EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Vertical III-V semiconductor nanowires (NWs) have received a great interest towards realizing optoelectronic devices such as photodetectors[1] and LEDs[2]. These devices must be fabricated on CMOS-compatible substrates for large scale integration. Graphene on SiO2/Si is a good candidate to integrate the NWs as it ensures good back-contacting for electrical injection. Growth of vertical NWs on silicon substrates is well controlled[3], however the situation for graphene is different. The verticality of NWs is often low with a yield[4] below 5% and is mainly dominated by parasitic growth[5]. To understand this issue, we propose to investigate the early stages of the growth aiming at improving the epitaxial relationship between the NWs and graphene and thus increase the vertical yield.

Here we report our efforts to reach high verticality of Au-catalyzed GaAs NWs on graphene substrates. The NWs were grown using the vapour-liquid-solid mechanism in molecular beam epitaxy. By modifying the choice of the catalyst (Au colloids or thin films), a high NW density (~10-15 µm^2) was obtained for the case of Au thin film. Nevertheless, the vertical yield was still low ~35%. To increase the NW verticality, two different approaches were adopted: 1) Doping the Au droplets prior to the growth with Si or C to increase the wetting angle between the droplets and the graphene substrate[4] 2) Using a two-temperature growth procedure (450°C/610°C or 540°C/610°C) to improve the nucleation of the NWs at the early stages of the growth to reduce parasitic growth. Preliminary data show that doping doesn’t affect substantially the NW verticality whereas the two-temperature growth procedure leads to a higher yield. Our results will be compared in the near future to real-time in-situ TEM to better understand the Au-catalyzed growth mechanisms on Graphene. Photoluminescence measurements will be presented to assess the quality of the GaAs NWs on Graphene.