Ultrafast Dynamics of Photoexcitations in HWCVD Hydrogenated Amorphous Silicon Alloys

Abstract

ABSTRACTWe present femtosecond studies of carrier dynamics in hydrogenated amorphous silicon-germanium 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.