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S1.02 - Understanding the Mechanism of Plasma-Assisted Silicon Nanowire Growth Based on Low Surface Tension Metals 
April 7, 2015   8:30am - 8:45am

Radial junction thin film solar cells fabricated over silicon nanowire (SiNW) arrays benefit from an excellent light trapping and a reduced material consumption. A material efficient approach to grow the SiNWs on low cost substrates is a vapor-liquid-solid (VLS) method, where metal droplets assist the deposition of gaseous precursors and may play the role of catalysts depending on their nature. In order to optimize the device properties, it is very important to understand the growth mechanism of the NWs. Most of the studies available in the literature explain the mechanism for gold droplets, but gold is not a suitable candidate for making silicon electronic devices as it introduces deep band gap defects in silicon which act as recombination centres. On the other hand, low melting point metals, such as gallium (Ga), tin (Sn), indium (In), and bismuth (Bi) could be ideal choices for fabricating NWs at low temperature, but very few studies have been carried out to understand their exact role in the growth process. Moreover, they are not stable on the SiNW top and tend to wet the nanowire sidewall due to their lower surface energy than silicon. As a matter of fact, a thin metal layer on the nanowire sidewall could help to stabilize metal droplets on the top. We have carried out a series of experiments, studying the axial and radial growth of SiNWs during the plasma-assisted VLS process using Sn as a model system, and found that some of the experimental observations cannot be explained without the existence of the sidewall wetting layer. In this work, we will show how this wetting layer stabilizes the metal droplet on the top of the nanowire and even promotes the growth when the droplet from top is exhausted either due to the hydrogen radical assisted etching or the incorporation inside the nanowire.

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