A hexagonal network of cracks on (a) a single ∼50 nm thick AlN-on-GaN layer and on ((b) and (c)) a 20-period AlN/GaN DBR, as observed by AFM. In the former case, the cracks (width ∼700 nm) are observable by optical microscopy. In the latter case, the cracks are unobservable by optical microscopy, since their width (∼90 nm) is below the diffraction limit resolution (∼200 nm).
TEM reveals (a) a highly periodical structure with (b) spontaneously formed (Al,Ga)N TLs resulting in a quasi four-layer structure. The V-shape domains are sporadically observed. (c) HAADF image and (d) EELS map with Plasmon peak maximum of a V-shape domain confirm that formation of (Al,Ga)N TLs is highly reduced (sometimes fully suppressed) near defective regions (more details can be found in Ref. 23 ).
(a) ω/2ϴ scan around the  Bragg reflection reveals presence of (Al,Ga)N and many satellite peaks that confirm high periodicity of the structure. (b) XRD reveals that GaN grows compressively strained (∼−1%) on AlN, whereas AlN grows tensile strained (∼+1.2%) on GaN.
Evolution of DBR reflectivity with increasing number of periods, from 6 to 20.
Waves reflected across one period of (left) nominal DBR and (right) four-layer period structure with homogeneous TLs. The phasors of each structure, demonstrating the reflected wave phase conservation and intensity attenuation in the latter case, are also shown for clarity.
Comparison between experimentally measured reflectivity profile and simulations: nominal DBR with 2.05/2.50 refractive indices and a DBR with reduced refractive index contrast 2.175/2.347 that has optical properties equivalent to the formed four-layer period structure.
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