Understanding the generation of polytypism in semiconductor nanowires through in situ observations in a TEM
Polytypism is the fact of changing the stacking order of the densest atomic planes in a crystal. It leads to different metastable phases of that crystal, which are in perfect epitaxial relationships, but which have different electronic properties. In semiconductors in general and in silicon in particular, the piling up of those structures opens up the possibility to build up functional electronic devices where new electronic functionalities are provided by changing the structure without changing the chemical composition.
The VLS (vapour-liquid-solid) and VSS (vapour-solid-solid) modes of semiconductor nanowire (NW) growth present the possibility of naturally delivering metastable crystal polytypes directly, without any additional treatment [1,2]. The mechanisms, for that natural occurrence of out-of-equilibrium structures, are fairly well understood for compound semiconductors [3,4]. However, if such polytypes sometimes also appear in elemental semiconductor nanowires, like in very narrow SiNWs [2,5], they are much rarer, and the conditions for their growth remain unknown.
We launched, four years ago, the “HexaNW” ANR project with the goal of understanding the occurrence of hexagonal polytypes in the case of SiNWs. An interesting point was that the 2H polytype, precisely the one which we had observed in SiNWs a few years ago  would have quite specific opto-electronic properties, offering for instance a larger absorption of the solar spectrum than standard cubic Si .
As we had observed this polytype in NWs grown with Sn catalyst on a Cu substrate , we now study Cu-Sn alloy catalysts. As we had used plasma-enhanced chemical-vapour-deposition (PECVD), we implemented an electron cyclotron resonance plasma source (Aura-wave from SAIREM) on the H2 line of the “NanoMAX” in situ transmission electron microscope . We finally obtain 2H domains in narrow SiNWs, both ex-situ in a standard PECVD reactor (Fig. 1)  and in NanoMAX , where we can watch the nucleation of that phase atomic plane by atomic plane.
This work is funded by the ANR, through the TEMPOS Equipex (ANR-10-EQPX-50), pole “NanoMAX” and the HexaNW project (ANR-17-CE09-0011). Electron microscopy observations are all performed at the CIMEX – Centre interdisciplinaire de microscopie électronique de l’X (École polytechnique).
Collaborators: Éric Ngo (PhD student, 2018-2021), Weixi Wang (PhD student, 2017-2021), Ileana Florea (email@example.com), Pavel Bulkin (firstname.lastname@example.org), Martin Foldyna (email@example.com), Pere Roca i Cabarrocas (firstname.lastname@example.org)
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