(Invited) High Efficiency Green-Yellow Emission from InGaN/GaN Quantum Well Structures Grown on Overgrown Semi-Polar (11-22) GaN on Regularly Arrayed Micro-Rod Templates

Last two decades have seen major developments in the field of III-nitride materials and optoelectronics, which is mainly based on polar GaN on sapphire. So far, the growth of c-plane GaN on sapphire has almost approached its theoretical limits and the major limit to the performance of GaN-based LEDs is due to the growth on c-plane. Consequently, this polar orientation leads to a number of fundamental issues, such as efficiency droop, and green and yellow gaps in wavelength coverage. One clear way forward, meeting the fundamental challenges, is to grow along semi-polar/non-polar direction. Semi-polar orientations, in particular, (11-22), offer another major advantage, namely, it can also significantly enhance indium incorporation into GaN, which cannot be achieved using c-plan GaN and thus is extremely important for growth of long wavelength emitters, such as green and even better for yellow. However, the current challenge is due to the crystal quality of semi-/non-polar GaN, which is far from satisfactory. So far, the best performing semi-polar emitters are grown on semi-polar GaN substrates which are typically ~ 10x10 mm in size. Therefore, it is necessary to develop a cost-effective approach to developing large size semi-polar GaN templates with major improvement in crystal quality but still based on sapphire substrates.

The Sheffield team has developed an overgrowth approach for 2" (11-22) semi-polar GaN with significantly enhanced crystal quality on nanorod templates fabricated by self-organised nickel nano-mask approach. Very recently, a new overgrowth approach based on optical lithography mask pattern technique has been developed, where regularly arrayed micro rod templates can be cost-effectively fabricated and the size of the micro rods can be well controlled. The new approach has led to further improvement in both crystalline quality and surface morphology. This has been confirmed by high resolution transmission microscopy (TEM) studies, demonstrating further significant reduction in both dislocation density and basal stacking fault density.

InGaN/GaN multiple quantum well structures with high In compositions have been grown on such semi-polar GaN templates, and we have achieved strong emission at long wavelengths, even down to ~600 nm. For the green-yellow emission at 555 nm, we have obtained an internal quantum efficiency (IQE) of up to 14%, and 7-10% for the pure yellow emission at ~570 nm. Con-focal photoluminescence (PL), micro-PL, time-resolved PL (TRPL), and micro-TRPL have been performed