Over the last decade, the solar photovoltaics (PV) industry has grown rapidly with increasing fossil fuel cost and greenhouse gas emissions. The applications of high quality, large grain hydrogenated microcrystalline silicon ($\mu$c-Si:H) thin film transistors and solar cells have become popular in recent years. To attain stability and excellent electronic properties as well as the low manufacturing cost, one must produce $\mu$c-Si with the maximum possible grain sizes, ideally as large as the film thickness itself. However, the traditional plasma enhanced chemical vapor deposition (PECVD) process for $\mu$c-Si:H growth is limited by poor control over structural quality.This work studies a new approach to synthesizing $\mu$c-Si:H with a lower thermal budget and better control of the microstructure, with potentially larger grain sizes. Such unique technique uses a low-pressure radio-frequency plasma reactor to synthesize silicon nanocrystals and embeds these nanocrystals in a-Si:H matrix, where they serve as starting points for crystal grain growth upon thermal annealing. In studying the crystallization process of seeded films, two important criteria are to be optimized: the maximization of crystal grain size, and minimization of annealing time. This work expects to reduce the annealing time by eliminating the incubation time by implanting nanocrystal seeds into the amorphous matrix. In regards to grain structure optimization, the final grain size can be controlled via controlling the initial seed concentration. Furthermore, a new phenomenon of nanovoid enhanced crystallization is observed during heated stage TEM annealing of seeded films. Nanocavity regions, referred as "nanovoids", form at the interface between the nanoseeds and the amorphous matrix. Those nanovoids propagate through the film during annealing at a speed higher than the solid-phase epitaxy (SPE) of the seeds and leave crystalline region behind, which further enhance the crystallization of amorphous Si films. The formation of nanovoids is related to film nanoporosity and the shadowing effect of the nanoparticles, and the void propagation mechanism is explained as a combination effect of atomic surface diffusion at its inner surface and the twin growth at its tail region. Efforts are made to control the nanovoid density as well as its crystallization enhancement effect, by tuning the film nanoporosity, the nanoparticle shape and introduction of a new seeded film structure.
University of Minnesota Ph.D. dissertation. November 2013. Major:Mechanical engineering. Advisor: Uwe Kortshagen. 1 computer file (PDF); ix, 124 pages. (some col.)
Enhanced crystallization of amorphous silicon films with silicon nanocrystal seeding.
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