HexaNW

The diamond hexagonal (2H) phase does not appear in the pressure-temperature phase diagram of silicon. However, the partner Laboratoire de physique des interfaces et des couches minces (LPICM, CNRS, École polytechnique, Université Paris-Saclay) has demonstrated that silicon nanowires having the 2H phase can be produced by the vapour-liquid-solid (VLS) method in a plasma-enhanced chemical vapour deposition (PECVD) reactor. The goal of the HexaNW project is to understand what stabilises that phase during growth in order to establish a protocol to reliably reproduce it. In order to understand that mechanism, we will use the atomic-scale observation of the growth, in situ, in the NanoMAX environmental transmission electron microscope (ETEM), and its modelling by thermodynamic calculations using surface and interface energies that will be ab-initio calculated. NanoMAX is the environmental part of the “EquipeX” TEMPOS. The calculated band structure of 2H Si makes it a better absorber of the solar spectrum than standard diamond-cubic (3C) silicon (Amato, Nano Lett. 16, 5694, 2016) and its band gap would be direct in nanowires or under stress (Rödl, …, and Guillemoles, Phys. Rev. B 92, 045207, 2015). These electronic properties, together with the natural abundance of the element and its environmental innocuity, make it a wonderful candidate for future nano opto-electronic devices. LPICM develops solar cells based on radial junctions around crystalline nanowires grown by tin-catalysed PECVD. Quite remarkably, the amorphous silicon deposited around a 2H core in such devices would apply a stress that may possibly be used to tune the band gap of the 2H phase. Although 2H Si does not exist at equilibrium, it has been found in various systems under stress. It has also been obtained deliberately by epitaxy on wurtzite GaP nanowires. But, before our observations, it had never been demonstrated in as grown silicon nanowires without ambiguity. Our observations are uniquely performed in a zone axis of the type [110] (3C)/ [11-20] (2H), where the characterisation of the phase is direct (Tang, Maurice, et al., Nanoscale, 2017). The 2H nanowires we prepare differ from those usually studied by (i) their small size (5 nm), (ii) their low-temperature fabrication by plasma-enhanced VLS and (iii) their liquid tin catalyst. In addition to understanding the mechanisms of growth, the goal of HexaNW will also be to know the properties that will come out of such objects, as a function of their size and polytype. Its prospects are to develop opto-electronic devices or photovoltaic cells taking advantage of the unique band structure of 2H Si nanowires. The project will run over a period of 42 months. The programme is the following: growth will be optimised at LPICM in the reactor that already produces the phase. Developing this part will be the task of the PhD student hired by the project. It will be observed in the NanoMAX ETEM in real time and at atomic resolution. Given the risk of this task, the in situ study will also include molecular beam epitaxy of Ge NWs by partner C2N in conditions where the 2H structure would form. Major opto-electronic characterizations of the material will be performed at IRDEP on nanowires grown ex-situ with the help of two interns. Cathodoluminescence (LPICM-C2N) will bring information on the individual luminescence properties. Nucleation and growth will be modelled at C2N (CNRS, Université Paris-Sud, Université Paris-Saclay), with the help of a one-year postdoctoral fellow paid by the project, using surface and interface energies ab-initio calculated at LSI (CEA-CNRS-École polytechnique, Université Paris-Saclay), with the help of a second one-year postdoctoral fellow. The correlation of atomic-scale observations with this modelling will lead to a fine understanding of the ways the Si 2H phase stabilises itself.