a-plane GaN templates and coalesced epitaxial lateral overgrown (ELOG) films on r-plane sapphire substrates were investigated by x-ray diffraction (XRD). The a-plane GaN templates were found to have [0001]-oriented stripe-features, which is related to anisotropic mosaicity. For the mosaic blocks, the mosaicity reached the largest and the smallest values along the [11̄00] and the [0001] directions. The ELOG procedure with the SiO2 mask stripes perpendicular to the [0001] direction limits the preferable growth along this direction, and thereby enhances the [11̄00] growth. This leads to large-area, featureless, a-plane GaN films for which the wing tilt and not the fine mosaic block size becomes the major XRD line-broadening mechanism.
We report on an AlN/AlGaN superlattice approach to grow high-Al-content thick n+-AlGaN layers over c-plane sapphire substrates. Insertion of a set of AlN/AlGaN superlattices is shown to significantly reduce the biaxial tensile strain, thereby resulting in 3-μm-thick, crack-free Al0.2Ga0.8N layers. These high-quality, low-sheet-resistive layers are of key importance to avoid current crowding in quaternary AlInGaN multiple-quantum-well deep-ultraviolet light-emitting diodes over sapphire substrates.
Polarization effects have been studied in GaN/AlGaN multiple quantum wells (MQWs) with different c-axis orientation by means of excitation-dependent photoluminescence (PL) analysis. Quantum structures were grown on [0001]-oriented sapphire substrates (C plane) and single-crystalline [11̄00]-oriented freestanding GaN (M plane) using the metalorganic chemical vapor deposition technique. Strong PL spectrum line blueshifts (up to 140 meV) which are correlated with the excitation intensity have been obtained for C-plane MQWs, whereas no shift has been observed for M-plane MQWs. Theoretical calculations and comparison with the PL data confirm that the built-in electric field for C-plane structures is much stronger than the field present for M-plane MQWs. In the former case, the excitation-induced blueshift of the PL line is due to the screening of the built-in electric field by photoinjected carriers, which is consistent with the field strength of 1.23 MV/cm in the absence of excitation.
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