Plasma Synthesis of Si Nanoparticles for Co-Deposition of Mixed Phase Thin Films

Curtis Anderson (ME), advisors: Uwe Kortshagen (ME) and Jim Kakalios (Phys)

Thin films of amorphous Si are very attractive for their use in various photovoltaic devices. The ability of a-Si to convert visible light to electrical energy has been studied for many years. The major drawback to this material is that upon exposure to light, these films tend to breakdown, ie. high defect densities are introduced which prohibit the movement of charge carriers through the crystal lattice. This breakdown is known as the Staebler-Wronski effect. In order to deal with this problem, one can anneal an affected film at high temperatures, after which the film will recover its photoconductivity to nearly its original behavior. Of course this is not practical for a film that is already incorporated into a device. A more feasible solution is to introduce nanocrystalline inclusions into the film, effectively stabilizing the film structure when exposed to light, hence minimizing the Staebler-Wronski effect. The exact reason for the latter stabilization method is not fully understood yet.

Several methods exist to incorporate nanoparticles into an a-Si:H film. The most popular and widely studied technique involves the production of a 'dusty plasma'. Using a capacitively-coupled plasma (CCP) reactor, a gas mixture containing silane (SiH4) is ionized, creating a gaseous environment consisting of reactive species (electrons, ions, and radicals). These reactive species are capable of breaking apart a silane molecule into SiH3, SiH2, and
so on. The Si-containing molecules then recombine at the substrate surface, resulting in the growth of a Si film. Under specific conditions, the plasma can be operated such that small crystallites (~ 2-5 nm) of Si form in the gas phase, and are quickly passivated due to a proper H2 dilution in the gas admixture. These crystallites that are close enough to the substrate are transported to the surface by various mechanisms, and are deposited in the film. While several research groups have studied the reactor parameters necessary to produce films of this kind, as well as the properties of these films, the limitations of this process are quite clear. Namely, one is limited to producing particles and films that are composed of a single material, such as silicon, although more novel applications involving Si particles embedded in Si-N films have emerged more recently. In the current study, the goal is to develop a process by which nanoparticles are produced separately from the film growth conditions, and are later introduced into the film. In this way, one could imagine tailoring the nanoparticle properties (size and morphology), as well as gaining control over the final mass fraction of crystalline/amorphous material.