A report is presented on the fabrication of light emitting diodes (LEDs) based on GaN core/shell wires on conductive substrates by metal organic vapour phase epitaxy. Catalyst-free GaN-based wires are grown spontaneously on 2-inch n-doped silicon substrates without any thick buffer layer. The LED wire heterostructure consists of an n-type GaN:Si core covered radially by five InGaN/GaN quantum wells and a p-type GaN:Mg shell. Macroscopic devices that integrate around 10 6 wire-LEDs have been obtained thanks to a simple, direct and full-wafer scale contacting process. For the first time, continuous-wave electrical injection at room temperature through the Si substrate into a cm 2 -chip of GaN-based core/shell wire-LEDs is successfully demonstrated, producing blue electroluminescent emission at 450 nm.Introduction: During the last decade, the performanced of planar light emitting diodes (LEDs) based on gallium nitride (GaN) has been strongly improved in such a way that the efficiency of current state-of-the-art white LED exceeds 200 lm/W. However, this conventional approach still suffers from technological issues that hamper its development for solid state lighting applications [1]. LEDs based on nanowires (NWs) may be one promising solution to meet general lighting market requirements in terms of cost and performance. Indeed, their one-dimensional morphology intrinsically offers high extraction efficiency. In addition, InGaN/GaN quantum wells (QWs) with high crystalline quality and large In incorporation should be obtained thanks to strain relaxation at free surfaces even on large size silicon substrates [2].Among the various epitaxial techniques used to grow GaN-based NWs, catalyst-free or selective area growths have been demonstrated using molecular beam epitaxy (MBE), hydride vapour phase epitaxy (HVPE) and metal organic vapour phase epitaxy (MOVPE) [3 -5]. Since 2004, several NW-based LEDs grown by MBE on Si using axial InGaN/GaN heterostructures have been reported in the literature, demonstrating the possibility to tune the emission wavelength over the entire visible spectrum range [6,7]. Using MOVPE growth techniques, radial InGaN/ GaN quantum wells can be deposited on the m-plane sidewalls of GaN NWs, which are grown on sapphire or GaN templates [8]. These core/ shell structures grown on nonpolar facets provide the opportunity to significantly increase the effective surface emission while eliminating the quantum-confined Stark effect. Although the core/shell wire geometry is promising to improve quantum efficiencies of LEDs structures, their fabrication remains an issue considering both the difficulty to grow high quality nonpolar quantum wells on m-plane facets and the challenging technological integration of high aspect ratio structures on insulating substrates.In this Letter, we report on the first fabrication of core/shell GaN-based wire-LEDs on Si substrate by MOVPE. Basically, this configuration allows a simple integration process, since carrier injection across the p-n core/shell heterojunction is ach...