Green light emitting diodes based on group-III nitrides still suffer from fairly low performance as compared to shorter wavelength blue emitters. One possible reason is the lattice mismatch induced strain of the GaInN quantum wells in the active region in such devices having a comparably large In content. This causes the formation of huge piezoelectric fields within the GaInN quantum wells separating electrons and holes locally and hence reducing their recombination probability. By changing the main epitaxial growth direction from the conventional polar c-direction into less polar crystal directions, the internal fields can be strongly reduced. This approach is currently investigated by growing on semipolar GaN wafers cut from thick c-plane material grown by other methods like hydride vapour phase epitaxy or ammonothermal crystal growth. However, owing to the limited size of those c-plane wafers, the semipolar substrates are limited in size to a few square millimetres. On the other hand, approaches to grow semipolar GaN on flat foreign substrates of accurate orientation other than c-plane typically result in highly defective layers. Obviously, growth in c-direction leads to lowest defect densities. Therefore, we currently study some hetero-epitaxial approach where the epitaxial process starts from c-plane-like sidewalls of trenches etched into sapphire wafers. To this end, either n-plane or r-plane sapphire wafers can be used. In both cases, the c-direction is inclined by about 60° from the surface normal. In a later stage, these inclined stripes coalesce forming (10-11) or (11-22) semipolar surfaces for the n-plane and r-plane wafers, respectively. This procedure can be easily applied to large size sapphire wafers. In this contribution, we will describe how to optimize such large area semipolar GaN layers grown by metalorganic vapour phase epitaxy. Similar as for regular c-plane growth, we start with an optimized low-temperature oxygen-doped AlN nucleation layer. After growing few 100 nm GaN, a very thin SiN nanomask layer is deposited in-situ acting as a defect-blocking layer. Hence, excellent semipolar layers have been achieved. Although the (10-11) semipolar planes show a much lower roughness than the (11-22) planes, we found lowest stacking fault densities below 10⁴ cm^-1 on the latter samples. On those surfaces, GaInN multi quantum well structures have been deposited which emit bright luminescence in the green spectral range. We observed a higher In incorporation efficiency on the (10-11) plane as compared to c-plane and (11-22). Moreover, first p-doping experiments revealed a fairly low Mg incorporation efficiency on our (11-22) planes.

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1. Optical properties of high-quality cubic AlN, GaN, and AlN/GaN MQWs grown on 3C-SiC