In plasma-enhanced chemical vapour deposition (PECVD), reactive species generated by the plasma have the effect, among others, of changing the surface energies of the growing materials, which allows one to prepare objects that would not stabilize otherwise. Here, we use this property to stabilize Sn droplets used as catalysts in the growth of Si nanowires (SiNWs) [1] with the vapour-liquid-solid (VLS) method [2]. However, the conditions necessary to ignite a plasma are not compatible with transmission electron microscopy (TEM), so that adapting the growth conditions of a PECVD reactor to in-situ TEM is hardly imaginable.

The TEM equipment we use is the environmental transmission electron microscope “NanoMAX”, a modified Thermo Fisher Titan ETEM. To circumvent the incompatibility between PECVD and TEM, we implemented, on the H₂ line of that ETEM, an electron-cyclotron-resonance plasma source (Aura-wave from SAIREM), to remotely generate the H atoms necessary for the growth.

Our in situ observations (Fig.) clearly show that, when decreasing the plasma  power (“plasma 10 W –> 5 W”, in the figure), the liquid Sn at the top of the NW becomes unstable so that no growth is allowed. This effect is associated with the fact that the H atoms generated by the plasma drastically decrease the Si surface energies [3], so that the Sn droplet cannot wet the Si walls and remains on top of the NW; this, in turn allows for the VLS mechanism of growth [4]. Inversely, when the plasma power decreases, the H-atom flux decreases, until it is not sufficient to insure the coverage of Si surfaces: the energy of the latter gets back to its bare value, which is much higher, and liquid Sn, which has a relatively low surface tension, wets it [1].

Moreover, we have recently found [5,6] that the addition of Cu to Sn, to make Cu-Sn bi-catalysts, allows one to reduce the size of the liquid nanoparticles below 10 nm, which appears to be a necessary condition for obtaining SiNWs with the hexagonal 2H polytype [3,5,7], a metastable structure that would have interesting optical propertie [8]. Thanks to our unique setting, we have been able to obtain Sn-Cu-catalysed SiNWs in-situ, with 2H regions [9].

Main collaborators: Pavel Bulkin (pavel.bulkin@polytechnique.edu), Éric Ngo (PhD student 2018-2021), Weixi Wang (PhD student 2017-2021), Pere Roca i Cabarrocas (pere.roca@polytechnique.edu), Martin Foldyna (martin.foldyna@polytechnique.edu), Ileana Florea (lenuta-ileana.florea@polytechnique.edu).

This work is funded by the French National Research Agency – ANR: TEMPOS-NanoMAX (ANR-10-EQPX-50); HexaNW (ANR-17-CE09-0011).

[1] J.-L. Maurice, P. Bulkin, É. Ngo, W. Wang, M. Foldyna, I. Florea, P. Roca i Cabarrocas, R. Béjaud and O. Hardouin Duparc, Visualizing the effects of plasma-generated h atoms in-situ in a transmission electron microscope, Euro. Phys. J. Appl. Phys. 97, 7 (2022), https://doi.org/10.1051/epjap/2022210276.

[2] R. S. Wagner and W. C. Ellis, Vapor-liquid-solid mechanism of single crystal growth, Appl. Phys. Lett. 4, 89-90 (1964), http://scitation.aip.org/content/aip/journal/apl/4/5/10.1063/1.1753975.

[3] R. Béjaud and O. Hardouin Duparc, Stabilizing the hexagonal diamond metastable phase in silicon nanowires, Comput. Mater. Sci. 188, 110180 (2021), http://www.sciencedirect.com/science/article/pii/S0927025620306716.

[4] S. Misra, L. Yu, W. Chen, M. Foldyna and P. Roca i Cabarrocas, A review on plasma-assisted vls synthesis of silicon nanowires and radial junction solar cells, J. Phys. D: Appl. Phys. 47, 393001 (2014), http://stacks.iop.org/0022-3727/47/i=39/a=393001.

[5] W. Wang, É. Ngo, I. Florea, M. Foldyna, P. Roca i Cabarrocas and J.-L. Maurice, High density of quantum-sized silicon nanowires with different polytypes grown with bimetallic catalysts, ACS Omega 6, 26381-26390 (2021), https://doi.org/10.1021/acsomega.1c03630.

[6] É. Ngo, W. Wang, P. Bulkin, I. Florea, M. Foldyna, P. Roca i Cabarrocas and J.-L. Maurice, Liquid-assisted vapor–solid–solid silicon nanowire growth mechanism revealed by in situ TEM when using Cu–Sn bimetallic catalysts, J. Phys. Chem. C 125, 19773-19779 (2021), https://doi.org/10.1021/acs.jpcc.1c05402.

[7] J. Tang, J. L. Maurice, F. Fossard, I. Florea, W. Chen, E. V. Johnson, M. Foldyna, L. Yu and P. Roca i Cabarrocas, Natural occurrence of the diamond hexagonal structure in silicon nanowires grown by a plasma-assisted vapour-liquid-solid method, Nanoscale 9, 8113-8118 (2017), http://dx.doi.org/10.1039/C7NR01299C.

[8] M. Amato, T. Kaewmaraya, A. Zobelli, M. Palummo and R. Rurali, Crystal phase effects in si nanowire polytypes and their homojunctions, Nano Lett. 16, 5694-5700 (2016), http://dx.doi.org/10.1021/acs.nanolett.6b02362.

[9] É. Ngo, In situ growth of silicon and germanium nanowires in the metastable hexagonal-diamond phase, PhD thesis, Institut Polytechnique de Paris, 2021, https://tel.archives-ouvertes.fr/tel-03284317.