Controlled Growth And Related Modulation Mechanism Of 1D Silicon Nanostructures

One-dimensional (1D) silicon (Si) nanostructures including silicon nanowires, silicon nanotubes, etc. are novel semiconductor materials, which exhibit significant electrical, optical, and thermal properties, due to the quantum confinement effect, and are well compatible with modern integrated circuit technology. That is why 1D Si nanomaterials are promising to be widely used in optoelectronic, biological, energy harvesting and magnetic devices in the near future. However, to reach the requirement of different applications, the controllability of the morphology of 1D Si nanomaterials is still a problem to be fully explored and developed. There are two kinds of preparation techniques, one is the "bottom-up methods" including laser ablation, chemical vapor deposition, thermal evaporation, sacrificial template method; the other is the "top-down methods" using chemical etching to selectively remove the material from Si substrates, which also involves lithographic technology. Although all of them could produce one-dimensional silicon nanostructures under certain conditions, they still generate some questions to be solved. For example, there will be metal-pollution in the products under a laser ablation condition and the morphology of silicon nanotube will be easily to be damage when templates is removed in sacrificial templates method, which obstacles the further application of the raw materials.In this paper, thermal evaporation technique and metal-assisted chemical etching method are chose to investigate and reveal the relationship between processing parameters and morphology of the final products based on the systematic experimental research and theoretical analysis.Three allotropes of one-dimensional silicon nanostructures as silicon nanowires, silicon nanotubes and silicon nanochains are synthesized by a lanthanum assisted thermal evaporation technique with various quantities of lanthanum in the source powder of silicon and silicon monoxide. There was no lanthanum detected in the final products. The role of lanthanum is decreasing the concentration of gaseous silicon through reacting with SiO in reactants, by which modulating the morphologies of one-dimensional silicon nanomaterials. On the other way, we reduce the evaporation surface area of reaction powder to decrease the concentration of gaseous silicon. Through characterizing the products we find that different one-dimensional silicon nanomaterials are generated. The results show that the lanthanum powder can modulate the concentration of gaseous silicon which dominantly affects the morphologies of final products.Based on the experimental results, a three phase interfaces preferential precipitation growth model is proposed to explain the relationship between the concentration of gaseous silicon source and the morphologies of final products based on the oxide-assisted growth theory combined with vapor-liquid-solid mechanism. A negative-feedback mechanism is also involved. This integrated growth model could be widely effective as theoretical support to the growth of other kinds of one-dimensional nanomaterial.As to the "top down method", we adopted metal-assisted chemical etching method to fabricate silicon nanowire arrays. We study how the metal elements and assemble method, the doping type and doping level of silicon wafers influence the morphologies of 1D Si nanomaterials. We find that the length of silicon nanowire arrays is proportional to the etching time. The doping type of silicon wafer has little impact on the morphologies of final products. The lightly doped wafer is etched faster than the heavy doped wafer, and morphologies in vertical orientation of the two kinds wafers are different. Finally, a micro-electrolytic cell arrays mechanism involving local cathodes and anodes is proposed to explain the grow mechanism of metal-assisted chemical etching method.Transmission electron microscopy, scanning electron microscopy, selected area electron diffraction, energy dispersive X-ray spectroscopy and X-ray diffraction were used to characterize the products and source residue.This systematic research on modulating methods and mechanisms of one-dimensional silicon nanostructures provides a new idea to realize the controllable preparation of different morphology of 1D Si nanomaterials and to improve the current processing. What’s more, the integrated oxide-assisted mechanism and three phase interfaces preferential precipitation mechanism consistent well with the experimental results and will be a useful model to support the growth mechanism of wide range of 1D nanomaterials.