Abstract
Localized and site-specific bottom-up synthesis techniques for one-dimensional nanostructures provide a solution to current challenges in nanostructure placement, integration and system-level manufacturing. This work extends localized nanowire synthesis techniques to a new material system and for the first time assesses localized nanowire synthesis techniques through a comparative study. This work contains the first demonstration of localized germanium nanowire synthesis. Additionally, this work begins to investigate the role of temperature and temperature gradients on the bottom-up synthesis of silicon and germanium nanowires. More specifically, the synthesis of silicon nanowires and germanium nanowires using a highly localized heat source is compared to silicon nanowires and germanium nanowires synthesized in a uniform, elevated temperature environment. With the exception of the thermal environment, identical synthesis parameters are maintained in the experiments. The localized heat source, a suspended silicon microscale heater, enables the localized heating and thus localized synthesis of the nanowires. The evaluation is focused on a comparative quantitative study of resulting nanowire geometry (assessed in terms of diameter, length and taper), nanowire growth rates and nanowire quality (assessed in terms of kinking levels). Due to the extreme sensitivity of bottom-up synthesis reactions to process conditions, temperature and partial-pressure-dependent experiments are conducted to better understand general process attributes and local heating effects. In contrast to the uniform temperature processes, nanowire kinking and bending is found to be prevalent in locally heated reactions. Additionally, branching behavior which hints of instabilities in the synthesis process as nanowires grow away from the heat source is reported for the first time with localized synthesis reactions. Branching is never observed in uniform temperature synthesis reactions explored in this work. Interestingly, differences in the response of silicon and germanium nanowire growth processes in the localized heating environment are also noted suggesting that material-system specific investigations are necessary. For example, it has been observed that locally synthesized silicon nanowires typically exhibit a taper which is absent in uniform temperature processes. On the other hand, for germanium nanowires where tapered growth is often a distinctive characteristic, the taper is largely eliminated in the localized processes. Finally, this study demonstrates a new lower temperature limit for the synthesis of germanium nanowires in a uniform temperature environment as synthesis is successfully carried out at 240 °C.
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