Ultrafast carrier dynamics in nanocrystalline silicon through the amorphous to nanocrystalline phase transition

Abstract

Summary form only given. Thin film nanocrystalline silicon (nc-Si), a promising new 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 these materials, which are prepared by hot-wire-assisted chemical vapor deposition, can be controlled by varying the hydrogen dilution ratio (R = H/sub 2//SiH/sub 4/) during deposition, yielding materials that span the transition from the amorphous to the nanocrystalline state, with increasingly larger grain size and crystalline fraction at higher dilution values. Low hydrogen dilution values result in amorphous materials, and the phase boundary between the amorphous and nanocrystalline states is characterized by protocrystalline structure with enhanced medium-range order and electronic properties that are particularly promising for photovoltaic applications. We have investigated the dynamics of photoexcited carriers in these materials at ultrafast time scales to address the underlying physics of the carrier trapping and carrier recombination processes. Measurements of the time-resolved change in transmission following photoexcitation were made on nc-Si thin films prepared at a series of hydrogen dilutions.