Low temperature spatially resolved cathodoluminescence was carried out on GaN films grown by the epitaxial-lateral-overgrowth (ELO) technique with the nonpolar (11-20) and the semipolar (11-22) orientations on R- and M-sapphires, respectively. Defect related optical transitions were identified and their localization was correlated to different regions of ELO. The sample microstructure was further investigated by plan-view and cross-section transmission electron microscopies. It is shown that the defect related emissions are mainly localized in the seed of the samples, but different defects occur as well in the wings, especially in the case of nonpolar GaN. The structural defect densities are lowest in the overgrown wings of semipolar GaN. In particular, the [0001] wing region of semipolar ELO-GaN is almost defect-free with a cathodoluminescence spectrum dominated by the GaN band-edge emission at 3.476eV.

The microstructure of GaN templates and epitaxial lateral overgrown (ELO) films deposited on M-plane sapphire is investigated by transmission electron microscopy. The epitaxial relationship is ½11 2 20 sapphire k ½1 1 100 GaN in-plane with a growth plane close to ð11 2 22Þ GaN . The microstructure of the templates is dominated by the presence of basal stacking faults with a density of around 3 Â 10 5 cm À1 . Basal stacking faults are either terminated by partial dislocations or connected by prismatic stacking faults. Perfect dislocations are also observed. ELO films with stripes parallel to ½11 100 GaN allow us to reduce the density of basal stacking faults in the overgrown regions down to 5 Â 10 3 cm À1 . In these overgrown materials, the density of perfect dislocations is 10 8 cm À2 while neither partial dislocation nor prismatic stacking faults are observed.

New conditions for one-step ELO were implemented to grow coalesced (1120) non-polar and (1122) semi-polar GaN layers starting, respectively, from Rand M-plane sapphire. A great part of the stacking faults (SFs) and dislocations are filtrated resulting in GaN material with better structural and optical properties. In the ELO-like (1120) and (1122) films, the near band edge emission dominates photoluminescence spectra and is in the range 3.45 -3.48 eV depending on lattice deformation. The strongest emission is met for the semi-polar (1122) ELO. When mask stripes are not normal to the c-axis, a singular ELO is developed with inclined coalescence facets. However, in this case, SFs overgrow above the mask and so lead to poor optical properties, dominated by SF and dislocation related peaks. In any case, the internal electric field reduction in (Al,Ga)N/GaN non-or semi-polar quantum-wells stacks is better viewed when the heterostructures are grown on ELO with stripes normal to the c-axis. OriginalPaper way to improve the microstructure is by filtrating most of these extended defects via the use of epitaxial lateral overgrowth (ELO) techniques [8][9][10]. In this paper we review our study on the MOVPE epitaxy optimisation and on the microstructure and optical improvements of (1120) a-plane and (1122)-plane semipolar GaN layers using a one-step ELO technique. The consequence on polarisation in (Al,Ga)N/GaN MQWs structures is then presented.