Nitride nanowires (NWs) are today actively explored as an active material for a large number of optoelectronic devices (light emitting diodes, photodetectors, solar cells). The NWs are usually grown on bulk crystalline substrates (Si, sapphire) but these substrates impose their properties which may not be adapted to the device functionality (low electrical or thermal conductivity, opacity, weigh, rigidity, cost,…). Recently, graphene has been proposed as an attractive candidate to grow III-V semiconductor NWs [1]. Graphene is transparent, flexible and it has high thermal and electrical conductances and it can be synthesized at low cost on large areés. Furthermore, graphene films are easily transferable to almost any carrier substrate, including amorphous and/or flexible materials.

In this study, epitaxial growth of GaN nanowires on graphene is demonstrated using molecular beam epitaxy without any catalyst or intermediate layer. We have grown GaN NWs on isolated mono-crystalline graphene flakes transfered onto SiO2/Si carrier substrates. The nanowires grow vertically and X-Ray diffraction (XRD) show that the nanowires have grown along their c-axis. We have observed by SEM that locally, the NW factes have the same in-plane orientation over several µm². The 10-10 facets of the NWs have been determined by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Cross-sections have been prepared by focussed ion-beam (FIB). HAADF images of NW-graphene/silica interface show that the base of the NWs is defect-free. Selected area electron diffraction (SAED) patterns taken from different zones show identical patterns indicating that the nanowires from all the zones have same in-plane orientation. Since it is well established that the hexagonal periphery of the graphene flakes corresponds to zig-zag edges of the graphene lattice, we can conclude that the epitaxial relationship is 11-20 GaN parallel to the graphene zig-zag. It leads to an apparent mismatch of 29% between the graphene and the GaN wurtzite structure. This value is very large. We show that an in-plane super cell coincidence between 4 unit cells of graphene and 3 unit cells could lead to a more favorable mismatch of -3.1%. The optical properties of NWs arrays have been probed by photoluminescence spectroscopy.

[1] Munshi A.M., Dheeraj D.L., Fauske V.T., Kim D.C., Van Helvoort A.T., Fimland B.O., Weman H. NanoLetters, 12 (9), 4570 (2012)

ACKNOWLEDGEMENTS

We acknowledge Agence Nationale de la Recherche (ANR), program of investment for the future, TEMPOS project (n°ANR-10-EQPX-50) for having funded the acquisition of the NANOTEM platform (Dualbeam FIB-FEG FEI SCIOS system and TEM-STEM FEI Titan Themis equipped with the Super-X Chemistem EDX detectors) used in this work

Figures:

SEM images of the sample during the FIB process of TEM preparation. a) GaN NWS on a graphene flake. b) after Pt mask deposition. c) after FIB milling d) after lift-out and sticking to the Cu TEM grid

TEM cross view images taken across different monolayers of graphene and their corresponding SAED patterns showing the presence of a unique in-plane orientation

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EMC Abstracts - https://emc-proceedings.com/abstract/epitaxy-of-gan-nanowires-on-graphene/