Impacts of anisotropic tilt mosaics of state-of-the-art m -plane freestanding GaN substrates on the structural and luminescent properties of m -plane Al_x Ga1-x N epilayers

Impacts of grown-in anisotropic tilt mosaics of state-of-the-art m -plane freestanding GaN (FS-GaN) substrates on the structural and luminescent properties of m -plane Al_x Ga1-x N epilayers are described. The results of cross-sectional transmission-electron microscopy and transmission-electron diffraction analyses reveal the following plane defects formation, even in mostly pseudomorphic epilayers: (i) The formation of basal-plane stacking faults (BSFs) is hard to avoid in the case of lattice-mismatched Al_x Ga1-x N thick film growth and (ii) the anisotropic greater m -plane tilt mosaic along the a -axis (twist in c -plane) of FS-GaN gives rise to pronounced formation of surface striations along the c -axis and planar-defect network (PDN) located on prismatic { 10 1- 0 } planes and pyramidal { 1 1- 0n } planes, which are often terminated by BSF, especially in the case of the epilayers grown by NH₃ -source molecular-beam epitaxy. These PDNs are assigned by spatially resolved cathodoluminescence (CL) and time-resolved photoluminescence measurements to associate with a characteristic emission peak approximately 200-300 meV lower than the near-band-edge (NBE) emission peak. Based on our database made with the aid of the positron-annihilation technique, three deep-state CL bands are correlated with cation vacancies (VIII) and cation-vacancy complexes. The intensity of the emission band originating from VIII -oxygen complexes was weaker by more than two orders of magnitude than the NBE peak.