▎ 摘　　要

We investigate by in situ X-ray diffraction the early stage of GaN nanowire growth on a single layer of polycrystalline graphene transferred onto a Si(100)/SiO2 carrier substrate. The experiment was carried out in a molecular beam epitaxy chamber installed on a synchrotron line equipped with a X-ray diffractometer and a nitrogen plasma source. The evolution of the in-plane lattice parameters of the crystals is monitored. The in-plane lattice parameter of graphene shows only a little variation between room temperature and GaN growth temperature (720 degrees C). As for GaN, an incubation time of about 1 h precedes nucleation. Just after nucleation, the GaN nanocrystals are under tensile strain, and they relax as growth proceeds. The critical nucleus size and the parameters governing nanowire growth dynamics are obtained from modeling. We discuss the possible incorporation of N atoms in the graphene lattice during the incubation time. These atoms could be involved in chemical bonds between GaN and graphene, which could explain the initial strain observed experimentally.