dynamics in hydrogenated amorphous silicongermanium alloys grown by the recently developed hot-wire assisted chemical vapor deposition (HWCVD) technique, which is promising for producing high-quality device-grade materials. We have used wavelength-resolved femtosecond pump-probe techniques, in which an intense pump pulse excites carriers in the sample and a time-delayed probe pulse measures the resulting change in optical properties as a function of time delay following the pump pulse, to study the dynamics of photoexcitations in these materials. Femtosecond dynamics measurements have been carried out on thin film samples under experimental conditions with varying sensitivity to carriers in extended states or in band tail states. The relaxation dynamics of carriers associated with extended states show a strong dependence on excitation density and follow a bimolecular recombination law, consistent with a number of earlier studies on related amorphous materials. In contrast, measurements involving carriers excited directly into band tail states reveal significantly altered dynamics, characterized by a marked deviation from simple bimolecular recombination at short times. INTRODUCTION Thin-film amorphous silicon and silicon-germanium alloys are promising materials for lowcost, high-efficiency solar cells, yet a range of scientific issues relevant to their development and application as photovoltaic materials are not yet fully understood. Important unresolved issues include the detailed nature of the electronic states in these disordered semiconductors, and in particular, the relation of the band tail and extended states and their influence on the carrier dynamics. A number of previous time-resolved studies of photoexcited carriers have revealed complex carrier dynamics in these systems on picosecond time scales [for example, Refs. 1-5 and citations therein]. These results have been interpreted in terms of a number of physical processes, including bimolecular recombination and lattice heating, or in terms of multiple trapping models involving band tail states. For technical reasons, much of this earlier work focused on the time-resolved optical response of carriers photoexcited into extended states well above the band gap in a-Si:H. In this work, we investigate the dynamics of photoexcited carriers as a function of excitation energy relative to the band gap. The experiments were carried out in a series of a-SiGe:H alloys in which the alloy composition has been systematically varied to control the band gap. By varying the relative energy of the photoexcitation and the band gap, we
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selectively excite carriers either into extended states or into progressively deeper band tail states, and
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