TEM
characterisation of diamond‐hexagonal silicon nanowires
Authors: Maurice Jean‐Luc , Tang Jian , Florea Ileana , Fossard Frédéric , Roca i Cabarrocas Pere , Johnson Erik V. , Foldyna Martin 
Published: December 2016

The diamond‐hexagonal phase (lonsdaleite) does not appear in the equilibrium pressure‐temperature phase diagram of silicon. However, it has been observed in several cases (see review in
1
), especially in samples that had undergone a stress treatment.
2, 3
Its presence in semiconductor nanowires does not make consensus. In such objects, the occurrence of the parent phase of wurtzite has been observed and explained in the case of (normally cubic) III‐V semiconductors,
4
but lonsdaleite has been ascertained in only very few cases: in Ge nanowires prepared under stress
5
or after epitaxy of Si on GaP nanowires.
6
In most cases of silicon nanowires grown by the vapour‐liquid‐solid (VLS) method, the proof of existence of that phase, given under the form of a transmission electron microscopy (TEM) high‐resolution image or diffraction pattern, was disputable.
1
This is due to the fact that, in most zone axes, hexagonal close packing (hcp) is very difficult to differentiate from face centred cubic (fcc) multiple twinning.
1, 7
Even in the [110] cubic axis where hexagonal stacking can easily be recognised, the superposition of twinned orientations in the beam direction can lead to erroneous conclusion.
1

Here, we present TEM recordings in the [110]
C
/[11‐20]
H
zone axis of VLS silicon nanowires that cannot be interpreted in terms of cubic stacking. Those are the first unambiguous TEM observations of the lonsdaleite phase in VLS SiNWs to our knowledge. These nanowires differ from those usually studied by (i) their small size, as we find the phase in nanowires with diameters around 5 nm, (ii) the
plasma‐enhanced
chemical vapour deposition technique (PECVD) we use for bringing gas radicals to the catalyst particles and (iii) the liquid tin we use as catalyst. Their small diameters, in particular, make it very unlikely that several different twin orientations be superimposed in the beam direction.

The figures present silicon nanowires (SiNWs) obtained after 2 min of growth. The SiNW in Fig. 1 has several polytypes, including 2H (lonsdaleite) and 6H silicon. The simulated images
8
superimposed to the two regions of hexagonal symmetry confirm the structure. The amorphous contrast at the NW surface indicates the presence of oxide. Figure 2 shows the Fourier transform of the experimental image in Fig. 1(top‐right) and diffraction patterns: (i) with a 10‐nm beam centred on 6H (middle‐right) and (ii) with a 20‐nm beam encompassing both polytypes (bottom‐right). These experimental diffractograms exhibit reflections characteristic of the 6H and 2H phases, as shown in the simulated diffractograms of the 6H (top‐left) and 2H (bottom‐left) geometries. Figure 3 exhibits the structure of a pure 2H silicon nanowire. The 90° bend is performed with no defect: the growth axis just changes from [0001] to [10‐10]. The image at the bottom‐left is in a slightly different orientation, giving a Fourier transform (bottom‐right) that demonstrates that the structure is pure 2H. The nanowire in Fig. 4 presents the interesting feature of having started with a (twinned) cubic phase and having then switched to hexagonal phase, as if the latter were energetically favoured in the dynamic conditions of the growth.