Different types of buffer layers like InGaN underlayer (UL) and InGaN/GaN superlattices are now well-known to significantly improve the efficiency of c-plane InGaN/GaN based light emitting diodes (LEDs). The present work investigates the role of two different kinds of pregrowth layers (low In-content InGaN UL and GaN UL namely "GaN spacer") on the emission of core-shell m-plane InGaN/GaN single quantum well (QW) grown around Si-doped !̅-GaN microwires obtained by silane-assisted MOVPE. According to photo-and cathodoluminescence measurements performed at room temperature, an improved efficiency of light emission at 435 nm with internal quantum efficiency > 15 % has been achieved by adding a GaN spacer prior to the growth of QW. As revealed by scanning transmission electron microscopy, an ultra-thin residual layer containing Si located at the wire sidewall surfaces favors the formation of highdensity of extended defects nucleated at the first InGaN QW. This contaminated residual incorporation is buried by the growth of GaN spacer and avoids the structural defect formation, therefore explaining the improved optical efficiency. No further improvement is observed by adding the InGaN UL to the structure, which is confirmed by comparable values of the effective carrier lifetime estimated from time-resolved (TR) experiments. Contrary to the case of planar cplane QW where the improved efficiency is attributed to a strong decrease of point defects, the addition of an InGaN UL seem to have no influence in the case of radial m-plane QW.

The present work reports high quality non-polar GaN/Al0.6Ga0.4N multiple quantum wells (MQWs) grown in core-shell geometry by metalorganic vapor phase epitaxy on the m-plane sidewalls of ̅ -oriented hexagonal GaN wires. Optical and structural studies reveal UV emission originating from the core-shell GaN/AlGaN MQWs. Tuning the mplane GaN QW thickness from 4.3 to 0.7 nm leads to a shift of the emission from 347 to 292 nm, consistent with Schrödinger-Poisson calculations. The evolution of the luminescence with temperature displays signs of strong localization, especially for samples with thinner GaN QWs and no evidence of quantum confinement Stark effect, as expected for non-polar m-plane surfaces. The internal quantum efficiency derived from the photoluminescence intensity ratio at low and room temperature is maximum (~7.3 %) for 2.6 nm-thick quantum wells, emitting at 325 nm and shows a large drop for thicker QWs. An extensive study of the PL quenching with temperature is presented.Two non-radiative recombination paths are activated at different temperatures. The low temperature path is found to be intrinsic to the heterostructure, whereas the process that dominates at high temperature depends on the QW thickness and is strongly enhanced for QWs larger than 2.6 nm, causing a drop of the internal quantum efficiency.