Growth and characterization of high-quality 10-period AlGaN/GaN Bragg reflectors grown by molecular beam epitaxy

We report on the fabrication and characterization of efficient Bragg reflectors, based on 10-periods AlGaN/GaN stacks. Reflectors are designed with a target center wavelength of 510 nm that is applicable to resonant-cavity light emitting diodes for the plastic optical fiber communications. The heterostructures have been grown on GaN/Al2O3 templates by plasma-assisted molecular beam epitaxy using Al concentrations in the range between 15 and 45%. Atomic force microscopy shows smooth surfaces with average roughness of about 19 إ, a value well below the starting template one. Crack-free surfaces have been obtained when the Al content is below 30%. X-ray diffraction spectra reveal well-defined multiple satellite peaks which are characteristic for very good interfaces and periodicity. Structural analysis by high-resolution transmission electron microscopy shows a highly periodic structure with abrupt interfaces. In addition, threading dislocations do not affect the superlattice periodicity and quality, even though they run across the whole structure. Reflectivity measurements have also been performed and compared with simulated reflectivity spectra. Measured reflectivity values around 50% are in very close agreement with simulations.