Tapering-free monocrystalline Ge nanowires synthesized via plasma-assisted VLS using In and Sn catalysts

In and Sn are the type of catalysts which do not introduce deep level electrical defects within the bandgap of germanium (Ge). However, Ge nanowires produced using these catalysts usually have a large diameter, a tapered morphology, and mixed crystalline and amorphous phases. In this study, we show that plasma-assisted vapor-liquid-solid (PA-VLS) method can be used to synthesize Ge nanowires. Moreover, at certain parameter domains, the sidewall deposition issues of this synthesis method can be avoided and long, thin tapering-free monocrystalline Ge nanowires can be obtained with In and Sn catalysts. We find two quite different parameter domains where Ge nanowire growth can occur via PA-VLS using In and Sn catalysts: (i) a low temperature-low pressure domain, below similar to 235 degrees C at a GeH4 partial pressure of similar to 6 mTorr, where supersaturation in the catalyst occurs thanks to the low solubility of Ge in the catalysts, and (ii) a high temperature-high pressure domain, at similar to 400 degrees C and a GeH4 partial pressure above similar to 20 mTorr, where supersaturation occurs thanks to the high GeH4 concentration. While growth at 235 degrees C results in tapered short wires, operating at 400 degrees C enables cylindrical nanowire growth. With the increase of growth temperature, the crystalline structure of the nanowires changes from multi-crystalline to mono-crystalline and their growth rate increases from similar to 0.3 nm s(-1) to 5 nm s(-1). The cylindrical Ge nanowires grown at 400 degrees C usually have a length of few microns and a radius of around 10 nm, which is well below the Bohr exciton radius in bulk Ge (24.3 nm). To explain the growth mechanism, a detailed growth model based on the key chemical reactions is provided.

Automated summary

This study demonstrates that plasma-assisted vapor-liquid-solid (PA-VLS) method can be used to synthesize monocrystalline Ge nanowires with In and Sn catalysts, avoiding sidewall deposition issues.