Visualizing the effects of plasma-generated H atoms in situ in a transmission electron microscope

Jean-Luc Maurice,Pavel Bulkin,Ileana Florea,Martin Foldyna,Romuald Béjaud,Éric Ngo,Pere Roca I Cabarrocas,Weixi Wang,Olivier Hardouin Duparc

doi:10.1051/epjap/2022210276

Abstract

The radicals and atoms generated by a plasma have the effect, among others, of changing the surface energies of materials, which allows one to prepare nano-objects that would not stabilise in other conditions. This is the case of the Sn catalysed silicon nanowires (NWs) we present in this paper: without plasma, the liquid Sn at the top of NWs is unstable (because Sn naturally wets the Si) so that no growth is allowed, while in presence of the H atoms generated by the plasma, the balance of surface energies is drastically changed; the Sn droplet stabilises and can be used efficiently by the vapour-liquid-solid (VLS) mechanism of growth. Thus, if one wants to study the growth mechanisms of such NWs in situ in the transmission electron microscope (TEM), one has to adapt a plasma system on the TEM. This is precisely what was done at École polytechnique on the NanoMAX environmental TEM. The paper reports on the plasma effects, on the catalyst and on NW growth, recorded in situ in real time, at atomic resolution. The results are discussed in the light of density functional calculations of bare and hydrogenated Si surface energies.

Highlights

In plasma-enhanced chemical vapour deposition (PECVD), several species are generated in addition to the parent gas molecules (e.g. SiH4 to deposit Si), including in particular radicals and ions that allow one to perform deposition at lower temperature than with standard CVD
For our first in situ observations of the effects of the remote electron cyclotron resonance (ECR) plasma, we ignited the plasma while Sn nanoparticles (NPs) deposited on SiC were annealed in situ at 400 °C in
We discovered a new growth mechanism, which we called liquid-assisted vapour-solid-solid (LAVSS), where the catalyst is made of liquid Sn(Cu) and solid Cu3Si [7,9]

Summary

Introduction

In plasma-enhanced chemical vapour deposition (PECVD), several species are generated in addition to the parent gas molecules (e.g. SiH4 to deposit Si), including in particular radicals and ions that allow one to perform deposition at lower temperature than with standard CVD (where it is the heat of the substrate that cracks the molecules). Some effects of the plasma are related to the presence of ions and of an electric field, while others are due to the radicals and atoms, like changing the surface energies of the growing materials. The liquid Sn at the top of NWs is unstable (because Sn surface energy is low compared to that of Si) so that no growth is allowed according to Nebol’sin stability criterion [3]. In presence of the H atoms generated by the plasma, the Si surface energies are drastically lowered [4]; the Sn droplet stabilises and can be used by the vapour-liquidsolid (VLS) mechanism of growth [5]. We implemented on the H2 line of our in situ TEM (“NanoMAX”, a modified Thermo Fisher Titan Environmental TEM À ETEM) an electron-cyclotron-resonance plasma source, to remotely generate the H atoms necessary for the growth

Experimental setup

Results and discussion

Conclusion