Ultrafast carrier dynamics in thin film nanocrystalline silicon

Abstract

Thin film nanocrystalline silicon (nc-Si), a promising material for photovoltaic and optoelectronic applications, is comprised of nanometer-scale crystals of silicon embedded in a matrix of hydrogenated amorphous silicon. The degree of crystallinity of the material can be controlled by varying the deposition conditions, yielding materials that span the transition from the amorphous to the nanocrystalline state, and yielding variable grain size and crystalline fraction. Pump-probe measurements using optical pulses 35 fs in duration in the near-infrared were carried out on a series of nc-Si films of varying composition. Photoexcitation results in an induced absorbance signal with a nonexponential time dependence that is strongly dependent on excitation density. The response can be understood in terms of a multicomponent model that includes distinct contributions from each phase of the heterogeneous material. We observe a 240-fs exponential relaxation process associated with intraband relaxation in the silicon crystallites, a response characteristic of bimolecular recombination in the amorphous silicon matrix, and a long-lived component assigned to grain boundary states.