Controlled Growth of Amorphous Silicon Nanowires Via a Solid-Liquid-Solid (SLS) Mechanism
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Mat. Res. Soc. Symp. Proc. Vol. 581 © 2000 Materials Research Society
energy dispersive spectrum (EDS) were employed for analysis of the morphology and microstructure of the product. RESULTS AND DISCUSSION
Fig.1 (a): SEM image showing the general morphology of the SiNWs grown via a SLS growth mechanism. (b): TEM image revealing that the SiNWs have smooth morphology and average diameter around 40 rum. The SAED pattern shown in inset reveals characteristic diffusive ring pattern, showing that the nanowires are completely amorphous Fig. l(a) shows in plan view an SEM image revealing the general morphology of the Si nanowires grown on a large area (10 mm x 10 mm) of 111 Si substrate after one hour's growth. The nanowires grew directly on the substrate without introduction of other Si source in the vapor phase. It is visible that the deposit consists of nearly pure SiNWs. The growth rate of the nanowires is estimated about 30 nm/second. The TEM image shown in fig. 1(b) reveals that the SiNWs have smooth morphology, and have a diameter of 10-50 nm and length up to a few tens of micrometers. The highly diffusive ring pattern (inset) of selected area electron diffraction (SAED) revealed that the Si nanowires are completely amorphous (a-SiNWs). EDS analysis proved that there exists a small amount of oxygen in the a-SiNWs, which was attributed to the surface oxidation when the freshly-made nanowires were exposed in air, because the growth
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process was controlled under a reduction atmosphere with a mixture of argon and hydrogen. The growth environment is quite different from that of laser ablation, or physical evaporation, and it indeed reveals a different growth mechanism. In the case of oven-laser ablation7 approach, silicon source for SiNWs growth was supplied from the vapor phase in which atomic Si species were ablated off by the laser beam. While in the high temperature evaporation method8 , sufficient silicon atoms were evaporated at high temperature from the powder target due to the extremely high specific ratio of surface/volume compared to bulk silicon. Such a high specific surface/volume ratio guarantees a Si concentration high enough in the vapor phase. In these two cases, the growth of the SiNWs is controlled by the well-known vapor-liquid-solid (VLS) mechanism, in which the vapor phase plays an important role in the growth of the SiNWs. The central idea of the VLS growth of SiNWs is that, the catalysts (usually Ni, or Fe as impurity) act as a liquid-forming agent, which react with the vapor phase, and forms the NiSi 2 eutectic liquid droplets. The vapor phase is rich in Si atoms. With the further absorption of Si atoms into the droplets from the vapor phase, the droplets become supersaturated, resulting in the precipitation of SiNWs from the droplets. Ni thin Film on Si (111)
Si substrate (a)
Formation of Si-Ni liquid droplets
:~
:cc (b)
(c)
(d)
Fig. 2 Schematic depiction of the SiNW growth via the SLS mechanism: (a) Deposition of a thin layer of Ni on the Si (111) substrate; (b) Formation of
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