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 ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N/GaN}$ distributed Bragg reflectors (DBRs) with high-Al composition $\text{(x=0.5).}$ This is achieved by introducing a thick AlN interlayer and strain mediating ${\mathrm{Al}}_y{\mathrm{Ga}}_{\text{1−y}}\mathrm{N}$ layer between the substrate and DBR. The relatively larger refractive index ratio between ${\mathrm{Al}}_{\text{0.5}}{\mathrm{Ga}}_{\text{0.5}}\mathrm{N}$ 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.