Crack-free AlGaN/GaN Bragg mirrors grown on Si (111) substrates by metalorganic vapor phase epitaxy

Crack-free distributed Bragg reflectors (DBR) with high reflectance and high Al content were grown over Si (111) substrates by metalorganic vapor phase epitaxy. This is achieved by introducing a set of nine period AlN / GaN short-period superlattices (SL) with average Al content of 50% instead of the bulk Al_0.5Ga_0.5N. We found that the AlN / GaN superlattices not only decreased the tensile strain but also improved the reflectance. Moreover, this structure also improves the surface roughness and reduces the dislocation densities. The larger refractive index ratio between high-Al-content Al_xGa_1-xN and GaN permits one to obtain a spectral stopband width of 25 nm and a reflectance value 62% at 480 nm for 7 mirror periods. We present the experimental and simulation results and the characterization by normal direction reflectance spectrum, high resolution x-ray diffraction (HRXRD), field emission scanning electron microscopy (SEM) and atomic force microscopy (AFM).