Growth of GaN micro- and nanorods on graphene-covered sapphire: Enabling conductivity to semiconductor nanostructures on insulating substrates

The self-catalyzed growth of vertically aligned and hexagonally shaped GaN micro- and nanorods on graphene transferred onto sapphire is achieved through metal-organic vapor phase epitaxy. However, a great influence of the underlying substrate is evident, since vertically aligned structures with a regular shape could not be grown on graphene transferred to SiO₂. The optical properties of the regular GaN nanorods were investigated by spatially and spectrally resolved cathodoluminescence showing defect related emission only near the interface between the sapphire substrate and nanorods but not from their upper part. Micro-raman spectroscopy confirms that the single-layer graphene remains virtually unchanged in terms of the Raman signal, even after undergoing high temperatures (∼1200 °C) during nanorod growth. Furthermore, Raman mapping demonstrates that GaN structures predominantly grow on defective parts of graphene, giving new insight into the nucleation and growth mechanism of semiconductors on graphene. To validate the conductivity of graphene, when being attached to the sapphire substrate and after the nanorod growth, current-voltage investigations were carried out on single, as-grown, GaN nanorods with a nanoprober in a scanning electron microscope. These measurements demonstrate the viability of graphene as a conductive electrode, for example, as a back contact for GaN nanorods grown on insulating sapphire.