Characterization of high crystal quality (11-22) semi-polar GaN overgrown on m-plane sapphire for long wavelength emitters

High quality semi-polar (11-22) GaN has been successfully achieved by means of developing a cost-effective overgrowth approach on either nano-rod or micro-rod arrayed templates on m-plane sapphire using Metalorganic Chemical Vapour Deposition (MOCVD). The nano-rod or micro-rod arrayed templates are fabricated by means of using self-organized Ni nanomasks or a standard photolithography technique, respectively. Based on x-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements, significant improvement in crystal quality has been confirmed as a result of the overgrowth approach. On these two kinds of templates, a fast coalescence has been obtained within a layer of less than 1 μm thick, demonstrating a much quicker coalescence process than those required by any conventional overgrowth technique (typically 10-20 μm).
The average diameter of the nano-rod template has been found to strongly influence the crystal quality and the electrical properties of the overgrown layer, which are characterized by the line-width of XRD rocking curves, strain relaxation, wafer bowing and electronic mobility. By increasing nano-rod diameter, the crystal quality of the overgrown layer is improved. In addition, by investigating the chemical etching semi-polar (11-22) GaN micro-rods using a potassium hydroxide solution on, an anisotropic and selective wet-etching has been obtained, facilitating to reduce basal stacking faults in the overgrown GaN.
Owing to the high crystal quality of the overgrown semi-polar GaN, stimulated emission has been achieved with a maximum optical gain of 130cm-1, which is the first report worldwide on any semi-polar GaN grown on sapphire. Furthermore, InGaN based light emitting diodes (LEDs) from green to amber have been achieved by growth on the overgrown GaN, demonstrating a clear reduction in efficiency droop and significantly contributing to resolve the challenging "green/yellow gap" issue.