High-Al-content crack-free AlGaN/GaN Bragg mirrors grown by molecular-beam epitaxy

We report on the growth by molecular-beam epitaxy on 2 in. sapphire substrates of crack-free $AlxGa1−xN/GaN$ distributed Bragg reflectors (DBRs) with high-Al composition $(x=0.5).$ This is achieved by introducing a thick AlN interlayer and strain mediating $AlyGa1−yN$ layer between the substrate and DBR. The relatively larger refractive index ratio between $Al0.5Ga0.5N$ and GaN permits one to obtain a quite large spectral stopband width (49 nm) and a high reflectance value (69%) for only eight mirror periods.