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 {1122}, {1011}, and {2021} 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. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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. 13–22) and Meisch et al. (pp. 164–168) in this issue.

GaN based laser diodes with semipolar quantum wells are typically grown on free-standing pseudo-substrates of small size. We present an approach to create a distributed-feedback (DFB) laser with semipolar quantum wells (QWs) on c-oriented templates. The templates are based on 2-inch sapphire wafers, the method could easily be adapted to larger diameters which are available commercially. GaN nanostripes with triangular cross-section are grown by selective area epitaxy (SAE) and QWs are grown on their semipolar side facets. The nanostripes are completely embedded and can be sandwiched inside a waveguide. For optical pumping, open waveguide structures with only a bottom cladding are used. Using nanoimprint lithography, stripe masks with 250 nm periodicity were fabricated over the whole wafer area. The periodicity corresponds to a 3rd order DFB structure for a laser emitting in the blue wavelength regime. These samples were analyzed structurally by high-resolution transmission electron microscopy (HRTEM), and spatio-spectrally by cathodoluminescence (CL) inside a scanning transmission electron microscope (STEM). Samples with an undoped cap are pumped optically for stimulated emission. To prove the feasibility of realizing a 2nd order DFB structure with this approach, stripes with a 170 nm periodicity are fabricated by electron beam lithography and SAE. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The electroluminescence (EL) output power of c-plane InGaN/GaN-based light-emitting diodes (LEDs) is much higher than that of semipolar {10 (1) over bar 11} and {11 (2) over bar2} LEDs at the same operation current. In order to elucidate the reasons for this behavior, we have fitted the pulsedELdata by the well-knownABCmodel to extract the internal quantum efficiency (IQE) and the carrier injection efficiency (CIE) to clarify which parameter weighs more for the poor EL output power of the semipolar LEDs. The CIE shows large differences, 78%, 4%, and 4% for the c-plane, {10 (1) over bar1} and {11 (2) over bar2} LEDs, respectively, whereas the IQE values are fairly the same for all three structures. The fit of resonant photoluminescence (PL) data at room temperature confirms the similar IQE values for all three structures. The CIE was increased from 4% to 10% for the planar {11 (2) over bar2} LED with better electrical conductivity of the p-GaN layer. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Polar and semipolar LED structures with InGaN quantum wells (QWs) were investigated in a scanning electron microscope (SEM) using simultaneously electron beam induced current (EBIC) and cathodoluminescence (CL). EBIC yields important information about depletion region, leakages, and the overall functionality of the pn-junction and allows to determine the diffusion length of the generated minority carriers on both sides of the pn-junction. Spectrally and spatially resolved CL measurements yield both, information about the QW quality and about centers of non-radiative recombination. EBIC measurements were carried out in top-viewand in cross-section between 26K and room temperature. The diffusion lengths measured for polar and semipolar structures prove the better crystal quality of the polar structures in terms of the higher carrier diffusion length and its increase for lower temperatures, as expected. On the semipolar structures, top-view EBIC measurements detect specific areas, where the pn-junction seems to be disturbed. The comparison of EBIC and CL measurements shows interesting correlations of the EBIC signal and the emission wavelengths of the quantum wells. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The optical and crystalline properties of a c-plane GaN-based LED structure with embedded semipolar InGaN quantum wells (QW) were investigated using highly spatially resolved cathodoluminescence spectroscopy (CL) directly performed in a scanning transmission electron microscope (STEM). Direct correlation of the cross-sectional STEM image with the simultaneously recorded spatially resolved CL mapping at room-temperature reveals the most intense emission coming from the semipolar InGaN QWs. We observe an inhomogeneous wavelength distribution due to local indium fluctuations and varyingQW thickness. In contrast, the donor-acceptor pair recombination (DAP) becomes the dominating luminescence process at 16K resulting in a superposition of the DAP luminescence and the InGaN QW emission. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Recent advances of the performance of GaN based devices with semipolar quantum wells have been realized homoepitaxially on pseudo bulk substrates which are typically small in size and high in cost. These limitations fuel the search for cheap and large area alternatives. Heteroepitaxial growth on sapphire substrates is well established with excellent results for polar GaN structures - the growth of semipolar gallium nitride on sapphire, however, presents unique challenges. In order to profit from our expertise in c-plane samples, our semipolar gallium nitride growth experiments are based on growth in c-direction. Using selective area epitaxy (SAE) on c-oriented templates, we can grow 3D structures with semipolar side facets. These structures are typically several μm in size which constitutes further challenges for device processing. Reducing the size of the structures to a sub-μm scale, we are able to bury our semipolar QWs within planar layers resulting in flat samples with c-plane surfaces. In this contribution, we present our results concerning the structural quality and spectral properties of quantum wells emitting in the blue and green spectral range as well as light emitting diodes. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Group III nitrides are promising materials for light emitting diodes (LEDs). The occurrence of structural defects strongly affects the efficiency of these LEDs. We investigate the optical properties of basal plane stacking faults (BFSs), and the assignment of specific spectral features to distinct defect types by direct correlation of localized emission bands measured by cathodoluminescence in a scanning electron microscope with defects found in high resolution (scanning) transmission electron microscopy and electron beam induced current at identical sample spots. Thus, we are able to model the electronic structure of BSFs addressing I_1, I_2, and E type BSFs in GaN and AlGaN with low Al content. We find hints that BSFs in semipolar AlGaN layers cause local changes of the Al content, which strongly affects the usability of AlGaN as an electron blocking layer in nitride based LEDs.

The effects of different Mg doping concentrations in the main p-GaN layer and the p-GaN capping layer on the electroluminescence (EL) properties of three-dimensional semipolar InGaN/GaN light emitting diode structures grown on GaN stripes with triangular cross-section were investigated. Secondary ion mass spectrometry analysis revealed the Mg concentration of the 3D semipolar p-GaN, indicating a higher Mg incorporation efficiency on the {10 (1) over bar1} facet as compared to the {11 (2) over bar2)over bar2} facet. The EL output power is low with a too low Mg concentration of 3x10(19)cm(-3), probably due to the inferior hole injection efficiency and stays almost constant with the Mg concentration ranging from 4x10(19)cm(-3) until 1.3x10(20)cm(-3) for the 3D LEDs with the {10 (1) over bar1} facet. Heavy Mg doping in the p-GaN capping layer is required to achieve good ohmic contact performance.

Using nanoimprint lithography, we fabricate GaN nano-structures with semipolar quantum wells on 2-inch c-oriented substrates and embed them within a planar waveguide to create a separate confinement heterostructure. Electroluminescence, transmission electron microscopy (TEM) and spatially resolved cathodoluminescence inside a scanning TEM (STEM-CL) is applied.