This paper reviews our recent investigations about semipolar GaN‐based optoelectronic heterostructures grown on foreign substrates. Two basically different approaches are discussed, both making use of epitaxial growth in the polar c‐direction to minimize any crystalline defects. By selective area growth, stripes with triangular cross‐section have been formed with semipolar side‐facets, on which quantum well and electroluminescence test structures have been deposited. By careful optimisation of many growth parameters, we could drastically increase the growth temperature of GaInN quantum wells emitting beyond 500 nm. In the second approach, the GaN growth starts on inclined sapphire c‐planes, which form the side facets of trenches etched into the substrates. After coalescence, planar semipolar GaN layers can be achieved. We investigated various sapphire wafer orientations leading to {112‾2}, {101‾1}, and {202‾1} layers. After careful optimisation with a major focus on the decrease of the stacking fault density, we have also investigated the doping behaviour of such semipolar structures. Eventually, full electroluminescence test structures could be grown.
Semipolar GaN heterostructures are promising for future green light emitters, because respective GaInN quantum wells are characterized by a reduced piezo‐electric field as compared to their polar counterparts, which is expected to be advantageous for the radiative recombination probability of the carriers in LEDs and laser diodes. However, such structure, requiring an epitaxial growth direction in other than the conventional polar c‐direction, are typically blamed by huge defect densities, particularly if grown on foreign substrates like sapphire. Several papers in this Special Issue (e.g. Scholz et al., Meisch et al., Leung et al., Hashimoto et al., de Mierry et al.) concentrate on a method where the eventually semipolar growth initially proceeds in c‐direction by etching trenches into the sapphire wafer which have a c‐plane side facet. Hence lower defect densities can be realized. The cover figure shows schematically such structure including an in‐situ deposited SiN nanomask layer for further defect reduction (red) and semipolar quantum wells on top (green). Such semipolar LEDs emit quite intense light (bottom pictures) in the green spectral range. See more details in Scholz et al. (pp. http://doi.wiley.com/10.1002/pssb.201552386) and Meisch et al. (pp. http://doi.wiley.com/10.1002/pssb.201552241) in this issue.
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